Soil: acidification 256 alkalinity

Ammonia has a lifetime of only a few hours to a few days in the atmosphere. It and its reaction products are transported through the atmosphere and deposited on terrestrial snrfaces elsewhere. It is the main gaseous alkaline species in the atmosphere and neutralizes a large part of the acid produced in oxidation of sulfur and nitrogen oxides, probably up to a half though its dry-deposition is much faster than that of NO and SO2 (Dentener and Crutzen, 1994). Dry- and wet-deposition of ammonia contribute to soil acidification because 2 mol of H+ are produced in the nitrification of Imol of NH4+. Also a large part of the ammonia deposited on moist forest soils may be re-emitted as N2O (Section 8.2). 

The contribution of root and microbial respiration to rhizosphere acidification depends on the amount of C02 released, the amount of organic material oxidized to C02, and the initial pH of the soil (Hinsinger et al., 2003). However, since the first pK of H2CO3 is 6.36, the contribution of these processes to soil acidification will be significant only in neutral and alkaline soils. 

Pollutant deposition may increase soil acidity, decrease nutrient availability, and increase the solubility of toxic ions. In central and northern Europe, increased pollutant deposition over several decades has induced soil acidification. In the soil, acid deposition undergoes many reactions, and this leads to a reduced alkalinity and increased aluminum content in the soil solution. The exchange complex of the soil becomes dominated by aluminum, the exchange acidity increases, bases are leached in association with acid anions due... 

In alkaline soils, acidification is required and realized by fertilization with sulfur or... 

If the weathering rate equals or exceeds the rate of H+ release by the biota, such as would be the case in a calcareous soil, the soil will maintain a buffer in base cations and residual alkalinity. On the other hand, in noncalcareous "acid" soils, the rate of H+ release by the biomass may exceed the rate of H+ consumption by weathering and cause a progressive acidification of the soil. In some instances, the acidic atmospheric deposition may be sufficient to disturb an existing H+ balance... 

Acid deposition can cause acidification of soils and waters. Acidification can be defined as a reduction of the acid-neutralizing capacity of soils or waters. The acidneutralizing capacity is also known as alkalinity (AUc) and indicates the sensitivity of a water system toward acidification. It can be expressed as [9] ... 

Long-term acidification related to soil depletion of soluble alkalinity producing minerals is irreversible without soil scarification or substantial... 

Finally, we consider acidification mechanisms specific to the continents so that ocean acidification is not an issue. Both enhanced respiration by surviving plant roots and bacterial decomposition of dead biomass witliin soils following Uie impact may have increased soil carbonic acid concentrations and soil weathering. (Dead biomass is also a source of alkalinity as Ca and other cations are released into the soil solution, but this process neutralized only a fraction of Uie total carbonic acid produced.) The subsurface soil biomass presently contains 2 x 10 moles C, which, if multiplied by -4 in the late Cretaceous, may liave been able to supply just enough carbonic acid to explain the foram Sr data. However, nearly all of the subsurface soil biomass would liave to have been decomposed by bacteria. Furthermore, the vast majority of CO2 released in soils diffuses out of the soil and joins the atmosphere [27], The numbers for plant root respiration are even less favorable. Presently, respiration accounts for 0.5 X 10 mol C yr released in soils. In the post-K/T impact atmosphere photosyntliesis was very likely interrupted for at least several months by dust and aerosols [7,8], so surviving plants would have had to respire at four times present biomass. One year of respiration yielded 2 x 10 mol CO2, not enough to weather Sr even if the CO2 remained in the soil. 

Decrease in alkalinity (acidification and cation depletion in soils) occurs whenever the production of organic matter (assimilation of NH ) is larger than the decomposition. This takes place, for example, when peat bogs or forest peats are formed these systems are very acidic. The harvest of crops on agricultural and forest land often causes discrepancies between production and decomposition. ... 

Acid atmospheric deposition causes acidification of waters and soils if the neutralization of the acids by weathering is too slow. Biologically mediated redox processes are important in affecting the H balance. Among the redox processes that have a major impact on H" production and consumption are the synthesis and mineralization of biomass. Any uncoupling of linkages between photosynthesis and respiration affects acidity and alkalinity in terrestrial and aquatic ecosystems (Table 15.1). 

This reaction may not actually acidify unless the NO3" is lost from the soil by leaching along with exchangeable base cations, leaving H to occupy exchange sites. Other possible fates of the NOs, such as plant uptake (see equation 5.42), generate alkalinity in the soil and may result in no net acidification. Similarly, biological denitrification of the NO ... 

One clear prediction from the simple acidification model developed in the last section is the loss of alkalinity (as defined by equation 5.68) and base cations from soil solution by leaching. High the soil should naturally result in base cation loss... 

Soil organic matter (SOM) is often referred to as humus and is derived primarily from the degradation of plant material lignin, carbohydrates, protein, fats, and waxes. Mineral soils may contain 0.5-3.0% of soil organic matter while muck soils and peat contain 50% and higher. Operationally, the material that cannot be extracted by alkaline agents is called humin. The material that precipitates from the alkaline extract on acidification is called humic acid, and what remains in solution fulvic acid. Felback summarized some of the properties of these complex polymeric materials as follows ... 

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