Effects of the application of wood ash and straw on the mineralization of soil C, soil chemical properties, and enzyme activities
Previous studies that examined the effects of wood ash on the emissions of CO2 have primarily concentrated on forest soils, and the results are contradictory. In some studies, long-term data on the effects of wood ash on the CO2 emissions of forest peatland increased owing to an increase in the pH and nutrient contents of the soil (Moilanen et al. 2012). In contrast, Klemedtsson et al. (2010) found that amendment with wood ash could be an appropriate mitigation measure for CO2 emissions from a spruce forest. This study found that the application of wood ash significantly decreased the total emissions of soil CO2 in the order of straw > straw plus wood ash > Control > wood ash (Table 2). The decrease in CO2 emissions in straw plus wood ash treatment was much higher than the amount of SIC generated, which could be partly owing to the formation of SIC and partly owing to the conversion of more straw to newly formed SOC (e.g., the labile organic C fraction). All of those results indicated that the addition of wood ash did reduce CO2 emissions in acidic and alkaline soils. In addition, the data showed that the addition of wood ash promoted the formation of new organic carbon and the retention of net organic carbon, but did not affect the mineralization of native SOC. This result suggests that when wood ash was present, the increase in the content of SOC might be owing to the progressive breakdown and transformation of straw by the enhanced enzyme activities. However, this result was inconsistent with that of Reed et al. (2017), who noted that wood ash brough about a lasting adverse impact on SOM turnover owing to the CEC and the ability of the speciଁc surface area of wood ash to chemically stabilize the pH of SOM.
The addition of straw and wood ash obviously enhanced the contents of MBC and DOC relative to treatment with the addition of straw (Table 2; Fig. 2), which may be directly affected by the enhanced enzyme activity and a change in the pH of soil. The application of wood ash apparently increase the pH, which was influenced largely by the dissolution of several oxides, hydroxides, carbonates, and bicarbonates contained in wood ash (Vassilev et al. 2013). Consistent with the change in pH, the increase in soil EC after wood ash was added was owing to the dissolution of wood ash during cultivation that contained high amounts of alkaline ions (Table 2). The addition of straw and wood ash had an effect on mineral nitrogen with the order: straw plus wood ash < straw < wood ash ≈ Control (Table 2). This indicated the combination increased the immobilization of mineral nitrogen, which may further promote the formation of soil organic nitrogen, improving the inorganic nitrogen sequestration, reducing the leaching loss of nitrogen.
The fact that catalase and dehydrogenase were only reduced by wood ash could be attributed to at least two reasons. First, the wood ash adsorbed the substrate used by catalase and dehydrogenase and inhibited the progress of enzyme reaction. Second, the high pH of the wood ash itself could also produce changes in the microbial community, resulting in a decrease in the secretion of catalase and dehydrogenase. The promotion of enzyme activity by straw plus wood ash was owing to the high pH caused by added wood ash to promote the hydrolysis of straw to produce more catalase and dehydrogenase reaction matrix (Gömöryová et al. 2016). Interestingly, the invertase activity was enhanced under wood ash compared with the Control. This was probably caused by the increase in pH owing to wood ash, which aided in the release of more sucrose from the straw or soil (Table 2).
Effects of the application of wood ash and straw on soil bacterial diversity
There were no differences of bacterial species richness among the treatments. Toke et al. (2017) recently found that the EC and pH had an obviously adverse impact on bacterial diversity, while our results are contrary to theirs, we found that the labile carbon, pH and EC have an important role in the increase in bacterial diversity based on Pearson’s correlation coefficients (Fig. 4). Indeed, the decomposition of C provides energy for most soil microorganisms, and recent studies found that soil bacterial diversity is driven by soil C storage (Delgado-Baquerizo et al. 2013; Maestre et al. 2013). However, the long-term experiment combining straw with wood ash used in the field merits further study.
Effects of the application of wood ash and straw on the soil bacterial community structure
Figure 5 shows that some speciଁc bacterial groups obviously varied among different treatments. Soil pH is a primary factor regulating microbial community structure. High pH values were favourable to Bacteroidetes and Actinobacteriota, while low pH values could be better suited for Acidobacteriota (Lauber et al. 2009). Bacteroidota and Actinobacteriota increased in relative abundance following the straw plus wood ash and wood ash alone relative to the straw alone (Fig. 5). Noyce et al. (2016) indicated that applying wood ash could increase the abundance of Bacteroidota. The possible increase in DOC and the more neutral pH at applications of 22 t ha−1 of ash improves the conditions for the growth of the copiotrophic Bacteroidota. Acidobacteriota was enhanced by the straw alone and is a heterotroph that can utilize extensive sources of carbon (Ward et al. 2009), thus, displaying a crucial role in the carbon cycle. This result was comparable to those of Rousk et al. (2010), who found that Acidobacteriota predominate in conditions of relatively lower pH values. The Control treatment in concert with the lack of effects of the application of degradable C can help to explain the lower bacterial numbers and the increased abundance of Firmicutes in this treatment that lacked amendments. The dominant phylum Proteobacteria maintains a high relative abundance (32.4–36.5%) throughout all the treatments (Fig. 5), which might be explained by the general resistance of Proteobacteria to environmental changes (Barnard et al. 2013). This study found that the highest percentage of Alphaproteobacteria was identified in the treatment that lacked amendments (lower pH), which in consistent with those of Ding et al. (2016), who found that the populations of Alphaproteobacteria negatively correlated with the soil pH. In addition, soil moisture is a critical factor in governing soil community composition. In this study, the relative abundance of Myxococcota was significantly lower in the Control treatment compared with those of the other three treatments, which had a strongly positively correlated with the soil moisture.
Previous studies indicated that changes that wood ash caused in the EC and pH were important determinants of the changes observed in the community composition of bacteria (Toke et al. 2017; Fierer et al. 2006; Fierer et al. 2007). This study found that soil properties, such as water soluble K, pH, EC, and soil moisture, obviously contributed to the variation in bacterial community structure (Fig. 6a). The relationship among the enzymatic activity, SOC, and bacterial community composition was studied. For example, the newly formed and sequestered SOC were obviously bound up with dehydrogenase, invertase and hydrolase activities based on the Distance-based redundancy analysis (Fig. 6b).