Symbiotic dinitrogen fixation in forage legumes amended with high rates of de-inking paper sludge

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