Browsing by Subject "Agroecosystem"

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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.