Potentially Mineralizable Nitrogen in Disturbed and Undisturbed Soil SamplesM. L. CABRERA* AND D. E. KISSEL

ABSTRACTCalculation of the correct nitrogen (N) fertilizer rate for a crop

requires an estimate of the amount of N that mineralizes from soilorganic matter during the growing season. A method proposed byStanford and Smith for estimating N mineralization sometimes re-sults in overpredictions which may be due to the use of disturbedsoil samples. To study the effect of soil disturbance, we measuredN mineralized in disturbed and undisturbed soil samples, and de-termined models that properly described the data for each type ofsample. A double exponential model was required for disturbed sam-ples, whereas a single exponential model was adequate for undis-turbed samples. Although in general, models fitted to disturbed sam-ples could not be used to predict N mineralized in undisturbedsamples, they could be used to estimate the parameters of the singleexponential model for undisturbed samples. Parameters estimatedusing this approach, together with daily values of soil temperatureand water content, allowed reasonable predictions of the amounts ofN mineralized in fallow plots on Haynie (coarse-silty, mixed, cal-careous, mesic Mollic Udifluvent) and Wymore (fine, montmorillon-itic, mesic Aquic Argiudoll) soils. Parameters of the single expo-nential model for undisturbed samples could also be estimated fromtotal N and clay contents of the soil. The results suggest that it maybe possible to obtain accurate estimates of N mineralized in the fieldby estimating, instead of determining, the parameters of the singleexponential model for undisturbed samples.

Department of Agronomy, Throckmorton Hall, Kansas State Univ.,Manhattan, KS 66506. Received 22 June 1987. "Corresponding au-thor.

Published in Soil Sci. Soc. Am. J. 52:1010-1015 (1988).

IN ORDER TO CALCULATE the correct N fertilizer ratefor a crop, it is necessary to estimate the amount

of N that mineralizes from soil organic matter duringthe growing season. Accurate methods for making suchestimates, however, are not presently available. Amethod proposed by Stanford and Smith (1972) in-volves the incubation of a dried and sieved soil sam-ple to determine the soil's N mineralization potential,Nm and its first-order rate constant of mineralization,/Co- To predict N mineralized in the field, the rate con-stant of mineralization is adjusted by soil temperature(Stanford et al., 1973), and the amount of N miner-alized predicted with the N mineralization potentialand adjusted rate constant is further adjusted by soilwater content as described by Stanford and Epstein(1974). Although some evaluations of this method haveshown promising results (Stanford et al., 1977; Her-lihy, 1979; Marion et al., 1981), others have shownconsiderable overpredictions of the amounts of Nmineralized in the field (Verstraete and Voets, 1976;Cabrera and Kissel, 1988). These overpredictions maybe related to the treatment of the samples before in-cubation (drying and sieving) and/or to the physicalconditions of the samples during incubation (soilmixed with sand or vermiculite). It has been shownpreviously that drying of soil samples can increase theamount of N mineralized after rewetting (Seneviratneand Wild, 1985), and that disruption of soil aggregatescan increase organic N mineralization (Craswell and

CABRERA & KISSEL: MINERALIZABLE NITROGEN IN DISTURBED AND UNDISTURBED SOIL SAMPLES 1011

Table 1. Some physical and chemical properties of soil samplesused for incubations.

Soil Soilseries Core weightt pH Total N Clay Sand

Haynie

Kahola

Ladysmith

Wymore

IIIIIIIVIIIIIIIVIIIIIIIVIIIIIIIV

kg0.2710.2700.2670.2510.2410.2400.2520.2470.2810.2720.2770.2620.2670.2530.2550.249

6.4 (6.4 C6.3 (6.2 (5.35.25.45.55.45.35.45.55.45.65.55.9

(.75(.761.811.78.24.23.17.12.19.13.08.23

1.491.50L.501.34

g kg ' soil120120120120220230220220290290280300300310340350

320340350350805080

10014013014014050404040

t Weight of soil in undisturbed core.

Waring, 1972; Hiura et al., 1976). Thus, incubation ofundisturbed samples may be more desirable to betterdepict field conditions.

