Estimating total N2 fixation by legumes in long-term rotation studies

Abstract

The amount of N2 fixed by legumes is most often estimated from only the above ground parts (`tops') of plants. The amount which remains in the roots and contributes to subsequent crops and soil N status is often not included in estimates of N2 fixation. The aim of the present study was to evaluate a rotation method of calculating fixed N2 removed in the tops of legumes and subsequent crops and, along with soil N changes, to estimate total N2 fixation in a vetch for hay-barley for grain long term rotation. Other methods of calculating total N2 fixation in the above system are also considered. Continuous barley for grain and barley cut for hay-barley for grain rotations were used as reference rotations to estimate soil N uptake and soil N changes at the end of four cycles of the rotations. N fertilizer at the rate of 60 kg N ha-1 was applied to barley for hay and rates of 0, 30, 60 and 90 kg N ha-1 were applied to sub-plots in all the barley crops for grain. The experiments were conducted on clay loam soils at two locations with average rainfalls of 250 and 350 mm. The total amount of N2 fixed was estimated to be 150 and 210 kg N ha-1 per year at the 250 and 350 mm rainfall sites, respectively. Of the total N2 fixation, 58 to 65 per cent was estimated to have remained in the roots after the vetch was harvested for hay. The amount of fixed N left in the roots had an N use efficiency for the next crop of 25 to 28 per cent, with the remaining amount found in the soil total N. Factors which might have influenced the methods which were used to estimate total N2 fixation include the input of fertilizer and the measurement of soil N status at the end of the experiment. The other components of the equation used were based on total N in the tops and could be measured accurately. It is concluded that for a precise estimation of total N2 fixation, the fertilizer use efficiency should be known when fertilizer is applied to the reference crop and the final soil N status should be measured as precisely as possible.