Impact of nitrogen deposition on forest plants
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Human activities such as combustion of fossil fuels, intensive agriculture and stockbreeding, have significantly altered the global nitrogen cycle in the last several decades, increasing the concentrations of nitrogenous compounds in the atmosphere and rising the consequent N deposition to the global surface by several folds. As N is usually a limiting nutrient in most terrestrial ecosystems, additional N inputs to forest ecosystems may influence the growth, functioning and dynamics of those receiving forests. Based on available scientific evidence, we reviewed the impacts of N deposition on forest plants. The potential impacts were related to the following six aspects: (1) N deposition to some degree promotes photosynthetic rate, which is significantly correlated with leaf N concentration, as N deposition increases the leaf N concentration. However, when N deposition exceeds the nutritional demands of plants, it would reduce net photosynthesis by decreasing the concentrations of chlorophyll and Rubisco and activity of Rubisco in the leaves. (2) As N deposition increases the availability of N in soils, it would increase the productivity of plants at least in the short term. However, excess N inputs would lead to a reduction of productivity of plants. (3) Excess N deposition lead to an increasing leaching loss of cations as counterbalancing ions of leached nitrate, resulting in reduced amounts of exchangeable cations in forest soil. . Moreover, soil acidification promoted by N deposition increases the mobilization of toxic Al 3+ . These combined with disproportionately high N concentration in soil lead to nutritional imbalance in plants. (4) N deposition is likely to cause a morphological change of plants, especially on the shoot/root ratios, because N deposition tends to promote growth of aboveground plants and inhibit root growth. (5) The susceptibility of plants to secondary stress factors such as frost, drought and fungal pathogens or insect pests, is increased by high N loading. (6) N deposition changes species composition and decrease plant diversity in forest ecosystems.