Nitrogen mineralization in samples that had beenpreviously dried and sieved has been described usinga single exponential model (Stanford and Smith, 1972)and a double exponential model (Deans et al., 1986).In contrast, authors working with soil samples thatwere kept moist after field collection have found Nmineralization to be linear with time of incubation(Tabatabai and Al-Khafaji, 1980; Addiscott, 1983).

The objectives of the present study were to measureN mineralized in disturbed and undisturbed soil sam-ples, to find models that would properly describe thedata obtained with each type of sample, and to deter-mine whether the models obtained for disturbed sam-ples could in any way be used to predict N mineralizedunder undisturbed conditions.

MATERIALS AND METHODSSoil Sampling

Disturbed and undisturbed soil samples were collectedfrom areas mapped as Haynie very fine sandy loam, 0 to1% slopes (coarse-silty, mixed, calcareous, mesic Mollic Udi-fluvent), at the Ashland Agronomy Farm; Kahola silt loam,0 to 1% slopes (fine-silty, mixed, mesic Cumulic Hapludoll),and Wymore silty clay loam, 1 to 4% slopes (fine, mont-morillonitic, mesic Aquic Argiudoll), at the North Agron-omy Farm; and Ladysmith silty clay loam, 0 to 1% slopes(fine, montmorillonitic, mesic Pachic Argiustoll), at theHarvey County Exp. Field of Kansas State Univ. The sam-ples for each soil were taken from an area not larger than20m2.

Undisturbed cores were obtained with a sampler com-monly used to determine bulk densities by the core method(Blake, 1965). The sampler consists of two cylinders that fitone inside the other, with the outer one extending aboveand below the inner to accept a hammer at the upper endand to form a cutting edge at the lower end. The inner cyl-inder (0.076-m long, 0.07-m-i.d.), made of plexiglass, holdsthe undisturbed sample and can be replaced by an emptyone after a sample is taken.

Several disturbed soil samples were taken right next to

ftlPerforated plate

'Sand

Soil

Diatomaceous earthCheesecloth

rFig. 1. Cross-sectional view of undisturbed core prepared for incu-

bation.

each undisturbed sample. For that purpose, after the sam-pler had been hammered into the soil and before retrievingit, six cores 0.019 m in diameter and 0.05-m long were takenaround the periphery of the sampler. These samples werecentered at the middle of the length of the inner cylinder.

The disturbed and undisturbed samples were taken fromthe 0.03 to 0.08-m layer of Kahola and Haynie soils on 4October, from the 0.12 to 0.17-m layer of Ladysmith soil on31 October, and from the 0.17 to 0.22-m layer of Wymoresoil on 3 October, 1984. In Kahola and Haynie soils, thesesamples were taken between the rows of sorghum crops(Sorghum bicolor L.) that had been planted (after rototillingthe soil to a depth of 0.1 m) on 31 May and 6 June, re-spectively. In Ladysmith and Wymore soils, these samplescame from fallow plots that had been rototilled to a depthof 0.1 m on 18 May and 6 July, respectively. Some physicaland chemical properties of the samples are presented in Ta-ble 1.

Cylinders containing undisturbed cores were placed inplastic bags inside cylindrical cardboard boxes and stored atapproximately 10 °C until use in February 1985. Disturbedsamples were dried at 35 to 40 °C for 72 h, crushed to passthrough a 0.002-m screen, and stored in air-tight, plasticcontainers.

Laboratory IncubationsUndisturbed cores were prepared for incubation as fol-

lows. Approximately 0.013 m of soil was removed from bothends of each core, leaving about 0.05 m of soil to be incu-bated. A layer of diatomaceous earth, approximately 0.008-m thick, was carefully packed at the lower end of the core,and then a piece of cheesecloth and a perforated plexiglassplate (that fitted tightly inside the cylinder) were placed ontop of the diatomaceous earth to hold it (Fig. 1). The sameprocedure was repeated at the upper end, using acid-washedsand instead of diatomaceous earth. Next, the cylinder wascapped by upper and bottom plexiglass covers, with rubbergaskets sealing the area of contact between the cylinder and

1012 SOIL SCI. SOC. AM. J., VOL. 52, 1988

the covers. The lower cover had a central stem that couldreceive a perforated stopper so that the cylinder with the soilcore could be placed on top of a side-arm flask. The uppercover had a plexiglass cylinder that could hold at least 100mL of leaching solution.

