Human activities have increased nitrogen (N) and phosphorus (P) inputs in terrestrial ecosystems and altered carbon (C) availability, shifting the stoichiometry of microbial substrates in soils, such as the C:N:P ratios of the dissolved organic matter pool. These stoichiometric deviations between microbial biomass and its substrate may control microbial processes of N cycling. We studied the effects of this stoichiometric mismatch using a full factorial N and P addition experiment replicated in six grassland ecosystems in South Africa, the USA, and the UK. We found that N and P addition changed the dissolved organic matter C:N ratio, but not the C:N ratio of the soil microbial biomass. Compared to P addition, N addition decreased microbial N acquisition via non-symbiotic N₂ fixation by −55% and increased microbial N release via net N mineralization by +134%. A possible explanation is that the dissolved elements, e.g., dissolved organic C (DOC) and dissolved total N (DN), serve as the main microbial substrate and its C:N ratio defines whether N is scarce or abundant with respect to microbial demands. If N is available in excess relative to microbial demands, net N mineralization increases. In contrast, when N is scarce, immobilization outweighs release decreasing net N mineralization. However, the activity of leucine aminopeptidases, which decompose peptides, was not affected by nutrient additions. Further, C rather than P availability may control the rates of non-symbiotic N₂ fixation in the six studied grassland sites. In conclusion, globally increasing nutrient inputs change processes of microbial N acquisition and release in grassland ecosystems and these changes are largely driven by shifts in substrate stoichiometry.

人类活动增加了陆地生态系统中的氮（N）和磷（P）投入，改变了碳（C）的利用率，改变了土壤中微生物底物的化学计量，例如溶解有机物池的C：N：P比。微生物生物量及其底物之间的这些化学计量偏差可控制氮循环的微生物过程。我们使用在南非，美国和英国的六个草地生态系统中复制的全因数氮和磷添加实验研究了这种化学计量失配的影响。我们发现氮和磷的添加改变了土壤微生物生物量的溶解有机物碳氮比，但并未改变碳氮比。与添加磷相比，添加氮减少了通过非共生氮₂引起的微生物氮的吸收固定-55％，通过净氮矿化增加+ 134％的微生物氮释放。一个可能的解释是，溶解的元素（例如，溶解的有机碳（DOC）和溶解的总氮（DN））用作主要的微生物底物，其C：N比率定义了相对于微生物需求而言，氮是稀缺还是丰富。如果氮相对于微生物需求过量，则净氮矿化增加。相反，当N稀缺时，固定化作用大于释放的净N矿化作用。然而，分解肽的亮氨酸氨基肽酶的活性不受营养添加的影响。此外，C而不是P的可用性可以控制非共生N ₂的比率固定在六个研究过的草地地点。总之，全球营养素输入的增加改变了草地生态系统中微生物氮的吸收和释放过程，而这些变化很大程度上是由底物化学计量的变化驱动的。