## 4 Discussion  
Both liming and plant residue incorporation can elevate the pH of acidic soils [3; 4; 5; 6]; however, only a few experiments were designed to compare their effects [37; 38]. In this study, we compared the effects of CaCO\({}_{3}\) and OM in increasing the pH of subtropical orchard soils with an initial pH of  
Figure 4: Canonical correspondence analysis (CCA) of the explanatory variables (pH and DOC) and the response variables, soil chemical properties (**A**) or soil enzyme activities (**B**). TN, total nitrogen; AN, available nitrogen; TP, total phosphorus; AP, available phosphorus; TK, total potassium; AK, available potassium; \(\alpha\)-GLU, \(\alpha\)-glucosidase; \(\beta\)-GLU, \(\beta\)-glucosidase; \(\beta\)-XYL, \(\beta\)-xylosidase; CEL, cellobiosidase; URA, urase; NR, nitrate reductase; CHI, chitinase; ACP, acid phosphatase; ALP, alkaline phosphatase. 4.98. It is clear that CaCO\({}_{3}\) is more effective in the amelioration of acidic soils than OM. When the effects of CaCO\({}_{3}\) and OM are compared, it should be born in mind that their primary effects are much different. CaCO\({}_{3}\) directly increases the soil pH in via soil chemical pathway, which can always be fast and obvious; however, OM indirectly increases it via soil biochemical pathway with the involvement of soil microbes. Elevating pH with plant residue incorporation involves microbial degradation of organic matter in the plant residues and the subsequent release of alkalinity (excess cations), and thus, can be time-consuming [8]. In this scenario, the effectiveness of plant residue incorporation depends to a certain degree on the compositions and types of plant residues and characteristics of soils [5]. Although some abiotic factors, such as temperature, soil moisture, pH are the important determinants of the decomposition process of plant residues, soil microbes are the key biotic factor contributing much to the decomposition. It is generally accepted that plant residues can be decomposed by a variety of soil microbes with priming effect [39; 40]. In fact, the direct effect of OM is increasing DOC due to the decomposition of plant residues [41; 42; 43; 44], and then the promoted microbial activity. In this study, the increased pH together with promoted microbial activity, further mobilized the TN, TP, TK, leading to the increased levels of AN, AP, or AK. It is suggested that the extra-cellular hydrolysis enzymes of soil microbes are mainly responsible for the mobilization of N, P, K [45], with _Bacillus_ the most effective. Our results demonstrate that the microbial community structure in lined soil is closer to that in control soil than that in the soil with plant residue incorporation, indicating a greater regulating effect on bacterial community composition by OM than that by CaCO\({}_{3}\). This difference suggests that, in parallel with the amelioration of acidic soil, OM can significantly reshape the microbial community structure while CaCO\({}_{3}\) can not. This regulated microbial community structure can further improve the microbial activity, as revealed by the soil enzyme activity in this study. This indicates that CaCO\({}_{3}\) exerted its influence on a limited array of soil chemical properties, while OM exerted its influence on a diverse array of soil chemical properties. Overall, the effects of OM in improving the soil are more comprehensive than CaCO\({}_{3}\), including not only the increased pH, but also the mobilization of nutrients and the promoted enzyme activity. In this study, although the manifest difference in the microbial community composition was observed according to DGEE profiling, we did not characterize the dominant species or the modified species. According to [PERSON] et al. [46], liming significantly decreased the diazotroph abundance while plant residue did not, highlighting their difference in regulating specific microbial taxa.