Interactions between residues of maize and pigeonpea and mineral N fertilizers during decomposition and N mineralization

Nitrogen mineralization patterns of maize and pigeonpea (Cajanus cajan) res idues were examined in leaching tubes, both in isolation and mixtures, in M alawian soils of varying texture. Senesced pigeonpea leaves (C-to-N ratio 2 4) induced a short period of nitrogen immobilization which was followed by steady net nitrogen mineralization in all of three soils. The immobilizatio n period lasted between 14 and 28 days and was longer in soils with larger clay contents. Maize residues contained 30% of their N in the form of water -soluble nitrate. Both the sole maize residue (C-to-N ratio 75 after adjust ment for nitrate which constituted 28% of the N) and the mixture of maize a nd senesced pigeonpeas leaves revealed a similar prolonged strong net N imm obilization up to 130 days before the two treatments started to diverge sli ghtly. Mixing maize with pigeonpea residues with equal amounts of N failed to substantially alleviate the N immobilization capacity of the maize resid ues. N immobilization in the mixture was much greater than that predicted f rom the mineralization patterns of the individual components. When increasi ng amounts (50, 100 and 150 mg N kg(-1) soil) of green pigeonpea leaves, se nesced pigeonpea leaves and ammonia-N were added to 50 mg N kg(-1) soil of maize residues, N released in the mixtures increased with the increasing am ounts of N added to the maize residues with greater increases from residues with larger N concentrations. There was evidence that microbial degradatio n of maize carbon was limited by N availability. The implications of the re sults for management of crop residues and mineral N fertilizers in the fiel d are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.