| Soil organic carbon?SOC?is not only an important part of soil quality,but also plays a key role in crop production,which depends on many physical and biological properties.The fractions of SOC with highly heterogeneous nature have different responses to environmental and management measures,and sequestration characteristics and protection mechanisms of different soil are also different.Therefore,a deeper study on SOC stabilization and its protection mechanisms is of great significance to better understand how soil organic carbon improves soil fertility.For these scientific problems,A data set on SOC was set up based on 319 pairs of data collected using the same soil organic carbon fractionation method?the wet sieving method?that were presented in 43 published papers.We conducted a meta-analysis to quantify the response of different fractions of organic carbon associated with >2,2–0.25,0.25–0.053,and <0.053 mm aggregates?Agg-OC?to manure?Org-M?or chemical fertilizer?Min-F?application compared to that without fertilizer?Non-F?application under different cropping systems,farmland type,and soil texture conditions.The characteristics of soil organic carbon sequestration and protection mechanisms in typical cropland under long-term fertilization in China were examined using physical-chemical fractionation techniques.The study not only can enrich the basic theory of soil organic carbon cycle,but also improve soil fertility;increase food production and increase carbon sink as well as control air temperature in China.The major findings obtained were as follows:1.Manure application benefited accumulation of soil Agg-OC,especially in >0.25 mm aggregates.Manure application is recommended for soil carbon sequestration in aggregates,especially for upland and light texture soil.Results showed that SOC in bulk soil and Agg-OC in the >2,2-0.25,and 0.25-0.053 mm aggregates were significantly increased by both Org-M and Min-F compared to by Non-F.As for as total SOC is concerned,compared to Non-F,the improvement rate of Org-M over Non-F?38.0%?was 4.3 times higher than that of Min-F over Non-F?8.8%?.As for the Agg-OC in different aggregates,the improvement rate of Org-M over Non-F?39.7%-72.3%?was 4.6-9.2 times higher?P<0.05?than that of Min-F over Non-F?4.3%-15.8%?,respectively.There was no significant effect of Min-F on the <0.053 mm Agg-OC,whereas the effect of Org-M was significant.As for different size aggregates under the same conditions,compared with Non-F,the increase in applying Org-M on the 2-0.25 mm Agg-OC was significantly higher than those in the >2,0.25-0.053,and <0.053 mm aggregates for double-cropping,upland,sandy,and clay conditions.In the same-sized particles under different conditions,compared to Non-F,Agg-OC was significantly increased by Org-M application.The orders of the increment were as follows: sandy > loam,clay,upland > paddy,paddy-upland for 2-0.25 mm;monocropping > double-cropping,upland > paddy,paddy-upland for both 0.25-0.053 and <0.053 mm.The effect of Min-F over Non-F on Agg-OC was similar to that of Org-M application,with no significant difference between them.Moreover,Min-F markedly decreased the <0.053 mm Agg-OC by 16.4% compared to Non-F in paddy.There was significant positive correlation between >0.25 and <0.25 mm Agg-OC and total SOC.An increase of 1.00 g·kg-1 in total SOC resulted in a 0.61 g·kg-1 increase in >0.25 mm Agg-OC,which was higher than that of <0.25 mm Agg-OC(0.23 g·kg-1).2.Application of mineral fertilizer plus manure significantly improved stock of total soil organic carbon in 0-20 cm under upland soil.The proportion of stock of total soil organic carbon in 20-100 cm in whole profile was 65.9%.The c POC in unprotected carbon pool is the most sensitive to agricultural management measures and is the main fixed form of new organic carbon in 0-20 cm under upland soil.Application of fertilizer plus manure had significantly increased stock of i POC in physical-protected carbon pool,while fertilizer has no significant effect.Stock of organic carbon of H-silt in chemically-and biochemically-protected carbon pool under long-term fertilization were significantly increased,among which application of fertilizer plus manure was most effective.Organic carbon pool in 20-100 cm soil layer is relatively stable compare with 0-20 cm,and there has little impact on stock of soil organic carbon in 20-100 cm.3.The stock of total soil organic carbon of 0-20 cm in upland soil were significantly higher than those of 20-40 cm in each treatment,while there was no difference between treatments of 0-20 cm and 20-40 cm in paddy soil.Application of mineral fertilizer plus manure had significantly increased stock of c POC in unprotected carbon pool of 0-20 cm in paddy soil,and there was no difference in stock of other fractions organic carbon among the treatments.It showed that the c POC in unprotected carbon pool and is the most sensitive to agricultural management measures.However,there were significant differences in the response of different fractions of organic carbon stocks to different fertilizations in upland soils.The proportion of unprotected carbon pool in upland soil to total organic carbon stock was the highest in 20-40 cm,while that of stable carbon pool?chemically-and biochemically-protected carbon pool?in paddy soil was the highest.There were no difference in stock of unprotected and physically-protected carbon pool among the treatments in upland and paddy soil and application of fertilizer plus manure had significantly increased stock of stable carbon pool.4.The relationship between soil organic carbon stock and cumulative carbon input in different soil types,0-20 cm in upland soil,stock of total soil organic carbon and unprotected carbon pool showed a significant linear increase with cumulative carbon input,which indicated that stock of these fractions organic carbon is not affected by carbon saturation limit and has a large potential of carbon sequestration and has not yet reach carbon saturation under the current conditions.Stock of total soil organic carbon,c POC in unprotected carbon pool and H-silt in biochemically-procected carbon pool of 0-20 cm in paddy soil showed a significant "saturation curve" increase with cumulative carbon input.The newly added organic carbon in paddy soil was stored in the stable carbon pool at first,and then transferred to unprotected and physical-protected carbon pool.The newly added organic carbon in upland soil was fixed in physical-protected carbon pool at first,and then transferred to unprotected carbon pool after reaching the stable state.In conclusion,manure application benefited accumulation of soil Agg-OC,especially for upland and light texture soil,and the increase of Agg-OC in >0.25 mm aggregates was more significant.Application of mineral fertilizer plus manure significantly improved stock of total soil organic carbon in 0-20 cm under upland and paddy soil under different farmland types.However,there are differences in soil stability mechanisms between upland and paddy soil.For paddy soil,the newly added organic carbon was stored in the stable carbon pool at first,and then transferred to unprotected and physical-protected carbon pool.For upland soil,the newly added organic carbon was fixed in physical-protected carbon pool at first,and then transferred to unprotected carbon pool after reaching the stable state.Therefor,manure application is more beneficial to the accumulation and stability of soil organic carbon. |
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