Comments on Nitrate Reduction in Unsaturated Soil1

A. DOUGLAS McLAREN2

ABSTRACTCyclic oxidation and reduction of nitrite and nitrate in soil are

analysed in terms of first- and zero-order microbial kinetic reactions,respectively. A reversible reaction step involving these ions can ac-count for a low, but constant concentration of nitrate in an unsatu-rated field soil for long periods or for considerable depths in a labora-tory soil column.

Additional Index Words: nitrification, soil nitrite, microbial ecology.

IN A RECENT STUDY in which nitrate was applied to an un-saturated soil, denitrification was not pronounced and ni-

trite was < 1 ppm in the Unsaturated profile. Nitrite ox-idizers increased substantially in numbers however, and thisindicated that nitrite was being formed from nitrate and thenreoxidized (Volz et al., 1975). It was also concluded thatany organically derived soil ammonium was not sufficientto supply enough nitrite to account for the growth of the ni-trite oxidizers. A low concentration of nitrite with little netreduction of nitrate must therefore be explained. It seemedworthwhile to examine some kinetic properties of a nitrogentransformation model that includes a reversible step involv-ing nitrite and nitrate and that can account for growth of ni-trite oxidizers in a nonponded soil.

THE MODELAmmonium nitrogen added to an Unsaturated soil can react,

with no ensuing translocation of reactants or products except bydiffusion, according to Scheme I.

Scheme I

NH4 NO2-

Oxidations may be taken as first-order, but within anaerobic crumbmicrosites, reductions are presumed to take place with zero-orderrates (Broadbent and Clark, 1965; McLaren, 1970; Doner et al.,1974). P is the sum of N2O, NO, and N2 generated. In a batch sys-tem, the ratios of all species of nonvolatile nitrogen, N f , are every-where the same in the bulk volume. In a soil column with a flow-ing solution, however, the ratios of N, change with depth andScheme I can be rewritten as

NH/ NO,- NO2

The forms of the equations to follow are the same in either case;however, in this Scheme the reversible step may be thought to takeplace a few centimeters below the surface, in contiguous anaerobicand aerobic microsites through which solution flows downward(Greenwood, 1962).

'As subschemes, the reactionsNO3~ N02-

'Contribution from the Dep. of Soils and Plant Nutrition, Univ. of Cali-fornia, Berkeley. Supported by NSF Grant GI-34733X. Received 29 Dec.1975. Approved 5 May 1976. ,

"Professor of Soil Biology.

(Scheme II) and NO3- NO2~ > P (Scheme III)will also be considered.

With the aid of rate constants, kt, corresponding to the reactionsteps indicated in Scheme I, we may write

) [la]

= *i (NH4+) - k2 (N02-) + ks - k4 [2a]

/dt = k2(N02~) - k3 [3a]

dPIdt = k4 [4a]

and

P = (AW4+)0 - t(AW4

+) + (N02-) + (N03-)] [5]

at any time. For simplicity it is assumed that populations mt corre-sponding to each step are unchanging, and perhaps maximal afterprolonged infiltration (i.e., dm/dt = 0; Volz et al., 1975), sincethe calculations to follow are of heuristic value only (McLaren,1973). Incorporation of Nf by the corresponding biomasses issmall in any case (2 to 6% of Nf metabolized) (McCarty and Haug,1971); i.e., Eq. [5] may be taken as the materials conservationequation. The rate constants are assumed to be invariant with timein soil considered as an idealized system of immobilized microor-ganisms. (NH4

+)0 is the initial concentration of ammonium at timeof infiltration, t = 0.

Solutions to these equations are (c.f., Capellos and Bielski,1972)

(NH4+) = (NH4

+)0 exp - k,t [Ib]

(N02-) = - *i)[exp - kit - exp -

[2b]

(N03-) = £2(NH4+)0/(fc2 - *,)[! - exp - ^t]

-fc1(NH,+)o/(*2 - *i)[l - exp - *2f]

-fe - *4)/*2 [1 - exp - k2t] -ktt, [3b]

and

P = k4t. [4b]

For Scheme II,

d(NO3-)/dt = -its and (NO3-) = (NO3-)0 -k3t [6a,b]

and d(NO2~)/dt = k3 - k4 and (NO2~) = (*3 - k4)(, [7a,b]

and P = k4t= (NOr)o - [(NO,-) + (NO,-)]. [8]

For Scheme III

(N03-) = (N03-)0 - (k3 - k4)/k2 [1 - exp - k.2t] ~ktt [9]

and (NO2-) = (k3 - k4)/k2 [1 - exp - *2r], [10]

andP = k4t. [8]

Overall the approach is similar to that of Mehran and Tanji(1974) except that here the solutions may be presented without theaid of a computer. For a general treatment see Atkins (1969).

