Legume production and nitrogen fixation in hill pasture communities

Abstract

Abstract The production, species composition, and nitrogen (N) fixation in five resident communities varying in pasture species and land slope were examined for a developed hill country paddock. The communities (and main legume species) were stock camp (white clover), gentle slope (white clover), gentle slope (subterranean clover), steep slope (subterranean clover), and steep slope (Lotus angustissimus L. and L. suaveolens Pers.). Annual dry matter (DM) production was 14310, 9640, 6420, 5330, and 4150 kg/ha respectively. These differences were largely caused by the grass component, in particular ryegrass, which yielded 7830, 3160, 1860, 1010, and 110 kg DM/ha per yr respectively. There was relatively little difference in annual production by legumes (1480–2090 kg DM/ha), but marked seasonal differences occurred. Most white clover growth occurred during summer — autumn, most subterranean clover growth during spring, and most lotus spp. growth during winter—spring. There was relatively little variability in legume N concentration (mean 4.9%) and in the proportion of legume N fixed from the atmosphere on all sites excluding stock camps (mean 82%). Thus, for sloping sites, the amount of N fixed could be estimated from kg legume DM/ha × 0.040, whereas for stock camps the multiplication factor was 0.030. In stock camps, the proportion of legume N fixed from the atmosphere varied with time between 42 and 87%, being lowest following a period of heavy grazing. The amounts of fixed N in legume shoots were 55, 82, 67, 85, and 65 kg N/ha per yr respectively. Estimates of N input, removal by grazing, and transfer to camp areas illustrate that the accumulation of soil N in the steep sites is likely to be a very slow process (e.g., as little as 7 kg N/ha per yr). In contrast, the camp sites received large N inputs via transfer from other areas by the grazing animal and by in situ N2 fixation. It is suggested that N cycling in hill country pasture—soil systems is very inefficient.