P l a n t and Soil X X I X , no. 1 Augus t 1968

S O I L O R G A N I C N I T R O G E N M I N E R A L I Z A T I O N

AS A F F E C T E D B Y

L O W S O I L W A T E R P O T E N T I A L S

by R. WETSELAAR

Division of Land Research, CSIRO, Canberra, Australia

INTRODUCTION

When no nitrogen fertilizers are used soil nitrogen availability is dependent on the rate at which soil organic nitrogen can be con- verted to mineral nitrogen. This conversion is largely a micro- biological process which is to a great extent dependent on the water content of the soil. It is well known that nitrate is formed in the not precisely formulated permanent wilting point (P.W.P.) - field capaci- ty range, but virtually no reliable data are available on the effect of lower soil water contents on soil organic nitrogen mineralization. R o b i n s o n 9 in Africa incubated soil at 1, 2/3, 5/6, and 1 × the soil water content at P.W.p. He maintained soil water content at the required level by daily additions of water and incubating at a :k 1 °C temperature variation. D o m m e r g u e s 5 incubated soil at soil water potentials between -- 0.5 and -- 500 bars in closed vessels which were opened twice a week for several minutes for aeration, and tem- perature was controlled at the ± I°C level. In both cases either temperature and/or soil water content control could have affected the results. J u s t i c e 6 overcame this by incubating in a water bath and aerating the soil with a predetermined relative humidity. How- ever, his results were restricted to nitrification since he applied am- monium sulphate to the soil, which, in addition, could have affected the osmotic component of the total soil water potential.

Soil from two soil types of the dry-monsoonal area of north- western Australia was therefore incubated under strict soil water and soil temperature control in order to evaluate the rate of miner-

- - 9 - -

10 R. WETSELAAR

alization of soil organic nitrogen around the -- 15 bar water po- tential. This evaluation was regarded as important to understand the fluctuations in mineral nitrogen found in soils of the Katherine region of the Northern Territory. In this region the topsoil dries out during the rainless five months period of the dry season. Periods of light showers, alternated with high evaporation, can occur at the start and end of the wet season, when the water content of the soil is regularly reaching tensions below the -- 15 bar level.

MATERIAL AND METHODS

The two soil types selected were Tippera clay loam and Manbulloo sand. T ippera clay loam is a lateri t ic red ear th wi th a clay co n t en t of 15 per cent , and a p H of 6.5, i ts organic n i t rogen con ten t is abou t 0.067 per cent, and its wa te r con t en t a t - - 0.1 and -- 15 bars is 17.4 and 10.2 per cen t respect ively. Manbul loo sand is der ived f rom river deposits . I t has a clay co n t en t of 11 per cen t and a p H of 7.2, its organic n i t rogen con ten t is abou t 0.04 per cent, and i ts wa te r con t en t a t -- 0.1 and -- 15 bars is 21.8 and 5.4 per cent respect ively. For bo th soils the 0-6 inch layer was selected f rom areas t h a t had been under cu l t iva t ion for a t least six years. The samples were s tored air d ry for four years. J u s t prior to incuba t ion t h e y were l ight ly crushed, and only the aggre- ga tes be tween 2 and 1 m m were used. The long s torage per iod was used to ob ta in soil which would have an init ial ly low micro-biological popu la t ion af ter re-wet t ing. This would allow for a long equi l ibr ium t ime a t a given wa te r po ten t i a l w i thou t an init ial high ra te of mineral izat ion. In this expe r imen t an equi l ibr ium t ime of th ree weeks was taken.

