Labile organic matter fractions and aggregate stability under short-term, grass-based leys

Effects of 5 yr of continuous grass/clover (Cont grass/clover) or grass (Cont grass) pasture or 5 yr of annual grass pasture under conventional (Ann grass CT) or zero tillage (Ann grass ZT) on labile soil organic matter fractions were compared with 5 yr of continuous barley (LT arable) on a site which had been under continuous arable production for 11 yr prior to the study. For comparison, a site nearby under long-term pasture (LT pasture) was also sampled. Organic C and total N in the soil profile to 20 cm followed the order LT pasture > Cent grass=Cont grass/clover > Ann grass ZT greater than or equal to Ann grass CT=LT arable. Differences were greatest in the surface 2.5 cm of soil. Microbial biomass C and N showed a similar trend except differences were more pronounced. Readily mineralizable C and N (10 d aerobic incubation) also showed a similar trend except mineralizable N was similar for the Ann grass ZT and Cent grass and Cent grass/clover treatments. N uptake by ryegrass in a greenhouse experiment showed the trend LT pasture > Ann grass ZT > Cent grass = Cent grass/clover > Ann grass CT = LT arable. Light fraction (LF) dry matter and LF organic C were very high under LT pasture and little affected by other treatments. The C concentration of LF and its C/N ratio were high for the annual cultivation treatments (LT arable and Ann grass CT) reflecting the recent incorporation of crop and grass residues into the soil prior to sampling. In comparison with LT arable, under Cent grass and Cent grass/clover, micro aggregates (< 0.25 mm) became aggregated into stable macro aggregates. Aggregate stability was almost as high under these two treatments as under LT pasture. The results demonstrate the positive effects that a short-term (5 yr) pasture can have on soil organic matter quantity and quality and its attendant benefits on N fertility and soil structure. (C) 1999 Elsevier Science Ltd. All rights reserved.