Browsing by Subject "Stabiles Isotop"

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Biochar amendment for C sequestration in a temperate agroecosystem : implications for microbial C- and N-cycling
(2018) Bamminger, Chris; Kandeler, Ellen
Climate warming will have great impact on terrestrial ecosystems. Different soil properties such as temperature and moisture will be altered, thereby influencing C- and N-cycles, microbial activity as well as plant growth. This may contribute to the observed increase in soil greenhouse gas (GHG) emissions under climate change. Therefore, new options are needed to mitigate theses projected consequences. Biochar is primarily suggested to be effective in long-term C sequestration in agricultural soils due to its long-term stability. In addition, it could be applied to improve various soil properties, plant growth and to reduce soil GHG emissions. To date, knowledge about such beneficial biochar effects in soil under predicted warming climate is extremely scarce. In the first study, a slow-pyrolysis biochar from Miscanthus x giganteus feedstock (600 °C, 30 Min.) was incubated for short time (37d) under controlled laboratory conditions in agricultural soil in the presence of earthworms and N-rich litter (Phacelia tanacetifolia Benth.). Biochar increased microbial abundances and the fungal-to-bacterial PLFA ratio after 37 days in arable soil applied with litter suggesting improved living conditions for microorganisms with biochar. Fungi may benefit most from newly created habitats due to colonizable biochar pores and surfaces. Additionally, fungi could have also mineralized small amounts of recalcitrant biochar-C during plant litter decomposition. Without litter, biochar led to interactions between earthworms and soil microorganisms resulting in enhanced bacterial and fungal abundances. This indicates better growth habitats for soil microbes in earthworm casts containing biochar. Soil respiration and metabolic quotients (qCO2) and N2O emissions (in litter treatments) were decreased after biochar application suggesting a more efficient microbial community and underscoring the GHG mitigation potential of the used biochar. The field experiment, investigated in the second and third study, focused on the stability and long-term soil C sequestration potential of comparable Miscanthus biochar (850 °C, 30 Min.). Related effects on soil GHG emissions, physical, chemical and microbiological soil properties as well as plant growth were determined in an agroecosystem at year-round elevated soil temperature (+2.5 °C, since 2008). The second study investigated the short-term effects of biochar on microbial abundances and growth of winter rapeseed during the first year after field application to a warmed temperate arable soil. It was found that fungal biomass and the fungal-to-bacterial ratio were increased in the warmed biochar plots only after three months in the presence of spring barley litter from the previous growing season. The disappearance of this effect points to an overall high stability of the investigated biochar. Moreover, biochar proved to be effective in mitigating negative effects of seasonal dryness on microbial abundances and early plant growth in the dry spring period in 2014. However, biochar had no effect on final aboveground biomass of winter rapeseed at harvest in the first growing season. As shown in the third study, after two vegetation periods of winter rapeseed and spring wheat, the assumption that plant productivity in already fertile temperate arable soils is unlikely to be further enhanced with biochar amendment, was confirmed. Total CO2 emissions after two years were not reduced by biochar and remained unchanged even under warming suggesting a high degradation stability of the used biochar. N2O emissions were increased in biochar-amended soil at elevated soil temperature, presumably due to enhanced water and fertilizer retention with biochar. By using the global warming potential (GWP100) of total soil GHG emissions, the storage of biochar-C in soil was estimated to compensate warming-induced elevated soil GHG emissions for 20 years. To conclude, this thesis revealed that biochar may have only minor influence on soil microorganisms and crop growth in temperate, fertile arable field soils. However, it was shown that biochar could be a valuable tool for C sequestration in temperate arable soils, thus potentially offsetting a warming-induced increase in GHG emissions. In order to face climate change impacts, more long-term studies on microbiological effects and the C sequestration potential of biochar in cultivated soil under warming are urgently needed.
