Nitrogen fixation by peas and lupins

Abstract

In a field experiment at Lincoln College, Canterbury, nitrogen fixation by spring sown vining peas (pisum sativum cv. Puke), grain peas (P. sativum cv. Huka) and lupins (Lupinus angustifolius cv. Uniharvest) was measured on a soil of low nitrogen status. Acetylene reduction assays were performed at intervals during the growth of legumes, and an uncultivated fallow treatment was included to facilitate the estimation of nitrogen fixation by a partial nitrogen balance method. Barley (Hordeum sativum cv. Manapou) was sown to compare the effect of this crop and legumes on the yield of ryegrass (Lolium multiflorum cv. Grasslands Tama), established in the succeeding autumn. Nitrogen at nil and 80 kg N haˉ¹ was applied to all treatments.

Biomass dry matter and nitrogen accumulation was greater for peas than lupins during vegetative growth, but a rapid increase in the rate of nitrogen fixation with the onset of the reproductive phase in lupins resulted in maximum yields which exceeded those of peas. Higher biomass yield at plant maturity was a contributing factor to the higher seed yield of Uniharvest lupins (3600 kg DM haˉ¹) compared with Huka peas (2250 kg DM haˉ¹) and was sufficient to compensate for the lower harvest index of lupins (0.49) in comparison with Huka peas (0.66).

The harvest index of Puke peas at the green pea stage was 0.24, which, combined with a lower biomass yield than for Huka peas, resulted in a seed yield of 475 kg DM haˉ¹ (2.5 t haˉ¹ of green peas).

At maturity, nitrogen harvest indices of Uniharvest lupins and Huka peas were 0.91 and 0.86 respectively, which resulted in low non-seed nitrogen yields. The nitrogen harvest index of Puke peas was 0.43, but non-seed nitrogen yield was similar to that for Uniharvest lupins.

Nitrogen balance estimates of nitrogen fixation by Uniharvest lupins, Huka and Puke peas were 183, 74 and 25 kg N haˉ¹ respectively, while seed nitrogen yields were 164, 66 and 15 kg N haˉ¹. Estimates of nitrogen fixed from acetylene reduction assays were close to those from the nitrogen balance method for Huka and Puke peas (60 and 34 kg N haˉ¹ respectively) but not for Uniharvest lupins (759 kg N haˉ¹). Nitrogen fertiliser had no consistent effect on the quantity of nitrogen fixed by legumes or legume yields, but increased the yield of Manapou barley.

With near total removal of legume and barley biomass, mean ryegrass yields were similar for the fallow, Huka and Puke pea treatments (2850 - 3290 kg DM haˉ¹) , lowest after Manapou barley (2080 kg DM haˉ¹), and highest after Uniharvest lupins (3680 kg DM haˉ¹). Soil nitrogen removal by barley appeared to reduce the quantity available to ryegrass, while the return of some fixed nitrogen via leaf and pod abscission in lupins increased nitrogen supply.