Nitrogen mineralization by maize from previously added legume residues following addition of new legume residues using 15N labelling technique

Although organic matter is applied repeatedly over crop cycles, the relationship between quality of legume residues and N release has been generally determined over single crop cycles (8 to10 weeks). To evaluate the N recovery by maize (Zea mays L.) from legume residues over two planting cycles, freshly chopped (1 to 2 mm) and 15N labeled residues of Centrosema pubescens Benth. and Calopogonium mucunoides Desv. were applied at the rate of 100 mg N-kg-1 soil in a glasshouse study. After the harvest of the maize plants (8 weeks), soil in the pots was split into two halves, one of which recieved a further addition of unlabelled legume residues (100 mg N-kg-1 soil), while the other did not. Recovery of N from the residues ranged from 12 to 44% , most of which was from the first application. Repeated addition of unlabelled N rich and low pholyphenol Centrosema residues, led to a stimulation effect on the previously applied 15N labelled legume residues, whereas new addition of N poor, high polyphenol legume residues led to a retardation effect. There was no compensation for initial N release from N poor, high polyphenol legume residues at second application.