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.