Impact Of Nitrogen On Karst Carbon Cycle In The Lijiang River Basin

The net atmospheric CO₂ sink from terrestrial carbonate weathering is 477 million t C a^-1.With changes in land use and increased rainfall,there may be an increase of 9.8 to 17.1 percent.However,nitric and sulfuric acid from human activities and their dissolution of carbonates are deducted from the carbon sink flux.The average amount of nitrogen fertilizer input is huge in China.It can promote the decomposition or accumulation of soil organic matter,promote the production and emission of soil CO₂,and have an indirect regulatory effect on the karst carbon cycle.When the application of nitrogen fertilizer is greater than the amount of plant uptake,the excessive nitrogen fertilizer input will produce HNO₃ by nitrification,and directly involved in the karst carbon cycle through the dissolution of carbonate.But what is the extent of this indirect or direct effect,what is the impact on the karst carbon sink,how calcium-rich and alkalic karst soil respond to nitrogen fertilizer input,and the migration and transformation of nitrogen in soil-epikarst-underground system in the watershed and its impact on the karst carbon cycle are still to be further studied.Therefore,a series of potted plant simulation experiment with different nitrogen application were carried out in this study.In combination with the observation of groundwater in natural watersheds,the effects of nitrogen on the intensity of soil carbon cycle and its ratio of source and sink,and the impact of nitrogen fertilizer on karst carbon cycle in soil-groundwater were studied.The migration and transformation of nitrogen at the watershed scale and its mechanism involved in the karst carbon cycle were studied,and the following main conclusions were found:1.Simulation experiments on potted plants with different nitrogen concentrations found that nitrogen fertilizer increased calcareous soil CO₂ by 10.5%³0.6%and the dissolution rate of the tablets was increased by 1.8-3.6 times.The soil respiration rate also increased with the amount of fertilization,averaging 26.97 to 48.95 mg C m^-2h^-1,which was increased 7%⁶0%compared with the blank without fertilization.Fertilization led to an increase in both carbon source and sink in the soil.With the increase in nitrogen fertilizer applied,the sink/source ratio increased from0.44%to 0.91%.2.Three acid buffering mechanisms were present in the calcareous soils:carbonate dissolution by carbonic acid,carbonate dissolution by nitric acid and cation exchange.The lower concentration of nitrogen fertilizer(100 kgN ha^-1a^-1)mainly indirectly participated in the karst carbon cycle by increasing the soil CO₂ concentration.The proton from nitrification was buffered by cation exchange,and the soil carbonate dissolution came from soil CO₂.At fertilization concentration 250⁷00 kgN ha^-1a^-1,45%of H⁺directly participated in the calcium carbonate dissolution,and 55%of H⁺was buffered by cation exchange.The?¹³C_DICIC of leachate was controlled by soil CO₂ partial pressure rather than by the strength of nitrification.3.The concentration of inorganic carbon,calcium and magnesium in groundwater increased with NO₃^-concentration in Lijiang River basin.Three processes were derived from carbonate dissolution by carbonate acid,nitric acid and cation exchange.When the anthropogenic input of NO₃^-was less than 0.2⁰.3 mmol L^-1,the plant would fully absorb nitrogen,stimulate microbial respiration and organic mineralization,and increase the soil CO₂ concentration to dissolve carbonate rock dominated;When the anthropogenic input of NO₃^-was higher than 0.3 mmol L^-1,carbonate dissolution by nitric acid or cation exchange dominated.Both the HCO₃^-concentration and?¹³C_DIC of underground rivers were controlled by CO₂ partial pressure.4.In underground river system,the average ratio of the carbonate dissolution by nitric acid calculated by the end member mixing model of isotope was 4.34%,while calculated by the water chemical equilibrium method was 8.83%.The difference of 4.49%between two methods was probably entirely due to the cation exchange.The results of the study are helpful to improve the theory of carbon and nitrogen coupled cycle in karst dynamic system,provide data support for accurately calculation of the nitrogen fertilizer application on karst carbon cycle and karst carbon sink,and provide scientific support for the rational use of nitrogen fertilizer to reduce nitrogen pollution.