Soil degradation under cropping and its influence on wheat yield on a weakly structured New Zealand silt loam

Results from the first phase of a long-term experiment showed that, after 6 years under pasture, several soil quality attributes had improved compared with soil cropped annually. The objectives of this study were to quantify the effects of pasture-induced increases in structural stability and organic matter (N fertility) on wheat grown in 3 successive seasons following pasture cultivation. Growing winter wheat after the ploughing of land that had previously grown perennial grass resulted in gradual reductions in soil organic C and total N. Reductions in soil microbial biomass C and earthworm populations were much more rapid. Soil aggregate stability declined rapidly in the first year after ploughing, but more slowly after that. Soil macroporosity increased after ploughing, mainly due to the relief of compaction caused by sheep treading during grazing. The contrasting soil conditions that existed at the end of the first experimental phase significantly affected the harvest yield of the first and second wheat crops, with yields 2-3 t/ha greater after perennial grasses than after annual crops. Variations in harvest yield and N uptake were explained by differences in soil N fertility and soil structural conditions. Treatment effects on yield were not detected in the third wheat crop. For the structural condition and N fertility of this soil, the extent of improvement during 3 years under perennial pasture was similar to the extent of decline under 3 years of cropping. This suggests that similar lengths of pastoral and arable cropping are needed in crop rotations for the long-term maintenance of these properties in weakly structured silt loam soils in New Zealand.