3. RESULTS
3.1 Soil erosion rate (Ɛr)
For validation, Ɛr calculated for present conditions with the CSLE model (Equation 2) were compared with those measured by137Cs tracer method (ƐrCs) for the three BSR watersheds (Table 3). Each watershed was overlaid on the soil erosion rate map and mean Ɛr was calculated. The mean ƐrCs was 1.9 mm/a in Fang et al. (2005), 1.6 mm/a in Liu et al. (2008) and 1.2 mm/a in Fang et al (2012), respectively (Table 3). Mean predicted Ɛr was 1.8 mm/a, 1.4 mm/a, and 1.2 mm/a, respectively. The Ɛr was very close to ƐrCs for all three watersheds validating the Ɛr predictions of this study at watershed scale.
Currently, the mean soil erosion rate (Ɛr) across all the BSR was 0.90 mm/a. Distribution of Ɛr is positively skewed. About 70% of the sloping croplands had Ɛrvalues below 1 mm/a, while 20% had Ɛr values between 1 mm/a and 2 mm/a. Only 11% of the sloping croplands had Ɛr values above 2 mm/a. The Ɛr was linearly and positively correlated with slope gradient (Figure 6). Townships with Ɛr above 2 mm/a were mainly located in the northwest counties and southeast counties, where landforms are dominated by hills (Figure 7). The relatively steeper landforms with more plentiful rainfall resulted in the larger erosion rates. Townships with Ɛr between 1 mm/a and 2 mm/a were mainly located in steeper table-lands dominated by black soil. Townships with Ɛr below 1 mm/a were mainly located in gentler table-lands, with mainly chernozem soils.
Mean Ɛr values, in descending order, were 0.90 mm/a (Present), 0.79 mm/a (Straw), 0.64 mm/a (Contour), 0.39 mm/a (Combo 1), 0.31 mm/a (Combo 2) and 0.08 mm/a (No-till, Figure 8). Compared to Present, Ɛr was reduced by 12% by Straw, 29% by Contour, 57% by Combo 1, 66% by Combo 2 and 91% by No-till. No-till was the most effective practice in reducing erosion. No-till can reduce erosion by 93% to 97%, according to runoff plot studies in the BSR (Yang, 2019; Chen et al., 2019). The area of sloping croplands with Ɛr above 1 mm/a under present conditions was 31% and fell to 26% under Straw, 17% under Contour and became negligible under Combo 1, Combo 2 and No-till. The areas with Ɛr between 0.2 mm/a and 1 mm/a were rather close under Present, Straw, Contour, Combo 1, Combo 2, ranging from 49% to 62% but only 9% under No-till. The most significant difference was in sloping croplands with Ɛr below 0.2 mm/a. The area ratio was 11% under Present and increased to 15% under Straw, 21% under Contour, 34% under Combo 1, 48% under Combo 2, but leaped to 91% under No-till. It can be concluded that No-till is the most effective at saving A horizons, with Combo 2, Combo 1, Contour and Straw following in sequence.
3.2 A horizon thickness
Current \(\text{Th}_{A}\) averaged 31 cm. The \(\text{Th}_{A}\) in early stage of cultivation was reported to be 35 cm to 59 cm with a mean of 44 cm (CAS-FSI, 1980). Thus, 13 cm of A horizon has been eroded since cultivation began (Ɛr≈1 mm/a). The area ratio of\(\text{Th}_{A}\)≤20 cm, 20<\(\text{Th}_{A}\)≤30 cm, 30<\(\text{Th}_{A}\)≤40 cm, 40<\(\text{Th}_{A}\)≤50 cm and \(\text{Th}_{A}\)>50 cm was 8.4%, 36.0%, 46.6%, 6.1%, and 3.0%, respectively. More than 80% of the croplands have\(\text{Th}_{A}\) between 20 cm and 40 cm. \(\text{Th}_{A}\) was negatively correlated with \(\theta\) (Figure 6). Townships with mean\(\text{Th}_{A}\) less than 20 cm were mainly located in the northwest counties (ML and AR etc.), where landforms are dominated by hills (Figure 9). The relatively thinner original A horizon and larger erosion rate in the northwest counties resulted in the small\(\text{Th}_{A}\).
