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 μm) continued to increase with manure application rate, while the silt (2-53 μm) and clay (<2 μm) particles showed no further increase with greater C inputs, exhibiting the C saturation. Further physical separation of small macro-aggregates (250-2 000 μm) into subpools showed that the non-protected coarse particulate organic matter (cPOM, >250 μm) 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 macroaggregates) 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.

The influences of tillage systems on soil carbon (C) stocks have been studied extensively, but the distribution of soil C within aggregate fractions is not well understood. The objective of this study was to determine the influences of various tillage systems on soil aggregation and aggregate-associated C under wheat (Triticum aestivum L.) and corn (Zea mays L.) double cropping systems in the North China Plain. The experiment was established in 2001, including four treatments: moldboard plow (MP) with residue (MP+R) and without residue (MP-R), rotary tillage with residue (RT), and no-till with residue (NT). In 2007 soil samples were collected from the 0-5, 5-10, and 10-20 cm depths, and were separated into four aggregate-size classes (>2 000, 250-2 000, 53-250, and <53 μm) by wet-sieving method. Aggregate-associated C was determined, and the relationships between total soil C concentration and aggregation-size fractions were examined. The results showed that NT and RT treatments significantly increased the proportion of macroaggregate fractions (>2 000 and 250-2 000 μm) compared with the MP-R and MP+R treatments. Averaged across all depths, mean weight diameters of aggregates (MWD) in NT and RT were 47 and 20% higher than that in MP+R. The concentration of bulk soil organic C was positively correlated with MWD (r=0.98; P=0.024) and macroaggregate fraction (r=0.96; P=0.036) in the 0-5 cm depth. In the 0-20 cm depth, comparing with MP+R, total C occluded in the >2 000 μm fraction was increased by 9 and 6% under NT and RT, respectively. We conclude that adoption of conservation tillage system, especially no-till, can increase soil macro-aggregation and total C accumulation in macroaggregates, which may improve soil C sequestration in the intensive agricultural region of the North China Plain.