Transformation And Priming Effect Of Straw In Various Fertilized And Salinized Fluvo-aquic Soils

Soil carbon pools are critical for maintaining soil fertility and mitigating the greenhouse effect.Carbon sequestration is regulated by the balance between carbon input and output.The degradation of plant residue and mineralization of soil organic carbon dominate soil carbon input and output processes,respectively.Microbial carbon use efficiency(CUE)determines the fate of plant residue degraded carbon,and priming effect(PE)strongly influences the mineralization of soil organic carbon.Soil carbon availability,nutrient effectiveness,and soil salinity may affect the utilization efficiency of exogenous carbon and PE by influencing microorganisms,but relevant studies are scarce.Fertilization is an important agricultural management practice.Soils developed from the same parent material often have different carbon effectiveness(carbon fraction characteristics),nutrient effectiveness,and microbiological properties after long-term different fertilizations,which provide a platform for unraveling the relationships between carbon,nutrient effectiveness,CUE and PE.Soil salinization is a common environmental phenomenon in fluvo-aquic soils,which act as important agricultural soil in China.Soil salinization is one of the most important problems threatening the sustainable development of agriculture.It is well-known that straw return is considered as an effective measure to improve soil fertility.However,the underlysing mechanisms of salinization degree on soil organic carbon mineralization and PE is not clear.There is still a lack of systematic and comprehensive understanding of the extent to which straw returning can increase soil carbon sequestration.Besides,how salinization affects straw conversion and PE is also unclear.Therefore,we investigated the effects of different fertilization patterns(eight kinds of patterns:no fertilization CK,application of nitrogen fertilizer alone N,application of nitrogen and phosphorus fertilizers NP,application of nitrogen,phosphorus and potassium fertilizers NPK,application of organic manure M,application of nitrogen fertilizer plus organic manure MN,application of nitrogen and phosphorus fertilizers plus organic manure MNP,and application of nitrogen,phosphorus and potassium fertilizers plus organic manure MNPK)and salinization levels and straw returning(five kinds of saline soils,lightly saline soil LCK,lightly saline soil+straw returning LS,moderately saline soil MCK,moderately saline soil+straw returning MS,and heavily saline soil HCK)on soil carbon fraction,microbial characteristics,¹³C straw degradation,mineralization of soil organic carbon and PE through field trial sample collection and indoor¹³C straw degradation test with the help of long-term localized fertilization test platform and salinized fluvo-aquic soils continuous straw returning test platform.The study was conducted to find the linkages between soil carbon effectiveness,microbiological characteristics,microbial carbon utilization efficiency and PE,reveal the internal mechanism of fertilization and salinization affecting soil organic carbon turnover,so as to provide the basis for the model application of microbial carbon utilization efficiency and PE,and provide guidance for the regulation and management of farmland soil carbon pool.The main findings are as follows.(1)Fertilizer application significantly increased soil easily oxidized organic carbon(EOC)content,with the most significant increase of 72.17%in the MNPK treatment.The NPK treatment had a higher enhancement effect on particulate organic carbon(POC)and mineral-bound organic carbon(MOC)content than the N treatment,but lower than the organic fertilizer treatment.Compared with the non-organic fertilizer treatment,the POC content in the organic fertilizer treatment increased on average by 92.69%.Under the condition of organic fertilizer application,the contribution of the increased total organic carbon to MOC was significantly higher,and the increased total organic carbon from MNPK treatment could enter the POC and MOC fractions 1:1.(2)Microbial carbon source metabolic function analysis(Biolog analysis)showed that the average well color development(AWCD)value of the organic fertilizer treatment(M?MN?MNP?MNPK)was 23.82%higher than that without organic fertilizer treatment(CK?N?NP?NPK).The organic fertilizer application alone M showed the highest microbial functional diversity and microbial carbon source utilization capacity.The organic fertilizer+inorganic fertilizer treatment reduced soil microbial functional diversity compared to M treatment.Long-term nutrient deficiency negatively affected the microbial capacity to utilization of carbon sources,and the CK treatment had the lowest AWCD values,Shannon richness and Simpson index.Multiple stepwise regression analysis showed that the effectiveness of soil phosphorus and phosphorus-related stoichiometric ratios under different long-term fertilization patterns had a greater effect on microbial activity and functional diversity than carbon,nitrogen,and potassium.