Study On Aggregate Stability And Its Carbon Sequestration Mechanism For The Irrigated Desert Soil

The irrigated desert soil is a typically cultivated soil of arid inland regions, which is distributed ininland river basin and the Yellow River Basin of the desert border region and occupies more than half ofthe total land area in China. In recent years, irrigated desert soil was severely damaged due to vegetationdegradation, soil erosion, etc, which results in the destruction of soil structure and the decline of soilfertility. In order to improve the soil structure and enhance the ability of soil resisting erosion, this studyselected a long-term fertilizer experiment located in Wuwei City of Gansu Province as the researchobjective to study the physical and chemical properties, mineral composition, organic matter (OM)composition, of water stable aggregates (WSA) size fractions and organic-mineral complex status underdifferent fertilizater treatments, further clarifying the protective effect of WSA size fractions on organiccarbon (OC) and Clearing the mechanism of aggregates stability and OC sequestration of irrigateddesert soil. The main results were showed in the following：1. The application of chemical N fertilizer alone had no significant effect on SOC concentrationsand the OC concentrations of WSA size fractions, but the application of organic manures alone or incombination with chemical N fertilizer could significantly increase the OC concentrations of>2mm,0.25–2mm,0.053–0.25mm and <0.053mm WSA size fractions by19.5%–51.6%，24.8%–71.0%，22.8%–65.3%and12.2%–83.2%. The OC concentration of0.25–2mm WSA size fraction was higherthan those of the other WSA size fractions. In addition, the percentages of>0.25mm WSA sizefractions showed significantly positive correlation with SOC contcentrations (r=0.485, n=24, P<0.05).The correlation analysis also indicated that the significantly positive relationships between thepercentage of0.25–2mm WSA size and its OC concentration (r=0.75, n=24, P<0.01).This indicates thatthe OC plays important role in the formation and stability of macroaggregates.2. Freeze-thaw process decreased the percentage of>2mm WSA size fractions by14.5%–66.1%,increased the percentages of <2mm WSA size fractions, correspondingly. The SOC concentrations werepositively and significantly correlated with the percentages of>0.25mm WSA fraction. However, thisrelationship was not significant after soils experiencing freeze-thaw process (r=0.143, n=24, P>0.05),indicated freeze-thaw process accelerates macroaggregates degradation and disrupt aggregate stability.The freeze-thaw process also barely changed the percentages of>0.25mm WSA size fractions in the GNand SN treatments, but decreased the percentages of this fraction in the other treatments by5.8%–17.1%.In addition, freeze-thaw process increased the OC enrichment ratios by6.7%and3.2%, and Nenrichment ratios by8.2%and10.2%in>0.25mm WSA fraction in the GN and SN treatments, butdecreased the OC enrichment ratios by1.5%–13.9%and N enrichment ratios by0.9%–8.4%in thisfraction in the other treatments. Results indicate that the fertilization modes of GN and SN could resistthe destruction of freeze-thaw process on aggregate stability and protect soil nutrients from erosion. Inaddition, before soil freezing, the percentages of0.25–2mm WSA size fractions were significantlycorrelated with its OC concentration, but this relationship was still significant after soils after soil thawing. Moreover, the percentages of the other WSA size fractions were not significantly correlatedwith their OC concentrations. This demonstrated that the ability of0.25–2mm WSA size fractionsresisting to freeze-thaw process was higher than those of the other WSA size fractions.3. The application of chemical N fertilizer alone could not significantly affect the OC and Nconcentrations of particulate OM (POM) and heavy fraction OM (HFOM), the application of organicmanures alone or in combination with chemical N fertilizer significantly increased the OC and Nconcentrations of POM by26.5%–191.7%and20.2%–175.7%, respectively, and increased the OCand N concentrations of HFOM by19.2%–70.1%and11.2%–60.6%, respectively. Fertilization had nosignificantly effect on OC concentration of Light fraction OM (LFOM), but significantly increased theN concentration of LFOM. Compared to the CK, the application of chemical N fertilizer could notsignificantly affect the OC concentrations of loosely combined humus(LCH), stably combinedhumus(SCH) and tightly combined humus(TCH), while the application of organic manures alone or incombination with chemical N fertilizer significantly increased the OC concentrations of the three typesof combined humus by23.1%–146.2%、11.1%–61.1%and20.6%–57.1%, respectively. The