Nitrogen yield advantage from grass-legume mixtures is robust over a wide range of legume proportions and environmental conditions

Current challenges to global food security require sustainable intensification of agriculture through initiatives that include more efficient use of nitrogen (N), increased protein self-sufficiency through homegrown crops, and reduced N losses to the environment. Such challenges were addressed in a continental-scale field experiment conducted over 3years, in which the amount of total nitrogen yield (N-tot) and the gain of N yield in mixtures as compared to grass monocultures (N-gainmix) was quantified from four-species grass-legume stands with greatly varying legume proportions. Stands consisted of monocultures and mixtures of two N-2-fixing legumes and two nonfixing grasses. The amount of N-tot of mixtures was significantly greater (P0.05) than that of grass monocultures at the majority of evaluated sites in all 3years. N-tot and thus N-gainmix increased with increasing legume proportion up to one-third of legumes. With higher legume percentages, N-tot and N-gainmix did not continue to increase. Thus, across sites and years, mixtures with one-third proportion of legumes attained similar to 95% of the maximum N-tot acquired by any stand and had 57% higher N-tot than grass monocultures. Realized legume proportion in stands and the relative N gain in mixture (N-gainmix/N-tot in mixture) were most severely impaired by minimum site temperature (R=0.70, P=0.003 for legume proportion; R=0.64, P=0.010 for N-gainmix/N-tot in mixture). Nevertheless, the relative N gain in mixture was not correlated to site productivity (P=0.500), suggesting that, within climatic restrictions, balanced grass-legume mixtures can benefit from comparable relative gains in N yield across largely differing productivity levels. We conclude that the use of grass-legume mixtures can substantially contribute to resource-efficient agricultural grassland systems over a wide range of productivity levels, implying important savings in N fertilizers and thus greenhouse gas emissions and a considerable potential for climate change mitigation.