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.