Soil carbon storage and stratification under different tillage systems in a semi-arid region

Abstract

Changes in the agricultural management can potentially increase the accumulation rate of soil organic carbon (SOC), thereby sequestering CO₂ from the atmosphere. In a long-term experiment (1992–2008) we examined the effects of various tillage intensities: no-tillage (NT), minimum tillage with chisel plow (MT), and conventional tillage with mouldboard plow (CT), on the topsoil profile distribution (0–30 cm) of SOC, on a semi-arid loamy soil from Central Spain. The crop sequence established was cheap pea (Cicer arietinun L.) cv. Inmaculada/barley (Hordeum vulgare L.) cv. Volley. Soil organic carbon in the various tillage treatments was expressed on a content bases and the equivalent soil mass approach. Measurements made at the end of 17 years showed that in the 0–30 cm depth, stocks of SOC had increased under NT compared with MT and CT. Most dramatic changes occurred within the 0–5 cm layer where plots under NT had 5.8 and 7.6 Mg ha⁻¹ more SOC than under MT or CT respectively. No-tillage plots, however, exhibited strong vertical gradients of SOC with concentrations decreasing from 0–5 to 20–30 cm. Stratification ratios of SOC in 1992 showed no significant differences between tillage systems. On the contrary, from 1993 onwards all stratification ratios were significantly higher in NT than in the other two tillage systems. In addition, since 2003 stratification ratios of SOC obtained under NT were systematically >2 and more than 2-fold those obtained under MT and CT. Stratification ratios >2 are uncommon under degraded conditions and could suggest that NT management system may have the most benefits to soil quality in semi arid regions with low native soil organic matter.

Introduction

It has been widely recognised that world soils play a key role in the global C cycle and that agricultural activities constitute a major source of C emission to the atmosphere (Hall, 1989, Batjes, 1998, Mosier, 1998, Rosenzweig and Hillel, 2000). Since an important part of the present atmospheric C pool came from the soil, there exist a potential to reverse the trend and sequester C into the pedosphere through appropriate land use, farming systems and management practices (Lal et al., 1998). The potential of different ecoregions of the world to sequester C is climatic dependent, being higher in tropical and temperate regions where crop growth conditions are more favorable. Due to the droughts, arid and semiarid regions have lower capacity to sequester C on an unit area basis. However, because of the large extent of such areas, the total capacity may be significant (Woomer, 1993).

Important practices for improving soil productivity and at the same time for enhancing C sequestration include: organic residue management, mulch farming, tillage systems, adapting crop rotations and cropping/farming systems with avoidance of bare fallow. Soil tillage practices are of particular significance to the soil C status because they affect C dynamics directly and indirectly. Following long-term tillage, soil C stocks can be reduced as much as 20–50% (Murty et al., 2002, Ogle et al., 2003). Conservation tillage on the contrary, reduced the negative impact of tillage and has proved to have the potential for converting many soils from sources to sinks of atmospheric C (Freibauer et al., 2004, Baker et al., 2007, Moreno et al., 2006, Martín-Rueda et al., 2007) and can be considered one of the largest potential sources of greenhouse gas mitigation within the agricultural sector. Lal et al. (1998) estimated that widespread adoption of conservation tillage on some 400 million ha of cropland by the year 2020 may lead to total C sequestration of 1.5–4.9 Pg.

Changes in SOC as influenced by tillage are expected to be more noticeable under long-term rather than short-term tillage practices. In an analysis of 17 experiments Kern and Johnson (1993) concluded that a change from CT to NT sequester the greatest amount of C in the top 8 cm of soil, a lesser amount in the 8- to 15-cm depth, and no significant amount below 15 cm. They also assumed the duration of C sequestration to be between 10 and 20 years. Paustian et al. (1997) compared 39 paired tillage experiments, ranging in duration from 5 to 20 years, and estimated that NT resulted in an average soil C increase of 285 g m⁻² with respect to CT. In an analysis of 17 European tillage experiments Smith et al. (1998) found that the average increase of SOC, with a change from CT to NT, was 0.73 ± 0.39% year⁻¹, and that SOC may reach a new equilibrium in approximately 50–100 years. West and Post (2002) reported that a change from CT to NT can sequester an average 57 ± 14 g C m⁻² year⁻¹, and based on regression analyses indicate that, within 5–20 year, C sequestration rates can be expected to have a delayed response, reach peak sequestration rates in 5–10 years, and decline to near zero in 15–20 years. Recently, Hernanz et al. (2009), evaluated SOC variations in three tillage systems over a period of 20 years and concluded that the average SOC was 14% higher in NT than in MT and CT and that the steady state of SOC sequestration was reached after 11 years of starting the experiment in NT and 12 years in MT and CT. However, some authors (Halvorson et al., 2002, Al-Kaisi et al., 2005) after several years of experimentation, have encountered no significant differences between CT, MT and NT in relation to the SOC stored either in the top soil layer or in deeper layers.

The objectives of this study were to investigate over a period of 17 years the effects of three tillage systems: no-tillage (NT), minimum tillage with chisel plow (MT), and conventional tillage with mouldboard plow (CT), on SOC status and on SOC stratification ratio as indicators of management-induced changes in soil quality.