ABSTRACT
Soil erosion decreases topsoil depth, and hence soil fertility and crop productivity in agricultural systems. However, it is not clearly elucidated how different crop species adapt to the soil erosion regarding root function, nutrient uptake and rhizosphere biochemical properties, which is pivotal to cropping strategy under soil erosion. We established three simulated erosion severities with topsoil depths of 10, 20 and 30 cm in a Mollisol farmland under a maize-soybean rotation system with no-tillage. After three consecutive years of field experiment, the decrease in topsoil thickness from 30 to 10 cm resulted in 9−22% of decrease in maize yield but not soybean. Compared to the 30 and 20-cm topsoil thickness, the 10-cm topsoil significantly lowered root and shoot biomass of maize at the jointing (V7) and milk stages (R3) and of soybean at the mid-seed filling stage (R6). Compared to the 30-cm topsoil, the 10-cm topsoil decreased available nitrogen and phosphorus in soil by 42% and 36% under maize, and by 25% and 19% under soybean, respectively, while the shallow topsoil also decreased N, P and K uptake per unit root length with the decreases being less for maize than soybean. Compared to the 30-cm topsoil, the 10-cm and 20-cm topsoil significantly increased the activities of urease, phosphatase and invertase in maize-grown soil, but not in soybean-grown soil except for the activity of urease in 10-cm topsoil. Maize was more sensitive to soil erosion than soybean due to the greater decreases in soil nutrient availability and its capability of nutrient uptake. The greater stimulation of nutrient mineralization processes in soil did not alleviate the nutrient constraint to maize yield under severe erosion conditions.
Key words : soil erosion; topsoil depth; black soil; soil enzymes
Introduction
It is estimated that 15.1% of global land suffers degradation, of which 83.6% results from soil erosion (Lal, 2001; Gao et al., 2015). Soil erosion is a gradual process of removal, transport and deposition of topsoil caused by wind and water (Liu et al., 2010). The reduction of topsoil thickness is one of significant impacts of erosion on farmland. Topsoil thickness is a major indicator of soil quality and productivity (Sui et al., 2013). The reduction of topsoil thickness worsens soil physical, chemical and biological properties, and thus reduces crop productivity (Christensen and McElyea, 1988; Larney et al., 2000; Sarapatka et al., 2018). Therefore, preventing soil erosion and understanding the response of crop yields to the erosion is important in assessing the vulnerability of agriculture to erosion (Lal et al., 2000; Bakker et al., 2004).
It is difficult to establish the relationship of soil erosion and crop productivity as the process of degradation is slow (Bakker et al., 2004; Zhou et al., 2015). Previous studies have been conducted to assess the effects of soil erosion on crop productivity through artificial erosion approaches such as cut-and-fill experiments (Larney et al., 2000; Jagadamma et al., 2009), transects characterized by different erosion phases (Kosmas et al., 2001; Rejman and Iglik, 2010) and variable past erosions (Graveel et al., 2002; Fenton et al., 2005). Furthermore, simulation modeling has been established to estimate the long-term effect of soil erosion on plant growth (Pierce et al., 1983; Duan et al., 2016). For instance, Wang et al. (2009) demonstrated that soybean yield was exponentially decreased with increasing soil erosion depth due to the loss of soil organic matter and nutrients using the Agriculture Land Management Alternatives with Numerical Assessment Criteria (ALMANAC) model. However, these researches did not clarify the relationship of soil erosion and crop productivity, which may exhibit linear, concave and convex response curves (Bakker et al., 2004). Besides different experimental methods, the response of crop yield to soil erosion also depends on variables such as crop type, soil properties, management practices and climate characteristics (Bakker et al., 2007).
The negative effects of soil erosion on crop productivity can be attributed to the loss of soil organic carbon and plant nutrients, especially nitrogen (N), phosphorous (P), and potassium (K) (Kaspar et al., 2004; Herbrich et al., 2018). Quinton et al. (2010) reported that topsoil erosion decreased 23–42 Tg of nitrogen, 2.1–3.9 Tg of organic P and 12.5–22.5 Tg of inorganic P per year on the global scale. Plants obtain nutrients from soil mainly through roots; however, root growth in the soil vary with soil physical, chemical, and biological properties (Wang et al., 2015; Shinohara et al., 2016; Herbrich et al., 2018). It has been reported that a reduction of topsoil thickness caused by soil erosion results in decreases in soil organic matter (Liu et al., 2003; Srinivasan et al., 2012; Li et al., 2019a; Miao et al., 2019), available nutrients (Bakker et al., 2004; Sui et al., 2013; Xiong et al., 2018) and the effective rooting zone (Graveel et al., 2002). In addition, soil enzymes play an important role in soil quality and soil nutrient availability to plants (Jia et al., 2005; Stott et al., 2010; Sarapatka et al., 2018). Researchers have used soil enzyme activities as indicators of soil deterioration caused by erosion (Chaer et al., 2009; Sui et al., 2013; Nosrati, 2013). However, there are limited studies on nutrients uptake of crop and activities of enzymes as affected under zero tillage and residue cover in the eroded areas of China (Sui et al., 2013).
As an important grain production area, northeast China produces 18.7% of China’s total grains of which 45% and 33% are soybean, and maize, respectively (National Bureau of Statistics of China, 2012). However, soil erosion increasingly becomes a major limiting factor for productivity of agricultural land in this region (Liu et al., 2010). Zhang et al. (2008) showed that the average thickness of topsoil layer decreased from 43.7 cm in 1982 to less than 30 cm in 2002. Sui et al. (2009) reported that the yield reduction in maize and soybean were 13% and 9% for the removal of every 10-cm topsoil. A survey with soil erosion in cropland in northeastern China over the past 300-year history conducted by Xie et al. (2019) showed that the soil loss of cropland exponentially increased due to the increasing cropland acreage in the steep slope areas.
In this study, we constructed three levels of topsoil depth in a Mollisol with the same physical, chemical and biological properties to explore how maize and soybean responded to topsoil erosion under zero tillage, residue cover and maize-soybean rotation conditions. The objectives were (1) to quantify the influence of topsoil thickness on crop yield; (2) to compare nutrient uptake of crops under three topsoil depths; (3) to analyze the effect of topsoil thickness on root development and biochemical characteristics in the rhizosphere of the crop plants.