Shulan Zhang Renjie Wang, Junyu Xie, Benhua Sun, Xueyun Yang

　　College of Natural Resources and Environment, Northwest A & F University

　　Understanding soil organic carbon (SOC) sequestration in aggregates and SOC stability is of key importance in improving soil quality and crop productivity and in mitigating the causes of climate change. A long-term field experiment established in 1990 was used to assess effect of cropland abandonment, keeping bare fallow and cropping on aggregate-associated SOC and its mineralization and on the SOC stock in an Anthrosol in North China. The cropping system was combined with the following nutrient management treatments: control (CK, no nutrient input), nitrogen and phosphorus and potassium (NPK) and manure (M) plus NPK (MNPK).We analyzed the size distribution of aggregates by dry sieving; the concentrations of OC in aggregates and bulk soils; and their respiration after 21 years. Abandonment increased the abundance of macroaggregates (> 2 mm) in the surface soil layer (0-10 cm) and reduced that of microaggregates (<0.25 mm) relative to cropping, whereas the fallow treatment reduced the abundance of macroaggregates at depths of 0-10 and 10-20 cm. Fertilization did not affect the size distribution of aggregates in the surface soil layer, but there was a significant change in the subsurface layer in response to manure addition. The SOC content of aggregate size fractions in the surface soil from the abandoned plots were greater (by 1.2-4.8 g kg^-1) than those of the corresponding fractions from the cropped plots, but the opposite trend was observed in the subsurface soils. Conversely, the fallow treatment reduced the SOC content of every aggregate size fraction. Application of NPK and MNPK significantly and evenly augmented SOC sequestration in aggregate classes tested. The SOC mineralization rate was significantly higher in micro- than in macro-aggregates under both tested systems. Abandonment did not change OC mineralization rate from tested three aggregate classes as compared with Cropping at both soil depths. Whereas, fallow showed significantly lower OC mineralization rate from 2-0.25 mm fraction than Abandonment, and from <0.25 mm fraction than both Abandonment and Cropping at surface layer. Application of NPK and MNPK significantly increased SOC mineralization rates of <0.25 mm fraction compared with CK at surface layer; MNPK treatment also yielded significant higher mineralization rates of >2 mm and 2-0.25 mm fractions than NPK or NPK and CK at subsurface layer. Overall, soils treated for 21 years with contrasting management significantly affected the SOC pools, compared with Cropping with fertilization of NPK, mean SOC sequestration rate was 232 and -203 kg C ha^–1 yr^–1, respectively, under cropland abandonment and bare fallow. While long-term application of NPK and MNPK had average sequestration rates of 353 and 1087 kg C ha^–1 yr^–1 respectively, relative to the control. Thus, cropland abandonment or bare fallow after 21 years showed similar rates in OC gains or losses. Under cropping application of NPK, alone or in combination with manure, can lead to improved SOC sequestration by evenly augmenting aggregate-associated C and providing at least similar protection for macro-aggregate-associated C and micro-aggregate-associated C on Anthrosol.