Estimated N2–Fixation of Cool‐Season Annual Clovers by the Difference Method

Abstract

Information on the N2–fixation (NF) capacity of cool‐season annual clovers in the southeastern United States is lacking. Estimates of NF were determined on arrowleaf (Trifolium vesiculosum Savi), crimson (Trifolium incarnatum L.), rose (Trifolium hirtum All.), and subterranean (Trifolium subterraneum L.) clovers using the difference in N accumulation of the clovers and annual ryegrass (Lolium multiflorum Lam.) as the nonfixing plant at 2‐wk intervals for 2 yr. Initial estimates of NF were negative because N accumulation in ryegrass exceeded that in clovers. The general trend was for NF and percent plant N from NF to increase as dry matter (DM) yield increased. Percentage of plant N fixed from the atmosphere exceeded 75% for all clover species at some point in spring. The 2 yr average maximum NF values using herbage mass (HM) were 296 kg N ha−1 for arrowleaf clover, 189 kg N ha−1 for crimson clover, 215 kg N ha−1 for rose clover, and 192 kg n ha−1 for subterranean clover. Using HM plus roots increased maximum NF values from 4 to 12 kg N ha−1. The root NF contribution was greatest during the winter after NF values turned positive ranging from 8 to 83% of plant NF. Then they decreased with plant maturity from 1 to 9% of total plant NF. Crimson clover had the greatest NF values until mid March after which the later‐maturing arrowleaf clover had the greatest NF value.