Advantages Of Carbon Sequestration And Their Maintaining Practices In Paddy Soils: Based On Analyses Of Carbon Fractions

Global mean temperature has increased by 0.74℃since 1850 and is predicted to further increase by 1.8-4.0℃by the end of this century. Global climate change is very likely (>90%) due to anthropogenic activities that cause large emissions of CO2 and other greenhouse gases. Global soil carbon (C) is 2-3 times the size of the atmospheric pool and the biotic pool. Thus, C sequestration in soils may be of great importance for mitigating the increase of atmospheric CO2 concentration. Compared to that in soils of natural ecosystems, soil organic C (SOC) in cropland soils is much lower. Numerous studies have shown that cropland soils may serve as a large sink for atmospheric CO2 by enhancing SOC under appropriate management practices. In addition, the level of SOC in cropland soils is an important indicator for soil fertility and quality. Thus, soil C sequestration in croplands may be also an important practice for enhancing the productivity of agroecosystems.China is the most important rice-producing country in the world. Despite of being a main source for CH4, increasing evidence has shown that C density in paddy soils was higher than that in upland soils, and that great C sequestration has occurred in paddy soils over the last two decades in China. However, previous studies concentrated largely on emissions of greenhouse gases and the dynamics of total SOC pool. Little attention was paid to the responses and dynamics of SOC fractions in paddy fields and its protection mechanisms. Physical protection is one of the mian mechanisms for SOC stabilization. Thus, in this study, from the view of differences in soil C stocks between paddy and upland soils, we combined the C natural abundance and the physical fractionation techniques of soil organic matter to investigate the mechanisms for C sequestration in paddy fields, especially its physical protection mechanism, and the quality and stability of paddy soil C. Then, we employed a long-term field experiment to investigate the effect of fertilization on SOC and its fractions. Furthermore, a meta-analysis was performed to examine SOC accumulation in different rice cropping systems and its time dependence in China. Through above analyses, we are aimed to determine the effects of organic matter input, the duration of continuous flooding and the duration of rice cropping on soil C sequestration, thereby providing practices that maintaining the advantages of SOC storage. Main results are as follows:Responses of SOC and its fractions to long-term land-use conversion from paddy fields to upland fields (corn cultivation) were investigated in the Northeast China. Results showed that land-use conversion from paddy fields to upland fields led to significant decreases in contents of SOC and total nitrogen (TN). Concentrations of total organic carbon (TOC) and TN were respectively greater by 76.7% and 47.6% in the paddy field than those in the corn field. Concentrations of occluded particulate organic matter (oPOM) and mineral-associated organic matter (MOM) on a whole soil basis were two times higher in the paddy field than those in the upland field, while no significant difference was found in free particulate organic matter (fPOM). Carbon concentrations of oPOM and MOM fractions on their own weight basis were significantly greater in the paddy field than those in the upland field, especially the oPOM fraction which was 6 times higher in the former than that in the latter. It could be concluded that SOC protection exerted by soil aggregates in the paddy soil was greater than that in the upland soil. After a 19-year conversion from paddy fields to corn fields,813C values of SOC fractions significantly increased. Corn-derived C accounted for 54.6%,24.7%, and 19.0% in fPOM, oPOM and MOM, respectively. Mean residence time (MRT) of the initial rice-derived C increased in the order fPOM (24 years)< oPOM (67 years)< MOM (90 years).Long-term (18 years) upland (corn-wheat cropping, C-W) conversion to rice-wheat rotations (R-W) led to a significant increase of soil organic carbon (SOC) by 12.9% in the North China Plain. Changes in cropping systems significantly altered soil aggregate distribution. Compared to C-W, the proportion of microaggregaes (53-250μm) was significantly lower and that of silt and clay particles (<53μm) higher in R-W, with no marked differences in that of macroaggregates (>2000μm and 250-2000μm). Carbon concentrations of various aggregates were significantly increased following long-term conversion from C-W to R-W, except the silt and clay. In comparison to C-W, the C content of coarse particulate organic matter (cPOM) and fPOM was significantly higher in R-W. As cPOM and fPOM fractions represent unprotected and labile C pools with high