Browsing by Subject "Soil respiration"

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Carbon cycling in a future agroecosystem
(2024) Leyrer, Vinzent; Kandeler, Ellen
Climate change is putting increasing pressure on the soil microbiome, driver of vital ecosystem functions. Predicting its future state is of great interest as it provides insight into how rising temperatures and increasing water scarcity will affect a fundamental process in terrestrial ecosystems: carbon (C) cycling between soil, vegetation and the atmosphere. However, climate is changing in both moderate and extreme ways. Mean conditions are changing with increases in temperature and shifts in precipitation patterns. In addition, an increased frequency of extreme droughts raises the question of how ecosystem functions will be affected in the future. In this thesis, we investigated soil microbial abundance and functions and the associated soil C dynamics under long-term predicted warmer and drier mean climatic conditions. By then exposing the soils of this system to drought, we aimed to highlight the difference in how drought affects C cycling between soil and the atmosphere under future compared to current mean climatic conditions. This allowed us to make a realistic prediction of how future drought will affect soil C dynamics if the mean climatic conditions are also warmer and drier overall. Our study is conducted in an ecosystem that is still strongly underrepresented in related studies, namely agricultural ecosystem soils. In addition to their importance in terms of total area, annual cropping in agricultural ecosystems defines their unique characteristics in terms of how C enters, transforms and exits the system. This is fundamentally different from comparable natural ecosystem soils such as forests or grasslands. The first study built the foundation of the thesis. A future mean climatic scenario was simulated at field scale for one decade, to assess broad patterns of future microbial abundance and activity. The key elements of soil C dynamics were monitored: its dynamic part — substrate availability, soil microbial biomass and respiration — in relation to the background, the total soil organic carbon (SOC) content. We then exposed this field to extreme drought and studied the response of the microbiome, which we assumed was adapted to warmer and drier conditions. Would this response be different from how the soil microbiome responds today, and can we derive a future prediction from this? If so, what are the driving forces behind the different response? First, we again used integrative parameters such as soil respiration to identify differences in the big picture. We expanded on this by using an in situ 13C pulse labelling experiment, tracing C from plants to soil and back to the atmosphere during extreme drought. From the first year, the change in mean climatic conditions led to a shift in the C dynamics of the arable soil. Whereas reducing the summer precipitation had no effect at all, warming was a strong accelerator of soil respiration and there was no sign of acclimation to the warmer regime. The microbial biomass C pool turned out to be unresponsive to the changing conditions and, therefore, the consistent increase in respiration indicated a shift in the microbial physiology in a warmer environment. Yet the simultaneously higher labile C input weighed up for the higher respiration and consequently, the total SOC content remained unchanged. This suggested that in a warmer world, arable soils of the temperate zone may exhibit relatively high stability regarding their C budget. An overall warmer world in the future will in addition increasingly be challenged by extreme drought. First of all, exposing the arable soil to drought showed expected patterns of a general decline in microbial activity. However, it also revealed unexpected findings of stable or even increasing microbial biomass C pools. As bacterial levels decreased and fungal levels remained unaffected, part of this net increase in microbial biomass C was to be attributed to intracellular C accumulation, a microbial measure to avert death under extreme drought. Another unexpected observation was that warming stimulated respiration even under drought. This is counterintuitive because the stimulus of warming acts via increasing reaction rates between enzymes and C compounds, which requires a liquid phase to allow their contact. Still, this observation was consistent in both drought studies and showed that the effect of drought on microbial related parameters depends on the overall temperature conditions in which the drought occurs. Apart from that, both drought experiments revealed that it is not only the direct effect of warming that modifies the microbial response to drought. We also found indications that exposing the soil microbiome to a decade of reduced summer precipitation left a legacy on how microorganisms use and acquire C during drought. In specific, in soils with a history of drier summer months we found the stimulatory effect of warming on respiration to be limited. This was a remarkable finding, which we explained by the highest fungal biomass in these soils, as fungi are known to have a high C-use efficiency. However, this remained largely elusive as no other parameter — neither substrate availability nor enzyme dynamics — could contribute to an explanation. Interestingly, the legacy effect of reduced summer precipitation also appeared elsewhere in the second study, where microbes pre-exposed to drier summer months shifted their C acquisition by using SOC rather than rhizodeposits to meet their C demand when conditions became extremely dry. Both observations, the first on respiration and the second on C acquisition, showed that not only repeated extreme droughts can create a legacy effect that drives microbial abundance and activity during drought. Even relatively moderate shifts in precipitation patterns can. Taking a step back, our study suggests that future C cycling between temperate arable soils and the atmosphere will be driven mainly by temperature increases, rather than by shifts in moderate precipitation patterns. However, it is both factors taken together that can alter the response of the system to drought. In the future, the impact of drought on microbially mediated C cycling will be different from what we observe today based on (1) expected warmer temperatures that will co-occur with drought and (2) legacy effects from historical precipitation patterns. Importantly, we show that both factors interact to modify the impact of drought on microbial abundance and activity. This suggests that the anticipation of realistic future scenarios requires an equal consideration of past legacies.
