Topsoil SOC Storage Reduction Under Cultivation And SOC Distribution And Binding In Particle-Size Fractions Of Selected Paddy Soils

Soil organic carbon (SOC) affects the global carbon cycling and the climate change. It has profound influence on the global environment. Topsoil is very sensitive to human disturbance under the climate change. China is a country with long history of cultivation. However, there have been a few reports about the influence of cultivations and fertilizations on the SOC sequestration and its mechanism in China. Some researches showed that the SOC in topsoil of paddy soils is higher than that of the dryland. The SOC of paddy soils has been rising in recent years in China. The particle-size fractions are the most basic and functional units in soils and their structure and composition are the keys in the physical and chemical process of soil. Therefore, it is important to estimate the topsoil SOC pools and its loss by cultivation, and analyze the influence of tillage managements and fertilizations on the SOC distribution, binding form and its stability in particle-size fractions of paddy soils. Two parts are studied in this paper. The first is "Topsoil organic carbon storage of China and its loss by cultivation" and the second is "SOC distribution and binding in particle-size fractions of the studied paddy soils".In the first part of this study, all the data was obtained by the 2nd State Soil Survey conducted in 1979-1982. The original soil date identified by the soil survey was grouped into uncultivated and cultivated soils according to the sampling records. The SOC pool was calculated to the recorded depth of A horizon (or Ap and P horizon in case of paddy soils) or a depth of 30cm in case of A horizon thickness exceeding 30cm. The SOC density and C pool were estimated by bulk density, depth of A horizon and SOC content. The cultivation-induced SOC was deduced by subtracting the SOC amount of uncultivated soil by that of cultivated soil for individual soil type. Thus, the topsoil SOC storage of China and its loss by cultivation and its geographical distribution were estimated. The results are as follows:1. The topsoil SOC density ranged from 0.77 to 1489 tC ha^-1 in uncultivated soils and 3.52 to 591 tC ha^-1 in cultivated soils of China, with the average being 50±47 tC ha^-1 and 35±32 tC ha^-1, respectively. Cultivation of China's soils had induced a decrease of SOC density of 15 tC ha^-1. The mean topsoil SOC density of China was lower than the world average value given by Batjes (1996). China may be considered as a country with low SOC density and may have great potential for C sequestration.2. The SOC density, SOC pool and its SOC loss by cultivation were different from regions to regions. The maximum mean topsoil SOC density was found in Northeast China, being of 70±104 tC ha^-1 for uncultivated soils and of 57±54 tC ha^-1 for cultivated soils, respectively. The lowest topsoil SOC density for uncultivated soils was found in East China, being of 38±33 tC ha^-1, and the lowest topsoil SOC density for cultivated soils was found in North China, being of 30±30 tC ha^-1. The maximum SOC pool loss induced by cultivation was found in Northeast and Southwest China, and then the minimum SOC pool loss induced by cultivation was found in South China with the increasing SOC after cultivation in low SOC area. This indicates the different sensitivity of SOC to the cultivation and global changes in different regions.3. China topsoil SOC pool was about 40Pg according to the soil area in 1980s. It was about 35Pg for uncultivated soils and 5Pg for cultivated soils. On average, cultivation of China's soils had induced a loss of SOC about 2Pg. This is significantly smaller than the total SOC pool decline of 7Pg due to cultivation of natural soils in China reported by Wu et al. (2003), who made a pool estimation of whole soil profile assuming 1m depth for all soils.4. The SOC density of paddy soils (the larger area of cultivated soil in China) is higher than that of the mean SOC content of cultivated soils. The paddy soils have great potential for C sequestration according to the research reports in recent years. So, It is important to study the SOC changes of the paddy soils and its sequestration mechanism under the global changesIn the second part of this study, three typical paddy soils of South China were selected to study the SOC distribution and its binding form and the effect of different tillage managements and fertilizations in particle-size fractions. In this study, the low energy ultrasonic wave technique, CNS element analyzer and TOC