Institut für Bodenkunde und Standortslehre

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Improvement of the Barometric Process Separation (BaPS) technique to measure microbial C and N transformation rates in arable high-pH soils
(2023) Munz, Hannah; Streck, Thilo
The Barometric Process Separation (BaPS) technique provides a simple way to determine the rates of heterotrophic microbial respiration, gross nitrification and denitrification in soils by crossbalancing CO2 and O2 production and consumption rates in a closed incubation system via gas balances. The BaPS measuring system has some methodological limitations, especially in soils of pH above 6.5. In these soils, the CO2 balance of the incubation system is strongly influenced by abiotic fluxes driven by thermodynamic equilibration of the CO2 - carbonate system of the soil solution, i.e. a non-negligible fraction of CO2 produced via respiration is buffered by the soil solution. Correct quantification of this flux is necessary to correctly determine the microbial process rates. It has been shown that the thermodynamic calculation of CO2 dissolution does not deliver accurate results, leading to uncertainty in and considerable over- and underestimations of the microbial process rates. In this dissertation, this problem has been solved by developing a method to experimentally determine abiotic CO2 buffering, the Sterilization-CO2-Injection (SCI) method. Moreover, the soil specific adaptation of the Respiratory Quotient (RQ) has been studied in detail in order to reestablishes the advantage of the BaPS of operating isotope-free. Furthermore, in this dissertation we present an easy on-site calibration method for the BaPS sensor set in order to garantee optimal data quality although the maintenance service by the manufacturer has been canceled. Overall, the presented adaptations and improvements enhance the accuracy of BaPS measurements and might enhance its value as a tool for measuring gross nitrification rates in the future.
Seed dispersal by wind decreases when plants are water‐stressed, potentially counteracting species coexistence and niche evolution
(2021) Zhu, Jinlei; Lukić, Nataša; Rajtschan, Verena; Walter, Julia; Schurr, Frank M.
Hydrology is a major environmental factor determining plant fitness, and hydrological niche segregation (HNS) has been widely used to explain species coexistence. Nevertheless, the distribution of plant species along hydrological gradients does not only depend on their hydrological niches but also depend on their seed dispersal, with dispersal either weakening or reinforcing the effects of HNS on coexistence. However, it is poorly understood how seed dispersal responds to hydrological conditions. To close this gap, we conducted a common‐garden experiment exposing five wind‐dispersed plant species (Bellis perennis, Chenopodium album, Crepis sancta, Hypochaeris glabra, and Hypochaeris radicata) to different hydrological conditions. We quantified the effects of hydrological conditions on seed production and dispersal traits, and simulated seed dispersal distances with a mechanistic dispersal model. We found species‐specific responses of seed production, seed dispersal traits, and predicted dispersal distances to hydrological conditions. Despite these species‐specific responses, there was a general positive relationship between seed production and dispersal distance: Plants growing in favorable hydrological conditions not only produce more seeds but also disperse them over longer distances. This arises mostly because plants growing in favorable environments grow taller and thus disperse their seeds over longer distances. We postulate that the positive relationship between seed production and dispersal may reduce the concentration of each species to the environments favorable for it, thus counteracting species coexistence. Moreover, the resulting asymmetrical gene flow from favorable to stressful habitats may slow down the microevolution of hydrological niches, causing evolutionary niche conservatism. Accounting for context‐dependent seed dispersal should thus improve ecological and evolutionary models for the spatial dynamics of plant populations and communities.
Effects of farmland conversion to orchard or agroforestry on soil organic carbon fractions in an arid desert oasis area
(2022) Wang, Weixia; Ingwersen, Joachim; Yang, Guang; Wang, Zhenxi; Alimu, Aliya
In southern Xinjiang province, northwest China, farmland is undergoing rapid conversion to orchards or agroforestry. This has improved land-use efficiency but has also caused drastic ecological changes in this region. This study investigated the effects of farmland conversion to orchard or agroforestry on soil total organic carbon (TOC) and several soil labile fractions: readily oxidizable carbon (ROC), light fraction organic carbon (LFOC), and dissolved organic carbon (DOC). Soil samples were collected from seven cropping treatments: a monocultured wheat field (Mono), a 5-year-old jujube orchard (5 J), a 5-year-old jujube/wheat alley cropping system (5 JW), a 10-year-old jujube orchard (10 J), a 10-year-old jujube/wheat alley cropping system (10 JW), a 15-year-old jujube orchard (15 J), and a 15-year-old jujube/wheat alley cropping system (15 JW). The results show that the ROC concentrations varied from 0.17 ± 0.09 g/kg to 2.35 ± 0.05 g/kg across all land-use types and soil depths studied. It was higher in the 0–10 cm and 10–20 cm layers of treatment 10 JW than in other treatments and significantly greater than in the Mono treatment. The highest value of DOC was reached at 593.04 mg/kg in the 15 JW treatment at 0–10 cm. Labile organic carbon decreased with increasing depth in all treatments. The proportion of ROC and LFOC to TOC decreased with increasing soil depth. In all treatments, the ratio of DOC to TOC generally decreased initially and then increased again with increasing depth. Correlation analysis showed that ROC, LFOC, and DOC were closely correlated with TOC (p < 0.01). The ROC, LFOC, and DOC concentrations were significantly correlated with each other (p < 0.01). Following conversion of farmland to jujube orchard or agroforestry, the content and activity of soil organic carbon tended to increase due to augmentation of plant residues. Thus, jujube orchards and agroforestry systems are effective methods to restore soil organic carbon.
Do agricultural advisory services in Europe have the capacity to support the transition to healthy soils?
