Stratification of microbial biomass C and N and gross N mineralisation with soil depth in two contrasting Western Australian agricultural soils

The distribution of microbial biomass C and N and the decline in gross N mineralisation and NH 4 + consumption with soil depth was investigated in 2 soils with different soil texture and land use. Soils were from an annual pasture on a loamy sand and from a sandy clay loam previously cropped with wheat. Intact soil cores were collected from the surface 0–10 cm in steel tubes and were sampled in 2·5 cm layers. Disturbed soil down to 50 cm was collected in 10 cm sections using a sand auger. Microbial biomass was estimated by chloroform fumigation and 0·5 M K 2 SO 4 extraction. Microbial biomass C was determined from the flush in ninhydrin-positive compounds, and microbial biomass N from the flush in total soluble N after K 2 S 2 O 8 oxidation. Gross N mineralisation and NH 4 + consumption were estimated by 15 N isotopic dilution using 15 NH 3 gas injection to label the soil 14 NH 4 + pool with 15 N. The pattern of distribution of the microbial biomass and the rate of N transformations were similar for both soils. There was a rapid decline in microbial biomass C and N and gross N mineralisation with soil depth. Approximately 55% of the microbial biomass, 70–88% of gross N mineralisation, and 46–57% of NH 4 + consumption was in the surface 0–10 cm in both soils. There was also a stratification of microbial biomass and gross N mineralisation within the 0–10 cm layer of intact soil cores. It was estimated that one-quarter of the total microbial biomass and at least one-half of the total gross N mineralisation within the soil profiles (0–50 cm) was located in the surface 2·5 cm layer. These results demonstrate the importance of the surface soil layer as a major source of microbial activity and inorganic N production. There was a strong correlation between the distribution of microbial biomass and the gross rate of mineralisation of soil organic N within the soil profile. Keywords: gross mineralisation, 15 N isotopic dilution. Australian Journal of Soil Research 36(1) 45 - 56 Full text doi:10.1071/S97045 © CSIRO 1998