NITROGEN CYCLING IN AN ALFALFA AND BROMEGRASS SWARD VIA LITTERFALL AND HARVEST LOSSES

Below ground transfer of N from legumes to associated grasses has been extensively documented; however, transfer of N via decomposition of p lant material and reabsorption of released N remains poorly understood . This study was conducted to (i) assess the intensity of litterfall a nd harvest losses of alfalfa (Medicago saliva L.) and meadow bromegras s (Bromus riparius Rhem.) under field conditions and (ii) determine th e quantity of N transferred from aboveground plant components. Litterf all and harvest losses were quantified over 3 yr. Nitrogen transfer wa s estimated using the N-15 isotope dilution technique. The return of N to the soil via litterfall losses of alfalfa and bromegrass was 13 an d 4 kg N ha(-1) yr(-1), respectively. Alfalfa returned an additional 1 5 kg N ha(-1) yr(-1) to the soil via harvest losses; bromegrass contri buted an additional 7 kg N ha(-1) yr(-1). Bromegrass remained a source of N to the N-2-fixing alfalfa, even though it was N-limited, and alf alfa successfully competed with the N-stressed bromegrass for availabl e N derived from litterfall and harvest losses. Alfalfa accumulated tw ice the amount of N from litterfall and harvest losses compared with b romegrass (2 vs. 1 kg N ha(-1) yr(-1)). Clearly, alfalfa was a strong competitor for available N and acted as both a source and a strong sin k for recycled N. The total amount of N transferred from alfalfa to br omegrass and vice versa via decomposition of aboveground plant compone nts was found to be remarkably similar (approximate to 1 kg N ha(-1) y r(-1)). Therefore, the net flow of N between alfalfa and bromegrass wa s negligible, and no net N contribution of alfalfa to associated brome grass from the decomposition of litterfall and harvest losses was dete cted. We concluded that, although alfalfa released more N through the decomposition of litterfall and harvest losses than bromegrass, alfalf a also reabsorbed more recycled N. Published estimates of net N transf er betw een alfalfa and bromegrass, therefore, could not be explained by the input of N from litterfall and harvest losses.