Difference In The Stability Of Soil Organic Carbon Between Paddies And Uplands Developed From Quaternary Red Clay Under Long-term Fertilization

Soil carbon?C?sequestration in croplands is of great importance to mitigate climate change and improve soil fertility and crop productivity.Carbon sequestration means the process of transfer and secure storage of atmospheric CO₂ into long-lived C pools.Furthermore,the stability of soil organic C?SOC?plays a vital role in long-term soil C sequestration and feedbacks of soil C stocks to climate change.A great amount of research indicates that compared with uplands,rice paddies contain higher SOC stocks and have a higher SOC sequestration potential,and thus SOC sequestration in rice paddies could mitigate the net global warming potential?GWP?of rice cropping.However,despite of the high SOC stocks,it is not clear whether or not the sequestered SOC can be stabilized in the long term.The stability of SOC is affected by both environmental conditions and inherent stabilization mechanisms.It has not been well understood whether the low SOC decomposition in paddies is mainly controlled by anaerobic conditions or by stabilization mechanisms.Higher SOC stocks in rice paddies can be reflected by comparing those between paddies and uplands and among different fertilization regimes,particularly organic amendments.However,it is not clear whether or not the sequestered SOC in paddies relative to uplands or in the organic-amended treatment can be stored over the long-term.Clarifying factors affecting the stability of SOC in paddies could deepen our understanding regarding the mechanisms underlying SOC sequestration and stabilization.Therefore,the objectives of this study are to?1?estimate C inputs and C sequestration efficiency?C stock increase per unit C input?in a pair of adjacent paddy?double rice?and upland?double corn?long-term field experiments,thus revealing whether higher C inputs or higher C sequestration efficiency contributes to the higher C stocks in paddies than in uplands;?2?evaluate the difference in C stability between the paddy and upland soils by both soil incubation and chemical oxidation methods;?3?employ different SOC fractionation methods to clarify the difference in SOC stabilization mechanisms?physical and chemical protections?;and?4?incubate soils under different temperature and moisture regimes to determine the response of SOC decomposition to temperature and moisture.Results are as follows:?1?The meta-analysis on long-term experimental results?>5 years?showed that both SOC and total nitrogen?TN?contents showed declining trends in both paddies and uplands in the unfertilized control?CK?treatment,but not statistically significantly.Inorganic N,P,and K fertilizer application alone?NPK?and along with organic manure?NPKM?could significantly increase SOC and TN contents.The content of SOC significantly decreased in the CK treatment in uplands,but showed an increasing trend in paddies without statistical significance.The content of SOC was significantly increased in the NPK and NPKM treatments in both paddies and uplands.The content of SOC increased with increasing experimental duration in both paddies and uplands.However,there were no growing trends in SOC content at experimental duration>20 years in uplands but with markedly continuously increasing trends in paddies.?2?Results from two adjacent paddy and upland long-term fertilization experiments indicated that,compared to the initial level,long-term corn cropping leaded to a decline in SOC content in the CK treatment,while continuous rice cropping significantly increased SOC content.Compared with the CK and NPK treatments,SOC content was significantly higher in the NPKM treatment in both the paddy and the upland.Relative to the CK,inorganic NPK application alone significantly increased SOC content in the upland,but had no significant effect in the paddy.Carbon sequestration efficiency was significantly higher in the paddy than in the upland,regardless of fertilization regimes.Soil incubation?30?and 60%water-holding capacity?showed that cumulative CO₂ release per gram of soil C was significantly higher in the paddy than in the upland,suggesting that the stability of SOC was lower in the paddy than in the upland.Compared with the NPK treatment,NPKM application significantly reduced SOC stability in both the paddy and the upland,while no significant difference was observed between the CK and NPK treatments.?3?Chemical oxidation showed that the C content of the very labile,labile,and less labile fractions were significantly higher in the paddy than in the upland,while no significant difference was found in that of the non-labile fraction.Fertilization had no significant impacts on the C content of the less labile and non-labile fractions in both the paddy and the upland,while organic amendments increased the C content of the very labile and labile fractions.The proportion of active C?very labile and labile fractions?was significantly greater in the paddy than in the upland.Organic amendments tended to increase the proportion of active C in both the paddy and the upland,but not significantly.Thus,Chemical oxidation could reveal the difference in SOC stability between paddies and uplands,but could not detect the effect of organic fertilization.?4?The proportion of SOC in large aggregates was significantly higher in the paddy than in the upland,while the proportion of SOC in microaggregates showed the opposite trend.The proportion of SOC in large aggregates decreased in the order in both the paddy and the upland:CK<NPK<NPKM,with the opposite in the proportion of SOC in microaggregates.Neither land use nor fertilization had significant effect on the proportion of SOC in the clay and silt.The paddy soil had higher clay content than the upland soil,but with no significant differences among fertilization regimes.The content of free iron oxides was significantly greater in the upland soil than in the paddy soil,but fertilization regimes had no significant impact.?5?Soil incubation under different temperature?20?and 30??and moisture regimes?60%water-holding capacity and keeping 1 cm water depth?showed that warming significantly promoted cumulative CO₂ release per gram of soil C,whereas anaerobic conditions significantly reduced that.Cumulative CO₂ release per gram of soil C was higher in the paddy than in the upland.Compared with the CK and NPK treatments,cumulative CO₂ release per gram of soil C was significantly greater in the NPKM treatment,while no significant difference was detected between the CK and NPK treatments.Significant CH₄ release was observed only in the paddy soil under anaerobic conditions.Compared with the CK and NPK treatments,cumulative CH₄ release per gram of soil C was significantly greater in the NPKM treatment.Warming significantly increased the GWP per gram of soil C.The GWP per gram of soil C was higher under anaerobic conditions than under aerobic conditions and in the paddy than in the upland.Compared with the CK and NPK treatments,NPKM application significantly increased the GWP per gram of soil C.?6?The temperature sensitivity(Q₁₀)of CH₄ release was significantly greater than that of CO₂ release under anaerobic conditions.The Q₁₀ of CH₄ release was significantly greater in the NPKM treatment than in the NPK treatment.The Q₁₀ of total C release and CO₂ release showed the similar trends:anaerobic conditions significantly reduced the Q₁₀ of CO₂ release;the paddy soil had the lower Q₁₀ of CO₂ release than the upland soil;the Q₁₀ of CO₂ release was lower in the NPKM treatment than in the CK and NPK treatments,but with no significant difference between the CK and NPK treatments.The Q₁₀ of the GWP was higher under anaerobic conditions than under aerobic conditions.The Q₁₀ of the GWP was significantly lower in the NPKM treatment than in the NPK treatment in the upland,but not significantly in the paddy.The Q₁₀ of total C release was significantly negatively correlated with SOC content and the content of O-alkyl-C,while showing a significant positive correlation with the aromaticity of SOC.In conclusion,paddy fields not only have higher SOC content and greater SOC sequestration capacity,but have longer C sequestration duration and higher C sequestration efficiency than uplands.However,SOC stability is significantly lower in the paddy than in the upland.Organic manure application significantly increases SOC content,but reduces SOC stability.The lower C stability in the paddy soil may be associated with the higher SOC proportion in large aggregates,the lower content of free Fe-oxides,and the lower degree of SOC decomposition.Thus,comapered with uplands,the higher C sequestration efficiency in paddies is mainly attributed to the anaerobic condition rather than the stability of SOC.There are significant interactions between temperature,moisture,and fertilization regimes on SOC decomposition.