Abstract:

Global climate change is altering precipitation patterns, profoundly affecting gross primary productivity (GPP) and aboveground biomass (AGB) in temperate grasslands. Water and nitrogen are key limiting factors that significantly influence ecosystem productivity in the study region. However, the effects of nitrogen and water addition on GPP and AGB under different precipitation conditions remain debated. Therefore, this study is based on a six-year (2017-2022) nitrogen and water addition control experiment in a temperate grassland in Inner Mongolia, and uses the Standardized Precipitation Evapotranspiration Index (SPEI) to categorize years as normal or dry. The purpose is to explore the effects and regulatory pathways of nitrogen and water addition on GPP and AGB under different precipitation conditions. The results indicate that there are differences in the response of GPP and AGB to nitrogen-water addition. The water addition treatment group significantly increased GPP (by 59% and 52%, respectively) in both normal and drought years, but had no significant effect on AGB. Secondly, the nitrogen addition treatment group significantly increased AGB (by 53% and 64%, respectively) in both normal and drought years, but its effect on GPP was precipitation dependent, with only a significant increase in normal years (35%). The nitrogen-water combined treatment group showed a greater (116%) improvement in GPP. Furthermore, the structural equation modeling results revealed that in normal years, the water addition treatment group increased GPP by increasing soil moisture volume fraction; the nitrogen addition treatment group increased GPP by increasing plant nitrogen concentration and aboveground biomass; in dry years, the water addition treatment group increases GPP by increasing Soil moisture volume fraction and underground biomass. In summary, this study reveals the differential responses of GPP and AGB to nitrogen and water addition in temperate grassland ecosystems and clarifies the mechanisms by which water and nitrogen influence grassland productivity under different precipitation conditions. These findings provide an important theoretical basis for adaptive management of grasslands under global change.

Key words: nitrogen addition, water addition, gross primary productivity, aboveground biomass, temperate grassland of Inner Mongolia

摘要：

全球气候变化导致降水格局的改变，深刻影响温带草原总初级生产力（GPP）和地上生物量（AGB）。水分和氮素作为该区域生产力的关键限制因子，对生态系统生产力具有重要影响，但不同降水背景下氮水添加对GPP和AGB的影响如何仍存在争议。该研究基于内蒙古温带草原6 a（2017－2022）氮水添加控制实验，并结合标准化降水蒸散指数划分正常与干旱年份，探究不同降水背景下氮水添加对GPP和AGB的影响及调控途径。结果表明，GPP与AGB对氮水添加的响应存在差异。正常和干旱年份水添加均显著增加GPP（分别为59%和52%），但对AGB无显著影响；正常和干旱年份氮添加均显著增加AGB（分别为53%和64%），但对GPP的影响具有降水依赖性，仅在正常年份显著提升（35%），而氮水共同添加对GPP的提升作用更显著（116%）。结构方程模型显示，正常年份时水添加通过提高土壤水分体积分数增加GPP，氮添加则通过增加植物氮质量分数和AGB增加GPP；干旱年份时水添加通过增加土壤水分体积分数和地下生物量增加GPP。该研究揭示了温带草原生态系统GPP与AGB对氮水添加的差异化响应，明确不同降水背景下水分和氮素对草原生态系统生产力的影响机制，为全球变化背景下草地的适应性管理提供了重要理论依据。

关键词: 氮添加, 水添加, 总初级生产力, 地上生物量, 内蒙古温带草原