To fractionate the contribution of plant to N2O emission from soil-plant system, the N2O emissions from soybean seedling, maize seedling, sand, soil, sand-plant system and soil-plant system were measured by closed chamber method in greenhouse. At the same time the correlation of plant N2O emission with nitrate reductase activity, nitrate content and nitrite content of the plant leaves was also analyzed. The results clearly indicated that N2O could be a by-product of plant metabolism. The emissions of N2O from maize and soybean accounted for 93%-100% of the total N2O emissions in the sand-plant systems, and 79.18%-92% in the soil-plant systems. The flux of plant N2O emission was significantly correlated (R2 > or = 0.97, n = 6) with the nitrate reductase activities, nitrate content and nitrite content of the maize and soybean leaves.