Effects Of Monovalent Cations On Soil Aggregates And Organic Carbon Mineralization In Loessial Soil

The Guanzhong Plain of Shaanxi Province is one of the main grain producing areas in China,and loessial soil is an important soil type in this area.The application of manure is one of the important means to improve loessial soil fertility and crop yield.However,the large application of manure may cause the accumulation of monovalent cations in soil,thus disperses soil aggregates,reduces loessial soil aggregates stability,and affects organic carbon mineralization.Therefore,this study took loessial soil as the research object to study the effect of organic supplements and exogenous monovalent cation of different concentrations on it.Organic supplements include two treatments:no straw and added straw(S);monovalent cations include 13 treatments:potassium gradient(K~+:0,120,200,280,370,540 mg/kg with different concentration gradients,represented by Nil,K1～K5,respectively),sodium gradient(Na~+:90,180 and 270 mg/kg,represented by Na1,Na2 and Na3,respectively)and sodium gradient at a specific potassium level(on the basis of adding 370 mg/kg K~+,add 0,90 and 180mg/kg Na~+,represented by KNa0,KNa1,KNa2 and KNa3,respectively).The soil was cultured at 25℃for 105 days.We measured the dynamics of soil organic carbon(SOC)mineralization and modeled SOC mineralization with double exponential model to research the effect of monovalent cation accumulation on SOC mineralization during incubation;separated the soil aggregates and determined their organic carbon(OC)content at the end of the incubation to research the effect of monovalent cation accumulation in soil on aggregates stability;determined the chemical properties of soil after incubate,and combined with correlation analysis and redundancy analysis clarified the effect of exogenous monovalent cation addition on soil organic carbon mineralization and aggregate stability with or without exogenous organic matter.The main results are as follows:(1)After adding straw,all treatments increased the cation ratio of soil structural stability(CROSS),sodium adsorption ratio(SAR),potassium adsorption ratio(PAR),exchangeable potassium percentage(EPP),soil organic carbon(SOC)and soil total nitrogen(TN),but decreased exchangeable sodium percentage(ESP),C/N and p H.Under the condition of no straw,all treatments spiked with K~+,Na~+or both increased PAR,EPP and ESP and reduced the C/N and p H.Under the condition of straw addition,all treatments of K~+increased CROSS,PAR and EPP.all treatments of Na~+increased CROSS,SAR and ESP and decreased PAR and EPP.All treatments spiked with K~+and Na~+increased CROSS、SAR、PAR、ESP and EPP(2)Regardless of whether straw was added or not,0.25-2 mm aggregates proportion was the highest in all treatments.The proportion of>2 mm aggregates was the second,which were significantly higher than the other two aggregates.All treatments with straw addition increased the proportion of>2 mm aggregates,decreased the proportion of 0.053-0.25 mm aggregates and<0.053 mm fraction,thus increased the mean weight diameter(MWD)and geometric mean weight(GMD)..Under the condition of no straw,all treatments spiked with K~+,Na~+or both reduced the proportion of>2 mm aggregates,and increased the proportion of 0.25-2 mm aggregates and<0.053 mm fraction.Under the condition of straw addition,all treatments spiked with K~+,Na~+or both decreased the proportion of>2 mm aggregates and<0.053 mm fraction(except SKNa0 and SKNa3),and increased the proportion of 0.25-2 mm aggregates.Therefore,MWD and GMD of each treatment were reduced,especially the treatment with added K~+and Na~+.Redundancy analysis and correlation analysis showed that MWD and GMD were significantly negatively correlated with CROSS,ESP,SAR and PAR.(3)Regardless of whether straw was added,organic carbon content of>2 mm,0.25-2mm and 0.053-0.25 mm aggregates was significantly higher than that of<0.053 mm fraction.All straw treatments improved the organic carbon content of>2 mm aggregate(except SK3)and<0.053 mm fraction.Under the condition of no straw,All treatments of K~+,Na~+or both increased the organic carbon content of>2 mm,0.25-2 mm aggregates and<0.053 mm fraction.Under the condition of straw addition,All treatments of K~+,Na~+or both reduced the organic carbon content of>2 mm、0.25-2 mm aggregates and<0.053 mm fraction and improved the organic carbon content of 0.053-0.25 mm aggregates(except SK1).(4)Straw supplementation significantly increased the cumulative amount and rate of organic carbon mineralization,and correspondingly increased the amount of mineralization from activated carbon pool(Ca)and stable carbon pool(Cs).Under the condition of no straw,All treatments spiked with K~+or both spiked with K~+and Na~+decreased the amount of organic carbon mineralization by 1.2%～22.3%.The results of double exponential model simulation showed that all treatments spiked with K~+,Na~+or both increased Ca,decreased Cs.Under the condition of straw addition,All treatments of K~+,Na~+or both decreased the amount of organic carbon mineralization by 0.9%～7.3%.All treatments spiked with K~+or both spiked with K~+and Na~+decreased Ca and Cs.Redundancy analysis and correlation analysis showed that the cumulative mineralization of organic carbon was significantly positively correlated with MWD and EPP,but negatively correlated with CROSS,SAR and ESP.In conclusion,whether straw was added or not,the accumulation of monovalent cations will disperse soil aggregates and reduce the proportion of>2mm macroaggregates,so as to reduce the stability of aggregates and inhibit the mineralization of organic carbon.Compared with no straw,adding straw can promote the formation of large aggregates,improve the stability of aggregates,and reduce the dispersion of monovalent cations to a certain extent.