(2022) Ingram, Julie; Mills, Jane; Black, Jasmine E.; Chivers, Charlotte-Anne; Aznar-Sánchez, José A.; Elsen, Annemie; Frac, Magdalena; López-Felices, Belén; Mayer-Gruner, Paula; Skaalsveen, Kamilla; Stolte, Jannes; Tits, Mia
The need to provide appropriate information, technical advice and facilitation to support farmers in transitioning towards healthy soils is increasingly clear, and the role of the Agricultural Advisory Services (AAS) in this is critical. However, the transformation of AAS (plurality, commercialisation, fragmentation, decentralisation) brings new challenges for delivering advice to support soil health management. This paper asks: To what extent do agricultural advisory services have the capacity to support the transition to healthy soils across Europe? Using the ‘best fit’ framework, analytical characteristics of the AAS relevant to the research question (governance structures, management, organisational and individual capacities) were identified. Analysis of 18 semi-structured expert interviews across 6 case study countries in Europe, selected to represent a range of contexts, was undertaken. Capacities to provide soil health management (SHM) advice are constrained by funding arrangements, limited adviser training and professional development, adviser motivations and professional cultures, all determined by institutional conditions. This has resulted in a narrowing down of access and content of soil advice and a reduced capacity to support the transition in farming to healthy soils. The extent to which emerging policy and market drivers incentivise enhanced capacities in AAS is an important area for future research.
A new framework to assess sustainability of soil improving cropping systems in Europe
(2022) Alaoui, Abdallah; Hallama, Moritz; Bär, Roger; Panagea, Ioanna; Bachmann, Felicitas; Pekrun, Carola; Fleskens, Luuk; Kandeler, Ellen; Hessel, Rudi
Assessing agricultural sustainability is one of the most challenging tasks related to expertise and support methodologies because it entails multidisciplinary aspects and builds on cultural and value-based elements. Thus, agricultural sustainability should be considered a social concept, reliable enough to support decision makers and policy development in a broad context. The aim of this manuscript was to develop a methodology for the assessment of the sustainability of soil improving cropping systems (SICS) in Europe. For this purpose, a decision tree based on weights (%) was chosen because it allows more flexibility. The methodology was tested with data from the SoilCare Horizon 2020 study site in Germany for the assessment of the impact of the integration of cover crops into the crop rotation. The effect on the environmental indicators was slightly positive, but most assessed properties did not change over the short course of the experiment. Farmers reported that the increase in workload was outweighed by a reputation gain for using cover crops. The incorporation of cover crops reduced slightly the profitability, due to the costs for seeds and establishment of cover crops. The proposed assessment methodology provides a comprehensive summary to assess the agricultural sustainability of SICS.
Soil-improving cropping systems for sustainable and profitable farming in Europe
(2022) Hessel, Rudi; Wyseure, Guido; Panagea, Ioanna S.; Alaoui, Abdallah; Reed, Mark S.; van Delden, Hedwig; Muro, Melanie; Mills, Jane; Oenema, Oene; Areal, Francisco; van den Elsen, Erik; Verzandvoort, Simone; Assinck, Falentijn; Elsen, Annemie; Lipiec, Jerzy; Koutroulis, Aristeidis; O’Sullivan, Lilian; Bolinder, Martin A.; Fleskens, Luuk; Kandeler, Ellen; Montanarella, Luca; Heinen, Marius; Toth, Zoltan; Hallama, Moritz; Cuevas, Julián; Baartman, Jantiene E. M.; Piccoli, Ilaria; Dalgaard, Tommy; Stolte, Jannes; Black, Jasmine E.; Chivers, Charlotte-Anne
Soils form the basis for agricultural production and other ecosystem services, and soil management should aim at improving their quality and resilience. Within the SoilCare project, the concept of soil-improving cropping systems (SICS) was developed as a holistic approach to facilitate the adoption of soil management that is sustainable and profitable. SICS selected with stakeholders were monitored and evaluated for environmental, sociocultural, and economic effects to determine profitability and sustainability. Monitoring results were upscaled to European level using modelling and Europe-wide data, and a mapping tool was developed to assist in selection of appropriate SICS across Europe. Furthermore, biophysical, sociocultural, economic, and policy reasons for (non)adoption were studied. Results at the plot/farm scale showed a small positive impact of SICS on environment and soil, no effect on sustainability, and small negative impacts on economic and sociocultural dimensions. Modelling showed that different SICS had different impacts across Europe—indicating the importance of understanding local dynamics in Europe-wide assessments. Work on adoption of SICS confirmed the role economic considerations play in the uptake of SICS, but also highlighted social factors such as trust. The project’s results underlined the need for policies that support and enable a transition to more sustainable agricultural practices in a coherent way.
