Changes in soil organic matter composition after afforestation of arable farmland in northeast China

Since the 1970s, a substantial area of arable farmland in a semi-humid area of northeast China has been planted as deciduous forests (Populus tomentosa Carr.). This study investigates the effects of afforestation on soil organic carbon (SOC) content and the chemical composition of the soil. Soil samples (Calcic chernozem) were collected from the upper 10?cm of paired arable land and secondary forests established 5 and 25 years previously. Carbon isotope analysis and pyrolysis–gas chromatography/mass spectrometry were used to determine SOC composition. The results show that (i) compared to the arable land, five years of afforestation caused a decrease in SOC and N concentration, while 25 years of afforestation resulted in an increase in SOC content; (ii) stable isotope δ¹³C analyses of the forest soils show gradual loss of crop-derived C and an accumulation of forest-derived C; and (iii) afforestation increased lignin abundance and decreased decompositional activity in the 25-year-old forest topsoil. Higher amounts of short-/mid-chain aliphatic compounds were observed in the 5-year-old forest (5–10?cm); and (iv) the arable soil contained substantially higher amounts of decomposed plant material and microbially derived substances. The results obtained suggest that long-term afforestation increases the SOC concentration, and alters the chemical composition of SOC.