Browsing by Subject "Bodenphysik"

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Constraints on microbial pesticide degradation in soils
(2023) Wirsching, Johannes; Kandeler, Ellen
Pesticides are an essential component of intensified agriculture and have contributed significantly to the increase in food production observed in recent decades. Since 1960, pesticide use has increased by a factor of fifteen to twenty, representing a market value of $40 billion in 2016. Soil monitoring campaigns to track pesticide contamination of croplands across Europe are quantifying pesticide residues whose residence times in soils exceed expected values. Diffuse contamination by pesticide residues raises concerns about soil functions, soil biodiversity, and food safety, as well as the transport of contaminants by wind and water to surface waters or to adjacent, organically managed croplands. Data on the frequency of occurrence and concentrations of pesticide residues in soil demonstrate a discrepancy between the determination of persistence and subsequent approval and their actual fate in soil. This raises the question of whether degradability of individual organic compounds has been adequately studied. Microbiological degradation is the most important process for reducing pesticide loads in soils. A reliable estimate of pesticide residence time requires an expanded understanding of the factors limiting microbial degradation. Degradation of anthropogenic organic chemicals in soils occurs much more slowly than would be expected based on their physicochemical properties. While processes that determine the fate of pesticides in soil have often been studied at different spatial and temporal scales, reasons for discrepancies between the observed complete degradation of pesticides under laboratory conditions and their persistence in the field remain unclear. This thesis addresses this challenge by focusing on the central question of why inherently biodegradable compounds in soils display increased persistence under field conditions. Organic contaminants in low but detectable and environmentally significant concentrations could remain in the soil once available concentrations fall below a threshold where bioenergetic growth restrictions come into play. In addition, potential microbial and biophysical limitations and environmental factors such as soil temperature and soil moisture are often examined separately in current degradation studies. Combinations of temperature and soil moisture changes associated with different concentration levels have been less well examined, resulting in an incomplete understanding of the degradation process. Another key factor in the demonstrated discrepancy between predicted and actual persistence in the field could be due to laboratory experiments that cannot account for field-scale processes. Therefore, degradation rates determined in laboratory experiments cannot be confidently extrapolated to the field scale. . This thesis identified further important regulatory mechanisms for microbially mediated pesticide degradation. The previously unknown concentration-dependent degradation dynamics and the concentration-dependent influence of limiting environmental conditions on microbial degradation emphasize the importance of studies using a realistic concentration range. Evidence of deep transport of a highly sorptive pesticide such as glyphosate primarily via preferential flow pathways into the subsoil with lower degradation dynamics underscores the need to include processes that can only be verified in field studies as part of risk assessments. The results of this thesis suggest that the biodegradation rates of pesticides are not homogeneous at field scales and may account in part for the discrepancy between complete degradation of pesticides under laboratory conditions and their persistence in the field. Laboratory studies in which soil samples are pooled and mixed to obtain a single "representative" sample can provide a simplified understanding of the process, but the complexity, particularly that of soil heterogeneity, of pesticide distribution and microbial degradation associated with prevailing climatic conditions, requires calibration of previously used methods in field studies and possibly at landscape, watershed, or regional scales. The scale-dependent degradation aspect will become even more important in the future; as soil properties and processes that control the toxicological aspects of contaminants include temperature and moisture, and changes associated with climate change will lead to an increase in extreme precipitation, longer dry periods, and soil erosion.
