ABSTRACT
Summary Some recent findings affecting nitrogen (N) cycling in grassland ecosystems are discussed, and an indication is given of major problems requiring research and development. The importance of modern techniques is stressed to enable further quantification of the pathways and processes of the nutrient in grassland; such data are needed both to improve understanding and to enable more efficient use of N in practice. A brief description of the N in soil organic matter is followed by an account of symbiotic N fixation by legumes, including estimated levels and rates of fixation and factors affecting the process. Data are presented on N transfer from legumes to soil and availability for grass growth. Studies on nutrient uptake indicate that grass can attain its potential growth rate with very low N concentrations in solution, provided they are kept constant; this suggests a possible role for slow-release fertilizers. Selection for more efficient conversion by grasses of N to dry matter (DM) also appears to be a possibility. DM response to N fertilizer and its recovery in harvested herbage are considered. The importance of pathways of N loss are evaluated, the most significant being volatilization of ammonia, both from excreta deposited directly onto the pasture and from slurry. Losses from denitrification and leaching are generally less than from volatilization, though they can be significant under certain conditions. Legumes and fertilizers are compared as sources of N in grassland ecosystems.
