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Introduction
The global process of soil water erosion has been identified as a net source of C to the atmosphere of about 1.3 Pg∙yr-1 (Lal, 2020). Bolaños et al. (2016) exposed that 76 % of the surface in Mexico has some degree of water erosion damage and pointed out that losses of soil organic carbon (SOC), related to this phenomenon, have been little studied and quantified. Water erosion influences the redistribution of SOC and positive or negative CO2 fluxes to the atmosphere (Deumlich et al., 2018; Yue et al., 2016), since, during the transport phase, water favors the loss and, in the deposition phase, the resulting sedimentation can facilitate storage (Wang et al., 2014b)
A high proportion of SOC is the C left in the soil after partial decomposition of any organic residues produced by living organisms (Food and Agriculture Organization of the United Nations [FAO], 2017). According to Lal et al. (2021), soil has the largest terrestrial organic C reserves; the global estimate of SOC is between 1 500 and 2 400 Pg C. Etchevers et al. (2006) expressed that SOC accumulation is an important process for mitigating the effects of climate change, because soil, being a sink, forms a stable C reservoir. In this regard, Cotler et al. (2016) mentioned that better agricultural and soil conservation practices increase SOC considerably, decrease erosion, and mitigate greenhouse gas (GHG) emissions. Mechanical soil and water conservation practices include terraces, ditches, ponds, accommodated stone dams, gabion dams, stone cordons, and living barriers.
The main objective of this study was to identify the main research trends regarding the influence of water erosion on SOC redistribution and its relationship with soil and water conservation practices at the micro-watershed level. A literature review was carried out to recognize the processes and relationships involved, which will allow the promotion of sustainable agronomic practices oriented to C sequestration based on evidence and experiences.
Materials and Methods
A search for scientific articles was made by matching the title, abstract and keywords of the terms: 1) water erosion, 2) soil organic carbon (SOC) and 3) micro-watersheds and dams. Databases consulted included Web of Science, Scopus, Scientific Electronic Library Online (SciELO), ResearchGate, Google Scholar and Redalyc. In the last four, a filter was applied to show results for Mexico.
The search covered the period 2012-2022 (preferred, but not restricted). Publications were reviewed to establish research trends, referring to the way in which researchers have addressed the relationship between erosion, SOC and mechanical soil and water conservation practices in high impact journals globally and those recognized by the National Council of Science and Technology (CONACYT) in Mexico. These publications were classified into the following groups: (I) SOC redistribution caused by the effect of water erosion, (II) effect of soil management and soil and water conservation practices on SOC, (III) estimation of SOC storage in sediments trapped in erosion control projects, and (IV) modeling of SOC storage using spatial analysis and erosion scenarios.
We exported the publications to the NVivo program (QSR International, 2018) as suggested by Ayala et al. (2020), in order to perform a word frequency analysis where at least five characters were considered to identify the most representative keywords of each document. The results were subjected to a cluster analysis to establish the relationship between research trends, classification parameters called nodes were created and the Jaccard coefficient similarity test was run between data sets to statistically validate the research trends in discussion groups. In this test, values greater than 0 and closer to 1 have similarity and values of 0 have no similarity.
Results and Discussion
Clustering of bibliographic references in discussion groups
The search for bibliographic references yielded a total of 48 scientific articles of interest, which were grouped based on research trends to generate discussion groups as shown in Figure 1.
Based on research trends, discussion groups 1, 2, 3 and 4 concentrated 40 %, 40 %, 12 % and 8 % of the scientific articles, respectively. This means that 80 % of the research (38 articles) studied the redistribution of SOC caused by water erosion and the effect of soil management and conservation practices on SOC (Tables 1 and 2). The remaining research has estimated SOC storage in sediments trapped in erosion control structures and the modeling of SOC storage using spatial analysis and erosion scenarios (Tables 3 and 4). It is important to highlight that, in discussion groups 1 and 3 (Figure 1), no research done in Mexico was found, suggesting the need to explore these topics. The greatest contributions from Mexico were observed in discussion group 2.
The NVivo program generated nine most frequent terms: carbon, erosion, organic, sediment, soils, water, suelo, global and China, which is the country with the most research published on the subject in the period studied (Figure 2).