Bayesian multi-purpose modelling of plant growth and development across scales
(2024) Viswanathan, Michelle; Streck, Thilo
Crop models are invaluable tools for predicting the impact of climate change on crop production and assessing the fate of agrochemicals in the environment. To ensure robust predictions of crop yield, for example, models are usually calibrated to observations of plant growth and phenological development using different methods. However, various sources of uncertainty exist in the model inputs, parameters, equations, observations, etc., which need to be quantified, especially when model predictions influence decision-making. Bayesian inference is suitable for this purpose since it enables different uncertainties to be taken into account, while also incorporating prior knowledge. Thus, Bayesian methods are used for model calibration to improve the model and enhance prediction quality. However, this improvement in the model and its prediction quality does not always occur due to the presence of model errors. These errors are a result of incomplete knowledge or simplifying assumptions made to reduce model complexity and computational costs. For instance, crop models are used for regional scale simulations thereby assuming that these point-based models are able to represent processes that act at regional scale. Additionally, simple statistical assumptions are made about uncertainty in model errors during Bayesian calibration. In this work, the problems arising from such applications are analysed and other Bayesian approaches are investigated as potential solutions. A conceptually simple Bayesian approach of sequentially updating a maize phenology model, an important component in plant models, was investigated as yearly observation data were gathered. In this approach, model parameters and their uncertainty were estimated while accounting for observation uncertainty. As the model was calibrated to increasing amounts of observation data, the uncertainty in the model parameters reduced as expected. However, the prediction quality of the calibrated model did not always improve in spite of more data being available for potentially improving the model. This discrepancy was attributed to the presence of errors in the model structure, possibly due to missing environmental dependencies that were ignored during calibration. As a potential solution, the model was calibrated using Bayesian multi-level modelling which could account for model errors. Furthermore, this approach accounted for the hierarchical data structure of cultivars nested within maize ripening groups, thus simultaneously obtaining model parameter estimates for the species, ripening groups and cultivars. Applying this approach improved the model's calibration quality and further aided in identifying possible model deficits related to temperature effects in the post-flowering phase of development and soil moisture. As another potential solution, an alternative calibration strategy was tested which accounted for model errors by relaxing the strict statistical assumptions in classical Bayesian inference. This was done by first acknowledging that due to model errors, different data sets may yield diverse solutions to the calibration problem. Thus, instead of fitting the model to all data sets together and finding a compromise solution, a fit was found to each data set. This was implemented by modifying the likelihood, a term that accounts for information content of the data. An additive rather than the classical multiplicative strategy was used to combine likelihood values from different data sets. This approach resulted in conservative but more reliable predictions than the classical approach in most cases. The classical approach resulted in better predictions only when the prediction target represented an average of the calibration data. The above-mentioned results show that Bayesian methods with representative error assumptions lead to improved model performance and a more realistic quantification of uncertainties. This is a step towards the effective application of process-based crop models for developing suitable adaptation and mitigation strategies.
Microbial drivers of plant richness and productivity in a grassland restoration experiment along a gradient of land‐use intensity
(2022) Abrahão, Anna; Marhan, Sven; Boeddinghaus, Runa S.; Nawaz, Ali; Wubet, Tesfaye; Hölzel, Norbert; Klaus, Valentin H.; Kleinebecker, Till; Freitag, Martin; Hamer, Ute; Oliveira, Rafael S.; Lambers, Hans; Kandeler, Ellen
Plant–soil feedbacks (PSFs) underlying grassland plant richness and productivity are typically coupled with nutrient availability; however, we lack understanding of how restoration measures to increase plant diversity might affect PSFs. We examined the roles of sward disturbance, seed addition and land‐use intensity (LUI) on PSFs. We conducted a disturbance and seed addition experiment in 10 grasslands along a LUI gradient and characterized plant biomass and richness, soil microbial biomass, community composition and enzyme activities. Greater plant biomass at high LUI was related to a decrease in the fungal to bacterial ratios, indicating highly productive grasslands to be dominated by bacteria. Lower enzyme activity per microbial biomass at high plant species richness indicated a slower carbon (C) cycling. The relative abundance of fungal saprotrophs decreased, while pathogens increased with LUI and disturbance. Both fungal guilds were negatively associated with plant richness, indicating the mechanisms underlying PSFs depended on LUI. We show that LUI and disturbance affect fungal functional composition, which may feedback on plant species richness by impeding the establishment of pathogen‐sensitive species. Therefore, we highlight the need to integrate LUI including its effects on PSFs when planning for practices that aim to optimize plant diversity and productivity.
The role of crop management practices and adaptation options to minimize the impact of climate change on maize (Zea mays L.) production for Ethiopia
(2023) Feleke, Hirut Getachew; Savage, Michael J.; Fantaye, Kindie Tesfaye; Rettie, Fasil Mequanint
Climate change impact assessment along with adaptation measures are key for reducing the impact of climate change on crop production. The impact of current and future climate change on maize production was investigated, and the adaptation role of shifting planting dates, different levels of nitrogen fertilizer rates, and choice of maize cultivar as possible climate change adaptation strategies were assessed. The study was conducted in three environmentally contrasting sites in Ethiopia, namely: Ambo, Bako, and Melkassa. Future climate data were obtained from seven general circulation models (GCMs), namely: CanESM2, CNRM-CM5, CSIRO-MK3-6-0, EC-EARTH, HadGEM2-ES, IPSL-CM5A-MR, and MIROC5 for the highest representative concentration pathway (RCP 8.5). GCMs were bias-corrected at site level using a quantile-quantile mapping method. APSIM, AquaCrop, and DSSAT crop models were used to simulate the baseline (1995–2017) and 2030s (2021–2050) maize yields. The result indicated that the average monthly maximum air temperature in the 2030s could increase by 0.3–1.7 °C, 0.7–2.2 °C, and 0.8–1.8 °C in Ambo, Bako, and Melkassa, respectively. For the same sites, the projected increase in average monthly minimum air temperature was 0.6–1.7 °C, 0.8–2.3 °C, and 0.6–2.7 °C in that order. While monthly total precipitation for the Kiremt season (June to September) is projected to increase by up to 55% (365 mm) for Ambo and 75% (241 mm) for Bako respectively, whereas a significant decrease in monthly total precipitation is projected for Melkassa by 2030. Climate change would reduce maize yield by an average of 4% and 16% for Ambo and Melkassa respectively, while it would increase by 2% for Bako in 2030 if current maize cultivars were grown with the same crop management practice as the baseline under the future climate. At higher altitudes, early planting of maize cultivars between 15 May and 1 June would result in improved relative yields in the future climate. Fertilizer levels increment between 23 and 150 kg ha−1 would result in progressive improvement of yields for all maize cultivars when combined with early planting for Ambo. For a mid-altitude, planting after 15 May has either no or negative effect on maize yield. Early planting combined with a nitrogen fertilizer level of 23–100 kg ha−1 provided higher relative yields under the future climate. Delayed planting has a negative influence on maize production for Bako under the future climate. For lower altitudes, late planting would have lower relative yields compared to early planting. Higher fertilizer levels (100–150 kg ha−1) would reduce yield reductions under the future climate, but this varied among maize cultivars studied. Generally, the future climate is expected to have a negative impact on maize yield and changes in crop management practices can alleviate the impacts on yield.
