模拟氮沉降对不同类型生物土壤结皮生长和光合生理的影响

生物土壤结皮是荒漠生态系统的重要组分,其如何响应氮沉降的增加还鲜见报道。以古尔班通古特沙漠中3种不同类型生物土壤结皮为研究对象,设置0(N0)、0.3(N0.3)、0.5(N0.5)、1.0(N1)、1.5(N1.5)和3.0(N3)g N m-2a-16个不同氮素处理浓度,研究氮素增加对生物土壤结皮生长和光合生理的影响。结果表明,经过3a的模拟增氮实验,藻类结皮、地衣结皮和苔藓结皮的总叶绿素、实际光化学效率YII、可溶性糖含量以及苔藓个体生物量随着氮素增加先增加后减少,但各指标的最大值位于不同的浓度处理。氮素增加对藻类和地衣结皮类胡萝卜素影响不显著,而低氮(N0.3-N0.5)对苔藓类胡萝卜素具有促进作用。高氮(N3)对3种类型结皮的最大光化学效率Fv/Fm均具有抑制作用。氮素增加对藻类结皮和地衣结皮的可溶性蛋白影响较小,但对苔藓结皮可溶性蛋白的影响表现为先增加后降低的趋势。3种结皮类型中,苔藓结皮对氮素增加的响应最为敏感,受影响最大,其次是藻类和地衣结皮。研究表明,低浓度氮沉降对3种类型结皮生长的影响较小,但是高浓度氮沉降则具有明显的负效应。

The effects of simulated nitrogen deposition on growth and photosynthetic physiology of three types of biocrusts

Increasing nitrogen (N) deposition has become a threat in many terrestrial ecosystems, and the effects of N deposition have been widely studied. Nitrogen deposition can increase plant biomass, decrease richness of plant species, and alter soil microbial composition and activity. Biocrusts, which consist of communities of bacteria (cyanobacteria), fungi, algae, lichens and mosses, are important for the functioning of desert ecosystems. Biocrusts can stabilize sand surfaces, greatly increase the threshold friction velocity. Biocrusts can also fix N₂ and serve as the major source of N for plants and microbes in desert ecosystems. Both N limitation and N excess can result in stress environment for biological activity, especially for physiological responses. However, how biocrusts respond to N physiologically is unclear. In 2010, sixty 2 × 2 m plots were established at the study site, with six plots in each of 10 blocks. Five concentrations of N were applied to each block of the plots in addition to a control without added N. Six simulated N deposition treatments, i.e. 0(N0), 0.3(N0.3), 0.5(N0.5), 1.0(N1), 1.5(N1.5) and 3.0 (N3) g N m^-2 a^-1, were applied on the biocrust plots in the center of the Gurbantunggut desert. The N was added annually in March after the snow thaw and again in October before the first snowfall. NH₄NO₃ and NH₄Cl were mixed at a ratio of 2 : 1 NH₄⁺ : NO₃^-, which approximated the composition of N deposition in the nearby city.The growth and physiological indicators of algal, lichen and moss crusts were determined after three year'' exposure to N addition. Chlorophyll (a+b) contents, actual photochemical efficiency YII, soluble sugar concentration of the three biocrust types and moss individual biomass increased and then decreased with the enhancements of N, with peak value at different N applied rates. Carotenoid content of algal and lichen crusts were not significantly affected by N additions (P > 0.05). However, low N additions (N 0.3-0.5) had positive effects on the carotenoid contents. High N additions (N3) significantly decreased maximum photochemical efficiency Fv/Fm. N addition non-significantly increased chl a/b of lichen crusts at N1.5 treatments and significantly decreased at N1 treatments compared to N0, while no significant effects of N addition were found in algal crusts. Chlorophyll/carotenoid of algal crusts increased and then decreased with increasing N addition. Lichen and moss crusts did not show significant effects of N addition on the chlorophyll/carotenoid. The highest value of soluble sugar content of moss occurred at N0.3 treatments, while the algal and lichen crusts occurred at N1.5 treatments. N addition had no significant effect on the proline contents of cyanobacterial and moss crusts. Lichen crusts had relatively lower proline contents in response to higher N addition rates, with a significant decrease being observed in response to N1.5 and N3 relative to N0.3. The soluble protein of algal and lichen crusts were not significantly affected by N addition, but moss soluble protein increased and then decreased after a gradient of N addition. Among the three biocrusts, moss were the most sensitive to N treatments, followed by algal and lichen crusts. Our results suggest that low levels of N addition do not significantly affect biocrust performance, but high N pollution negatively affects the growth. Therefore, Chlorophyll and its fluorescence of biocrusts, could be used for the assessment of high N pollution.