Table 1 Global-scale literature references associated with discussion group 1: Redistribution of soil organic carbon because of water erosion.
| No. | Reference | Title |
|---|---|---|
| 1 | Holz and Agustín (2021) | Erosion effects in soil carbon and nitrogen dynamics on cultivated slopes: A meta-analysis |
| 2 | Gaspar et al. (2020) | Lateral mobilization of soil carbon induced by runoff along karstic slopes |
| 3 | Tong et al. (2020) | Sediment deposition changes the relationship between soil organic and inorganic carbon: Evidence from the Chinese Loess Plateau |
| 4 | Lal (2019) | Accelerated soil erosion as a source of atmospheric CO2 |
| 5 | Wang et al. (2019). | Selective transport of soil organic and inorganic carbon in eroded sediment in response to raindrop sizes and inflow rates in rainstorms |
| 6 | Zhang et al. (2019) | The role of dissolved organic matter in soil organic carbon stability under water erosion |
| 7 | Liu et al. (2018) | Soil carbon and nitrogen sources and redistribution as affected by erosion and deposition processes: A case study in a loess hilly-gully catchment, China |
| 8 | Xiao et al. (2018) | The mineralization and sequestration of organic carbon in relation to agricultural soil erosion |
| 9 | Deumlich et al. (2018) | Estimating carbon stocks in young moraine soils affected by erosion |
| 10 | Doetterl et al. (2016) | Erosion, deposition and soil carbon: A review of process-level controls, experimental tools and models to address C cycling in dynamic landscapes |
| 11 | Novara et al. (2016) | Understanding the role of soil erosion on CO2-C loss using 13C isotopic signatures in abandoned Mediterranean agricultural land. |
| 12 | Olson et al. (2016) | Impact of soil erosion on soil organic carbon stocks |
| 13 | Yue et al. (2016) | Lateral transport of soil carbon and land-atmosphere CO2 flux induced by water erosion in China |
| 14 | Müller and Chaplot (2015) | Soil carbon losses by sheet erosion: a potentially critical contribution to the global carbon cycle |
| 15 | Kirkels et al. (2014) | The fate of soil organic carbon upon erosion, transport and deposition in agricultural landscapes - A review of different concepts |
| 16 | Wang et al. (2014b) | Soil organic carbon redistribution by water erosion –The role of CO2 emissions for the carbon budget |
| 17 | Berhe and Kleber (2013) | Erosion, deposition, and the persistence of soil organic matter: mechanistic considerations and problems with terminology |
| 18 | Mchunu and Chaplot (2012) | Land degradation impact on soil carbon losses through water erosion and CO2 emissions |
| 19 | Lal (2003) | Soil erosion and the global carbon budget |
Table 2 Bibliographic references according to scale associated with discussion group 2: Effect of soil management and soil and water conservation practices on soil organic carbon.
| No. | Reference | Title |
|---|---|---|
| 1 | Holz and Agustín (2021) | Erosion effects in soil carbon and nitrogen dynamics on cultivated slopes: A meta-analysis |
| 2 | Gaspar et al. (2020) | Lateral mobilization of soil carbon induced by runoff along karstic slopes |
| 3 | Tong et al. (2020) | Sediment deposition changes the relationship between soil organic and inorganic carbon: Evidence from the Chinese Loess Plateau |
| 4 | Lal (2019) | Accelerated soil erosion as a source of atmospheric CO2 |
| 5 | Wang et al. (2019). | Selective transport of soil organic and inorganic carbon in eroded sediment in response to raindrop sizes and inflow rates in rainstorms |
| 6 | Zhang et al. (2019) | The role of dissolved organic matter in soil organic carbon stability under water erosion |
| 7 | Liu et al. (2018) | Soil carbon and nitrogen sources and redistribution as affected by erosion and deposition processes: A case study in a loess hilly-gully catchment, China |
| 8 | Xiao et al. (2018) | The mineralization and sequestration of organic carbon in relation to agricultural soil erosion |
| 9 | Deumlich et al. (2018) | Estimating carbon stocks in young moraine soils affected by erosion |
