Abstract: The analysis of the variation of soil organic nitrogen fractions in growing season and its response to current year and long-term straw returning could provide scientific basis for reasonable regulation of soil organic nitrogen pool and the improvement of soil fertility. A field plot experiment with two treatments was conducted at the National Field Observation and Research Station of Shenyang Agroecosystems, i.e., single application of nitrogen fertilizer and straw returning combined with nitrogen fertilizer. Soil samples were collected at before-seeding, jointing, silking, filling, and maturity stages. Bremner method was used to classify soil organic nitrogen fractions. The results showed that each of the organic nitrogen fractions in the soil showed obvious dynamic patterns in the whole growing season of maize. Hydrolyzable ammonium content increased firstly and then decreased, indicating that it may act as a temporary pool during soil nitrogen transformation. Amino acid nitrogen did not vary significantly across the growing season. Amino sugar nitrogen was significantly lower in the late growing season than that in the early growing season. Hydrolyzable unknown nitrogen increased and then decreased with high activity across the growing season. Compared with single nitrogen treatment, straw returning increased the contents of soil total nitrogen and hydrolyzed nitrogen at the end of the growing season and in the long term, increased the proportion of hydrolyzed nitrogen to total nitrogen, and decreased the proportion of no hydrolyzed nitrogen to total nitrogen by 7.2% and 3.8% in the current season and long term, respectively. The results indicated that straw returning could not only improve soil nitrogen availability, but also improve the capability of soil nitrogen storage and supply.

Key words: soil organic nitrogen fraction, straw returning, seasonal dynamics, maize, fertilizer application.