Crop yield and fate of nitrogen fertilizer in maize-based soil conservation systems in Western Thailand
(2021) Wongleecharoen, Chalermchart; Cadisch, Georg
The increase in food demand and land scarcity in high-potential lowland areas have forced cropping intensification with a transformation of land use from subsistence to permanent agriculture in remote hillside in Southeast Asia. This change and inappropriate land use are the prime cause of soil degradation by erosion, which have negatively affected the agricultural systems productivity and sustainability in Thailand. Therefore, vulnerable land in sloping terrain is classified as unsuitable for continuous production of arable crops unless conservation measures are introduced to stabilize the landscape. Even though conservation practices can stabilize sloping land, farmers have not been widely adopted the measures due to various constraints, such as crop area loss and crop-tree competition. To improve land use management, a two-year study (2010-2011) was conducted at the Queen Sirikit research station (13°28’N, 99°16’E), Ratchaburi Province, Thailand, on a hillside with a slope of around 20%. The treatments consisted of (T1) maize (Zea mays L.) mono-crop under tillage and fertilization, (T2) maize intercropped with chili (Capsicum annuum L.) under tillage and fertilization, (T3) maize intercropped with chili, application of minimum tillage plus Jack bean (Canavalia ensiformis (L.) DC) relay cropping and fertilizer application, (T4) maize intercropped with chili, application of minimum tillage with Jack bean relay cropping and fertilizer application plus perennial hedges of Leucaena leucocephala (Lam.) de Wit, (T5) as T3 but without fertilization, and (T6) as T4 but without fertilization. There was an additional plot of chili sole cropping to calculate the land equivalent ratio (LER). The first part of the study evaluated yield performance and nitrogen use efficiency (NUE) of crops using the 15N isotope technique under diverse fertilized cropping systems during the first year. Maize grain yields were lower in T2 (3.1 Mg ha-1), T3 (2.6 Mg ha-1) and T4 (3.3 Mg ha-1) than in the control (T1) (6.7 Mg ha-1). The total returns from maize and chili yields were 1,914, 5,129, 3,829, 3,900, 3,494, and 2,976 USD ha-1, for T1, T2, T3, T4, T5 and T6, respectively. Higher economic returns in mixed crop systems, by selling both maize and chilies, compensated for the maize area loss by intercropping. Maize 15NUE was highest in T2 (53.5%), being significantly higher than in T1 (47.0%), T3 (45.5%), and T4 (45.7%). Overall system’s NUE in T2 (56.8%) was comparable to T1 (53.8%) and T4 (54.5%) but significantly lower in T3 (48.6%). Minimum tillage and hedgerows (despite their positive filter effect) did not increase NUE but adversely affected maize growth during the establishment phase. The second part of the study examined nitrogen fertilizers fate and quantified partial nitrogen budgets at plot level over two cropping seasons for various maize-based cropping systems with or without fertilizer application. Overall plant uptake of fertilizer 15N applied to maize was 48.6-56.8% over the first season, while residual fertilizer 15N recovery of plants was only 2.3-4.9% over the subsequent season. The quantity of applied labelled N remaining in the soil at the end of season 1 and season 2 was 6.2-28.1% and 7.7-28.6%, respectively. Thus, 60.0-76.0% in season 1 and 12.7-31.3% in season 2 of the applied fertilizer 15N were accounted for within the plant-soil system. Consequently, 24.0-40.0% and 12.9-16.1% of labelled fertilizer N were not accounted for at the end of season 1 and season 2, respectively. The derived N balance over two years revealed severe soil N depletion under T1 (-202 kg N ha-1), T5 (-86 kg N ha-1) and T6 (-48 kg N ha-1), and a slightly negative N budget under T2 (-5 kg N ha-1). In contrast, T3 (87 kg N ha-1) and T4 (62 kg N ha-1) had positive N balances. The increase of N input via additional N fertilizer applied to chili and symbiotic N2 fixation of legumes, and the reduction of N losses by soil erosion and unaccounted fertilizer N (probably lost via leaching, volatilization and denitrification) were the main factors of the positive N balances under maize-chili intercropping systems with conservation measures and fertilization (T3 and T4). Maize yield decline under T1, T2, T5 and T6 in season 2 was related to negative N balances, while maize yield increase under T3 and T4 was related to positive N