## Materials and Methods
### Analysis of Soil Extracellular Enzyme Activities  
In this experiment, five soil extracellular enzyme activities were determined according to a modified version of the standard fluorometric techniques [20]. The functions of different soil extracellular enzymes are shown in Table 1. Briefly, the principle of enzyme activity determination is that the enzymes hydrolyze the artificial substrates and produce 4-Methylumbelliferone (MUB), which could be determined by fluorescence [30]. The specific measurement steps were as follows: 1 \(\pm\) 0.01 g of fresh soil was weighed into a 250 mL plastic bottle, and 125 mL of deionized water was added, then the soil particles were all broken by shaking on a rotary shaker. Then, we added 200 uL of soil suspension liquid and 50 uL of substrate (200 umol\(\cdot\)L\({}^{-1}\)) to the microplates (6 replicates per sample), and a blank control was added with 200 uL of soil suspension liquid and 50 uL of deionized water. The microplates were then incubated for 4 h at 25 \({}^{\circ}\)C in the dark. Finally, 10 uL of NaOH (concentration: 0.5 mol\(\cdot\)L\({}^{-1}\)) was added to each sample well to terminate the reaction. The fluorescence value was measured using a microplate reader after 1 min. After correcting for negative controls and quenching, all activities were expressed in units of nmol-g\({}^{-1}\).h\({}^{-1}\). The reason for selecting the five soil extracellular enzymes is that their potential activities are usually related to the rate of microbial metabolism and biogeochemical processes. Among them, BG, NAG, and AKP are commonly used as indicators of microbial nutrient demand [16] and were also used as the indicators of C, N, and P acquiring enzymes in previous studies about soil extracellular enzyme stoichiometry [10; 15]. Because the other two major C-acquiring enzymes (CBH and BX) were much lower than the activity of BG, they had no effect on the soil extracellular enzyme stoichiometry results. In addition, vector analysis (Vector length, VL; Vector angle, VA) of soil extracellular enzyme stoichiometry is a means for visualizing the relative C, N, and P controls on soil microbial communities, which can reflect soil nutrient limitations [31]. Our analysis of the enzyme activity vectors clearly showed the changes in relative resource demands caused by decomposer communities during the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Enzyme** & **EC** & **Abbr.** & **Substrate** & **Function** \\ \hline \(\beta\)-1,4-glucosidase & EC & 3.2.12.21 & BG & 4-MUB-\(\beta\)-D-glucose & Hydrolysis of cellobiose to glucose during carbon cycling \\ \(\beta\)-1,4-xylosidase & EC & BX & 4-MUB-\(\beta\)-D- & Hydrolysis of cellulose to form hemicellulose during carbon cycle \\ Cellobiohydrolase & EC & 3.2.191 & CBH & 4-MUB-\(\beta\)-D- cellobioside & Hydrolysis of cellulose to produce sucrose during carbon cycle \\ \(\beta\)-N-acetylglucosamine & EC & NAG & 4-MUB-N-acetyl-\(\beta\)-D- & Participation in the hydrolysis of chitin during the carbon-nitrogen cycle \\  & 3.2.1.14 & NAG & glucosmimide & Hydrolys phosphate from phosphosaccharides and phospholipids during the phosphorus cycle \\ Alkaline phosphatase & EC 3.1.3.1 & AKP & 4-MUB-phosphate & phosphosaccharides and phospholipids during the phosphorus cycle \\ \hline \hline \end{tabular} EC: enzyme commission classification; MUB: methylumbelliferyl.  
\end{table} 
Table 1: Abbreviations, substrates, and functions of soil extracellular enzymes. decomposition process, which cannot be observed by alternative analyses [32]. This method is thus considered to compensate for the fact that pairwise comparisons of enzyme activities cannot not reveal resource co-limitations [33].