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Nitrogen Cycling in an Alfalfa and Bromegrass Sward via Litterfall and Harvest Losses

EMBRAPA, P.O. Box 569, 99001-970 Passo Fundo, RS, Brazil

Frances L. Walley and Chris van Kessel^*

Dep. of Soil Science, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, SK S7N 5A8, Canada

Alfred E. Slinkard

Slinkard, Dep. of Crop Science and Plant Ecology, Univ. of Saskatchewan, 51Campus Dr., Saskatoon, SK S7N 5A8, Canada

^* Corresponding author.

Belowground transfer of N from legumes to associated grasses has been extensively documented; however, transfer of N via decomposition of plant material and reabsorption of released N remains poorly understood. This study was conducted to (i) assess the intensity of litterfall and harvest losses of alfalfa (Medicago sativa L.) and meadow hromegrass (Bromus riparius Rhem.) under field conditions and (ii) determine the quantity of N transferred from aboveground plant components. Litterfall and harvest losses were quantified over 3 yr. Nitrogen transfer was estimated using the ¹⁵N isotope dilution technique. The return of N to the soil via litterfall losses of alfalfa and bromegrass was W and 4 kg N ha^–1 yr^–1, respectively. Alfalfa returned an additional 15 kg N ha^–1 yr^–1 to the soil via harvest losses; bromegrass contributed an additional 7 kg N ha^–1 yr^–1. Bromegrass remained a source of N to the N₂-fixing alfalfa, even though it was N-limited, and alfalfa successfully competed with the N-stressed bromegrass for available N derived from litterfall and harvest losses. Alfalfa accumulated twice the amount of N from litterfall and harvest losses compared with bromegrass (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 sink for recycled N. The total amount of N transferred from alfalfa to bromegrass and vice versa via decomposition of aboveground plant components was found to be remarkably similar ( 1 kg N ha^–1 yr^–1). Therefore, the net flow of N between alfalfa and bromegrass was negligible, and no net N contribution of alfalfa to associated bromegrass from the decomposition of litterfall and harvest losses was detected. We concluded that, although alfalfa released more N through the decomposition of litterfall and harvest losses than bromegrass, alfalfa also reabsorbed more recycled N. Published estimates of net N transfer between alfalfa and bromegrass, therefore, could not be explained by the input of N from litterfall and harvest losses.

Received for publication January 20, 1995.

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