4.1 The effect of cattle manure on SOC chemical composition
In this study, cattle manure addition altered SOC chemical structure as well as SOC content (Fig. 1, Table 2). Irrespective of the soil layer (topsoil v/s subsoil) and sampling date (45d or 90 d) as well as with or without manure, SOC in this study was dominated by O-alkyl C (Table 2). Carbohydrates and lignin are particularly rich in this functional group, while proteins and lipids only contain minimal quantities (Ng et al. 2014). Consistent with previous research, this observation shows that carbon molecules generated from manure may have accumulated as O-alkyl C in the soil (Yan et al. 2013). For example, C compounds of the O-alkyl form were discovered to be the most abundant in an annual wheat-maize double-cropping system (He et al. 2018). In contrast, alkyl C was shown to be the most prevalent SOC type in a different research conducted in North China by (Zhang et al. 2009). These discrepancies in SOC functional group proportions may be linked to shifts in soil types, organic residue input quantities and types, and agricultural practices (He et al. 2018; Zhang et al. 2009). Further, the SOC also includes Alkyl C and Aromatic C, which contribute to the following group after O-alkyl C. The Aromatic C from cellulose and hemicellulose (Solomon et al. 2007), which might be associated with the lower activity of lignin degrading microbes, such as fungi, and thus selectively preserving OC substances from microbial decomposition (Mustafa et al. 2022). Alkyl-C may be present in proteins as well as lipids, waxes, cutins, suberins, and lignin (Baumann et al. 2009). Aromatic carbon often used together with Alkyl C to represent carbon compounds that are difficult to be utilized by microorganisms.
The SOC chemical nature of topsoil and subsoil was significantly different (Table 2). Topsoil induced higher alkyl C proportion but relatively lower O-alkyl C compared with subsoil. However, it was contrary to the findings of others studies, for example, Zhang et al. (2015) found that there was a lot more Alkyl C in the deep soil than in the top soil, but there was a lot less O-alkyl C in the deep soil (Zhang et al. 2015b). O-alkyl C typically decreases and alkyl C typically increases as decomposition proceeds (Baldock et al. 1992). According to these findings, which are in line with those of other research, topsoil sped up the decomposition of labile C group whereas subsoil slowed it down (Tian et al. 2016). Also, the alkyl C/O-alkyl C ratio (A/OA) was greater in topsoil than in subsoil, with a larger value indicating greater decomposition of soil C (Zhao et al. 2012) and subsequently greater rates of C loss (Shrestha et al. 2015). This finding provided evidence for the concept that the microbial community preferentially decomposes labile C, and that the resulting compounds help produce stable C (Cotrufo et al. 2013). As a result, the aliphatic C/aromatic C ratio (AL/AR) could be confirmed, indicating that the chemical composition of SOC in topsoil was more complicated than subsurface (Zhao et al. 2012).
The chemical composition of SOC in both the topsoil and the subsoil was drastically changed when cattle manure was added (Table 2). The decreased A/OA ratio of topsoil treated with manure and the decrease in Alkyl C and rise in O-alkyl C reveal that SOC the decomposition of topsoil with manure was delayed. While in another study, soil particle organic matter had minimal O-alkyl C after 18 years of organic fertilisation, but major levels of aromatic C and alkyl C (Zhou et al. 2010). Possible causes for the elevated O-alkyl C content in topsoil include the increase of root aboveground biomass after manure addition (Hu et al. 2023; Li et al. 2015), and, He et al. (2018) discovered that polysaccharides generated by bacteria led to the synthesis of O-alkyl C during the degradation of plant litter (He et al. 2018). Soil physical conditions, cation exchange, metal complexing processes, production of humic compounds, biological activity, and adsorption of polysaccharides by clay minerals may all be affected by the elevated O-alkyl C proportions of topsoil under manure (He et al. 2018). Soil C sequestration might result via improved particle aggregation and structure as a result of these alterations (Kundu et al. 2007). Due to the considerable potential for C sequestration in Chinese croplands, a delay of this type should be seriously examined (Di et al. 2017). However, the subsoil was the opposite. Decomposition of SOC in subsoil with cattle manure may have been facilitated by the observed rise in alkyl C and decrease in O-alkyl C. Further, the greater A/OA ratio of subsoil with manure shown that the labile C forms was consumed and resulted a greater recalcitrant C in subsoil. Thus, subsoil with manure was more favour SOC accumulation compare to than that without manure. Pisani et al. (2016) found that long-term inputs of organic wastes accelerated the decomposition of labile SOC and improved the sequestration of aliphatic chemicals in soil, and our findings corroborated these findings (Pisani et al. 2016). In summary, the topsoil with manure amended was more favour SOC accumulation, and the subsoil amended manure had high potential to sequestrated C.
