Nitrogen Transfer from White Clover, Red Clover, and Birdsfoot Trefoil to Associated Grass

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Abstract
Information on N transfer from field‐grown forage legumes to associated grass would contribute to elucidation of N budgets and improved N management of pasture mixtures. This research evaluated N transfer by white clover (Trifolium repens L.) (WC), red clover (Trifolium prutense L.) (RC) or birdsfoot trefoil (Lotus corniculatus L.) (BT), grown with tall fescue (Festuca arundinacea Schreb.) (TF) at various legume proportions. Two identical experiments were established on a Typic Argiudoll soil in Uruguay, one in 1983 (Exp. 1) and one in 1984 (Exp. 2), and each was evaluated for approximately 2 yr. The soil of the area was a fine, montmorillonitic, mesic, Typic Argiudoll. Transfer of N to TF was estimated by 15N isotope dilution, using TF grown alone as the reference crop. Mean values for the proportion of grass N derived from legumes (%NT) were 20% for the 60% harvest after seeding, and later increased up to 60%. The increase on %NT over time was greater for WC‐TF and RC‐TF in Exp. 1 and for WC‐TF in Exp. 2. The annual uptake of transferred N (TNY) averaged 18 and 34 kg ha−1 in 1st and 2nd yr, respectively. The WC‐TF mixture usually showed greater annual TNY than RC‐TF on BT‐TF. Transfer by WC in summer often was greater than for RC or BT. Increasing legume proportion generally resulted in linear increases of %NT for the three mixtures, but TNY either did not change or decreased. We conclude that these legumes transferred small amounts of N shortly after seeding, but transfer increased to significant values with time. The concentration of legume‐derived N in TF herbage increased when legume proportion increased, but the amount transferred was not affected or reduced.