SUBSTRATE AMENDMENTS CAN ALTER MICROBIAL DYNAMICS AND N-AVAILABILITY FROM MAIZE RESIDUES TO SUBSEQUENT CROPS

The release and recovery of N from poor-quality (0.35% N), N-15-labell ed maize residues was studied over five cropping cycles in a pot exper iment in a sandy (7% clay) Quarzipsamment and a sandy-clay (31% clay) Acrorthox from Brazil. During the cropping cycles the soils were amend ed with bean residues high in N (4.4%), cellulose, or KNO3. Total reco very of maize-N in plants at the end of the experiment was greater in the sandy soil (23-37%) compared with the sandy-clay soil (19-30%) in all treatments. Additions of bean residues increased the recovery of m aize-N consistently in both soils compared with control treatments, es pecially when applied from the first cropping cycle onwards. At the en d of the five cropping cycles, sequential bean residue additions resul ted in a 38% increase in maize residue-N recovery in the sandy soil an d a 32% increase in the sandy-clay soil. When applied from the second cycle onwards they resulted in only 27% and 9% increases respectively. The increased maize-N recovery resulted from greater microbial activi ty due to the bean-C application, followed by fast turnover of the new ly-formed and largely unprotected microbial biomass. However, rather t han producing a ''real priming effect'' on maize residue decomposition , this seems to have produced a ''pool substitution'' effect, where mi crobial metabolites of bean instead of maize origin were stabilized in soil. KNO3-N applications had the opposite effect to that of bean app lications (16% average decrease in maize-N recovery compared with cont rol treatments) because available-C was not added to stimulate microbi al activity, and NH4-N from the decomposing residues was probably recy cled in preference to NO3-N by the soil microorganisms. Cellulose appl ication considerably reduced availability of both soil-N and residue-N to plants but the effect lasted for only one cropping cycle. The resu lts clearly showed the potential for manipulation of long-term recover ies of N from recalcitrant plant residues, particularly when better qu ality residues are also applied to satisfy part of the N demand of pla nts and microbial biomass, but this probably occurs at the expense of the recovery of the N from the other substrate. (C) 1998 Elsevier Scie nce Ltd. All rights reserved.