1. INTRODUCTION
1.1 Soil erosion, A horizon thickness and crop yield
Soil erosion is a serious problem due to inadequate and inappropriate
management of land (Benaud et al., 2020). By 2011, 1.3 million
km2 of China’s lands were still suffering from water
erosion (Ministry of Water Resources of China, 2013). The area for
moderate erosion (1.9 to 3.7 mm/a), strong erosion (3.7 to 5.9 mm/a),
very strong erosion (5.9 to 11.1 mm/a) and extremely strong erosion
(≥11.1 mm/a) was 351400 km2, 168700
km2, 76300 km2 and 29200
km2 by 2011, respectively.
Given that the A horizon is located at soil surface, it is most affected
by human activities. The A horizon is richer in organic matter and more
fertile than the underneath horizons. In the Black Soil Region (BSR) of
China, the soil organic matter content (SOM) of the A horizon is
significantly higher than those of the underneath horizons (Figure 1).
The mean SOM of the A horizon is 3.5%, 2.5% and 4.3% for black soil,
chernozem and dark brown soil, respectively. On average, SOM of the A
horizon is 2.4 times that of the underneath horizons. This ratio can be
more than 10 for some newly reclaimed dark-brown soil (Figure 1C,
profile 2).
The A horizon is the main source of nutrients for crops. For soils
without a B horizon, such as is common in the BSR, the A horizon serves
as the main source of water as the available water capacity (AWC) of
mollic epipedon is extremely high while C horizon material is extremely
low. The crop yield is, therefore, closely related to A horizon
thickness (\(\text{Th}_{A}\)). Liu et al. (2013) found that for every 1
cm decrease in A horizon thickness in the BSR, there was a corresponding
2% decrease in crop yield. The crop yield declined
30%~40% when the A horizon eroded away completely in
the humid eastern United States (NSESPRPC, 1981). Some studies indicated
that crop yield will decline sharply as \(\text{Th}_{A}\) falls below a
critical threshold (Wen and Easter, 1987; Sparovek and Schnug, 2001;
Fenton et al., 2005; Zhang et al., 2006; Sui et al., 2009; Feng et al.,
2018). Soil desurfacing experiments in the BSR showed the critical
thickness to minimize crop yield loss is about 20 cm (Figure 2). When\(\text{Th}_{A}\) is above 20 cm, crop yield declines by 7% with every
10 cm loss of \(\text{Th}_{A}\), but 17% when \(\text{Th}_{A}\) is
below 20 cm. Mollisols in the USA similar to the soils in the BSR showed
less dramatic trends as the BSR, with crop yields declining by 3% for
every 10 cm loss for \(\text{Th}_{A}\) above 25 cm. The ratio will
become 5% as \(\text{Th}_{A}\) is below 25 cm (Fenton et al., 2005).
The critical thickness of 20~25 cm is mainly due more
than half of crop roots being distributed in the top 20 cm of soils (Fan
et al., 2016). Compacted layers will motivate roots to grow horizontally
and cause almost all roots to distribute in the top 20 cm (Cannell et
al., 1985, Ball-Coelho et al., 2010). This phenomenon is prevalent and
significant in the BSR and other parts of China. For 82% of cornfields
in China, 90% of the roots are distributed in the top 20 cm (Wang et
al., 2018). As a result, in soils with original A horizon thicker than
20 cm, most roots will distribute in the fertile and high AWC A horizon.
Such crops will grow in a more favorable condition. In contrast, in
soils with an A horizon thinner than 20 cm, roots will have to
distribute in the infertile and low AWC underneath horizon or be limited
in nutrient and water availability. Thus, crops will grow in a stressed
condition.
The situation is more complicated for ploughed croplands. Ploughing can
mix materials from infertile horizons underneath with the A horizon
(Figure 3), thereby diluting the A horizon benefits. The diluted A
horizon will be lighter in color, less fertile and lower in AWC. Even
with a thickness 20 cm the A horizon becomes less productive (Figure 1
and 3). The decline in crop yield is obvious and irreversible in large
part to loss of AWC and potential increase leaching of the fertilizers
applied (Figure 2). For those soils with underneath horizons mainly
composed of sands or rocks, soil productivity loss will be even more
dramatic as the A horizon is eroded away and become negligible when the
A horizon is gone. This explains why Liu et al (2013) found a 20% loss
in crop yield per 10 cm decrease in A horizon. Once the A horizon is
gone or diluted by C material underneath mixing, the degradation is
irreversible.
The erosion hazard should not be evaluated merely by erosion rate but
also according to the thickness of the A horizon. Suppose region 1 has a
higher erosion rate (2 mm/a) than region 2 (1 mm/a), but has a thicker A
horizon (100 cm) than region 2 (30 cm). In less than 300 years, region 2
cannot be farmed due to soil erosion, while region 2 is still
productive. A better index for evaluation is soil life expectancy (SLE)
expressed in years (a). SLE is the time until a critical soil thickness
for sustaining crop production is reached (Sparovek and Schnug, 2001;
Paroissien et al., 2015). SLE of A horizon (SLEA) can be
calculated with following equation.
\(\text{SLE}_{A}=\frac{10\left(\text{Th}_{A}-\text{Th}_{\text{CR}}\right)}{E_{r}}\)(1)
Where \(\text{Th}_{A}\) is A horizon thickness (cm),\(\text{Th}_{\text{CR}}\) is critical horizon thickness (cm), and\(E_{r}\) is the soil erosion rate (mm/a). \(\text{Th}_{\text{CR}}\) is
related to root distribution and may vary among crops and regions. A\(\text{Th}_{\text{CR}}\) of 20 cm was used for Crotalaria junceain Brazil (Sparovek and Schnug, 2001). A SLE of 50 indicates that crop
yield will decline sharply, after 50 years. SLE has already been applied
to evaluate erosion hazard in Brazil and France (Sparovek and Schnug,
2001; Paroissien et al., 2015). Reducing crop yield is the ultimate soil
erosion hazard for agriculture regions. .The objective of this study was
to predict A horizon thickness and crop yield reduction under different
soil conservation scenarios for the BSR during 2020 to 2200 to evaluate
erosion hazard.