Environmental Problems, Their Causes, and Sustainability 1

when the soil pH falls below 5.1. Effects of acid depositionon trees and other plants are caused partly bychemical interactions in forest and cropland soils (Figure20-11).At first, sustained acid precipitation adds nitrogenand sulfur to the soils, which stimulates plant growth.But continued acid inputs can cause several problems.One is that the acids leach essential plant nutrientsand calcium and magnesium salts (which can reduceacidity) from soils. This reduces plant productivityand the ability of the soils to buffer or neutralize acidicinputs.Another problem is that calcium deficiencies inplants produced in such nutrient-depleted soils can bepassed on to herbivores. For example, birds eatingcalcium-deficient plant material could have less calciumfor egg production, mammals could haveweaker bones, and insects could have weaker exoskeletons—anotherexample of connections and unintendedconsequences.In addition, some air pollutants can harm sometypes of trees through synergistic effects. For example,studies show that no visible injury occurs to whitepine seedlings when they are exposed individually tolow concentrations of sulfur dioxide and ozone. However,if the seedlings are exposed to the same concentrationsof both pollutants simultaneously, visibledamage occurs, apparently because the two pollutantsinteracted synergistically.Acid inputs can also dissolve insoluble soil compoundsand release ions of metals such as aluminum,lead, cadmium, and mercury. When absorbed fromsoil, these ions are highly toxic to plants and animals.In addition, acid deposition can promote growth ofEmissionsAciddepositionSO 2H 2 O 2PANsNO xO 3OthersDirect damage toleaves and barkReducedphotosynthesisand growthIncreasedsusceptibility todrought, extremecold, insects,mosses, anddisease organismsSoil acidificationTree deathLeachingof soilnutrientsAcidsReleaseof toxicmetal ionsRootdamageReduced nutrientand water uptakeLakeGroundwaterFigure 20-11 Natural capital degradation: air pollutants are one of several interacting stresses that can damage,weaken, or kill trees and pollute surface and groundwater.http://biology.brookscole.com/miller14447