The effect of different crops and slopes on runoff and soil erosion

Accelerated soil erosion rate is a major cause of land degradation and unsustainable agriculture. It can lead to loss of soil fertility and reduce agricultural yield and, hence, farmers' incomes (Novara et al. 2013; Colazo & Buschiazzo 2015; Yan & Cai 2015). Soil erosion is also a serious and challenging environmental issue related to land management worldwide. It is a complex natural process altered by anthropogenic activities including land clearance, agricultural practices, surface mining, construction and urbanization (Krasa et al. 2005; Illangasinghe & Hewawasam 2013; Hewawasam & Illangasinghe 2015). Studies have shown that the key factors controlling soil erosion on hillslopes are erodibility, slope, and land use cover, and that changing these factors can reduce soil erosion (Diyabalanage et al. 2017).

High erosion rates from agriculture land are usually due to lack of vegetation cover, which is a key factor to understand in soil erosion (Ola et al. 2015; Zhao et al. 2016). The structure of soil without vegetation is easily broken by the impact of raindrops, increasing runoff and soil erosion rates (Cerdà 2000). Much research on runoff and soil erosion has been based on simulated natural rainfall in laboratories as the conditions are easier to control (Cerdà et al. 2016; Prosdocimi et al. 2017; Vaezi et al. 2017). There has also been research at field stations where conditions may be closer to natural (Tanner et al. 2016; Anache et al. 2017). Although there has been much work on runoff and soil erosion from agricultural land, comprehensive studies of the impacts of slope, rainfall intensity, rainfall and leaf area index (LAI) are uncommon.

Cultivation of sloping farmland is common in western and central Jilin Province, China. Excessive deforestation for farming, and serious disconnection between crop types and soil and water conservation measures, have caused severe soil erosion. It is very important, therefore, to study different crop types there. In this paper, the effects of rainfall, rainfall intensity, slope and LAI, on runoff and soil loss from sloping farmland are considered. Field experiments were carried out at Xingmu experimental station, Jilin, and the effects of different crops assessed on water and soil loss from farmland.

The study was carried out at the National Soil and Water Conservation Science and Technology Demonstration Park, Xingmu, south-central Jilin Province, China. Its coordinates are 42°17′40″ to 43°13′40″ N latitude and 124°51′22″ to 125°49′52″ E longitude. The soil is a clay loam, with 35.9% clay, 35.4% silt, and 28.7% sand, and bulk density 1.13 gm/ml. Annual sunshine typically exceeds 2,500 hours, the average temperature is 5.2 °C, and annual precipitation 658 mm. The experimental station is in a small watershed typical of the low mountain and hilly landform type, and is representative of the central part of Jilin Province.

The contribution of rainfall, rainfall intensity, slope, LAI, runoff, and soil loss for each plot was determined using the difference between the variables during the experimental runs. Comparison of the variables between the three crop types was done using the independent-samples t-test, and the effects of each on runoff and soil loss were determined using Pearson's correlation matrix (r).

The average annual rainfall from 1978 to 2008 and the rainfall in 2012, the year of the study, are shown in Figure 2(a), which shows that rainfall in 2012 basically conformed to the 30-year (1978–2008) annual average. The heaviest rainfall occurs in July and August, with rainfall intensity up to 78.31 mm/h (Figure 2(b)). The LAIs of maize and soybean were up to 3.73 and 5.69, respectively, and that of soybean always exceeds that of maize. The LAIs increased in the early period but began to decline later in the study (Figure 2(c)).

The soil loss and runoff from the plots are shown in Figures 3–5. With increased slope, the amount of soil loss and runoff increased from the maize plots. When the slope of the soybean plots increases from 3 to 5°, the amount of soil loss and runoff does not change greatly, but it changes significantly when the slope increases to 8°. Compared with the same slopes on maize and soybean plots, the soil loss and runoff from uncultivated plots can be higher by two orders of magnitude or more.

Table 2 shows significant positive correlation between soil loss, and rainfall intensity and slope, but there is no significant relationship between soil loss and rainfall. Only intense rainfall splashes much, destroying the soil structure and making it easier to move the soil away. Vaezi et al. (2017) showed that at 20 to 30 mm/h rainfall intensity, raindrop impact was the dominant factor controlling soil loss, while rainfall intensity below 20 mm/h has no significant soil erosion effect. Plant leaves can cushion the raindrops' fall, reducing splashing and thus soil loss (Römkens et al. 2002; Novara et al. 2011; Balota et al. 2016), and the correlation between soil loss and LAI is significantly negative (Table 2), so that the runoff from cultivated and uncultivated land is different. The steeper the slope, the more likely soil erosion is to occur, so the slope is positively correlated with soil erosion (Table 2) and this study's results are consistent with those of others (Defersha & Melesse 2012; Sajjadi & Mahmoodabadi 2015; Mahmoodabadi & Sajjadi 2016).

The relationship between different crops, and runoff and soil loss on different slopes, is shown in Table 3. During the experiments, the runoff and soil loss from uncultivated land were much higher than from cultivated plots, and soybeans gave better protection from runoff and soil loss than maize on the same slopes.

The study revealed the variation in runoff and soil erosion on different slopes under different crops in a small watershed in Xingmu. Both the slope and crop have significant effects, and the effect of the crop can be indicated by its LAI. Soybeans have a better effect on soil erosion than maize, as soybean's LAI is greater. The crop type should be considered when solving runoff and soil erosion problems.

All relevant data are included in the paper or its Supplementary Information.