Organic matter storage in a sandy clay loam Acrisol affected by tillage and cropping systems in southern Brazil

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Abstract

Soil organic matter decline and associated degradation of soil and environmental conditions under conventional tillage in tropical and subtropical regions underline the need to develop sustainable soil management systems. This study aimed first to evaluate the long-term effect (9 years) of two soil-tillage systems (conventional tillage: CT, and no-tillage: NT) and two cropping systems (oat (Avena strigosa Schreb)/maize (Zea mays L.): O/M; and oat+common vetch (Vicia sativa L.)/maize+cowpea (Vigna unguiculata (L.) Walp): O+V/M+C without N fertilization on total organic carbon (TOC) and total nitrogen (TN) concentrations in a sandy clay loam Acrisol in southern Brazil. The second objective was to assess soil potential for acting as an atmospheric CO2 sink. Under NT an increase of soil TOC and TN concentrations occurred, in both cropping systems, when compared with CT. However, this increase was restricted to soil surface layers and it was higher for O+V/M+C than for O/M. The O+V/M+C under NT, which probably results in the lowest soil organic matter losses (due to erosion and oxidation) and highest addition of crop residues, had 12 Mg ha−1 more TOC and 0.9 Mg ha−1 more TN in the 0–30.0 cm depth soil layer, compared with O/M under CT which exhibits highest soil organic matter losses and lowest crop residue additions to the soil. These increments represent TOC and TN accumulation rates of 1.33 and 0.10 Mg ha−1 per year, respectively. Compared with CT and O/M, this TOC increase under NT and O+V/M+C means a net carbon dioxide removal of about 44 Mg ha−1 from the atmosphere in 9 years. NT can therefore be considered, as it is in temperate climates, an important management strategy for increasing soil organic matter. In the tropicals and subtropicals, where climatic conditions cause intense biological activity, in order to maintain or increase soil organic matter, improve soil quality and contribute to mitigation of CO2 emissions, NT should be associated with cropping systems resulting in high annual crop residue additions to soil surface.

Introduction

Conventional tillage with intensive soil disturbance promotes rapid decrease of soil organic matter and subsequent CO2 emission increase. A chemical, physical and biological soil degradation process then develops, negatively affecting crop productivity. In tropical and subtropical areas, where high temperatures and humidity accentuate soil degradation a main agricultural research goal should be the development of management systems that increase soil conservation and crop productivity. Therefore, knowledge of the main factors involved in soil organic matter increase is fundamental.

Tillage is the principal agent producing soil disturbance and subsequent soil structure modification, increasing potential soil organic matter loss by erosion and biological decomposition (Langdale et al., 1992, Carter et al., 1994). Quantitatively, the latter is thought to be the primary source of organic matter loss triggered by soil tillage (Rasmussen et al., 1998). Cambardella and Elliot (1992) determined that the winter wheat-fallow system after 20 years under no-tillage (NT) resulted in 6.7 Mg ha−1 more organic carbon than plow disk. In addition, management systems promoting soil organic carbon accumulation provide an atmospheric CO2 sink.

The quantity of residue addition by cropping systems can affect soil organic matter accretion in degraded soils. A close relationship between crop residue addition during 5 years and carbon and nitrogen concentrations in an Acrisol under NT in southern Brazil was observed by Testa et al. (1992) and Teixeira et al. (1994). Other authors have also emphasized the great importance of residue addition in restoring soil organic matter (Angers et al., 1997, Larney et al., 1997, Potter et al., 1997, Huggins et al., 1998, Janzen et al., 1998).

Despite general soil organic matter increases under NT and cropping systems having high residue addition, increases also depend on other factors such as climate, mainly temperature and rainfall (Alvarez and Lavado, 1998), soil texture and mineralogy (Sollins et al., 1996, Hassink and Whitmore, 1997, Parfitt et al., 1997). Management systems need to be investigated regionally under different climate and soil conditions to fully clarify both conservation management potential and effect on global changes.

This study aimed at (1) evaluating the long-term effect (9 years) of two soil-tillage systems with different soil disturbance intensities and two cropping systems with different C and N additions on TOC (total organic carbon) and TN (total nitrogen) storage in a sandy clay loam Acrisol in southern Brazil; and (2) assessing soil potential for acting as a C sink, mitigating CO2 emissions to atmosphere.

Section snippets

Site descriptions

This study was based on a long-term soil management experiment started in September 1985 in an area previously cultivated for 15 years under conventional tillage at the Experimental Station of the Federal University of Rio Grande do Sul, Eldorado do Sul, Rio Grande do Sul State, Brazil. Station area geographic coordinates are 30° 50′ 52″ S and 51° 38′ 08″ W. The soil is a sandy clay loam Acrisol (Dark red podzolic by Brazilian taxonomy and Paleudult by US soil taxonomy) whose physical, chemical

Results

The O/M and O+V/M+C cropping systems added 4.35 and 7.95 Mg C ha−1 per year to soil, respectively (data not shown). The N recycled or added in the same cropping systems was 74 and 213 kg ha−1 per year, mainly due to biological fixation by legumes and recycling by crops.

Soil-tillage and cropping systems both affected TOC and TN concentration in soil profile (Fig. 1, Fig. 2). Soil TOC and TN concentration under NT in both cropping systems (Fig. 1, Fig. 2) were higher than under CT. However, this

Discussion

In our experiment, soil organic matter storage in sandy clay loam Acrisol was largely dependent on tillage and cropping systems (Fig. 1, Fig. 2) and their effects on organic matter loss and addition rates. NT resulted in higher soil organic matter content in soil profile (0–30 cm) than CT in both cropping systems (Table 2), an increase occurring mainly on surface layers and when using O+V/M+C system (Fig. 1, Fig. 2).

Long-term experiments connected to studies of C and N storage in agricultural

Conclusions

In tropical and subtropical regions, NT is the primary management strategy for increasing soil organic matter. Large quantities of crop residue additions by cropping systems under NT accentuate soil organic matter accumulation. The highest increase occurs at soil surface layers, but a net increase of soil organic matter storage is observed in total soil profile (0–30 cm). In these management systems, soil acts as a C sink, contributing to mitigating CO2 emissions to the atmosphere.

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