correlationanalysis showed that the OC concentrations of the three types of combined humus were significantlyand positively correlated with the quantity of organic-mineral complex (P<0.01). This demonstrated thatthe the quantity of organic-mineral complex was better than the degree of organic-mineral complex inmeasuring the organic-mineral complex status of irrigated desert soil. In addition, the OC concentrationsof the three types of combined humus was significantly and negetively correlated with the exchangeableCa content, while they were significantly and positively correlated with exchangeable Mg andCa–humus contents. The correlation relationship between organic-mineral complex status and thepercentages of WSA size fractions showed that the degree of organic-mineral complex was notsignificantly correlated with the percentages of WSA size fractions, but the quantity of organic-mineralcomplex was significantly and positively correlated with the percentage of0.25–2mm WSA sizefraction, and was significantly and negatively correlated with the percentage of <0.053mm WSA sizefraction. This indirectly indicated that the quantity of organic-mineral complex of0.25–2mm WSA sizefraction was higher than those of the other WSA size fractions.4. Fertilization had no effect on the total Ca and Mg contents of WSA size fractions, but it couldaffect the exchangeable Ca, exchangeable Mg, CEC, and Ca–humus contents to some extent. Theapplication of chemical N fertilizer could increase the OC concentrations of LCH, SCH and TCH in the<0.053mm WSA size fractions, but the application of organic manures alone or in combination withchemical N fertilizer could significantly increase the OC concentrations of LCH, SCH and TCH in the<0.053mm WSA size fractions. This study indicates that CEC and exchangeable Mg play an importantrole in the formation of>2mm WSA size fraction. CEC, exchangeable Ca, exchangeable Mg andCa–humus cement OC to form the0.25–2mm WSA size fraction, which could protect OC in the0.25–2mm WSA size fraction from degradation. CEC, exchangeable Mg and Ca–humus play a protectiveeffect on the OC in the0.053–0.25mm WSA size fraction. It could see that the difference amongchemically protective mechanism of organic carbon in different WSA size fractions. The contents of O–alkyl–C in the total OC area of WSA size fractions were the highest. In addition, the mean content ofO–alkyl–C in the <0.053mm WSA size fraction was lower than those of the other WSA size fractions.Moreover, the mean aromaticity of the WSA size fraction followed the order:<0.053mm=2mm>0.053–0.25mm>0.25–2mm, which indicated that the Humification degree of OC in the <0.053mmWSA size fraction was higher than those of the other WSA size fractions and OC in the0.25–2mmWSA size fraction still had a huge mineralization potential.5. Fertilization mainly increased the OC stocks of <2mm WSA size fractions, the application oforganic manures significantly increase the OC contents inputting into soils. The correlation analysisshowed that the C sequestration rate of irrigated desert soil was significantly and positively correlatedwith the annually OC input amount (r=0.680, n=24, P<0.01). The C sequestration rate increased withthe increase of the inputted OC, showing that SOC did not reach the saturation point and the soil stillhad a huge potentiality for C sequestration in this region. During the23years of fertilization, about11.51%of the inputted OC was converted into SOC, and the inputted OC amounting to0.66Mg ha–1year–1could sustain the OC content at initial level of9.48g C kg–1in this experiment. This studyshowed that in the wheat and maize rotation system, the maize–derived C was mainly distributed in the0.25–2mm WSA size fraction, and the wheat–derived C was mainly distributed in the <0.053mm WSAsize fraction. The correlation analysis showed that the OC stocks of0.25–2mm and0.053-0.25mmWSA size fractions were significantly and positively correlated with the total OC input amountindicated (P<0.01) that the amended OM is mainly converted into the OC stocks of0.25–2mm and0.053-0.25mm WSA size fractions. The application of organic manures alone or in combination withchemical N fertilizer generally increase the OC stocks of LFOM, LCH, SCH and TCH in the WSA sizefractions of irrigated desert soil(P<0.01). Moreover, TCH was the main component for OC sequestrationin the various WSA size fraction. In addition, The correlation analysis showed that the percentage of0.25–2mm WSA size fraction was significantly and positively correlated with the OC stocks of LFOM,LCH, SCH and TCH, which also demonstrated that0.25–2mm WSA size fraction was the maincomponent for the OC sequestration in the irrigated desert soils.