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

Abstract

Reforestation and afforestation of agricultural land can lead toincreased storage of C and N. Few studies have determined the effect ofreforestation on N compared with C accumulation or the impact of forestcommunity structure on soil organic C (SOC) and N stocks. We selected achronosequence of six forested study sites to investigate whether SOCand 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 eachstudy site, measured SOC and N concentrations, SOC/N ratios, the minimumtime since partial and full reforestation, and tree composition andstructure. The SOC and N concentrations increased with time sincepartial 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 limitedanalyses below 20 cm). The SOC and N concentrations were more closelylinked to the onset of reforestation. The SOC/N ratio increased withtime since partial and full reforestation in deeper soils (p < 0.01) butnot in shallow soils (p > 0.05). There was no relationship betweeneither SOC or N and forest composition or structure (p > 0.05). Ourresults demonstrate that the onset of reforestation is a key factorcontrolling SOC and N concentrations, which increased from partial tofull reforestation, particularly in shallow soils. The more pronouncedincrease in the SOC/N ratio in deeper soils reflected an accumulation ofSOC with time in these soils, while N content remained comparativelystable.