Acid hydrolysis organic N content and the distribution in different sizes of soil complexes in Loess Plateau dryland under long-term no-tillage and straw mulching

【Objectives】 Conservation tillage measurements have different effect on the organic N content and its distribution in different sizes of soil complexes. This study will provide scientific base for the evaluation of different conservation tillage methods in the dryland in Loess Plateau.【Methods】 Soil samples were collected from a 14-years’long-term experiment in dryland in Loess Plateau, in which, four tillage treatments were set up including conventional tillage (T), no-tillage (NT), straw mulching (TS) and no-tillage + straw mulching (NTS). The Bremner method was used to determine the organic N contents and their proportion in different sizes of soil complexes.【Results】Under all the three conservation tillage treatments, the percentage of soil complexes in 2-10 μm was increased by 20.0%-31.7%, while that of 0-2 μm was decreased by 27.6%-31.0%. The percentage of different soil size complexes was all in order of 10-50 μm＞2-10 μm＞0-2 μm＞50-100 μm＞100 μm. The amino sugar nitrogen content in 0-2 μm of soil complexes was significantly increased by 46.9%-107.1%, while the NH+ 4-N content decreased by14.8%-27.0%; the contents of acid hydrolysis nitrogen, amino acid N and amino sugar N in 2-10 μm of soil complexes were increased by 8.2%-14.3%, 16.2%-31.5%, 154.9%-184.3%, respectively, while the NH+4-N content was decreased by 28.7%-46.6%. The contents of soil complexes combined organic nitrogen was in order of 10-50 μm＞0-2 μm＞2-10 μm＞50-100 μm＞100 μm under all the conventional treatments, while it followed 10-50 μm＞2-10 μm＞0-2 μm＞50-100 μm＞100 μm under conservation tillage. Compared to conventional tillage measures, the contents of acid hydrolysis nitrogen, amino acid N and amino sugar nitrogen were significantly promoted and increased by 6.6%-20.4%, 11.9%-31.6% and 89.0%-113.0%, respectively, while the NH+ 4-N content decreased under conservation tillage treatments. 【Conclusions】In the long term, all the no-tillage and straw-mulching are effective in increasing the percentage of soil complexes in 2-10 μm while decreasing that in 0-2 μm, and increasing the contents of organic nitrogen while decrease NH+4-N content in dryland in Loess Plateau. So both no-tillage and straw mulching in long run will improve soil texture and the capability of regulating nutrient supply to crops.