Center for Nuclear Energy in Agriculture, University of São Paulo, Av. Centenário 303, 13416-000 Piracicaba, SP, Brazil.
Center for Nuclear Energy in Agriculture, University of São Paulo, Av. Centenário 303, 13416-000 Piracicaba, SP, Brazil.
Sci Total Environ. 2015 May 15;515-516:30-8. doi: 10.1016/j.scitotenv.2015.02.025. Epub 2015 Feb 14.
Historical data of land use change (LUC) indicated that the sugarcane expansion has mainly displaced pasture areas in Central-Southern Brazil, globally the largest producer, and that those pastures were prior established over native forests in the Cerrado biome. We sampled 3 chronosequences of land use comprising native vegetation (NV), pasture (PA), and sugarcane crop (SC) in the sugarcane expansion region to assess the effects of LUC on soil carbon, nitrogen, and labile phosphorus pools. Thirty years after conversion of NV to PA, we found significant losses of original soil organic matter (SOM) from NV, while insufficient new organic matter was introduced from tropical grasses into soil to offset the losses, reflecting in a net C emission of 0.4 Mg ha(-1)yr(-1). These findings added to decreases in (15)N signal indicated that labile portions of SOM are preserved under PA. Afterwards, in the firsts five years after LUC from PA to SC, sparse variations were found in SOM levels. After more than 20 years of sugarcane crop, however, there were losses of 40 and 35% of C and N stocks, respectively, resulting in a rate of C emission of 1.3 Mg ha(-1)yr(-1) totally caused by the respiration of SOM from C4-cycle plants. In addition, conversion of pastures to sugarcane mostly increased (15)N signal, indicating an accumulation of more recalcitrant SOM under sugarcane. The microbe- and plant-available P showed site-specific responses to LUC as a function of different P-input managements, with the biological pool mostly accounting for more than 50% of the labile P in both anthropic land uses. With the projections of 6.4 Mha of land required by 2021 for sugarcane expansion in Brazil to achieve ethanol's demand, this explanatory approach to the responses of SOM to LUC will contribute for an accurate assessment of the CO₂ balance of sugarcane ethanol.
土地利用变化（LUC）的历史数据表明，在全球最大的糖料生产国巴西中南部，糖料种植的扩张主要取代了牧草地，而这些牧草地是在塞拉多生物群落的原生林地上建立的。我们在糖料种植扩张区采样了 3 个土地利用时间序列，包括原生植被（NV）、牧场（PA）和糖料作物（SC），以评估 LUC 对土壤碳、氮和生物有效磷库的影响。在 NV 转化为 PA 30 年后，我们发现 NV 中原始土壤有机质（SOM）大量损失，而热带牧草向土壤中引入的新有机质不足以抵消损失，反映出净碳排放量为 0.4 Mg ha(-1)yr(-1)。这些发现加上（15）N 信号的减少表明，PA 下 SOM 的生物有效部分得以保留。之后，在 PA 向 SC 转变后的最初五年内，SOM 水平没有明显变化。然而，在种植糖料 20 多年后，C 和 N 储量分别损失了 40%和 35%，导致总碳排放量为 1.3 Mg ha(-1)yr(-1)，这主要是由于 C4 循环植物的 SOM 呼吸作用造成的。此外，牧场向糖料的转化主要增加了（15）N 信号，表明在糖料下积累了更多的难降解 SOM。微生物和植物可利用的 P 对 LUC 的响应随不同 P 输入管理的变化而变化，生物库在两种人为土地利用中均占生物有效磷的 50%以上。随着巴西到 2021 年实现乙醇需求所需的 640 万公顷土地的糖料扩张计划，这种解释 SOM 对 LUC 响应的方法将有助于准确评估甘蔗乙醇的 CO₂平衡。
