Biomass used for fuel can be derived from different sources, one of them being ethanol production via fermentation of biomass. Sugarcane Saccharum spp. The current trend is for growth in the biofuel sector, mainly due to geopolitical, economic and environmental issues further promoting the use of ethanol as alternative to fossil fuel. For instance, possible negative effects include enhanced greenhouse gas GHG emissions, especially of the potent N 2 O with a global warming potential of as compared to CO 2 , emitted from synthetic fertilizer application.
On the other hand, positive environmental consequences might result, if soil C and N stocks could be increased due to adaptation of agricultural management such as improved treatment of crop residues.
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Thus, biofuel production is an important component in any sustainable regional management strategy, requiring the balancing of ecological and economic demands. In Brazil, burning the dry leaves and tops in order to facilitate the sugarcane harvesting and transportation of the stalks is still a common practice. Due to a combination of pressure from changes in the public opinion and economical reasons, sugarcane is changing from a burned into an unburned system.
Mechanical harvesters have been developed that can take the stalk and leave the residues on the field, forming mulch, in a system called green cane management. The conversion from burning sugarcane to green management of sugarcane will have impacts on the biogeochemical cycling of C and N in the plant soil system. Instead of burning, "green" management will result in the deposition of large amounts of plant litter on the soil surface after harvest, ranging from 10 to 20 tons of dry matter per hectare, which will have impacts on the whole production process of sugarcane, influencing yields, fertilizer management and application, soil erosion, soil organic matter SOM dynamics as well as GHG emissions CO 2 , N 2 O, CH 4.
From a GHG perspective, the conservation of sugarcane residues prevents emissions from the burning process, may promote C sequestration in soils and fixes N during the decomposition process replacing the need for, and GHG emissions from, fossil fuel based nitrogen fertilizer sources. Measurements of soil C and N stocks and associated greenhouse gas emissions from the burned and unburned sugarcane systems and in the sugarcane expansion areas are still scarce.
Therefore, the main objective of this study was to propose appropriate methodologies for the determinations of soil organic C and GHG fluxes in sugarcane production.
The proposition of appropriate methodologies is important since strategies to mitigate climate change through the use of biofuels such as ethanol are associated not only to the increase in the amount of C stored in soils but also to the reduction of GHG emissions to the atmosphere. Soil organic matter is an important pool in the C cycle, comprising Pg of C in the first meter of soil, of which one quarter is in tropical soils Crasswell and Lefroy, The terrestrial biosphere, which also includes Pg in vegetation, represents three times as much C as present in the atmosphere.
Soil C stock dynamics in agricultural systems is conditioned by factors such as climate, soil texture, topography, type and amount of organic material added as soil and surface litter, and the degree of disturbance by tillage operations.
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Soil C stock changes are usually not detectable in the short term days and months , but in the time frame of years Minasny et al. The effect of management practices and land use change on soil C on a specific site can be detected through two main ways. In the first approach, measurements are taken in the same area at different moments in time, in long term experiments. A second approach, a chronosequence, is possible when there is a set of areas with similar topography, soil type and soil texture, with different land uses and periods of conversion.
A reference area is selected, which is usually native vegetation or the previous land use site. Then, areas representing the main land uses with different times since conversion are sampled, providing an estimate of the temporal changes in soil C. Two examples of chronosequences are described in Figure 1. In the first example, a native vegetation area was converted to sugarcane with pre-harvested burning, which caused a decline in soil C stocks.
After 20 years of burning, there was a change in harvest system to unburned sugarcane, with maintenance of crop residues on the soil, which led to an increase in soil C stocks. Areas with 10 and 20 years of unburned management were used to estimate this increase in soil C with time.
In the second example, degraded pastures, with low baseline soil C stocks, were converted into unburned sugarcane, and there was a steady increase in soil C stocks, demonstrated by sampling in fields under unburned sugarcane for 1, 10 and 20 years. The main advantage of the chronosequence methodology is that it makes it possible to concentrate the sampling in one occasion, instead of waiting several years to resample.
In the characterization of the soil C content of an area, it is generally not possible to examine the whole and it is therefore necessary to take samples. The soil samples collected must be as representative as possible of the whole area being characterized. The pre-selection of the area where samples will be taken could be made using soil maps, land use maps, aerial photographs, satellite images and interviews about land use history. In parallel to the desk-top work, site visits can be used to evaluate the exact location of the sampling points.
