Soil Organic Matter Quality and Storage Under Different Land-Use Systems Following Primary Deciduous Forest Conversion.

Humic substances are one of the important components of total organic C (TOC) to influence soil quality. The goal of our study was to improve our understanding on how the impact of land-use changes upon primary forest conversion influence TOC content and lability under similar soil and climatic conditions. Geo-referenced soils from primary deciduous forest (Forest), Miscanthus giganteus plantations (Bioenergy), conventionally-tilled agricultural crop fields (Agriculture), Conservation Reserve Program (CRP), and wetlands (Wetlands) at 0 to 30 cm depth were analyzed. Results showed that TOC, humin, humic acid (HA), fulvic acid (FA), FA_glucose, HA_glucose, E₄/E₆, humification index (HI), humification ratio (HR), and degree of humification (DH) were significantly affected by the impacts of land-use changes. Among the land-use systems, Forest and the CRP had the highest TOC, humin, FA, FA_glucose and HA_glucose contents with respect to lowest in Agriculture and Bioenergy, respectively. However, the HA contents were higher than the FA contents in both Bioenergy and Agriculture when compared with other land-use systems. Soil profile-wise TOC, FA, and humin stocks under Agriculture were significantly lower than other land-use systems, especially when compared with Forest and the CRP. Higher TOC, humin, FA, HA, FA_glucose and HA_glucose stratification was observed in both Forest and the CRP when compared to Agriculture. Spectral analysis have shown higher values of E₄/E₆ for FA (9.7 to 16.2) than the HA (4.5 to 5.9) in all the land-use systems. The E₄/E₆ of FA was highest in Forest and lowest in Agriculture. The HI was highest in Bioenergy (36.5%) followed by Forest (29.6%) as compared with lowest in Agriculture (20.9). In contrast, the DH and HR responded differently by the impact of land-use changes. The TOC and extracted C contents except FA/HA, E₄/E₆ of FA and HA, HI, DH and HR decreased with depth. However, the TOC, TEC, FA, HA, and humin contents, FA/HA, E₄/E₆ of FA and HA, HI, DH and HR were significantly influenced by a land-use x depth interaction. Results suggested that primary forest conversion to intensive agriculture significantly affected the aliphatic nature of the TOC contents towards more aromatic quality.