E

of y ar ier

FIGURE 13.6 Environmental controls on nitrification (from Robertson, 1989, after Groffman et al., 1988). The most proximal scale (right side) is at the cellular level.

Robertson, 1989), although the physiological basis for this is still not well understood (DeBoer and Kowalchuck, 2001).

inhibition of nitrification

Nitrification is unaccountably slow in some soils, and in some circumstances it may be inhibited by natural or manufactured compounds. A wide variety of plant extracts can inhibit culturable nitrifiers in vitro, for example, although their importance in situ is questionable. Likewise, commercial products such as nitrapyrin and dicyandimide can be used to inhibit nitrification in soil with varying degrees of success. Most commercial compounds are pyridines, pyrimidines, amino triazoles, and sulfur compounds such as ammonium thiosulfate. One recent innovation is paraffin-coated calcium carbide (CaC2; Freney et al., 2000). Calcium carbide reacts with water to form acetylene (C2H2), which inhibits nitrifiers at very low partial pressures, ca. 10 Pa. As the paraffin wears off, CaC2 is exposed to soil moisture and the C2H2 formed inhibits nitrification. Likewise, neem oil, extracted from the Indian neem tree (Azadirachta indica), has been used commercially to coat urea fertilizer pellets to slow its nitrification to NO-.

The potential value of managing nitrifiers in ecosystems can be easily seen from the position of nitrification in the overall N cycle (Fig. 13.1). Nitrogen is lost from ecosystems mainly after its conversion to NO- and prior to plant uptake, so keeping N in the NH+ form keeps it from being lost by nitrate leaching and deni-trification, the two principal pathways of unintentional N loss in most ecosystems. Because many plants prefer to take up N as NO-, it is not desirable to inhibit nitrification completely even in intensively managed ecosystems such as fertilized row crops, but slowing nitrifiers or restricting their activity to periods of active plant growth is an attractive—if still elusive—management option.

denitrification

Denitrification is the reduction of soil nitrate to the N gases NO, N2O, and N2. A wide variety of mostly heterotrophic bacteria can denitrify, whereby they use NO-3 rather than oxygen (O2) as a terminal electron acceptor during respiration. Because nitrate is a less efficient electron acceptor than O2, most denitrifiers undertake denitrification only when O2 is otherwise unavailable. In most soils this occurs mainly following rainfall as soil pores become water-saturated and the diffusion of O2 to microsites is slowed drastically. Typically denitrification starts to occur at water-filled pore space concentrations of 60% and higher (Fig. 13.2). In wetland and lowland rice soils diffusion may be restricted most of the time. Oxygen demand can also exceed supply inside soil aggregates and in rapidly decomposing litter.

Denitrification is the only point in the N cycle at which fixed N reenters the atmosphere as N2; it thus serves to close the global N cycle. In the absence of denitrification, N2 fixers (see Chap. 14) would eventually draw atmospheric N2 to nil, and the biosphere would be awash in nitrate. Denitrification is also important as the major source of atmospheric N2O, an important greenhouse gas that also consumes stratospheric ozone.

From a management perspective, denitrification is advantageous when it is desirable to remove excess NO32 from soil prior to its movement to ground or surface waters. Sewage treatment often aims to remove N from waste streams by managing nitrification and denitrification. Typically wastewater is directed through sedimentation tanks, filters, and sand beds designed to remove particulates and encourage decomposition and the mineralization of organic N to NH4, which is then nitrified under aerobic conditions to NO3. The stream is then directed to anaerobic tanks where denitrifiers convert the NO33 to N2O and N2, which is then released to the atmosphere. Part of the nitrification/denitrification management challenge is ensuring that the stream is exposed to aerobic conditions long enough to allow nitrifiers to convert most NH4 to NO3 but not so long as to remove all dissolved organic C (known as biological oxygen demand or BOD to wastewater engineers), which the denitrifiers need for substrate.

Denitrification can also remove nitrate from groundwater prior to its movement to streams and rivers. In most wetlands and riparian areas nitrate-rich groundwater must move across a groundwater-sediment interface that is typically anaerobic and carbon-rich. As nitrate moves across this interface it can be denitrified to N2O and N2, keeping it from polluting downstream surface waters.

In managed ecosystems it is usually desirable to minimize denitrification in order to conserve N further for plant uptake; in regions with ample rainfall ecosystem N losses due to denitrification can rival or exceed losses by nitrate leaching. There are no technologies designed to inhibit denitrification per se; usually deni-trifiers are best managed indirectly by manipulating water levels (e.g., in rice cultivation) or nitrate supply (e.g., nitrification inhibitors).

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