For incuba t ion at the requi red wa te r po ten t i a l the soil aggregates were first spread in a 3-ram th ick layer on a glass p la te and then w e t t ed wi th dis t i l la ted wa te r using an a tomizer unt i l a requi red weight was obta ined . The layer was t h e n allowed to d ry out on a balance and 10-g samples were collected a t selected soil wa te r contents , cor responding to -- 2.7, -- 5, - - 15, -- 20, -- 35, and -- 50 bars wa te r potent ia l . The re la t ion be tween wa te r co n t en t and wa te r po ten t i a l was ob ta ined using a pressure m e m b r a n e for the -- 0.1 to - - 15 bar range. For t he lower po ten t ia l s a modif ica t ion of Richards and O g a t a ' s s m e t h o d was used ( B a r f s and S l a t y e r 1). The 10-g samples were placed into pe r spex vials (P in Fig. 1), 5-cm high, 1.7 cm diameter . Four of these vials (2 soils X 2 replications) were f i t ted into a 500 ml jar, 0, on the b o t t o m of which was placed NaC1 solution of which the concen t ra t ion cor responded to the requi red wa te r po ten t i a l above the solution. The jar 0 was f i t ted wi th lead T, s toppered wi th out le t R and spiral M, placed in t he wa te r b a t h S, and spiral M was then connec ted to the glass capi l lary L. The W a t e r in the wa te r b a t h was kep t a t 25 ° 4- 0.02°C by ag i ta t ing and hea t ing wi th a Braun T h e r m o m i x II (Q in Figure 1). Twelve jars, two for each selected water potent ia l , were thus p laced in the wa te r b a t h and connected via capil lary L to the i r cor responding

SOIL WATER POTENTIAL AND N-MINERALIZATION 1 1

Fig. 1.

air A

qf3t

s

~ R

A p p a r a t u s for i n c u b a t i n g soil a t c o n s t a n t t e m p e r a t u r e a t t he r equ i red wa te r po ten t i a l . Fo r deta i ls see t ex t .

h u m i d i f y i n g c h a m b e r s G (90 cm long). Air was t h e n le t ill u n d e r s l ight pres- sure v ia f i l ter A a n d i ts flow regu la t ed a t 25 ml mil l -1 pe r j a r O, us ing t he needle va lve B a n d air flow m e t e r D. Ai r p ressure was r ead off a t m a n o m e t e r C. The NaC1 c o n c e n t r a t i o n s in t he t ubes G were a d j u s t e d to t he r equ i red w a t e r p o t e n t i a l us ing R o b i n s o n a n d S t o k e s ' 10 d a t a g iv ing t he molaI v a p o u r pressure lowerings (Po -- P)/(MPo) for va r ious c o n c e n t r a t i o n s of NaC1. For ttlis t h e a i r pressure decrease due to cap i l la ry L, was t a k e n in to a c c o u n t accord ing to R .H. = Pr /P i ( B a r t h o l o m e w a n d B r o a d b e n t 2), where R.H. = re la t ive h u m i d i t y , Pr = t h e f ina l pressure (in th i s case equa l to a tmospher ic ) , a n d Pt t he pressure u n d e r which t he air was humid i f i ed in t he c h a m b e r s G. T he whole a p p a r a t u s was k e p t in a c o n s t a n t t e m p e r a t u r e room a t 25 ° ~= 2°C.

For each soil w a t e r p o t e n t i a l level one j a r was d i sconnec ted a f te r t h r ee weeks a n d t he second one a f te r six weeks. The soil was e x t r a c t e d w i t h 1 N CaC12, a n d n i t r a t e n i t rogen was d e t e r m i n e d us ing a modi f i ca t ion of M i d d l e - t o n ' s 7 Orange I m e t hod . A m m o n i u m n i t rogen was d e t e r m i n e d us ing t he cold micro-d i f fus ion t e c h n i q u e ( B r e m n e r a n d S h a w 4). I n p r e l im ina ry t r ia l s n i t r i t e could n o t be de tec ted . All r esu l t s are expressed on a n oven d ry basis .

