Estimating the contribution of nitrogen from legume cover crops to the nitrogen nutrition of grapevines using a 15N dilution technique

Abstract

Cover cropping is increasingly being used as a form of sustainable soil management in grapevine (Vitis vinifera L.) production. The objective of the current study was to determine the effect of a legume cover crop on a set of growth and production variables of a vineyard, and to quantify the relative importance of legume nitrogen (N) as a source of N for the vine plants and to compare this to the current recommended practice of annually applying 40 kg fertilizer-N ha-1. The study was carried out in a 5-yr-old vineyard cv. Cabernet Sauvignon at the Cauquenes-INIA Experimental Center in one of the wine production areas of Chile. The treatments were: control without cover crop (C); a legume mixture of early maturing cultivars of subterranean clover (Trifolium subterraneum L.) and burr medic (Medicago polymorpha L.) (EMC); or a legume mixture of late maturing cultivar of subterranean clover and balansa clover (T. michelianum Savi) (LMC). Average inputs of legume N generated by the EMC and LMC treatments represented 112 and 161 kg above-ground legume N ha-1yr-1, respectively. Grape dry matter production was increased significantly (P ≤ 0.05) by 48–61% and the amount of N accumulated in grape bunches was enhanced by 74–105% after 2 years of legume cover crop. However, no significant (P > 0.05) effects of cover cropping were observed on vine leaf, canes, roots or trunk biomass. To estimate the relative contribution of legume and fertilizer N to the N nutrition of grapevines 15N-enriched fertilizer (10 atom% 15N excess) was applied to the soil in micro-plots within all treatments as two split applications of 20 kg N ha-1. The 15N composition of vines grown with or without cover crops were subsequently compared. Any measured reduction in the 15N enrichment of the tissues of cover cropped vines relative to the nil cover crop control was assumed to have resulted from the uptake of legume N. Recovery of 15N in vine plant parts was used to calculate the efficiency of use of fertilizer N. Surprisingly the 15N data indicated that the fruit had the lowest reliance upon legume N for growth (only 7–13% cf 14–56% of the N in other organs), even though grape bunches were the only plant part to positively respond to the cover cropping treatments. This suggested that either the temporal patterns of N mineralization in soil and N uptake by vines over the growing season was such that much of the mineral N utilized during fruit development was not directly derived from legume residues, or that N remobilized from other plant parts within the vines was the major source of N for grape production rather than N newly assimilated from soil. The amount of legume N estimated to be recovered by cover cropped vine plants (12–15 kg N ha-1), was similar to the calculated contribution from 40 kg of fertilizer-N applied to vines (11-12 kg ha-1). This amount of N represented <10% of the amounts of N annually returned to the soil in above-ground legume material in the case of the cover cropping treatments, and was equivalent to a recovery of 27–30% of the N applied in fertilizer.