Shoot pruning of a hedgerow perennial legume alters the availability and temporal dynamics of root-derived nitrogen in a subtropical setting

There is increasing interest in the ecology of tree-based intercropping systems, particularly alley cropping, in temperate settings. Shoot pruning of the tree component is an integral practice to alley cropping. Shoot pruning likely results in an input of belowground C and N to soil, with potential, but unknown impacts on the time pattern of N availability. We evaluated the impacts of pruning of Amorpha fruticosa L. on (a) its root dynamics; (b) mineral N and microbial N, and (c) the fate of root-derived N and its importance in determining N availability. For this, we conducted repeated partial pruning and foliar labeling with 15N under field conditions in Georgia, USA. Pruning altered root dynamics by initially decreasing biomass and later preventing increases, likely due to root death. Pruning modified the time pattern of mineral N by substantially increasing concentrations during the 3 months following pruning. 15N labeling indicated that root-derived N was an important source of this extra N and that pruning caused the availability of root-derived N to microbes to be extended until the following growing season. Greater concentrations of mineral N, a greater pool of microbial N and prolonged microbial access and storage of root-derived N with pruning has the potential to positively impact system productivity.