D u r i n g i n c u b a t i o n ou t l e t R was d ipped in to w a t e r twice da i ly to t e s t if air was f lowing t h r o u g h a t t he r equ i r ed ra te . T he h u m i d i f y i n g c h a m b e r s G were f i t t ed w i t h in le t I a n d ou t l e t J a n d t aps F a n d H, wh ich were used to replace t he NaC1 so lu t ions in t he c h a m b e r s OllCe a week. A glass f loa te r K, w i t h a piece of m e t a l inside, was d r a w n twice da i ly over the surface of t he NaC1

12 R. WETSELAAR

solution, using a magnet, to disturb possible NaC1 concentration gradients near the surface of the solution.

A separate jar 0, with Tippera clay loam in jar P, was connected to the -- 15 bar humidifying chamber G, and put in the water bath to test temper- ature fluctuations within the soil column. Changes greater than 0.006°C could not be detected within a 24-hour period, indicating that the tempera- ture control in the soil columns was at a desirably accurate level.

RESULTS AND DISCUSSION

The n i t ra te and a m m o n i u m ni t rogen concentra t ions , a f te r deduc- t ion of the values a t zero t ime, are given in Fig. 2, for 3 and 6 weeks incubat ion at the different wa te r potent ials , for bo th soil types . In T ippe ra c lay loam ve ry li t t le n i t ra te and a m m o n i u m was fo rmed in the first three weeks. Af ter six weeks n i t r a te was increased, par t icu- lar ly at the higher wa te r potent ia ls , while a m m o n i u m was increased m a r k e d l y a t the lower ones. In Manbulloo sand subs tan t i a l amoun t s

of n i t ra te were fo rmed at the higher wa te r potent ia ls up to and in- cluding a t - - 20 bars. This was also the case af ter six weeks. Am- m o n i u m fo rmat ion was negat ive in the first three weeks, but was subs tan t i a l at - - 20 bars and lower af ter six weeks.

I f the first three weeks are regarded as a period of equi l ibr ium for the soil wa te r po ten t i a l to be ad jus ted to exac t ly the required level, and also for micro-organisms to br ing their popula t ion to a level more comparab le with field condit ions, then the changes be tween

three and six weeks in minera l n i t rogen can be regarded as repre- sen ta t ive for the effect of the wa te r potent ia ls inves t igated. These changes are p lo t t ed in Figure 3, in which l inear regressions w e r e f i t ted for the n i t ra te and a m m o n i u m changes, for bo th soil types . T h e y suggest t h a t for T ippe ra c lay loam no more n i t r a te is fo rmed below - - 24.3 bars wa te r potent ia l , bu t a m m o n i u m fo rma t ion con- t inues down to - - 50 bars, the lowest po ten t i a l applied. Manbul loo sand shows a s imilar t rend. The odd resul ts at - - 15 bars for this soil t y p e t ended to increase the n i t ra te values and decrease the am- m o n i u m values. I f disregarded, n i t ra te fo rmat ion would have been zero a t - - 34.8 bars. For b o t h soil types there was a high nega t ive corre la t ion be tween n i t ra te and a m m o n i u m ni t rogen of r = - - 0.933 for T ippe ra c lay loam, and r = - - 0.948 for Manbulloo sand.

These results are only p a r t l y in ag reement wi th those of D o m - m e r g u e s 5 and R o b i n s o n 9. The former found t h a t ammoni f i ca -

25 T ippe rs c tay loom

\ \ \

\ \ /

\ x / /

\ \ \ 11 -

°\ / >/ .

S --_/ \ \ / -

'r.-..~ o -

0 Monbut to sand

"C

.i o% \ X

//

I /

/ //

~o

/

/

t / ° ~ /

lo ~ / V A

• I

0 [ ~ p i I i / , "',. ~ l / ", !

l J Xo I o /

_.I 'J l I I _ L I -so -3rs -20 -115 -S -2.7

w a t e r p o t e n t i a l , "~ ( ba rs }

Fig . 2. C h a n g e s ill n i t r a t e a n d a m m o n i u m n i t r o g e n (ppm) a f t e r t h r e e a n d

s i x w e e k s i n c u b a t i o n a t d i f f e r e n t w a t e r p o t e n t i a l s , for t w o soil t y p e s .

