DIVISION S-4-SOIL FERTILITY& PLANT NUTRITION

Nitrogen Mineralization Potential of Arid and Semiarid Soils of MoroccoM. El Gharous, R. L. Westerman,* and P. N. Soltanpour

ABSTRACTAn accurate estimate of N mineralized from soil organic matter

during the growing season is required to make correct N fertilizerrecommendations for crops. In Morocco, very little information re-garding the N-supplying power of soils is available. These studieswere conducted to-determine N-mineralization potential (N0) andrate constant (k) values, N flux, and instantaneous rate of reactionfor soils of the Chaouia region of Morocco. Fourteen soils wereselected and net mineralizable N during 16 wk of incubation at op-timum moisture and temperature was determined for each soil. Ex-ponential and hyperbolic models were used to describe net miner-alization, and N0 and k values for each soil were determined usingnonlinear least-squares regression. Values using the exponentialmodel ranged from 120 to 241 mg N kg-1 of soil for N0 and 0.06 to0.274 week-1 for k. The values ranged from 164 to 391 mg N kg-1

of soil for N0 and 0.024 to 0.212 week ' for k, using the hyperbolicmodel. The lowest average N0 values for different soils were shownto occur in Palexerolls, which are shallow soils approximately 35cm deep that have received very low N input. Calculations using theexponential and hyperbolic models indicated the active fraction oftotal N in these soils varied from 7 to 22% and 10 to 36%, respec-tively. This study supported earlier reports that a single exponentialmodel as well as a hyperbolic model can be used to estimate reliableN0 and k values in soils if a nonlinear least-squares fitting techniqueis used.

THE INTENSIFICATION OF CROP PRODUCTION in thearid and semiarid regions of Morocco requires

the use of N fertilizers, or at least more efficient useof the N mineralized by the soil. In recent studies,Soltanpour et al. (1989) reported grain yield in theregion was increased 76% due to N fertilization.

Nitrogen-mineralization potential values can be cal-culated based on the hypothesis that the rate of Nmineralization is proportional to the quantity of NM. El Gharous, INRA-MIAC Project B.P. 290, Settat, Morocco;R.L. Westerman, Dep. of Agronomy, Oklahoma State Univ., Still-water, OK 74078; and P.N. Soltanpour, Dep. of Agronomy, Colo-rado State Univ., Ft. Collins, CO 80523. Joint contribution fromINRA-MIAC and the Oklahoma Agric. Exp. Stn. Journal no. 5289.Received 16 June 1988. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 54:438-443 (1990).

comprising the mineralizable substrate (Stanford andSmith, 1972). However, N0 is affected by many com-plex interactions of moisture, aeration, temperature,nature and quantity of organic matter, nature andquantity of the previous crop residue, and other soilphysical, chemical, and biotic properties. Althoughoptimum conditions for N mineralization are seldomachieved for long periods of time under field condi-tions, it is important to understand the effect of thesefactors on the overall process.

The effect of moisture on N mineralization has beeninvestigated by many researchers. Optimum moisturehas been reported to vary from 0.015 to 0.05 MPa byMiller and Johnson (1964) and from 0.01 to 0.033MPa by Stanford and Epstein (1974). However, Cass-man and Munns (1980) found that moisture tensionof 0.03 MPa gave the maximum mineralization rate.Similar results were reported by others (Chiang et al.,1983;Myersetal, 1982).

Soil N-mineralization rate is affected profoundly bytemperature within the range that is normally en-countered under field conditions. Cassman andMunns (1980) reported the optimum temperature forN mineralization to be from 30 to 35 °C. Using a tem-perature coefficient of Qw « 2, Stanford et al. (1973)found similar mineralization rates in different soils foreach temperature studied in the 5 to 35 °C range.

Nitrogen-mineralization rate is also affected by thenature of organic matter. Stanford (1968) reported theexistence of two general pools of organic N. The firstpool is relatively easily decomposable through micro-bial action. The second pool, however, is somewhatresistant to further rapid decomposition and contrib-utes a small proportion of N mineralization during ashort-term incubation or even within a cropping sea-son.