Development of MidDRIFTS methodologies to support mapping of physico-chemical soil properties at the regional scale
(2014) Mirzaeitalarposhti, Reza; Müller, Torsten
Changing climate conditions and land-use change severely affect key ecosystem processes in soils. Hence, regular monitoring of essential soil properties are required to implement appropriate soil management in agro-ecosystems. However characterizing soil properties at different spatial scales remains challenging, requiring a large amount of geo-referenced data by intensive sampling. Mid-infrared diffuse reflectance infrared Fourier transform spectroscopy (midDRIFTS) in combination with partial least square regression (PLSR) was applied as a rapid-throughput method to quantify soil properties and to assess soil spatial patterns at the regional scale in two agro-ecological areas. A pre-sampling at the regional scale was done to develop the most efficient midDRIFTS-PLSR prediction models by testing two different calibration procedures, i.e. cross-validation and independent validation, to quantify essential soil properties with 126 sample points. A generic MidDRIFTS-PLSR prediction model was developed to predict most soil properties of “unknown” samples accurately using independent validation approach. The next step was the integration of midDRIFTS-PLSR with geostatistics to facilitate regional soil property mapping. Developed midDRIFTS-PLSR models were used to predict TC, TIC, TOC and soil texture contents (clay, silt and sand) of the 1170 soil samples. The midDRIFTS-PLSR models accurately predicted all soil properties. Furthermore, the integration of midDRIFTS-PLSR-based predicted data with geostatisitcs resulted in high resolution maps of soil carbon and texture at the regional scale which are an improvement over the existing maps. As a further development of midDRIFTS approaches for soil quality assessment, spectral-based indexes for characterizing SOM quality and quantifying carbonate at regional scale were explored. MidDRIFTS peak areas corresponding to SOM functional groups (2930, 1620, 1520 and 1159 cm-1) were assessed to study the composition of SOM. The peak assigned for aliphatic C-H bond (2930 cm-1) was an appropriate index to investigate SOM fractions if the interference of carbonates was taken into consideration. Regression performance obtained between the peak at 2930 cm-1 and SOM fractions (e.g., R2 = 0.31 for Cmic) increased to R2 = 0.65 when high carbonate containing samples (total inorganic carbon > 1%) were excluded. The most accurate spectral index for carbonate was the peak area at 713 cm-1 when relating to TIC obtained by Scheiblers method (R2 = 0.98). In conclusion, it was demonstrated that midDRIFTS-PLSR is a rapid-throughput method for providing high-quality predictions of soil properties to update regional digital soil property mapping by integration with geostatistics. It opens a new possibility to gain high resolution data coverage of soil C and N pools, which is relevant for the application of SOM simulation models on a regional scale. However, to up-scale the approach for extended geographical areas, further efforts are needed to establish a national level spectral library by considering standardization of sampling, analytical reference analyses and midDRIFT spectroscopy techniques.
Relationship between soil physical properties and crop yields in different cropping systems in southern Cameroon
(2014) Tueche, Jacques Roberto; Cadisch, Georg
Crop yields in sub-Saharan Africa (SSA) has been more or less stagnant since 1961. This can be connected to the traditional slash-and-burn agricultural based system. A growing population has forced most farmers to cultivate the same fields repeatedly. The resulting rapidly declining crop yields led eventually to an accelerated conversion of forest land into agricultural land to cope with food demand. However, the integration of leguminous species, the use of fertilizer and tillage have been proven to increase yield especially in intensive cropping systems. Although, depending on its frequency and kind, tillage can destroy soil aggregates resulting in degradation of soil organic matter. Else, it is known that improved crop varieties can be higher-yielding and more yield responsive to increased fertilizer application than traditional crop varieties. Information is scarce on the effects of soil physical properties on plantain, maize and tomato yield formation and on their changes during their cropping phase. This study aimed at understanding the relationships between soil physical parameters and crop yields in different cropping systems in southern Cameroon with the goal to identify improved management strategies. This led to the setup of 4 experiments: In a first experiment, the effects of soil physical properties on plantain yield were determined in a factorial trial in three southern Cameroonian villages comparing four cropping systems comprising two planted legumes (1) Flemingia macrophylla, (2) Pueraria