Abstract:

The development of alfalfa industry is an important strategy for enhancing the comprehensive benefits of agricultural and animal husbandry, and improving the ecological environment in the valley regions of Xizang. Soil nutrients and microbial communities serve as key indicators for assessing ecological environment, but the effects of long-term planting of alfalfa on soil properties in the Lhasa River Valley of Xizang are still lack of in-depth study. We used alfalfa artificial grasslands with different cultivation years (2, 4, 5, 6 and 7 years) as the research object, with oat artificial grasslands and natural floodplain areas as control, to explore changing trends of the soil nutrient level and microbial community characteristics. The results showed that: (1) planting alfalfa increased soil C and N contents, and the content of soil organic carbon, total nitrogen, nitrate nitrogen, total soluble nitrogen, dissolved organic carbon, dissolved organic nitrogen and microbial biomass nitrogen were significantly different from that of the oat artificial grassland and river bank land after 5 and 6 years of cultivation (P<0.05). (2)There were no significant differences in bacterial OTU richness and diversity among the plots, while fungal communities showed significant variations. Among them, the alfalfa plots with 5, 6 and 7 years cultivation showed significantly higher fungal OTU richness, fungal species number, Shannon-Wiener index, and Pielou evenness index compared to the oat artificial grassland (P<0.05). In contrast, the 4-year alfalfa plot only exhibited a significant increase in fungal OTU richness. Moreover, both fungal OTU richness and diversity indices gradually increased with increasing the planting year of alfalfa. (3) The dominant bacterial and fungal phyla/genera were highly similar across all plots, but their relative abundances varied significantly. Notably, the relative abundance of beneficial microbial phyla and genera gradually increased with longer alfalfa cultivation duration. (4) The richness of bacterial and fungal OTUs exhibited significant positive correlations with soil nitrogen and carbon metrics, including TN, NO₃^--N, STN, DOC, DON and MBN, indicating that microbial community richness was enhanced with elevation of nutrient content. Dominant bacterial genera were primarily driven by SOC and available phosphorus (AP), whereas fungal genera showed stronger associations with soil carbon-to-nitrogen (C/N) ratios, highlighting distinct nutrient response strategies between microbial kingdoms. Meanwhile, the bacterial Simpson diversity index increased significantly with soil nitrogen content, which indicated that nitrogen enrichment promotes bacterial community diversity. In terms of planting years, the combined benefits of alfalfa planted for 5 and 6 years were significantly better than the treatment of 2 years in terms of improving soil quality and enhancing microbial diversity (e.g., nutrient cycling and biodiversity restoration). This study underscores alfalfa’s pivotal role in plateau ecosystem recovery of the Lhasa River Valley Plateau, and clarifies that the optimal planting period is 5-6 years, which provides scientific basis and practical guidance for soil improvement, ecological reconstruction and sustainable development of alfalfa grass industry in this area. The research results can provide reference for the construction of artificial grassland in alpine valleys and promote ecological restoration and sustainable development of agriculture and animal husbandry.

Key words: valley regions of Xizang, alfalfa, artificial grassland, soil nutrients, soil microbial diversity, community structure