Change in Carbon Storage in Soil Physical Fractions after Afforestation of Former Arable Land

In the Loess Hill region of China, a large amount of arable land has been replaced by forest plantation, resulting in increased storage of soil organic C (SOC). To elucidate the mechanisms of SOC storage among the afforested lands, we separated SOC into four specific size/density fractions: coarse free particulate organic C (cfPOC), fine free POC (ffPOC), intra-microaggregate POC (iPOC), and mineral–associated organic C (MOC). Soils were collected from arable land and from areas with four tree species—robinia (Robinia pseudoacacia L.), poplar (Populus tomentosa Carriere), caragana (Caragana korshinskii Kom.), and buckthorn (Hippophae rhamnoides L.)—in 15-yr-old stands. The total soil C stock to a depth of 100 cm was in the order robinia > poplar > caragana = buckthorn, and C increased by 7.9 to 18.2 Mg C ha⁻¹ compared with the arable land. Stocks of the cfPOC, iPOC, and MOC in the robinia and poplar land were also higher than that of the caragana and buckthorn land. Compared with the arable land, the MOC and cfPOC increased by 4.8 to 11.9 and 1.9 to 5.1 Mg C ha⁻¹, respectively, in all of the afforested lands, and the iPOC only increased by 0.8 Mg C ha⁻¹ in the robinia and poplar areas. However, the ffPOC increased by 0.9 Mg C ha⁻¹ in all four afforested lands. The differences of the SOC fractions among the four afforested lands occurred primarily because the planting of robinia and poplar increased the cfPOC and MOC in the 0- to 100-cm soil profile, whereas caragana and buckthorn only increased cfPOC and MOC at depths of 0 to 20 cm and 0 to 60 cm, respectively. The total SOC fraction was composed of MOC (75.8%) > cfPOC (15.8%) > iPOC (4.0%) = ffPOC (4.3%). We conclude that the afforestation of former arable land with trees (robinia and poplar) can provide a greater increase in the SOC in the physical structure of the soil than shrubland (caragana and buckthorn), especially for the MOC.