Texture is known to be an important factor affectingsoil organic-matter content. Bremner (1965) reportedthat total N increased as texture became finer. Also,it was reported that losses of organic matter, poten-tially mineralizable N, and the active N fraction weregreater in coarse-textured soils (Campbell and Souster,1982; Herlihy, 1979).

EL GHAROUS ET AL.: NITROGEN MINERALIZATION IN ARID AND SEMIARID SOILS 439

Some reports have successfully established a rela-tionship between estimates of N0 and k values thatwere corrected for soil temperature and water contentwith N mineralized in situ under field conditions(Smith et al, 1977), and in fallow plots (Cabrera andKissel, 1988), in short growth periods under green-house conditions (Stanford et al., 1973) and for cropgrowth under field conditions (Stanford et al., 1977).However, Cabrera and Kissel (1988) noted that, in allcases except fallow, N mineralized was overestimatedwhen rate constants were adjusted for temperatureand moisture. This was attributed to an improper soilwater content factor and the drying and sieving of thesamples before incubation.

The objective of this study was to determine N0 andk values, N flux, and instantaneous rate of reactionfor soils of the Chaouia region of Morocco.

Table 1. Classification and chemical and physical properties of Mo-roccan soils.

Soilclassification

Orthent 1Orthent 2Chromoxerert 1Chromoxerert 2Palexeroll 1Palexeroll 2Palexeroll 3Palexeroll 4Palexeroll 5CalcixerollXeralf 1Xeralf 2Xeralf 3Xeralf 4

PH

8.28.08.08.18.08.38.28.28.08.47.17.67.37.1

Organic TotalC N

-gk13.0 1.16.4 0.9

13.012.722.724.016.718.718.412.511.113.816.617.2

.0

.1

.0

.9

.4

.7

.4

.4

.2

.4

.1

.4

CaCO3

g-1 ——15203030

17070

11015

107940530

Claycontent

180420510640330390420450350370340330160440

Cation-exchangecapacity

cmolc kg"1

10.218.542.069.142.043.739.949.035.144.728.523.518.028.9

MATERIALS AND METHODSSoil moisture and temperature are two major factors that

affect N-mineralization rates (Cassman and Munns, 1980;Myers et al., 1982; Stanford et al., 1973). The yearly meansof rainfall in the arid and semiarid regions of Morocco havetwo gradients. The first decreases from the northern part ofthe region at Settat (408 mm) to the southern part at Mach-raa Ben Abou (232 mm). The second increases from thewestern part at Imfout (258 mm) to the eastern part atKhouribga (395 mm). Fall and winter rainfall comprises ap-proximately 70% of the annual rainfall. Approximately 26to 28% of the annual rainfall occurs during the spring, and2 to 6% occurs during the summer. Distribution and occur-rence of rainfall in the region is erratic and varies widelyfrom year to year. Temperature in the region is more stable,with an annual mean of 17 °C. The mean temperature ofthe coldest month (January) is 11 °C and the hottest month(August) is 27 °C. Organic N mineralization is enhanced bythe light rainfall (10-20 mm) that generally occurs after sum-mer drought (Chiang et al., 1983).

Fourteen soil types were selected for this study, whichrepresented five of six orders mapped in the Chaouia regionof Morocco (Stitou, 1984). The classification and certainchemical and physical characteristics of the 14 soils areshown in Table 1.

Surface soil at each location was sampled (0-20 or 30 cm)and air dried without delay. Total N and organic C in soilwere determined by the Kjeldahl (Bremner, 1965) and Walk-ley and Black procedures (Allison, 1965), respectively. SoilpH was measured using a 1:1 (w/v) soil/water suspensionratio.

The N-mineralization procedure followed the methodused by Stanford and Smith (1972). Triplicate samples of30 g of each soil plus 5 g of vermiculite were mixed thor-oughly. Soil-vermiculite mixtures, n = 42, were moistenedusing a fine spray of distilled water, mixed thoroughly, andtransferred to 50-mL leaching tubes. Soil was retained in thetubes by placing a glass-wool pad at the bottom of each tube.A thin (0.25-cm) glass-wool pad was placed over the top ofthe soil to avoid dispersing the soil when solutions werepoured into the tubes.