balances. However, maize-chili intercropping with fertilization had some advantage (LER > 1.0) relative to sole species cropping. Moreover, total returns from crop yields in season 2 of all maize-chili intercroppings (1,378-1,818 USD ha-1) were higher than chili sole cropping (1,321 USD ha-1), which pointed to its crucial role in decreasing production risk by reducing yield loss by pests and diseases observed in chili plants. The third part of the study used combined data of stable isotope discrimination and electrical resistivity tomography (ERT) to improve understanding of competition at the crop-soil-hedge interface. Hedges significantly reduced maize grain yield and aboveground biomass in rows close to hedgerows. ERT revealed water depletion was stronger in T1 than in T4 and T6, confirming time domain reflectometry (TDR) and leaf area data. In T4, water depletion was higher in maize rows close to the hedge than rows distant to hedges and maize grain δ13C was significantly less negative in rows close to the hedge ( 10.33‰) compared to distant ones ( 10.64‰). Lack of N increased grain δ13C in T6 ( 9.32‰, p ≤ 0.001). Both methods were negatively correlated with each other (r= 0.66, p ≤ 0.001). Combining ERT with grain δ13C and %N allowed identifying that maize growth close to hedges was limited by N and not by water supply. In conclusion, the results suggested a significant positive interaction between mineral N fertilizer, intercropping systems and soil conservation measures in maintaining or improving crop yields and N balances in Thailand’s hillside agriculture. Simultaneously, combining ERT imaging and 13C isotopic discrimination approaches improved the understanding of spatial-temporal competition patterns at the hedge-soil-crop interface and pointed out that competition in maize-based hedgerow systems was driven by nitrogen rather than water limitation. Therefore, sustainable agriculture might be achieved if farmers in Thailand combine soil conservation measures with appropriate and targeted N fertilizer use.
Effects of resource availability and quality on soil microorganisms and their carbon assimilation
(2014) Kramer, Susanne; Kandeler, Ellen
Soil microorganisms play a pivotal role in decomposition processes and therefore influence nutrient cycling and ecosystem function. Availability and quality of resources determines activity, growth and identity of substrate users. In agricultural systems, availability of resources is dependent on, for example, crop type, management, season, and depth. At depth substrate availability and microbial biomass decrease. However, there remain gaps in our understanding of C turnover in subsoil and how processes in the topsoil may influence abundance, activity, and function of microorganisms in deeper soil layers. With respect to substrate quality it is thought that bacteria are the dominant users of high quality substrates and more labile components whereas fungi are more important for the degradation of low quality and more recalcitrant substrates (i.e. cellulose, lignin). Therefore, this thesis was designed to increase our understanding of C turnover and the influence of both availability and quality of substrates on microorganisms in an agricultural soil. In the first and second studies, a recently established C3-C4 plant exchange field experiment was used to investigate the C flow from belowground (root) and aboveground (shoot litter) resources into the belowground food web. Maize plants were cultivated to introduce a C4 signal into the soil both by plant growth (belowground / root channel) and also by applying shoot litter (aboveground litter channel). To separate C flow from the shoot litter versus the root channel, maize litter was applied on wheat cultivated plots, while on half of the maize planted plots no maize litter was returned. Wheat cultivated plots without additional maize litter application served as a reference for the calculation of incorporated maize-C into different soil pools. Soil samplings took place in two consecutive years in summer, autumn and winter. Three depths were considered (0-10 cm: topsoil, 40-50 cm: rooted zone beneath the plough layer, 60-70 cm: unrooted zone). In the third study a microcosm experiment with substrates of different recalcitrance and complexity was carried out to identify primary decomposers of different plant litter materials (leaves and roots) during early stages of decomposition (duration of 32 days) and to follow the C flow into the next higher trophic level (protozoa).