phaseoloides, as well as (3) a crop, i.e. hot pepper, and (4) natural regrowth, all planted with plantain, established after conversion of old forest versus young bush fallow. Between 2002 and 2006, clay and silt content, MWD, GMD and the proportion of macroaggregates increased, whereas relative sand content, bulk density, the proportions of mesoaggregates and microaggregates decreased (not absolute decreased for sand content) in all villages, fallows and cropping systems. Changes of aggregate MWD and GMD were larger in the F. macrophylla and natural regrowth systems than in Pueraria systems. Plantain fresh bunch yield was 107 unaffected by village, fallow, and cropping systems. Plantain cultivation did not lead to a degradation of the determined soil physical properties. In a follow up second trial at Mfou, it was evaluated, if maize cropped immediately after plantain was affected by the previous plantain systems and if tillage or N fertilizer would affect maize growth and grain yield and soil physical properties. In 2006, all plantain plots were cleared and split into 4 subplots, to assess the response of maize to tillage versus no–till, and of 60 kg ha-1 of N as urea compared to no N in a 2 x 2 factorial design. Freshly cleared eight years old bush fallow served as control. Maize grain yield was highest in the previously not cropped bush control and lowest in the previous Flemingia system. Grain yield in the previous Pueraria and natural regrowth systems were not different from the control. Maize grain yield was highest, when tillage was combined with fertilizer application, being significantly higher than in individual tillage or fertilizer application treatments. Soil physical properties were affected by tillage but did not remain different until the end of the maize growing phase. In a third experiment the response of different tomato cultivars to different cultivation practices in an on-farm factorial trial was tested at Essong Mintsang in the central region of Cameroon on a Rhodic Kandiudult. Treatments were: current farmer practice of manual tillage yet not destumped, with either reduced input (no tillage, not destumped) or increased input (no tillage yet destumped, manual tillage and destumped, mechanical tillage and destumped). Yields of three tomato varieties were determined to assess, if changes in intensity of land preparation can improve soil physical properties and thus yields. At harvest, across land preparations the cv. Rossol produced higher yields (8.12 Mg ha-1) than cv. Roma (6.05 Mg ha-1) and cv. Rio Grande (4.46 Mg ha-1). Tomato total and marketable yields were significantly higher on the destumped tractor till, destumped manual till and stumps-retained manual till treatments than in the stumps retained no-till treatment. Total fresh yields of cvs. Roma and Rossol increased, when the soil was tilled, while cv. Rio Grande had no response to land preparation. Soil aggregates were least stable in the destumped, tractor till treatment, with significantly lower MWD (p=0.02) and higher mesoaggregate proportions (p=0.05) than in the other treatments. Across tomato cultivars and treatments, the marketable fruit yield could be predicted by clay, macroaggregates and bulk density. Early flowering and fruit production combined with nematode resistance were probably the main contributing factors to the high yields of cv. Rossol. 108 In a fourth experiment, the residual effects of the previous land preparation methods on maize growth and yield as well as impacts on soil physical properties were assessed. Land preparation methods had been applied to the preceding tomato crop. At harvest, maize fresh cob yield was significantly (P<0.05) lowest in the stump retained no till treatment. The equivalent maize dry grain yields varied from 2.35 Mg ha-1 in the stump retained no-till treatment to 4.16 and 4.33 Mg ha-1 in the manual till stump retained and destumped treatments, respectively. Soil aggregates were the least stable in the destumped tractor till treatment, with significantly lower (P=0.10) GMD than in the destumped manual till treatment. Maize fresh cob yield showed a strong correlation (R2~0.50, P=0.037) with soil aggregation and cone resistance to soil penetration. In summary, the transition from shifting to permanent cultivation with acceptable yields is possible if an appropriate combination of crops (cultivar), use of leguminous species, tillage and fertilizers is implemented. Soil physical properties can control crop yield and hence can be manipulated to maximise yield. Tillage can contribute to yield increase if there is an adequate SOM content and a suitable crop cultivar is chosen. Yet, tillage is labour intensive and degrades soil physical properties. Therefore, it is crucial to identify a minimum tillage frequency for low labour demand and minimal soil degradation, but with improved yields in conjunction with optimised fertilization and the development of improved crop cultivars adapted to a wide range of soil conditions.