Nitrogen-15 natural abundance is robust to quantify nitrogen transfer from clover to grass in temporary grassland

Abstract

Nitrogen (N) transfer from clover to grass may present a significant input of symbiotically fixed clover N in grasslands. We determined grass N transferred from clover over two years using both 15N natural abundance (NA) and 15N labelling (LAB) methods. These methods have not previously been compared under identical environmental and management conditions. We established a model clover-grass mixture in a field experiment that had been fertilised with no, organic, or mineral N for three decades. With the NA method we used procedures differing in proxies that either represented the δ15N of the N sources clover N and plant available soil N while LAB procedures differed in labelling strategies and/or calculation. Different proxies in NA had little impact on the proportion of grass N transferred from clover. The δ15N of the two N sources differed significantly under all fertilisation treatments, by 4.0‰–5.9‰ under zero and organic fertilisation, though often less than 2‰ under mineral fertilisation. The by-treatment average proportion of N transferred from clover ranged from 27% to 55% of grass N for all NA procedures, and from 21% to 68% for all LAB procedures. The LAB procedures were affected by temporally non-uniform 15N enrichment of clover roots. For both methods, all treatments, and both years, about 44% of grass N was transferred from clover. As a result, from 1.5 to 6.3 g N m−2 a−1 of clover N was transferred to grass. The NA method was found to be robust and can be applied to determine the N transfer in temporary grasslands established across a range of fertilisation treatments, including under moderate mineral N fertilisation. The NA procedure that agreed best with the average results from all NA and LAB procedures requires only the δ15N of the shoots of clover and grass growing in the mixture.