Nitrogen mineralization of legume residues: interactions between species, temperature and placement in soil

Aim of study: To assess the interactive effects of legume species, residue placement and temperature on the net nitrogen (N) mineralization dynamics in a sandy loam soil. Area of study: Northern Portugal. Material and methods: Cowpea (Vigna unguiculata L. Walp), faba bean (Vicia faba L.) and pea (Pisum sativum L.) residues were incorporated or applied to the soil surface at typical field yields in Europe and incubated in aerobic conditions for up to 240 days, either at 10 degrees C or 20 degrees C. Initial chemical characteristics of the soil and residues were determined. Net N mineralization was estimated at eight time intervals. Main results: Cowpea residues caused no negative changes in soil mineral N contents and were able to release the equivalent of 2145 kg N ha(-1) in 240 days. Net N immobilization (up to 17 kg N ha(-1)) was observed throughout most of the trial in soil with faba bean and pea residues. Differences in mineralization patterns could be attributed to the higher quality (lower carbon to nitrogen (C:N) ratios) of cowpea. Surface placement increased net N mineralized by as much as 18 kg N ha(-1). The sensitivity of N mineralization to changes in temperature and residue placement varied with legume species, likely due to effects associated with differences in C:N ratios. Research highlights: Adding cowpea residues to soil is suitable when high N availability is immediately required. Faba bean or pea residues are better suited for conservation of soil N for later release.