Soil and water conservation measures reduce soil and water losses in China but not down to background levels: Evidence from erosion plot data
Introduction
Soil erosion is a widely spread environmental problem (Lal, 2003). Globally, ca. 22–35 Gt of soil is annually mobilized by water and tillage erosion on agricultural land (Govers et al., 2013; Van Oost et al., 2007). Severe soil erosion not only threatens the sustainability of agriculture by reducing the soil's water holding capacity and its nutrient and soil organic carbon content (Bakker et al., 2004; Quinton et al., 2010; Zhao et al., 2016a) but also causes off-site reservoir siltation and water pollution (Lal, 1998). Also in China, water erosion is widely occurring and is acknowledged as a serious problem in many agricultural areas. Erosion rates exceeding 100 t ha−1 yr−1 have been reported on the Chinese loess plateau (Li et al., 2015; Shi and Shao, 2000; Yang et al., 2006; Zhao et al., 2016b) while erosion rates can exceed 50 t ha−1 yr−1 in the south and southwest of China (Lu et al., 2004; Zhang et al., 2003, Zhang et al., 2004).
A range of soil conservation technologies is now applied around the world to combat the loss of soil and water, including conservation tillage, terracing, hedgerow planting and mulching (Maetens et al., 2012; Mason et al., 2015; Prosdocimi et al., 2016; Taye et al., 2013). Soil and water conservation measures (SWCMs) can be effectively used to reduce and control soil erosion and sediment mobilization (Maetens et al., 2012; Montgomery, 2007). Since antiquity (ca. 5000 BP) terracing was used to protect the soil in China (Wei et al., 2016). Since ca. 1960, other SWCMs are also being used and the rate of implementation of soil conservation measures has accelerated: Liu et al. (2013) identified 53 different types of SWCMs that are being tested and applied in China. Some of these techniques are nowadays widely applied: for instance, ca. 50% of the arable land with a slope gradient >5% on the Chinese Loess Plateau has now been terraced (Zhao et al., 2016a).
As is the case elsewhere, the efficacy of soil conservation measures in China was tested by establishing paired erosion plot experiments. Soil loss and runoff rate on one or more erosion plots where SWCMs were applied was compared with the soil loss and runoff rate on one or more conventional erosion plots without SWCMs. Several of such experimental studies were conducted in different agro-ecological settings and testing different soil conservation techniques (Cai, 2004; Xu et al., 2010; Zhou et al., 2011). The measured soil loss (SLR) and runoff (RR) ratios, i.e. the ratio of soil loss and runoff of SWCMs plot to the reference (or conventional agricultural management) plot with same environmental condition except for the application of an SWCMs, can then be used as an index for assessing for the efficacy of SWCMs (Maetens et al., 2012).
While individual experimental studies do provide important information, they do not always allow to gain a full understanding of the efficacy of SWCMs. Erosion is known to be highly variable in both space and time and the same is true when the efficacy of SWCMs is considered (Leys et al., 2010; Maetens et al., 2012). Thus, the efficacy of a particular SWCMs as measured in a single study is inevitably subject to a large uncertainty. Furthermore, land managers should have information on the efficacy of different SWCMs when making a choice, taking also into account other important factors such as cost and social acceptability while most empirical studies only test a single SWCM. Evaluating and comparing the performance of different SWCMs while considering uncertainty is only possible when the results of individual experimental studies are combined in meta-analysis.
Gaining insight into the overall efficacy of SWCMs also allows gaining insight into the degree to which soil conservation measures do indeed effectively protect the soil resource. It is well known that, when correctly implemented, SWCMs can considerably reduce soil erosion and runoff from plot to catchment scales (Fu et al., 2005; Maetens et al., 2012). Montgomery (2007), using main data from the United States, reported that soil erosion rates, which are strongly accelerated by conventional agricultural activities, can be reduced to values close to the background erosion rates by using conservation tillage. This implies that conservation tillage allows re-instating an equilibrium between soil production and soil loss. However, this conclusion may not be universally valid: variations in crops, climate, topography and soil type may affect the efficacy of conservation tillage and other SWCMs. Quantifying and understanding this variability will allow to evaluate under what conditions SWCMs are indeed effective in protecting agricultural land from further degradation.
A comprehensive study to assess the efficacy of SWCMs in China is, at present, absent. While valuable information may be derived from existing meta-analyses of SWCMs, the results of these studies cannot directly be applied to China. First, environmental conditions in China are different from those in the North America or Europe where earlier meta-studies were carried out: in comparison to these regions, arable land plots in China is located on much steeper slopes and rainfall erosivity is, in general, high (Fig. 2; Panagos et al., 2015; Qin et al., 2016). Second, some soil conservation technologies that are being used in China are not being applied elsewhere in the world (Table 1).
In this study, we compiled a relatively extensive erosion plot database of erosion plot studies carried out in China in which the soil loss ratio and runoff ratio for one or several SWCMs was assessed under field conditions. Based on this database, the overall efficacy of these SWCMs was analysed. We also explored the effect of environmental factors such as climate and topography on the efficacy of SWCMs. Finally, we compared the reduction in soil loss with literature information on soil formation rate (similar to Montgomery, 2007) so that we can assess to what extent soils in China can effectively be protected against erosion by soil and water conservation technology.
Section snippets
Erosion plot database
Data on plot experiments wherein the annual soil loss rate (SL, t ha−1 yr−1) and annual runoff rate (R, mm yr−1) as on erosion plots under natural rainfall in China with and without SWCMs were compared were collected from the literature. The main sources of information were peer-reviewed papers and so-called hydro-station reports about SWCMs experiments. The hydro-station reports were obtained from the National Data Sharing Infrastructure of Earth System Science, China (http://www2.geodata.cn/)
Overall efficacy of SWCMs in China
On average, the soil loss rate on conventional erosion plots (28.63 ± 3.42 t ha−1 yr−1, n = 155) was ca. 3.5 times larger than the rate of soil loss under SWCMs (8.63 ± 0.89 t ha−1 yr−1, n = 499), which suggested that the implementation of SWCMs, on overall average, reduced soil loss in China by ca. 70%. The average weighted SLR for all pair's SWCMs plots suggests a somewhat lower efficacy than the direct comparison of absolute soil erosion rates (0.39 ± 0.02 (n = 323)). However, the variation
Efficacy of soil conservation measures in China
Clearly, soil conservation measures, on average, allow to considerable reduce soil losses in China. This is consistent with the results of Leys et al. (2010), Maetens et al. (2012) and Montgomery (2007) as well as many other studies that reported that soil loss rates under conventional agriculture were generally one to two orders of magnitude larger than soil loss rates under soil conservation practice (conservation tillage) worldwide. When assuming a soil loss tolerance level of 10 t ha−1 yr−1
Conclusions
Our study evaluated the efficacy of soil and water conservation measures in reducing soil loss and runoff at erosion plots in China based on a review of erosion plot data. We presented the overall efficacy of SWCMs in controlling soil loss and runoff and also compared the efficacy of single SWCMs. Our results suggested that in general, applying the SWCMs can reduce soil loss by ca. 70% (overall SLR is 0.39 ± 0.02) and runoff by ca. 50% (overall RR is 0.54 ± 0.05). The engineering measures were,
Acknowledgments
This study is supported by the Fundamental Research Funds for the Central Universities, China (300102268303) and the China Scholarship Council (CSC).
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