Fate of microbial carbon derived from biogas residues applied to arable soil
(2015) Coban, Halil; Kandeler, Ellen
Soil organic matter (SOM) is the major determinant of soil fertility as it has a number of positive impacts such as improving soil physical parameters, providing nutrients for crops, and supplying energy for the microbial biomass activity in soil. Loss of organic matter is a soil threat observed worldwide. Also, bioenergy crop cultivation may accelerate SOM loss due to higher biomass harvesting compared to food crops. It is necessary to supply adequate organic matter input to arable soils in order to maintain sustainable food and biofuel production. Biogas residues (BGRs), the side-products of biogas production, are rich in microbial and plant biomass; they thus can be used as a soil conditioner and contribute to replenishing the carbon (C) pool in soil. However, our knowledge on the contribution of BGRs particularly the microbial residues present in it to SOM formation is limited, even though scientific interest on SOM formation via microbial inputs is growing. Therefore, the objective of this thesis were i) developing an approach to label microbial biomass of biogas residues, ii) tracing the fate of labelled BGRs in arable soil, iii) determining the C flux within microbial food web, and iv) determining the impacts of other soil conditioners on the mineralization of BGRs. In the first study a method was developed to label the autotrophic microorganisms in a biogas reactor using KH13CO3-amended cow manure as substrate. Analyses of phospholipid fatty acids (PLFA) and ether lipids confirmed the successful labelling of microorganisms, especially Gram-positive bacteria and methanogenic archaea. After removal of unused labelled carbonates by an acid fumigation approach, the labelled BGRs were incubated in soil for 378 days. The fate of 13C was traced in CO2 and in bulk soil with a mass balance having 93% mean recovery. Results showed that about 40% of the C derived from BGRs was rapidly mineralized within the first seven days, and mineralization reached 65% at the end of experiment. The data could be fitted to a two-pool exponential degradation model assuming two C pools each decaying exponentially. The proportions of readily degrading and stable C pools were determined to be 51% and 49%, respectively, with half-lives of 3 days and 1.9 years, respectively. The long half-life of the stable C pool in BGRs may indicate a mid-term contribution to SOM. In addition, the mineralization of SOM was enhanced by BGR-application, i.e. priming effects were detected, thus their extensive application should be avoided. A differential fatty acid approach was used in the second study for the separation of C input from BGRs to living biomass and non-living SOM. Phospholipid fatty acids (PLFA) as indicators of living biomass were compared with total fatty acids (t-FA), which are found also in necromass. Using PLFA as biomarkers of specific microbial groups, C redistribution within the microbial food web was determined. Results showed that BGRs increased the microbial biomass in soil. The sum of 13C-labelled PLFA and t-FA decreased during incubation to 60% and to 33%, respectively. The level of enrichment was different for the individual PLFA and indicated that Gram-negative bacteria were predating on Gram-positive bacteria. A contribution of ether lipids was also detected indicating C flow from decaying methanogens. This study confirmed that microbial biomass in BGRs applied to arable soil significantly contributes to SOM formation. After determining the fate of microbial C derived from BGRs in arable soil, the impacts of other soil conditioners on the mineralization of BGRs were tested in the third study. For this, labelled BGRs were incubated in soil both alone and together with compost, biochar and untreated manure. The amount of C mineralized to CO2 and the degradation rate constant of stable C pool were not affected by any of the co-amendments. However, manure resulted in a higher mineralization rate constant of the readily degrading C pool. C flow within microbial food web was from Gram-positive bacteria and methanogenic archaea to mainly Gram-negative bacteria and slightly to fungi in all treatments. This study showed that co-amending BGRs with other soil conditioners brings neither benefits nor harms in terms of the formation or the mineralization of soil organic matter. The proposed labelling approach using KH13CO3 may be useful for tracing the fate of BGRs. The enrichment in both bacteria and archaea were sufficient to be measured in an incubation experiment lasting for more than one year. However, there are disadvantages of the proposed approach such as presence of highly enriched residual carbonates. The fumigation method should be optimized for a complete removal of the highly labelled residual carbonates which will increase the precision of the overall approach.