The undisturbed cores were leached before incubation byplacing each cylinder on top of a 1 L side-arm flask con-nected to a vacuum line, and then allowing 1800 mL of 0.01M CaCl2 to percolate through the soil (in 100 mL incre-ments), followed by 100 mL of a N-free nutrient solution.Preliminary work had showed that leaching with 1900 mLof solution was enough to remove all nitrate-N from thesoils. The nutrient solution, prepared with KH2PO4, K2SO4,MgSO4, and CaSO4, contained 100, 24, 113, 0.5, and 4 mg/L of Ca, Mg, S, P, and K, respectively; its pH was approx-imately 7. The leachate was made up to a volume of 2 Lwith 0.01 M CaCl2, and inorganic N concentrations (am-monium nitrite, and nitrate) were determined with colori-metric procedures (Technicon Industrial Systems, 1977 a,b).After the leaching procedure, the cores were allowed to drainunder vacuum (approx. 500 mm Hg) until either a certainweight was achieved or a total draining time of 3 h wascompleted.

Sixteen undisturbed cores (four for each soil) were incu-bated in a high humidity, plexiglass box placed inside anincubator at 35 °C. Humidified air was circulated throughthe box at a rate of 0.333 mL s~'. Preliminary work showedthat the average respiration rate would produce approxi-mately 1 ML CO2 g"1 soil h~'. Calculations made with thisaverage rate indicated that the air flow provided would keepthe O2 concentration in the box very similar to that of thelaboratory air, and that the CO2 concentration would in-crease up to a maximum of 0.004 L L'' air, if no CO2 trap-ping took place. Since two containers with 100 mL 2 MKOH each were placed inside the box to trap CO2, and sincethe KOH was changed every 35 d, CO2 concentrations insidethe box should have remained at values equal to or lowerthan those commonly found in the soil atmosphere (Buy-anovsky and Wagner, 1983). The undisturbed cores wereretrieved from the incubator and leached as previously de-scribed, at 20, 55, 90, 125, 160, 195, and 224 days of incu-bation. No ammonium-N was detected at any of these leach-ings.

On the average, the soil water content during incubationwas 0.250, 0.282, 0.239, and 0.195 kg/kg in cores from Ka-hola, Wymore, Ladysmith, and Haynie soils, respectively.Gravimetric water content at 0.033 MPa was 0.202, 0.266,and 0.214 kg/kg for Kahola, Wymore, and Ladysmith soils,respectively; gravimetric water content at 0.02 MPa was 0.178kg/kg for Haynie soil. The average water loss in 35 d ofincubation, expressed as a percentage of the dry soil, was0.8, 1.1, 1.3, and 2.8 for Ladysmith, Haynie, Kahola, andWymore soils, respectively.

Disturbed samples were incubated at 35 °C, as describedelsewhere (Cabrera and Kissel, 1988). Briefly, 15 g of soiland 15 g of sand were mixed, placed in a leaching tube, andthen leached with 70 mL of 0.01 M CaCl2 followed by 25mL of the same nutrient solution used for undisturbed sam-ples. All tubes were covered with 0.03-mm thick polyeth-ylene and placed in an incubator at 35 °C. Subsequent leach-ings were conducted in a similar manner after 14, 28, 56,84, 112, 140, 168, 196, and 224 d of incubation. Two rep-licates were incubated for each disturbed sample and themeans were used to fit the N mineralization models consid-ered. The average water loss in 28 d of incubation was 1.2g per tube.

