Contribution of Tamarix aphylla to soil organic matter evolution in a natural semi-desert area in Tunisia

Highlights

• The contribution to SOM of Tamarix spp. residues was studied in a dryland.

• In drylands natural vegetation plays a key role in SOM accumulation.

• Tamarix spp. residues (litter) are rich in aromatic and aliphatic carbon.

• The so called soil ligno-humic fraction directly derives from the litter.

Abstract

A soil, classified as Arenosol (Eutri-Aridic Arenosol (Calcaric)), located in Neffatia (Tunisia) and populated by the shrub tamarisk (Tamarix aphylla), was studied to assess how the litter deriving from tamarisk can affect its characteristics. Several parameters were considered: particle size distributions (PSD), pH, cation exchange capacity (CEC), total CaCO₃ content, total nitrogen (TKN), phosphorus and total organic carbon (TOC). Down the soil profile the pH increased from 7.83 to 8.32 as a probable consequence of salts accumulation deriving from the mineralization of the organic matter and the limited leaching due to low rainfall. As expected, TOC and TKN decreased, from the top downwards, and the two parameters were well correlated (TOC vs TKN: R² = 0.98; p < 0.05; n = 3). CEC assumed progressively lower values reflecting the decreasing organic matter content (CEC vs TOC: R² = 0.93; p < 0.05; n = 3). PSD showed that the presence of roots influenced the quantity of fine particles down the profile and the PSD cumulative curves were indicative of an aeolian origin for the soil parent material, confirming the hypothesis that tamarisk interacts with the environment, trapping sediment and forming the so-called phytogenic dunes. By chemical and spectroscopic analyses, it was possible to assess that tamarisk plant residues directly contributed to the soil organic matter (SOM) accumulation and characteristics. Stable SOM (ligno-humic fraction) closely resembles that of the plant (leaves and stems) and is chemically lacking in the more easily degradable organic components such as fats, hemicellulose, cellulose and proteins. ¹³C CPMAS NMR spectroscopy showed that the so-called soil ligno-humic fraction consists of aromatic molecules such as tannins, and aliphatic carbon (i.e. cutins and suberins) already present in the plant and preserved by mineralization processes because they are the most resistant to biological degradation.

Introduction

Drylands constitute about 41% of the world's land area, and are distributed mainly in Africa and Asia (Prăvălie, 2016). These areas are characterized by average rainfall lower than the potential moisture losses through evaporation and transpiration, with the consequence that the negative balance between precipitation and evapotranspiration results in a short growing season for crops (UNEP, 1992). One of the main problems in drylands is the degradation of the natural ecosystems due to loss of soil organic matter (SOM) and desertification processes (Berdugo et al., 2020). To preserve soil in natural ecosystems, the most important strategy is to limit desertification and erosion by vegetation cover, the residues of which are also able to maintain an adequate concentration of SOM, improving soil quality and fertility (Lal, 2001). In drylands, plant litter residues accumulating on the soil surface are the primary source of SOM (Castellano et al., 2015) and they play a key role in reducing sand transport, improving the deposition of finer soil particles, and in increasing soil quality and fertility (Yair et al., 2018). Typically, soils of drylands are characterized by low organic matter (OM) content, sandy texture and limited water holding capacity (Lal, 2001) and, as consequence, only xerophytic vegetation adapted to the drought and salinity of arid soils and characterized by a high water-use efficiency is able to grow. Perennial vegetation varies considerably and tends to be sparse and sporadic and the land use is, above all, for nomadic pastoralism (Heathcote, 1983). Generally, soils' organic carbon in drylands ranges between 1% to less than 0.5% (Lal, 2001). However, some environmental aspects of arid soils, such as limited soil moisture, can work in limiting organic matter mineralization and, consequently, carbon dioxide (CO₂) emission to the atmosphere (Glenn et al., 1993). For this reason, the residence time of organic carbon in soils of drylands is long and, sometime, longer than in forest soils. An estimate (Klemmedson, 1989) of the soil organic carbon (SOC) content, based on different ecosystems and considering the inputs through the litter, showed average emissions equal to 19.2 kg OC m⁻² for temperate grassland ecosystems and 5.6 kg OC m⁻² for semi desert ecosystems. In drylands, where natural vegetation constitutes the primary source of input of organic residuals, the accumulation as OM is determined by both biomass productivity and plant residues degradation rate (Klemmedson, 1989). Other factors, able to influence biodegradation and/or conservation of organic residues in soils, are moisture, aeration, temperature, abundance and typology of clay, microbial population and, obviously, amount and chemical characteristics of the organic residues (Klemmedson, 1989). In general, environmental and biological conditions seem to control SOM conservation rather than the intrinsic quality of litter in terms of molecular structure and chemical composition (Kögel-Knabner, 2002). Litter quality assumes higher importance in soil carbon storage when soils show a deficit of OM (Wieder et al., 2014). In desert and semi-desert contexts, many grasses, shrubs and trees, such as Acacia tortilis, Argania spinosa, Peganum harmala and Tamarix spp. have ecological and economic importance and, at the same time, can play a key role in soil protection and stabilization against wind or water erosion (Cremaschi and Di Lernia 2001), while also providing sources of forage for animals (El-Beheiry and El-Kady, 1998; You et al., 2016). Among these plants, Tamarix aphylla (tamarisk) is a shrub or small tree species, widespread from North Africa to the Sind desert, adapted to live in arid climates with an average annual rainfall of 100–150 mm (Arbel et al., 2005). The plant is characterized by fast-growth and, by the continuous addition of its residues to the soil (leaves and stems), contributes to the formation of a litter layer in a short time. As consequence, tamarisk can play a crucial role in improving SOM in dryland and, as recently reported, it is able to promote soil carbon sequestration (Iranmanesh and Sadeghi, 2019).

The aim of this paper was the characterization of the litter and the soil, at different depths, underneath tamarisk plants growing in a semi-desert site located in the center of Tunisia. The contribution that the plant is able to give to the formation of stable OM (ligno-humic fraction) was investigated by using chemical and spectroscopic techniques.