3 Results
3.1 Data overview
Thirteen studies were available for meta-analysis, and 9 studies
involved multiple cases. From these, we obtained a total of 70
correlations between erosion depth and crop yield (Supplementary
material Data S1), and 205 crop yield data with erosion treatments
(Supplementary material Data S2). Among them, maize, wheat, and soybeans
were subjects of 8, 50, and 12 studies, respectively (Figure 3). The
following erosion depths were included (Figure 5): 5 cm, 6 cm, 10 cm, 12
cm, 15 cm, 18 cm, 20 cm, 25 cm, 30 cm, 40 cm (9 cases), 50 cm (9 cases),
60 cm (6 cases), and 70 cm (7 cases). Both positive and negative
correlation coefficients were reported between erosion depth and yield
reduction, with r ranging from -0.4739 to 0.9999.
Geographically, research areas were located in Asia (34%), North
America (63%), and Africa (3%). None of the studies were located in
Europe or Australia. Relatively, soil erosion gets more attention in
high-yield areas. Both negative and positive values of the Pearson
correlation coefficient were reported for studies in Asia and North
America.
3.2 Meta-analysis
We found evidence that crop yield followed consistent patterns of change
in response to soil erosion worldwide. Crop yield negativity responded
to soil erosion no matter the intra-assay variance existed or not, which
supports our hypothesis. Overall, soil erosion had a significant
positive effect on crop yield reduction (estimate: 1.7861;
p<.0001; 95%CI [1.4396, 2.1327]), and a significant
negative effect on crop yield (estimate: -0.5523; p=.0002; 95%CI
[-0.8473, -0.2574]). Since crop yield is usually associated with
plant height and seed size, we did meta-analysis for the two indexes
using relevant publications (Supplementary material Data S3). The
results definitely showed that both of them significantly and negatively
responded to erosion. For Data S1, the effect was highly heterogeneous
among experiments (Qt=608.4312, p < .0001). The reliability of
this result was tested by a fail-safe number (Fail-safe N: 1432840). As
expected for a correlation between erosion and percent yield loss, there
was a significant residual heterogeneity (Qt=608.43, p<.0001)
in the hierarchical mixed-effects meta-analysis for grain type (Qt=
9039.7297, p<.0001), soil type
(Qt=8231.2124,p<.0001), measure (Qt=
8579.4537,p<.0001), latitude (Qt= 9031.5350, p <
.0001), and longitude (Qt = 9029.1165, p < .0001); we tried to
explain this with different modifiers. Grain_type (Qm= 0.9014, p =
0.6372), soil_type (Qm= 6.5215, p = 0.4803), measure (Qm= 5.7817, p =
0.2161), latitude (Qm = 0.0766, p = 0.7820), and longitude (Qm = 0.2347,
p = 0.6281) had no significant effect on effect size. There were no
significant differences between multiple levels of soil types, grain
types, and measures (Figure 3).
All erosion depths except 5 cm and 6 cm exhibited statistically
significant effects on crop yield (p < 0.05) (Figure 5).
Effect sizes in 8.8% of cases were negative. When erosion depth was
higher than 20 cm, crop yield was reduced. When erosion depth was higher
than 60 cm, the effect sizes (lnRR) were similar (Figure 4). The
cumulative effect size of erosion at 5, 10, 15, 20, 25, and 30 cm was
-0.17, -0.38, -0.78, -0.76, NA, and -1.01, respectively, indicating a
decreasing trend with increasing erosion depth (Figure 5). The effect
degree of erosion was significantly different between 5 - 10 cm and 10 -
15 cm erosion depth intervals (Figure 6).