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), 15N-labelled maize residues was studied over five cropping cycles in a pot experiment in a sandy (7% clay) Quarzipsamment and a sandy–clay (31% clay) Acrorthox from Brazil. During the cropping cycles the soils were amended with bean residues high in N (4.4%), cellulose, or KNO3. Total recovery 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 maize-N consistently in both soils compared with control treatments, especially when applied from the first cropping cycle onwards. At the end of the five cropping cycles, sequential bean residue additions resulted in a 38% increase in maize residue-N recovery in the sandy soil and 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 activity due to the bean-C application, followed by fast turnover of the newly-formed and largely unprotected microbial biomass. However, rather than producing a “real priming effect” on maize residue decomposition, this seems to have produced a “pool substitution” effect, where microbial metabolites of bean instead of maize origin were stabilized in soil. KNO3–N applications had the opposite effect to that of bean applications (16% average decrease in maize-N recovery compared with control treatments) because available-C was not added to stimulate microbial activity, and NH4–N from the decomposing residues was probably recycled in preference to NO3–N by the soil microorganisms. Cellulose application considerably reduced availability of both soil-N and residue-N to plants but the effect lasted for only one cropping cycle. The results clearly showed the potential for manipulation of long-term recoveries of N from recalcitrant plant residues, particularly when better quality residues are also applied to satisfy part of the N demand of plants and microbial biomass, but this probably occurs at the expense of the recovery of the N from the other substrate.