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Symbiotic Dinitrogen Fixation in Forage Legumes Amended with High Rates of De-Inking Paper Sludge

^a Département de Phytologie, Faculté des Sciences de l'Agriculture et de l'Alimentation, Université Laval, Québec, QC, Canada, G1K 7P4
^b Present address: Dep. of Agron. and Plant Breeding, Aburaihan Campus, P.O. Box 4147, Univ. of Tehran, Tehran, Iran

^* Corresponding author (francois-p.chalifour{at}plg.ulaval.ca).

Received for publication July 3, 2001. The paper de-inking process produces a waste by-product, de-inking paper sludge (DPS), which contains paper fibers, clay particles, and inks and has high C and Ca and low N and P concentrations. Use of high rates of DPS to increase the soil organic matter thus requires provision of high rates of N for adequate plant growth. Using dinitrogen (N₂)-fixing forage legumes is an alternative to the N fertilization under such circumstances. In a 2-yr field study (1995 and 1996), the effect of different rates of DPS (0, 50, or 100 Mg dry matter ha^–1), applied once in October 1994, were evaluated on symbiotic N₂ fixation of forage legumes established on two soil types in Eastern Quebec, Canada. Symbiotic N₂ fixation was measured in alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus corniculatus L.), red clover (Trifolium pratense L.), and sweetclover (Melilotus officinalis L.); bromegrass (Bromus inermis L.) was used as the reference (non N₂–fixing) crop. Dinitrogen fixation was estimated by the ¹⁵N natural abundance method. The percentages of N derived from the atmosphere increased significantly with DPS in the year of establishment (1995). In the first production year (1996), the effects of DPS on N₂ fixation were mainly observed at the first cut. Our results show that DPS used as an organic amendment generally led to similar or greater forage legume productivity and greater N₂ fixation compared with unamended controls in the first production year and is compatible with sustainable agricultural practices.

Abbreviations: CL, clay loam • DM, dry matter • DPS, de-inking paper sludge • DPS_L, linear effects of de-inking paper sludge • DPS_Q, quadratic effects of de-inking paper sludge • DPS₀, de-inking paper sludge applied at 0 Mg dry matter ha^–1 • DPS₅₀, de-inking paper sludge applied at 50 Mg dry matter ha^–1 • DPS₁₀₀, de-inking paper sludge applied at 100 Mg dry matter ha^–1 • Ndfa, nitrogen derived from atmosphere • ¹⁵N-E, ¹⁵N enrichment (method) • ¹⁵N-NA, ¹⁵N natural abundance (method) • SCL, silty clay loam • ¹⁵N, natural abundance of ¹⁵N