Browsing by Subject "Soil enzyme activity"

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    Effects of elevated soil temperature and altered precipitation patterns on N-cycling and production of N2O and CO2 in an agricultural soil
    (2016) Latt, Yadana Khin; Kandeler, Ellen
    Both temperature and precipitation regimes are expected to change with climate change and are, at the same time, major environmental factors regulating biogeochemical cycles in terrestrial ecosystems. Therefore, crop water availability, soil nitrogen transformations, losses, and uptake by plants as well as CO2 emissions from soil are likely to be changed by climate change. Agriculture is known to be one of the most important human activities for releasing significant amounts of N2O and CO2 to the atmosphere. Due to global concern about the changing climate, there has been a great interest in reducing emissions of N2O and CO2 from agricultural soils. CO2 and N2O are produced in soil primarily by microbial processes. Their production and emissions from the soil are controlled by a number of environmental variables including inorganic N availability, soil temperature and water content. Agricultural management practices, such as irrigation, affect these environmental variables and thus have the potential to dramatically alter N2O and CO2 emissions from the soil. The present study is titled "Effects of elevated soil temperature and altered precipitation patterns on N cycling and production of N2O and CO2 in an agricultural soil". The objectives of this study were: to determine the effects of elevated soil temperature on N cycling in a winter wheat cropping system, to investigate the short-term response of N2O and CO2 fluxes during rewetting of soils after extended dry periods in summer, and to determine the effects of different degrees of rewetting on the CO2 emission peaks after rewetting in laboratory incubations. In the 1st experiment, we used the Hohenheim Climate Change (HoCC) experiment in Stuttgart, Germany, to test the hypothesis that elevated soil temperature will increase microbial N cycling, plant N uptake and wheat growth. In the HoCC experiment, soil temperature is elevated by 2.5°C at 4 cm depth. This experiment was conducted at non-roofed plots (1m x 1m) with ambient (Ta) and elevated (Te) soil temperature and with ambient precipitation. In 2012, winter wheat (Triticum aestivum) was planted. C and N concentrations in soil and aboveground plant fractions, soil microbial biomass C and N (Cmic and Nmic), mineral N content (NH4+ - N and NO3- - N), potential nitrification and enzymes involved in nitrogen cycling were analyzed at soil depths of 0-15 and 15-30 cm at five sampling dates. The plants were rated weekly for their phenological development and senescence behavior. We found that an increase in soil temperature by 2.5oC did not have a persistent effect on mineral N content and the activity of potential nitrification within the soil. Plant growth development also did not respond to increased soil temperature. However microbial biomass C and N, and some enzyme activities involved in N-cycling, tended to increase under elevated soil temperature. Overall, the results of this study suggested that soil warming by 2.5oC slightly stimulates soil N cycling but does not alter plant growth development. In the 2nd experiment, in 2013, the effects of a change in the amount and frequency of precipitation patterns on N2O and CO2 emissions were studied after the two dry periods in summer in the HoCC experiment. N2O and CO2 gas samples were taken from four subplots (1m x 1m) of each roofed plot exposed to ambient (Ta) or elevated (Te) soil temperature and four precipitation manipulations (ambient plot, reduced precipitation amount, reduced precipitation frequency, and reduced precipitation amount and frequency). We found that CO2 emissions were affected only by temperature, but not by precipitation pattern. It can be said that N2O and CO2 emissions after rewetting of dry soil were not altered by changing precipitation patterns during dry periods in summer. In the year 2014, using laboratory incubations, we also measured the short-term response of CO2 production to a rewetting of dry soil to different volumetric water contents for 24 hours. This study was conducted by manipulating microcosms with agricultural soil from the HoCC experimental site, which had been exposed to severe drought conditions of three months duration for each of the last six years. The results showed that CO2 production increased with increases in the water content of soils by rewetting at 5%, 15%, 25%, 35% and 45% VWC. With increasing water additions more peaks in CO2 production were detected and different temporal patterns of CO2 emission were affected by adding different amounts of water. It might be due to the fact that with greater water additions successively larger pore sizes were water filled and therefore different bacterial groups located in different pore size classes might have contributed to CO2 production. In summary, the results from field study suggested that climate warming will affect N cycling in soils in an agricultural cropping system. The results from both field and microcosm rewetting experiments contribute to a better understanding of C and N dynamics in soil by investigating the effect of varying soil water content on the emission of N2O and CO2.
