<Fig. 2>
Descriptive statistics of the SOC contents on the three sampling dates
(Fig. 2) showed that the mean topsoil SOC contents increased from 13.34
g kg-1 in 1980 to 16.99 g kg-1 in
2000, and then decreased to 14.67 g kg-1 in 2015. The
net increment of SOC contents during the period of 1980-2000 was 3.65 g
kg-1, while the net decrement during the subsequent 15
years (2000-2015) was 2.32 g kg-1. The ranges between
the minimum and maximum values for SOC1980,
SOC2000, and SOC2015 were 35.22 g
kg-1, 29.29 g kg-1, and 30.62 g
kg-1, respectively. The coefficients of variation (CV)
for SOC1980, SOC2000, and
SOC2015 were all moderate (CVs ranging from 25% to
36%). However, all CVs exceeded 20%, indicating considerable
variations in the SOC contents among the three sampling dates. The value
distribution of SOC1980 was slightly positively skewed,
with a skewness of 1.39. While for SOC2000 and
SOC2015, their value distributions were symmetrical,
with small skewness values of 0.36 and 0.06, respectively. The normal
Q-Q graphs further indicated that SOC2000 and
SOC2015 were both approximately normal distributions,
while the value distribution of SOC1980 was
approximately normal after 2 outliers were removed (Figure S1).
The experimental variograms of SOC
in the three sampling dates were presented in Fig. 3. Fig. 3 showed that
the nugget to still ratios for the SOC contents in 1980, 2000, and 2015
were 38%, 55%, and 84%, respectively, indicating an overall trend of
decreasing SOC spatial autocorrelation over the past 35 years. The range
of the spatial autocorrelation continuously decreased from 38km in 1980
to 33km in 2015 (35km in 2000), and the R-squared values for the fitted
variogram models of SOC1980, SOC2000,
and SOC2015 were 0.95, 0.83, and 0.29, respectively.
Therefore, compared to the spatial structures of SOC1980and SOC2000, the relatively short range of spatial
autocorrelation and the worse goodness of fit for
SOC2015 indicated that external factors, such as
anthropogenic activities, may have a significant impact on the spatial
variability of SOC2015 in the area.