Soil organic matter and related physical properties in a Mediterranean wheat-based rotation trial

Abstract

Under semi-arid Mediterranean conditions, limited moisture is the main constraint to rainfed cropping with wheat (Triticum aestivum), barley (Hordeum vulgare), and food and forage legumes. With increasing land-use pressure, moisture-conserving fallowing is being replaced by continuous cropping, which is considered an unsustainable practice. Thus, a long-term trial with durum wheat (T. turgidum var. durum) was established in 1983 at Tel Hadya, Aleppo, Syria (mean annual rainfall 330 mm) to assess alternative rotation options to fallow and continuous cropping. Nitrogen (N) and grazing/residue management were secondary factors. Soil aggregation, infiltration, hydraulic conductivity, and total soil organic matter and component fractions (fulvic and humic acids and polysaccharides) were determined at the end of 12 years. Some rotations, e.g., medic (Medicago sativa) and vetch (Vicia faba), significantly increased soil organic matter (12.5–13.8 g kg⁻¹ versus 10.9–11 g kg⁻¹ for continuous wheat and wheat/fallow). All measurements, or indices, indicated parallel trends with increasing organic matter, e.g., coefficients of macro-structure, micro-aggregation, and water-stable aggregates, and decreasing dispersion. Similarly, legume rotations had higher infiltration rates (16.2–21.8 cm h⁻¹ versus 13.9–14.4 cm h⁻¹ with continuous wheat and wheat/fallow) and hydraulic conductivity rates (8.7–12.4 cm h⁻¹ versus 6.2–7.4 cm h⁻¹ with continuous wheat and wheat/fallow). We conclude that cereal/legume rotations, in addition to being biologically and economically attractive, also enhance soil quality and thus promote soil use sustainability in fragile semi-arid areas as in the Mediterranean zone.

Introduction

Dryland crop production in any part of the world is beset by inherent constraints, notably drought and a fragile soil environment. The semi-arid lands bordering the Mediterranean represent such an environment (Cooper et al., 1987, Kassam, 1981), and have been cultivated for millennia, sustaining mankind and civilization since the beginning of settled agriculture. With increasing land-use pressure, land degradation has become a serious threat under such conditions (Lal, 2001). Indeed, the sustainability of cropping system in the Mediterranean zone is a major concern due to the risk of land degradation (Lal, 2002).

A critical edaphic factor in dryland agroecosystem productivity is total soil organic matter concentration, which is partly controlled by crop biomass input via residues and roots (Parton et al., 1987). Soil organic matter affects soil physical, chemical, and biological processes. In cultivated soils, physical processes are of major importance; a decline in soil organic matter invariably leads to a decrease in soil porosity and an increase in bulk density (Tisdall and Oades, 1982). Under rainfed conditions, reduced soil organic matter can decrease the effective utilization of limited precipitation by decreasing infiltration and hydraulic conductivity and increasing runoff and erosion.

The specific effects of organic matter on soil aggregation have been more closely linked to various fractions of organic matter, especially polysaccharides and humic and fulvic acids, particularly the latter (Hayes and Swift, 1990). These components cause physico-chemical linkages with soil clay particles to form stable soil aggregates, a process enhanced where soils are high in iron oxides (Payne, 1988), as in typical Mediterranean soils.

Tillage is a major factor dictating loss of soil organic matter (Rasmussen et al., 1989), in addition to frequent fallowing (to conserve moisture) and crop residue removal by grazing or harvesting (Rasmussen and Collins, 1991). Repeated inverting and pulverizing soil exposes soil organic matter to aeration and thus mineralization (Cannell and Hawes, 1994). While the process leads to reduced potential biological and biochemical activity (Doran et al., 1998, Riffaldi et al., 2002), the main problem is aggregate destruction (Golchin et al., 1994, Bossuyt et al., 2002, Plante and McGill, 2002, Achmed et al., 2003). Although tillage is used to prepare a seedbed and control weeds, a concern is to minimize any harmful effects of tillage while maintaining beneficial effects. Thus, the development of conservation tillage over the past few decades has indicated that soil organic matter can be maintained or improved (Zibilske et al., 2002, Franzluebbers, 2002) and contribute to carbon sequestration (Curtin et al., 2000). Manipulation of cropping systems using appropriate rotations can also enhance soil organic matter (Katsvairo et al., 2002, Whitbread et al., 2000) and thus improve soil quality and land-use sustainability.

With this background, a relatively long-term cereal/legume rotation trial was established in the rainfed wheat/barley belt in northern Syria in the early 1980s (Harris, 1995, Harris et al., 1995). Clear crop yield trends had become evident (Pala et al., 1999, Ryan et al., 1999). Observations of Ryan (1998) indicated that some legumes such as medic and vetch had produced higher levels of organic matter than continuous cereal cropping and fallow. Our objective was to relate changes in soil organic matter due to crop rotations to various soil physical processes that control water utilization in a semiarid climate.