Effect of organic carbon and total nitrogen distribution in alpine meadow soil aggregates with different nitrogen addition level

Abstract: The alpine meadow, mainly distributed in cold and high altitude region in the Qinghai-Tibetan Plateau, is a grassland ecosystem with the largest area. In the past decade, the grassland and soil ecological environments were degraded continuously, which have been paid high attention by human beings. The reasons of grassland degradation are complex, and from the view of ecology, the degradation is mainly caused by the unbalance of energy flow and material circulation in grassland ecosystem. Grazing together with other human activities, lead to soil nutrient loss with the output of grass and livestock products. Due to the insufficient supply of nutrients, the decline of soil fertility seriously affects the grass growth in pasture, leading to grassland ecosystem health deteriorating. Nitrogen (N) is the main limiting factor of soil nutrient in the alpine meadow. N addition is an important means to maintain the balance of grassland soil nutrient, which in turn can increase soil N content, and stimulate the growth and distribution of aboveground biomass and belowground root system, thereby affecting the soil structure. Soil aggregates are the basic unit of soil structure, while carbon (C) and total N are the most important factors affecting the structure of soil aggregates. Therefore, relying on N addition experiment of 3 consecutive years located in Xiahe County in eastern Qinghai-Tibet Plateau, in order to explore the effects of N addition on the changing process of soil aggregates, organic C and total N, and its impacts on soil structure, the paper attempted to seek methods for maintaining the stability of soil structure in the alpine meadow. The experiment consisted of 4 treatments with different N addition levels: CK (0), LN (50 kg/hm2), MN (100 kg/hm2) and HN (200 kg/hm2), in which the randomized block design was applied and each processing was repeated for 3 times. The result showed that LN, MN and HN treatments improved the content of ≥0.25 mm soil aggregate in 0-30 cm, and compared to CK treatment, LN, MN and HN increased by 4.74%, 6.42% and 1.96%, respectively; meanwhile, LN and MN treatments improved the mean weight diameter (MWD), and compared to CK treatment, LN and MN increased by 9.79% and 12.63%, respectively; at the same time, LN and MN improved the contents of the aggregate of ≥5 and ≥2-5 mm. The results showed that reasonable N addition not only enhanced soil stability but also enabled glued micro-aggregate to form the larger soil aggregate, and MN treatment was the most effective among the 4 treatments, followed by LN. In the 0-30 cm depth, N addition produced the difference in organic C content, and the trend was MN≈LN > CK > HN; and N addition significantly increased the total N content of LN, MN and HN treatments, which trend was MN > LN ≈ HN > CK. In soil aggregates with different sizes, soil organic C content of <0.25 mm aggregate was the highest, while that of ≥0.25-2 mm aggregate was the lowest; the total N content was in the opposite of organic C content, and ≥0.25-2 mm aggregate was the highest, while <0.25 mm aggregate was the lowest. The correlation between ≥2-5 mm soil aggregate content and their soil organic C content was significantly positive (P<0.05). The correlations between ≥5 and ≥2-5 mm soil aggregate contents and their total N contents were significantly positive with the correlation coefficient of 0.865 and 0.547, respectively. Therefore, the contributing rates of organic C, total N and aggregate content among soil aggregates with different sizes are the same, which shows that aggregate content of different sizes mainly causes the changing of the contributing rates of organic C and total N. In conclusion, 50-100 kg/hm2 N addition improves the soil stability and the nutrient status.