Manure addition has different effects on SOC chemical composition in topsoil and subsoil under different stage. In the 45 day stage, the alkyl C and O-alkyl C proportion, as well as the A/OA and AL/AR ration were similar between topsoil amended manure and subsoil amended manure (Table 3). It indicated that the biochemical properties and chemical composition of topsoil and subsoil with application manure were similar. While at 90 day stage, the subosoil with manure amended had a greater proportion of alkyl C and a lower proportion of O-alkyl C, which indicated that the decomposition degree of subsoil amended manure is greater than that of topsoil amended manure. And the higher A/OA ratio of subsoil amended manure than that of topsoil amended manure could also be verified this. Our research showed that adding manure to subsoil resulted in a more complex SOC structure, while adding manure to topsoil resulted in a less complex SOC structure. It follows that topsoil amended with manure can serve to preserve SOC and promote the formation of a more resilient SOC structure.
4.2 The effect of cattle manure on microbial communities composition
The total PLFA, bacterial, fungi and actinomycete content of topsoil were greater than that of subsoil (Fig. 2). Decreased levels of soil nutrients including carbon, nitrogen, and phosphorus have an effect on the total amount of microorganisms in the soil (Naylor et al. 2022). Soil microbial biomass is reported to be lower in N-limited soil than in N-rich soil (Liu et al. 2022). Some researchers have found that soil C/N determined microbial community structure (Wan et al. 2015). Since the topsoil's C/N ratio was 11.70, which was significantly lower than the subsoil's (16.14), there should be sufficient N available for microbes in the topsoil. The significantly greater SOC content under treatments with the inclusion of cattle manure corresponded well with the significantly higher soil microbial biomass (bacterial, fungal, actinobacterial, G+ and G− PLFAs content) (Fig. 2). Previous reports have shown comparable results (Wang et al. 2012; Zhao et al. 2010). As soil microbes are generally C limited, the addition of manure can offer a substrate that is either reasonably stable or labile, which is necessary to promote the development of microorganisms (van Groenigen et al. 2014), and thereby improve the quantity of total microorganisms abundances (Zhang et al. 2015). It is quite likely that changes in the composition of manure and SOM, and consequently, differences in the availability of substrates, are the basis for the observed variances in the abundance of particular microbial groups and community composition. There have been a variety of conclusions produced about the impact that fertilizations have had on the levels of G + and G-bacteria. For instance, Peacock et al. (2001) found that the addition of manure over a period of five years led to an increase in the G- bacterial biomass while leading to a decrease in the G + bacterial biomass (Peacock et al. 2001). On the other hand, Ai et al. (2012) presented evidence that a 31-year application of organic fertiliser led to an increase in the amount of G + bacteria (Ai et al. 2012). However, the percentage of actinomycetes in total PLFA was decreased in the top-, and subsoil applied manure, which was consistent with other studies (Clegg et al. 2003; Zhang et al. 2015). This is due to the fact that actinobacteria make relatively little use of newly generated or added organic C, but are highly well suited to metabolise older organic matter (Zhang et al. 2015). The increase in total PLFAs content was much larger in the subsoil compared to the topsoil, which showed that the microbes in the subsoil utilised the organic compound from manure to a greater extent than those in the topsoil (Angst et al. 2019). Similarly, research has shown that there is a trade-off between the production of microbial biomass and the maintenance of microbial activity, with the increase in microbial activity following labile C addition in subsoil being greater than that in topsoil (Tian et al. 2016).
