Soil organic C (SOC) pools have various stabilization mechanisms and turnover times, which change with depth and are highly influenced by land use. Total SOC (TOC), particulate organic carbon (POC), mineral-associated organic carbon (MOC), and carbon management index (CMI) were used as indicators to compare cropland with manure application (CM) and continuous pasture (CP) to a hardwood forest (HF) at soil 0–10 and 10–25 cm depths. Land use, depth, and their interactions had significant influences on TOC, POC, MOC, MOC/TOC ratio, and CMI except for land use main effect on TOC. Compared to HF and CP soils, CM showed significantly larger POC (12.4 g kg⁻¹) and smaller MOC (8.36 g kg⁻¹) at 10–25 cm; CM had significantly smaller TOC (32.3 g kg⁻¹) and MOC at 0–10 cm (11.81 g kg⁻¹). CM soil at 10–25 cm had higher soil quality and SOC lability as indicated by a significantly larger CMI value (419.2), while CP soil had lower soil quality and SOC lability at 0–10 cm (83.7) and 10–25 cm (73.6) compared to HF soil. This study implied high sensitivity of cropland and pasture surface SOC to degrade under disturbance, which implies that better management strategies are still needed to improve soil quality for these agricultural systems.

The essential roles of Fe oxides in stabilizing long-term soil organic carbon (SOC), especially aromatic dissolved organic carbon (DOC_aro), are well-established in forest soils and sediments. We chose to focus on these processes in agricultural soils in which the input and translocation of native DOC to deeper soils are impacted by management practices. We quantified SOC, Fe oxide bound SOC (Fe-bound OC), and the DOC_aro sorption in a forest, a cropland, and a pasture soil at 0–10 and 10–25 cm. Significantly larger amounts of Fe oxides in the cropland soil were observed compared to the forest and pasture soils at both depths (p < 0.05). Land management practices and depth both significantly influenced the Fe-bound OC percentage (p < 0.05). Larger maximum sorptions of DOC in the cropland (315.0 mg kg⁻¹) and pasture (395.0 mg kg⁻¹) soils than the forest soil (96.6 mg kg⁻¹) at 10–25 cm weres found. DOC_aro sorption decreased in the three soils at 0–10 cm (slope of −0.002 to −0.014 L² mg⁻² m⁻¹) as well as the forest soil at 10–25 cm (−0.016 L² mg⁻² m⁻¹) with increasing equilibrium DOC concentration. Conversely, the cropland and pasture soils at 10–25 cm increased (0.012 to 0.014 L² mg⁻² m⁻¹). The different sorption behaviors of DOC_aro in these surface soils indicate that the forest, cropland, and pasture-managed soils may have more complex and various sorption behaviors in stabilizing DOC_aro and non-DOC_aro.

Mitigating climate change and enhancing fertility in agricultural systems require the adoption of more sustainable fertilizer management practices. Applications of recycled organic materials, such as animal and green wastes, can promote soil carbon stabilization via changing extracellular enzyme activities while providing the necessary nitrogen (N) for plant growth. The goals of this study were to quantify the effects of compost type (cow manure, green manures, mixtures of green and cow manure at various proportions, and inorganic fertilizers) on (1) enzyme activity (phenol oxidase, peroxidase, and urease), and (2) mineralized N under laboratory incubation at 30 °C over an eight-week period. The lowest oxidative enzyme activities (phenol oxidase and peroxidase) were found in the soil treated with a mixture of 50% cow manure and 50% green manure (2.45 μmol h⁻¹ g⁻¹) and a mixture of 30% cow manure and 70% green manure (3.21 μmol h⁻¹ g⁻¹) compared to all other fertilizer treatments. The highest phenol oxidase activity was found in soils amended with green manures (3.52 μmol h⁻¹ g⁻¹), while the highest peroxidase activity was found in soils amended with a mixture of 70% cow manure and 30% green manure (5.68 μmol h⁻¹ g⁻¹). No significant differences were found in total net mineralized N content among all organic fertilizer treatments, but these were significantly lower than total net mineralized N in soil treated with inorganic fertilizers. These results indicate similar effects of organic amendments and mixtures in providing plant-available N, but different effects on lignin-degrading enzyme activities, which may lead to differences in soil organic carbon cycling and long-term C storage depending on which organic amendment is utilized.