Soil water status shapes nutrient cycling in agroecosystems from micrometer to landscape scales
(2022) Bauke, Sara L.; Amelung, Wulf; Bol, Roland; Brandt, Luise; Brüggemann, Nicolas; Kandeler, Ellen; Meyer, Nele; Or, Dani; Schnepf, Andrea; Schloter, Michael; Schulz, Stefanie; Siebers, Nina; von Sperber, Christian; Vereecken, Harry
Soil water status, which refers to the wetness or dryness of soils, is crucial for the productivity of agroecosystems, as it determines nutrient cycling and uptake physically via transport, biologically via the moisture‐dependent activity of soil flora, fauna, and plants, and chemically via specific hydrolyses and redox reactions. Here, we focus on the dynamics of nitrogen (N), phosphorus (P), and sulfur (S) and review how soil water is coupled to the cycling of these elements and related stoichiometric controls across different scales within agroecosystems. These scales span processes at the molecular level, where nutrients and water are consumed, to processes in the soil pore system, within a soil profile and across the landscape. We highlight that with increasing mobility of the nutrients in water, water‐based nutrient flux may alleviate or even exacerbate imbalances in nutrient supply within soils, for example, by transport of mobile nutrients towards previously depleted microsites (alleviating imbalances), or by selective loss of mobile nutrients from microsites (increasing imbalances). These imbalances can be modulated by biological activity, especially by fungal hyphae and roots, which contribute to nutrient redistribution within soils, and which are themselves dependent on specific, optimal water availability. At larger scales, such small‐scale effects converge with nutrient inputs from atmospheric (wet deposition) or nonlocal sources and with nutrient losses from the soil system towards aquifers. Hence, water acts as a major control in nutrient cycling across scales in agroecosystems and may either exacerbate or remove spatial disparities in the availability of the individual nutrients (N, P, S) required for biological activity.
Nitrogen dynamics of grassland soils with differing habitat quality: high temporal resolution captures the details
(2023) Kukowski, Sina; Ruser, Reiner; Piepho, Hans‐Peter; Gayler, Sebastian; Streck, Thilo
Excessive nitrogen (N) input is one of the major threats for species‐rich grasslands. The ongoing deterioration of habitat quality highlights the necessity to further investigate underlying N turnover processes. Our objectives were (1) to quantify gross and net rates of mineral N production (mineralization and nitrification) and consumption in seminatural grasslands in southwest Germany, with excellent or poor habitat quality, (2) to monitor the temporal variability of these processes, and (3) to investigate differences between calcareous and decalcified soils. In 2016 and 2017, gross N turnover rates were measured using the 15N pool dilution technique in situ on four Arrhenatherion meadows in biweekly cycles between May and November. Simultaneously, net rates of mineralization and nitrification, soil temperature, and moisture were measured. The vegetation was mapped, and basic soil properties were determined. The calcareous soils showed higher gross nitrification rates compared with gross mineralization. In contrast, nitrification was inhibited in the decalcified soils, most likely due to the low pH, and mineralization was the dominant process. Both mineralization and nitrification were characterized by high temporal variability (especially the former) and short residence times of N in the corresponding pools (<2 days) at all sites. This illustrates that high temporal resolution is necessary during the growing season to detect N mineralization patterns and capture variability. Parallel determination of net N turnover rates showed almost no variability, highlighting that net rates are not suitable for drawing conclusions about actual gross turnover rates. During the growing season, the data show no clear relationship between soil temperature/soil moisture and gross N turnover rates. For future experiments, recording of microbial biomass, dissolved organic matter, and root N uptake should be considered.
Comprehensive assessment of climate extremes in high-resolution CMIP6 projections for Ethiopia
(2023) Rettie, Fasil M.; Gayler, Sebastian; Weber, Tobias K. D.; Tesfaye, Kindie; Streck, Thilo
Climate extremes have more far-reaching and devastating effects than the mean climate shift, particularly on the most vulnerable societies. Ethiopia, with its low economic adaptive capacity, has been experiencing recurrent climate extremes for an extended period, leading to devastating impacts and acute food shortages affecting millions of people. In face of ongoing climate change, the frequency and intensity of climate extreme events are expected to increase further in the foreseeable future. This study provides an overview of projected changes in climate extremes indices based on downscaled high-resolution (i.e., 10 × 10 km2) daily climate data derived from global climate models (GCMs). The magnitude and spatial patterns of trends in the projected climate extreme indices were explored under a range of emission scenarios called Shared Socioeconomic Pathways (SSPs). The performance of the GCMs to reproduce the observed climate extreme trends in the base period (1983–2012) was evaluated, the changes in the climate projections (2020–2100) were assessed and the associated uncertainties were quantified. Overall, results show largely significant and spatially consistent trends in the projected temperature-derived extreme indices with acceptable model performance in the base period. The projected changes are dominated by the uncertainties in the GCMs at the beginning of the projection period while by the end of the century proportional uncertainties arise both from the GCMs and SSPs. The results for precipitation-related extreme indices are heterogeneous in terms of spatial distribution, magnitude, and statistical significance coverage. Unlike the temperature-related indices, the uncertainty from internal climate variability constitutes a considerable proportion of the total uncertainty in the projected trends. Our work provides a comprehensive insight into the projected changes in climate extremes at relatively high spatial resolution and the related sources of projection uncertainties.
