| Soil carbon and nitrogen cycle are basic processes in farmland ecosystem.Soil organic carbon(SOC)and total nitrogen(TN)content are key indicators to soil fertility.Returning organic materials to the field can not only increase soil organic matter content and enhance soil fertility,but also reduce waste pollution to the environment that plays a crucial role in alleviating greenhouse gas increase and global climate change.Two filed trials under rainfed and irrigated dryland were set up to study the effects of different organic material inputs on soil carbon and nitrogen.Six treatments were included in each trial,such as non-fertilizer(CK),single application of nitrogen and phosphate(NP),organic fertilizer(MNP),straw return(SNP),low-level biochar(B1NP)and high Biochar(B2NP).The results showed as follow:1)Under rainfed condition,overall,SOC at the harvest stage of the third crop season immediately declined to the initial level in the organic fertilizer(MNP)treatment,indicating that it has no post-effect.The post-effect of SNP was slightly stronger than that of MNP.SOC at the harvest stage of the third crop season decreased slightly in biochar treatments(B1NP and B2NP),and then were kept stable with fluctuation,which showed a strong post-effect.Biochar had a stronger effect than straw,while organic fertilizer had the worst effect in improvement and maintenance of SOC content.Soil TN at the harvest stage of the fourth crop season was increased great in the return of straw(SNP),and followed a downward trend,indicating that the straw has a certain post-effect.Biochar treatment(B1NP and B2NP)had a tendency in increasing soil TN content at the harvest stage of the third crop season,and then kept stable with fluctuation,which showed a much stronger post-effect.The mean soil microbial biomass carbon and nitrogen(SMBC and SMBN)content at the harvest stage of the second crop season was significantly higher than that of the first crop season(P<0.05)in all organic material input treatments,and the average of that of SNP treatment in the initial two crop seasons was the highest.The average SMBC of the four-year fertilization treatments(NP,MNP,SNP,B1 NP,B2NP)were significantly higher than that of CK(P<0.05),which increased by 18.6%,17.0%,47.3%,35.8% and 20.5% respectively.SMBN only were increased in SNP and B1 NP by 43.5% and 35.3%,respectively(P<0.05).Compared with chemical fertilizer(NP),biochar(B1NP and B2NP)basically reduced the accumulation of nitrate-N in the deep layer(100-200cm)at the harvest stage of each crop season,indicating that biochar has better effects than straw and organic fertilizer in reducing nitrate leaching risk of soil profile.2)Under irrigated conditions,SOC and TN content were the highest at the harvest stage of the third crop season in all organic material treatments.SOC and TN of MNP treatment were decreased and returned to the initial level at the harvest stage of the fifth crop season.SOC and TN in SNP were decreased more at the harvest stage of the fifth crop season,and then were remained stable with fluctuation,which were always higher than the initial level.SOC of biochar treatments(B1NP and B2NP)were decreased at the harvest stage of the fifth crop season,and then were remained stable with fluctuation,which were always higher than that of SNP.Overall,Biochar had a stronger effect than straw,while organic fertilizer had the worst effect in improvement and maintenance of SOC content under the irrigated condition,which is the same with the rainfed condition.SMBC contents at the harvest stage of the second crop season were significantly higher than that of the first crop season in all organic material input treatments(P<0.05).After statistical analysis of results at the harvest stage of the eight crop seasons,it was found that NP,SNP and B2 NP had the most significant effect on increasing of SMBN content.In the harvest stage of the second season crop(October 2014),soil nitrate-N accumulated in the 100-200 cm soil layer in treatments of SNP,B1 NP and B2 NP were significantly lower than that in NP treatment,indicating that the application of organic material inputs could prevent nitrate-nitrogen to leach down with soil water.After the organic material inputs were stopped,compared with NP,biochar treatments(B1NP and B2NP)reduced the accumulation of nitrate-N in the deep layer(100-200cm),while straw and organic fertilizer had no obvious reducing effect,which means that biochar had a better function than straw and organic fertilizer,and also shows that biochar had a strong post-effect in reducing nitrate leaching risk in soil profile.3)Under rainfed condition,it was fine for DNDC model to simulate dynamics of SOC and crop yields under the straw returning and organic fertilizer application on the whole.The normalized root mean square error(NRMSE)is almost within an acceptable range.The DNDC model can be used to simulate long-term dynamic changes of SOC and yield in the Guanzhong area of Shaanxi Province.Using the calibrated DNDC model,the dynamics of SOC storage in the soil surface(0-20cm)were simulated under the application of different organic materials in the study area for 30 years.The results showed that the treatment of straw returning and organic fertilizer significantly increased the organic carbon storage in the surface soil compared with the single application of nitrogen and phosphorus fertilizer.By comparing the results of SOC storage simulated after the application of organic fertilizer and straw stopped,it was showed that organic fertilizer has no obvious post-effects,but straw return to soil had a significant post-effect in increasing soil organic carbon storage in a relatively longer time(4 years and 6 years).In conclusion,applying organic materials is conducive to improve the soil fertility,to increase the nitrate-N content of the surface layer(0-20cm),and to reduce the leaching of nitrate-N in the profile.After organic material applications were stopped,biochar(B1NP and B2NP)could still effectively inhibit the accumulation of nitrate in the deeper layers of the soil.Compared with organic fertilizer and straw,biochar had a strong post-effect.The long-term post-effect of organic material inputs simulated by the DNDC model need to be further verified in the field experiment. |
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