Effects of Long-term Fertilization on Soil Organic Nitrogen Fractions in Vegetable Soil

To reveal the impact of fertilization on soil, organic nitrogen (N) forms in vegetable soil were quantified using the method proposed by Bremner. Results showed that compared with the plots without fertilizer, no single chemical fertilizer had profound effects on the soil organic N content and composition. Compared with chemical fertilizer applied alone, organic manure mixed with chemical fertilizer greatly increased the contents of total N and acid hydrolysable N by 19.7% and 26.7% respectively; Among the forms of acid hydrolysable N, there was an increase of amino acid N and unidentified hydrolysable N by 36.6% and 32.1% respectively, while the increases in amino sugar and ammonia N were smaller. The contents of different forms of soil organic N in all treatments followed the order of amino acid N > non-hydrolysable N > unidentified hydrolysable N> ammonia N> amino sugar N. Since manure mixed with chemical fertilizer increased the content of most organic N forms, it was considered the best way for maintaining soil fertility and ensuring sustainable agriculture.