关键词: 农牧交错带, 盐渍化, 降水, 刈割, 氮添加, 净初级生产力

Abstract: Aims In the saline–alkaline grassland of the agro-pastoral ecotone with limited nutrients, the increase of nitrogen input induced by agricultural fertilization and the change of land use patterns usually cause changes in soil available nutrients, which further affect the net primary productivity of plants. However, due to the impact of inter-annual precipitation, there is still great uncertainty about whether the response of net primary productivity of plants to nitrogen addition and mowing will vary with changes in rainfall. Methods This study takes saline-alkali grassland in the agro-pastoral ecotone of northern China as the research object, relies on the experimental platform for nitrogen forms and mowing at the National Research Station of Grassland Ecosystems on the Loess Plateau in Youyu, Shanxi Province, and experiments with six treatments including control and simulated nitrogen deposition experiments with two common nitrogen compounds (ammonium nitrate and urea), with and without mowing. The response of plant net primary productivity to nitrogen addition and mowing was studied. Important findings The results showed that: (1) Regardless of mowing or un-mowing treatments, short-term addition of ammonium nitrate and urea significantly increased the content of inorganic nitrogen in the soil, thereby increasing the aboveground (ANPP), belowground (BNPP), and total net primary productivity (NPP); (2) ANPP, BNPP, NPP, inorganic nitrogen content, and soil water content showed significant interannual differences, with higher values observed in the wet year (2018) than in the dry year (2017); (3) The interaction between short-term nitrogen addition and year had a significant impact on NPP. In the wet year, NPP was significantly higher under nitrogen addition than in the dry year. In the wet year, the positive effect of nitrogen addition on NPP was significantly higher than that in the dry year, which was mainly related to the synergistic effect of soil nitrogen and water; (4) Mowing decreased NPP and had a significant interactive effect with the year on the BNPP/ANPP. In the dry year, mowing generally decreased BNPP/ANPP. However, in the wet year, this negative effect gradually weakened and even turned into a positive effect. These results emphasize the crucial role of natural precipitation in regulating the response of net primary productivity of saline-alkali grassland in the agro-pastoral ecotone to anthropogenic disturbances, and further indicate that the saline-alkali grassland ecosystem in the agro-pastoral ecotone is jointly limited by nitrogen and water.

Key words: agro-pastoral ecotone, saline-alkaline, precipitation, mowing, nitrogen addition, net primary