1 Introduction
Soils provide multiple services for ecosystems and humans (Brevik et al., 2015; Gu et al., 2018). It was evaluated that 95% of food on the earth comes from soil (Borek et al., 2018). However, human activities, including but not limited to unreasonable farming (Amundson et al., 2015), deforestation, fire, and overgrazing usually increase the rate of ecological degradation, destroy the natural ecological balance, and lead to continuous expansion and intensification of soil erosion worldwide (Stanchi et al., 2015). Soil erosion caused by or stimulated by human activities is reaching rates that are tens to hundreds of times those of natural erosion (Wuepper et al., 2019). Because of soil erosion, fertile soils are disappearing at a faster rate than natural replenishment. Some severely eroded soils are not useable for further crop production. Mathieu et al. (2019) reported that approximately 75 billion metric tons of soil is removed annually from arable lands by wind and water, and around 80% of the world’s land suitable for cultivation is moderately or severely eroded (Pimentel, 2006). According to Wuepper et al. (2019), global average soil erosion rate reached 2.4 t ha−1 yr−1.
Soil erosion affects carbon cycling (Lugato et al. 2016) by redistribution of soil organic matter (SOM) (Wang et al., 2009; Gu et al., 2018), increases water runoff, and decreases water-storage capacity of soils (David and Michael, 2013). The most severe negative impacts of soil erosion include decreased soil fertility (Mahdi et al., 2001; Gu et al., 2018), degraded soil structure (Tenberg et al. 2014), and reduced effective plant rooting depth. When nutrient resources are depleted by erosion, plant growth is stunted and productivity is reduced (Pimentel, 2005; Dominati et al., 2010; Li et al. 2015). Erosion-induced variabilities of soil nutrient significantly alters agricultural productivity in cropping systems (Montanarella et al., 2015; Gu et al., 2018). In May 2019, The Food and Agricultural Organization (FAO) led Agriculture and food reports that the impact of erosion on crop production has been estimated at a 0.4 percent reduction in global crop yields per year due to erosion. It is clear that soil erosion is a threat to world food production (David and Michael, 2013; Liu et al., 2015; Zhou et al., 2015; Xie et al., 2019). Pierce and Lal (2017) established the essential for research on erosion’s impact on productivity. Consequently, a thorough understanding of soil erosion effects on crop yield is critical for assessing agricultural production dynamics (Lin et al., 2019) to ensure food security (Bakker et al., 2004; Pierce and Lal, 2017; Gu et al., 2018).
Quantitative relationships between erosion variables and crop yield are the basis for determining soil loss tolerance and evaluating regional sustainable development (Zhou et al., 2012). Since the 1930s, American scientists have taken the lead in observing effects of soil erosion on crop yields at soil and water conservation observation stations (Musgrave et al., 1923; Hays et al., 1949). In 1950s, research on the impacts of soil erosion on soil productivity has received a wider attention with the enactment of the US Soil and Water Conservation Act and the establishment of the National Soil Erosion-Soil Productivity Research Planning Committee (Williams et al., 1981). Using various research methods including field observation as well as model estimation, research area has extended from the United States to other developed countries and regions (Schmidt et al., 1982; Olson and Nizeyimana, 1988; Sasal et al., 2010; Gao et al., 2015; Duan et al., 2016). Although global efforts to assess degradation by soil erosion often measure degradation in terms of erosion rate rather than by its impact on productivity (Pierce and Lal, 2017). So far, numerous studies focused on erosion-crop yield relations (Larney et al., 1995; Larney et al., 2000; den Biggelaaret al., 2004; Bakker et al., 2007), and included research methods, climatic conditions, agricultural systems, soil characteristics (Bakker et al., 2004), and soil types.
Most research confirmed the negative effects of soil erosion on land productivity (Bakker et al., 2004; den Biggelaaret al., 2004). For example, Larney et al. (2008) showed that 16-year average yield reductions were 10.0, 19.5, 29.0, and 38.5% for 5, 10, 15, and 20-cm topsoil removal treatments, respectively. Additionally, the observed relationships between crop yield and soil erosion varied in different studies from linear (Lal, 1981; Rosa et al., 2000), exponential (Wang et al., 2009; Zhao et al., 2012) to quadratic (Larney et al., 1995). Although several publications confirmed the influence of erosion on crop yield, their quantitative relationships at global scale have yet to be investigated. This is the key to developing practices and policies for the restoration of eroded soils. To address this, which is essential to assess degradation of soil, we tested the overarching hypothesis that soil erosion leads to a decline in crop yield and follows a common yield-erosion relationship across different soil types and grain types worldwide. We conducted a meta-analysis of crop yield variability along erosion gradients worldwide using data from 13 published studies. Our primary objectives were to (a) demonstrate variability of those relationships across different regions; (b) determine the overall effects of erosion on crop yields; and (c) identify quantitative relationships between erosion and crop yield.