Laboratory AnalysesUndisturbed cores were taken apart after the last leaching

and the soil was dried at 40 to 45 °C for 22 h. Twenty grams

of soil were extracted with 60 mL of 2 MKC1 for 1 h, filteredthrough Whatman # 41 filter paper, and washed twice with15 mL of 2 M KC1. The leachate was made up to 100 mLwith 2 M KC1, and nitrite + nitrate N concentrations weredetermined as described for CaCl2 leachates. The inorganicN measured (approximately 0.3 mg/kg) was added to theinorganic N of the last leaching. Total soil N in disturbedsamples was measured with the same colorimetric procedureused for ammonium N determinations in CaCl2 leachates,following a salicylic-sulfuric acid digestion (Bremner andMulvaney, 1982). Soil clay content was determined fromhydrometer readings (Day, 1956) and sand content was de-termined by sieving the dispersed soil sample through a 50micrometer sieve. Soil pH was measured using a 1:1 (soil/water) ratio. All results are expressed on an oven-dry (105 °C,24 h) weight basis.

N Mineralization ModelsThe Marquardt option of NLIN, a nonlinear curve-fitting

procedure (SAS Institute, Inc., 1982), was used to fit one-pool and two-pool models of N mineralization to the cu-mulative N mineralized with time in disturbed and undis-turbed samples. The one-pool model (Stanford and Smith,1972) is of the form

whereas the two-pool model (Molina et al., 1980) is of theform

Nm = N{[1 - <?-*•'] + 7V2[1 - trk-*\where 7V,n is N mineralized in time t, N0, N,, and N2 arepools of mineralizable N, and ko, k,, and k2 are rate con-stants of mineralization. For disturbed samples, the one-pool model was fitted to the original data and to the datawithout the first 28 d of incubation (that is, the cumulativeN mineralized during the first 28 d was subtracted from allsubsequent values of cumulative N mineralized, and timezero was assumed to be at 28 d). Two linear models of thefollowing form were also fitted to the undisturbed samples:Nm = a + bt, and N,,, = ct (zero-intercept model), wherea, b, and c are regression coefficients and t is time.

RESULTS AND DISCUSSIONFor each soil, the amount of N mineralized in dis-

turbed samples was larger than that in undisturbedsamples at any time. Data presented in Fig. 2 are typ-ical of the results obtained with all four soils. Marionand Miller (1982) also found larger net N minerali-zation in air-dried than in field-moist intact samplesof a tussock tundra soil incubated at 35 °C and at 0.02MPa of soil water potential.

The two-pool model described the data obtainedwith disturbed soil samples much better than the one-pool model, whereas the one-pool model was the bestmodel for undisturbed samples (Table 2). The two-pool model fit the data from undisturbed samples aswell as the one-pool model, but in all cases the pa-rameters for one of the pools were not significantlydifferent from zero. Values for the parameters of thebest models for each type of sample are given in Table3.

The appearance of a small pool of mineralizable N(that decomposes relatively fast) in disturbed samplesmay be a consequence of drying the soil, since it hasbeen shown that drying of soil samples can cause aflush of N mineralization upon rewetting (Seneviratneand Wild, 1985). Thus, if the small pool of mineral-

CABRERA & KISSEL: MINERALIZABLE NITROGEN IN DISTURBED AND UNDISTURBED SOIL SAMPLES 1013

izable N of disturbed samples is considered an artifactof drying the samples, it would be of interest to de-termine whether the model for the large pool of mi-neralizable N of disturbed samples is equal to the one-pool model of undisturbed samples. As an alternative,since most of the effect of the pretreatment of the sam-ples seems to disappear within the first 28 d of incu-bation (Stanford and Smith, 1972), it would be of in-terest to determine if the one-pool model fitted to datafrom disturbed samples without the first 28 d of in-cubation is equal to the one-pool model of undis-turbed samples. These comparisons have to be basedon the assumption that incubation conditions wereoptimum for N mineralization in both types of sam-ples. The appeal of using parameters determined withdisturbed samples to predict the amount of N min-eralized under undisturbed conditions lies in the rel-ative ease with which disturbed samples can be in-cubated, compared to undisturbed ones.

