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Extreme weather events, such as extreme rainfall and flooding events, are expected to increase in frequency and intensity. Such events are known to disturb major biogeochemical cycles such as the nitrogen (N) cycle (Greaver et al. 2016). As a result, N losses from the agroecosystem to the environment will increase, leading to the alteration of soil, air, water, and biodiversity. Hence the adoption of biodiversity-based practices has gained attention as a way to increase internal regulations that limit N losses. By combining trees and grasslands, silvopastoral agroforestry is adopted in temperate regions with high livestock densities as a solution to prevent N losses while providing other ecosystem services. However, little is known about the regulation of the N cycle in temperate silvopastures, especially under extreme rainfall and flooding events (Kim et Isaac 2022). Here, we present recent results obtained on nitrification, a key process involved in the regulation of N losses, and its stability under flooding stress in two silvopastoral systems (hedgerow versus alley-cropping) of Brittany, France.
Nitrification potential and stability were assessed ex situ for soils sampled at three distances from the trees (0.5, 1.5 and 10m) and for two silvopastoral systems (hedgerow and alley-cropping). Nitrification potential was first measured in stress-free conditions and was explained by variables related to vegetation, soil physicochemical properties, and soil organisms (Mettauer et al. 2023). Then, the resistance and resilience of nitrification potential were measured immediately after four weeks of flooding stress and four weeks after the end of the stress, respectively. Resistance and resilience were considered as proxies of nitrification stability, which was explained by soil physico-chemical properties using a multigroup latent structural equation modeling (ML-SEM). The used ML-SEM enabled to test the causal relations that may explain nitrification stability and examine if similar causal patterns are shared among the distances to the trees and the silvopastoral systems.
Under stress-free conditions, spatial patterns of nitrification potential differed between the two agroforestry systems. Nitrification potential was on average 1.5 times higher in the tree row as compared to the grass-alley in the alley-cropping system, while in the hedgerow system, nitrification potential was on average 40% lower in the grass-alley next to the trees as compared to under the trees and the middle of the grass alley. These results were mostly correlated with the spatial patterns of soil pH. Under flooding stress, resistance was significantly higher under the trees of both systems while a boost of nitrification (27% to 35%) was recorded in the grass-alleys. Resilience did not differ among the distances and the two systems. The ML-SEM approach revealed that nitrification stability was positively related to higher soil organic carbon content and lower soil bulk density in the alley-cropping system. Yet, these relations were not confirmed in the hedgerow systems as none of the tested ML-SEM model fitted to the results obtained in this system.
Our results underlined the impact of the silvopastoral design on the regulation of nitrification. If the adoption of silvopastoral systems can mitigate spikes in N losses from grass alleys right after a flooding stress, results under stress-free conditions revealed higher risks for N losses in the tree rows of the alley-cropping system. Hence, trade-offs between higher risk of N losses under stress-free conditions and their limitation under stressed conditions may exist in these systems, while hedgerows seemed to promote more stable and lower nitrification in both conditions. Through the ML-SEM, our study further encourages the adoption of specific management options (improvement of soil organic matter and bulk density) as sources of external regulation of N losses under extreme climate events.
References:
Greaver, T.L., C.M. Clark, J.E. Compton, D. Vallano, A.F. Talhelm, C.P. Weaver, L.E. Band, et al. 2016. « Key ecological responses to nitrogen are altered by climate change». Nature Climate Change 6: 836‑43. https://doi.org/10.1038/nclimate3088.
Kim, Dong-Gill, Marney E. Isaac. 2022. « Nitrogen Dynamics in Agroforestry Systems. A Review ». Agronomy for Sustainable Development 42 (4): 60. https://doi.org/10.1007/s13593-022-00791-7.
Mettauer, Romane, Lukas Beule, Zita Bednar, Margaux Malige, Olivier Godinot, Edith Le Cadre. 2023. « Influence of Two Agroforestry Systems on the Nitrification Potential in Temperate Pastures in Brittany, France ». Plant and Soil. https://doi.org/10.1007/s11104-023-06309-8.
| Keywords | Nitrogen cycle; Agroforestry; Microbial resilience; Extreme weather events; Structural equation modeling |
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