4 Discussion
4.1 Response of crop yield to soil
erosion
Our analysis supported the overarching hypothesis and illustrated that
crop yield varied significantly with an increase in soil erosion
thickness. This was in agreement with a previous integrated analysis of
soil types, regions, and crops, which showed that soil erosion led to an
average global productivity loss of 0.3% (den Biggelaar et al., 2004).
Similarly, Wang et al. (2009) showed an average soybean yield reduction
of 14.9% for every 10 cm increase in eroded soil depth. Pimentel,
(2005) supported that nutrients deficient soils produce are 15 to 30%
lower crop yields than non-eroded soils. In this study (Figure 5), three
erosion depths (i.e., 6, 12, and 18 cm), data from Allen et al. (2009)
were different than the rest. This may have been due to a reduced
erosion effect as crop yield was reported 16 years after erosion
simulation experiment. With these data removed, the cumulative effect
size showed a consistently decreasing yield with increasing erosion
depth. The early scientific evidence of the detrimental effects of
erosion on crop productivity was considered sufficient (Pierce and Lal,
2017).
We found that crop yields did not change significantly when erosion
affected less than 5 cm of soil (Figure 5), and this agreed with the
results of previous studies (Sui et al., 2009; Rejman and Iglik, 2010).
For example, slightly eroded soil (5 cm of topsoil removal) did not
affect crop yields (Sui et al., 2009; Rejman and Iglik, 2010). Lack of
effects may be due to the fact that similar levels of available
nutrients are stored in both eroded and non-eroded topsoil (0 to 10 cm
depth) (Christensen and McElyea, 1988; Larney et al., 2000; Larney et
al., 2003). Many processes contributing to accumulation of nutrients
occur in the surface soil regardless of erosion. For example, storage of
decaying leaves and stems (David and Michael, 2013), restorative
tillage, and addition of chemical fertilizers (Wang et al., 2009; Guo et
al., 2010). Consequently, plant nutrients, such as alkali nitrogen,
available phosphorus, and available potassium are mainly concentrated in
surface soils (Huang, 2016). While water-stable soil aggregates, SOM,
and particulate organic matter are found in the top 15 cm soil was not
significantly different (Allen et al., 2011). In addition, bacterial
biomass in the first 6 cm of soil was unaffected by the removal of 0 - 6
cm of topsoil (Allen et al., 2011).
4.2 Response of crop yield to other
factors
No significant difference was observed between the cumulative effects of
multiple levels of factors like Grain type and Measure in our study.
This is different from other studies. For example, Sui et al. (2009)
found that yield reduction in maize was greater than that in soybean.
This was in agreement with the results of Lin et al. (2019), who also
observed that maize yields decreased significantly with increasing
erosion, but soybean yields did not. But there are obvious differences.
Figure 3 is worth mentioning that soybean and maize yields are more
sensitive to soil erosion than wheat yields. This finding indicates that
if soil becomes less productive for one crop, it may achieve more
productivity for another that is better able to exploit adverse or
resource-limiting conditions.
In addition, several studies indicated that addition of manure or
chemical fertilizers into eroded soils could achieve the same yield
level of non-eroded soil (Schmitt et al., 2001; Hichman, 2002; Azeez,
2010; Zhou et al., 2012). Especially, a general conclusion drawn by
Pierce and Lal, (2017) from 50 years of erosion and productivity
research in the United States is that management inputs were sufficient
to restore production to levels of undisturbed soils. Sui et al. (2009)
demonstrated the value of manure to restore eroded soil productivity for
corn production. For example, manure addition reduced the effect of
topsoil loss, and increased corn yield by 57% in 2005 and 37% in 2006
in a 20-cm topsoil removal treatment (Sui et al., 2009). However, our
results indicated that no matter what the fertilizer/manure measures
used, crop yield was clearly reduced when soil was severely eroded
(erosion depth > 20 cm) (Figure 4); in this case, land
productivity could not be restored. Wang et al. (2009) showed that
yields following fertilizer application did not reach the levels
obtained in non-eroded soil. Further, the addition of fertilizer did not
offset yield loss in the absence of topsoil (Allen et al., 2011). This
may be due to a substantial loss of organic matter, which is mainly
distributed in topsoil (0 - 20 cm) (Gu et al., 2018; Allen et al.,
2011). Further, eroded soil contains about three times more nutrients
per unit weight than that in the remaining soil (David and Michael,
2013). In severe erosion conditions, soils without adequate nutrients
may limit plant growth, and result in yield reduction. It has been also
shown that crop yield declined with an increase in topsoil removal depth
(den Biggelaar et al., 2004; Wang et al., 2009; Gao et al., 2015). A
decline in SOM, total nitrogen, and saturated water capacity may be the
main soil factors affecting crop productivity (Sui et al., 2009; Allen
et al., 2011; Gou et al., 2020). Meanwhile, significant reductions in
soil clay content, significant increases in sand content, and
significant decreases in water retention capacity were observed with
increasing erosion levels (Duan et al., 2016; Lin et al., 2019).
Decreases in nutrient levels (Wang et al., 2009), and loss of topsoil
and root-restrictive layer (Gao et al., 2015) were the primary reasons
for the decline in soil productivity resulting from soil erosion.
4.3 Crop yield response curve to
erosion
Bakker et al. (2004) classified yield response curves to erosion into
four groups: linear, convex, concave, and S-shaped. Linear curves were
widely used to assess the effect of soil erosion on crop yield. However,
we found significant differences in the variability in effect sizes
across erosion depths of 5 cm to 15 cm. Our results indicated a larger
negative response of yield in erosion depths of 5 - 15 cm than at other
erosion depths. These findings support the results of Wang et al. (2009)
who showed that the initial rate of decline in productivity due to early
stages of soil erosion (10 cm - 20 cm) was high and significant at 15 cm
of soil removal (Gorji et al., 2008). Gao et al. (2015) also showed the
reduction in soybean yield was highest when the first 10 cm soil was
eroded. Thus, linear relationship cannot be used to describe the
relationship between erosion and crop yield. Likewise, Christensen and
McElyea (1988) suggested that a linear relationship was a simplistic
representation of a yield-topsoil depth relationship, as constant slope
assumed that loss of each incremental depth of topsoil exerts a constant
incremental decrease in soil productivity. Den Biggelaar et al. (2001,
2004) pointed out that even though the relationship between soil
degradation and soil productivity was linear, linear relationships may
not always represent reality; the effect of soil erosion on crop
productivity depends to a large extent on intrinsic soil
characteristics, especially soil loss tolerance or effective soil depth.
When there is no soil to erode, crop yield is 0, or reduction in crop
yield is 1. Based on various erosion depth-yield studies, our result
preliminarily suggested that the crop yield response curve was convex
when erosion depth was < 5 cm, and concave in other cases.