Mitigation of soil organic carbon mineralization by soil redistribution - An erosion-deposition plot study under natural rainfall over five years
Introduction
Soil erosion-deposition is a widespread form of lateral soil redistribution (Lal, 2019; Zhao et al., 2015). Global estimates of erosion- and deposition-induced carbon emissions are widely debated, ranging from a soil carbon sink of 1 Pg C yr−1 (Rosenbloom et al., 2006, Van Oost et al., 2005, Van Oost et al., 2007) to a soil carbon source of the same magnitude (Lal, 2003, Polyakov and Lal, 2008). Such contrasting results generate uncertainty in predicting the land-atmosphere CO2 flux of the eroding landscapes (Lal, 2003, Lal, 2019, Quine and van Oost, 2007, Stallard, 1998, Van Oost et al., 2007).
Various mechanisms of erosion and deposition influencing soil organic carbon (SOC) mineralization are responsible for these contradictory results ( Kirkels et al., 2014). The breakdown of aggregates, which leads to the exposure of hitherto encapsulated C to microbial processes, is primarily responsible for increased SOC mineralization on eroding slopes (Polyakov and Lal, 2008, Wei et al., 2016). Meanwhile, erosion-induced depletion of SOC stock (Gao et al., 2018; Wang et al., 2017) and the deficiency of both soil moisture and microbial population size ( Li et al., 2015; Wang et al., 2017) can restrict SOC mineralization. Conversely, soil deposition can enrich a large amount of labile SOC (Gregorich et al., 1998, Lal, 2003, Tong et al., 2021) and increase soil moisture and microbial population size (Li et al., 2015, Wang et al., 2017) to promote SOC mineralization. The buried SOC (Chaopricha and Marin-Spiotta, 2014, Van Oost et al., 2005, Van Oost et al., 2007) and the soils above some threshold moisture content (Davidson et al., 1998, Wei et al., 2014) diminish the SOC mineralization rate. Actually, the displacement of runoff and sediment is a critical factor driving the redistribution of soil environmental variables during the processes of soil erosion and deposition (Girmay et al., 2009, Ma et al., 2016, Zhang et al., 2016). Therefore, fluctuations in the amounts of runoff and sediment are expected to be closely related to the responses of SOC mineralization to erosion and deposition. However, because of the spatiotemporal variability and difficulty in collecting runoff and sediment data under natural conditions, runoff and sediment processes are not considered when assessing the effects of erosion and deposition on SOC mineralization.
Erosion plots are useful and flexible tools (Boix-Fayos et al., 2006, Li et al., 2021) that have been widely used in case studies considering the interplay of soil erosion and soil carbon dynamics (Gao et al., 2018, Hu and Kuhn, 2016, Novara et al., 2016, Polyakov and Lal, 2008, Shi et al., 2017). These plots can help researchers to homogenize the initial soil conditions of eroding slopes and depositional zones and can exclude disturbances from lateral soil redistribution to ensure longitudinal soil redistribution along the slope during erosion events. In addition, they are convenient for monitoring the individual erosion-deposition processes according to different test requirements. Therefore, a simulated experimental plot can focus on the core purpose of estimating the effects of erosion and deposition on SOC mineralization, rather than involving all relevant factors. Based on a local eroding landscape, we systematically constructed erosion-deposition plots. By observing the spatiotemporal changes in soil CO2 emission rate, runoff, sediment, and other soil environmental variables from 2015 to 2019, we aimed to 1) compare the respective and combined soil CO2 emissions of the eroding slopes of different gradients and depositional zones; and 2) investigate the effects of environmental variables on soil CO2 emissions in the erosion-deposition plots when considering the displacement of runoff and sediment. This study will provide essential slope scale data necessary to quantify land-atmosphere CO2 fluxes induced by soil erosion-deposition.
Section snippets
Site description and experimental design
The study site was established at Wangdong Catchment (35°13′ N, 107°40′ E, altitude 1220 m), Changwu, southern Loess Plateau, China. The mean annual precipitation is approximately 560 mm with 60% of rainfall occurring in the period from July to September, and the mean annual air temperature is 9.4 °C (provided by the Shaanxi Changwu Agroecosystem National Observation and Research Station). Local soil is dominated by loam (Cumulic Haplustoll; the USDA Soil Taxonomy System), originated from loess
Variations in soil biochemical properties
Throughout the experiment years of 2015–2019, annual runoff, sediment yield and erosion rate increased with slope gradients (Table 1). Compared to the 0.14–0.30 m3 runoff on the 5° slope, it was 30–115% higher on the 10° slope and 48–207% higher on the 20° slope respectively. Meanwhile, the annual sediment yield of the 10° and 20° slopes increased by 146–505% and 241–742% respectively when compared to the 5° slope (Table 1). The accumulated sediment depths after 73 erosional events within the
Contrasting responses of SOC mineralization to soil erosion and deposition
Compared to the eroding slopes, the depositional zones had higher soil CO2 emissions (Fig. 3). The contrasting responses of soil CO2 emissions to erosion and deposition were largely attributed to the variations in DOC, soil moisture, and microbial population sizes (MBC and MBN) induced by spatially displaced runoff and sediment (Table 1, Table 2, and 4). Specifically, the selective removal of DOC by soil erosion can partly explain the divergence of SOC mineralization between the eroding slopes
Conclusions
In this study, the changes in soil CO2 emission rates and other biogeochemical properties of the eroding slopes of three gradients and their respective depositional zones were regularly monitored. The results showed that greater soil CO2 emissions were observed in the depositional zones, as compared with the eroding slopes. As the eroding slopes steepened, the soil CO2 emissions decreased in the eroding zones but increased in the depositional zones. When combining the eroding slopes and
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
This project was supported by the National Natural Science Foundation of China (No. 41371279, No. 42107360), the West Light Foundation of the Chinese Academy of Sciences (No. XAB2020YN03).
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