Tippera clay loam NHa-N y = 22.18 log (-- x) -- 9.9710 o N H 4 - - N N O 3 - N y ~ 7 .44 l o g ( - - x) + 10 ,3136 - N O 3 - - N

Manbul loo sand N H 4 - N y = l l . 3 9 1 o g ( - - x) - - 3 .1252 - - A f t e r 3 w e e k s

N O a - N y = 7 .87 l o g ( - - x) + 13.3335 - - - A f t e r 6 w e e k s

14 R. WETSELAAR

Fig. 3.

3[ Tippera clay loam

o o

2O

le

-g

o

20 Nanb ul[o sond

10 ~ ~

.

~,..-- ~ i i r

° i J

-;o -3~5 -~o -; -5 _27

woter potenliat, ~ (bars)

Relation between soil water potential and changes in mineral nitro- gen (ppm) between three and six weeks incubation.

tion always exceeded nitrification between - - 10 and - - 400 bars, and that both gradually decreased as the water potential was lowered. The latter found no nitrate formation and a decreased am- monium Iormation below P.W.P. J u s t i c e 6 found evidence of nitrification of applied ammonium sulphate proceeding still at a slow rate at - - 70 bars water potential.

No doubt ammonium formation must cease at a water potential somewhere between - - 50 bars and at the air dry state, since air dry

SOIL W A T E R POTENTIAL AND N - M I N E R A L I Z A T I O N 15

soil was used for the determination of the control values. As am- monium formation is a necessary step in the formation of nitrate the totals (NOa -t- NH4)N are of interest. These totals, for the changes between three and six weeks, are given in Table 1. Those for Man- bulloo sand did not differ significantly when the water potential was altered, but those for Tippera clay loam increased significantly from -- 2.7 bars down to -- 35 bars. The data for Tippera clay loam sug- gest that there are micro-organisms which utilize a part of the am- monium when mineralization at high water potential is proceeding, and that their activity is more susceptible to low water potential than that of the ammonium formers.

TABLE 1

Increases in (NOa + NH4)N (ppm) between three and six weeks incubation

Bars

- - 5 0 So~ type

- - 3 5 - - 2 0 - - 1 5 - - 5 - - 2 . 7

Tippera clay loam 23.2 25.5 18.7 19.3 9.3 7.0 6.61 Manbulloo sand 16.7 17.0 17.7 6.2 14.9 12.3 12.80

L.S.D.

(P = 0.05)

The fact that ammonium formation continues at a lower soil water potential than does nitrate formation would explain the relatively high ammonium values found by the author in bare fallow topsoil in the Katherine region immediately after the last rains of the wet season when the soil dries out. R o b i n s o n 9 found a similar increase in ammonium nitrogen in African soils. At the beginning of the following wet season scattered light showers normally precede the main rains at Katherine. During this period the topsoil will be lightly wetted and be dried out again, and ammonium nitrogen could thus be increased. This might partly explain why nitrate formation is high when it is followed by heavier rains at the begin- ning of the wet season. This effect could be stimulated by the in- creased micro-biological activity when soil is rewetted after drying (Bi rch 3). In addition, mineralization of organic nitrogen will be stimulated by aggregate disruption (R o v i r a and G r e a c e n 11) from the ploughing, which normally takes place after the early rains of the wet season. These phenomena combined would explain why at least 50 per cent of the total amount of nitrate made available in the growing season is formed in the early part of the wet season on Tippera clay loam, as found by W e t s e l a a r and N o r m a n 12

