ARTICLE IN PRESS

0038-0717/$ - se

doi:10.1016/j.so

�CorrespondIndooroopilly,

fax +617 3214

E-mail addr

Soil Biology & Biochemistry 40 (2008) 1013–1018

www.elsevier.com/locate/soilbio

Citation classics

Relationships between soil respiration and soil moisture

Freeman J. Cooka,b,c,�, Valerie A. Orchardd

aCSIRO Land and Water, 120 Meiers Road, Indooroopilly, Qld. 4074, AustraliabThe University of Queensland, Australia

cCooperative Research Centre for Irrigation Futures, AustraliadInstitute of Environmental Science and Research Limited (ESR), Kenepuru Science Centre, 34 Kenepuru Drive, PO Box 50-348, Porirua, New Zealand

Received 3 December 2007; accepted 5 December 2007

Available online 15 January 2008

Abstract

The interaction of soil microbes with their physical environment affects their abilities to respire, grow and divide. One of these

environmental factors is the amount of moisture in the soil. The work we published almost 25 years ago showed that microbial

respiration was linearly related to soil-water content and log-linearly related to water potential. The paper arose out of collaboration

between two young researchers from different areas of soil science, physics and microbiology. The project was driven by not only our

curiosity but also the freedom to operate without the constraints common to the current system of science management. The citation

history shows three peaks, 1989, 1999 and from 2002 to the present day. Interestingly, the annual citation rate is as high as it has ever

been. The initial peak is due to the application of the work to studies on microbial processes. The second peak is associated with

the rise of simulation modelling and the third with the relevance of the findings to climate change research. In this article, our paper is

re-evaluated in the light of subsequent studies that allow the principle of separation of variables to be tested. This re-evaluation lends

further credence to the linear relationship proposed between soil respiration and water content. A scaled relationship for respiration and

water content is presented. Lastly, further research is suggested and more recent work on the physics of gas transport discussed briefly.

Crown Copyright r 2007 Published by Elsevier Ltd. All rights reserved.

Keywords: Soil respiration; Soil moisture; Microbes; Soil physics; Soil biology

1. Introduction

It is worthwhile to start with the first line of the Orchardand Cook (1983) paper: ‘‘In the study of soil microbialactivity and its relationship to soil moisture, microbialecologists have rarely collaborated with physicists.’’ This isstill true to a certain extent today although a welcometrend towards multi-disciplinary research is making thesevaluable collaborations more common. Nonetheless, wewould contend that considerably more progress in the areaof soil ecology, microbiology, agronomy and soil physicscould be made by more collaboration especially if curiositydriven science was given funding.

e front matter Crown Copyright r 2007 Published by Elsevie

ilbio.2007.12.012

ing author at: CSIRO Land and Water, 120 Meiers Road,

Qld. 4074, Australia. Tel.: +617 3214 2840;

2855.

ess: freeman.cook@csiro.au (F.J. Cook).

Dr. Valerie Orchard and I started at DSIR Soil Bureauin 1976 on almost the same day and became friends. Valhad recently completed her PhD in the UK at NewcastleUniversity, and I had completed a post-doctoral diplomain soil science, a necessary step having come from a purescience background (physical chemistry and mathematicsat Massey University). At that time, the DSIR soil biologysection was strong with established researchers, such asDes Ross, John Stout, Kevin Tate and Gregor Yeates, andfuture stars including Val Orchard, Graham Sparling, TomSpeir and Andy West. I was lucky in that the soil physicssection was run by Rick Jackson and he allowed me a lot offreedom to pursue my own ideas. I went back to Massey in1978 and completed an M.Phil. in Soil Physics in 1980 andit was when I returned to Soil Bureau that Val and I startedto work together.The research had its beginnings in discussions and

arguments with Val about the inappropriate way that soilbiologists measured soil-water status using the water-holding

r Ltd. All rights reserved.

ARTICLE IN PRESSF.J. Cook, V.A. Orchard / Soil Biology & Biochemistry 40 (2008) 1013–10181014

capacity (WHC) method (see Section 3) when makingrespiration measurements. These exchanges often took placeat the dinner parties held by the biology section, and to whichI was invited as I was married at the time to a member of thisgroup. Val’s challenge was for us to ‘‘do it better’’ andmeeting this challenge led eventually to the 1983 paper.

