The fate of cumulative applications of 15N-labelled fertiliser in perennial and annual bioenergy crops

Abstract

The fate of nitrogen (N) fertiliser applied to bioenergy crops is a key issue to allow high biomass production while minimising environmental impacts due to N losses. The aim of this study was to follow the fate in the soil-plant system of N fertiliser applied to perennial (Miscanthus×giganteus and switchgrass), “semi-perennial” (fescue and alfalfa) and annual (sorghum and triticale) bioenergy crops. Crops received 15N-labelled fertiliser (urea ammonium nitrate solution) during 4 or 5 successive years on the same subplots, at a rate varying from 24 to 120kgNha−1yr−1. Biomass production, N and 15N removal at harvest were measured each year. The 15N recovery in crop residues, non-harvested crop parts and soil was measured at the end of the 15N-labelling period. Perennial crops had higher biomass production but generally lower 15N recovery in harvested biomass than other crops, particularly when harvested late (end of winter). At the end of the 4 or 5-year period, the proportion of 15N recovered in harvested biomass was 13–34% for perennials, 23–38% for semi-perennials and 34–39% for annual crops. Perennial crops stored large amounts of N in their belowground organs; the mean 15N recovery in these organs was 12%, corresponding to a N storage flux of 14kgNha−1 yr−1. The 15N recovery in soil (including crop residues) was higher for perennials (average 36%) than semi-perennials (28%) and annual crops (19%), corresponding to a N immobilisation rate of 43, 15 and 12kgNha−1yr−1 respectively. The mean overall 15N recovery in the soil-plant system was 69% in perennials, 61% in semi-perennials to 56% in annual crops, suggesting that important fertiliser losses occurred through volatilisation and denitrification. Perennial bioenergy crops had the better efficiency by storing fertiliser-N in soil organic matter and living belowground biomass used as N reserves for succeeding years.