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.
Impact of land use change on soil respiration and methane sink in tropical uplands, Southwestern China
(2020) Lang, Rong; Cadisch, Georg
Land use conversion could modulate soil CO2 emissions and the balance between CH4 oxidation and production via changing soil physical, chemical and biological properties. Large areas of natural forests have been converted to rubber plantations in Southeast Asia, but its impact on soil CO2 and CH4 fluxes has not been sufficiently understood. This study was conducted in Xishuangbanna, Southwestern China, aiming to quantify the impact of this land use change on soil CO2 and CH4 fluxes and to clarify mechanisms responsible for the differences between natural forests and rubber plantations. Dynamics of soil respiration rates in two land uses were compared, and a mixed effect model was used in studying the interference of soil moisture on estimating temperature sensitivity (Q10) of soil respiration (Chapter 2). The land use change impact on the ability of soils to function as CH4 sink was firstly assessed with surface CH4 fluxes measured by static chambers, and then assessed with gas concentration profiles determined from soil probes. Confounded controlling factors and land use effects were disentangled, and the pathway of interactions between CH4 processes and mineral nitrogen was identified (Chapter 3). The concentration gradient method and one-dimensional diffusion-oxidation model were applied to quantify the vertical distribution of CH4 uptake in soil profiles, and to separate the relative control by gas diffusivity and by methanotrophic oxidation on CH4 uptake (Chapter 4). Distinct different temporal patterns of soil respiration were observed on sites during most of the rainy season: forest maintained a high soil respiration rate, while soil respiration in rubber plantations became suppressed (by up to 69%). Forest soils thus emitted the highest amount of CO2 with an annual cumulative flux of 8.48 ± 0.71 Mg C ha-1 yr-1, compared to 6.75 ± 0.79, 5.98 ± 0.42 and 5.09 ± 0.47 Mg C ha-1 yr-1 for 22-year-old rubber, rubber-tea intercropping, and 9-year-old rubber, respectively. Adding a quadratic soil moisture term into the regression model accounted for interference of moisture effect on the effect by soil temperature, therefore, improved temperature sensitivity assessments when high soil moisture suppressed soil respiration under rubber plantations. The static chamber method showed that soils under natural forest were stronger CH4 sinks than soils under rubber plantations, with annual CH4 fluxes of -2.41 ± 0.28 kg C ha-1 yr-1 and -1.01 ± 0.23 kg C ha-1 yr-1, respectively. Water-filled pore space was the main factor explaining the differences between natural forests and rubber plantations. Although soils under rubber plantations were more clayey than soils under natural forest, this was proved not to be the decisive factor driving higher soil moisture and lower CH4 uptake in the former soils. Concentration gradients method showed that CH4 consumption in 0-5 cm soil was significantly higher in natural forests than in rubber plantations, with a mean CH4 flux of -23.8 ± 1.0 and -14.4 ± 1.0 ug C m-2 h-1 for forest and rubber plantations, respectively. The atmospheric CH4 oxidized by top 10 cm soil accounted for 93% and 99% of total consumption for forest and rubber plantations, respectively. CH4 diffusivity at four sampled depths were significantly lower in rubber plantation than in forest. This reduced CH4 diffusivity, caused by altered soil water regime, predominately explained the weakened CH4 sink in converted rubber plantations. Estimated isotopic fractionation factor for carbon due to CH4 oxidation was 1.0292 ± 0.0015 (n=12). Modeling 13CH4 distribution in soil profiles using a diffusion-oxidation model explained the observations in the dry season, but suggested CH4 production in subsoil in the rainy season. In summary, converting natural forests into rubber plantations tended to reduce soil CO2 emissions, but this conversion substantially weakened CH4 uptake by tropical upland soils. The altered soil water regime and conditions of soil aeration under converted rubber plantations appear to have a pronounced impact on processes of gaseous carbon fluxes from soils. The clarified mechanisms in this study could improve the regional budget of greenhouse gases emissions in response to land use change and climate change.