698

MC LAREN: COMMENTS ON NITRATE REDUCTION IN UN SATURATED SOIL 699

« Returning to the nitrification-denitrification Scheme I,10 ->^^ ^ Fig. 1-Part B, we note first an increase in nitrite concentra-

^^^-^ .̂̂ ^ tion and then a decline to a value of about 0.125 ppm which8 - ^^v-Cr"~~~--—. persists from about 300 hours, (cf. [2b], [3b]). That is, at

^^^^^^^^ "~~ .̂ x~\ long times the exponentials approach zero and concentra-| 6 - ^^"^^N^^^ ~-) ti°ns are determined mainly by the zero-order rate con-^ 5 —— ^*"'̂ >Z '̂O3 slants. Ammonium decreases with time throughout and ni-z 4 - ^^>^~ trate concentrations decline eventually (not shown in Fig.

^̂ -̂ "̂""""̂ ^ IB) as P increases.2 - ^ _ -r- It is the second term in Eq. [2b] that distinguishes the

^————— ^-NO • / present evaluation of the fate of nitrite in soil from that of an-^=zZ--~~~ ——£-.——, earlier study (McLaren, 1969). This term can help account

o 100 200 300 40O for ^e persistence of nitrite for long periods or at low con-10 r—————————————————————————————I centrations in an irrigated soil.

\ B Heretofore consecutive reaction rate theory has been8 -\ NH + -N applied to the conversion of nitrite to nitrate and of ammo-

\ 4 J ^^———~————— nium to nitrate via nitrite as an intermediate. By including a|̂ 6 - \ _/ S*1^ 3 back reaction of nitrate to nitrite in the theory, a reaction°- 5 __ \^ .s taking place in anoxic microsites in an unsaturated soil,

2 4 - NO • N. / growth of nitrite oxidizers can be accounted for along with/ 2^/\.^ the persistence of low concentrations of nitrite. In the ab-

2 _(x'"7^^>^^ p sence of added organic materials, the back reaction con-/ / ^^^"-̂ i__———-~~ sumes soil organic matter, the consequences of which havel^~-~-T~—— i^—^T"=-— —. been discussed (Volz et al., 1975).

O 5O 1OO 15O 2OOTime (hours)

Fig. 1. (Part A) Rates of change of NO3~, NO2~ and P for Scheme H,£4- [6], [7], and [8] (solid lines) and for Scheme HI, Eq. [9], [10](broken lines) and [8]. (Part B) Course of reactions of nitrogenspecies according to Scheme I and Eq. [Ib], [2b], [3b], and [4b].

COMPUTATIONS AND DISCUSSIONTo evaluate the equations, arbitrary numerical values for

kt were assigned that are reasonable as to order of magni-tude (McLaren, 1976). They are ^ = 0.02/hour, k2 =0.04/hour, &3 =0.015 ppm-N/hour, and &4 = 0.01 ppm-N/hour. Curves obtained with these constants for the twomost simple Schemes, II and III, are in Fig. 1, Part A.

The rate of disappearance of nitrate is, of course, slowerin Scheme III because of the back reaction 2, and the levelof nitrite is comparatively low. In fact, the concentration ofNO2~ is nearly constant from about 100 hours to about1,000 hours. When the nitrate is gone (about 1,000 hours inScheme III and about 670 hours in Scheme II) the level ofnitrite must fall (McLaren, 1970). The rate of P formation isthe same in both schemes.

Following application and translocation of a solution con-taining nitrate to a soil profile, by Eq. [10] the nitrite con-centration should be small and nearly constant for a consid-erable depth (cf. Volz et al., 1975).