Cycling of grain legume residue nitrogen

Symbiotic nitrogen fixation by legumes is the main input of nitrogen in ecological agriculture. The cycling of N-15-labelled mature pea (Pisum sativum L.) residues was studied during three years in small field plots and lysimeters. The residual organic labelled N declined rapidly during the initial weeks of decomposition, due to high rates of residue N net mineralization and subsequent leaching and denitrification losses of N. Lysimeter experiments showed that pea residues may reduce leaching losses of N, probably due to their effect on the mineralization-immobilizalion turnover of N and denitrification. Winter barley succeeding field pea recovered 13% of the incorporated pea residue N by early December; the recovery was found to be 15% at maturity in July. A spring-sown crop of barley recovered less than half the amount of pea residue N recovered by winter barley. The residue N-use efficiencies were 36% and 15% in three sequences of cropping systems with autumn- or spring-established crops, respectively. The results highlight the requirement for establishment of crops after cultivation of pea which are efficient in taking up N in the autumn/early spring and the need for improved residue management methods in order to conserve grain legume residue N sources within the soil-plant system.