A quantitative analysis of the variability in the activity of nitrifying organisms in a soil under pasture : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University

Copyright is owned by the Author of the thesis. Permission is given for

a copy to be downloaded by an individual for the purpose of research and

private study only. The thesis may not be reproduced elsewhere without

the permission of the Author.

A thes is presented in partial ful f i lment of the requirement s for

the degree of Doctor of Phi losophy at Massey University

Robert G.V. Bramley

1 9 89

i ABSTRACT

vari ab i l i t y in the inputs , outputs and trans formations of mineral N under f ield condit ions makes the predict ive mode l l ing of the leaching of soil nitrate very d i f f icult . In an attempt to understand and quant i f y this variabi l i t y , the activity of nitr i fying organi sms in the Tokomaru s i l t loam ( a Typic f ragiaqua l f ) under pas ture was measured us ing a short-term nitr i f ication assay ( SNA ) . Spatial dependence of the variab i l i ty in SNA was examined using geos tatistical methods , and the e f fect on SNA of soil pH change through l iming , and of seasonal changes in soil temperature and moi s ture were inves t igated .

Nitr i f ier act ivity and associated soi l properties such as the amount of exchangeable ammonium and the soi l ni trate concentrat ion , were found to decrease in value w i th depth between 0 - 2 4 cm . The greatest decrease in SNA was observed between 0 - 9 cm depth , but due to the need for suf f icient quanti ties of soi l to make SNA measurements , and the des ire to avoid the poss ibi l ity of inhibitory ef fects of grass roots on nitr i f ication , soil was s ampled from the 3 -9 cm depth range for the bul k of the work reported here . Results i ndicated that the technique of sieving and mi xing samples was s at is factory f or removing de pth-dependen ce from the r esults for spa t i a l vari ab i l i ty and other analyses .

The spa t ial variab i l i ty of SNA , soi l No3 - , soi l mois ture content and the pH of the SNA incubation , which was assumed to approximate the f i eld soil pH , was invest igated over areas of 9 m2 and 6 2 5 m2 us ing a regular 1 1 x

1 1 square grid sampl ing des ign with minimum sample separat i ons of 30 cm and 2 . 5 m respect ively . However , the results of these analyses proved inconclusive , apparent ly due to the lack of samples separated. by lags that were suf f iciently short in relat ion to the overal l dimens ions of the sampl ing area . Accordingly , spa t i a l ana lysis of the above proper ties , together w i th exchangeable ammonium , was carried out over 6 2 5 m2 using a n es ted sampl ing design that permitted an adequate number of observation p a i n t s a t l a gs r anging f rom 1 2 . 5 c m t o 2 5 m . Thi s d e s i gn was a cons iderable improvement on the regular square des i gn , al though i t had a number of shor tcomings , not ably bias caus ed in the estimat ion of the samp l e vari ance d ue to t he nes ting o f a large number of data points

w i thin a sma l l area , and bias caused in the estimat ion o f va lues of the s e m i va r i ance a t s ome l a gs- due to m i s s i ng samp l i n g points at some pos i t i ons in the sampl ing grid .

The v a l ues o f S N A , N0 3 - and ex chan geab l e ammonium were a l l highly variable and conformed to lognormal distribut ions . The range of spatial dependence in the variabi l i t y of SNA , soil N03 - and incubation pH was 2 . 4 , 5 . 4 and 6 . 1 m respect ive ly . Exchangeabl e ammonium , SNA , soil No3 - and incubation pH varied i sotropi cally w ithin the sampl ing area but Ex­

NH4• showed no spatial dependenc e . Soi l moisture content was s trongly anisotropic , and showed no spa t ial dependence in one d irection , but clear evidence o f dr i f t in a p er pend i cular d i r ec t i on . These results are d i s c u s s e d in r e l a t i o n t o t he mos t e f f i c i ent s ampl ing s trategy for est imation of the mean field N03 - concentration . I t was concluded that suf f ic i ent sma l l loca l i zed c lusters of samples should be taken to give a low s tandard error of the mean , w i th each cluster separated by at l east 5 m . I n the case of the Tokomaru s i l t loam , 20 clus ters , each compris ing 5 samples ( bulked ) , would be required for estimation of the mean f ield n itrate concentrat i on with 9 5 % probability of

0

being wi thin ± 5% of �' the true mean . This represents a l arge sampl ing ef fort .