We first tested the null hypothesis that the modelfor the large pool of mineralizable N of disturbed sam-ples was equal to the one-pool model of undisturbedcores. For each soil, we simultaneously fitted a two-pool model to data from disturbed samples and a one-pool model to data from undisturbed cores, with theconstraint that the large pool of disturbed samples wasthe same as the single pool of undisturbed cores. Thisprocedure provided an estimate of the residual sumof squares for the null hypothesis. Then, we fitted thetwo-pool model to the data from all four disturbedsamples and the one-pool model to the data obtainedfrom all four undisturbed cores. The addition of theresidual sum of squares of each model provided anestimate of the residual sum of squares for the alter-native hypothesis. The difference between the residualsum of squares of the null and alternative hypothesesprovided an estimate of the residual sum of squaresthat was due to deviations from the null hypothesis.This residual sum of squares was used in an F testagainst the residual sum of squares of the alternativehypothesis to determine if the models under consid-eration were significantly different. The second null

Table 2. Root mean square errors for models fitted to data fromdisturbed and undisturbed soil samples.

Disturbedsamples

Soilseries

Haynie

Kahola

Ladysmith

Wymore

Sam-ple

IIIIIIIVIIIIIIIVIIIIIIVIIIIIIIV

Onepool

3.22.63.53.42.73.54.53.86.26.64.95.78.57.66.98.5

Twopools

0.40.30.40.20.80.40.70.40.30.40.30.10.20.40.20.1

Undisturbed samplesLinear model

With Withoutintercept intercept

— mg N4.43.54.74.55.36.25.27.51.31.51.51.31.51.71.41.4

kg'1 soil —5.34.56.25.76.87.66.59.51.31.51.61.51.51.81.31.3

Onepool

1.71.91.61.61.72.31.92.70.90.80.80.50.70.80.80.8

Twopools

1.92.11.81.81.92.52.13.01.00.90.90.60.80.90.90.9

,100

80

60

40

5 20

Disturbed

Wymore

40 20080 120 160

INCUBATION TIME, daysFig. 2. Cumulative N mineralized with time in disturbed and undis-

turbed samples of Wymore soil.

hypothesis, that the one-pool model fitted to data fromdisturbed samples without the first 28 d was equal tothe one-pool model of undisturbed samples, was testedin a similar way.

The results show that the model for the large poolof mineralizable N of disturbed samples was differentfrom the one-pool model of undisturbed cores for allsoils (Table 4). In general, the model for the large poolof mineralizable N predicted higher amounts of Nmineralized than the one-pool model of undisturbedsamples. This may partly explain the overpredictionsof 67 to 343% obtained when the large pool of mi-neralizable N of disturbed samples was used to predictN mineralized in the field (Cabrera and Kissel, 1988).

The one-pool model fitted to data from disturbedsamples without the first 28 d was also different fromthe one-pool model of undisturbed cores for all soils,except Haynie. In general, the model fitted to dis-turbed samples consistently predicted higher amountsof N mineralized than the one-pool model of undis-turbed cores. The overpredictions at 224 d of incu-bation showed a linear relationship with the ratio ofclay/total N (Fig. 3). The ratio of clay/total N can beviewed as an index of the degree of protection againstmicrobial attack that clays provide to organic matterTable 3. Parameters for the two-pool model of disturbed samples

and for the one-pool model of undisturbed samples.

OU11series

Haynie

Kahola

Sample

IIIIIIIVIIIIIIIV

Ladysmith I

Wymore

IIIIIIVIIIIIIIV

N,— mg150144162190243226276264201253491185160285112118

N,kg- -

15121817221129262125162830403837

k,

—— d"0.003560.003650.003090.002560.002860.003870.002100.002400.001420.001060.000550.001330.002490.001030.002680.00296

k,

' ——0.074220.070960.065050.062770.036470.170960.047440.043730.168490.116910.151740.082950.164440.077080.086030.12855

Namg kg'111611211712515017312012282716052135677769

kod-

0.004400.004000.004940.004450.004470.004320.004960.006360.002020.002580.002970.003370.001860.003170.002320.00244

1014 SOIL SCI. SOC. AM. J., VOL. 52, 1988

Table 4. Test of null hypotheses.

Soilseries

SSres(H.)

