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) residues were examined in leaching tubes, both in isolation and mixtures, in Malawian soils of varying texture. Senesced pigeonpea leaves (C-to-N ratio 24) induced a short period of nitrogen immobilization which was followed by steady net nitrogen mineralization in all of three soils. The immobilization 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 adjustment for nitrate which constituted 28% of the N) and the mixture of maize and senesced pigeonpeas leaves revealed a similar prolonged strong net N immobilization up to 130 days before the two treatments started to diverge slightly. Mixing maize with pigeonpea residues with equal amounts of N failed to substantially alleviate the N immobilization capacity of the maize residues. N immobilization in the mixture was much greater than that predicted from the mineralization patterns of the individual components. When increasing amounts (50, 100 and 150 mg N kg(-1) soil) of green pigeonpea leaves, senesced 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 amounts of N added to the maize residues with greater increases from residues with larger N concentrations. There was evidence that microbial degradation of maize carbon was limited by N availability. The implications of the results for management of crop residues and mineral N fertilizers in the field are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.