Short-term legacy effects of long-term nitrogen and water addition on soil chemical properties and micro-bial characteristics in a temperate grassland.

Nitrogen deposition and summer precipitation in eastern Inner Mongolia are predicted to increase in recent decades. However, such increases in nitrogen inputs and precipitation may not be continuous under the future new patterns of global change, with the direction and magnitude of which may change or weaken. The legacy effects of nitrogen and water addition after cessation on ecosystems are still unclear. Based on a 13-year nitrogen and water addition experiment in temperate grassland of northern China, we examined the short-term (2 years) legacy effects of historical nitrogen and water addition on soil physicochemical properties and microbial properties after the cessation of nitrogen and water addition in the 14th year. The results showed that the positive effects of historical nitrogen addition on most of soil nutrient variables diminished after two years of cessation, including ammonium nitrogen, nitrate nitrogen, dissolved organic carbon and nitrogen, and Olsen-P concentrations. In contrast, there were legacy effects on soil microbial characteristics. For example, the historical nitrogen input of 15 g N·m-2·a-1 reduced microbial biomass carbon, respiration, and alkaline phosphomonoesterase activity by 73.3%, 81.9%, and 70.3% respectively. It implied that microbial parameters restored slowly in comparison with soil nutrients, showing a hysteresis effect. Results of Pearson's correlation and redundancy analysis showed that the legacy effects of historical nitrogen addition on microbial parameters could be attributed to the negative effects of nitrogen addition on soil pH. Historical water addition showed significant legacy effects on soil pH, ammonium nitrogen, dissolved organic carbon and nitrogen, respiration, and soil enzyme activities, which significantly interacted with historical nitrogen addition. These results are of great significance to predict the changes in grassland ecosystem functions and services under the local environmental improvement conditions, and to reveal the restoration mechanism of degraded grassland.据预测,内蒙古东部地区未来将出现氮沉降加剧和夏季降水增加的趋势,但大气氮沉降和降水增加都是不可持续的,增加强度可能减弱,也可能出现降低趋势。目前,历史氮、水输入停止后对生态系统的遗留效应尚不明确。本研究基于中国北方温带草原长达13年的氮、水添加实验平台,于处理第14年开始定向停止氮、水添加处理,探讨了历史氮、水添加在短期(2年)内对土壤理化性质和微生物学特性的遗留效应。结果表明: 停止添加两年后,历史氮添加对铵态氮、硝态氮、可溶性有机碳、氮和速效磷等土壤养分指标的正向遗留效应消失;但对大部分土壤微生物学特性指标仍存在遗留效应,历史15 g N·m-2·a-1添加处理微生物生物量碳、土壤呼吸和碱性磷酸单酯酶活性分别降低73.3%、81.9%和70.3%,表明氮输入停止后微生物学指标的恢复比土壤养分指标慢,具有迟滞效应。相关分析和冗余分析表明,氮对微生物学特性的负向遗留效应与对土壤pH的负向遗留效应有关。历史水添加在处理停止两年后,对土壤pH、铵态氮、可溶性有机碳、氮等以及土壤呼吸、酶活性等仍表现出显著的遗留效应,并与氮沉降的遗留效应存在交互作用。研究结果对预测局域环境改善条件下草地生态系统功能和服务变化趋势及揭示退化草地恢复机制具有重要意义。.