Mineral N initially present was removed by leaching with100 mL of 10 mM CaQ2 in 5- to 10-mL increments, fol-lowed by 25 mL of a nutrient solution devoid of N (2.0 mMCaSO4; 2.0 mM MgSO4; 5.0 mM Ca(H2PO4)2-H2O; and 2.5mM K2SO4). Excess water was removed under vacuum (0.02MPa). The tubes were then incubated at 32 ± 1 °C. After2 wk, mineral N was recovered by leaching with 100 mL of10 mMCaC!2 and 25 mL of "minus-NT" nutrient solution,followed by applying suction as described above. Tubes were

returned to the incubator for periods of 2, 4, 6, 8, 10, 12,14, and 16 wk cumulative, with intermittent leachings ofmineral N. Optimal soil water content was maintained atapproximately 0.02 MPa throughout the incubations.

Mineral Nitrogen DeterminationAfter steam distillation of 20 mL of leachate with MgO

and Devarda's alloy into H3BO3 indicator solution, NH4-Nand NO3-N were determined by titration using 25 mMH2SO4 (Bremner, 1965).

CalculationsTwo models were used to describe the data of this study.

The first model utilized was the first-order exponential equa-tion describing net N mineralization proposed by Stanfordand Smith (1972):

N = - exp(-A:E0]where N is mineralized N in time t, and N| and If are theN-mineralization potential and rate constant values. Thesecond model used was the hyperbolic equation proposedby Jumaetal. (1984):

N = 0where Tc is the half time for mineralization and other termsare as described above. Half time for mineralization is re-lated to the rate constant, kH, by the following:

Tc = ln2/A:H

A Systat nonlinear least-squares regression program wasused to evaluate N0 and k in both models (Wilkinson, 1986).

RESULTS AND DISCUSSIONNitrogen Mineralization

Cumulative N mineralized during the incubationperiod exhibited the same general trend for all soils(Fig. 1). The rate of mineralization was rapid at first,then declined with the length of the incubation period.The lowest quantity of mineral N produced during theincubation period was observed in the Palexerolls.The highest quantity, however, was produced by Xer-alfs.

The NO values (120-241 mg kg-1) obtained by theexponential model (Table 2) were within the range ofthe recalculated data of Stanford and Smith (1972)reported by Talpaz et al. (1981), which included 39

1711fInlllfiw

jglx

l"S

-Sltt

ttS

1 -B

B-SB

-B-S

BSSa

SL *re

^ re

n>

—

£.<

1>'o

rtre

oX

g5

£-.

ill1

cl*S

?v

£)r

fao

'~i

^a

ls (Tableere consid and Smy are gene. (1980). I) obtainen were inford and

.S'

.o

idit

er. Iedin

raith (rally

In geinth

Smi

3 .

W.

2

6T

I

estimateably highe(1972) anly in agreeeneral,this st

e rangith (1

der

ndree-, the

studynge of

972)

><

en 0

>

i_i

O

.^

X

*"

^^

Sk^

ry ^

r i*

. —

4. ,-!

||

IS

eS |

|p|| |

§'S

I-9

Q 1

- P

2^2°

" "

i?r »

3&»

II a B

B s.

•o o

rrr m

(n m

mX

X

X X

X

m m

m m

m3J

DO

33

DO 3

3o o

o o

o

ACCU

MUL

ATED

MIN

ERAL

N (m

g kg

~1)

ACCU

MUL

ATED

MIN

ERAL

N (m

g kg

-1)

ACCU

MUL

ATED

MIN

ERAL

N (m

g kg

-1)

ACCU

MUL

ATED

MIN

ERAL

N (m

g kg

"1)

EL GHAROUS ET AL.: NITROGEN MINERALIZATION IN ARID AND SEMIARID SOILS 441

Table 2. Nitrogen-mineralization potentials (N0), rate constants (k), and active N fraction of Moroccan soils.Exponential model

Soil classification

Orthent 1Orthent 2Chromoxerert 1Chromoxerert 2Palexeroll 1Palexeroll 2Palexeroll 3Palexeroll 4Palexeroll 5CalcixerollXeralf 1Xeralf2Xeralf 3Xeralf 4LSD (0.05)

Nf— mg

157178174184145144120169145164184165241172

17

SEkg- —

6.835.16.65.54.04.86.7

15.411.712.517.89.1

14.41.7

*E SE

0.1090.1230.1000.1680.1640.1270.1390.0730.0960.0680.0600.0820.0760.2740.019

0.0090.0860.0070.0130.0120.0090.0170.0100.0140.0080.0080.0070.0070.004

N?

