Paddy soil is a substantial source of methane emissions, contributing 31 to 112 Tg/y annually, accounting for 9–19% of worldwide methane emissions [31]. Soil moisture is one of the important factors that influences methane production and has a crucial influence on CH4 formation. The results of the present study showed that methane fluxes were lower under 60% WFPS conditions, while methane emissions were higher in flooded soil compared with 60% WFPS. Our findings are consistent with those of previous research, which found that a high level of soil moisture led to increased methane fluxes [32]. Higher levels of water-filled pore space in soil led in higher emissions of methane [32, 33]. In recent study, methane fluxes were high as compared with those of the 60% WFPS moisture in this study. The methanogenic activities in soil rise with soil moisture content, but methanotroph activities drop with soil oxidized zone reduction [34]. We believe that in the present study, increased soil moisture caused anaerobic conditions, which promoted the activities of methanogenic rather than methanotrophic bacteria, resulting in the detected methane fluxes [35]. Increased soil moisture content assists in the breakdown of native SOM, which acts as a stimulant for methanogens to produce CH4 [34, 35].
The presence of both aerobic and anaerobic microsites in the soil is directly correlated with its water content, about 60–70% WFPS provides suitable conditions to assist nitrification and denitrification at the same time and thus produce more N2O emissions [36]. According to our findings, the increase in N2O emissions in 60% WFPS soil was likely because of the higher nitrate concentration in the soil and the reduction in NH4+-N during nitrification processes in all treatments. Microbial nitrification and denitrification processes use soil ammonium and nitrate nitrogen as substrates, resulting in the production of nitrous oxide as an intermediate product. In 60% WFPS soil moisture content, the nitrous oxide emissions decrease [37]. Higher water-filled pore space improves anaerobic soil conditions, which encourages denitrification and converts nitrous oxide into nitrogen, resulting in lower or no nitrous oxide emissions [38–40]. In flooded soil, a vital process known as denitrification utilizes nitrogen oxides as substitute electron acceptors in the absence of oxygen. In this particular research, at 60% WFPS, more nitrous oxide was produced than in flooded soil (Figs. 1, 2). Flooding often creates an anaerobic environment in the soil and alters the chemical and biological processes that limit organic carbon and nitrogen mineralization, subsequently lowering substrates for N2O emissions [41]. Our results are consistent with the findings of Shang et al. who reported low N2O emissions from flooded soils [42]. The findings of previous study observed the insignificant N2O emissions from flooded soils, are in line with our results. Flooded soils are usually referred to as anaerobic because the water-filled soil pores limit the amount of oxygen available [42]. The explanation for this is that the majority of denitrifying bacteria are facultative anaerobes, meaning they prefer to accept oxygen as an electron acceptor but will also absorb nitrogen oxides as an electron acceptor in the event that oxygen becomes scarce [42, 43]. N2O emissions increased during the incubation study's early phases and reduced as it carried on.
According to Fig. 2 of the current investigation, N2O emissions were slightly lower in the CK treatment than in the NPK and NPKM treatments. According to Shaaban et al. (2015) N2O emissions were much lower at 55% WFPS than at 90% WFPS [39]. The amount of SOC breakdown produced by variable soil moisture levels might explain the differences in cumulative N2O emissions between moisture level treatments [43]. Furthermore, the 60% WFPS treatment provided aerobic soil conditions, but the flooding conditions created anaerobic soil conditions. Increasing the moisture level of soil from 60–100% resulted in a significant reduction in nitrous oxide (Fig. 2), which is consistent with previously reported outcomes [44].
