Effects of grazing on plant and soil nitrogen relations of pasture-crop rotations

Abstract

Plant and soil nitrogen (N) fluxes were assessed in subterranean clover (Trifolium subterraneum L.) based pastures set-stocked at 8 sheep per hectare (light grazing) or grazed at a much higher, but variable, intensity to maintain 1400 kg standing dry matter per hectare (intensive grazing) through the addition or removal of sheep. Pasture composition and biomass production, herbage N concentration, plant nitrate (NO^-₃) utilisation, and N₂ fixation by clover were assessed at 3-weekly intervals over the growing season. Soil ammonium (NH⁺₄) and NO^-₃ availability were assessed at similar intervals using soil coring and in situ incubation cores. Seasonal pasture yield under light grazing was 11·5 t dry matter/ha compared with 7·9 t/ha under intensive grazing, the difference being mostly attributable to reduced grass growth under intensive grazing. However, there was essentially no difference between the pastures in total N accumulation (300 kg N/ha in the lightly grazed and 302 kg N/ha in the intensively grazed pastures). The lesser dry matter production under intensive grazing was compensated for by higher N concentration and increased clover content of the sward, and faster clover growth late in the growing season. N₂ fixation by clover under intensive grazing (153 kg N/ha) was slightly greater than under light grazing (131 kg N/ha). Proportional dependence of clover on N₂ fixation (%Ndfa) was similar under intensive grazing (78%) and light grazing (84%), despite higher continued availability of soil mineral N under intensive grazing. Uptake of soil N by the grass component amounted to 147 kg N/ha under light grazing v. 96 kg N/ha in the intensively grazed pasture, and for the clover was 18 and 40 kg N/ha, respectively. Capeweed (Arctotheca calendula L.), a common weed of south-west Australian pastures, was extraordinarily active in absorbing, storing, and reducing soil NO^-₃, especially when subjected to intensive grazing. After the 3 years of the grazing trial, the pastures were cultivated and cropped to oats, triticale, and canola and the biomass and N uptake of each crop assessed. Intensive grazing in the previous pasture resulted in increased availability of soil mineral N in the subsequent cropping phase and accordingly augmented crop N uptake and eventual grain protein levels relative to crops following lightly grazed pasture. The study indicated that intensive grazing before cropping may offer a useful management tool for improving N nutrition and yields of non-leguminous crops in pasture-crop rotations under the conditions prevailing in the south-west of Australia.