FATE OF NITROGEN FROM CROP RESIDUES AS AFFECTED BY BIOCHEMICAL QUALITY AND THE MICROBIAL BIOMASS

Net mineralization of N from a range of shoot and root materials was d etermined over a period of 6 months following incorporation into a san dy-loam soil under controlled environment conditions. Biochemical ''qu ality'' components of the materials showed better correlation with net N mineralization than did gross measures of the respiration and N con tent of the soil microbial community during decomposition. The quality components controlling net N mineralization changed during decomposit ion,with water-soluble phenolic content significantly correlated with net N mineralization at early stages, and water-soluble N, followed by cellulose at later stages. C-to-N and total N were correlated with ne t N mineralization towards the end of the incubation only. Cumulative microbial respir ation during the early stages of decomposition was co rrelated with net N mineralizalion measured after 2 months, at which t ime maximum net N mineralization was recorded for moat residues. Howev er, there was no relationship between microbial-N and net N mineraliza tion. Biochemical quality factors controlling the C and N content of t he residue remaining at the end of the incubation as light fraction or ganic matter (LFOM) were also investigated. Both C and N content of LF OM derived from the residues were correlated with residue cellulose co ntent, and the chemical characteristics of LFOM were highly correlated with those of the original plant material. Incorporation of low cellu lose, high water-soluble N-containing shoot residues resulted in more N becoming mineralized than had been added in the residues, demonstrat ing that net mineralization of native soil organic matter had occurred . Large amounts of N were lost from the mineral-N pool during the incu bation, which could not be accounted for by microbial immobilization. (C) 1998 Elsevier Science Ltd. All rights reserved.