Undecomposed litter mixed in the soil can increase interrill erosion on hillslopes: A laboratory study
Introduction
Soil erosion caused by water is a serious environmental problem worldwide. This type of erosion leads to on-site land degradation and off-site nonpoint source pollutants, thereby attracting much attention from soil and environmental scientists. Water erosion on a slope can be divided into rill erosion and interrill erosion (Zhang et al., 2018). The interrill erosion process is caused by raindrop impact and overland flow. Interrill erosion provides sediment to rill systems, thus greatly contributing to sediment yield (Zhang et al., 2018, Nouhou Bako et al., 2017). The intensity of soil erosion is controlled by the energy of rainfall and flow and the resistance of soil to these effects (Parwada and Van Tol, 2019, Nouhou Bako et al., 2017). Plant litter is one of the most important components for both agriculture and natural ecosystems (Almagro and Martínez-Mena, 2014, Cotrufo et al., 2015, Seitz et al., 2015). It plays an important role in alleviating soil erosion. Thus, substantial attention has been given to the effect of litter on soil erosion (Brown et al., 1989, Brown et al., 1990, Franti et al., 1996, Pannkuk and Robichaud, 2003, Sun et al., 2016a, Sun et al., 2016b, Liu et al., 2020a).
Plant litter can be artificially and naturally mixed into topsoil (Brown et al., 1990, Tsukamoto, 1991, Franti et al., 1996, Hewins et al., 2013, Šamonil et al., 2015). The presence of litter was found to affect soil loss on hillslopes (Sun et al., 2016a, Sun et al., 2016b, Parwada and Van Tol, 2019, Wang et al., 2019, Wang et al., 2020, Yang et al., 2021). By litter incubation and laboratory rainfall experiments, Parwada and Van Tol (2019) found that litter added to the soil led to higher soil resistance to splash erosion. Wang et al. (2019) reported that plant litter mixtures decreased interrill erosion. On farmland, studies found that crop residue incorporated into soil reduced rill erosion resulting from concentrated overland flow (Brown et al., 1989, Brown et al., 1990, Franti et al., 1996). By implementing flow-shear experiments, Sun et al., 2016a, Sun et al., 2016b and Liu et al. (2020a) found that litter incorporation into topsoil led to reduced erodibility. Namely, rill erosion decrease can probably be attributed to soil resistance to erosion. The effect of litter mixtures on soil erosion is strongly controlled by litter rate (amount of litter involving mixing process per unit area), litter morphology (e.g., litter length density, litter surface area density, and litter volume ratio), and environmental factors (e.g., soil type, soil texture, bulk density, slope gradient, and slope length) (Foster et al., 1982a, Foster et al., 1982b, Sun et al., 2016b, Liu et al., 2020a). Taken together, soil-litter mixing has been found to reduce soil erosion on hillslopes.
In the field or farmland, the soil-litter mixing process inevitably causes litter exposure on the soil surface and litter buried in the soil. However, previous studies did not distinguish the two kinds of litter (Brown et al., 1989, Brown et al., 1990, Franti et al., 1996, Wang et al., 2019, Wang et al., 2020). This may lead to an unclear understanding of the overall effect of soil-litter mixing on soil loss because of the distinctive mechanisms of the two kinds of litter on soil erosion, which vary as the litter decomposes. Studies have shown that the effect of litter on reducing soil erosion diminishes with time after litter is mixed with soil (Brown et al., 1989, Sun et al., 2016b, Parwada and Van Tol, 2019). After mixing, the exposed litter can shield the surface soil from raindrop splashing, similar to the effect of litter cover, whose magnitude depends on the surface coverage (Brown et al., 1989, Franti et al., 1996, Wang et al., 2020). The effect is greatest immediately after the litter is mixed because the coverage of exposed litter is highest, while it decreases as the litter is decomposed. Litter buried in the soil can change soil resistance to erosion by modifying soil hydrological conditions and bonding soil particles (Sun et al., 2016a, Sun et al., 2016b, Parwada and Van Tol, 2019). In the early period after mixing, the bonding effects are probably not notable because of the low degree of litter decomposition; in contrast, the impervious surface of the undecomposed litter may play a role in impeding water percolation in the soil. In this condition, the soil moisture content above the buried litter can increase faster than that without litter buried, which creates oversaturated soil conditions. This condition of the soil was shown to reduce soil resistance to erosion compared with bare hillslopes (Smets et al., 2011). Therefore, soil loss may be greater for slopes with litter buried in the soil in this period. Thus, the previously proposed effect of litter mixtures on reducing soil erosion may mostly be a consequence of the surface-exposed litter in the early period after soil-litter mixing. As litter decomposes, the role of surface-exposed litter diminishes, while the bonding effects derived from buried litter are expected to be enhanced (Brown et al., 1989, Sun et al., 2016b, Parwada and Van Tol, 2019, Liu et al., 2020a). The latter is probably the dorminant effect on reducing soil loss in the period.