In this short article, I will look at the interesting citationhistory of Orchard and Cook (1983), as its popularitycould not have been predicted at the time of publication. Iwill also re-examine the paper in the light of subsequentpublications and explain the research directions this hasled us.

2. Citation history

It is interesting to follow the history of citations for thepaper (Fig. 1). It shows an initial rapid rise to a peak in1990, which is associated with similar studies on microbialresponse to soil moisture (e.g., Ramsay and Orchard, 1984;Sparling et al., 1985; Groffman and Tiedje, 1988) followedby a decline until 1995. Most of the early citations in thetwo years following publication were by other researcherswithin the biology section of Soil Bureau. Taking intoaccount the modern obsession with impact factors (Mon-astersky, 2005), our paper would have had a veryrespectable individual impact factor of 10. The rate ofcitations rises again from 1995 through to a second peak in1999 in part due to the surge of papers presentingsimulation models of soil processes (e.g., Azzalini andDiggle, 1994; Svendsen et al., 1995; Qiu and McComb,1996). There is another dip in popularity until 2001 andthen a rise to the present peak which has continued since2003. This latest prolonged high water mark is due to thepaper being cited by climate change researchers (e.g.,Thornton and Rosenbloom, 2005; Campos, 2006) as wellas the modellers (e.g., Wang et al., 2004; Petersen et al.,2005) and those studying soil biological processes (e.g., Liu

Year

1980

Annual citation r

ate

(no. of citations)

2

4

6

8

10

12

14

16

Citation h

alf-life

0

100

200

300

400Annual citation rate

Citation half-life

1985 1990 1995 2000 2005 2010

Fig. 1. Annual citation rate with time from 1984 to 2007 and citation half-

life (h) (defined as h ¼ (y(t)�y0)Ci(t), where y(t) is the year in which

citations Ci(t) occurred and y0 is the year of publication) with time (t).

Total citations 231 (as of November 2007).

et al., 2005; Carlisle et al., 2006). The half-life (number ofcitation� years since publication) of papers in a journal isanother biblometric commonly used to assess a journalsquality. The continued high citation rate for our papersome 24 years since it is publication is of value to Soil

Biology & Biochemistry.

3. Soil-water status

In order to compare the effect of water on microbialrespiration, a measure of the soil-water status is required.There are two parameters that have been used in soilphysics to describe the soil-water status: the water contentand the soil-water potential (Hillel, 1980, pp. 123–147). Thewater content is the quantity of water, expressed as eithermass water per unit mass (w, [M M�1]) or volume of waterper total soil volume (y, [L3 L�3]. The soil-water potentialis the potential energy of the water compared to a referencestate per unit dry mass, weight or volume of soil. Soilbiologists had come up with another soil-water statusmethod, WHC (Ross and Boyd, 1970; Howard andHoward, 1979; Foster et al., 1980). WHC is measured byfilling a container with sieved soil, saturating this withwater, allowing it to drain to equilibrium, and thendetermining the water content on a mass basis. Thisnumber is taken as 100% WHC and the soil-water contentof other samples of the same soil are referenced to thisvalue. It is obvious that this method will not give consistentresults as WHC is dependent on a large number of factorsincluding the: degree of packing (dry bulk density), heightof the soil in the pot (as this will determine the soil-waterpotential profile), diameter of the pot, aggregation of thesoil, organic matter content, and the soil textural proper-ties. The limitation of this method has been recognised byearlier workers (Rixon and Bridge, 1968; Miller andJohnson, 1964). It is surprising to find the WHC methodstill in use an example being the recent paper by Castilloand Torstensson (2007).We then embarked on a series of experiments where we

measured respiration and the soil-water potential andwater content. It was both a fun and frustrating time, as Ispent considerable part of my time initially getting thedew point hygrometers to measure soil-water potentialaccurately calibrated. The efforts were worth it as is shownby the continuing interest in this paper over the last24 years.

4. Re-examination of results from Orchard and Cook (1983)

In the light of further developments and subsequentexperiments it is worthwhile re-examining our 1983 paper.Experiments were performed to measure the waterpotential, water content and respiration rate simulta-neously and at a constant temperature (T). The experi-ments were conducted at a constant temperature (T). Theresults showed clearly that respiration rate was a log-linearfunction of soil-water potential in the range over which the

ARTICLE IN PRESSF.J. Cook, V.A. Orchard / Soil Biology & Biochemistry 40 (2008) 1013–1018 1015

measurements were made. The relationship (R) betweensoil respiration and gravimetric water content (w) wasfound to be linear, and are rewritten here more generally as

R ¼ f ðwÞ. (1)

Soil Biology & Biochemistry was chosen as the appro-priate journal for the publication due to its internationalstatus. Although we did not initially consider the relation-ship of respiration with water content as of greatsignificance one of the referee’s suggested we should thinksome more about this; without doubt this close scrutinymade our publication better.

