Seasonal Responses Of Soil Respiration, Soil Microbial And Enzyme Activity To Nitrogen Deposition In A Citrus Plantation And Masson Pine Forest In Mt. Jinyun, Chongqing, China

With the rapid development of the modernizing agriculture, livestock and industry, nitrogen-containing compounds in atmosphere are increasing Several times than several years ago, which lead the global nitrogen deposition increased and continued to grow. The increasing of nitrogen deposition make carbon cycle in terrestrial ecosystem changed, however, disagreements over the effects of nitrogen deposition on soil respiration still exist, thereby many uncertainties still need to further study. In the present study, we designed an experiment to explore the seasonal responses of soil respiration, soil temperature, soil moisture, soil microbial and soil enzyme activity to various levels of nitrogen deposition both in the Masson pine forest and in the Citrus plantation at Mt. Jinyun, Chongqing, China. We measured soil respiration, soil temperature, soil moisture and soil microbial phospholipid fatty acids(PLFA) by using the Automated Soil CO2 Exchange Station(ACE, UK) and Gas Chromatograph(Aligent 6850). The results showed that :(1) The two forests underwent similar seasonal changes in soil respiration, with the most change being observed during the summer, followed by spring, autumn, and winter, which yielded the least change. However, the citrus plantation presented significantly higher levels of soil respiration than the Masson pine forest across all seasons(P<0.05). Generally, nitrogen deposition suppressed soil respiration in both forests and these inhibitory effects were strengthened with increasing levels of nitrogen deposition. The only exception in which nitrogen deposition enhanced soil respiration was in the Masson pine forest during the winter, while soil moisture was very low. Soil respiration showed significantly positive exponential relationships with soil temperature(P<0.01) and significant quadratic relationship with soil moisture(P<0.05). Besides, exponential power function model are more suitable to describe the complex relationship among soil temperature, soil moisture and soil respiration, which showed that soil temperature and soil moisture are determinating the variation of soil respiration from 56.6% to 61.4% jointly in both two forests. Both factors(soil temperature and moisture) are influenced by the levels of nitrogen deposition in subtropical forests. With the increased of the concentration of nitrogen deposition, temperature coefficient of sensitivity of soil respiration decreased, so did the fitting degree of soil moisture and soil respiration in the Citrus plantation, yet the fitting degree of soil moisture and soil respiration in the Masson pine forest performed opposite.(2) Nitrogen deposition has remarkable effect on soil microbial(P < 0.05), different season and different concentrations of nitrogen deposition had different results. In general, low and high nitrogen inhibit soil fungi in two forests, medium nitrogen showed a promotion action. Low nitrogen and high nitrogen reduced soil bacteria in two forests, however medium nitrogen made soil bacteria increased in the Citrus plantation, and decreased in the Masson pine forest. Low nitrogen restrained the soil actinomyces in the Citrus plantation and promoted in the Masson pine forest, medium nitrogen had a positive effect on soil actinomyces in the Citrus plantation and had a negative effect on soil actinomyces in the Masson pine forest, the soil actinomyces in both forests were inhibited by high nitrogen. The total PLFA in the Citrus plantation were suppressed by low and high nitrogen, but promoted by medium nitrogen. In spring and autumn, total PLFA in the Citrus plantation suppressed by low and high nitrogen deposition, promoted by medium nitrogen simultaneously. total PLFA in the Masson pine forest were inhibited under each nitrogen treatment in spring and autumn, yet promoted in summer. While in winter, total PLFA in the Masson pine forest were promoted under low and medium nitrogen treatment, and inhibited under high nitrogen treatment.(3) Nitrogen deposition had a different influence on soil enzyme activity in two forests. In spring and summer, soil urease in the two forests were reduced from 0.37% to 40.47% under low nitrogen treatment, from 5.98% to 30.67% under medium nitrogen treatment, and from 13.20% to 60.25% under high nitrogen treatment, while no consistent variation were appearanced in autunm and winter. Soil invertase, catalase and acid phosphatase in two forests were inhibited by low and high nitrogen, yet promoted by medium nitrogen.(4)Principal component analysis showed that 12 influencing factors of soil respiration may be simplified into four main components, the first principal component was composed of the soil catalase, forest type, soil invertase, soil acid phosphatase and soil actinomycetes, the contribution rate reach to 39.98%. The second principal component was made up of soil bacteria, soil moisture and the phospholipid fatty acids of soil microbial, the contribution rate reach to 20.33%. The third principal component was made up of soil temperature and soil urease, the contribution rate reach to 12.26%. The fourth principal component was made up of soil fungi and the concentration of nitrogen deposition, the contribution rate reach to 9.54%. Except for the concentration of nitrogen deposition, other influencing factors had a positive effect on soil respiration, which indicated with the increased of the concentration of nitrogen deposition, the soil respiration decreased(and the additional indexes promoted soil respiration). The main component of the corresponding variance contribution rate λi(i = 1,2,3,4) as the weights, the resulting soil respiration Comprehensive principal components calculated as follows: F=0.487F1+0.248F2+0.149F3+0.116F4.