| 10 | Doetterl et al. (2016) | Erosion, deposition and soil carbon: A review of process-level controls, experimental tools and models to address C cycling in dynamic landscapes |
| 11 | Novara et al. (2016) | Understanding the role of soil erosion on CO2-C loss using 13C isotopic signatures in abandoned Mediterranean agricultural land. |
| 12 | Olson et al. (2016) | Impact of soil erosion on soil organic carbon stocks |
| 13 | Yue et al. (2016) | Lateral transport of soil carbon and land-atmosphere CO2 flux induced by water erosion in China |
| 14 | Müller and Chaplot (2015) | Soil carbon losses by sheet erosion: a potentially critical contribution to the global carbon cycle |
| 15 | Kirkels et al. (2014) | The fate of soil organic carbon upon erosion, transport and deposition in agricultural landscapes - A review of different concepts |
| 16 | Wang et al. (2014b) | Soil organic carbon redistribution by water erosion –The role of CO2 emissions for the carbon budget |
| 17 | Berhe and Kleber (2013) | Erosion, deposition, and the persistence of soil organic matter: mechanistic considerations and problems with terminology |
| 18 | Mchunu and Chaplot (2012) | Land degradation impact on soil carbon losses through water erosion and CO2 emissions |
| 19 | Lal (2003) | Soil erosion and the global carbon budget |
Table 3 Literature references according to scale associated with discussion groups 3 (Estimation of soil organic carbon [SOC] storage in sediments trapped in erosion control studies) and 4 (Modeling SOC storage using spatial analysis and erosion scenarios).
| No. | Cite | Title | Scale | Trend |
|---|---|---|---|---|
| 1 | Mekonnen and Getahun (2020) | Soil conservation practices contribution in trapping sediment and soil organic carbon, Minizr watershed, northwest highlands of Ethiopia | Global | Group 3 |
| 2 | Addisu and Mekonnen (2019) | Check dams and storages beyond trapping sediment, carbon sequestration for climate change mitigation, Northwest Ethiopia | Global | Group 3 |
| 3 | Liu et al. (2017) | Do land use change and check-dam construction affect a real estimate of soil carbon and nitrogen stocks on the Loess Plateau of China? | Global | Group 3 |
| 4 | Zhang et al. (2016) | Loess Plateau check dams can potentially sequester eroded soil organic carbon | Global | Group 3 |
| 5 | Wang et al. (2014a) | Carbon sequestration function of check-dams: A case study of the Loess Plateau in China | Global | Group 3 |
| 6 | Lü et al. (2012) | Carbon retention by check dams: Regional scale estimation | Global | Group 3 |
| 1 | Márquez et al. (2020) | The use of remote sensing to detect the consequences of erosion in gypsiferous soils | Global | Group 4 |
| 2 | Bolaños et al. (2016) | Mapa de erosión de los suelos de México y posibles implicaciones en el almacenamiento de carbono orgánico del suelo | Mexico | Group 4 |
| 3 | Ji et al. (2014) | Assessment of the redistribution of soil carbon using a new index—a case study in the Haihe River Basin, North China | Global | Group 4 |
| 4 | Segura et al. (2005) | Carbono orgánico de los suelos de México | Mexico | Group 4 |
Based on the 48 selected articles, the results of similarity between keyword codes, shown by Jaccard's similarity coefficients (Table 4), ratify the thematic classification and the proportion of research in each discussion group. Thus, there is more research in the thematic group 1 and its relationship with SOC compared to less research in group 4.
Table 4 Jaccard's similarity coefficients between keyword codes related to the topic water erosion and soil organic carbon (SOC).
| Code A | Code B | Coefficient |
|---|---|---|
| SOC redistribution | Carbon-erosion | 0.66 |
| Conservation practices | Carbon-erosion | 0.61 |
| Check dams | Carbon-erosion | 0.24 |
| SOC modeling | Carbon-erosion | 0.22 |
Group 1. Soil organic carbon redistribution caused by water erosion
Soil erosion and its influence on carbon release to the atmosphere is uncertain in the processes of estimating global C fluxes (inventories) and is a component that is not considered in determining the potential of soils for C sequestration (Mchunu & Chaplot, 2012; Novara et al., 2016; Wang et al., 2014a).
