Quantifying biological nitrogen fixation of different catch crops, and residual effects of roots and tops on nitrogen uptake in barley using in-situ 15N labelling

Contributions of legume-based catch crops (LBCCs) to succeeding cereals may be significant. We quantified biological N fixation (BNF) and residual N effects of contrasting CC tops and roots. BNF of three LBCCs (red clover, winter vetch, perennial ryegrass-red clover mixture) was quantified in microplots by N-15 labelling. Their residual effects on spring barley were tested against two non-LBCCs (perennial ryegrass, fodder radish) after spring incorporation of CC tops or roots in monoliths. Total N accumulated in LBCCs was 153-226 kg N ha(-1), of which 62-66 % was derived from BNF in tops and 31-46 % in macro-roots (0-18 cm soil). Macro-roots represented 31-50 % of total plant N. LBCCs showed similar capacity for soil N extraction as non-LBCCs. After incorporation of LBCC residues, the dry matter and N yields of spring barley were comparable to the effect of 50 kg N fertilisation ha(-1), whereas no extra N uptake was derived from non-LBCCs. The N-15-based N fertiliser values of LBCC tops were 34-47 % against 26-29 % for non-LBCCs. LBCC roots contributed substantial amounts of N to the system, a source that is usually underestimated. N immobilisation after incorporation of non-LBCCs may hamper the growth of following main crops especially after removing tops.