## 4 Discussion
However, in our study the soil MBC:MBP and MBN:MBP in sand vs. a mix of sand and feldspathic sandstone were lower than the global research results, which may be related to soil P infertility in the study area [44]. Simultaneously, soil MBC:MBN, MBC:MBP, and MBN:MBP increased in mixed soil. The first possible explanation for this observation is that MBC was more sensitive to environmental changes than MBN and MBP [45]. The second possible explanation is related to the increase in biodiversity and the change in soil properties after adding feldspathic sandstone. The increase in soil MBN:MBP may have also been due to the relative decrease in the demand for P-rich ribosomal RNA by microbial growth [13]. Likewise, microbes can take up excess resources and store them in the form of glycogen or polyphosphates, thereby inducing changes in MBC:MBN ratio [46]. Additionally, [PERSON] and [PERSON] [47] pointed out that MBC:SOC varied between 0.27% and 7.0%. However, the variation range of MBC:SOC was 7.35%-10.45% in our study, which was higher than previous studies. This difference may be because SOC was poor in our study area, and the microbial metabolic cycle is short. In order to maintain soil nutrient content, the proportion of MBC in SOC must be increased to maintain high organic matter metabolism and nutrient cycling [47]. We also found that the MBC:SOC in mixed soil was higher than in sand soil. This may be because the turnover rate in MBC after adding feldspathic sandstone is higher than that in soil C. Another possible explanation is that the quantity, quality, and cycle of the organic matter input into the mixed soil were better than those in sand soil, resulting in higher soil microbial activity and microbial biomass and promoting the turnover and decomposition rate of soil organic matter. Also, the RDA analysis indicated that the stoichiometry in microbial biomass C, N, and P were all correlated with SOC, SWC, and microbial biomass C, N, and P, which was consistent with existing studies [36; 48], indicating that there was an adaptive response of the soil microbial stoichiometry to resource cycling [11]. These results supported our hypothesis that the increase in soil microbial biomass and its ecological stoichiometry was closely related to the changes in SOC and SWC. Explaining Variance in Soil Extracellular Enzyme Activity Metrics in Sand vs. a Mix of Sand and Feldspathic Sandstone  
Soil extracellular enzymes are very sensitive to changes in biotic and abiotic environments and are active participants in the material circulation and energy conversion of soil system, and are often regarded as important indicators for determining soil quality [31; 49]. Studies have shown that soil extracellular enzymes are the proximate agents of nutrient decomposition [20]. Therefore, in our study, soil enzyme activity in mixed soil was significantly higher than that in sand soil, probably because more soil organic carbon enrichment after adding feldspathic sandstone for 10 months provided relatively abundant C sources for microbes [50; 51]. This result may also be related to the increase of SWC [52]. Our results showed that there was a significant and positive correlation between the five soil extracellular enzyme activities (Table S3), indicating that the soil extracellular enzymes were interrelated and interact with each other, and there was a symbiotic relationship during the interaction process. Moreover, it is generally accepted that soil nutrients are the basis of soil extracellular enzyme activity, and soil extracellular enzyme activity is the driving force of soil nutrient cycling, which are closely related to each other [53]. Therefore, it is not surprising that there is a significant correlation between the activity of five extracellular enzymes and SOC, microbial biomass C, N, and P, and its stoichiometry in our study (Table 4). It is implied that soil extracellular enzyme activity can be used as an important indicator to measure soil nutrient status [52].