The area ratios of \(\text{Th}_{A}\)≤20 cm decreased under all scenarios (Figure 10). By 2200, the area ratios of \(\text{Th}_{A}\)≤20 cm, in descending order, were 54% (Present), 50% (Straw), 42% (Contour), 33% (Combo 1), 32% (Combo 2) and 12% (No-till). Croplands with\(\text{Th}_{A}\)≤20 cm are classified as “Damaged” in erosion hazard degree (Table 1). If the present situation was maintained, the ratio of “Damaged” will increase from the current 8% to 16% in 2050, 29% in 2100, 43% in 2150 and 54% in 2200. Therefore, the present practices must be improved.
The area ratio of 20 cm≤\(\text{Th}_{A}\)<30 cm increased initially and then decreased under all conservation scenarios, expect for No-till (Figure 10). This pattern was due to the distribution of\(\text{Th}_{A}\). At first, the loss of croplands with 20 cm≤\(\text{Th}_{A}\)<30 cm was offset by the erosion of croplands with \(\text{Th}_{A}\)>30 cm being incorporated into this class. As the >30 cm class became depleted, supplementing the 20-30 cm class could no longer offset the loss. No-till is an exception as the area ratio of 20 cm≤\(\text{Th}_{A}\)<30 cm kept on increasing under No-till at a rate of 0.3% per decade. This is because the erosion rate is very low and the offset can be sustainable. The area ratio of 30 cm≤\(\text{Th}_{A}\)<40 cm and\(\text{Th}_{A}\)>40 cm decreased under all scenarios. The decrease ratios were all below 0.5% per decade, with a minimum of 0.1% (No-till) and maximum of 0.4% (Present).
3.3 Erosion hazard degree
The area ratios presently for “Damaged”, “High hazard”, “Moderate hazard”, “Low hazard” and “No hazard” were 8%, 5%, 22%, 61% and 4%, respectively (Table 1). “Damaged” croplands were mainly located in northwest and southeast counties, with black soil and aeolian sand soil as the main soils (Figure 11). “High hazard” and “Moderate hazard” croplands were mainly located in northern counties with black soil and dark-brown soil as the main soils. “Low hazard” croplands were mainly located in eastern counties with black soil and chernozem as the main soils. “No hazard” croplands were mainly located in southern counties with chernozem and black soil as the main soils. Sadly, 8% of sloping croplands in the BSR are already damaged and 27% will be damaged in 100 a. Note that these are very conservative estimates as they account for sheet and rill erosion only and do not include gully erosion or wind erosion.
3.4 Future corn yield
The relative corn yield decreased under all soil conservation scenarios (Figure 12). The decrease rate under Straw was highest (1.3% per decade). In this scenario, straws were shredded and incorporated into tillage layer (20 cm) in the fall. Incorporation of straw can provide some reduction in rill erosion by increasing the resistance of the soil to erosion (Van Liew and Saxton, 1983; Brown et al., 1989; Wei et al., 2013; Yang, 2019). However, incorporation in fall provides limited erosion protection during the winter and spring when erosion can be significant. According to Zou et al (2016), Straw can reduce the corn yield by 11%. This ratio will increase to 16% if the tillage layer is 15 cm thick. The reduced but still rather high erosion rate under Straw (0.79 mm/a), together with its adverse effect on crop yield, results in the largest decrease rate of crop yield among all scenarios. By 2200, the corn yield will be 77% of current value. The decrease in crop yield under Present ranked second highest (0.8% per decade). By 2200, the corn yield will be 85% of current value. Nearing et al. (2017) recognized the problem as being the use of conventional tillage with long rows and concluded that “unless major changes are made to tillage and management (residue) practices” the land will not remain under production. This study places time-frames on the soil life expectancy and spatial distributions of the degree of soil erosion hazard to assess the effectiveness of different conservation practices.
