The metabolic activity of soil microorganisms is often limited by soil nutrient availability. Fertilization can increase available nutrient content, but nutrient limitations may persist because of imbalances in nutrient inputs. However, the mechanisms driving the response of soil microbial nutrient limitation to N and P application in grasslands remain unclear. To address this issue, we applied fifteen fertilization treatments composed of five N levels (0, 1.55, 4.65, 13.95, 27.9 g N m y) and three P levels (0, 5.24, 10.48 g P m yr) to a meadow steppe in Inner Mongolia across three years using a split-plot experiment design. Soil microbial biomass and extracellular enzyme activities were analyzed in samples collected from each plot in May, July, and August. The addition of N significantly reduced microbial biomass carbon (MBC) in samples collected in May and increased microbial biomass nitrogen (MBN) in July samples, thus decreasing the ratio of MBC:MBN in both months. P addition significantly increased microbial biomass phosphorus (MBP), whereas it reduced the ratio of MBC:MBP and MBN:MBP. Using vector analysis, we found that vector angle was less than 45° across all sampling dates, indicating that soil microbial metabolism was predominately limited by N rather than P. The severity of microbial N limitation was attenuated by N addition, but was worsened by P addition in May and July. The severity of microbial C limitation was significantly intensified by N addition in May and July, and forced by P addition in July and August. Visual partitioning analysis showed that soil physicochemical and microbial properties explained 37% and 70% of variation in microbial C and N limitation, respectively. Besides soil available nutrient concentrations, soil water content (SWC) and pH were identified as the key factors driving microbial C and N limitations. The relative influence of SWC on microbial N limitation was highest across all sampling dates. According to PLS-SM modeling, SWC had a total effect of −0.349 on microbial N limitation, which was significantly higher than the effects than N addition (−0.192) and P addition (0.131). Overall, this study indicates that soil moisture was the primary control over the response of microbial nutrient limitation to N and P additions in a meadow steppe in Inner Mongolia.

中文翻译：

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内蒙古草甸草原土壤水分驱动土壤微生物养分限制对氮磷添加的响应

土壤微生物的代谢活动通常受到土壤养分有效性的限制。施肥可以增加可用养分含量，但由于养分投入不平衡，养分限制可能持续存在。然而，土壤微生物养分限制对草地氮磷施用的响应机制仍不清楚。为了解决这个问题，我们在内陆地区的草甸草原上施用了十五种施肥处理，其中包括五个氮水平（0、1.55、4.65、13.95、27.9 g N m y）和三个磷水平（0、5.24、10.48 g P m y）。蒙古使用裂区实验设计跨越三年。对五月、七月和八月各地块采集的样品进行了土壤微生物量和细胞外酶活性分析。添加N显着降低了5月份收集的样品中的微生物生物量碳（MBC），并增加了7月份样品中的微生物生物量氮（MBN），从而降低了这两个月份的MBC：MBN比率。P的添加显着增加了微生物生物量磷（MBP），同时降低了MBC:MBP和MBN:MBP的比率。通过矢量分析，我们发现所有采样日期的矢量角都小于45°，这表明土壤微生物代谢主要受到N而不是P的限制。微生物N限制的严重程度因N的添加而减弱，但因P而恶化5月和7月增加。5 月和 7 月的氮添加显着加剧了微生物碳限制的严重性，而 7 月和 8 月的磷添加则显着加剧了微生物碳限制的严重程度。视觉分配分析表明，土壤理化和微生物特性分别解释了微生物碳氮限制变化的 37% 和 70%。除了土壤有效养分浓度外，土壤含水量 (SWC) 和 pH 值被确定为驱动微生物碳和氮限制的关键因素。SWC 对微生物氮限制的相对影响在所有采样日期中最高。根据PLS-SM模型，SWC对微生物氮限制的总影响为-0.349，显着高于氮添加（-0.192）和磷添加（0.131）的影响。总体而言，本研究表明，在内蒙古草甸草原中，土壤湿度是微生物养分限制对氮和磷添加的响应的主要控制因素。