Hyperbolic model

SE

23526226425519920816426622326631726639121036

18.223.225.614.69.7

11.110.38.7

22.440.862.423.131.93.5

ft" SEwk~'

0.0540.0630.0490.0950.0990.0680.0830.0340.0460.0300.0240.0370.0350.2120.014

0.0080.0050.0080.0130.0120.0070.0100.0060.0090.0060.0070.0090.0080.016

Active N fractionExponential

modelHyperbolic

model

14.2319.7517.3916.777.267.578.549.93

10.3611.7415.2911.8021.8712.273.27

21.3829.1526.4123.229.93

10.9211.7215.6715.9119.0026.3119.0335.5214.975.55

Table 3. The instantaneous rate of reaction in Moroccan soils using exponential and hyperbolic models.Weeks

10Soil classification Exp. Hyp. Exp. Hyp. Exp. Hyp.

14

Exp. Hyp.

Orthent 1Orthent 2Chromoxerert 1Chromoxerert 2Palexeroll 1Palexeroll 2Palexeroll 3Palexeroll 4Palexeroll 5CalcixerollXeralf 1Xeralf 2Xeralf 3Xeralf 4r

13.7617.1214.2522.0917.1314.1912.6310.6611.499.739.79

11.4815.7327.27

13.6916.9114.2821.5617.1614.2112.6910.7211.569.779.74

11.4715.6724.710.99

8.9010.479.55

11.288.898.547.277.967.837.427.708.27

11.619.11

————— mgN8.509.949.18

10.538.148.076.647.747.587.277.618,09

11.348.000.98

ke~' —————•*6

5.756.406.405.764.615.144.155.955.335.656.065.968.573.04

5.786.536.406.224.825.204.075.855.355.616.106.018.583.920.98

3.723.914.292.942.393.052.384.443.364.304.774.296.321.02

4.194.624.714.103.163.632.754.583.984.475.014.646.722.310.97

= NE fcE exp(dN/d/ = N» TJ(TC + t)2 (exponential)

(hyperbolic)The instantaneous rates of reaction computed by

the two equations for each soil are highly correlatedfor each period of incubation with correlation coeffi-cients (r) ranging from 0.97 to 0.99 (Table 3). Thesehigh r values are further evidence that both modelscan be used in laboratory incubation studies to esti-mate N0 and k values for the soils of the Chaouiaregion of Morocco using a nonlinear least-squares fit-ting technique.

The Active Nitrogen FractionThe active N fraction of organic matter (N0/Nt),

where Nt is total N content, considered as the portionof the organic matter that supplies a major portion ofplant-available N for crop growth. The active N frac-tion in these soils varied from 7 to 22% (Table 2) andwas of the same order as those reported in other coun-tries (Stanford and Smith, 1972; Feingin et al., 1974;Herlihy, 1979; Campbell and Spuster, 1982). Thehighest value of the active N fraction was observed inXeralf 3, as was expected since this soil had not beencultivated for many years. The lowest values of active

N fraction were found in Palexerolls. Differences inthe active N fraction estimated using NJ/N, rangedfrom 2.6 to 13.6% higher than the estimates obtainedfrom NE/Nt.

Soils studied were divided into two groups accord-ing to the active N fraction calculated using NE/Nt.The first group included Palexerolls that had an activeN fraction < 10%. All other soils were included inthe second group, which had an active N fraction be-tween 10 and 20%. This subdivision was not the sameas the one made according to kE.