The a c t i v i t y o f n i t r i f i e r s w a s s tud ied in relat ion to soil pH and seasona l changes in soil mois ture and temperature over two consecut ive years i n an attempt to explain the spatial variabi l i ty in SNA values . The pH opti mum for nitrif ier activity ( pHopt ) was def ined for four var iates of the Tokomaru s i l t loam w i t h d i f ferent l i ming hi s torie s . Values of pHopt which varied between the four soi ls in the range 5 . 9 2 - 6 . 4 5 did not v a r y m a r ke d l y w i th s ea s on , and i t was found that the form of the rel a t i onship bet ween SNA and pH remained c ons tant wi th t ime . I t was further observed that the add i t ion of l ime in 1 9 87 had the ef fect of rais ing the mean soil pH and pHopt in previously-unl imed soi l , but had

negligible e f fect on ei ther the soi l pH or pHopt in soi l that had been l imed in 1 9 8 2 . The significance of heterotrophic relat ive to autotrophic n i t r i f icat ion could not be discerned .

iii

No sign i f icant relationships could be found for the four soils between soil pH , pHopt, SNA, soil moisture content and soi l temperature at 3 0 cm dept h . Va lues of SNA ( pmol N g -1 so i l h- 1 ) at pHopt ( SNAopt) wer e calcul a t ed fr om equa tions f i t ted t o p l o t s of SNA vs . the pH of SNA

incuba t ion , a nd thes e show a more obvious seasona l trend . SNA va lues calculated for the prevail ing so i l pH ^(SNApH) were never very d i f ferent from v alues of SNAopt and follow a 1 : 1 rela t i onship over a range of values f rom 0 . 0 1 5 - 0 . 1 1 0 pmol g-1 h-1 ; that i s , the nitr i f ier activity in the s o i l , i r r e spe c t i ve o f va r i a t i o n s t ha t were ra ndo m ( unknown i n f l uences ) or a s s oc i a ted w i th s e a s o n a l var iables ( temperature and moi sture ) , was near the opt imum with respect to the soi l pH a t the time of samp l ing .

The e f f ect of soil moisture variat ion on n i trif ier act ivity was further invest i gated in an experiment in which soil samples were s tored for 1 2 4 days a t d i f ferent soi l mois ture tens ions . The opt imum moisture conditions for n i t r i f ier activity in the Tokomaru s i l t loam prevai led at pF 3 . 3 9 . However , thi s optimum was less clearly def ined than was the pHopt • S ince the s o i l moisture status changes cons iderably with season, whi ls t soi l pH does not , it was concluded that nitrif iers were more tolerant of changes

in pF than changes in pH .

Compa r i son of these with published results indicates that not only is the soi l n i trif ier popul ation dynamic , and changes in response to changes in i t s environmen t , but the degree to which n i trif ier activi t y i s a f fected by var ious soi l properties is soi l - spec i f ic . It is therefore concluded that the spatial variabi l i ty of nitrif ier activi ty will a l so be so i l ­ speci f ic , and that dif ferent soils are l ikely t o have dif ferent ranges o f spa t i a l dependence for the parameters of mineral N. Furthermorer the f act that SNA i s not the only factor governing the soi l No3- concentration , and t h a t o t h e r f ac tors s uch as p l a n t uptake and leaching are a l so impo r t an t , indicates that ^SNA var i ab i l i t y i s not necessar i ly a good e s t i m a t o r of s o i l N03- var i abi l i t y . Th i s conc l us i o n i s c e r t a inly suppor ted by the geostatist ical aspects of this work .

ACKNOWLEDGEMENTS

I would like to thank the fol lowing people for their part in seeing this proj ect through to complet ion :

P r o f e s s o r R . E . Wh i t e f o r h i s s u p e rv i s i o n a nd gui dance throughout . The support given in l oco paren ti s by Bob and his w i fe , Annet t e , on arrival in this s trange ( ! ) country and their friendship thereaf ter has been very greatly appreciated .

D r A . N . Macgregor for hi s supervis ion .

