Grassland biomass production and plant species diversity in response to nitrogen and phosphorus addition in central and southwestern Tajikistan 后印本

摘要: Nitrogen (N) and phosphorus (P) are essential nutrients regulating plant growth, yet their long-term impacts on grassland ecosystems in Tajikistan remain poorly understood. This study conducted a five-year (2018–2022) field experiment across four grassland sites (Tabakqi, Balkhi, Luchob, and Ziddi) along an elevation gradient in central and southwestern Tajikistan to explore the effects of varying N (0, 30, and 90 kg N/(hm2•a)) and P (0 and 30 kg P/(hm2•a)) additions on aboveground biomass (AGB) and plant species diversity. Nutrient addition significantly increased AGB across all sites. Compared with the control (without N or P addition), AGB increased by 20%–80% under moderate N treatment (adding 30 kg N/(hm2•a)) and by up to 190%–200% under high N and P addition treatment (adding 90 kg N/(hm2•a) and 30 kg P/(hm2•a)). In 2022, AGB at the low-elevation site (Tabakqi) increased from 494 g/m2 under the control to 650 g/m2 under high N and P treatment, while at the high-elevation site (Ziddi), it rose from 552 to 1614 g/m2. In contrast, biodiversity responses were elevation-dependent: species richness declined at mid-elevation grassland sites (Balkhi and Luchob) but showed little change at low-elevation (Tabakqi) and high-elevation (Ziddi) sites. Shannon-Wiener index, Simpson's dominance index, and Pielou's equitability index also varied, reflecting complex interactions among nutrient addition, precipitation, and temperature. The structural equation model (SEM) confirmed that nutrient addition directly enhance AGB but generally suppress plant species diversity, while precipitation promotes AGB, and temperature effects are inconsistent across sites. Overall, our findings demonstrate that nutrient enrichment can increase productivity but reduce biodiversity, with responses strongly mediated by elevation and climate. These results provide the first long-term experimental evidence from Tajikistan's grasslands and underscore the need to balance productivity gains with biodiversity conservation in sustainable grassland management.