Water erosion comprises three processes: (i) initiation or detachment of soil particles, (ii) transport and redistribution of sediments over the landscape, and (iii) deposition on depression or concavity sites and in aquatic ecosystems (Doetterl et al., 2016). Derived from these processes, water erosion has significant impacts on the distribution and transformation of SOC at the micro-watershed scale (Liu et al., 2018; Wang et al., 2014b).
Soil water erosion, when analyzed using the micro-watershed approach, can be considered as a CO2 emitting source, in terms of particle initiation and transport due to the effect these processes have on soil organic matter (SOM) mineralization, but also as a source of storage at deposition sites (Doetterl et al., 2016).
Erosion initiation or start site
Forces causing splashing and collisions induced by raindrops and runoff initially decrease the stability of soil aggregates and accelerate the mineralization of SOM (Wei et al., 2017). These forces can induce the death of microorganisms on the soil surface, driven by aggregate dispersion and nutrient loss, which will result in a positive effect on soil mineralization (Xiao et al., 2018). On the other hand, the destruction of structural aggregates exposes SOM as it is no longer encapsulated and protected against microbial processes (Lal, 2003, 2019; Olson et al., 2016). In eroded areas of the landscape, SOC stores are reduced by erosion with speed and extent related to the gradient of slope and convexity (Singh & Benbi, 2018).
Transport and redistribution on the landscape
Transport is a link between erosion and sedimentation, which results in deposition at an eroded or intact site, even in an aquatic environment, but always elsewhere on the landscape (Kirkels et al., 2014). Two seemingly contradictory processes have been highlighted in transport: (i) increased C mineralization due to aggregate breakdown and (ii) enrichment of C in sediments relative to source soils, due to deposition and selective transport of other components with high organic load. The possible release of CO2 to the atmosphere during the transport of soil removed by erosion is related to the particle size of the sediment source soil; those with higher clay and silt content are transported more easily (where SOC is most concentrated), since coarse particles are the first to be deposited (Doetterl et al., 2016; Wang et al., 2019).
Reservoir sites
Redistribution of sediment from erosion results in C concentration at reservoir sites (Holz & Agustín, 2021; Kirkels et al., 2014; Zhang et al., 2019). According to Olson et al. (2016), deep burial by deposition is responsible for a removal of 0.4 to 0.6 Pg C∙yr-1 at a global level. SOC in deep-buried sediments can be considered as a sink for CO2, as C is stabilized by being in an area with little or no oxygen present, with organisms responsible for its decomposition (Müller & Chaplot, 2015) and, many times, out of reach of moisture. Gaspar et al. (2020) established that the highest SOC gains were recorded in sediments buried on concave slopes of the landscape. In this respect, Tong et al. (2020) determined a 24 % increase in SOC at reservoir sites compared to boot sites.
Water erosion and its relationship with CO2 emissions to the atmosphere
Figure 3 shows the scheme proposed by Lal (2019), which exposes the fate of the transported SOC in the stages of the erosive process, from the in situ impact of rain erosivity to the burial of a small fraction, emissions to the atmosphere and transport to the ocean.
Lal (2019) mentions that soil erosion would be a net source if the on-site replacement of eroded C were less than the new organic matter added by photosynthesis and if all GHGs (CO2, CH4 and N2O) were accounted for. However, data based on measurements of SOC transported in a watershed are scarce and measurements of GHGs in different landscape units are even scarcer.
Group 2. Effect of land use and soil and water conservation practices on soil organic carbon
Lal et al. (2021) indicated that the adoption of soil management practices can lead to recarbonization of depleted soils and convert them into a sink of atmospheric C. The most suggested practices include conservation tillage and maize intercropped with fruit trees (Cotler et al., 2016; González et al., 2014; Lal, 2018; Srinivasarao et al., 2015); additionally, Lal (2013) and Gallardo (2021) recommend the use of cover crops, green manures, biochar, agroforestry, fertilization, irrigation, compost addition, reduced tillage, crop rotation, soil covered by organic residues and mulching.
Regarding land uses and redistributed SOC at a landscape scale by land use changes, several studies show that grassland has higher SOC reserve compared to agricultural use (Barrales et al., 2020; Cantú & Yáñez, 2018; Seifu et al., 2021; Shi et al., 2019; Velásquez et al., 2016).
