Key Messages on Soils and Nutrient Cycling effects D.W. Johnson University of Nevada, Reno Natural acidification processes Capacity vs Intensity effects (soil solid vs soil solution) Differences in sulfur and nitrogen behavior

NRES 497/697 Acidic deposition Agricultural activities (fertilization, manure) releases large amounts of NH3 In soils or on foliage: NH3 + H2O  NH4+ + OH- (initially basifying) Later acidifying, as NH4+ is taken up by plants Direct deposition of NH4+ in rain is acidifying because of later uptake by plants and/or nitrification

Capacity Effect: A change in the soil solid phase (Loss of exchangeable bases) – takes a long time

A- = strong acid anion (Cl-, NO3-, SO42-) Intensity Effect: A change in soil solution but not soil solid phase Addition of mineral acid anions to an acid soil Takes a very short time C+ = any cation A- = strong acid anion (Cl-, NO3-, SO42-)

Intensity Effect: A change in soil solution Takes a very short time (minutes) and is easily reversible (once MAA are removed) Next figure shows an example of how soil solution Al3+ varies by 100 fold over the season with variations in NO3- in the Smokies red spruce site Note: Extremely acidic soils are a necessary but not sufficient condition for the mobilization of Al into soil solution You can have very extremely acid soils but high soil solution pH and low Al if there are no mineral acid anions around (pollution is low)

Intensity effect: short-term (weekly) change in soil solution Al as a result of fluctuations in nitrate and sulfate (no possibility of changes in base saturation over this short interval) Soil solution Al, NO3-, and SO42- in a Red Spruce Stand in the Great Smoky Mountains, NC (Johnson et al., 1991)

Ca: Al molar ratios Ca:Al can be reduced by either capacity or intensity effects Al3+ increases to the 3/2 power of Ca2+ as soil solution mineral acid anion concentrations increase

Modeling study by Reuss: As base saturation declines, there is little change in Al over a long range until a critical threshold is reached (usually between 5 and 15% base saturation Critical base saturation threshold

Field studies show the same pattern (Johnson and Lindberg, 1992)

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Resin techniques for soil nutrient availability – Question Over what time period do resins shift from infinite sink to equilibrium? Presumably depends on capacity and concentrations of ions exposed to and therefore varies with time and place What is the shape of the curve? How long does it take? Resin recovery Equilibrium Infinite sink ? Time

Resins measure both capacity and intensity effects To what degree do they measure each? 90% intensity, 10% intensity? Depends strongly on soil water flux, which of course varies over different collection periods Resin recovery Equilibrium Infinite sink ? Time

Differences in controls on sulfate and nitrate mobility NO3- mobility is controlled entirely uptake by plants and microbes Adsorption is unimportant Once microbial and plant demand for N is satisfied, N will always leach as nitrate Form of N input (NH4+, NO3-, labile organic N) does not matter in the end Sulfur demand by plants and microbes is <10% of N on a weight basis Sulfate mobility is largely controlled by adsorption to Fe and Al hydrous oxides

Differences in controls on sulfate and nitrate mobility Additions of more N to an N-saturated system will probably lead to an equivalent increase in nitrate leaching Additions of more S to an “S-saturated” system will not lead to equivalent increases in leaching until the adsorption system moves up to the new steady-state condition. (Thus, there really is no “S-saturation”

The Nitrogen Problem Nitrogen has a very narrow “sufficiency or optimum plateau” after which bad things start to happen and before which N is deficient (soil quality is low) Sufficiency plateau for N Nitrogen Toxicity Deficiency Sufficiency plateau for non-N nutrients Too much – growth inhibited, negative effects on soil and water Growth-limiting Enough but, possibly more Then enough, but not too much Response P, K, Ca, Mg, S Nutrient Supply