Enhanced biological nitrogen fixation and competitive advantage of legumes in mixed pastures diminish with biochar aging

Soil amendment with biochar can increase biological nitrogen fixation (BNF) and improve growing conditions for legumes, while such effects may alter when biochar properties change with time (biochar aging). We examined BNF and competitiveness of legumes in a mixed pasture (dominated by clover, Trifolium repens) after 26 months of biochar amendment (field aged biochar) and in a pot study with grass (Lolium perenne) and clover mixture using fresh and chemically aged biochars. In both studies, the fraction of N derived from symbiotic fixation (%Ndfa) was not affected by aged biochar treatments, while it reduced from 73% in the control to 68% with fresh biochar addition in the pot study. However, this reduction in %Ndfa was compensated with greater biomass production resulting in the largest amount of N fixed with fresh biochar application. The amount of N fixed was reduced by field aged biochar, although it was not affected by chemically aged biochars. Moreover, in the field study grasses became more competitive than legumes with biochar, while such an advantage by grasses was not observed in the pot study. Aging of biochar can reduce BNF and competitiveness of legumes in mixed pastures with significant implications for pasture management.