Context: Land use change (LUC) is considered one of the main factors associated with soil carbon (C) loss worldwide.

Aim: Evaluate changes in labile and non-labile soil organic matter (SOM) fractions in different land use systems – native vegetation (NV), agriculture with conventional cultivation (CC) and pasture (PA) – in five soil classes in the Brazilian semi-arid region.

Methods: Soil samples were collected to a depth of 100 cm, and soil C and nitrogen (N) content and stocks were determined and also stocks of labile (LC) and non-labile (NlC) C fractions. In addition, the Carbon Management Index (CMI) was used to evaluate soil health changes.

Key results: SOC stocks significantly decreased after conversion from NV to CC (by 23%; 0–30 cm) and PA (by 22%; 0–100 cm). Losses due to LUC were greater in the LC than the NlC fraction, and this was reflected in the CMI reducing by 37% for PA and 57% for CC in the 0–100 cm layer. Regarding the different soil classes, LUC reduced SOC stocks only in the Luvisol, Planosol and Leptosol classes, while in the LC fraction, changes were observed in Acrisols, Cambisols and Planosols.

Conclusions: Clearing NV areas for CC systems and PA reduced the SOC stocks and SOM fractions and, consequently, decreased soil quality.

Implications: These findings underscore the importance of considering the quality of SOM when evaluating LUC impacts on SOC stocks in the different classes of soil in the Brazilian semi-arid region.

背景：土地利用变化（LUC）被认为是与全球土壤碳（C）损失相关的主要因素之一。

目的：评估不同土地利用系统（本地植被（NV）、传统耕作农业（CC）和牧场（PA））中不稳定和非不稳定土壤有机质（SOM）比例的变化——巴西半干旱地区五个土壤类别-干旱地区。

方法：采集 100 厘米深的土壤样品，测定土壤碳和氮 (N) 含量和储量，以及不稳定 (LC) 和不稳定 (NlC) C 部分的储量。此外，碳管理指数（CMI）用于评估土壤健康变化。

主要结果：从 NV 转换为 CC（下降 23%；0–30 cm）和 PA（下降 22%；0–100 cm）后，SOC 储量显着下降。LC 中 LUC 造成的损失大于 NlC 部分，这反映在 0-100 cm 层中 PA 的 CMI 降低了 37%，CC 降低了 57%。对于不同的土壤类别，LUC 仅减少了 Luvisol、Planosol 和 Leptosol 类别中的 SOC 储量，而在 LC 部分中，在 Acrisols、Cambisols 和 Planosols 中观察到了变化。

结论：为 CC 系统和 PA 清理 NV 区域减少了 SOC 储量和 SOM 分数，从而降低了土壤质量。

影响：这些发现强调了在评估土地利用面积对巴西半干旱地区不同类别土壤中有机碳储量的影响时考虑土壤有机质质量的重要性。