Soil organic matter is a key contributor to soil due to its capacity
to affect plant growth indirectly and directly [45]. As SOC and
TN constitute heterogeneous mixtures of organic substances, they
are widely used as the main parameters for evaluating soil fertility
[46]. Meanwhile, human activities such as fertilizer practices and
cropping systems play a key role in the regulation of C and N
contents in agricultural soils [47]. In our experiments, SOC and
TN contents increased considerably in the fertilization treatments
compared with CK, especially in OM and NPK treatments (Fig.3),
suggesting that organic and chemical fertilizer are beneficial to the
accumulation of soil organic matter and thus improves soil fertility.
This may be because both the application of organic manure and
chemical fertilizer can improve soil aggregation, soil water
retention, and reduce bulk density of the soil in the plough layer,
promoting crop growth and the return of more root residues to the
soil [48]. Under the SR application, SOC content was increased
gradually with time, suggesting that the continuous SR supply had
a positive effect in sustaining SOC, in accordance with the finding
of Nie [49]. In farmland ecosystems, pig manure is easy to
accumulate in soil and has lower ammonia losses than other
fertilizers [33], so the TN concentration was significantly increased
by continual annual OM applications compared to the other
treatments (Fig.4B).