Fenn M E, Jovan S, Yuan F, Geiser L, Meixner T, Gimeno B S
USDA Forest Service, Pacific Southwest Research Station, Riverside, CA 92507, USA.
Environ Pollut. 2008 Oct;155(3):492-511. doi: 10.1016/j.envpol.2008.03.019. Epub 2008 May 21.
Empirical critical loads (CL) for N deposition were determined from changes in epiphytic lichen communities, elevated NO(3)(-) leaching in streamwater, and reduced fine root biomass in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) at sites with varying N deposition. The CL for lichen community impacts of 3.1 kg ha(-1) year(-1) is expected to protect all components of the forest ecosystem from the adverse effects of N deposition. Much of the western Sierra Nevada is above the lichen-based CL, showing significant changes in lichen indicator groups. The empirical N deposition threshold and that simulated by the DayCent model for enhanced NO(3)(-)leaching were 17 kg N ha(-1) year(-1). DayCent estimated that elevated NO(3)(-) leaching in the San Bernardino Mountains began in the late 1950s. Critical values for litter C:N (34.1), ponderosa pine foliar N (1.1%), and N concentrations (1.0%) in the lichen Letharia vulpina ((L.) Hue) are indicative of CL exceedance.
根据附生地衣群落的变化、溪流中硝酸盐淋失量的增加以及不同氮沉降水平下黄松（Pinus ponderosa Dougl. ex Laws.）细根生物量的减少，确定了氮沉降的经验临界负荷（CL）。预计3.1千克公顷⁻¹年⁻¹的地衣群落影响CL能够保护森林生态系统的所有组成部分免受氮沉降的不利影响。内华达山脉西部大部分地区高于基于地衣的CL，地衣指示组出现了显著变化。经验性氮沉降阈值以及DayCent模型模拟的硝酸盐淋失增强的阈值为17千克氮公顷⁻¹年⁻¹。DayCent估计，圣贝纳迪诺山脉硝酸盐淋失增加始于20世纪50年代末。地衣Letharia vulpina（(L.) Hue）中凋落物碳氮比（34.1）、黄松叶片氮含量（1.1%）和氮浓度（1.0%）的临界值表明CL已被超过。