Manure application not only increased the biomass of all microbial groups, but also changed microbial community structure (Fig. 2). In contray with previous research on agricultural soil (Marschner et al. 2003), we saw a considerable improvement in the F/B ratio after manure was added. Soil C sequestration has also been described using the F/B ratio. Fungal predominance in the microbiome is associated with slower rates of SOC turnover and greater SOC accumulations (Ananyeva et al. 2015; Malik et al. 2016). Networks of fungal hyphae contribute in the development of soil aggregates that are both strong and resistant to breakdown by microbes (Peng et al. 2013). Furthermore, necromass produced by fungi is chemically more stable than that produced by bacteria (Liang et al. 2017). The higher F/B ratio in both the top-, and subsoil after manure management was indicative of a larger prevalence of fungal growth. Fungi are the most abundant decomposers of external carbon input due to their wide arsenal of extracellular enzymes (Li et al. 2020). Before manure was applied, the F/B ratio of the topsoil was greater than that of the subsoil, but after manure was applied, the reverse was true. It showed that the fungal growth was stimulated by the increased availability of C substrates in the manure-amended subsoil. A decreased F/B ratio in manure-applied topsoil was consistent with previous research suggesting that bacteria would respond favourably to the increased availability of C substrates present in these soils (Wei et al. 2017; Zhong et al. 2010). In our research, manure was shown to increase fungal biomass in both the top and subsoil and to support SOC accumulation, particularly in the subsoil.
When the G+/G- ratio alterations, it shows that the surrounding environment is stressful, and the decreases in soil C availability result in a higher G+/G- ratio (Fanin et al. 2019; Rankoth et al. 2019). Since the G+/G- ratio in topsoil is higher than in subsoil, it is more favourable to G + bacteria. This is consistent with the chemical composition of SOC in topsoil that the lower O-alkyl C and the greater alkyl C (Table 3). At the 45 and 90 day, we discovered that the G + bacterial population increased in manure-applied soils, both in the topsoil and the subsoil (Fig. 2). Previous research has shown that when organic matter is added, a community succession effect occurs, wherein fast-growing G- bacteria proliferate initially, but then decrease, making way for slower-growing microbes like G + bacteria (Lazcano et al. 2013). G + bacteria are well suited to soils with poor substrate availability, hence the more resistant material in cattle manure could improve their competitive abilities (Zhang et al. 2015). More N-acetylglucosamine, a precursor of relatively decay-resistant soil organic matter, is found in the peptidoglycan of G + bacteria than in those of G- bacteria (Zhang et al. 2015). Therefore, our finding that the SOC content in the treatments under study was significantly positively linked with the G+/G bacterial ratio (Fig. S1).
4.3 Linking microbial community to SOC chemistry
The relationship between microbial community structure and soil C forms was studied using Principal component analysis (PCA), Redundant analysis (RDA) and Multivariate regression tree (MRT) (Fig. 5, 6, 7). We discovered that the microbial community was significantly (P < 0.05) related with SOC content, which was consistent with previous research (Dong et al. 2014; Schnecker et al. 2015). RDA analysis revealed that changes in the composition and diversity of soil microbial communities were best described by the chemical composition of SOC, correlating with comparable findings in earlier investigations (Wang et al. 2017). This confirmed our prediction that fertilization-induced changes in soil C chemistry would lead to changes in the composition and diversity of microbial communities. Therefore, compared to prior studies (> 50%), C forms accounted for a higher fraction (> 85%) of the variability in soil microbial composition in the current study (Ng et al. 2014).
