CONTRIBUTION AND TRANSFER OF NITROGEN FROM COVER CROPS TO RASPBERRY PLANT USING ISOTOPIC TECHNIQUES WITH 15N

Abstract

The objective of this study was to determine the effect of a legume cover crop on raspberry production and to quantify the eventual nitrogen contribution from the legumes to raspberry plants. The study was carried out in a 5 year-old organic orchard of raspberry cv. Heritage. Cover crop treatments were: a) white clover (Trifolium repens), b) a mixture of white clover and tall fescue (Festuca arundinacea) and c) a control without cover crop. To estimate the nitrogen recovery by raspberry plants derived from the legume, a solution of ammonium sulfate 10% 15 N atom excess (80 kg N ha -1 ) was applied to raspberry plants growing with and without cover crops. During the growing season in summer the 15 N content (% 15 N atom excess) and total N was determined in leaves, stems and fruits, and the percentage of N derived from the legume in raspberry plant calculated. Pastures of white clover and white clover + tall fescue produced high levels of biomass, 8.6 to 9.4 t DM ha -1 , with more than 3.3% of N. In soil mineral N content important differences were detected, with significant higher N content in the legume cover crop treatments. Total raspberry biomass production was significantly higher (P≤ 0.05) when they grew for two years in association with a legume cover crop than when grown on bare soil. On average 18–22% of the stem and leaf N content of raspberry plants growing with white clover cover crop derived from the legume. Evaluations made at the end of raspberry plant growth cycle showed that recovered N in fruits was small (2.5 to 7.0%).