ABSTRACT

Understanding the long-term effects of land use on soil organic carbon (SOC) is of interest due to the possibility of using soils as carbon (C) sinks. We determined how the C levels in the labile (C₀) and stable (C_r) fractions of the organic matter of soils of the Pampas were affected by land use, applying the inverse analysis methodology. Soil samples (0–50 cm) taken from 198 sites under different land uses (forest, grassland, and cultivated soil) were incubated for six months, adjusting a one pool exponential model to C mineralization data. The model allowed a good estimation of C₀ (R² = 0.96–0.98) and C_r was calculated by difference with SOC. The stocks of C₀ and C_r were calculated on an equivalent mass basis (5000 t soil ha⁻¹) for comparisons. About 80% of the SOC is found in the C_r pool in the Pampas soils. Both C₀ and C_r were higher in forest and grassland than in cultivated soils. Soil cultivation produced an average reduction of 30% of the C₀ pool and 18% of the C_r pool in relation to grassland soils. Consequently, most of the SOC lost due to agriculture came from the C_r pool. In mixed production systems, no differences of the C pools were detected between the crop or pasture phase of the rotation. The parameters C₀ and k (mineralization rate constant) of the exponential model were negatively correlated, so the comparison of C₀ between different land uses must be careful. The k parameter was similar among land uses when the comparison was made for similar values of C_0, suggesting that land use affected the size of the C pools not their stability. Forest and grasslands were effective systems for sequestering C in stable forms. Expanding the cultivated area in the Pampas should be avoided if it is desired not to emit C into the atmosphere from the soil.

摘要

由于有可能将土壤用作碳 (C) 汇，因此了解土地利用对土壤有机碳 (SOC) 的长期影响很有意义。我们应用逆向分析方法确定了潘帕斯土壤有机质的不稳定 (C ₀ ) 和稳定 (C _{r ) 部分中的 C 水平如何受土地利用的影响。}从不同土地利用（森林、草地和耕地）的 198 个地点采集的土壤样品（0-50 厘米）培养了六个月，将单池指数模型调整为 C 矿化数据。该模型可以很好地估计 C ₀ (R ²  = 0.96–0.98)，并且通过与 SOC 的差异计算C _{r 。C 0}和 C _r的股票以等效质量为基础（5000 t 土壤 ha ^-1）进行计算以进行比较。大约 80% 的 SOC 存在于潘帕斯土壤的 C _r池中。森林和草地的C ₀和C _r均高于耕地。与草地土壤相比，土壤耕作平均减少了 30% 的 C ₀库和 18% 的 C _r库。因此，由于农业而损失的大部分 SOC 来自 C _r池。在混合生产系统中，在轮作的作物或牧草阶段之间没有检测到碳库的差异。参数 C ₀和指数模型的k（矿化速率常数）呈负相关，因此不同土地利用之间的C ₀比较必须谨慎。当对相似的 C _{0 值进行比较时，土地利用中的 k 参数相似，}这表明土地利用影响了 C 池的大小而不是它们的稳定性。森林和草地是以稳定形式封存 C 的有效系统。如果不希望 C 从土壤中排放到大气中，则应避免扩大潘帕斯草原的耕地面积。