The fate of tree root and pruning nitrogen in a temperate climate alley cropping system determined by tree-injected 15N

Abstract

 Nitrogen (N) fluxes through the major plant pools of an alder (Alnus sinuta)-sweet corn (Zea mays) alley cropping system were determined over the course of two cropping seasons. Alder trees were injected with ¹⁵NO₃–N to directly follow the flow of N between alder and corn. The contribution of the above- and below-ground tree N to corn was determined by exchanging the labeled above-ground prunings (green manure) with those from unlabeled plots. During the first growing season after coppicing of the injected alders, 18% of the alder ¹⁵N was taken up by the corn with 12% coming from the above-ground prunings. Of the ¹⁵N remaining in the tree/stump following coppicing, the majority was recovered by corn plants within the rows next to the labeled trees during the first growing season. Earlier recovery of ¹⁵N by corn in the labeled root plots compared to the labeled pruning plots indicated the importance of root turnover in supplying N to corn, especially following coppicing. By the end of the first and second growing seasons, 34% and 38% of the ¹⁵N initially present in prunings was recovered in corn plants, respectively. Approximately 80% of the total injected ¹⁵N was found in the soil during the second growing season; however, the turnover of above- and below-ground alder components supplied only 3–4% of the N required by corn during the year of green manure application. Thus, most of the corn N demand was met by mineralization of residual soil N within the 2 years of coppicing and green manure additions. Continued internal cycling of tree N and movement of soil N into more labile pools would presumably allow more alder N to become available over time. The synchronization between N mineralization from the hedgerow green manure components and nutrient uptake of the alley crop remains a major challenge in alley cropping and other green manure systems.