4.1. Response of soil chemical and microbial properties to cultivation and N fertilization
In this study, time lapse (cultivation years) was found to be influential to most soil properties, including ECe, pH, SOC, CEC, AP, MBC, MBN, CMR, NMR and PNR (Table 5). Among these, soil ECe had significantly negative correlation with most of other soil properties, indicating that the changes of soil chemical and microbial properties were mostly ascribed to the decline of soil ECe induced by consecutive paddy rice- winter wheat rotation. Actually, most of previous literatures (Canfora et al., 2014; Chen et al., 2017) reported that soil salinity was a key determinant for soil microbial communities in desert and coastal ecosystems. Soil organisms, available nutrients, microbial diversity, biomass and metabolic activities linearly decreased along the increase of salinity gradient. This coincided with the findings of this study.
In addition to the change of soil ECe caused by rotation system, fertilization also played an important role in temporal changes of soil chemical and microbial attributes which was essential to transformation and cycle of soil organic matter and nutrients. Table 5 showed that SOC, TN, AN, AP, MBC, MBN and NMR exhibited clear response to N fertilization. Li et al. (2016) reported that N fertilization affected bacterial communities by strongly driving the shifts of dominant bacteria within an intensive greenhouse ecosystem. Chen et al. (2017) found that N fertilization had significant influence on soil microbial metabolic activity (MMA), and medium (350 kg ha−1) N fertilization coupled with medium amount of saline water irrigation could obtain the optimal MMA. Dong et al. (2015) reported that soil microbial community composition, soil microbial biomass (C and N) and enzyme activities were responsive to N fertilization, but responses often varied depending on N quantity added, and combined additions of N and P fertilization was suggested to promote soil fertility and microbial activity. In a more rigorous study, Liang and MacKenzie (1996) tracked the fate of nitrogen using15N fertilization, and revealed that higher N fertilization rates above normal increased microbial biomass N immobilization with greater N release, and high N fertilization rates significantly increased both the magnitude of soil microbial biomass N and microbial fertilization N recovery in the soil microbial biomass. This was consistent with our findings in this study that high chemical N fertilization rates increased both microbial biomass N (MBN) and N mineralization rate (NMR), and resulted in higher soil total nitrogen and available nitrogen.