(3)Cumulative CO₂release at 120 d was 131.44%higher in the treatment with straw addition than in the treatment without straw addition under different fertilization patterns.The cumulative straw degradation at 120 d was 26.42%,25.32%and 25.65%higher in the NPKL,ML and MNPKL treatments than that in the CKL treatment,respectively.The overall rate of soil organic carbon mineralization and 120 d cumulative mineralization of the straw addition treatment were 46.67%and 39.61%higher than those of the no straw addition treatment,respectively,and the cumulative 120 d soil excitation of the no organic fertilizer application treatment was 25.07%higher than that of the organic fertilizer application treatment.The soil microbial carbon use efficiency was 2.01 times higher in the treatment with organic fertilizer and inorganic fertilizer than in the treatment without fertilizer.(4)The content of total organic carbon and its components in the whole soil decreased with increasing salinity.The straw returning increased the stability of soil aggregates in saline soils and increased the total organic carbon content in the whole soil of light and key saline soils by 39.81%and 24.62%,respectively.In lightly saline soils,straw fielding decreased the EOC content,POC content and POC/SOC in 0.053?0.25 mm micro-aggregates by 12.08%,30.56%and 46.14%,respectively,and the contribution of micro-aggregates to the increment of SOC and MOC was greater than that of macro-aggregates.In moderately saline soils,straw return significantly increased the content of SOC components>0.25 mm macro-aggregates and increased POC/SOC in whole soil and micro-aggregates by 56.56%and 46.65%,respectively.And the contribution of macro-aggregates to the increment of SOC and MOC was greater than that of micro-aggregates.(5)Biolog analysis showed that soil microbial functional diversity decreased with increasing salinity,the microbial carbon source metabolic activity(AWCD)of heavily saline soils was 70.36%and 61.82%lower than that of lightly and moderately saline soils,respectively.The Shannon index H?was 22.93%and 20.76%lower than that of lightly and moderately saline soils,respectively,and the Mc Intosh index U was 54.51%and 53.78%lower,respectively.(6)The cumulative CO₂release,cumulative straw degradation and cumulative soil organic carbon mineralization at 180 d were significantly lower in heavily saline soils than in light and moderately saline soils.The overall rates of soil organic carbon mineralization and cumulative mineralization of soil organic carbon in the treatment with ¹³C straw were 18.40%and 16.35%higher than those in the treatment without straw addition.The overall PE induced by the addition of ¹³C straw was positive in all saline soils,with the highest cumulative excitation at 180 d in heavily saline soils,110.56%and 169.19%higher than in light and moderately saline soils,respectively.The microbial carbon utilization efficiency in light saline soils was32.53%and 29.58%higher than that of medium and heavy saline soils,respectively.The continuous straw return in the field increased the cumulative degradation of ¹³C straw in light saline soils by 12.71%,but reduced the microbial carbon utilization efficiency.In summary,the application of organic fertilizer can effectively improve soil chemical properties,enhance the content of soil carbon fractions,enhancing the metabolic function of microbial carbon source.And it can significantly improve the carbon utilization efficiency of exogenous straw by soil microorganisms.Under long-term fertilization,the intensity of priming effect triggered by ¹³C straw was positively correlated with the percentage of easily oxidized organic carbon.Soil organic carbon fraction content and microbial functional diversity decreased with the increase of soil salinity,but the percentage of easily oxidized organic carbon increased.Straw returning increased organic carbon content in saline soil,but the mechanism of carbon increasing was different between lightly and moderately saline soils.The intensity of priming effect and microbial carbon use efficiency of saline soil was closely related to the percentage of active organic carbon(EOC/SOC POC/SOC)in macro-aggregates and micro-aggregates,respectively.