Seasonal Dry Matter, Nitrogen, and Dinitrogen Fixation Patterns of Crimson and Subterranean Clovers

Abstract

Annual legumes are potential N sources in forage‐livestock systems, but information on the quantity and distribution of N2 fixed is lacking. Field experiments were conducted to determine the quantity and seasonal distribution of N2 fixed by two annual clovers, ‘Tibbee’ crimson clover (Trifolium incarnatum L.) and ‘Mt. Barker’ subterranean clover (T. subterraneum L.), having diverse annual dry matter (DM) production patterns. Plots were established on a Catalpa silty clay soil (fine, montmorillonitic, thermic Fluvaquentic Hapludoll) in each of 2 yr. Dinitrogen fixation was estimated by the I5N isotope dilution method with ‘Marshall’ annual ryegrass [Lolium multiflorum (L.) Lam.] as the non‐N2‐fixing control. A mean of 77% of the annual herbage yield provided by crimson clover was obtained by early April compared to 39% for subterranean clover. Subterranean clover produced greater annual herbage yield than crimson clover the first year (7.6 vs. 6.4 Mg DM ha−1), while yields under drought conditions were similar the second year (mean of 4.8 Mg DM ha−1). The clovers fixed an average of 196 and 114 kg N ha−1 in the first and second years, respectively. Depending on the harvest date, fixed N2 comprised 32 to 91% of total N in crimson clover and 75 to 91% of total N in subterranean clover. The average seasonal proportion of N derived from symbiosis by both clovers was equivalent in the first and second year (mean of 81 and 75%, respectively). Seasonal need for herbage production, rather than N2 fixation capability, may govern the value of crimson vs. subterranean clover in forage‐livestock systems.