Drs D . R . Scotter , P . R . Darrah , A . B . McBratney and A . Swi f t , and Messrs L . D . Currie, M . Eggel s , Mrs H . Murphy and Mrs A . Rouse for their assistance at various s tages of the proj ect .

The s t a f f and po s t graduate s tude n t s of t he S o i l S c ience Department for providing the re laxed and friendly a tmosphere which made l i fe at Massey so enj oyable .

The Vice-Chancel lor of Massey Univers ity , Dr T . N . M . Waters , for making the necessary funds avai lable for this work .

Fina l l y I would l ike to thank my parents , John and Rosal ind Bramley , for providi ng a l l that made starting this work possible , and Jo T o mp k i n s , who s e suppor t a nd a f f e c t i o n w a s t h e much n e eded inspirat ion for its complet ion.

V

A QUANTITATIVE ANALYSIS OF THE VARIABILITY IN

THE ACTIVITY OF NITRIFYING ORGANISMS IN A SOIL UNDER PASTURE

TABLE OF CONTENTS

Abstract ^{. . . . . . . . . . . . . . . . . .. . . . . . . . .. . . . . . . . . .. . . . . . . . .. .. . . . . . . . . . . . . . . .}i

Acknow ledgments ^{. . . . . . .. . . . .. . . . . . . .. . . . . . . . . . .. . . . . . . .. . . . . . . . . . . . . . . . .}iv

Table of contents ^{.. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . .}v

List o f Figures ^{. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . .}xi

List o f Tables ^{. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . .}xvi

SECTI ON I . INTRODUCTION CHAPTER 1. INTRODUCTION AND AIMS OF THE PROJECT ^{. . . . . . . . . . . . . . . .}1

i . Background to the proj ect - the nitrate leaching problem ^{. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .}1

i i . Nitrate leaching models ^{. . . . . . . . . . . . . . . . . . . . . . . . . . . . .}3

i i i . The aim of the proj ect ^{. . . . . . .. . . . . . . . . . . . . . . . . . . . . . .}5

CHAPTER 2 . NITRIFICATION IN SOILS: A REVIEW . . . ^. . . . 7

i . The factors a f fect ing ni tri f icat ion and

mineral i zation ^. . .^. . . ^.. ^. . . . ^{. .} . . ^{. . .} . . . ^{. .} . . . ^. . ^{. . . . .}9 i i . Mode l l ing of nitrif ication ^{. .} . . . . ^. . . . . ^. . . ^. . ^. . . ^{. .} . . . ^. 1 7 i i i . ^A comment on measured nitrif ication rates ^{. .} . . . ^{.. . .} . 2 3 iv . Conc lus ion . . . ^.. . . . ^. . . ^{. .} . . . .^. . ^{. . .} . .^{. .} . . . 2 4

CHAPTER 3 . A THEORETICAL CONSIDERATION OF SPATIALLY

DEPENDENT VARIABILITY . ^. . . ^. . . . -. . . 2 5

i . Why do we need geostatistics ? ^. . . . ^{. . . .} . . ^{. .} . . . 2 5

i i . Some pre l i minary data analysis using class ical

statist ics . . . ^.. . . . ^{.. .} . . . . ^. . . . ^{. . .} . . . ^... . ^. . . . ^. . . . ^. . . 28 i i i . Stationari ty and the semi -variance ^. . .^. . .^{. · .} . . . . ^. . . ^. . 3 2 i v . The var iogram . .^{. . . . .} . .^. . ^{. .} . ^. . ^{. . . .} .^{. . .} . . . ^{. .} . . . ^. . . . 3 5 v . Spatial variation in two dimens i ons .^. . ^{. .} . . ^{. . .} . . ^.. . . 40 vi . Other variogram models . . ^{. . . .} . ^{. . .} . ^{. .} . . .^. . . . ^. . . . . ^. . 4 7 v i i . Conclus ions ^. . ^.. ^{. .} . . . ^. . . . ^. . .^. . . . ^.. ^{. .} . . ^{. .}. . ^. . ^. . ^. 5 1

