Effects Of Iron Oxides On Organic Carbon Mineralization In Paddy Soils Under Different Moisture Regimes And Mechanisms Involved

Soil is an abundant storage of organic carbon,and as a source and sink of global greenhouse gases,it plays a central role in global carbon balance.Paddy soil,as an artificial wetland,contains greater organic carbon than upland,presenting large carbon retention potential.Mineralization of soil organic carbon?SOC?in paddy soil is important for global carbon cycle and climate change.Flooding and drying operations during rice planting and fallow periods may make the mineralization process of organic carbon in paddy soil different due to water changes.Iron oxides play an important role in SOC fixation.Moreover,in the process of water change,iron acts as a variable valence metal,and its redox process may affect the mineralization of SOC in paddy soil.Given the two important factors of water condition and iron oxides,this paper takes typical subtropical paddy soils as the object of study.three paddy soils with different total iron contents were collected,and incubation under different water regimes?dry-wet alteration,flooding and anaerobic?,to reveal the mineralization characteristics of SOC under different water conditions,to explore the changes of soil iron oxide morphology and valence state and the response of SOC mineralization to it.Data on soil physical and chemical properties,microbial biomass and enzymatic activity were collected and combined to help understand the mechanism on the effect of soil iron oxides on SOC mineralization.The results of the study could improve the theory of carbon cycle in paddy ecosystem,and to reveal the mechanism of long-term continuous carbon sequestration capacity of rice paddy soil.The main results are as follows:The effect of iron content and moisture regimes on the law of CO₂ release was significant.Higher iron oxide content leads to greater cumulative release of CO₂.Under all three different moisture regimes,soil CO₂ release were significantly different,but with the highest found under anaerobic culture,followed by those under flooded conditions,and then dry and wet conditions.After submerged culture of dry and wet culture conditions,the daily rate of carbon mineralization in high,medium and low iron soils increased by 1.53,1.28 and 1.88 times,respectively.?1?The total iron content and water regimes showed significant effect on CO₂ emission.Higher iron oxide content led to greater cumulative CO₂ emission.The difference in cumulative CO₂ emission among three soil was largest under anaerobic incubation and the lowest under dry-wet alteration.In the dry-wet alteration incubation,right after alteration to flooding,the daily rate of SOC mineralization in high,medium and low iron soils increased immediately by 1.53,1.28 and 1.88 times,respectively.?2?The effect of iron content and moisture on respiratory quotient and enzyme activities were significant.the higher the content of soil iron oxide was,the greater the soil respiratory quotient varied.The initial respiratory quotient under flooding and anaerobic conditions was higher than that under dry-wet alternation and decreased rapidly until reaching stable.While the respiratory quotient under dry-wet alternation was found less changeable during incubation.Under all three different water regimes,paddy soil iron content showed a significantly positive effect on PER activity.The PER activity was consistent with the soil iron content.Under the condition of dry and wet and flooded water,BG and NAG enzyme was dominant enzymes,while PER enzyme dominated in anaerobic condition.?3?The iron form and valence state were significantly correlated with DOC content under three water regimes.The higher the iron oxide content in paddy soil was,the greater the soil DOC varied;the change of DOC under flooding conditions was larger than those under dry-wet alteration.The partial correlation analysis of DOC with factors of different forms of soil iron showed that soil crystalline and amorphous iron affected soil DOC under non-flooding period of dry-wet alteration.Only soil crystalline iron affected soil DOC under the flooding stage of dry-wet alteration,flooding culture and anaerobic culture.?4?The iron form and valence state were significantly correlated with soil carbon mineralization under three different water regimes.The highest Fe²⁺content was found under anaerobic culture,and the least was found under alternating dry and wet conditions.cumulative CO₂ emission was significantly correlated with soil Fe²⁺content and complexation iron content,and was significantly positively correlated with crystalline iron content under anaerobic conditions,and with soil amorphous iron content under flooded and anaerobic conditions.Under anaerobic conditions,the cumulative mineralization was significantly positively correlated with the content of all three forms of iron oxide and Fe²⁺contents.The contents of complexed iron and amorphous iron were positively correlated with the cumulative mineralization under the conditions of wet and wet flooding culture and anaerobic culture.