Anecic earthworms generate more topsoil than they contribute to erosion – Evidence at catchment scale in northern Vietnam
Introduction
Earthworms ingest and transform huge amounts of organic and inorganic material, corresponding to up to 30 times their own weight per day (Lavelle, 1975). As a result they deposit organic matter rich aggregates at the soil surface (casts), which impact numerous soil functions, such as those associated with the dynamics of carbon and nutrients (Van Groenigen et al., 2019). High casting activity and stability of these structures are also considered to (1) shape the organization of soil structure (Lavelle et al., 2020), (2) explain the granular organization of A horizons in many ecosystems (Jongmans et al., 2003), and (3) buryarchaeological items (Darwin and bi.-39. tx., 1881, Stein, 1983). Despite their significant influence on main ecological functions in soil, a major obstacle to the quantification and modeling of the influence of earthworms on soil functioning lies in the fact that the amount and dynamics of casts produced at landscape scale remain poorly documented.
Surface earthworm casting activity is highly variable and reaches its record in the tropics with 250 t ha−1 year−1 measured in Nigeria (Madge, 1969). Once deposited on the soil surface, casts are degraded by rainfall and/or by trampling of large animals depending on environmental and ecological factors. For instance, the production and degradation of casts is expected to be higher in tropical than in temperate environments, because of higher temperature and higher precipitations and greater biological activity. Vegetation may protect casts from destruction though raindrop impacts and extend their lifespan (Decaëns, 2000). However, based on our visual assessment in the field, we hypothesized that the casts’ resistance to erosion also depends on soil properties such as their carbon and clay content or on the cast type (granular < past-like < globular) formed in different situations. If not degraded on site, casts can also contribute to the sediment load following water erosion, especially in tropical sloping land (Blanchart et al., 2004, Jouquet et al., 2010). Generally, the impact of erosion on the fate of earthworm casts was estimated from indicators, as for example soil aggregate stability to water or measured directly in the field at small scale on erosion plots under natural or simulated rainfalls (Hazelhoff et al., 1981, Jouquet et al., 2008b, Jouquet et al., 2012, Jouquet et al., 2013, Le Bayon et al., 2002, Le Bayon and Binet, 1999, Le Bayon and Binet, 2001, Podwojewski et al., 2008, Sharpley and Syers, 1976). These approaches are not necessarily representative of the processes taking place at the landscape scale.
Dry and water-stable casts are usually considered to be resistant to raindrop impact. They increase soil roughness and water infiltration (i.e. reduce water runoff), reduce soil detachment and protect soil from crusting (Jouquet et al., 2012). In contrast, freshly emitted casts are unstable, prone to dislocation and can increase seal formation and soil loss by erosion (Le Bayon et al., 2002, Le Bayon and Binet, 2001). Therefore, the casting activity could contribute to soil accrual or its erosion. The occurrence of both phenomena may vary in different landscape positions due to microclimatic conditions.
This study investigated the balance of both processes to answer the question if earthworms contribute more to soil accrual or its erosion. We addressed this issue by quantifying the impact of earthworm casting activity at the landscape scale in a tropical sloping catchment (46 ha). To this end, we established small plots (0.25 m2) under three vegetation units and quantified (1) casts produced by the anecic earthworm Amynthas adexilis and (2) their impact on water runoff and soil detachment during 2.5 years. Additionally, we assessed cast production in the entire catchment by grid sampling and measured soil transported out of the catchment. We hypothesized that (1) earthworm casting activity is influenced by climatic variables and shows highest activity during the rainy season, and (2) that cast production might counteract soil erosion.
Section snippets
Study site
This study was carried out from 2016 to 2018 in a tropical catchment located in Dong Cao village in the North of Vietnam (20° 57′N, 105° 29′E). The site belongs to the M−Tropics long-term observatory. The Dong Cao catchment with an area of 46 ha is located on sloping land with an average slope of 40%, locally exceeding 100%. Annual rainfall varied from 1770 mm in 2017 to 2224 mm in 2018. The annual air temperature was 23 °C in 2017 and 2018. The dry season lasted from November to April and was
Earthworm cast production at the plot scale and consequence on soil detachment
The production of casts at the plot scale varied from 16 to 219 t ha−1 year−1 and decreased in the order meadow > woodland > shrubland (p < 0.01) (Fig. 2). The production of casts was higher in 2017 than 2018 in woodland and meadow (p < 0.01 for in both cases), whereas the opposite was found in shrubland (p < 0.05). In shrubland and meadow, the production followed a periodicity of 6 months with lowest values occurring in December and July. In woodland, cast production followed a periodicity of
Earthworm cast production varies in space and time
The quantity of surface casts produced in the three vegetation units during the years 2017 and 2018 varied from 16 to 219 t ha−1 at plot scale (i.e., between 0.2 and 2.2 cm of soil considering the soil bulk density of 1 g cm−3), which is of the same order of magnitude as was found in other tropical regions (Hauser et al., 2012, Madge, 1969). The most important factor explaining this variability is vegetation. Differences are most likely linked to the abundance of earthworms and not their
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgments
This project was financially supported by the CNRS/INSU (VINAWORM) research program under the framework of the EC2CO program. Dong Cao catchment is part of Multiscale Tropical Catchments (M-Tropics) project (https://mtropics.obs-mip.fr/) supported by the French national research institute for sustainable development (IRD). We are very grateful to Kim Yen Do for her invaluable help in organizing the experiment. We also acknowledge the technical assistance of the farmers in Dong Cao village for
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