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

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(-1)yr(-1), respectively. Grape dry matter production was increased significantly (P a parts per thousand currency signaEuro parts per thousand 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 N-15-enriched fertilizer (10 atom% N-15 excess) was applied to the soil in micro-plots within all treatments as two split applications of 20 kg N ha(-1). The N-15 composition of vines grown with or without cover crops were subsequently compared. Any measured reduction in the N-15 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 N-15 in vine plant parts was used to calculate the efficiency of use of fertilizer N. Surprisingly the N-15 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.