Interaction of biochemical quality and particle size of crop residues and its effect on the microbial biomass and nitrogen dynamics following incorporation into soil

Abstract

Mineralization of N from organic materials added to soil depends on the quality of the substrate as a carbon, energy and nutrient source for the saprophyt- ic microflora. Quality reflects a combination of bio- chemical and physical attributes. We investigated how biochemical composition interacts with particle size to affect the soil microflora and N dynamics following in- corporation of crop residues into soil. Four fresh shoot and root crop residues were cut into coarse and fine particle sizes, and incorporated into sandy-loam soil which was incubated under controlled environment conditions for 6 months. In the case of the highest bio- chemical quality material, potato shoot (C/N ratio of 10 : 1), particle size had no effect on microbial respira- tion or net N mineralization. For lower biochemical quality Brussels sprout shoot (C/N ratio of 15 : 1), re- ducing particle size caused microbial respiration to peak earlier and increased net mineralization of N dur- ing the early stages of decomposition, but reduced net N mineralization at later stages. However, for the low- est biochemical quality residues, rye grass roots (C/N ratio of 38 : 1) and straw (C/N ratio of 91 : 1) reducing particle size caused microbial respiration to peak later and increased net immobilization of N. For Brussels sprout shoot, reducing particle size decreased the C content and the C/N ratio of residue-derived light frac- tion organic matter (LFOM) 2 months following incor- poration. However C and N content of LFOM derived from the other materials was not affected by particle size. For materials of all qualities, particle size had little effect on biomass N. We conclude that the impact of particle size on soil microbial activities, and the protec- tion of senescent microbial tissues from microbial at- tack, is dependant on the biochemical quality of the substrate.