Soil management strategies aimed at C sequestration are most effective when accompanied by conservation projects that reduce erosion and increase the capacity of farmers to offset emissions and adapt to climate change (Mengistu et al., 2016; Nadeu et al., 2015). Chen et al. (2020) reported that terraces caused 32.4 % increase in SOC after five years of construction. An important contribution to the generation of information on soil and water conservation, and its relationship with SOC storage at the micro-watershed level in Mexico, has been the Project Manejo Sustentable de Laderas, which highlights the practice of terraces with fruit trees as living retaining walls (Etchevers et al., 2006).
Group 3. Soil organic carbon storage in sediments trapped in erosion control structures
Sediment control structures have been used in watersheds to trap soil resulting from erosion. Studies by Lü et al. (2012) showed that check dams in Shaanxi Province, Loess Plateau region, retained about 42.3 million tons of SOC; this was about 4.0 % of the estimated C emission from fossil fuels in China in 2000. Wang et al. (2014c) estimated that 11 000 silt control structures trapped 219 109 m3 of sediment and stored 0.945 Tg SOC. Addisu and Mekomen (2019) found that storage and silt control dams constructed in a watershed in Ethiopia sequestered 4 468 Mg of SOC and 68.8 Tg of sediment. This research reveals that, in addition to conserving soil and water, these structures also store C.
In Mexico, there are still no studies that prove or disprove that mechanical soil and water conservation works (accommodated stone dams and gabion or stone fence dams), built in river tributaries, are CO2 sinks. Sediments trapped and accumulated behind the stone or earth walls, transported by water erosion, may be enriched with C as in other parts of the world. In Mexico, the construction of soil and water conservation structures has been subsidized by several government programs; therefore, it is suggested to evaluate the C storage capacity, whose value may be important in terms of retention of the stable C already mineralized that, because of the lack of oxygen inside the sediment, decreases the emission of CO2 to the atmosphere. Considering the large number of structures constructed nationwide, a significant amount of retained C that is not emitted to the atmosphere is expected, as long as it is not removed from the site.
Group 4. Modeling soil organic carbon storage by spatial analysis with erosion scenarios
SOC and water erosion can be analyzed and modeled using GIS. Ji et al. (2014) proposed a redistribution index of SOC on a watershed scale. In Mexico, remote sensing and GIS were used to generate information on SOC storage and soil loss caused by erosion at a national level (Bolaños et al., 2016; Segura et al., 2005).
In short, water erosion, in the processes that characterize it, is an emitter of CO2 as a GHG. Despite this, there are no research reports in Mexico on the redistribution of SOC as a result of erosion and no evaluations of sediment control projects as CO2 sinks (taking into account the bibliographic references reviewed on a global scale). Therefore, it is inferred that SOC redistribution may be associated with the redistribution of silt- and very fine sand-sized sediments resulting from rainfall transport and surface runoff from erosion initiation sites to deposition sites behind mechanical conservation practices. This results in significant accumulations of SOC at the depths of the storage sites that need to be quantified by further research.
Regarding the serious problem of water erosion in Mexico and the fact that several programs have been implemented to support the construction of soil and water conservation structures with significant success, but without having evaluated the impacts in terms of C storage, it is suggested to recognize this area of opportunity to focus research at different scales. The analysis of water erosion and its relationship with SOC can be studied with the use of remote sensing and GIS under a watershed approach. This can be used to suggest the best sustainable practices for soil and water conservation and C retention to contribute to the commitments acquired by Mexico in the nationally determined contributions. These practices would allow decision makers and public policies to further promote the implementation of mechanical structures and agronomic and vegetative practices for soil and water conservation to mitigate and adapt to climate change.
Conclusions
In the period 2012-2022, 80 % of global research focused on soil organic carbon (SOC) redistribution caused by water erosion, and the effect of soil management and conservation practices; however, no studies on this subject were found in Mexico, nor were there any evaluations of sediment control dams as CO2 sinks. These research trends suggest studying: I) SOC redistribution caused by water erosion at the watershed scale, II) the storage of buried SOC in sediments deposited by water erosion, III) the potential of mechanical soil and water conservation practices as carbon sinks, and IV) the generation of SOC loss risk indices using remote sensing. This research could generate the necessary knowledge to promote sustainable agronomic practices in Mexico, oriented to evidence-based carbon sequestration.
