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

Experimental estimates of amounts of foliage nitrogen (N) fixed in Australi an pastures range from 2 to 284 kg N/ha. year for annual and perennial legu mes growing in temperate and tropical environments. Differences in the amou nts of N-2 fixed relate primarily to the legume content and net productivit y 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 environ ments. Strategies likely to improve the potential for N-2 fixation include: (i) rhizobial inoculation at time of first sowing a new legume species; (i i) 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 i nputs from annual legumes. However, pasture response to such management tre atments and the subsequent availability of soil mineral N may be modified b y 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 estimate s 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 suffi cient to balance the N removed by at least 1 subsequent wheat crop provided estimates of below-ground N are included in calculations. Pasture type inf luences the duration of subsequent rotational benefits and while residual e ffects on mineral N are commonly exhausted within 2 years after an annual l egume-based pasture phase, N carry-over following lucerne generally lasts c onsiderably longer.