Increasing plant species richness by seeding has marginal effects on ecosystem functioning in agricultural grasslands
(2023) Freitag, Martin; Hölzel, Norbert; Neuenkamp, Lena; van der Plas, Fons; Manning, Peter; Abrahão, Anna; Bergmann, Joana; Boeddinghaus, Runa; Bolliger, Ralph; Hamer, Ute; Kandeler, Ellen; Kleinebecker, Till; Knorr, Klaus‐Holger; Marhan, Sven; Neyret, Margot; Prati, Daniel; Le Provost, Gaëtane; Saiz, Hugo; van Kleunen, Mark; Schäfer, Deborah; Klaus, Valentin H.
Experimental evidence shows that grassland plant diversity enhances ecosystem functioning. Yet, the transfer of results from controlled biodiversity experiments to naturally assembled ‘real world’ ecosystems remains challenging due to environmental variation among sites, confounding biodiversity ecosystem functioning relations in observational studies. To bridge the gap between classical biodiversity‐ecosystem functioning experiments and observational studies of naturally assembled and managed ecosystems, we created regionally replicated, within‐site gradients of species richness by seeding across agricultural grasslands differing in land‐use intensity (LUI) and abiotic site conditions. Within each of 73 grassland sites, we established a full‐factorial experiment with high‐diversity seeding and topsoil disturbance and measured 12 ecosystem functions related to productivity, and carbon and nutrient cycling after 4 years. We then analysed the effects of plant diversity (seeded richness as well as realized richness), functional community composition, land use and abiotic conditions on the ecosystem functions within (local scale) as well as among grassland sites (landscape scale). Despite the successful creation of a within‐site gradient in plant diversity (average increase in species richness in seeding treatments by 10%–35%), we found that only one to two of the 12 ecosystem functions responded to realized species richness, resulting in more closed nitrogen cycles in more diverse plant communities. Similar results were found when analysing the effect of the seeding treatment instead of realized species richness. Among sites, ecosystem functioning was mostly driven by environmental conditions and LUI. Also here, the only functions related to plant species richness were those associated with a more closed nitrogen cycle under increased diversity. The minor effects of species enrichment we found suggest that the functionally‐relevant niche space is largely saturated in naturally assembled grasslands, and that competitive, high‐functioning species are already present. Synthesis: While nature conservation and cultural ecosystem services can certainly benefit from plant species enrichment, our study indicates that restoration of plant diversity in naturally assembled communities may deliver only relatively weak increases in ecosystem functioning, such as a more closed nitrogen cycle, within the extensively to moderate intensively managed agricultural grasslands of our study.
The need to decipher plant drought stress along the soil-plant-atmosphere continuum
(2023) Schweiger, Andreas H.; Zimmermann, Telse; Poll, Christian; Marhan, Sven; Leyrer, Vinzent; Berauer, Bernd J.
Lacking comparability among rainfall manipulation studies is still a major limiting factor for generalizations in ecological climate change impact research. A common framework for studying ecological drought effects is urgently needed to foster advances in ecological understanding the effects of drought. In this study, we argue, that the soil–plant–atmosphere‐continuum (SPAC), describing the flow of water from the soil through the plant to the atmosphere, can serve as a holistic concept of drought in rainfall manipulation experiments which allows for the reconciliation experimental drought ecology. Using experimental data, we show that investigations of leaf water potential in combination with edaphic and atmospheric drought – as the three main components of the SPAC – are key to understand the effect of drought on plants. Based on a systematic literature survey, we show that especially plant and atmospheric based drought quantifications are strongly underrepresented and integrative assessments of all three components are almost absent in current experimental literature. Based on our observations we argue, that studying dynamics of plant water status in the framework of the SPAC can foster comparability of different studies conducted in different ecosystems and with different plant species and can facilitate extrapolation to other systems, species or future climates.
Linking horizontal crosshole GPR variability with root image information for maize crops
(2023) Lärm, Lena; Bauer, Felix Maximilian; van der Kruk, Jan; Vanderborght, Jan; Morandage, Shehan; Vereecken, Harry; Schnepf, Andrea; Klotzsche, Anja
Non‐invasive imaging of processes within the soil–plant continuum, particularly root and soil water distributions, can help optimize agricultural practices such as irrigation and fertilization. In this study, in‐situ time‐lapse horizontal crosshole ground penetrating radar (GPR) measurements and root images were collected over three maize crop growing seasons at two minirhizotron facilities (Selhausen, Germany). Root development and GPR permittivity were monitored at six depths (0.1–1.2 m) for different treatments within two soil types. We processed these data in a new way that gave us the information of the “trend‐corrected spatial permittivity deviation of vegetated field,” allowing us to investigate whether the presence of roots increases the variability of GPR permittivity in the soil. This removed the main non‐root‐related influencing factors: static influences, such as soil heterogeneities and rhizotube deviations, and dynamic effects, such as seasonal moisture changes. This trend‐corrected spatial permittivity deviation showed a clear increase during the growing season, which could be linked with a similar increase in root volume fraction. Additionally, the corresponding probability density functions of the permittivity variability were derived and cross‐correlated with the root volume fraction, resulting in a coefficient of determination (R2) above 0.5 for 23 out of 46 correlation pairs. Although both facilities had different soil types and compaction levels, they had similar numbers of good correlations. A possible explanation for the observed correlation is that the presence of roots causes a redistribution of soil water, and therefore an increase in soil water variability.