SSresdf df

Istff.Haynie 673 60 605 58Kahola 1724 60 1454 58Ladysmith 1492 60 393 58Wymore 1318 60 712 58

Haynie 525 54 506 52Kahola 3019 54 1605 52Ladysmith 2618 54 77 52Wymore 3500 54 717 52

3.3*5.4**81.1**24.7**

1.023.0**858.0**100.9**

*,** Indicate significance at the 0.05 and 0.01 levels of probability,respectively.

1st H,: The model for the large pool of mineralizable N of disturbed samplesis equal to the one-pool model of undisturbed samples.

2nd H0: The one-pool model fitted to data from disturbed samples withoutthe first 28 d of indubation is equal to the one-pool model of un-disturbed samples.

in undisturbed soil samples. Edwards and Bremner(1967) postulated that clays facilitate the formation ofmicroaggregates containing organic material physi-cally inaccessible to microorganisms. This theory issupported by experimental results showing that phys-ical disruption of soils increases the amounts of Nmineralized (Craswell and Waring, 1972). It has alsobeen shown that small aggregates contain a larger pro-portion of readily mineralizable organic N than largeraggregates (Craswell et al., 1970; Cameron and Posner,1979). Thus, physical disruption of soils seems to in-crease microbial accessibility to more readily miner-alizable N. This would explain the larger overpredi-tions obtained at higher clay/total N ratios. Similarresults to those presented in Fig. 3 have been reportedby Hiuraet al. (1976).

These results suggest that the one-pool model fittedto data from disturbed samples without the first 28 dof incubation may be used to predict N mineralizedunder undisturbed conditions in soils like Haynie, withpoor structure and low clay/total N ratios. For soilsin which disruption increases N mineralization con-siderably, however, it seems necessary to determinethe parameters of N mineralization corresponding toundisturbed samples.

The range of values obtained for the parameters ofundisturbed samples in each soil depicts the variabil-ity in N mineralization that exists even within a smallarea of a field (Table 3). Even though undisturbedsamples may provide the best representation of fieldconditions, the number of undisturbed cores thatwould have to be incubated to obtain an average ofthe N mineralization pattern of a field would makethe task impractical. Thus, the ideal situation wouldbe that in which the parameters needed to describe Nmineralization in a field could be estimated either fromthe N mineralization characteristics or from the chem-ical and physical properties of a composite (disturbed)sample of that field.

With respect to the possibility of estimating param-eters for undisturbed samples from the N minerali-zation pattern of disturbed samples, we found that thevalue of (N0 X k0)UD for undisturbed samples couldbe predicted from the corresponding value of the one-pool model fitted to data from disturbed samples

so

60

40

20

Ladysmith -

Wymore

r = 0.988*

Kahola

160 190 220 250CLAY/TOTAL N

Fig. 3. Overprediction of N mineralized by Day 224 of incubationvs. clay/total N of soils. The overprediction for each soil wascalculated as the relative difference between the N mineralizedpredicted with the one-pool model fitted to data from disturbedsamples without the first 28 d of incubation and the N mineralizedpredicted with the one-pool model fitted to undisturbed samples.

without the first 28 d of incubation [ (N0 X ^)D] andfrom clay content, as follows(N0 X fco)UD = -0.087 + 1.36 (N0 X £Q)D - 0.00163

(N0 X ko)D X clay R2 = 0.962; n = 16.The value of N0 for undisturbed samples, could inturn, be predicted from the value of (N{ X fc,) cor-responding to the large pool of mineralizable N ofdisturbed samples, as followsNO = 22.7 + 177.4 (N{ X fc,) R2 = 0.841;

n = 16.When these equations were used to estimate the pa-rameters of the one-pool model for the undisturbedsamples of each soil, the values of N mineralized pre-dicted with those parameters were very close to theobserved ones Obs = -0.5 + 1.006 Pred (n = 112;R2 = 0.984; range = 1.4 to 107.4 mg N/kg soil; theintercept and slope are not significantly different from0 (t = -0.83) and 1 (t = 0.48), respectively, at a 0.05level of probability).

