Factors regulating the contributions of fixed nitrogen by pasture and croplegumes to different farming systems of eastern Australia

On-farm and experimental measures of the proportion (%Ndfa) and amounts of N-2 fixed were undertaken for 158 pastures either based on annual legume sp ecies (annual medics, clovers or vetch), or lucerne (alfalfa), and 170 wint er pulse crops (chickpea, faba bean, field pea, lentil, lupin) over a 1200 km north-south transect of eastern Australia. The average annual amounts of N-2 fixed ranged from 30 to 160 kg shoot N fixed ha(-1) yr(-1) for annual pasture species, 37-128 kg N ha(-1) yr(-1) for lucerne, and 14 to 160 kg N ha(-)1 yr(-1) by pulses. These data have provided new insights into differe nces in factors controlling N-2 fixation in the main agricultural systems. Mean levels of %Ndfa were uniformly high (65-94%) for legumes growing at di fferent locations under dryland (rainfed) conditions in the winter-dominant rainfall areas of the cereal-livestock belt of Victoria and southern New S outh Wales, and under irrigation in the main cotton-growing areas of northe rn New South Wales. Consequently N-2 fixation was primarily regulated by bi omass production in these areas and both pasture and crop legumes fixed bet ween 20 and 25 kg shoot N for every tonne of shoot dry matter (DM) produced . Nitrogen fixation by legumes in the dryland systems of the summer-dominan t rainfall regions of central and northern New South Wales on the other han d was greatly influenced by large variations in %Ndfa (0-81%) caused by yea rly fluctuations in growing season (April-October) rainfall and common farm er practice which resulted in a build up of soil mineral-N prior to sowing. The net result was a lower average reliance of legumes upon N-2 fixation f or growth (19-74%) and more variable relationships between N-2 fixation and DM accumulation (9-16 kg shoot N fixed/t legume DM). Although pulses often fixed more N than pastures, legume-dominant pastures provided greater net inputs of fixed N, since a much larger fraction of the total plant N was re moved when pulses were harvested for grain than was estimated to be removed or lost from grazed pastures. Conclusions about the relative size of the c ontributions of fixed N to the N-economies of the different farming systems depended upon the inclusion or omission of an estimate of fixed N associat ed with the nodulated roots. The net amounts of fixed N remaining after eac h year of either legume-based pasture or pulse crop were calculated to be s ufficient to balance the N removed by at least one subsequent non-legume cr op only when below-ground N components were included. This has important im plications for the interpretation of the results of previous N-2 fixation s tudies undertaken in Australia and elsewhere in the world, which have eithe r ignored or underestimated the N present in the nodulated root when evalua ting the contributions of fixed N to rotations.