Impact of Agricultural Management on Quality of Soil, Carbon Storage and Carbon Stratification

The different systems of agricultural exploration can change the deposition of plant residues on the soil and may increase or decrease carbon storage in the soil, acting either as a source or a drain for carbon into the atmosphere (Cerri et al., 2009). The adopted management system acts directly on the amount of carbon in the soil. Intensive soil tilling as used in conventional agriculture enhances losses by erosion and causes macro aggregates to break into smaller units and favors the exposure of the labile fraction of organic matter oxidants, causing its mineralization. This system also exposes residues to microbial action and attacks by their enzymes, resulting in an increase of CO emissions (Bruce et al., 1999; Six et al., 1999). 

 Changes in land use may increase the losses of stored carbon, which may be bigger than its sequestration. The conversion of grasslands into crop production causes losses in stored carbon varying from 14% to 33% (Soussana et al., 2010). Furthermore the deposition of pasture residues, predominantly dead leaves, stems and roots with high C: N ratio represent an average time of permanency usually long for the carbon stored in the soil, and grasslands can be considered overall as a carbon sink (Goudriaan, 1992). A FAO report (2010) presents pastures (native and cultivated) as the second largest potential sink of global carbon sequestration (C), with ability to drain 1.7 billion tons of C per year from the atmosphere, followed by forests, whose estimated capacity reaches 2 billion tons of C per year. Brazil, with about 197 million ha of pastures, stands as the country with the largest capacity for contribute to mitigating global warming through biological carbon sequestration (Braga, 2006; Corsi and Goulart, 2006), by recovering degraded areas, using a crop-livestock integrated system. 

Common examples of change in the land use are forests turned into pastures for grazing or into conventionally cultivated crops. The conversion of forests into pastures with adequate management can enhance carbon storage, but if pastures were poorly managed it will induce C losses; however the conversion of native vegetation to conventional agriculture invariably reduces C stocks (Carvalho, 2010). 

The use of appropriate management grazing practices, especially the replacement of soil fertility, enables the accumulation of C in the soil at a rate of 0.3 t of C. ha-1. year - 1, which corresponds approximately to the mitigation of 1 t of CO equivalent 2 ha-1 year-1. This value, quite conservative, would be enough to nullify approximately 80% of the annual emission of methane from an adult beef cattle, estimated at 57 kg which is equivalent to 1.42 tons of CO (57 kg CH . year-1 x 25 potential global 2 4 warming gas = 1.42 t CO -eq) (Machado et al., 2011). 

The quantification of C stocks in the soil can point out the most appropriate land use and which can be more efficient and environmental-friendly. The different uses for grasslands (improved pastures, hay areas, corn silage areas) could be used to partially mitigate greenhouse gas emissions on ranches that raise beef cattle. The variations in labile and recalcitrant fractions of organic matter change with alterations in land use. The labile fraction is highly sensitive to changes in management, and represents an important nutrient reservoir, releasing them in the short term for plants, as well as energy and C to microorganisms in the soil (Conte et al., 2011; Silva et a.l, 2011). Moreover, recalcitrant fractions, especially humic substances, act as regulators of chemical and biological soil and plant processes (Loss et al., 2010) and are important to the sequestration of atmospheric C (Mendonça and Silva, 2007). Carbon Management Index (CMI) can be used as an indicator of the quality of soil management, enabling its evaluation, whether the quality of the soil is improving or not: higher CMI values indicate soil of higher quality (Blair et al. 1995; Campos et al, 2011). IMC can be used to measure changes in soil organic matter, and considers aspects of labile organic matter, making it possible to compare the changes that occur in total organic C and labile C indifferent land use systems (Loss et al., 2011). 

The stratification ratio of carbon (SR) is the ratio between the stock of soil organic carbon in two distinct layers, the surface usually with strong influence of soil management practices, and the adjacent layer, which is less affected by agricultural operations (Franzluebbers, 2002). Higher SR values suggest better soil quality. The values of SR in degraded soils of temperate climates are usually less than 2.0. For tropical soils studies relating SR and retention of organic matter and soil quality are rare. 

The objective In this study was to evaluate changes in carbon stocks, as well as in the carbon management index, in an Eutroferric Red Latosol soil under different land use and several management systems: Forest, artificial degraded pasture with Panicum sp. var. Tanzania, improved artificial pasture with Panicum sp. cv. Tanzania, hay area, and conventionally managed corn crop in the municipality of Sertaozinho, at São Paulo State, Brazil.