turnover rates, no significant shift in the sequestered SOC toward stable C pools was observed following long-term conversion from C-W to R-W.We identified the differences in SOC and its fractions between long-term double rice cropping and double corn cropping fields with similar cropping histories in red soils. Results showed that the concentrations of TOC and TN in the paddy field soil were 2.2 and 2.5 times higher than those in the upland soil, respectively. Compared to the initial level of SOC, long-term rice cultivation increased the concentration of SOC by 30.8%, while no significant effects were found in the upland soil after more than 20-year corn cropping. Furthermore, concentrations of aggregate-associated SOC in the paddy soil were significantly higher than that in the corn field soil. The most difference in aggregate-associated SOC was up to 3 times in the 53-250μm microaggregates. Concentrations of microaggregate protected C (iP0M_m) on a whole soil basis were 4.2 folds higher in the paddy field than that in the corn field. The proportion of iPOM_m in TOC reached 25.5% in the paddy soil with 2 times higher than that in the upland soil. Moreover, the difference in microaggregate protected C between the paddy field and the upland field could explain 42.8% of the difference in TOC. Hence, the shift of SOC toward microaggregates favors the long-term storage of SOC in the paddy field.Five soil C fractions were separated by physical fractionation in a subtropical paddy field following 27-year differential fertilization regimes (started in 1981) in a red soil. Results showed that, compared to the initial level, long-term rice cropping increased SOC concentrations by 28.8%,30.1%,30.8%, and 61.6% in the non-fertilized (Control), nitrogen (N), nitrogen-phosphorus-potassium (NPK), and NPK combined with farmyard manure (NPK+FYM) treatments, respectively. Application of FYM enhanced the formation of macroaggregates (>2000μm and 250-2000μm), whereas no significant differences in aggregate-size distribution were found among the Control, N, and NPK treatments. Inorganic fertilization (N and NPK) did not affect the concentration of either total SOC or any C fraction as compared with the Control, whereas application of FYM significantly increased the concentrations both in total SOC (25.5%) and in all C fractions, except cPOM. Carbon in the paddy soil was dominated by free silt and clay (s+c_f) and iPOM_m in all treatments that accounted for 46.4-49.6% and 25.1-27.2% of the total SOC, respectively. Furthermore, the differences in C in the iPOM_m and s+c_f fractions between the Control and NPK+FYM treatments accounted for 53.2% and 38.8% of the differences in total SOC stocks, respectively. These results indicate that SOC originating from manure is stored mainly in fractions with slow turnover (i.e., iPOM_m and s+c_f), which may benefit the long-term C sequestration in the paddy soil.A meta-analysis was performed to examine SOC accumulation in different rice cropping systems and its time dependence in China. Results showed that rice cropping without any nutrient application (Control) increased SOC stocks by 9% compared to the initial level in double rice cropping systems (DR), whereas no significant effects were observed in single rice cropping systems (SR) and rice-upland crop rotation systems (RU). Paddy soils sequestered C in all the three cropping systems under inorganic NPK fertilization, and the magnitude of the accumulation of SOC increased in the order DR> RU> SR. Soil C sequestration increased with the increasing experimental duration. Continuous rice cropping more than 20 years resulted in average SOC gains of 15% and 23% in the control and NPK treatments, respectively.The above results further indicate that paddy field soils hold a greater potential for C sequestration than corresponding upland soils, while the magnitude of the increase of SOC and the quality of the sequestered C significantly differed among rice cropping systems. While rice cropping favors SOC sequestration, long-term continuous anaerobic conditions, such as single rice and double rice cropping annually, facilitate the physical protection of the sequestered C as compared with rice-upland crop rotations. Organic amendments can not only enhance the content of SOC in paddies, but improve the physical proctection of C, thus benefiting the long-term storage of SOC in rice fields. Large-scale analyses indicate that balaced NPK fertilization can increase the content of SOC in rice paddies as compared with the unfertilized control, and that double rice cropping results in a larger accumulation of SOC relative to single rice cropping and rice-upland crop rotation. Thus, more inputs of organic matter, longer annual duration of continuous flooding, and greater duration of rice cropping favor the storage of SOC in paddy fields.