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    Spatial and temporal variations of microorganisms in grassland soils : influences of land-use intensity, plants and soil properties
    (2019) Boeddinghaus, Runa S.; Kandeler, Ellen
    Grassland ecosystems provide a wide range of services to human societies (Allan et al., 2015) and plants and soil microorganisms have been identified as key drivers of ecosystem functioning (Soliveres et al., 2016). Therefore, understanding soil microbial distributions and processes in agricultural grassland soils is crucial for characterizing these ecosystems and for predicting how they may shift in a changing environment. Yet we are only beginning to understand these complex ecosystems, which account for about 26% of the world’s terrestrial surface (FAOSTATS, 2018), making it especially urgent to gain better insights into the effects of land-use intensity on soil microbial properties and plant-microbe interactions. This thesis was conducted to evaluate the impact land-use intensity has on soil microbial biogeography of grasslands with respect to both spatial patterns and temporal changes in soil microbial abundance, function (in terms of enzyme activities), and community composition. It also investigated the relationships between plants and the spatial and temporal distributions of soil microorganisms. Thereby both, land-use intensity effects and plant-microbe interactions, were assessed in light of ecological niche and neutral theory. This thesis is based on three observational studies conducted on from one to 150 continuously farmed, un-manipulated grassland sites in three regions of Germany within the Biodiversity Exploratories project (DFG priority program 1374). The first study assessed the effects of land-use intensity and physico-chemical soil properties on the spatial biogeography of soil microbial abundance and function in 18 grasslands sites from two of the three regions, sampled at one time point. The second study analyzed spatial and temporal distributions of alpha- and beta-diversity of arbuscular mycorrhizal fungi in a low land-use intensity grassland with six sampling time points across one season. The third study investigated both legacy and short-term change effects of land-use intensity, soil physico-chemical properties, plant functional traits, and plant biomass properties on temporal changes in soil microbial abundance, function, and community composition in 150 grassland sites across three regions, with particular regard to direct and indirect land-use intensity effects. Although the three studies used different approaches and assessed different soil microbial properties, general patterns were detectable. Abiotic soil properties, namely pH, nitrogen content, texture, and bulk density played fundamental roles for spatial and temporal microbial biogeography. Since these factors were specific and unique for each investigated site, they formed the background based on which other processes occurred. In addition to abiotic soil properties, impacts of land-use intensity and plants were detected, though to various degrees in the three studies. Land-use intensity played a much smaller role than anticipated in the first and third study. No influence on the spatial distribution of soil microbial abundance and function could be detected in the first study. In the third study, short-term changes in and legacy effects of land-use intensity played a minor role with respect to short-term changes in soil microbial abundance, function, and community composition. Where detected, changes in land-use intensity had a direct and negative effect on soil microbial properties in structural equation modelling; i.e., increases in land-use intensity reduced, e.g., soil microbial enzyme activities, while legacy effects of land-use intensity were shown to act both directly and indirectly on soil microbial properties. Thereby indirect legacy effects were mediated via plant functional traits. Only one of the three studies detected minor plant diversity effects on soil microbial properties. Instead, functional properties of the plant communities, i.e., plant functional traits, biomass, and nutritional quality, were significantly related to spatial and temporal distributions of soil microorganisms. Finally, the findings of the three studies suggest that processes related to niche and neutral theory both drive spatial and temporal patterns of soil microbial properties at the investigated plot scale (up to 50 m × 50 m). This thesis concluded that in order to gain deeper insights into the complex functions and processes occurring in grassland ecosystems, a multidisciplinary approach investigating fundamental physico-chemical site characteristics, microbial soil properties, and plants is necessary. The results of the thesis suggest that focus be turned to functional properties of plant and microbial communities, as they are closely intermingled, provide more detailed insights into plant-microbe interactions, and are able to reflect effects of human impacts on grassland soils better than diversity measures.