Impact of Land-Use History and Forest Trees on Soil Organic Carbon and Nitrogen Stocks

Reforestation and afforestation of agricultural land can lead to increased storage of C and N. Few studies have determined the effect of reforestation on N compared with C accumulation or the impact of forest community structure on soil organic C (SOC) and N stocks. We selected a chronosequence of six forested study sites to investigate whether SOC and N were affected by land-use history or forest community structure. We took soil cores at depths of 15 to 35 cm from eight plots at each study site, measured SOC and N concentrations, SOC/N ratios, the minimum time since partial and full reforestation, and tree composition and structure. The SOC and N concentrations increased with time since partial and full reforestation in shallow soils (top 10 cm; p < 0.01) but less so in deeper soils (10-20 cm; p > 0.05; rocky soil limited analyses below 20 cm). The SOC and N concentrations were more closely linked to the onset of reforestation. The SOC/N ratio increased with time since partial and full reforestation in deeper soils (p < 0.01) but not in shallow soils (p > 0.05). There was no relationship between either SOC or N and forest composition or structure (p > 0.05). Our results demonstrate that the onset of reforestation is a key factor controlling SOC and N concentrations, which increased from partial to full reforestation, particularly in shallow soils. The more pronounced increase in the SOC/N ratio in deeper soils reflected an accumulation of SOC with time in these soils, while N content remained comparatively stable.