vi i CHAPTER 4. EXPERIMENTAL METHODOLOGY ^{. . . . . . . . .. .} . ^{. . . . . . . . . . . . . .} . 5 2

A . THE SHORT-TERM NITRIFICATION ASSAY . . . .^. . . . . ^{. .} . . ^. . . ^. . ^. . 5 2

i . Select ion of incubation media for SNA

analyses . . . 5 3

i i . Linearity of nitr i f ication rate in the

Tokomaru s i l t loam ^{. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .}5 4

i i i . Select ion o f ammonium subs trate concentration

for SNA analyses ^{. . . . . . . . . . . . . . . . . . ..} . ^{. . .} . ^{. . . . . .} . ^{. . .}5 8 iv . Ana lysis of exchangeable ammonium ^{. .. . .} . ^{. . . . ..} . . ^{. . . .}6 1

B . FIELD SAMPLING . ^{. . . . . . . . . . . . .} . ^{. .} . ^{. . . .} . ^{. . . .. .} . ^{. . . . . . . . .} . ^{. . .}6 2 i . S i te detai l s . ^{. .} . ^{. . . . . . . . . . . . . .} . ^{. . . .. .} . ^{. . . . . . . . .} . . . . 6 2 i i . Soil sampl ing ^{. . . . . . . . . .} . ^{. . . .} . . ^{. . . .. . . . . . .} . . ^{. . .} . ^{. . . .}6 3

i i i . Correlation between moisture contents o f

s i eved and uns ieved soil ^{. . . . .} . ^. . ^. . . ^{. ..} . ^. . . ^{. . . . .} . ^. . . 6 3

C . STORAGE OF SAMPLES PRIOR TO SNA MEASUREMENTS . ^{. .} . : . . . 6 7

i . Ef fects of drying and s torage on mineral

nitrogen ^{. . ..} . ^{. . . . .. .} . ^{. . . . . . . .} . ^.. ^. . ^{. . . . .} . ^{. . . . . . . . . . .}67 i i . Ef fects of drying and storage on soil biomass ^{. . . . . .}69

D . CONCLUS IONS ^{. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . ... . . . . . . . . . . .}7 6

SECTION I I . AN ANALYSIS OF SPATIAL VARIABILITY IN NITRIFIER 'ACTIVITY

CHAPTER 5 . VARIABILITY IN NITRIFIER ACTIVITY WITH DEPTH

AND DISTANCE ^{. . . . .. . .. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .}7 8

A . DEPTH DEPENDENT VARIABI LITY ^{. . . . .. . . . . . . . . . . . . . . . . . .} . ^{. . . . .}7 8 i . Methods and Materials ^{. . . . . . . .} . ^{. . . . .. .. . . . .} . ^{. . . . . . . .}7 9 i i . Results ^{. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .}8 1 i i i . D i scus s ion ^{. . . . . . . . . .. . . . . . . . . . . . . . . . . . .} . ^{. . . . . . . . . . .}9 3

B . SPATIALLY DEPENDENT VARIABILITY ^{. . . . .} , . . . , . . . 9 6 i . Methods and Materials ^{. . . . . .} . ^{. . . . . .} . ^{. . . . . . . . .}. ^. . ^{. . . .}9 7 i i . Results ^{. . . . . . . . . . . . . .. . . . . . . . . . .. . . . . .. . . . . . . . . . . . . .}9 8 i i i . Discuss ion ^{. . . . . .}. ^. . ^{. . .. . . . . . . . . . . . . . . .} . ^{.. . . . . . . . . .}1 0 8

CHAPTER 6 . SPATIAL VARIABILITY OF NITRIFIER ACTIVITY -

A MORE REFINED ANALYSIS ^{. . . . . .} , . . . 1 1 6

i . Methods and Materi als ^{. . . . . . . .} . ^{. . . . . . . . . . . . . . . . . . . .}1 2 1 i i . Resul t s ^{. . . . . . .} . ^{. . . .} .^{. . .} .^{. . . . . . . . . . . . . . . . . . . . . . . . . .}1 2 1 i i i . Di scuss ion ^{. .} . ^{. . . . . . . . .}. ^{. . . . .} . ^{. . . . .} . ^{. ..} . ^{. . . . .} . ^{. . . .} . 1 2 5 iv . Conc lus ions and recommendations for future

samp ling s trategy ^{.. . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. .}1 4 0

