Effects of termite sheetings on soil properties under two contrasting soil management practices

Highlights

• Termite sheetings substantially improved soil properties.

• Termite activity sequester organic matter in sheetings.

• Termite activity improves soil properties in organic tilled field.

• The positive effects are less pronounced under permanent raised beds.

Abstract

Soil organic matter (SOM) dynamics and termite activity have now been widely accepted as key players for improving soil properties in tropical agro-ecosystems. Numerous studies have described environmental impacts of aboveground termite mounds, while few data are available on temporary structures built for food foraging, called termite sheetings. The effects of termite activity on soil properties resulting from organic matter (OM) amendment under two contrasting management practices were studied in similar pedological and climatic conditions in Southern India (Auroville). Our results showed an increase in bio-available nutrients (K, Mg and P), organic carbon (OC) content, cationic exchange capacity (CEC), exchangeable base cations and water pH in the termite sheetings compared to the underlying and reference soils, in the organic tilled field. On the other hand, only bio-available K increased in the permanent raised beds. Aggregation processes were improved in termite sheetings for the organic tilled field, as the amounts of macroaggregates (250 μm – 2 mm) and protected microaggregates increased, whereas the amount of free microaggregates (50–250 μm) decreased. Moreover, termite activity favoured SOM storage in termite sheetings by increasing OC content in each aggregate fraction, while no differences were observed in the permanent raised beds. Our study demonstrates that termite activity can improve nutrient availability, carbon storage and pH conditions in agro-ecosystems but that the magnitude of the effect likely depends on the agronomic practices in use.

Introduction

The quantity and quality of soil organic matter (SOM) are key for soil functioning (Devine et al., 2014), improving soil water-holding capacity, cation exchange capacity (CEC), soil structure (Lehmann and Kleber, 2015) and nutrient sources for plant growth (Gao and Chang, 1996; Trumbore, 1997). In this regard, the mechanisms of SOM preservation in soil aggregates are of crucial importance to meet the climatic issues (increasing SOM stock in soils) and food security (improving soil functioning) while preserving the environment (decreasing groundwater and animal/plant contamination) (Devine et al., 2014; Lal, 2009). The relationship between SOM and aggregation is mutual: SOM improves soil aggregation, which in turn enhances organic carbon (OC) stabilisation. Macroaggregates are primarily formed by temporary binding agents such as roots and hyphae. When those aggregates degrade, the resulting fragments become encrusted with clays, forming microaggregates (Six et al., 2004; Waters and Oades, 1991). Soil C pool in microaggregates is the most stable (Lehmann et al., 2007), protecting SOM in the long term (Six et al., 2004). In contrast, macroaggregates have a low inertia, responding quickly to perturbations (Cammeraat and Imeson, 1998). Therefore, macroaggregate turnover is of major importance for SOM stabilisation (Six et al., 2004), as a decreased turnover leads to an increased OM stabilisation in stable microaggregates (Six et al., 2000, 1999, 1998).

Soil fauna are known to drive OC turnover and, therefore, soil aggregation. Termites are dominant insects in tropical soils, where they act as ecosystem engineers (Decaëns et al., 2006). In India, 300 termite species have been identified, among which only 35 have been reported as pests in agricultural crops (Chhotani, 1997; Jouquet et al., 2018). Besides termite negative effects, termites have been largely recognized as ecosystem services providers strongly controlling nutrient cycling by affecting soil-forming processes and more specifically OM decomposition and nutrient cycling (Hole, 1981; Jouquet et al., 2018). Their impact can be seen at four different scales: (i) at the landscape level, they are heterogeneity drivers (Donovan et al., 2001; Lavelle et al., 1992; Jouquet et al., 2016); (ii) at the soil profile scale, they are bioturbators (Bottinelli et al., 2015; Jouquet et al., 2016); (iii) at the aggregate level, they reorganize the litter cover, by covering it with wet soil particles enriched with clay (Bottinelli et al., 2015; Jouquet et al., 2016) ; (iv) and at the clay mineral level, they are weathering agents (Bottinelli et al., 2015; Jouquet et al., 2016), triggering soil weathering (Jouquet et al., 2007, 2002a). They also modify the properties of clay minerals (Mujinya et al., 2010), especially by increasing the content of expandable layers, as demonstrated by Jouquet et al. (2002b) in laboratory conditions.

Termites play a major role in bioturbation, which is always coupled with foraging (Kaiser et al., 2017; Malaisse, 1978). Termites cover OM with sheetings made up of fine soil particles cemented with saliva and/or faeces, creating a temperature-controlled environment (Ferrar and Watson, 1970; Jouquet et al., 2015). Those temporary structures have a higher OC content than the surrounding soil (Awadzi et al., 2004; Harit et al., 2017a, 2017b; Lavelle et al., 1992; Mora et al., 2003). By protecting SOC in biogenic structures, termites impact carbon dynamics (Lavelle et al., 2001). When these structures are degraded, weathering and decomposition reactions release nutrients available for plants (Bottinelli et al., 2015; Lavelle et al., 1992). Sustaining termites with another food source helps reducing the likelihood of crop damage (Jouquet et al., 2018). In the case of crusted soils, the combination of termite activity and organic inputs helps to soften the soil, since termites burrow through the crust (Jouquet et al., 2014; Léonard and Rajot, 2001). In general, all agricultural practices with return of plant residues to the soil are favourable for termite activity and for stabilisation of biogenic structures (Lavelle et al., 2001).

Compared to earthworms, termites have been less studied (Jouquet et al., 2018; Six et al., 2004). The impacts of the latter on soil properties have been explored mainly in Africa (Erens et al., 2015; Mujinya et al., 2013), whereas little is known in Asia, especially in an agricultural context (Jouquet et al., 2015). In Southern India, termite influence on soil aggregation has been studied in permanent mounds (Jouquet et al., 2016). On the contrary, the impact of temporary structures, such as termite sheetings, that will return to the soil after rainfall or watering is not well documented so far (Harit et al., 2017b). To our knowledge, only two studies on termite sheetings have been conducted in India (Harit et al., 2017a; Jouquet et al., 2015). Jouquet et al. (2015) suggested that long-lasting structures are the most different from the underlying soil. Harit et al. (2017a) showed that the physicochemical properties (OC content, pH, particle size distribution, concentration of soluble ions) of termite mounds were similar to the surrounding soil, whereas that of termite sheetings showed higher OC and clay contents. However, termite-induced beneficial effects on soil properties can be strongly affected by soil management practices. There is therefore a need to better understand the role of termite sheetings on soil properties in identical pedological context but for contrasting management practices.

We aim to determine the physico–chemical properties of termite sheetings as well as the distribution of OC in aggregate fractions. Our field approach has been implemented in the context of a soil restoration project started in Auroville (India) in 1968. Two contrasting soil practices have been studied: organic tilled land and permanent raised beds. In each of these agricultural contexts, we compare physico–chemical properties of the digested soil (termite sheetings) with the underlying soil and a reference soil. This approach allows us to assess how termites affect soil properties and SOM dynamics in two different agricultural contexts.