Production And Reduction Of Soil N₂O In The Mid-subtropical Forests Of China

The apparent soil N₂O fluxes and impacting factors were firstly investigated in situ and then the rates of N₂O production and reduction processes and biotic and abiotic affecting factors were researched in laboratory with methods of anaerobic incubation and dynamics models for the subtropical evergreen broad-leaved forests and Chinese fir plantations located in two nature reserves of Wuyi mountain and Wanmulin, northern Fujian, China, aimed at understanding forest soil N₂O source and sink processes in the region, providing a scientific basis for building modles of predicting N₂O emissions, developing reasonable measures to reduce emissions and increase uptake, evaluating forest ecological function value. The main results are as follows:（1） The apparent N₂O fluxes in Jian’ou was 0.13±1.38μg·m^-2·h-1～3.47±2.94μg·m^-2·h-1, yearly averaged 1.38±1.82μg·m^-2·h-1 and in Wuyishan was -0.45±0.94μg·m^-2·h-1-2.54±4.91 μg·m^-2·h-1, and yearly averaged 1.14±2.07μg·m^-2·h-1. The difference of N₂O flux was not significant in the two regions. N₂O flux was higher in summer and autumn, but lower in winter and for both sites. The frequency of N₂O negative flux was 33.18% in a year, with corresponding net flux range of -14.53～-0.016μg·m^-2·h-1, and mean of-2.02±0.45μg·m^-2·h-1 in Jian’ou, while in Wuyishan, the the three numbers were 36.11%,-10.3～-0.002μg·m^-2·h-1, and -2.03±0.62μg·m^-2·h-1 respectively. The frequency of N₂O negative flux was higher in winter and spring than that in summer and autumn. From the data of two sites together, N₂O emission flux is mainly affected by atmospheric temperature or soil temperature, soil total nitrogen （TN） and ammonium nitrogen （NH4+）; From the data of each site, N₂O emission fluxes were mainly affected by soil total nitrogen and ammonium nitrogen in Jian’ou, while in Wuyishan the main affecting factors were temperature and available phosphorus.（2） The amount of reduced NO₃^--N is 122-200mg·kg-1, and for most of the soils denitrification rates come up to more than 90%. The N₂O emission rates of all soils ranged 11.35%-49.78%, and averaged 29.02%. The results indicated that the N₂O concentration （N） in the headspace with incubation time （t） which was the net result of N₂O emission and N₂O reduction to N2, could be described with the equation: N=A·（1-exp（-kit））-B-exp（k2t）. The average of the determination coefficients of 45 soil samples （R2） was 0.77±0.13 （P<0.05 for all samples）. A is the total N₂O emission during the incubation, B is a constant, k1 and k2 are the N₂O emission rate constant and reduction rate constant, respectively.The vegetation types had a significant influence on the total N₂O emissions, percentage of N₂O emitted to nitrogen added and N₂O emission rates and Jian’ou vegetation types（65.9±14.99mg·kg-1,21.13±4.77%,0.43±0.22mg·kg-1·d-1） were significantly higher than that of Wuyishan vegetation types（30.12±8.97mg·kg-1, 10.47±2.98%, 0.29±0.13mg·kg·d-1）. The total N₂O emissions and percentage of N₂O emitted to nitrogen added were significantly greater in August（60.06±20.02mg·kg-1 , 19.55±6.03%） than that in January（53.79±21.47mg-kg’,17.38±6.07%）. N₂O emission rate of 0-10cm soil layer （0.49±0.17mg·kg-1·d-1） was greater than 10-20cm soil layer （0.29±0.19mg·kg-1·d-1）.The total N₂O emissions was mainly affected by soil total organic nitrogen (TON, and dissolved organic nitrogen （DON）, total P and clay contents in soil. The percentage of N₂O emitted to nitrogen added was influenced by pH, C/N, total P, and clay contents in soil. N₂O emission rate was mainly affected by ammonium nitrogen （NH4+） total organic carbon （TOC）, TON, and available P. N₂O reduction rates did not be detected to be impacted significantly by various factors observed in this research.（3） In the number of gram positive bacteria> gram negative bacteria> actinomycetes> fungi, in general, the area is dominated by bacteria. Total microbial biomass, gram positive bacteria, gram negative bacteria, actinomycetes, fungi and bacteria were greatly influenced by vegetation types, and the soil microbial biomass in the surface layer was greater than that in the bottom soil. In addition to gram positive bacteria, sampling time had a significant effect on other microorganisms and the total amount. During the denitrification process, the N₂O emission rate was controlled by the bacteria and actinomycetes, and the total N₂O emission and the percentage of N₂O emitted to nitrogen added was not significantly correlated with all kinds of microorganisms. The relationship between the N₂O reduction rate and the microbial biomass was not significant.