Despite the effectiveness of no-till at controlling erosion, the yield decrease under No-till ranked third highest. The corn yield abruptly decreased to 88% of present after application of No-till. Nearing et al. (2017) acknowledged that no-till does not work well in all environments and can have limitations when soil moisture or soil temperature becomes an issue. No-till was found to decrease corn yield by 15.7% on sloping croplands in the cold BSR by Chen et al. (2011) and 26.8% by Zou et al. (2016). Negative effects of no-till on crop yield are also reported in similar cold regions, such northern America and Canada (Defelice et al., 2006). In cooler BSR, the mean soil temperature at 20 cm depth in May is mostly below 12℃, with minimum below 8 ℃ (Figure 4B). In contrast, the mean value was mostly above 12℃ in warmer BSR, with maximum above 15 ℃. Studies in warmer BSR areas (Figure 4) indicate that no-till can slightly increase corn yield (Zhang et al., 2010; Zhou et al., 2015).Thus, the cause of the abrupt yield decrease predicted in this study for no-till is likely an issue with the low soil temperatures in the BSR.
The corn yield decrease rate under Contour, Combo 1 and Combo 2 ranked fourth, fifth and sixth, respectively. By 2200, crop yield decrease was only 4% under Combo 1 and Combo 2. Xu et al. (2018) recommended flat tillage systems for the BSR because in laboratory test it exhibited lower runoff and soil loss than ridge systems whether on contour or up and down slopes. An alternative that may improve the performance of conventionally tilled ridge systems is the use of furrow dikes which are essentially small check dams at intervals along the furrows. Strip tillage, in which residue is maintained on the top of rows (typically with flat tillage but can apply to ridge systems), shows great potential for erosion control (Licht and Al-Kaisi, 2005; Shi and Mi, 2018). However, none of these systems have sufficient in situ experimental data on erosion and crop yield in the BSR to be included in this analysis.
3.5 Optimum soil conservation practice
For “Damaged” croplands, current soils not only need to be saved but also improved. No-till with residue management is the optimum practice, as it can both reduce erosion and increase fertility. For those located in north BSR where no-till is less productive, manures could be used to offset residue removal and restore fertility (Singer et al., 2004). For those with aeolian sand soils, soil amendments, such as winter cover crops, manures, peat (Li et al., 2004) and biochar (Uzoma et al., 2001; Bruun et al., 2014) could improve soil structure and fertility. For “High hazard” croplands, conservation practices should be applied as soon as possible, as A horizons will all become thinner than 20 cm in just 20 years. These croplands are a priority in conservation planning. Contour farming is not recommended for them, as it only slightly increases the SLEA (Table 4). The reason is that “High hazard” croplands are mainly distributed on steeper areas, where contour farming has limited effectiveness. Contour farming can only reduce erosion by 43% on a 5°slope and by 26% on a 10°slope. In contrast, Combo 1, Combo 2 and No-till can all increase the SLEA to more than 100 a. These practices are recommended. Among them, Combo 2 may be the optimum practice. Compared to Combo 1, Combo 2 is less costly as less terracing is needed. Compared to No-till, Combo 1 is more productive. For “Moderate hazard” croplands, practices should be applied before 2040 to keep A horizons thicker than 20 cm. No-till is not recommended even though it can increase SLEA to more than 3000 a since most “Moderate hazard” croplands are located in the north BSR where no-till is less productive. For, “Low hazard” and “No hazard” croplands, present practices are acceptable although they should be used with care to maintain soil health. However, additional practices are necessary if erosion types other than sheet-rill erosion are prevalent or a better crop yield is desired.
3.6 Optimum conservation schedule
An optimum conservation schedule is proposed with the following principles. The plan is spatially precise so that it can tell where to act. The spatial resolution used here is 100 km2, which is approximately the size of townships. The plan is temporally precise so that it can tell when to act to make sure all soils have\(\text{Th}_{A}\) maintained above 20 cm. In our schedule, practices are applied immediately for the “Damaged” and “High hazard”, before 2040 for the “Moderate hazard”, before 2120 for the “Low hazard” and no changes applied to the “No hazard” areas. The start year for action depends on the township’s maximum erosion hazard degree. Sequential actions by township will reduce workload for related people in other counties and earlier-acting townships will provide training for later-acting townships. Based on these principles, an optimum conservation schedule is suggested (Figure13). About 46% of townships should act immediately, while 17% should act before 2040. Due to logistic, economic, social and other reasons, this schedule is an idealized one that can serve as a target/goal for government officials and land managers. However, the following alarm must be sounded, 180 km2 croplands will become “Damaged” and at least 0.8% of corn yield lost irreversibly with every 10 year delay.