| 英文摘要: | Sugar cane ethanol replaces fossil fuels, but changes in soil carbon could offset some of the benefit. Now, a study shows minor loss of soil carbon when pastures and croplands are converted to cane, but larger losses when converting native savannahs. As if growing enough food without clearing native forests and savannahs were not hard enough, an increasingly large land area is now being used to grow biofuel crops1. Brazil alone grew 98,000 km2 of sugar cane in 2012, harvesting 721 million tonnes of cane, about half of which was destined for producing ethanol2. Burning biofuels in lieu of fossil fuels may increase energy security and mitigate climate change, but fully accounting for net greenhouse gas (GHG) savings has proved challenging. Among other things, limited data exist on potential soil carbon losses or gains when sugar cane replaces other land uses. As they report in Nature Climate Change, Mello and colleagues3 use field measurements of soil carbon in Brazil to estimate the carbon debt and payback time associated with sugar cane expansion over a range of land covers. Accounting for the GHG savings associated with sugar cane ethanol production requires reliable estimates of: (1) fossil fuels used during sugar cane production, transport and ethanol processing; (2) carbon and other GHG emissions when native forests or savannahs are cut down for cane production; (3) changes in soil carbon stocks during land conversion and crop production. These emissions add up to a 'carbon debt' that must be 'paid back' before a biofuel crop can be considered a net benefit for climate change mitigation. The paybacks include the sequestering of carbon in soils as biofuel crops grow and the fossil fuel emissions avoided through ethanol use. Mello et al.3 report field measurements of changes in soil carbon when cattle pastures, croplands or native savannahs were converted to sugar cane. Earlier studies4, 5 were based on broad-brush assumptions about how much soil carbon would be lost with tillage. The current study supports those conclusions, but provides greater confidence based on soil data from 75 pairwise comparisons of land conversion in Brazil. An average soil carbon loss of 10% was indicated by a comparison of 57 pasture-to-cane conversions. Combining these carbon losses with the gains from avoided fossil fuel emissions, the authors estimate a three-year payback time for ethanol production to compensate for the soil carbon debt and become a net benefit for climate mitigation. All crops leave a portion of their biomass behind in the soil, but sugar cane is an especially prodigious biomass producer, building up soil carbon stocks compared with many other crops. Pairwise comparisons of thirteen cropland-to-cane conversions indicated an average increase of 16% in soil carbon stocks — a carbon surplus that requires no (or negative) payback time. In contrast, five pairwise comparisons of conversions from native Cerrado (woodland/savannah) to sugar cane indicated an average topsoil carbon loss of 25%. Carbon stocks in deep soils (deeper than 30 cm) also showed a declining trend after conversion, but there were too few data to establish statistical significance. Considering only topsoil carbon loss, the authors estimate that savannah-to-cane conversion incurs an eight-year payback time. Adding the carbon lost from aboveground woody vegetation means it would take 17 years for ethanol production to compensate the carbon lost due to converting Cerrado. Sugar cane has been the star of first-generation biofuels, boasting greater carbon savings than corn, soy or palm oil1. With the world's largest fleet of flex-fuel vehicles (more than 20 million), Brazil has been a leader in ethanol use, making it an important part of its national commitment to reduce GHG emissions. Brazilian ethanol production is poised for continued growth as flex-fuel technology becomes more widespread and global demand for biofuels increases. But the potential climate mitigation benefits of expansion must be weighed against the consequences of land-use change. Given the significant carbon debt resulting from conversion of Cerrado to sugar cane and the importance of the remaining Cerrado for biodiversity and other ecosystem services7, this land-use transition is unwise from both a climate and a conservation perspective. In contrast, the findings of Mello et al.3 suggest that croplands and pastures can be converted to sugar cane without significant carbon debts. Brazil has a large supply of under-used, low-productivity pasturelands that are suitable for crop production8, 9, but sugar cane is just one of many potential uses for these lands. Competition for land among food, fuel and beef production greatly complicates the application of the new findings presented by Mello et al. (Fig. 1). Converting large areas of existing pastures to sugar cane could preclude expansion of other crops and would restrict beef production to a shrinking area of pasturelands. Barring large-scale intensification of cattle production, there is a risk that such pasture-to-cane transitions could push the expansion of pasturelands and other croplands into new regions, spurring new deforestation. Such 'leakage' (as it is called in policy circles) is notoriously difficult to document, as it may occur across state and national boundaries, is subject to varying time lags, and is technically difficult to establish causality10.
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