Effects Of Application Of Nitrogen Fertilizer On Carbon Emissions And Their Sources From Calcareous Soils

As the largest carbon sink of terrestrial ecosystem,soil carbon pool has important influence on global carbon cycle and atmospheric CO₂ concentration.Soil carbon emission is influenced by natural environment and human factors.Nitrogen fertilizer application has important influence on soil carbon emission.The effects of nitrogen application on soil CO₂release are mainly concentrated in the mineralization of organic carbon.In fact,there are abundant inorganic carbon content of calcareous soil in the northern China,does the soil acidification caused by nitrogen fertilizer result in the release of inorganic carbon in calcareous soil?What is its contribution to carbon emission?The research is needed to answer these questions.Therefore,this study uses the indoor incubation and field experiment to study the application methods and type of nitrogen fertilization in different soils on soil CO₂ emission.And nitrification inhibitor?DCD?and ¹³ C methods were used to estimate the source of CO₂emission from soils.The main results obtained included:?1?The effects of different nitrogenous fertilizer application methods on soil pH and CO₂emission were studied by comparing uniform N-application?N100%?and partial N-application?N50%,N25%?.The results showed that compared with N0 treatment,the soil pH was significantly reduced and the soil CO₂ emission was significantly increased after applying nitrogen fertilizer.Compared different nitrogen treatments,the soil pH of partial N-application pH drop more significantly than the uniform N-application treatment after incubation,the soil also release more CO₂ in the former than the latter,the soil CO₂ emission of N25%,N50%treatment increased by 10.04%and 8.21%respectively than N100%treatment.It is indicated that local application of nitrogen fertilizer is more easily to cause soil acidification and CO₂ release.?2?In order to study the effect of N-fertilizer types??NH₄?₂SO₄,NH₄NO₃?and exogenous inorganic carbon on soil CO₂ emission and pH changes,an indoor airtight incubation experiment was done by using soils sampled from the horizon poor in SIC-argillic horizon in Yangling region.The results showed that nitrogen fertilizer increased the cumulative release of soil CO₂ and reduced the soil pH value,whether or not the external carbonates were put into it.Compared two types of nitrogen fertilizer,the soil pH of?NH₄?₂SO₄ treatment dropped larger than the NH₄NO₃ treatment,especially under no exogenous inorganic carbon input condition.The average soil CO₂ emission of?NH₄?₂SO₄treatment were overall higher than that of NH₄NO₃ treatment.The decrease of soil pH of?NH₄?₂SO₄ treatments was significantly lower than that of non-input carbonate treatments,while the CO₂ emission didn't show significantly difference.?3?An closed-jar incubation experiments was conducted to study the effects of adding nitrogen?N?fertilizer and nitrification inhibitor?DCD?on soil pH and carbon dioxide emissions from three soil types with different contents of soil inorganic carbon?SIC?,including paddy soil?PS?,lime concretion black soil?CS?and Lou soils?LS?.The treatments included a control without N fertilizer?N₀?,and 0.2g·kg^-1 N fertilizer(N_0.2),and its combination with DCD(N_0.2+DCD)for each soil type.The soil pH,contents of mineral N?NH₄⁺,and NO₃^-?,and CO₂ emissions were measured.The result showed that the application of N fertilizer reduced soil pH significantly;on the contrary,it increased soil CO₂ cumulative emissions in each type of soil.Compared to N₀ treatment,the CO₂ cumulative emissions after49 days incubation from the three soil types were enhanced by 39.4%,23.4%,and 71.8%for PS,CS,LS soils,respectively.The soil pH of N_0.2+DCD for three soil types were significantly higher than N_0.2 after 49 days incubation,indicating that DCD application inhibited soil nitrification process.There were no significant differences in the mean CO₂ cumulative emissions from PS and CS soils between N_0.2 and N_0.2+DCD treatments;however,N_0.2+DCD.2+DCD treatment significantly reduced CO₂ cumulative emissions from LS soil by 12.5%.?4?Field experiment was conducted to study the effect of N-application rate?N0,N160,N220?and its combination with DCD?N160+DCD,N220+DCD?on the soil acidification and CO₂ emission,and preliminary estimated the contribution ratio of soil inorganic carbon?SIC?to CO₂ by measuring the?¹³C value for SOC,SIC,urea and CO₂ released from soils within 10days after urea fertilizer application.The N-application treatments significantly reduced soil pH and promoted soil CO₂ cumulative amounts.Compared with the single N-application treatments,N-application combined with DCD significantly inhibited the soil pH reduction,and the soil CO₂ emission decreased by 15.91%on average.Nearly 30%of soil CO₂ emission was contributed to SIC for the N0 treatment within 10 days,while the ratio for only N-application treatments?N160,N220?was higher than the ratio of N-application combined with DCD treatments respectively.