[Profile of soil microbial biomass carbon in different types of subtropical paddy soils]

The soil microbial biomass carbon (C(mic)), one of the most active components of soil organic carbon (C(org)), is an effective indicator of soil quality. In the present study, five subtropical paddy soils developed from different parent materials were selected, and the distribution of C(mic) through the profiles was studied, as well as the relationship of C(mic) with C(org) and soil nutrients. The results showed that the contents of C(org) and C(mic) decreased markedly with increasing soil depth, ranging from 2.45 g x kg(-1) to 26.19 g x kg(-1) and from 4.55 mg x kg(-1) to 1 691.75 mg x kg(-1), respectively. They mainly concentrated in the surface layer (plough horizon and plough pan). The content of C(mic) varied significantly in paddy soils developed from different parent materials, with the highest one in yellow clayey soil, and the lowest ones in alluvial sandy soil and reddish yellow clayey soil. This was on the contrary to the distribution of C(org) in the surface paddy soils, since the reddish yellow clayey soil and alluvial sandy soil showed higher contents while other types of paddy soils exhibited similar contents of C(org). Notwithstanding, C(mic) was still controlled by the quantity of C(org) and positively correlated with C(org). The ratio of C(mic) to C(org)(C(mic)/C((org)) decreased with increasing soil depth and differed in the plough horizon between different paddy soils, with lower values in alluvial sandy soil (2.11%) and reddish yellow clayey soil (1.37%) but higher value in reddish yellow clayey soil I (8.24%). It indicated that the microbial substrate availability in alluvial sandy soil and reddish yellow clayey soil was lower than those in reddish yellow clayey soils. The content of C(mic) was significantly positively correlated with total nitrogen, alkali-hydrolyzable N and Olsen-P, but was irrelevant to available K. It is implied that the C(mic) was not only controlled by C(org), but also complicatedly interacted with soil nutrients in paddy soils.