Effects Of Nitrogen And Phosphorus Addition On Soil CO₂ And CH₄ Fluxes In Tropical Forests In Diaoluo Mountain

With the intensification of global nitrogen and phosphorus deposition,the issue of greenhouse gases emissions from forest soils in tropical regions is still a hot topic of current research.China has become one of the three major nitrogen deposition concentrated distribution areas in the world.With the rapid development of society,the amount of nitrogen deposition will continue to increase.There are abundant tropical forest vegetation resources in the Diaoluo Mountain,Hainan Island,which are of great research value.The experiments to simulate the effect of nitrogen and phosphorus deposition on the CO₂ and CH₄ fluxes in tropical forest soil is of great scientific significance to study the temporal and spatial variation of greenhouse gas fluxes in tropical forest soil and their response to nutrient addition,as well as to improve China's greenhouse gas emission inventory.This experiment respectively chosen tropical mountain forests and tropical lowland forests in Diaoluo mountain as the research objects.In August 2018,12 plots were set up respectively in the two forest type for in-situ nitrogen and phosphorus addition.Every sample plot area is 20 m×10 m,and the experiment was divided into four treatments,which were nitrogen addition(50 kg N hm^-2 yr^-1),phosphorus addition(50 kg P hm^-2 yr^-1),and nitrogen and phosphorus addition(50 kg N+50kg P hm^-2 yr^-1)and control treatment(CK),urea(CO(NH₂)₂)and superphosphate(Ca(H₂PO₄)₂)were selected as the nutrient addition types of nitrogen fertilizer and phosphate fertilizer.We used the LGR portable greenhouse gas analyzer to monitor the soil CO₂ and CH₄fluxes at monthly time scale between August 2018 and December 2019,and measured the changes of soil environmental factors.After long-term monitoring,the main results are as follows:(1)The variation of soil temperature and moisture factors in the mountain forest and lowland forest were mainly affected by monsoon climate,and both presented the same seasonal dynamic of higher in rainy season and lower in dry season.During the experiment,The variation ranges of soil temperature and soil water content in high-altitude forest were respectively as 15.87±0.11?to 23.64±0.36?and 9.00±2.29%to 31.18±1.95%, and in low-altitude forest were respectively as 19.71±0.14?to 26.73±0.26?and 2.43±0.22%to 21.17±1.42%.(2)After one year of nitrogen and phosphorus nutrient addition,whatever in mountain forest and lowland forest,there were both no significant difference in soil p H and carbon and nitrogen content(NH₄⁺-N?NO₃^--N?Inorganic-N?DOC)between the treatment plots(N addition,P addition,NP addition)and the control plots(CK)(P>0.05).(3)There were no significant difference in soil CO₂ flux between different treatments both in mountain forest and lowland forest(P>0.05).The soil CO₂ fluxes of mountain forest and lowland forest both exhibited a single-peak seasonal dynamic of higher in the rainy season and lower in the dry season,and the average annual flux ranges was 2.84±0.78 ?mol·m^-2·s^-1to 3.6±1.78?mol·m^-2·s^-1 and 2.98±0.33?mol·m^-2·s^-1 to 3.31±0.35 ?mol·m^-2·s^-1,respectively.These seasonal dynamics and flux sizes were both consistent with the results of other reseaches in tropical forests.(4)The soil CO₂ fluxes of mountain forest and lowland forest were both significantly correlated with soil temperature(P<0.05).Among them,the soils with different treatments in the mountain forest and the soils with N addition and P addition in the lowland forest had a positive linear relationship,with an interpretation rate of 15%to 25%.However,the relationship between soil CO₂ fluxes and soil water content in the two forest types under different treatments were both not significant(P>0.05).Moreover, the correlation between soil p H,carbon and nitrogen content and soil CO₂ fluxes were not significant in these two forest types(P>0.05).(5)There were no significant difference in soil CH₄ flux between different treatments both in mountain forest and lowland forest(P>0.05).The soil CH₄ fluxes of high-altitude forest and low-altitude forest both exhibited a single-peak seasonal dynamic of higher absorption in the dry season and lower absorption in the rainy season,and the average annual flux ranges was-0.45±0.09 nmol·m^-2·s^-1to-0.70±0.09 nmol·m^-2·s^-1 and-0.91 ±0.012 nmol·m^-2·s^-1 to-1.02±0.015 nmol·m^-2·s^-1,respectively.These seasonal dynamics and flux sizes were both consistent with the results of other reseaches in tropical forests.(6)The relationship between soil CH₄ absorption fluxes and soil water content in both two forests were negative linear correlation(P<0.05),with an interpretation rate of 41%to 76%.However,The absorption fluxes of soil CH₄ was negatively linearly correlated with soil temperature only under N addition and CK treatment in the mountain forest(P<0.05), but this relationship existed at all treatments in the lowland forest(P<0.05).Moreover, the correlation between soil p H,carbon and nitrogen content and soil CH₄ fluxes were not significant in these two forest types(P>0.05).Based on the above research results,we think that one year's nitrogen and phosphorus addition treatments had no significant effects on the soil CO₂ and CH₄ fluxes in the tropical forests in Diaoluo mountain,Hainan Island.Both the key control factor of soil CO₂ fluxes in mountain forest and lowland forest was soil temperature,and the key control factor of soil CH₄ fluxes was soil water content.