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

Abstract

The distribution of microbial biomass C and N and the decline in gross N mineralisation and NH₄⁺ 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 K₂SO₄ 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 K₂S₂O₈ oxidation. Gross N mineralisation and NH₄⁺ consumption were estimated by ¹⁵N isotopic dilution using ¹⁵NH₃ gas injection to label the soil ¹⁴NH₄⁺ pool with ¹⁵N. 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 NH₄⁺ 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.