analyzer were used to disperse the soil particle-size fractions and determinate the total SOC, the Ca-SOC and Fe(Al)-SOC. The results are as follows:1. The total SOC distribution in particle-size fractions of the studied paddy soils depends on the soil types. The SOC of the red paddy soil mainly exists in particle-size fractions of 2000-200μm and 200-20μm. The SOC of the Huangnitu mainly exists in the particle-size fractions of 200-20μm and 20-2μm and the SOC of the purple paddy soil mainly exists in the particle-size fractions of 2000-200μm and 20-2μm.2. The coarser particle-size fractions of the studied paddy soils were affected greatly by tillage managements and fertilizations, and they are the main carriers of the pools of younger carbon. The easily-oxidized organic carbon (one type of Labile Organic Carbon (LOC)) to SOC ratio is higher in coarser particle-size fractions but relative lower in small-size fractions. The LOC content cannot be used to judge the SOC pool stability. It was suggested that the LOC to SOC ratio might be used to judge the SOC pool stability.3. The difference of Ca-SOC or Fe(Al)-SOC to the total SOC ratio depends on the studied paddy soil types. The Ca-SOC content in the purple paddy soil is highest but lowest in the red paddy soil among the three paddy soils. The Fe(Al)-SOC content is highest in the red paddy soil but lowest in the purple paddy soil. The Ca-SOC mainly concentrates to the coarser particle-size fractions in which the SOC is unstable. But the Fe(Al)-SOC concentrates to the small-size fractions in which the SOC is relatively stable. The oxidation of SOC increased with increasing Ca-SOC to total SOC ratio in the studied paddy soils but dropped with increasing Fe(Al)-SOC to total SOC ratio. The increasing Fe(Al)-SOC in paddy soils may be one of the main reasons of why the SOC accumulating and steadying in South China.4. The contents of combined Fe, combined Al, Fed, Ald, Feo and Alo were highest in the particle-size fraction of＜2μm. This is consistent with the fact that the Fe(Al)-SOC concentrated to the same particle-size fraction. When the contents of combined Fe, combined Al, Fed, Ald, Feo and Alo are higher in the coarser particle-size fractions, there are higher SOC content and stability in these particle-size fractions.5. The tillage managements influence the SOC distribution and its stability in the purple paddy soil. The ridge-notillage influences the particle-size fractions content of 2000-200μm obviously. The Rice-Cole crop rotation reduces the content of particle-size fractions of 200-20μm. The ridge-notillage can increase the SOC content of coarser particle-size fractions and enhance its stability. But the influence of Rice-Cole crop rotation has opposite result.6. The fertilizers, particularly the organic-inorganic fertilizers, can increase the content of particle-size fractions of 2000-200μm and 200-20μm obviously and reduce the content of＜2μm. The fertilizers increased the total SOC content of the studied paddy soils. The organic-inorganic fertilizers not only increased the SOC content of all particle-size fractions obviously but also enhanced the SOC pool stability. The inorganic fertilizers can only increase the SOC content of the coarser particle-size fractions, and then cause the SOC instability. Different fertilizations have no influence on the small-size fractions stability.7. The invertase activity of the studied paddy soils is higher in small-size fractions but the peroxidase and phenoloxidase activity are higher in coarser particle-size fractions. The ridge-notillage enhances the invertase activity obviously. Fertilizer, especially the organic-inorganic fertilizers, can increase the soil invertase activity. The invertase, peroxidase and phenoloxidase activity of coarser particle-size fractions are affected more easily. There are more SOC in the coarser particle-size fractions with high invertase activity. When the phenoloxidase activity in the coarser particle-size fractions is low, the SOC pools of these particle-size fractions are stable.8. The inorganic fertilizer mainly made the Ca-SOC content increasing, and then the organic-inorganic fertilizers mainly made the Fe(Al)-SOC content increasing in the coarser particle-size fractions. The ridge-notillage mainly increases the Fe(Al)-SOC content in the coarser particle-size fractions and the Rice-Cole crop rotation mainly increase the Ca-SOC content in the purple paddy soil. Therefore, the ridge-no tillage, the organic fertilizer and the organic-inorganic fertilizers can increase the content and stability of SOC by the increasing of Fe(Al)-SOC.