The quantity and quality of soil organic matter and humic substances following dry-farming and subsequent restoration in an upland pasture

Abstract

Soil recovery, particularly soil organic matter (SOM), after land-use changes is crucial for the maintenance of ecosystem functioning and sustainability. The objective of this study was to investigate the influence of wheat dry-farming and subsequent abandonment of dry-farming on the quantity and quality of SOM and humic substances (HS) in a semi-humid upland pasture ecosystem. Soil samples were collected at 0–15 and 15–30 cm depths from cultivated pastures under dry-farming, restored pastures after dry-farming abandonment and never-cultivated pastures as a reference site. The samples were analyzed for total organic C (TOC), chemically labile organic C (LOC) and non-labile C (NLC) fractions; and further fractionated into conventional fulvic acid (FA), humic acid (HA) and humin (HU) components. Land-use changes in pastures altered both labile C and highly recalcitrant C fractions, depending on soil sampling depth. Long-term dry-farming reduced soil TOC (33%), LOC (64%) and NLC (29%) fractions, while dry-farming abandonment resulted in an increase in soil TOC (18–35%), LOC (45–65%) and NLC (17–33%), depending on the age of cultivation abandonment. The quantity of both FA and HU fractions decreased (40–43%) following dry-farming in pasture soils but increased (17–81%) after dry-farming abandonment when compared with cultivated pasture soils. Nevertheless, neither dry-farming nor abandonment of dry-farming affected the HA fraction. Although dry-farming practices increased the HA/FA ratio, (FA + HA)/TOC ratio and E4/E6 ratio of HA, the abandonment of dry-farming reduced these qualitative parameters of soil HS. Dry-farming abandonment in native pastures caused HA structure to be become more aromatic and stable. This study indicated that land-use changes in primary pastures can affect not only the quantity, but also the quality of SOM and its major fractions. Changes in both quantity and quality of SOM and HS could be used as sensitive indicators of soil degradation and ecological restoration in semi-humid pastures.