| 英文摘要: | The effects of land-use change (LUC) on soil carbon (C) balance has to be taken into account in calculating the CO2 savings attributed to bioenergy crops1, 2, 3. There have been few direct field measurements that quantify the effects of LUC on soil C for the most common land-use transitions into sugar cane in Brazil, the world’s largest producer 1, 2, 3. We quantified the C balance for LUC as a net loss (carbon debt) or net gain (carbon credit) in soil C for sugar-cane expansion in Brazil. We sampled 135 field sites to 1 m depth, representing three major LUC scenarios. Our results demonstrate that soil C stocks decrease following LUC from native vegetation and pastures, and increase where cropland is converted to sugar cane. The payback time for the soil C debt was eight years for native vegetation and two to three years for pastures. With an increasing need for biofuels and the potential for Brazil to help meet global demand4, our results will be invaluable for guiding expansion policies of sugar-cane production towards greater sustainability. Energy crops have expanded significantly in Brazil during recent years. Between 2000 and 2012, nearly 5 Mha of sugar cane were added, bringing the current total to 9.7 Mha (ref. 5), half of which is used for the production of energy. This expansion has made sugar cane the main source of renewable energy in Brazil6. However, the full impact of sugar cane on greenhouse gas (GHG) emissions requires that the effects of converting land to sugar cane also be considered. Several studies indicate that energy crop expansion may result in a carbon debt1, 2, 3 due to significant carbon losses as CO2, promoted by activities such as slash and burn of native vegetation7 or by the accelerated decomposition of soil organic matter (SOM) due primarily to the disturbance of the soil structure or reduced inputs8. The replacement of degraded lands with low soil carbon (C) stocks, with high productivity energy crops, may reduce the payback period of the C debt incurred from land-use change (LUC), or even eliminate the payback1, 2, 3, resulting in a positive soil carbon balance or a biofuel carbon credit. Gains in soil C could be achieved with proper soil management and high rates of organic matter input from plant residues, allowing soils to contribute to GHG mitigation of biofuel-related land use and LUC9, 10. Brazilian sugar-cane production is concentrated in the south-central region of the country, comprising almost 90% of the national production11. In terms of LUC to sugar cane, there are indications that more than 95% of recent expansion has been from pasture (~70%), grain crops (~25%) and citrus (~1%; refs 12, 13). The conversion of natural vegetation into sugar cane has occurred in the past, but represents less than 1% of the expansion in this area from 2000 to 200913. Here we investigate the effect of LUC on soil C stocks and calculate the carbon payback time for sugar-cane ethanol production in Brazil. Measurements from 135 study sites, forming 75 comparison pairs (CP), and ~6,000 soil samples in south-central Brazil were analysed, for three types of land use conversion into sugar cane from: native vegetation, pastures and annual cropland. Measurements were taken for multiple soil depth increments to facilitate comparisons with previous studies, which are often restricted to surface layers (for example, 0–30 cm), but also to provide a more complete C inventory encompassing the near full depth of rooting (for example, 0–100 cm). Measurements for the 75 CP were distributed across 13 regions in south-central Brazil (Fig. 1). The majority were areas in which sugar cane replaced pastures (57 CP) followed by conversions from annual cropland (13 CP) and cerrado (5 CP), known as Brazilian savannah. Soil C stocks were determined for each of the 75 CP for 0–30 cm, 0–50 cm and 0–100 cm depth increments (Supplementary Table 1). Soil C stock changes were calculated from response ratios, referred to as LUC factors, which represent the relative change in SOC stocks due to LUC. A response ratio equal to 1 represents no change, values <1 mean loss and values >1 mean gain. The LUC factors were derived for five-year time blocks, to coincide with sugar-cane regeneration cycles, for up to 20 years (IPCC timeframe to approximate equilibrium of soil C stocks). The LUC factors were calculated for a 20-year time span to estimate carbon debt (or credit) and payback times (Table 1).
| http://www.nature.com/nclimate/journal/v4/n7/full/nclimate2239.html
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