| Soil organic carbon(SOC)of cropland,as the most active component in soil carbon pool is employed as one of the important indexes for soil fertility evaluation.On the other hand,fertilization is an important factor affecting the soil quality and organic carbon pool,thus,understanding the effect of fertilization on variation of soil organic carbon pool and its fertility characteristics would provide valuable guideline for soil sustainable utilization.This research was conducted on fluvo-aquic soil to investigate its change of organic carbon pools and fertility characteristics under 35 years long-term fertilization experiment in Xuzhou of Jiangsu province.Eight fertilization treatments were employed,which included no fertilizer(CK),N fertilizer(N),combined N and P fertilizer(NP),combined N,P and K fertilizer(NPK),organic fertilizer(M),N fertilizer and organic fertilizer(MN),combination of N,P and organic fertilizer(MNP),combination of N,P,K and organic fertilizer(MNPK).Then,soil organic carbon storage and its variation,soil organic carbon and its components on biological were studied,and both the effects of long-term inorganic fertilizer and organic fertilizer application on soil organic carbon turnover were distinguished,the relationship between soil organic carbon and crop yield were estimated,fertilizing effect under different fertilization treatments were evaluated.All of these could provide scientific basis for the improvement of land productivity,crop yield,and carbon sequestration technology.The main results showed as follows:(1)Average contents of soil organic carbon was increased by all fertilizer treatments with the organic manure and chemical fertilizers treatments being the most significant.After 35 years of experiment,soil organic carbon storage were increased by 5.3-14.8 t C·hm-2,and the MN,MNP and MNPK treatments were higher than the organic manure and chemical fertilizers treatments,and M treatment was significantly higher than the chemical fertilizers treatments.When the cumulative carbon input was greater than 83 t C·hm-2,and the increment of organic carbon storage was decreased significantly with the increment of exogenous organic carbon input.Application of chemical fertilizer treatments has a great potential for carbon sequestration which is not limited by the saturation,and organic fertilizer treatments on soil carbon sequestration efficiency showed a phenomenon of "saturation curve effect" In addition,strongly positive linear correlation was observed between annual variable of organic carbon storage and carbon input(y=0.0893x-0.0257,R2=0.9456,p <0.01),and the least annual carbon input for maintaining the initial organic carbon level was about 0.29 t C·hm-2.(2)The cumulative mineralization in the MN,MNP and MNPK treatments were higher than the other fertilization treatments,and with the greatest increasing of 2.6-3.1 times in the non-fertilization treatment.Experimental study on indoor culture showed that the changes of mineralization rates of soil organic carbon of all the treatment presented similar changed trends which could be described by the logarithmic function relationship.Soil organic carbon mineralization was simulated by using three-level and one-order dynamic model,the results showed that apply fertilizer increased the proportion of the intermediate carbon pool and decreased Stable carbon plool ratio.Compared to CK treatment,the intermediate carbon content was increased by 12.0-12.5 times in combined application of organic and inorganic,which was much higher than both chemical fertilizer treatment(2.1-5.9 times)and single application of organic fertilizer treatment(9.8 times).The turnover time of labile organic carbon pool was decreased in the application of organic fertilizers.There was a significant positive correlation between soil organic carbon and carbon input,the highest correlation was found in the intermediate organic carbon pool.(3)All fertilizer treatments very significantly increase the aveage of wheat and maize yield,in which application of organic fertilizer and chemical fertilizer were higher than others.Fertilizer application could significantly increase sweetpotato yield with exception of N treatment.Sweetpotato average yield under application of MNPK and NPK were higher than other fertilization treatments.There was no significant difference between MNPK and NPK treatments.The stability and sustainability of crop yield can be characterized by crop yield coefficient of variation and sustainable yield index,respectively.Compared with CK,wheat,maize and sweetpotato yield stability of MNPK were better than any addition treatment.In a word,combination of N,P,K and organic fertilizer is the best fertilization mode in fluvoaquic soil region,which made soil nutrient more balanced,crop yield more stable and agricultural ecosystem quality better.(4)Soil organic carbon is closely related to crop yield,and the linear regression equations were established for soil organic carbon and wheat,maize and sweetpotato yield.There were significant positive correlations between organic carbon variable and the yield of wheat,maize and sweetpotato.The yield of wheat and sweetpotato were significantly positively correlated with soil labile organic carbon pool and intermediate organic carbon pool.The correlation between the CV,SYI value of wheat yield and soil organic carbon average content were significantly correlated.(5)Significant differences of soil properties were found among the different fertilization treatments.Compared with no organic manure treatment,treatments of organic manure or its combination with chemical fertilizers significantly reduced soil bulk density,and increased soil porosity,soil organic carbon,total nitrogen,alkali-hydrolyzable nitrogen,available phosphorus,microbial biomass,catalase,urease,alkaline phosphatase and invertase.Two principal components that could reflect 85.51% of the original information quantity were extracted from the initial 15 indices.As a comprehensive evaluation of Soil Fertility Indicators,biological indicators of microbial biomass and enzyme is better than that of urease,alkaline phosphatase and invertase,chemical indicators of soil nitrogen,phosphorus and organic matter plays a leading role.Hierarchical cluster analysis of the new indices extracted based on scores of the two principal components revealed the effects of different patterns of long-term fertilization,and displayed an order of MNPK >MNP >M,MN >NPK >N,NP >CK. |