Effectiveness of bio-effectors on maize, wheat and tomato performance and phosphorus acquisition from greenhouse to field scales in Europe and Israel: a meta-analysis
(2024) Nkebiwe, Peteh Mehdi; Stevens Lekfeldt, Jonas D.; Symanczik, Sarah; Thonar, Cécile; Mäder, Paul; Bar-Tal, Asher; Halpern, Moshe; Biró, Borbala; Bradáčová, Klára; Caniullan, Pedro C.; Choudhary, Krishna K.; Cozzolino, Vincenza; Di Stasio, Emilio; Dobczinski, Stefan; Geistlinger, Joerg; Lüthi, Angelika; Gómez-Muñoz, Beatriz; Kandeler, Ellen; Kolberg, Flora; Kotroczó, Zsolt; Kulhanek, Martin; Mercl, Filip; Tamir, Guy; Moradtalab, Narges; Piccolo, Alessandro; Maggio, Albino; Nassal, Dinah; Szalai, Magdolna Zita; Juhos, Katalin; Fora, Ciprian G.; Florea, Andreea; Poşta, Gheorghe; Lauer, Karl Fritz; Toth, Brigitta; Tlustoš, Pavel; Mpanga, Isaac K.; Weber, Nino; Weinmann, Markus; Yermiyahu, Uri; Magid, Jakob; Müller, Torsten; Neumann, Günter; Ludewig, Uwe; de Neergaard, Andreas
Biostimulants (Bio-effectors, BEs) comprise plant growth-promoting microorganisms and active natural substances that promote plant nutrient-acquisition, stress resilience, growth, crop quality and yield. Unfortunately, the effectiveness of BEs, particularly under field conditions, appears highly variable and poorly quantified. Using random model meta-analyses tools, we summarize the effects of 107 BE treatments on the performance of major crops, mainly conducted within the EU-funded project BIOFECTOR with a focus on phosphorus (P) nutrition, over five years. Our analyses comprised 94 controlled pot and 47 field experiments under different geoclimatic conditions, with variable stress levels across European countries and Israel. The results show an average growth/yield increase by 9.3% (n=945), with substantial differences between crops (tomato > maize > wheat) and growth conditions (controlled nursery + field (Seed germination and nursery under controlled conditions and young plants transplanted to the field) > controlled > field). Average crop growth responses were independent of BE type, P fertilizer type, soil pH and plant-available soil P (water-P, Olsen-P or Calcium acetate lactate-P). BE effectiveness profited from manure and other organic fertilizers, increasing soil pH and presence of abiotic stresses (cold, drought/heat or salinity). Systematic meta-studies based on published literature commonly face the inherent problem of publication bias where the most suspected form is the selective publication of statistically significant results. In this meta-analysis, however, the results obtained from all experiments within the project are included. Therefore, it is free of publication bias. In contrast to reviews of published literature, our unique study design is based on a common standardized protocol which applies to all experiments conducted within the project to reduce sources of variability. Based on data of crop growth, yield and P acquisition, we conclude that application of BEs can save fertilizer resources in the future, but the efficiency of BE application depends on cropping systems and environments.
Formation of mineral‐associated organic matter in temperate soils is primarily controlled by mineral type and modified by land use and management intensity
(2023) Bramble, De Shorn E.; Ulrich, Susanne; Schöning, Ingo; Mikutta, Robert; Brandt, Luise; Poll, Christian; Kandeler, Ellen; Mikutta, Christian; Konrad, Alexander; Siemens, Jan; Yang, Yang; Polle, Andrea; Schall, Peter; Ammer, Christian; Kaiser, Klaus; Schrumpf, Marion
Formation of mineral-associated organic matter (MAOM) supports the accumulation and stabilization of carbon (C) in soil, and thus, is a key factor in the global C cycle. Little is known about the interplay of mineral type, land use and management intensity in MAOM formation, especially on subdecadal time scales. We exposed mineral containers with goethite or illite, the most abundant iron oxide and phyllosilicate clay in temperate soils, for 5 years in topsoils of 150 forest and 150 grassland sites in three regions across Germany. Results show that irrespective of land use and management intensity, more C accumulated on goethite than illite (on average 0.23 ± 0.10 and 0.06 ± 0.03 mg m−2 mineral surface respectively). Carbon accumulation across regions was consistently higher in coniferous forests than in deciduous forests and grasslands. Structural equation models further showed that thinning and harvesting reduced MAOM formation in forests. Formation of MAOM in grasslands was not affected by grazing. Fertilization had opposite effects on MAOM formation, with the positive effect being mediated by enhanced plant productivity and the negative effect by reduced plant species richness. This highlights the caveat of applying fertilizers as a strategy to increase soil C stocks in temperate grasslands. Overall, we demonstrate that the rate and amount of MAOM formation in soil is primarily driven by mineral type, and can be modulated by land use and management intensity even on subdecadal time scales. Our results suggest that temperate soils dominated by oxides have a higher capacity to accumulate and store C than those dominated by phyllosilicate clays, even under circumneutral pH conditions. Therefore, adopting land use and management practices that increase C inputs into oxide-rich soils that are under their capacity to store C may offer great potential to enhance near-term soil C sequestration.
