Bacterial and fungal abundance and biomass in conventional and no-tillage agroecosystems along two climatic gradients

Microbial community composition may be an important determinant of soil org anic matter (SOM) decomposition rates and nutrient turnover and availabilit y in agricultural soils. Soil samples were collected from six long-term til lage comparison experiments located along two climatic gradients to examine the effects of no-tillage (NT) and conventional tillage (CT) management on bacterial and fungal abundance and biomass and to examine potential contro ls on the relative abundances of bacteria and fungi in these two systems. S amples were divided into 0-5 and 5-20 cm depth increments and analyzed for bacterial and fungal abundance and biomass, total C and N, particulate orga nic matter C and N (POM-C and N), soil water content, texture, pH, and wate r-stable aggregate distributions. Soil moisture, which varied by tillage tr eatment and geographically with climate, ranged from 0.05 to 0.35 g g(-1) d ry soil in the surface 0-5 cm and 0.15 to 0.28 g g(-1) dry soil at 5-20 cm. Measured organic matter C and N fractions and mean weight diameter (MWD) o f water-stable aggregates were significantly higher in NT relative to CT at three of the six sites. Fungal hyphal length ranged from 19 to 292 m g(-1) soil and was 1.9 to 2.5 times higher in NT compared to CT surface soil acr oss all sites. Few significant tillage treatment differences in soil physic al and chemical properties or in fungal abundance and biomass were observed at 5-20 cm. Bacterial abundance and biomass were not consistently influenc ed by tillage treatment or site location at either depth. The proportion of the total biomass composed of fungi ranged from 10 to 60% and was signific antly higher in NT compared to CT surface soil at five of six sites. Propor tional fungal biomass was not strongly related to soil texture, pH, aggrega tion, or organic C and N fractions, but was positively related to soil mois ture (r = 0.67; P < 0.001). The relationship between soil moisture and the degree of fungal dominance was due to the positive response of fungal bioma ss and the lack of response of bacterial biomass to increasing soil moistur e across the range of measured soil water contents. Tillage treatment effec ts on fungal biomass and proportional fungal abundance were not significant when the data were analyzed by analysis of covariance with soil moisture a s the covariate. These results suggest that observed tillage treatment and climate gradient effects on fungi are related to differences in soil moistu re. Further research is needed, however, to determine how tillage-induced c hanges in the soil environment shape microbial community composition in agr oecosystems. (C) 1999 Elsevier Science Ltd. All rights reserved.