Soil moisture plays an important role in regulating N2O emissions [45]. In this particular study, our results demonstrated that significantly lower N2O emissions were recorded in the CK than in the NPK and NPKM treatments (Fig. 2). The various magnitudes of SOC breakdown generated by varying soil moisture levels were primarily responsible for the differences in cumulative N2O emissions between treatments [44, 45]. In addition, there were aerobic conditions under 60% WFPS, while flooding under WFPS resulted in anaerobic conditions. It is well established that repeated aerobic and anaerobic conditions result in nitrification and denitrification, respectively [45, 46]. Previous research showed that lower N2O emissions were produced under nitrification than denitrification [46]. Our findings demonstrated that increasing the soil moisture content from 60–100% WFPS resulted in a considerable reduction in N2O emissions (Fig. 2), which is consistent with prior research findings.
Soil moisture is also linked to organic carbon breakdown and solubilization, releasing easily accessible organic carbon. For development and metabolism, soil microorganisms require easily accessible organic C [47]. The dissolved organic carbon and microbial biomass carbon stocks were greater under flooding conditions than under 60% WFPS (Table 1, 2), showing that soil moisture promoted the solubilization and decomposition of indigenous organic matter. The favourable relationship among CH4 emissions, DOC and MBC was further highlighted by correlation analysis. The results revealed that moisture content elevated soil pH from 5.3 to 5.7, 5.6, and 5.8 in the CK, NPK, and NPKM treatments, respectively. On the other hand, the soil pH of the 60% WFPS treatments declined throughout the incubation period. The present study's findings aligned with those of recent investigation, which demonstrated that reification in which two moles of protons are produced for every mole of NH4+ oxidized to NO3is the reason of the significant decrease in soil pH [48].
One important metric for assessing the quality of soil is the biomass of soil microbes. SMB is less resistant to soil management practices and environmental variables than soil organic matter [49]. Although SMB is a small portion of OM, so it plays a critical role in processes such as soil nutrient cycling, transformation of soil organic matter and insoluble materials [38–40]. Similar to the findings of the present study, the enriched stocks of microbial biomass under long-term fertilization have been found to exhibit increased metabolic activity of microorganisms [50]. In accordance with previous studies, long-term inorganic fertilisation combined with manure (NPKM) increased microbial biomass carbon and nitrogen as compared to the inorganic fertiliser and control treatments (Table 2). According to previous research, anthropogenic C input has effectively increased microbial biomass carbon and nitrogen due to strong activation of microbes under high soil carbon concentrations [23]. lower microbial biomass ratio between microbial biomass carbon and phosphorus may drive soil microorganisms to release nutrients and increase the availability of nitrogen and phosphorus pools [51, 52]. Similar to the findings of the present study, also found a strong link between microbial biomass stoichiometry and nutrient inputs in the soil [53]. In this study, compared with the NPK and NPKM treatments, the control (CK) treatment intensely reduced MBP (Table 2). Dai et al. (2019) discovered in an earlier investigation that N:P stoichiometry is the primary regulator of microbial biomass phosphorus. They also reported that adding phosphorus over time improved microbial phosphorus immobilization by lowering the relative abundance of phosphorus-depleted microbial communities [54].
Current study suggested that combining organic and inorganic fertilizers increased the soil SOC content, which influenced soil available nutrients and microbial biomass stoichiometry and its ratios (Table 2). These findings were in line with earlier research on the issue [53, 54, 55]. Additionally, in our present study, compared with the NPK and NPKM treatments, the control treatment substantially reduced MBP (Table 1, 2).
Compared with the NPK and CK treatments, application of inorganic fertilizers combined with manure (NPKM) significantly increased enzyme activities (Fig. 4), with the NPKM treatment showing the greatest increase in enzyme activities. These increases could be attributed to the use of a combination of inorganic fertilizer and manure [53]. According to a prior study, pig dung coupled with inorganic fertilizers substantially boosted extracellular enzymatic function in rice when in comparison with inorganic fertilizers and manure applications alone [55]. Increased soil acidity could contribute to reduced enzyme activity under inorganic fertilizer. The inorganic nitrogen input lowered soil pH, which resulted in a drop in soil microbial activity and influenced the quantity of the phosphatase-solubilizing microbial community [54–56].