Asynchronous Response of Plant Productivity and Soil Aggregate Nutrients to Grazing Exclusion Duration in Alpine Steppe

Abstract

Grazing exclusion is a critical restoration measure for degraded alpine steppe ecosystems. However, research remains limited on the key factors influencing soil aggregates that affect plant productivity and the mechanisms influencing nutrient distribution across aggregates of different size fractions during long-term grazing exclusion. We examined alpine steppe ecosystems in northern Tibet with a space-for-time substitution approach to analyze dynamic changes in plant productivity and soil aggregate nutrients across different exclusion durations (0, 2, 6, 13, and 18 years). Community aboveground biomass and Stipa purpurea biomass initially increased and peaked after 6 years of exclusion, followed by a gradual decline. In contrast, long-term exclusion (≥ 13 years) significantly increased the proportion of soil macroaggregates and their associated organic carbon, nitrate nitrogen, and available phosphorus contents. Spearman correlation analysis revealed that nitrate nitrogen stored in macroaggregates was negatively correlated with plant productivity. These results indicate an asynchronous response between vegetation recovery and soil aggregate nutrient dynamics during ecological restoration. Plant productivity recovered rapidly in the short term but did not synchronize with improvements in soil structure and nutrient pools. Although prolonged enclosure enhanced total soil nutrient stocks, nutrients became increasingly physically protected within stable macroaggregates, reducing their accessibility to plant roots. Soil aggregate-scale nutrient distribution regulates alpine steppe productivity under grazing exclusion. These findings deepen our understanding of alpine steppe ecosystem restoration dynamics and the coordination of ecological conservation with sustainable pastoral development.