SECTION I I I . FACTORS AFFECTING NITRIFIER ACTIVITY

ix

CHAPTER 7 . THE EFFECT OF pH , MOISTURE AND TEMPERATURE ON

NITRIFIER ACTIVITY . . . . . . . . 1 4 4

i . Methods and Materials ^. . ^{. . . . . . . . . .} . ^{. .. . . . . . . . . . .} . . ^. 1 4 4 i i . Results ^{. .} . . .^. . ^{. . . . .. . .} ; . . . ; . . . . ·. ; . . . 1 4 9 i i i . Di scussion ^{. . . . .} . . ^{. . . .. .} . ^{. .. . . .} . ^{. .} . . ^. . ^. . ^{. . . .} .^. . ^{. .}. ^. 1 68 iv . Conclusions ^{. . . . . .}. ^{.. .. .} . ^. . . . ^. . ^{. . .}. . ^{. .} . ^{. . . . . .} . . . ^. 1 7 6

CHAPTER 8 . A FURTHER INVESTIGATION OF THE EFFECT OF

MOISTURE ON NITRIFIER ACTIVITY ^{. . .}. ^. . ^. . . ^. . ^{. .} . . ^{. .} . . . 1 7 7

i . Methods and Materials . . . 1 7 8 i i . Results ^{. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .}1 8 0 i i i . Di scussion . ^{. . . . . .} . ^{. . . . . . . . .} . . ^{. .} . .^{. .} . . ^{. .} . . ^{. . . . . .} . ^{. .}1 9 3

CHAPTER 9 . HETEROTROPHIC NITRIFICATION - EXACTLY WHAT

HAS BEEN MEASURED BY THE SNA ? ^{. ..} . ^. . . . ^{. . .} . . ^{. . .} . . . ^. 1 9 8

i . Methods and Materials ^{. .} . ^{. . . . .} . ^{. . . .}. ^{. . . . . . .} , . . . 2 0 1 i i . Results . ^.. ^. . ^{. . .} . ^{. .} . . ^{. . .} . . ^{. .. . . . . .}. . ^{. .} . .^. . ^{. . . .} . ^. . ^{. .}2 0 3 i i i . D iscuss ion . ^{. .} . ^{. . . . . . .} . ^{. . . . .} . ^. . ^. . .^{. . .} . ^{. . . . . .} . ^. . ^. . ^{. .}2 0 9 iv . Conclus ion ^{.. . .} . . ^{. . . .. . . . ... .} . ^{. . . . .} . ^. . ^. . ^{. . .} . ^{. .} . ^{. . .} . ^. 2 1 2

SECTION IV .

CHAPTER 10. GENERAL DISCUSSION, CONCLUSI ONS AND

RECOMMENDATIONS FOR FURTHER WORK ^{.. . .} . ^{. .} . ^{. .}, . . . 2 1 3

A . A CRITIQUE OF THE GEOSTATI STICAL TECHNIQUES USED FOR THE ASSESSMENT OF SPATI AL VARIABILITY IN

NITRIFIER ACTIVITY ^{. . . . . . . .. . .} . ^{. .} . .^{. . . . . . . . . . . .} . . ^{. . . . . . . .}2 1 3 i . Problems caused by ani sotropy ^{. ..} . ^. . ^{. . .}. ^{. . . .} . . ^. . ^{. . .}2 1 4

i i . Problems caused by changes in the sample variance and variation in the variogram model

w i th time and the scale o f sampl ing . . . 2 1 8

i i i . The relationship between the s i l l and the

sample variance ^{. . . . .. .} . ^{. . . . .} . ^. . . ^{. . . . . . . . . . . .} . . ^{. . . .}2 2 5 iv . Cros svariogram ana lys i s . . . : . . . 2 3 0