16 R. WETSELAAR

SUMMARY

Two soils of the Ka the r ine region of the d ry monsoona l area of no r th - wes te rn Aust ra l ia were incuba ted a t - 2 . 7 , - 5 , - 15, - - 2 0 , - 35, a n d - 50 bars wa te r po ten t i a l for th ree and six weeks under s t r ic t t e m p e r a t u r e and soil wa te r control condit ions. The a m o u n t s of n i t r a t e and a m m o n i u m formed be tween three and six weeks incuba t ion were regarded as a good measure of t he effect of the d i f ferent wa te r po ten t ia l s on organic n i t rogen mineraJizat ion. N i t r a t e fo rmat ion ceased at - - 24.3 bars in Tippera clay loam and a t -- 50 bars in Manbulloo sand. A m m o n i u m fo rma t ion was increased up to - - 50 bars. The resul ts for Tippera clay loam suggest t h a t there are micro-organisms which util ize a m m o n i u m ni t rogen a t h igh wa te r potent ia l , b u t the i r ac t iv i ty is decreased a t a higher wa te r po ten t i a l t h a n t h a t of the a m m o n i u m formers. The impl ica t ions of the resul ts are discussed in re la t ion to high soil n i t rogen avai labi l i ty a t the beginning of the wet season.

ACKNOWLEDGEME NTS

I am indeb t ed to Mr. P. F i r t h for his con t inuous ass is tance wi th the de- s igning of the appa ra tu s and pe r fo rming the exper iment . The advice and help by Dr. R. O. S l a t y e r and Dr. H. D. B a r r s is acknowledged. Credi t is due to Mrs. D. B r o w n b i l l for de te rmin ing the soil wa te r po ten t ia l s using the pressure membrane .

Received April 13, 1967

REFERENCES

1 Barrs , H. D., and S la tye r , R. O., Experience with three vapour methods for measuring water potelltial in plants .Prec. Montpellier Syrup. Methodology Plant Ec0-physiol. Unesco, 369-84 (1965).

2 B a r t h o l o m e w , W. V., and B r o a d b e n t , F. E., Apparatus for control of moisture, temperature and air composition in microbiological respiration experiments. Soil Sci. Soc. Am. Prec. 14, 156-50 (1949).

3 Birch, H. F., The effect oi soil drying on humus decomposition and nitrogen availa- bility. Plant and Soil 10, 9-31 (1958).

4 Bremller , J. M., and Shaw, E., Determination of ammonia and nitrate in soil. J. Agr. Sci. 46, 320-28 (1955).

5 Dommergues , Y., Contribution ~ l'etude de la dynamique microbienne des sols en zone semi-aride et en zone tropicale s~che. Thesis. Bussi6re ~ Saint-Amand (Cher),

France (1962). 6 J us tice, J. K., Moisture and temperature effects on the transformations of nitrogen

from applied ammonium sulphate in a calcareous soil. Thesis. Utah State University (1961).

7 Middle ton , K. R., The use of Orange I method for determining soil nitrates and a comparison with the phenoldisulphonic acid method for certain soils of northern Nigeria. J. Sci. Food Agr. 10, 218-24 (1959).

SOIL W A T E R P O T E N T I A L AND N - M I N E R A L I Z A T I O N 17

8 R i c h a r d s , L. A., and O g a t a , G., Thermocouple for vapour pressure measurements in biological and soil sys tems at high humidi ty . Science 128, 1089-90 (1958).

9 R o b i n s o n , J. B. D., The critical relationship between soil moisture content in the region of wilting point and the mineralization of natural soil nitrogen. ,I. Agr. Sci. 49, 100-5 (1957).

10 R o b i n s o n , R. A., and S t o k e s , R. H., Electrolyte solutions. Butterworths, London (1955).

11 R o v i r a , A. D., and G r e a c e n , E. L., The effect of aggregate disruption on the ac- t ivi ty of microorganisms in soil. Austral ian J. Agr. Research 8, 659-73 (1957).

12 W e t s e l a a r , R., and N o r m a n , M. J. T., Recovery of available soil ni trogen by annual fodder crops at Katherine, Northern Territory. Austral ian J. Agr. Research 5. 693-704 (1960).