Most studies assessing land-use change (LUC) and management impacts on the soil organic carbon (SOC) budget mainly focused on total soil organic matter (SOM), neglecting its functional fractions such as water-extractable organic matter (WEOM). This may limit a better understanding of SOC dynamics along the soil profile as WEOM plays a key role in several soil and microbial processes. Here, we aimed to obtain a quantitative and qualitative assessment of WEOM dynamics to 1 m depth in areas after native vegetation conversion to long-term agriculture in Brazil. For this purpose, cropland areas under no-tillage (NT; ∼ 23 years old) and native vegetation (NV; i.e., Cerrado vegetation) were evaluated across a transect of 1000 km within Brazil’s new agricultural frontier – the so-called MATOPIBA region. We combined spectroscopic analyses (UV–Vis and DRIFT spectroscopies), C and nitrogen (N) determination in different SOM pools (i.e., bulk SOM, WEOM, and microbial biomass), and microbial measurements [e.g., microbial biomass C (MBC) and N (MBN), and β-glucosidase activity] to better understand changes in WEOM dynamics induced by LUC. We observed that although SOC decreased along the soil profile after NV (4.2–20.7 g kg⁻¹) conversion to NT (3.8–14.2 g kg⁻¹), water-extractable organic C levels (3.6–79.3 mg L⁻¹) were similar between land uses. On the other hand, an increase in water-extractable total N was observed after NV (0.4–4.6 mg L⁻¹) conversion to NT (0.5–19.3 mg L⁻¹). Although MBC and MBN levels decreased for most study sites, β-glucosidase activity increased after LUC indicating intensive microbial processing of SOM. As a result, WEOM had lower aromaticity and molecular weight (i.e., low values of SUVA254 and A2/A3) in NT than NV; and a relative decomposition of aliphatic and polysaccharides over aromatic and amine/amide was observed after LUC through DRIFT spectroscopy. Our study indicates that LUC had stronger impacts on WEOM quality than its quantity, with agricultural areas having more biochemical changes in WEOM than NV.

大多数评估土地利用变化 (LUC) 和管理对土壤有机碳 (SOC) 预算的影响的研究主要集中在土壤总有机质 (SOM) 上，而忽略了其功能部分，例如可提取水的有机质 (WEOM)。这可能会限制更好地理解沿土壤剖面的 SOC 动态，因为 WEOM 在几个土壤和微生物过程中起着关键作用。在这里，我们的目的是在巴西将原生植被转化为长期农业后，对 1 m 深度区域的 WEOM 动态进行定量和定性评估。为此，在巴西新的农业前沿——所谓的马托皮巴地区，在 1000 公里的横断面上对免耕（NT；～23 岁）和原生植被（NV；即 Cerrado 植被）的农田面积进行了评估。我们结合了光谱分析（UV-Vis 和 DRIFT 光谱）、不同 SOM 库（即散装 SOM、WEOM 和微生物生物量）中的 C 和氮 (N) 测定以及微生物测量 [例如，微生物生物量 C (MBC) 和N (MBN) 和 β-葡萄糖苷酶活性]，以更好地了解 LUC 诱导的 WEOM 动力学变化。我们观察到，尽管 NV（4.2-20.7 g kg^-1 ) 转换为 NT (3.8-14.2 g kg ^-1 )，水提取的有机碳水平 (3.6-79.3 mg L ^-1 ) 在土地利用之间相似。另一方面，在 NV (0.4-4.6 mg L ^-1 ) 转化为 NT (0.5-19.3 mg L ^-1）。尽管大多数研究地点的 MBC 和 MBN 水平降低，但 LUC 后 β-葡萄糖苷酶活性增加，表明 SOM 的微生物加工。因此，WEOM 在NT 中的芳香性和分子量（即SUVA254 和A2/A3 的值低）比NV 低；通过漂移光谱法观察到 LUC 后脂肪族和多糖在芳香族和胺/酰胺上的相对分解。我们的研究表明，LUC 对 WEOM 质量的影响大于其数量，农业区 WEOM 的生化变化比 NV 多。