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.