Long-Term Manure Amendments Enhance Soil Aggregation and Carbon Saturation of Stable Pools in North China Plain

Organic amendment is considered as an effective way to increase soil organic carbon (SOC) stock in croplands. To better understand its potential for SOC sequestration, whether SOC saturation could be observed in an intensive agricultural ecosystem receiving long-term composted manure were examined. Different SOC pools were isolated by physical fractionation techniques of a Cambisol soil under a long-term manure experiment with wheat-maize cropping in North China Plain. A field experiment was initiated in 1993, with 6 treatments including control (i.e., without fertilization), chemical fertilizer only, low rate of traditional composted manure (7.5 t ha(-1)), high rate of traditional composted manure (15 t ha(-1)), low rate of bio-composted manure (7.5 t ha(-1)) and high rate of bio-composted manure (15 t ha(-1)). The results showed that consecutive (for up to 20 years) composted manure amendments significantly improved soil macro-aggregation, aggregate associated SOC concentration, and soil structure stability. In detail, SOC concentration in the sand-sized fraction (>53 mu m) continued to increase with manure application rate, while the silt (2-53 mu m) and clay (<2 mu m) particles showed no further increase with greater C inputs, exhibiting the C saturation. Further physical separation of small macro-aggregates (250-2000 mu m) into subpools showed that the non-protected coarse particulate organic matter (cPOM, >250 mu m) was the fraction in which SOC continued to increase with increasing manure application rate. In contrast, the chemical and physical protected C pools (i.e., micro-aggregates and silt-clay occluded in the small macro-aggregates) exhibited no additional C sequestration when the manure application rate was increased. It can be concluded that repeated manure amendments can increase soil macro-aggregation and lead to the increase in relatively stable C pools, showing hierarchical saturation behavior in the intensive cropping system of North China Plain.