The soils studied were ranked according to the timerequired to mineralize a fixed quantity of mineral N(Table 4) using both the exponential and hyperbolicmodels. This classification showed no consistent re-lationship with soil type, or with physical or chemicalparameters. However, soils were ranked similarly us-ing both models. Time required to mineralize 10, 25,and 50 mg N kg-1 of soil using NE and if values rangedfrom 1.5 to 6.5, 4 to 17, and 8.8 to 37.4 d, respectively.Twenty-five to 50 mg N kg"1 of soil is representativeof the N requirement for cereal crops in the region(Soltanpour et al., 1989).

A linear relationship between total N and the in-verse of the active N fraction was found in this group

442 SOIL SCI. SOC. AM. J., VOL. 54, MARCH-APRIL 1990

Table 4. Time required to mineralize a fixed amount of N in Moroccan soils using exponential and hyperbolic models.Exponential model Hyperbolic Model

Soil classification Rank

Orthent 1 8Orthent 2 4Chromoxerert 1 5Chromoxerert 2 2Palexeroll 1 3Palexeroll 2 7Palexeroll 3 9Palexeroll 4 12Palexeroll 5 10Calcixeroll 14Xeralf 1 13Xeralf2 11Xeralf 3 6Xeralf 4 1

25| ——————— i —————r = 0.95n - 42

2 0 -

"̂ 1R -Z 15

£ * '$ 1 °- AAof*5 y1?^0 ~^r J0 1

10

4.233.293.372.333.053.974.385.855.216.486.525.343.901.53

/AK A

,2

mg N kg"1 mg N kg"1

25 50 Rank 10 25 50———————— d ___________ __________ d —-———_———

11.14 24.62 9 4.00 10.71 24.318.61 18.77 4 3.09 9.09 18.388.83 19.28 8 3.92 10.40 23.246.08 13.21 2 2.08 5.54 12.438.08 18.05 3 2.60 8.09 16.51

10.51 23.51 5 3.62 9.80 22.7111.76 27.14 6 3.81 10.64 25.9415.35 33.64 12 5.64 14.97 33.4113.80 30.83 10 4.71 12.64 28.7317.03 37.44 13 6.30 16.73 37.3317.04 36.99 14 6.48 17.03 37.2414.03 30.82 11 5.19 13.77 30.7410.09 21.42 6 3.81 9.93 21.314.01 8.77 1 1.15 3.10 7.18

/ environmental effects. Future work should include%r field calibration of estimates of N0.

A,' - In conclusion, this study supported earlier reports>^ that a single exponential model as well as a hyperbolic

A . model can be used to estimate reliable N0 and k valuesin soils if a nonlinear least-squares fitting technique isused.

"

3 4

TOTAL N (g kg"1)

Fig. 2. Relationship between total N and the inverse of the activeN fraction in Moroccan soils.

of soils. The slope and intercept values were 6.908 and-1.057, respectively, with an r of 0.95 (Fig. 2). Thisrelationship was probably due, in part, to the previousmanagement of Palexerolls. Palexerolls are shallowsoils with the lowest active N fraction and the highestorganic C levels and are fallowed in alternate yearsduring which animals graze the weeds. These soils re-ceive little or no N fertilizer during the cereal phaseof the fallow-cereal rotation.

Developing an index for N availability for plantshas proven to be relatively difficult, compared withthe success in testing soils for plant-available P. TheC/N ratio has been used to predict N availability, andit is generally reported that C/N ratios of decomposingmaterials have to be below 20 to 25 to obtain appre-ciable net mineralization (Harmsen and Kolenbran-der, 1965). The weakness of this assumption is thatstable organic fractions are relatively resistant to de-composition. The general rule that net mineralizationof organic N depends primarily on N content of thesubstrate holds only for the readily mineralizable partof the decaying materials.

Considering the limitation of C/N ratio and total-N indices for available N, N0 would be a better index.Although N0 and k values are useful in providing es-timates of the N-supplying power of soils, a majordisadvantage is that incubations are time consumingand do not take into consideration the fluctuation of

HADAS ET AL.: PHOSPHORUS EXTRACTABILITY IN ENRICHED MANURE PELLETS 443