Pathways of C and N turnover in soil under elevated atmospheric CO2
(2008) Dorodnikov, Maxim; Fangmeier, Andreas
In the present thesis the C and N transformations in soil as influenced by indirect effect of elevated atmospheric CO2, soil physical structure and land use change were studied in four laboratory experiments using stable-C and N isotopes, as well as soil microbiological techniques. To test the interrelations between chemical and biological characteristics of soil organic matter (SOM) as affected by land use change and elevated atmospheric CO2 an approach for SOM partitioning based on its thermal stability was chosen. In the first experiment C isotopic composition of soils subjected to C3-C4 vegetation change (grassland to Miscanthus x gigantheus, respectively) was used for the estimation of C turnover in SOM pools. In the 2nd (Free Air CO2 Enrichment ? FACE ? Hohenheim) and 3rd (FACE Braunschweig) experiments CO2 applied for FACE was strongly depleted in 13C and thus provided an opportunity to study C turnover in SOM based on its δ13C value. Simultaneous use of 15N labeled fertilizers allowed N turnover to be studied (in the 2nd experiment). We hypothesized that the biological availability of SOM pools expressed as the mean residence time (MRT) of C or N is inversely proportional to their thermal stability. Soil samples were analysed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). According to differential weight losses between 20 and 1000 °C (dTG) and energy release or consumption (DSC), SOM pools (4 to 5 depending on experiment) with increasing thermal stability were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ13C and δ15N by IRMS. For all three experiments the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N. A possible explanation for the inability of thermal oxidation for isolating SOM pools of contrasting turnover times is that the fractionation of SOM pools according to their thermal stability is close to chemical separation. In turn, it was found that chemical separations of SOM failed to isolate the SOM pools of different turnover time because different biochemical plant components (cellulose, lignin) are decomposed in a wide temperature range. Individual components of plant residues may be directly incorporated into, or even mixed with the thermal stable SOM pools and will so mask low turnover rates of these pools. To evaluate the interactions between availability of SOM for decomposition by soil microbial biomass (biological characteristic) under elevated atmospheric CO2 and protection of SOM due to the occlusion within aggregates of different sizes (physical property, responsible for SOM sequestration) we measured the activity of microbial biomass (indicated by enzyme activities) and growth strategies of soil microorganisms (fast- vs. slow growing organisms) in isolated macro- and microaggregates. The contribution of fast (r-strategists) and slowly growing microorganisms (K-strategists) in microbial communities was estimated by the kinetics of the CO2 emission from bulk soil and aggregates amended with glucose and nutrients (Substrate Induced Growth Respiration method). Although Corg and total Cmic were unaffected by elevated CO2, maximal specific growth rates were significantly higher under elevated than ambient CO2 for bulk soil, small macroaggregates, and microaggregates. Thus, we conclude that elevated atmospheric CO2 stimulated the r-selected microorganisms. Such an increase in r-selected microorganisms could increase C turnover in terrestrial ecosystems in a future elevated atmospheric CO2 environment. The activities of β-glucosidase, phosphatase and sulphatase were unaffected in bulk soil and in aggregate-size classes by elevated CO2, however, significant changes were observed in potential enzyme production after substrate amendment. After adding glucose, enzyme activities under elevated CO2 were 1.2-1.9-fold higher than under ambient CO2. This indicates an increased activity of microorganisms, which leads to accelerated C turnover in soil under elevated CO2. Significantly higher chitinase activity in bulk soil and in large macroaggregates under elevated CO2 revealed an increased contribution of fungi to turnover processes. At the same time, less chitinase activity in microaggregates underlined microaggregate stability and the difficulties for fungi hyphae penetrating them. We conclude that quantitative and qualitative changes of C input by plants into the soil at elevated CO2 affect microbial community functioning, but not its total content. Future studies should therefore focus more on the changes of functions and activities, but less on the pools. In conclusion, elevated CO2 concentrations in the atmosphere along with soil physical structure have a pronounced effect on qualitative but not quantitative changes in C and N transformations in soil under agricultural ecosystem. The physical parameters of soil such as aggregation correlate more with biological availability of SOM than the chemical properties of soil organic materials. The increase of soil microbial activity under elevated CO2 detected especially in soil microaggregates, which are supposed to be responsible for SOM preservation, prejudice sequestration of C in agroecosystems affected by elevated atmospheric CO2.
