DYNAMICS OF CARBON AND NITROGEN IN A LONG-TERM CROPPING SYSTEM AND PERMANENT PASTURE SYSTEM

On the brigalow lands of south-east Queensland, productivity of sown p erennial grasses is severely limited by N availability, whereas annual crops grown on the same soil are N-sufficient. The dynamics of C and N were compared under in these soils under permanent green panic (Pani cum maximum var. trichoglume cv. Petrie) pasture and continuous croppi ng with grain sorghum (Sorghum bicolor). Although the sorghum system w as more productive, it contained 18% less N and 29 degrees less C. Ann ual flows of C and N through the soil microbial biomass were, respecti vely, 4500 and 240 kg ha(-1) under sorghum, and 4050 and 60 kg ha(-1) under pasture. Over 80 degrees of C and N inputs to the sorghum system occurred after harvest. Under pasture, the continuous supply of resid ues of high C/N ratio (50-75) enabled the development of a large and a ctive microbial biomass, which competed with the pasture plant for N, resulting in slow net mineralization of N and low levels of inorganic soil N. Under sorghum, the size of the microbial biomass was limited b y C availability during the growing season. The sorghum residues had s lightly lower C/N ratios (36-46), and their rapid decomposition and ne t mineralization of N were promoted by the fallow period and soil cult ivation. Estimated annual C turnover through the soil microbial biomas s was slightly faster under sorghum, and annual N turnover was around seven times faster under sorghum than under green panic. The productiv ity of these soils under the two management systems was controlled by the amount, quality and timing of organic matter inputs. These in turn controlled the size of the soil microbial biomass and its C and N sup ply, and hence the balance between immobilization and mineralization o f N.