At the end of the experiments the soils were returned totheir original water content but to our surprise therespiration did not return to the value at the start of thestudy; in fact, it was as much as 60% lower for samplesexperiencing the lowest soil-water potential and varied withthe soil-water potential. It was suggested that thispermanent reduction, recorded on returning the watercontent to its original value, is due to a decline inrespiration rate with time as a result of substrate depletion.This means we have a functional relationship betweenrespiration (R) and water content (w) and time (t), i.e.:

R ¼ f ðw; tÞ, (2)

where f(w,t) is the relationship between time, water contentand respiration. It also implies that the relationship foundbetween R and w had the effects of a decrease in respirationas a function of time entangled in it. Here, time isaccounting for the decrease in available substrate due tomicrobial metabolism.

4.1. Separation of variables

In liming trials, it had been observed that the moisturecontent of the treated soil was greater than that of theuntreated soil, and that an organic mat of dead plantmaterial disappeared on liming (Edmeades et al., 1981;During et al., 1984). To see if these phenomena werelinked, further experiments measuring the relationshipbetween the respiration rate and the water content werecarried out (Cook et al., 1985). Experiments wereperformed using limed and un-limed soil equilibrated tothe same water potential. The limed soil had a higher watercontent and respiration rate than the un-limed soil andshowed that a single relationship existed between soilrespiration rate and water content.

To try and account for the decline in respiration withtime, one treatment in these experiments was kept at theinitial water content throughout the experiment. Thus anydecrease in respiration rate with time could be attributed tosubstrate depletion or other time-based effects, such aschanges in microbial numbers and species composition.These effects can be compensated for in the othertreatments where water content also varied with time ifthe principle of separation of variables is valid. The othertwo treatments (limed and un-limed) had their water

contents kept constant for the first 14 days and thenallowed to decrease. The relationship between respirationand time for all the treatments when the water content waskept constant were the same.If Eq. (2) can be split into two functions by the principle

of separation of the variables then:

R ¼ f ðw; tÞ ¼ f ðtÞgðwÞ (3)

and the data can be analysed for both the relationshipbetween R and t as well as R and w. The function g(w) inEq. (3), when w is constant (w�) also becomes a constanthence Eq. (3) then becomes:

R ¼ gðwnÞf ðtÞ. (4)

The relationship between R and t was found by regressionto be a power function:

f ðtÞ ¼ a0t�b ¼ gðwnÞat�b, (5)

where b was found to be 0.31, a0 ¼ ag(w�) and a is aconstant. The constant a0 was ignored in the subsequentanalysis by Cook et al. (1985), but will be used in theanalysis below to determine the validity of Eq. (3). Thelimed and un-limed treatments had different initial watercontents, 0.57 and 0.52 kg kg�1, respectively, which impliesthat g(w�) will be different and hence a0 should be differentfor the two treatments. This is in fact the case with valuesof a0 1.74 and 1.50 for the limed and un-limed treatments,respectively. If a is a constant then the ratio of these valuesof a0 represent the ratio of g(w� ¼ 0.57)/g(w� ¼ 0.52), andis 1.16.Dividing Eq. (4) by t�b results in

R=t�b ¼ agðwÞ ¼ acw� ad ¼ cnw� dn, (6)

where c� and d� are constants that were determined bylinear regression of R/t�b with w to give c� ¼ 11.9 andd� ¼ 1.5 for the units used in Cook et al. (1985).Substitution of the two values of w� for the limed andun-limed treatments into Eq. (6) allow the determination ofag(w�) and from these the ratio of g(w� ¼ 0.57)/g(w� ¼ 0.52) can again be determined and is 1.13.This gives two independent estimates of g(w� ¼ 0.57)/

g(w� ¼ 0.52), one from the decline of respiration with timeand the other from the decline of respiration with watercontent. The values for g(w� ¼ 0.57)/g(w� ¼ 0.52) arevirtually the same from both estimates indicating that theuse of separation of the variables is valid.The relationship found for f(t) suggests that second

order kinetics are involved in the reduction of respirationwith time. Second order kinetics is used to describe decayof microbial biomass in other models (Whitmore, 1996;Whitmore et al., 1997) but may not be universally valid.The linear decrease in respiration rate with w led another

group within the Soil Bureau led by Graham Sparling tostart work on this topic (West et al., 1988a, b). They foundthat the drop in respiration rate was initially due to adecrease in activity of the microbes. As the soil driedfurther the continuing decline was associated with a sharp