As previously established, the soil microbial community composition definitely responded differentially to topsoil and subsoil carbon forms. The higher relative abundance of O-alkyl C and lower of alkyl C of the SOC in the T1M treatment may relying on the Proteobacteria, Firmicutes, and Bacteroidetes abundances, which were shown to be positively associated to O-alkyl C. It is most likely that the application of cattle manure in topsoil reduced native SOC decomposition, as seen by the decreased A/OA ratio of topsoil treated with manure. As alkyl C is inadequate and has a negative connection to the abundance of Actinobacteria, Verrucomicrobia, and Cyanobacteria, it is possible that these bacteria participate in its degradation. Actinobacteria, considered r-strategists with high affinity to a specific substrate, and can provide glycoside hydrolases for exogenous organic matter degradation (Trivedi et al. 2013). Members of the Verrucomicrobia have been claimed to break down refractory biological materials in the past (Fierer et al. 2013). The higher relative abundance of Alkyl C and lower of O-alkyl C of the SOC in the T2M treatment may rely on the Acidobacteria, Gemmatimonadetes, and Patescibacteria abundances, which were shown to have a favourable correlation with alkyl C. It may be possible that application cattle manure in subsoil promoted native SOC decomposition, showing the greater A/OA ratio of subsoil with manure. The phylum Acidobacteria includes many oligotrophic species. After the addition of new organic matter to the soil, communities of resistant SOC decomposers came to dominate native soil organic matter-degrading organisms. This was due to the fact that these organisms acted as decomposers of SOC (Pascault et al. 2013). In addition, the low abundance of O-alkyl C, together with the negative connection between O-alkyl C content and abundance of both members of Cyanobacteria and Acidobacteria and the fungus of Ascomycota, implies that these bacteria may be engaged in the breakdown of O-alkyl C. This is supported by the fact that there is a negative correlation between O-alkyl C content and abundance of both of these bacterial groups. Subsoil has the lower F/B ratio and A/OA ratio than topsoil, while subsoil with manure has a greater F/B ratio and A/OA ratio than topsoil with manure. This is indicated that application cattle manure in subsoil promoted the microbial community composition transform from bacterial to fungal, especially the members of Ascomycota, suggested that fungal community of subsoil was more sensitive to organic manure. Therefore, the subsoil with cattle manure has more decomposition degree of SOC when compared with topsoil, and improving the stability of SOC.
The RDA and MRT analyses both showed that O-alkyl C was the most important factor in explaining the differences in microbial composition. In addition, PLFA indicators for Actinobacteria, Firmicutes, and Bacteroidetes were predominantly related with O-alkyl C and aromatic C forms that differentiated T2 from T2M based on MRT. When resources are plentiful, the phyla of Firmicutes are able to expand rapidly due to their copiotrophic features, which allow them to preferentially consume labile soil organic C pools (Fierer et al. 2007). And it was found that bacteria belonging to the group Bacteroidetes favour a C-rich substrate (Fierer et al. 2007). There was a positive correlation between the phyla Firmicutes and Bacteroidetes and manure-amended subsoil, which makes sense given that the SOC in the subsoil decomposed more rapidly following manure application. In addition, MRT showed that aromatic C and alkyl C separated T1 from T1M, with the former being mostly related with PLFA indicators for bacteria like Acidobacteria and fungi like Basidiomycota and Ascomycot. The phylum Acidobacteria includes many oligotrophic species, Acidobacteria, operating as resistant SOC decomposers, were found to dominate native soil organic matter-degrading communities after the addition of fresh organic matter to the soil, as demonstrated by the research of (Pascault et al. 2013). Ascomycota and Basidiomycota fungi were traditionally recognised as the two most common saprophytic decomposers in soil (Sanaullah et al. 2016). These fungi may be representative of the community's capacity to break down cellulose and ligno-cellulose (Sanaullah et al. 2016). Meanwhile, these bacteria and fungal were positive association with topsoil that has a greater alkyl C (recalcitrant C) proportion. Taken as a whole, the disparate associations between microbial community composition and soil C composition emphasize the necessity to take both into account simultaneously.