氮素补充对高寒草甸土壤团聚体有机碳、全氮分布的影响

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

土壤结构的稳定性对高寒草甸生态系统有重要意义.为研究不同水平氮素补充对高寒草甸草地土壤团聚体、有机碳和全氮含量及分布格局的影响,于2012-2014年在青藏高原东部夏河县进行3 a的试验研究.试验为随机区组设计,包括0（对照）、50（低氮）、100（中氮）和200（高氮）kg/hm2 4个氮素补充水平.研究结果表明：低、中、高水平氮素补充处理显著提高了0~30 cm土层≥0.25 mm大团聚体质量分数（P〈0.05）,比对照分别提高了4.74%、6.42%和1.96%;较之对照,低、中水平氮素补充处理显著增加了≥5 mm、≥2~5 mm粒级团聚体含量;低、中水平氮素补充处理显著提升了0~30 cm土壤团聚体平均质量直径,分别比对照提升了9.79%和12.63%.不同水平氮素补充处理有机碳、全氮含量大小排序分别为：中氮≈低氮＆gt;对照＆gt;高氮、中氮＆gt;低氮≈高氮＆gt;对照.不同粒级团聚体中〈0.25 mm微团聚体有机碳含量最高而全氮含量最低、≥0.25~2 mm粒级有机碳含量最低而全氮含量最高;低、中水平氮素补充提高了不同粒级团聚体0~30 cm土层有机碳含量而高水平氮素补充处理显著降低了有机碳含量;低、中、高水平氮素补充增加了不同粒级团聚体全氮含量,其中中水平氮素补充处理最高,低水平氮补充次之.不同粒级团聚体含量是影响团聚体养分贡献率的主要原因,≥2~5 mm粒级团聚体含量与相应粒级团聚体有机碳含量呈显著正相关关系,≥5 mm和≥2~5 mm粒级团聚体含量与相应粒级团聚体全氮含量分别呈极显著正相关、显著正相关关系.研究表明连续每年补充50~100 kg/hm2氮可以改善高寒草甸土壤结构并提高土壤肥力状况.

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 ＆gt; CK ＆gt; HN; and N addition significantly increased the total N content of LN, MN and HN treatments, which trend was MN ＆gt; LN≈HN ＆gt; 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.