Microbial carbon-use efficiency (CUE) is defined as the portion of carbon (C) incorporated into biomass relative to the total carbon consumed and plays a pivotal role in regulating microbially-mediated C and nutrient transformations in soil. However, little is understood about how CUE is impacted by edaphic properties, like soil moisture. Soil moisture physically regulates microbial activity through its effects on both water potential and water content. Low water potential can result in high, compensatory intracellular solute concentrations that may inhibit biochemical functions through cytoplasmic desiccation, whereas low soil water content results in thin water films that can limit substrate diffusion, reducing microbial access to dissolved substrates. Because these two aspects of soil moisture may affect microbial respiration differently than C assimilation, they may have different effects on CUE. The purpose of this research was to evaluate the relative importance of water potential and water content in regulating CUE of soil microbial communities. Moist soil incubations of a sandy loam soil were used to determine the impact of both aspects of soil moisture on CUE, and soil slurries were used to determine the impact of water potential alone. Both ¹³C-acetate and ¹⁵N-ammonium were added to moist soils and slurries to quantify gross rates of C and N transformations. In moist soils, acetate assimilation and respiration rates and gross N mineralization and immobilization rates increased exponentially with increasing soil moisture (−3.0 to −0.03 MPa). In contrast, acetate assimilation and respiration and gross N transformation rates remained constant in soil slurries across a similar water potential gradient, created by modifying solute concentrations. Similarly, values of CUE in moist soils increased exponentially with increasing soil moisture, whereas slurry values of CUE remained constant across the soil water potential gradient. Because no changes in rates and CUE were observed in slurries, changes observed in moist soils were attributed to limited substrate diffusion associated with low water contents rather than to adverse physiological effects associated with low water potentials. Results of this study demonstrate that limited substrate diffusion is the primary physical mechanism through which soil moisture regulates microbially-mediated C and N transformation rates and CUE in this sandy loam soil.

微生物碳利用效率（CUE）定义为掺入生物质中的碳（C）相对于消耗的总碳的比例，并且在调节土壤中微生物介导的C和养分转化方面起着关键作用。但是，人们对于土壤水分等CUE如何受到诸如土壤水分等土壤渗透性的影响知之甚少。土壤水分通过其对水势和水含量的影响来物理调节微生物活性。低的水势可能会导致高的代偿性细胞内溶质浓度，从而可能通过细胞质干燥而抑制生化功能，而低的土壤水含量会导致形成薄水膜，从而限制底物扩散，从而减少微生物对溶解底物的接触。由于土壤水分的这两个方面对微生物呼吸的影响与对C的吸收不同，因此它们对CUE的影响可能不同。这项研究的目的是评估水势和含水量在调节土壤微生物群落CUE中的相对重要性。沙质壤土的潮湿土壤温育用于确定土壤水分对CUE的两个方面的影响，土壤泥浆仅用于确定水势的影响。都 和泥浆仅用于确定水势的影响。都 和泥浆仅用于确定水势的影响。都¹³ C-乙酸盐和¹⁵将N铵添加到潮湿的土壤和泥浆中，以量化C和N转化的总速率。在潮湿的土壤中，乙酸盐的同化和呼吸速率以及总氮矿化和固定化速率随土壤湿度的增加（-3.0至-0.03 MPa）呈指数增长。相反，在通过改变溶质浓度而产生的相似的水势梯度下，土壤泥浆中的乙酸同化和呼吸作用以及总氮转化率保持恒定。同样，潮湿土壤中的CUE值随土壤湿度的增加而呈指数增加，而CUE的浆液值在整个土壤水势梯度中保持恒定。由于在浆液中未观察到速率和CUE的变化，在潮湿的土壤中观察到的变化归因于与低水含量相关的有限的底物扩散，而不是与低水势相关的不利生理影响。这项研究的结果表明，有限的底物扩散是土壤水分调节该沙壤土中微生物介导的C和N转化速率以及CUE的主要物理机制。