Release of glucose from dissolved and mineral‐bound organic matter by enzymatic hydrolysis
(2023) Lenhardt, Katharina R.; Brandt, Luise; Poll, Christian; Rennert, Thilo; Kandeler, Ellen
Sorption of dissolved organic matter (DOM) by poorly crystalline minerals during their formation may protect large amounts of carbon in soils from mineralization. We investigated the bioavailability of carbohydrates in DOM and after co-precipitation with short-range ordered aluminosilicates. Carbohydrates originated from soil solutions collected in situ at two depths of a Dystric Cambisol, and from litter extracts. Quantification of substrate-specific degradability was achieved by the addition of β-glucosidase at an optimal concentration and subsequent determination of glucose release. Depending on DOM composition, 0.6–41.4 mg g−1 C−1 of glucose was enzymatically released from dissolved carbohydrates. Co-precipitated carbohydrates were partially accessible, resulting in a glucose release of 0.7–5.2 mg g−1 C−1. Restricted enzymatic depolymerization due to co-precipitation may contribute to accumulation of easily degradable substrates in soils.
Associations of short-range ordered aluminosilicates and organic matter: formation, properties and stabilization of organic matter
(2023) Lenhardt, Katharina Raphaela; Rennert, Thilo
Short-range ordered aluminosilicates (SROAS) typically form during the weathering of volcanic ejecta by polymerization of released aluminium (Al) and silicon (Si). These minerals exhibit variable chemical composition and crystallinity. Tubular imogolite is a SROAS with long-range order; its locally defined Si configuration occurs also in poorly ordered SROAS. Interactions of SROAS and organic matter (OM) promote carbon (C) accrual by protecting OM from microbial degradation in the long term; however, the fundamental processes are poorly understood. Stable mineral-organic associations may form by adsorption of dissolved OM (DOM) on SROAS surfaces and by co-precipitation of DOM with SROAS during mineral formation. The objective of this study was to elucidate the chemical interactions between DOM and SROAS by both processes, and to assess the stability of OM sorbed by SROAS. Therefore, the impact of SROAS composition on DOM adsorption, partitioning of OM moieties by adsorption and co-precipitation, the structure of co-precipitates, and the degradability of co-precipitated OM was investigated. A method to synthesize SROAS at ambient conditions was developed to mimic the surface properties of natural weathering products. Characterization of SROAS structure by solid-state 27Al and 29Si nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy evinced a close similarity of synthetic SROAS to their natural analogues. Aluminium-rich SROAS (molar Al:Si>2) resembled proto-imogolite, with Al mainly in octahedral coordination and ≥38% of Si nuclei exhibiting an imogolite-like configuration. Silicon-rich SROAS (molar Al:Si = 1.4) contained tetrahedral Al and Si existed largely in ill-defined environments. Analyses of the specific surface area by nitrogen adsorption revealed marked aggregation of Al-rich SROAS, which was less pronounced in Si-rich SROAS. These results show that the poor crystallinity of Al-rich SROAS permits a very dense spatial arrangement of mass at the submicron scale, while Si incorporation restricts aggregation. Interactions of SROAS and DOM were studied using natural DOM with heterogeneous composition collected in situ from a Dystric Cambisol at two depths, and by water extraction of litter. Solid-state 13C-NMR and FTIR spectroscopy revealed a major contribution of oxidized aromatic moieties to soil DOM, likely originating from lignin degradation, while litter DOM was predominantly composed of carbohydrates. Soil DOM adsorption was driven by surface accessibility and was thus larger for Si-rich SROAS than for Al-rich SROAS, showing the importance of SROAS physical properties for OM retention. Aromatic products of lignin degradation preferentially adsorbed on SROAS, inducing relative enrichment of aliphatic substances, particularly carbohydrates, in the residual DOM. Topsoil DOM adsorption depended more strongly on contact time (1–168 h) than subsoil DOM adsorption, possibly due to qualitative differences of the aromatic fraction. Associations formed by co-precipitation contained more C than adsorption complexes. As Al interacted preferentially with oxidized aromatic compounds, co-precipitation of DOM increased as a function of aromatic C. Nevertheless, marked sorption of carbohydrates from litter DOM evinced possible retention of substances with low affinity for Al by co-precipitation, in contrast to adsorption. Time-dependent (1–72 h) structural evolution of SROAS in the absence of DOM was examined to resolve the mechanisms of SROAS formation. Irrespective of the initial molar Al:Si ratio, amorphous precursors formed by olation during the first hour. After 72 h, up to 50% of Si nuclei exhibited imogolite-like configuration, showing rapid development of short-range order. Dissolved OM interfered in condensation of Al and Si, causing partial exclusion of Si, and slowed crystallisation of the octahedral Al sheet, promoting ill-defined Si species in the co-precipitates. Hence, DOM likely impedes assembly of precursors into structurally ordered particles, in particular, oxidized aromatic DOM in topsoils. The binding strength of DOM to SROAS surfaces may be affected by Si incorporation due to structure modifications. Thus, a mechanistic adsorption study was conducted with oxalic, salicylic and octanoic acid as models of functional moieties in DOM. Adsorption of oxalic and salicylic acid was up to 80–90% lower for Si-rich SROAS than for Al-rich SROAS. Rapid (<1 min) release of hydroxyls, indicating ligand exchange, was observed only for oxalic and salicylic acid, suggesting octanoic acid interacted electrostatically with SROAS surfaces. Chelate complexes of oxalic acid and partial inner-sphere binding of salicylic acid on both SROAS were identified by FTIR spectroscopy. Fast adsorption kinetics were retraced by changes in electrical conductivity using a stopped-flow technique. Ligand exchange by oxalate proceeded at a similar rate as complexation of monomeric Al3+, showing its binding to octahedral Al. Hence, the much lower susceptibility of Si-rich SROAS to ligand exchange with carboxyl groups is caused by tetrahedral Al. Consequently, little OM may be stabilized by chemical bonds with Si-rich SROAS. The degradability of co-precipitated carbohydrates was tested by addition of b-glucosidase, a microbial extracellular enzyme, at optimal concentration and quantification of the reaction product glucose. Glucose release was analysed for initial DOM, co-precipitated OM and for residual DOM to account for compositional changes by co-precipitation. As a result of carbohydrate enrichment in residual DOM, its degradability by b-glucosidase increased. Minor amounts of glucose were released from co-precipitated carbohydrates, showing their restricted accessibility for enzymes due to occlusion. Formation of SROAS in soils likely induces preferential association of lignin degradation products with the mineral matrix and alters the composition of OM introduced to the subsoil. Otherwise easily degradable OM with low affinity for SROAS surfaces can be effectively protected from mineralization by co-precipitation. As the structure of SROAS reflects formation processes and affects their reactivity, future research characterizing natural SROAS will give mechanistic insights into C sequestration and potentially other vital soil functions.