As a preliminary test of these equations, we esti-mated N0 and /CQ for soil samples from the upper three0.15-m layers in fallow plots on Wymore and Hayniesoils. These plots had been used in a previous studyin which the average measured values of N mineral-ized were 107 kg N/ha in 104 d in Haynie soil, and75 kg N/ha in 84 d in Wymore soil (Cabrera and Kis-sel, 1988). The estimated values of N0 and ko wereused together with daily values of soil temperatureand water content to predict the amounts of N min-eralized in the field. For that purpose, the rate con-stant of mineralization was corrected by soil temper-ature using an average Qi0 of 2 between 15 and 35 °C,and the predicted N mineralized was corrected by soilwater content as described by Cabrera and Kissel(1988). The predicted values were, on the average, 11%larger than the values observed on Wymore soil, and6% lower than the values observed on Haynie soil.These errors were not significantly different from zeroat the 0.05 level of probability. When the amounts ofN mineralized in the same plots were predicted using

CABRERA & KISSEL: MINERALIZABLE NITROGEN IN DISTURBED AND UNDISTURBED SOIL SAMPLES 1015

the large pool of mineralizable N of disturbed sam-ples, we obtained a significant overprediction of 67%for Wymore soil and a nonsignificant underpredictionof 3.5% for Haynie soil (Cabrera and Kissel, 1988).Those results suggested that models describing N min-eralization in disturbed samples may be used to pre-dict N mineralized in the field in soils with low claycontent and relatively poor structure (like Haynie),but not in soils with high clay content and fairly goodstructure (like Wymore). The present results show thatit may be possible to obtain reasonable estimates ofN mineralized in the field for both types of soils, ifthe parameters for the one-pool model of undisturbedsamples are estimated from the parameters of modelsfitted to data obtained with disturbed soil samples.

With respect to the possibility of estimating the pa-rameters for the one-pool model of undisturbed sam-ples from soil chemical and physical properties, wefound that the value of(N0 X £O)UD could be predictedfrom total N and clay contents of the soil, and thatthe value of &o could be estimated from the value of(N0 X fc0)uD- as follows(N0 X ko)UD = -0.53 + 1.91 Total N - 0.0043 Total

N X clay R2 = 0.808k0 = 0.0017 + 0.00489 (N0 X /CO)UD R2 = 0.798,

n = 16.(In these equations and in the ones above, all variablesare significant at the 0.01 level of probability, N0 andNI are expressed in mg N/kg soil, ko in d~', and totalN and clay contents in g/kg soil). The equation de-scribing (N0 X k0)VD has a negative interaction termfor clay and total N contents, which indicates that iftwo soils have the same level of total N, the one withhigher clay content would have a lower value of (N0X ^O)UD- Since the value (N0 X fc0)uo can be viewedas an approximation of the amount of N that miner-alizes during the first day of incubation at optimumconditions, these results agree with findings that claysprotect organic matter against microbial attack. In ageneralization of this equation, however, it may benecessary to group soils according to major clay typebecause it has been shown that clay type has an effecton the degree of protection offered to the soil organicmatter (Craswell and Waring, 1972).

When the equations just described were used to es-timate the parameters of the one-pool model for theundisturbed samples of each soil, the values of N min-eralized predicted with those parameters were reason-ably close to the observed ones: Obs = — 3.7 + 1.054Pred (n = 105; R2 = 0.916; range = 1.4 to 107.4 mgN/kg soil; the intercept is significantly different from0 (t = —2.53) but the slope is not significantly differ-ent from 1 (t = 1.71) at a 0.05 level of probability).

In summary, the N mineralization pattern of dis-turbed samples differed considerably from that of un-disturbed samples. Although in general, parameters ofmodels fitted to data from disturbed samples couldnot be used to predict N mineralized in undisturbedsamples, they could be used to estimate the param-eters of the one-pool model for undisturbed samples.Parameters estimated using this approach providedreasonable estimates of the amounts of N mineralizedin fallow plots on Haynie and Wymore soils. It was

also possible to estimate the parameters of the one-pool model for undisturbed samples from the total Nand clay contents of the soils.

ACKNOWLEDGMENTWe are thankful to Martha Blocker for help with lab in-

cubations and chemical analysis of the samples, to GeorgeMilliken for help with the statistical analysis of the data,and to Mark Claassen for help with sample collection.