The grid sampling scheme provides a good coverage of the sampled area, while allowing for future identification of the area for re-sampling purposes. Each sampling area should be georeferenced using a GPS device, and the sampling pits should be plotted on a map of the area. However, several authors have emphasized the need to investigate soil C stocks at deeper layers when assessing the impact of management changes in sugarcane, preferably down to one meter Resende et al.
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Samples could be sampled from the following soil layers: Regression models or pedotransfer functions Benites et al. In the three deeper soil pits till 1. Once the sampling grid is established and marked on the field, soil sampling will consist in two steps: i gaining access to the point of sampling removing plant litter and digging the trenches down to the desired depth of sampling , and ii taking the soil sample.
The bulk of surface litter must be removed manually with care.
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The fine litter closer to the soil surface must be lightly dusted off, avoiding the removal of soil particles on the surface layer. The deeper trenches will measure 1. Once the pits are dug, soil samples will be taken in 0. In order to determine soil bulk density, some of the samples should be undisturbed, preserving soil structure.
When sampling in the sugarcane fields, samples should be taken both in the rows and the inter-rows, in order to detect possible spatial differences related to traffic of machines, tillage operations and root C turnover. The soil samples should be conditioned in plastic bags previously labeled and taken to the laboratory for sample preparation.
Samples will be air dried, homogenized and sieved in a 2 mm screen. The fraction greater than 2 mm needs to be weighted to calculate the adequate C stock for the given soil layer; although this fraction is considered as C-free. Subsamples of the soil sieved in 2 mm screens should be ground and sieved at 60 meshes for C determination by dry combustion Nelson and Sommers, This method provides total C, which is composed of inorganic from carbonates and organic C. In most Brazilian soils, the inorganic C content is small; therefore the total C content determined by dry combustion is mostly comprised of organic fraction.
However, in soils with high carbonate content, the organic C determination can be done by wet oxidation with dichromate Walkley and Black, or by dry combustion after a preliminary carbonates removal with acid, typically HCl 0. Several studies that focus on SOM dynamics under different soil and residue management systems present the results in C content, not in stocks. Nevertheless, the concept of stocks is more useful than C content, since it is a measurement of the mass in a specific volume of soil. Since C stock is also a function of soil bulk density, factors such as machine traffic and soil tillage, which affect soil density, could influence the results.
By correcting the density of all sites to a reference area, the stock comparison will be done considering the same mass of soil Ellert and Bettany, ; Moraes et al.
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Carbon stocks in the areas being evaluated should be calculated in an equivalent depth basis, i. The reference areas is generally the previous land use e. Another possible approach is to use the mass coordinate system Gifford and Roderick, ; McBratney and Minasny, Greenhouse gases emissions to the atmosphere and their contribution to climate change have attracted worldwide attention. This has resulted in a need for process based understanding of the main factors influencing the exchange of these gases between the soil and atmosphere.
The CO 2 concentration increased of to ppm parts per million in last decades, and actually increase the rate of 2. According to Forster et al. In the period from to , the increase in CO 2 emissions was 1. The increase in CO 2 emissions is due to a combination of factors such as deforestation, biomass burning, use of fossil fuels, cement production, tillage operations and lime application in agriculture Denman et al. Although CO 2 fluxes from the soil can be intensified in agricultural areas, this gas is not considered in the global balance of GHG.
According to the Intergovernmental Panel on Climate Change IPCC, a,b CO 2 fluxes are measured by mass balance; CO 2 is assimilated through photosynthesis in the next sugarcane cycle and, thus, does not contribute towards an increase in the GHG concentrations in the atmosphere. The remainder comes from natural sources, as such wetlands and termites IPCC, b. Methane emissions from cultivated soils in tropical conditions are usually very low or even negative Mer and Roger, thus, not contributing to the GHG balance and therefore will not be emphasized in the present work.
N 2 O is a potent GHG contributing to global warming and atmospheric ozone depletion. Despite its low concentration of 12 to ppb IPCC, a , it is the fourth largest contributor to enhance warming Denman et al. The N 2 O concentration in the atmosphere is substantially lower than CO 2.