B . SOME CONCLUDING COMMENTS ON VARIABILITY IN

NITRIFIER ACTIVITY ^{. . . . . .} . ^{. .} . ^{. . .}. ^{. . .} . . ^{. . . . . . .} . ^{. . . . .} . ^{. . . . .}2 3 2

REFERENCES ^{. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . .. . . . . .}2 3 5 APPENDIX I . COMPUTER PROGRAMS ^{. . . . . . . .. . . . . . . . . . .} . ^{. .} . ^{. .} . ^{. . . . . .} . . . ^{. .} . 2 5 2 GAMMAH ^{. . . . . . . . . . .} . ^. .^{. . . . . . . . . . . . .. . .} . ^. . ^{. . . . . . . . .} . ^. 2 5 3 COVGM . . . 2 5 8

Figure 3 . 1 Figur e 3 . 2

F igur e 3 . 3

Figure 3 . 4

Figure 3 . 5

Figure 3 . 6

Figure 3 . 7

Figure 3 . 8

Figure 3 . 9

F igure 3 . 1 0

x i

LIST O F F IGURES

Distribut ion o f a set of 2 5 pH data . ^. . .^. . . ^. . ^. . . . 2 9

Change in the mean and variance w i th increas ing

sample number for a set of 25 pH data . . . ^. . . . ^. . 3 1

Experimenta l variogram for the 2 5 pH. data , and the number of pairs of points s eparated by each

lag ^{. .}. . . ^. . . . ^. . ^. . . . . ^. . . ^. . ^. . . . ^. . . . .^{. .} . . . 3 6

Change in the value of ¥ ( h ) as the number o f pairs A

of data points used to calculate it increases . . ^. . . . 3 8

Experimental variogram for soi l pH with linear models f i t ted by ordinary and least squares

optimi zat ion . ^. . . . ^. . ^. . . ^. . . . . ^. . . . ^. . . . ^. . . .^. 3 9

1 2 1 equally spaced pH data sampled f rom a regular

1 1 x 1 1 square grid . . . . ^.. . ^. . . . ^. . ^. . . . ^.. . . . 4 1

Distribution of the 1 2 1 pH data shown in Figure

3 . 6 . . . 4 2

Change i n the mean and variance with increas ing

sample number for the set of 1 2 1 pH data . . . ^. . . . ^. . 4 4

Change i n the number o f pairs o f points separated by each lag as the length of the grid s ide

increases from 1 to 1 0 lag uni ts . ^. . . ^{. .}. . . ^. . ^. . ^. . . . . ^{. .} . 4 5

Devel opment o f an experimental vari ogram a s the number of samples used to est imate it increases as the length of the grid side increases from 2

( 4 samples ) to 1 0 lag units ( 1 2 1 samples ) . ^. . .^. . . . ^. . 4 6

Figure 3 . 1 1

F igure 4 . 1

Figure 4 . 2

Figure 4 . 3

Figure 4 . 4

Figure 4 . 5

Figure 5 . 1

F i gure 5 . 2

Figure 5 . 3

Experimental var iogram for the 1 2 1 pH data f i tt ed with a spher ica l model by weighted least squares

opt imization . . . 5 0

L inearity o f ni trificat ion rate over 1 9 hours i n

the Tokomaru s i l t loam a t 2 2 oc . . . . ^. . . . 5 6

Mean monthly weather data ( 1 92 8 - 1 98 0 ) for the D . S . I . R Grasslands weather stat ion , Palmers ton

North ( N . Z . Meteorologica l service , 1 98 3 ) . . . 6 4

Corre lation between the gravimetric moisture contents of s ieved ( < 2mm ) and unsieved Tokomaru

s i l t loam . . . 6 6

p H optima curves for nitri f i er activity in fresh

soil and soil that had been s tored for 3 weeks . . . 7 4

Common pH opt imum curve f i tted t o SNA data for

fresh and stored soil . . . 7 5

Change i n bul k dens ity and mean volumetri c moisture content with depth in the Tokomaru s i l t loam

sampled in mid May . . . 8 2

Depth pro f i les of ( a ) SNA , ( b ) No3 - , ( c ) Ex-NH4 · , ( d ) incubation pH , ( e ) total carbon , ( f ) total nitrogen , ( g ) C/N ratio , ( h ) tota l phosphorus , and ( i ) % mineral N in the Tokomaru �i l t loam.

sampled in mid May . . . 8 4

Distribution with depth of total carbon , ni trogen , phosphorus , C/N ratio and SNA expressed as a % of

thei r maximum values . . . ... . . 92