ARTICLE IN PRESSF.J. Cook, V.A. Orchard / Soil Biology & Biochemistry 40 (2008) 1013–10181016

reduction of in the number of viable microbes whenwo0.1 kg kg�1.

The decrease in water content is likely to be associatedwith a decrease in ‘habitable space’ for microbes to existwithin and also reduce the volume into which they candischarge there metabolic wastes (West et al., 1988a, b).There are still very many opportunities to explore andunderstand the processes that are occurring.

5. Other experiments

We resumed our research association in 1986 and someinvestigations were carried out at a site located on the SoilBureau farm at Lower Hutt, where long-term phosphatefertiliser trials had occurred (Orchard et al., 1992). In theseexperiments, field samples were brought to the laboratoryand the respiration rate, water content and water potentialwere measured at constant temperature. Although therewas more scatter in the data than in the earlier laboratoryexperiments, a linear relationship between water contentand respiration was again found. Further laboratorystudies of respiration were carried out and these showedsimilar, if less scattered, relationships between respirationand water content. Further studies were conducted tomeasure f(t) for these soils. In the fertile (Waikanae) soil apower relationship between respiration and time (substratedepletion) was again found, but there was no relationshipfor the low fertility (Pomare) soil (i.e. b ¼ 0 in Eq. (5)). ThePomare soil also respired at a lower rate than the Waikanaesoil and had a higher carbon content (Waikanae 3.9%;Pomare 5.6%). The tentative conclusion drawn was thatthe Pomare soil contained a microbial population that waslargely autochthonous. However, this work continued tosupport the validity of Eq. (3) and the linear relationshipbetween water content and respiration when aeration doesnot affect respiration.

We also performed a number of experiments that werenot been published, although with hindsight we should tryto get some of this into the literature. This was to an extentdue to major changes in research structures that took placein New Zealand from 1986 to 1993. We both changed roleswith one of us (Freeman Cook) immigrating to Australiaand the other (Valerie Orchard) moving to a role as Scienceand Research Manager in Environmental Science andResearch, one of the new Crown Research Institutes. I stillhave the data from these experiments and recently suppliedthem to a colleague in California, for a database he wascompiling.

There is one piece of information that was presented at aconference (Cook and Orchard, 1985) which readers mightfind useful. The relationship between respiration and watercontent can be scaled with reference values so long as weaccount for decreases in respiration due to time effects.This scaled relationship has of the form:

rðw; tÞ

rnðwn; tÞ¼ a

w

wnþ b

h i, (7)

where r�(w�,t) is the respiration at time, t, at the referencewater content w� and a and b are parameters. Cook andOrchard (1985) choose a value for w� as the value when thewater potential was �1m. This avoided the respirationbeing restricted by oxygen transport. This region of therespiration–water content relationship was found by Linnand Doran (1984) to be better described by a parabolicfunction. It would be easy enough to splice a parabolic andlinear function together to cover the total water contentrange.

6. Subsequent research

Following moving from New Zealand to Australia in1991 to take up an appointment at CSIRO Centre forEnvironmental Mechanics in Canberra, I started work onexamining the problem of oxygen transport. In this work,microbial respiration is treated as a distributed sink, i.e. therespiration is defined as a value per unit volume of soil andtransport to individual or communities of microbes is notdescribed in detail. This led to a number of publicationswith colleagues on soil aeration and root respiration and issummarised in Cook (2002). The area of the interfacebetween soil biology, physics and chemistry is one that stillfascinates me and increasingly others (e.g. IUSS Commis-sion 2.5) and is fertile ground for further research. Theproblem is nowadays getting funding to support suchstudies on research with no immediate benefits. I continueto work on the theoretical side of some of these issues(Cook and Kelliher, 2006; Cook et al., 2007). Our 1983paper is cited in some of our recent publications on root/soil respiration of carbon dioxide (Cook et al., 1998), soilaeration (Cook and Knight, 2003) and oxygen transportand rooting depth (Cook et al., 2007).

