Labile soil organic matter pools under a mixed grass/lucerne pasture and adjacent native bush in Western Australia

Total C and N were measured in whole soils (0-0.15, 0.15-0.35, and 0.35-0.65 m), light organic matter fractions (< 1 g/cm(3) (LF 1.0) and 1.0-1.7 g/cm(3) ( LF 1.7)) in surface soils, and in leaf litter collected from a mixed grass/lucerne pasture and adjacent native bush at Moora, Western Australia. The C content of the plant material and light fractions was characterised by C-13 cross-polarisation/magic angle spinning nuclear magnetic resonance (C-13 CP/MAS NMR) spectroscopy. Water-extractable organic C (WEOC) and N (WEON) were measured in soil, and dissolved organic C (DOC) and N ( DON) were measured in soil solutions. In addition, both NO3-N and NH4-N (SMN) were measured in soil solutions and water extracts.Total soil C (0-0.65 m) did not differ significantly between land uses, but there was clear evidence of N enrichment under the pasture system, which contained significantly (P< 0.05) more total N in the surface soil ( 0-0.15 m) compared with that under native bush. The significantly (P< 0.05) smaller C/N ratios of the surface soil, plant litter, and light fractions (LF 1.0 and 1.7) under the pasture provided further evidence of N enrichment. The 13C CP/MAS NMR spectra for plant material and light fractions did not differ greatly between landuses, but in both cases the O-alkyl : alkyl carbon ratio declined with increasing density. The decomposition and subsequent mineralisation of the relatively N-rich organic matter fractions in the pasture system may have contributed to the significantly ( P< 0.05) greater DOC, DON, and SMN concentration measured in soil solutions under pasture compared with those under native bush.