Nitrogen dynamics of pastures: nitrogen fixation inputs, the impact of legumes on soil nitrogen fertility, and the contributions of fixed nitrogen to Australian farming systems

Experimental estimates of amounts of foliage nitrogen (N) fixed in Australian pastures range from 2 to 284 kg N/ha. year for annual and perennial legumes growing in temperate and tropical environments. Differences in the amounts of N-2 fixed relate primarily to the legume content and net productivity of pastures. On average, close to 20-25 kg of shoot N are fixed for every tonne of legume herbage dry matter produced across a wide range of environments. Strategies likely to improve the potential for N-2 fixation include: (i) rhizobial inoculation at time of first sowing a new legume species; (ii) amelioration of nutritional problems (applications of superphosphate or lime); (iii) manipulation of pasture composition (herbicide applications to remove grasses in annual pastures in the year prior to cropping); and (iv) including lucerne to offset the year-to-year variability in N-2 fixation inputs from annual legumes. However, pasture response to such management treatments and the subsequent availability of soil mineral N may be modified by livestock effects on nutrient cycling, pasture productivity and botanical composition. Conclusions about the relative size of the contributions of fixed N to the N economies of Australian farming systems depend on whether or not estimates of fixed N are included for nodulated roots. Thus residual net inputs of fixed N after each year of a legume-based pasture are generally rated sufficient to balance the N removed by at least 1 subsequent wheat crop provided estimates of below-ground N are included in calculations. Pasture type influences the duration of subsequent rotational benefits and while residual effects on mineral N are commonly exhausted within 2 years after an annual legume-based pasture phase, N carry-over following lucerne generally lasts considerably longer.