Effects Of Applying Nitrogen Fertilizer On Carbon Emission And Accumulation In Calcareous Soil

Rapid climate change as a consequence of abundant emission of greenhouse gases leads to the global temperature increase. Among the greenhouse gases, carbon dioxide(CO2) is a critically player. Soil carbon pool as the biggest carbon pools in the terrestrial ecosystem which has played an important role in the carbon dynamic, and has a great potential of CO2 sequestration. Calcareous soil contains both soil organic carbon(SOC) and soil inorganic carbon(SIC). SOC is the main composition in the humid and sub-humid areas, while SIC is the main carbon pool in arid and semiarid areas. Compared with many researches onthe SOC, researched about the SIC are limited. Calcareous soils with pedogenic carbonate cover about 30% of China, mainly in the arid and semiarid regions in the Northwest. Applying nitrogen fertilizer has been one of the major aspects of global soil acidification, nowhere has that change been more dramatic than in China. So we studyed the carbon turnover and influencing factor in calcareous soil under the applying nitrogen fertilizer conditions. The study might give us a reference or basis about the stabilization of SIC and its role in the global carbon cycle.Therefore, we sampled the calcareous soils from different regions in Shaanxi Province, by means of closed-jar incubation and field experiment, combined with the methods to differentiate CO2 emission from SOC and SIC pools(mercuric chloride sterilization, “loss on ignition” method), to investigate the SIC emission, stock. The main conclusions showed as follows:(1) To evaluate the effect of N fertilizer application levels on CO2 emission from Lou soil during closed-jar incubation, five N rates were conducted: 0, 0.1, 0.2, 0.3 and 0.4g kg-1. To estimate the source of CO2 emission, we incubated the soil with and without addition of 1000 mg kg-1 mercuric chloride(HgCl2) which buffered by piperazine-1,4-bisethanesulfonic acid(PIPES). The result showed N fertilizer application resulted in a pH increase in the first few days and followed by a pH decrease. The content of NO3- increased with time in all soils without sterilization. While sterilization effectively inhibited the nitrification process and the reduction in soil pH, resulted in accumulations of high content of NH4+. After 46 d, soil CO2 emission significantly increased from 21.8% to 103.5% compared with control in the non-sterilized condition. The sterilized soils had less CO2 emission compared to the soils without sterilization due to the mineralization of SOC was inhibited. However, CO2 emission also increased from 5.3% to 17.8% compared with control for the sterilized soil. It indicated that although mineralization was inhibited, the sulfate radical(SO42-) in ammonium sulfate((NH4)2SO4) fertilizer could also promote acidification, which led to carbonate dissolution and CO2 emission. We estimated the approximate emission ratio from SIC was about 54.4%.(2) Four different types of soils(Lou soil, dark loessial soil, loessial soil and aeolian sandy soil) with various SOM, texture, and carbonate were used to evaluate the effects of(NH4)2SO4 fertilizer application on CO2 emission from different calcareous soils.. Soils were incubated with and without 1000 mg kg-1 HgCl2 which were buffered by PIPES under closed-jar conditions for 46 days at 25℃. The results showed soil pH and NH4+ decreased with(NH4)2SO4 application under the non-sterilized condition. While sterilization effectively inhibited the nitrification process and the reduction in soil pH, resulted in accumulations of high concentrations of NH4+. After 46 d, CO2 emission of four soils increased by 26.9%, 19.5%,68.8% and 46.1% compared with control in the non-sterilized condition, respectively. The sterilized soils had less CO2 emission compared with the soils without sterilization due to the mineralization of SOC was inhibited. While the average release increment was 25.5% compared with control soils under the sterilized condition. We concluded that the major mechanism for CO2 emission after addition of(NH4)2SO4 fertilizer is the protons produced by nitrification under the non-sterilized condition in calcareous soils. While production of H2SO4 may also contribute to the CaCO3 dissolution under the sterilized condition.(3) The effects of three N fertilizer types(NH4NO3,KNO3 and(NH4)2SO4) on CO2 emission from Lou soil were studied with the closed-jar incubation method. To differentiate CO2 emission from SOC and SIC pools, we incubated the soil with and without ignition to remove SOC. The results showed soil pH significantly increased at the beginning of incubation after ignition. The average pH was 9.66 on the first day. It was due to the SOM was removed, soil buffer capacity decreased rapidly. NH4+ content of(NH4)2SO4 and NH4NO3 treatments without ignition decreased rapidly with time. After ignition, although NH4+ content of(NH4)2SO4 treatment decreased, the degree was smaller than the treatment without ignition. There were no significant changes for the other two fertilizers. In contrast to NH4+, NO3- of ammonium nitrogen fertilizer treatments increased with time. For the soils without ignition,CO2 release amount were highest in(NH4)2SO4 treatment, followed by NH4NO3 and KNO3. The average release increment was 36.6% compared with CK. CO2 release amount significantly decreased compared with soils without ignition,(NH4)2SO4 treatment also had the highest release amount. The average release amount increased 24.5% compared CK. We estimated the emission ratio from SIC was about 27.2%.(4) A 11 years long-term field was conducted in Yangling of Shaanxi to investigate the effects of different cultivation methods and N fertilizer levels on SIC stock and its compositions in 0~10 cm, 10~20 cm and 20~40 cm soil profiles under wheat-maize cropping rotation system. The results indicated that SIC stock in three soil profiles of different cultivation methods were all significantly higher than fallow soil, the average increment was 7.25 t ha-1. The order of different cultivation patterns on 0~40 cm SIC stock is: furrow planting > conventional cultivation > straw mulching, while there was no significant difference among cultivation methods. The mineral compositions of SIC was the same as cultivation methods. The content of calcite was apparently higher than dolomite, it indicated that the major conponent of SIC in the field was calcium carbonate. The content of SIC stock and its compositions were all decreased with N fertilizer levels. The SIC stock of N 120 and N 240 kg ha-1 treatment was decreased by 0.60 and 1.45 t ha-1, respectively. We concluded high N fertilizer levels could decrease SIC content.