7. Future research

We would like to conclude with some ideas for researchthat some of you reading this may feel inspired to pursue.We can extend Eq. (3) for other variables that may controlto give

Rðz; t;T ;w; p;x;CÞ ¼ f ðtÞgðwÞhðTÞkðzÞlðpÞmðxÞnðCÞ, (8)

where h(T) is the relationship for temperature (T), k(z) isthe relationship with depth (z), l(p) is the relationship withpH (p), m(x) is the relationship with texture (x) and n(C) isthe relationship with substrate (C). Many of these relation-ships have been studied in isolation, so a good initialestimate of the functions will be possible. The novelty ofthe research lies in determining if it is possible to separatethe variables as shown in Eq. (8) and, if not, what is thecorrect form for Eq. (8)? We did explore this usinghysteresis in the water content/water potential relationshipto determine if water content was really the controllingvariable. At the time the effect of substrate depletion (time)complicated the results. However, given Eq. (3) and knowthat separation of the variables is valid it would be possible

ARTICLE IN PRESSF.J. Cook, V.A. Orchard / Soil Biology & Biochemistry 40 (2008) 1013–1018 1017

to use hysteresis to produce a soil sample at the same waterpotential but different water contents and explore thisproblem further. Our experiments indicated that watercontent was the controlling variable.

My concluding remarks echo a previous comment andare directed at those within the research funding commu-nity, especially economists, who have sometimes imposedthe sledgehammer of contested funding to crack the nut ofseemingly irrelevant and self-indulgent research. This topichas been covered with eloquence and humour by Philip(1991) and recently by Hamilton (2007). However, forthose who think they can pick winners in research projectsI doubt if the usefulness of our early 1980s research wasobvious and its application foreseen when the work started.The prospects of research and importance of simulationmodelling and climate change research were in theirinfancy in 1983. I sometimes wonder if some of the moneynow spent on the ‘machinery’ of contestable funding andaccountability would be better spent on a bit morecuriosity driven research, especially for researchers in theearly stages of their careers. This should be a time when wenurture their curiosity and fascination with the naturalworld by providing the space to make discoveries—andmistakes. It is interesting that similar sentiments wereexpressed by Jenkinson et al. (2004) in their citation classic!

Acknowledgement

The authors would like to thank Prof. Richard Burns forthe invitation to write this article and his useful commentswhich led to its improvement.

References

Azzalini, A., Diggle, P.J., 1994. Prediction of soil respiration rates from

temperature, moisture-content and soil type. Applied Statistics—

Journal of the Royal Statistical Society Series C 43, 505–526.

Campos, A., 2006. Response of soil surface CO2-C flux to land use

changes in a tropical cloud forest (Mexico). Forest Ecology and

Management 234, 305–312.

Carlisle, E.A., Steenwerth, K.L., Smart, D.R., 2006. Effects of land use on

soil respiration: conversion of oak woodlands to vineyards. Journal of

Environmental Quality 35, 1396–1404.

Castillo, M.D., Torstensson, L., 2007. Effect of biobed composition,

moisture and temperature on the degradation of pesticides. Journal of

Agriculture and Food Chemistry 55, 5725–5733.

Cook, F.J., 2002. Soil respiration and gas transport: modelling and

measurement of the phenomena. PhD thesis, University of Technol-

ogy, Sydney. 163pp.

Cook, F.J., Kelliher, F.M., 2006. Determining vertical root and microbial

biomass distributions from soil samples. Soil Science Society of

America Journal 70, 728–735.

Cook, F.J., Knight, K.H., 2003. Oxygen transport to plant roots:

modelling for physical understanding of soil aeration. Soil Science

Society of America Journal 67, 20–31.

Cook, F.J., Orchard, V.A., 1985. Soil water and microbial activity. In:

New Zealand Institute of Agricultural Science Convention, Water 85,

Abstracts, Lincoln College, Canterbury, 2–6 September 1985, p. S30.

Cook, F.J., Orchard, V.A., Corderoy, D.M., 1985. Effects of lime and

water content on soil respiration. New Zealand Journal of Agricultural

Research 28, 517–523.