Microbial regulation of soil organic matter decomposition at the regional scale
(2018) Ali, Rana Shahbaz; Kandeler, Ellen
The fate of soil organic carbon (SOC) is one of the greatest uncertainties in predicting future climate. Soil microorganisms, as primary decomposers of SOC, control C storage in terrestrial ecosystems by mediating feedbacks to climate change. Even small changes in microbial SOC decomposition rates at the regional scale have the potential to alter land-atmospheric feedbacks at the global scale. Despite their critical role, the ways in which soil microorganisms may change their abundances and functions in response to the climate change drivers of soil temperature and moisture is unclear. Additionally, most existing C models do not consider soil microorganisms explicitly as drivers of decomposition, one consequence of which is large variability in predicted SOC stock projections. This demonstrates the need for a better mechanistic understanding of microbial SOC decomposition at large scales. This thesis was designed to clarify the role of microbial SOC decomposition dynamics in response to climate change factors in two geographically distinct areas and land-use types. The hypothesis was that microbial communities would be adapted to climatic and edaphic conditions specific to each area and to the SOC organic quality in each land-use and would therefore exhibit distinct responses to soil temperature and moisture variations. Three studies were performed to address the goals of this thesis. The first study aimed to clarify temporal patterns of degradation in C pools that varied in complexity by modelling in situ potentials of microbially produced extracellular enzymes. Temperature and moisture sensitivity patterns of C cycling enzymes were followed over a period of thirteen months. The second study investigated group-specific temperature responses of bacteria and fungi to substrate quality variations through an additional incubation experiment. Here, complex environments were mimicked in order to determine the dependence of microbial responses not only on environmental conditions, but also under conditions of inter- and intra-specific community competition. Changes in microbial community composition, abundance, and function were determined at coarse (phospholipid fatty acid – PLFA, ergosterol) and relatively fine resolutions (16S rRNA, taxa-specific quantitative PCR, fungal ITS fragment). A third study investigated 1) the spatial variability of temperature sensitivity of microbial processes, and 2) the scale-specificity and relative significance of their biotic and physicochemical controls at landscape (two individual areas, each ca. 27 km2) and regional scales (pooled data of two areas). Strong seasonal dependency was observed in the temperature sensitivities (Q10) of hydrolytic and oxidative enzymes, whereas moisture sensitivity of β-glucosidase activities remained stable over the year. The range of measured enzyme Q10 values was similar irrespective of spatial scale, indicating a consistency of temperature sensitivities of these enzymes at large scales. Enzymes catalyzing the recalcitrant SOC pool exhibited higher temperature sensitivities than enzymes catalyzing the labile pool; because the recalcitrant C pool is relatively large, this could be important for understanding SOC sensitivity to predicted global warming. Response functions were used to model temperature-based and temperature and moisture-based in situ enzyme potentials to characterize seasonal variations in SOC decomposition. In situ enzyme potential explained measured soil respiration fluxes more efficiently than the commonly used temperature-respiration function, supporting the validity of our chosen modelling approach. As shown in the incubation experiment, increasing temperature stimulated respiration but decreased the total biomass of bacteria and fungi irrespective of substrate complexity, indicating strong stress responses by both over short time scales. This response did not differ between study areas and land-uses, indicating a dominant role of temperature and substrate quality in controlling microbial SOC decomposition. Temperature strongly influenced the responses of microbial groups exhibiting different life strategies under varying substrate quality availability; with soil warming, the abundance of oligotrophs (fungi and gram-positive bacteria) decreased, whereas copiotrophs (gram-negative) increased under labile C substrate conditions. Such an interactive effect of soil temperature and substrate quality was also visible at the taxon level, where copiotrophic bacteria were associated with labile C substrates and oligotrophic bacteria with recalcitrant substrates. Which physicochemical and biological factors might explain the observed alterations in microbial communities and their functions in response to climate change drivers at the regional scale was investigated in the third study. Here, it was shown that the soil C:N ratio exerted scale-dependent control over soil basal respiration, whereas microbial biomass explained soil basal respiration independent of spatial scale. Factors explaining the temperature sensitivity of soil respiration also differed by spatial scale; extractable organic C and soil pH were important only at the landscape scale, whereas soil texture as a control was independent of spatial scale. In conclusion, this thesis provides an enhanced understanding of the response of microbial C dynamics to climate change at large scales by combining field measurements with innovative laboratory assays and modelling tools. Component specific degradation rates of SOC using extracellular enzyme measurements as a proxy, group-specific temperature sensitivities of microbial key players, and the demonstrated scale-specificity of factors controlling microbial processes could potentially improve the predictive power of currently available C models at regional scale.