Effects of eight year rotations of grain sorghum with lucerne, annual legume, wheat and long fallow on nitrogen and organic carbon in two contrasting soils

Abstract

Soil nitrogen and organic carbon levels during 8-year rotations of grain sorghum (4 years) with lucerne (4 years), annual legume (faba beans or cowpeas in alternate years), wheat (4 years) and long fallow (alternate years) were measured on a black earth and a red clay in northern New South Wales. Total soil nitrogen (0-15 cm) increased at about 100 kg ha-1 yr-1 in the lucerne leys, but it maximized after 2 years on the red clay and then declined at about the same rate, suggesting that nitrogen fixation had ceased in response to heavy grazing pressure and drought conditions. The positive effects of lucerne on total nitrogen and organic carbon were maintained during 4 years of subsequent sorghum growth on the black earth, but effects on the red clay were not significant. Sorghum, when its residues were incorporated, had a much larger effect than lucerne in raising soil organic carbon. Concentrations of soil nitrate were much higher immediately after the lucerne than after continuous cereal, and they remained higher in the lucerne rotations after 2 years of sorghum. Accumulation of nitrate during long fallows was much larger after sorghum, with incorporated residues, than after no-tilled wheat, and the magnitude of this difference suggested biological nitrogen fixation by free-living bacteria during the sorghum long fallows. Fluctuations in nitrate were generally larger in the red clay than in the black earth, but sorghum yields and nitrogen uptake were less responsive to high nitrate in the red clay, apparently because of its lower nitrate buffering capacity and poorer soil structure. In comparison with continuous cereal growing, the annual legume rotation had no significant effect on organic carbon or total nitrogen, but it sometimes increased nitrate during the sorghum rotation, probably by a nitrate sparing effect during the cowpea crop.