Spatial patterns of causality in temperate silvopastoral systems: a perspective on nitrification stability in response to flooding

Extreme rainfall and flooding events are projected to increase in frequency and disturb biogeochemical cycles such as the nitrogen (N) cycle. By combining trees and grasses, silvopastoral agroforestry is expected to increase the stability of this cycle in response to flooding. However, little is known about the response of nitrification to flooding in silvopastoral systems. Aim of this study was to assess nitrification stability in response to flooding and identify the main causal relations that drive it in temperate silvopastures. The nitrification stability (i.e., resistance and resilience) was assessed in two silvopastoral systems (i.e., hedgerows and alley cropping) at three positions relative to the trees. The resistance and resilience of nitrification potential were measured in the laboratory after four weeks of flooding stress and four weeks after the end of the stress, respectively. For the first time, we used multigroup latent structural equation modeling (ML-SEM) to explore the spatial structure of causal relations between nitrification stability and soil properties across all positions of the two silvopastoral systems. Tree rows of both systems favored nitrification resistance, while the mean nitrification potential under flooded conditions was on average 27% and 35% higher as compared to non-stressed soils at the two positions assessed in the grass alleys. ML-SEM revealed that the causal relations that explained these results differed between the two systems. The ML-SEM models tested were unable to explain the causal relations in the hedgerow system. However, the model that considered a covariance between soil physical properties and soil resources availability (model A) was able to explain them in the alley-cropping system. It revealed that causal relations explaining nitrification stability varied according to the position relative to the trees: in the tree rows nitrification stability was associated with higher soil organic carbon concentration and earthworm abundance; in the grass alleys it was associated with higher soil organic carbon concentration and soil bulk density. This study indicates that silvopastoral systems help regulate the N cycle near the trees. The results further imply that improvements in soil organic carbon concentration and soil bulk density favor the regulation of N-related processes in grasslands.