Influence of microbial populations and residue quality on aggregate stability

Soil structure mediates many biological and physical soil processes and is therefore an important soil property. Physical soil processes, such as aggr egation, can be markedly influenced by both residue quality and soil microb ial community structure. Three experiments were conducted to examine (i) th e temporal dynamics of aggregate formation and the water stability of the o btained aggregates, (ii) the effect of residue quality on aggregation and m icrobial respiration, and (iii) the effect of fungi and bacteria on aggrega tion. In the first experiment, 250 mum sieved air-dried soil, mixed with wheat st raw, was incubated for 14 days to allow formation of water-stable macroaggr egates (>250 mum). Aggregate stability was measured by wet sieving after fo ur different disruptive treatments: (i) soil at field capacity: (ii) soil a ir-dried and slowly wetted; (iii) soil air-dried and quickly wetted; (iv) 8 mm sieved soil, air-dried and immersed in water (slaking). After 14 days o f incubation, maximum aggregation for soil sieved at field capacity was rea ched; however, these newly formed aggregates were not yet resistant to slak ing. During the second experiment, the effect of low-quality residue (C/N: 108) (with or without extra mineral nitrogen) and high-quality residue (C/N: 19. 7) (without extra mineral nitrogen) on macroaggregate formation and fungal and bacterial populations was tested. After 14 days, aggregation, microbial respiration, and total microbial biomass were not significantly different between the low-quality (minus mineral nitrogen) and high-quality residue t reatment. However, fungal biomass was higher for the low-quality residue tr eatment compared to the high-quality residue treatment. In contrast, bacter ial populations were favored by the high-quality residue treatment. Additio n of mineral N in the low-quality residue treatment resulted in reduced mac roaggregate formation and fungal biomass, but had no effect on bacterial bi omass, These observations are not conclusive for the function of fungal and /or bacterial biomass in relation to macroaggregate formation. In order to directly discern the influence of soil microflora on aggregation, a third e xperiment was conducted in which a fungicide (captan) or bactericide (oxyte tracycline) was applied to selectively suppress fungal or bacterial populat ions. The direct suppression of fungal growth by addition of fungicide led to reduced macroaggregate formation. However, suppression of bacterial grow th by addition of bactericide did not lead to reduced macroaggregate format ion. In conclusion, macroaggregate formation was positively influenced by f ungal activity but was not significantly influenced by residue quality or b acterial activity. (C) 2001 Elsevier Science B.V. All rights reserved.