The accurate quantification and source partitioning of CO_(2)emitted from carbonate(i.e.,Haplustalf)and non-carbonate(i.e.,Hapludult)soils are critically important for understanding terrestrial carbon(C)cycling.The two main methods to capture CO_(2)released from soils are the alkali trap method and the direct gas sampling method.A 25-d laboratory incubation experiment was conducted to compare the efficacies of these two methods to analyze CO_(2)emissions from the non-carbonate and carbonate-rich soils.An isotopic fraction was introduced into the calculations to determine the impacts on partitioning of the sources of CO_(2)into soil organic carbon(SOC)and soil inorganic carbon(SIC)and into C3 and/or C4 plant-derived SOC.The results indicated that CO_(2)emissions from the non-carbonate soil measured using the alkali trap and gas sampling methods were not significantly different.For the carbonate-rich soil,the CO_(2)emission measured using the alkali trap method was significantly higher than that measured using the gas sampling method from the 14 th day of incubation onwards.Although SOC and SIC each accounted for about 50%of total soil C in the carbonate-rich soil,SOC decomposition contributed 57%–72%of the total CO_(2)emitted.For both non-carbonate and carbonate-rich soils,the SOC derived from C4 plants decomposed faster than that originated from C3 plants.We propose that for carbonate soil,CO_(2)emission may be overestimated using the alkali trap method because of decreasing CO_(2)pressure within the incubation jar,but underestimated using the direct gas sampling method.The gas sampling interval and ambient air may be important sources of error,and steps should be taken to mitigate errors related to these factors in soil incubation and CO_(2)quantification studies.
Yi ZHAORoland BOLZhaoan SUNYuping ZHUGEXiaoxia SHIWenliang WUFanqiao MENG
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.
DU Zhang-liuWU Wen-liangZHANG Qing-zhongGUO Yan-binMENG Fan-qiao