Formation and properties of inorganic Si-contaminant compounds
(2023) Stein, Mathias; Rennert, Thilo
Environmental contamination is the most pressing issue of our global society. Among others, contamination with potentially toxic elements (PTEs) such as cadmium (Cd), copper (Cu), and lead (Pb) threatens organisms, humans, and entire ecosystems. Silicon (Si) is known to benefit the resilience to such abiotic stresses and its application showed to alleviate PTE toxicity. These beneficial effects are predominantly attributed to in planta processes, but PTE immobilization in soil induced by Si addition has also been reported. However, interactions between silicic acid and Cd, Cu, and Pb at undersaturation of their silicates and other mineral phases remains elusive. Silicic acid, which is dissolved Si, may interact with cationic PTEs in soil, altering their environmental fate. At oversaturation, PTEs and silicic acid may precipitate forming metal silicates, whereas at undersaturation PTEs may be incorporated into the network of polymerized silicic acid or inner-spherically complexed on the negatively charged surface of polymeric silicic acid, forming particulate Si-contaminant compounds. Aiming to elucidate the extent and the mechanism of the potential PTE immobilization, long-term formation experiments in aqueous solution, a soil column experiment, and batch adsorption experiments including isothermal titration calorimetry (ITC) experiments were conducted. Long-term formation experiments in aqueous solution were conducted at undersaturation of PTE silicates and other mineral phases. Time-dependent particle size and charge changes were measured in between 211 days using dynamic light scattering and phase analysis light scattering. Solid phases were characterized by Fourier transform infrared (FTIR) spectroscopy and 29Si nuclear magnetic resonance (NMR) spectroscopy. Particle size measurements revealed a positive effect of cationic PTEs on silicic acid polymerization (Cu>Cd>Pb). However, only traces (2.1‰ Cd, 2‰ Cu and 1.4‰ Pb of the initially added PTEs) were bound during the polymerization of silicic acid. Copper was incorporated in the polymeric network of silicic acid during its polymerization as indicated by FTIR spectra and 29Si NMR relaxation experiments. Cadmium was only outer-spherically adsorbed. The long-term formation experiments revealed that particulate compounds form due to silicic acid/PTE interactions at undersaturation of other mineral phases. Soil column experiments were conducted to investigate the formation of Si-contaminant compounds in an acidic soil (pH 4.6). Therefore, a Haplic Phaeozem was preconditioned with Cu and Cd in the absence and presence of additional monomeric silicic acid and subsequently irrigated with artificial rainwater. Interactions of silicic acid and PTEs were investigated by monitoring the elemental composition of the eluates, and the size and charge of the particles eluted. After irrigation, total and exchangeable Si and PTE contents were analysed. Silicic acid application resulted in larger particles in the eluates, indicating silicic acid polymerization. The molar metal:Si ratios of the eluates and the significant correlation between Si and exchangeable metals indicated that particularly Cu formed Si-contaminant compounds in the soil, enhancing its retention. However, translocation of PTEs in particulate form, associated with polymerized silicic acid, was indicated. The negative charge and the very small size of the formed compounds may facilitate translocation from soil into groundwater. Batch adsorption experiments and ITC experiments were conducted to examine mechanism and extent of PTE adsorption to polymeric silicic acid. These experiments did not reveal any adsorption of the metals on polymeric silicic acid at pH 4 to 6 and after 24 h, which was underpinned by the results of the ITC experiments. However, zeta-potential measurements indicated weak electrostatic interactions between the negatively charged silanol groups and the PTEs. These electrostatic interactions may be the initial step of Si-contaminant compound formation. This thesis elucidates extent and mechanisms of silicic acid, either mono- or polymeric, and PTE interactions, showing the formation of particulate compounds from the reaction between silicic acid and cationic PTEs in aqueous solution and in an acidic soil. Particularly Cu formed stable compounds during silicic acid polymerization. However, the interactions showed a low extent and mainly weak electrostatic interactions, concluding that the addition of monomeric silicic acid to acidic soils may not be a quantitatively effective measure to reduce PTE mobility in soils. Aggregation effects, resulting from freezing/thawing or drying/rewetting, however, could alter the mobility of Si-contaminant compounds. These effects may be subject of future research, as well as the spectroscopic detection of Si-contaminant compounds in soils.

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