The single most important factor regulating nitrification in the majority of soils is ammonium supply (Fig. 13.6). Where decomposition and thus N mineralization is low or where NH4 uptake and thus N-immobilization by heterotrophs or plants is high, nitrification rates will be low. Conversely, any ecosystem disturbance that increases soil NH4 availability will usually accelerate nitrification unless some other factor is limiting. Tillage, fire, clear-cutting, waste disposal, fertilization, atmospheric N deposition—all have well-documented effects on nitrate production in soils, mostly due to their effects on soil NH4 pools.
That nitrification usually accelerates only when the NH4 supply exceeds plant and heterotroph demand implies that nitrifiers are relatively poor competitors for NH4 in the soil solution. In fact this is the case: nitrification rates are typically low in midsuccessional communities and aggrading forests because of high plant demand for N, and also following the addition of high C:N residues to agricultural soils because of high microbial (heterotroph) demand for N. In old-growth forests and mature grasslands, plant N demand has diminished and consequently nitrification is usually higher than in midsuccessional communities in which plant biomass is still accumulating, but not usually as high as in early successional communities, in which N supply often greatly exceeds demand (Robertson and Vitousek, 1981).
Oxygen is another important regulator of nitrification in soil. All known nitri-fiers are obligate aerobes, and nitrification proceeds very slowly if at all in submerged soils. In flooded environments such as wetlands and lowland rice, nitrifiers are active only in the oxidized zone around plant roots and at the water-sediment interface, usually only a few millimeters thick. And although some nitrifiers have the capacity to use nitrite rather than O2 as an electron acceptor during respiration, O2 is still required for ammonia oxidation.
Nitrifiers are little different from other aerobic microbes with respect to their response to temperature, moisture, and other environmental variables (see Fig. 13.2). Nitrification occurs slowly but readily under snow and in refrigerated soils, and soil transplant experiments (e.g., Mahendrappa et al., 1966) have demonstrated an apparent capacity for nitrifiers to adapt to different temperature and moisture regimes. For many decades nitrifiers were thought to be inhibited in acid soils, probably because in many cases and especially in soils from cultivated fields, raising soil pH with calcium or magnesium carbonate stimulates nitrification, and cul-turable nitrifiers exhibit a pH optimum of 7.5-8 (Prosser, 1989). We now recognize that nitrification can be high even in very acid forest soils (pH < 4.5;
Rainfall Soil porosity Transpiration
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