Soil microbial assimilation and turnover of carbon depend on resource quality and availability
(2017) Müller, Karolin; Kandeler, Ellen
The decomposition of soil organic carbon (SOC), which is predominantly performed by soil microorganisms, is an important process in global carbon (C) cycling. Despite the importance of microbial activity to the global C budget, the effects of resource quality and availability on soil microorganisms are little understood. Most of this plant-derived C enters the soil organic C pool via incorporation into soil microorganisms, but the subsequent fate of C is rarely reported. Therefore, soil microbial biogeochemistry is still highly uncertain in earth system models. The study presented in Chapter 5 used a field experiment established in 2009 to investigate C flow at three soil depths over five consecutive years after a C3 to C4 crop exchange. Root-derived C (belowground pathway) was introduced by the cropping of maize plants, whereas shoot-derived C (aboveground pathway) was introduced by application of shoot litter to the soil surface. The proportion of maize-derived C varied between the different soil pools with lower incorporation into SOC and EOC (extractable organic C) and higher incorporation ratios of maize C into microbial groups. Although root-C input was three times higher than shoot-C input, similar relative amounts of maize-C were found in microorganisms. Both root and shoot C were transferred to a depth of 70 cm. At all three depths, fungi utilized the provided maize C to a greater extent than did either Gram-positive or Gram-negative bacteria. Fungal biomass was labeled with maize-C to 78% after the fifth vegetation period, indicating preferential utilization of litter-derived C by saprotrophic fungi. The second study investigated, in a microcosm experiment, the effects of decreasing resource quality on microorganisms during plant residue decomposition at the soil-litter interface. Reciprocal transplantation of labeled 13C and unlabeled 12C maize litter to the surface of soil cores allowed us to follow C transfer and subsequent C turnover from residues into microbial biomass of fundamental members (bacteria and fungi) of the detritivore food web during three stages of the litter decomposition process. Quality (i.e. age) of the maize litter influenced C incorporation into bacteria and fungi. Labile C from freshly introduced litter was incorporated by both groups of microorganisms, whereas saprotrophic fungi additionally used complex C in the intermediate stage of decomposition. Bacteria responded differentially to the introduced litter; either by turnover of litter C in their phospholipid fatty acids (PLFAs) over time, or by storage and/or reuse of previous microbially released C. Saprotrophic fungi, however, showed a distinct litter C turnover in the fungal PLFA. The mean residence time of C in the fungal biomass was 32 to 46 days; the same or shorter time than in bacterial PLFAs. In the third study, presented in Chapter 7, another field experiment was conducted to distinguish herbivore- from detritus-based food chain members over two consecutive years. Three treatments were established: maize as crop plant, maize shoot litter application, and fallow without C input. This provided root-derived C, shoot-derived C, and autochthonous organic matter, respectively, as the main C resource. The altered C supply due to plant removal had less severe effects on the micro-food web structure than expected. In the first growing season, nematode abundance under plant cultivation was similar to that under litter and fallow conditions. After the second harvest, the abundance of detritivore food chain members increased, reflecting the decomposition of root residues. Bacteria and fungi showed a marked resilience to changed C availability. Results of this experiment suggest considerable micro-food web resilience to altered C and nutrient availability, and indicate that organic matter from previous vegetation periods was successfully utilized to overcome C deprivation. In conclusion, this thesis provides new insights into microbially mediated decomposition processes at different time scales and at different soil depths. Stable isotope probing combined with biomarker analysis enabled us to study C fluxes between biotic and soil C pools to separate the contributions of bacteria and fungi to soil C cycling. These results can be used as a basis for an empirical model of C flow through the entire soil food web.