Altogether, the soil-litter mixing process results in two different existing forms of litter. Their mechanisms of controlling soil erosion may be different, particularly in the early period after the mixing process, due to distinctive effects on raindrop and runoff energy and soil water movement (Brown et al., 1990, Franti et al., 1996, Li et al., 2015, Liu et al., 2020a, Wang et al., 2020, Yang et al., 2021). However, previous studies have not identified these distinctions (Brown et al., 1989, Brown et al., 1990, Franti et al., 1996, Wang et al., 2020). This causes an elusive understanding of the overall effect of soil-litter mixing on soil erosion. In this study, undecomposed litters were mixed with topsoil, and the resulting litters loosely left on the soil surface were defined as the surface exposed litter, while those not exposed were defined as litter buried in the soil. We hypothesized that exposed litter plays a role in reducing soil erosion, while litter buried in the soil could increase soil erosion. The overall effect of litter on soil erosion varied depending on the ratio of the two components. To test this assumption, a control laboratory experiment was designed to isolate the effects of litter in the soil and that on the surface by analyzing the infiltration rate, runoff hydraulics, and soil loss amount. This study can enhance our understanding of the effects of litter on soil erosion and may help improve the modeling of soil erosion on hillslopes with litter mixtures.
Section snippets
Experimental treatments
Laboratory rainfall experiments were carried out in Beijing, China. The climate of this area is a warm temperate monsoon climate with a mean annual rainfall of ~500 mm, which mainly falls in summer. The dominant soils are cinnamon and fluvo-aquic soils derived from weathered rock and loose Quaternary sediment. Laboratory rainfall experiments were carried out to test our hypothesis using locust leaf litter (Robinia pseudoacacia Linn.). In the field, litter consists of components such as leaves,
Infiltration and runoff hydraulic properties
The infiltration rate decreased quickly over the first 10 min after runoff initiation for the different treatments. The infiltration rate reached a steady-state 50 min after runoff generation (Fig. 2). For the three kinds of treatments, the mean infiltration rate was negatively related to the litter rate, as presented in Fig. 3. This trend was especially notable for the S treatment. Because of the variation in the soil infiltration rate, the runoff changed correspondingly. Our data showed that
Effects of litter on infiltration and hydraulic properties
Our study showed that litter mixed in the soil caused reduced infiltration, particularly under conditions without surface cover (Fig. 3). These results contradict those of Yang et al. (2021), who found a higher infiltration rate in soil mixed with litter under laboratory rainfall experiments. This difference may be associated with the differences in litter type (broadleaf versus straw) and initial conditions of the experimental soil boxes (i.e. bulk density control and prewetting of soil). Yang
Conclusions
Litter mixed with soil leads to litter buried in the soil and exposed on the soil surface. Laboratory rainfall experiments were carried out to isolate the effects of mixed undecomposed litter on soil erosion with three litter treatments. Our results showed that the litter buried in the soil could decrease the infiltration rate and thus enhance the runoff coefficient. The results were associated with water repellency and pore disconnection because of the presence of litter in the soil.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Financial support for this study was provided by the National Key Research and Development Plan of China (2021YFD1500803), State Key Program of National Natural Science of China (42130701), Guizhou Provincial Science Technology Projects (QKHJC-ZK[2021]YB222), and the Introduced Talents Scientific Research Grants of Guizhou University (No. 2019 (27)). We also appreciate the helpful technical support from the rainfall simulation hall of the Fangshan Comprehensive Experimental Station.
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