Cook, F.J., Thomas, S.M., Kelliher, F.M., Whitehead, D., 1998. A model

of one-dimensional steady-state carbon dioxide diffusion from soil.

Ecological Modelling 109, 155–164.

Cook, F.J., Knight, J.H., Kelliher, F.M., 2007. Oxygen transport in soil

and vertical root distribution. Australian Journal of Soil Research 45,

101–110.

During, C., Jackson, B.L.J., Dyson, C.B., 1984. Lime effects on

hill country. 1. Effect of lime and monocalcium phosphate on

soil moisture. New Zealand Journal of Agricultural Science 15,

192–199.

Edmeades, D.C., Judd, M.J., Sarathchandra, S.U., 1981. Nitrogen

mineralisation as a mechanism for lime responses on two New

Zealand soils. Plant and Soil 60, 177–186.

Foster, N.W., Beauchamp, E.G., Corke, C.T., 1980. The influence of soil

moisture on urea hydrolysis and microbial respiration in Jack Pine

humus. Canadian Journal of Soil Science 60, 675–684.

Groffman, P.M., Tiedje, J.M., 1988. Denitrification hysteresis during

wetting and drying cycles in soil. Soil Science Society of America

Journal 52, 1626–1629.

Hamilton, S., 2007. Completing the loop: from data to decision and back

to data. Hydrologic Processes 21, 3105–3106.

Hillel, D., 1980. Fundamentals of Soil Physics. Academic Press, New

York, 413pp.

Howard, P.J.A., Howard, D.M., 1979. Respiration of decomposing litter

in relation to temperature and moisture. Oikos 33, 457–465.

Jenkinson, D.S., Brookes, P.C., Powlson, D.S., 2004. Measuring soil

microbial biomass. Soil Biology & Biochemistry 36, 5–7.

Linn, D.M., Doran, J.W., 1984. Effect of water-filled pore space on

carbon dioxide and nitrous oxide production in tilled and nontilled

soils. Soil Science Society of America Journal 48, 1267–1272.

Liu, Q., Peng, S.L., Bi, H., Zang, H.Y., Li, Z.A., Ma, W.H., Li, N.Y.,

2005. Decomposition of leaf litter in tropical and subtropical forests of

Southern China. Journal of Tropical Forest Science 17, 543–556.

Miller, R.D., Johnson, D.D., 1964. The effect of soil moisture tension on

carbon dioxide evolution, nitrification, and nitrogen mineralization.

Soil Science Society of America Proceedings 28, 644–647.

Monastersky, R., 2005. The number that’s devouring science. The

Chronicle of Higher Education, 52, /http://chronicle.com/weekly/

v52/i08/08a01201.htmS.

Orchard, V.A., Cook, F.J., 1983. Relationship between soil respiration

and soil moisture. Soil Biology & Biochemistry 15, 447–453.

Orchard, V.A., Cook, F.J., Corderoy, D.M., 1992. Field and laboratory

studies on the relationships between respiration and moisture for two

soils of contrasting fertility status. Pedobiologia 36, 21–33.

Petersen, B.M., Berntsen, J., Hansen, S., Jensen, L.S., 2005. CN-SIM—a

model for the turnover of soil organic matter. I. Long-term carbon and

radiocarbon development. Soil Biology & Biochemistry 37, 359–374.

Philip, J.R., 1991. Soils, natural science and models. Soil Science 151,

91–98.

Qiu, S., McComb, A.J., 1996. Drying-induced stimulation of ammonium

release and nitrification in reflooded lake sediment. Marine and

Freshwater Research 47, 531–536.

Ramsay, A.J., Orchard, V.A., 1984. Soil respiration and numbers in soil

under pasture 5 years after removal of topsoil. New Zealand Journal of

Science 27, 413–417.

Rixon, A.J., Bridge, B.J., 1968. Respiratory quotient arising from

microbial activity in relation to matric suction and air filled pore

space of soil. Nature 218, 961–962.

Ross, D.J., Boyd, I.W., 1970. Influence of moisture and aeration on

oxygen uptakes in Warburg respiratory experiments with litter and

soil. Plant and Soil 33, 251–256.

Sparling, G.P., Whale, K.N., Ramsay, A.J., 1985. Quantifying the contri-

bution from the soil microbial biomass to the extractable P-levels

of fresh and air-dried soils. Australian Journal of Soil Research 23,

613–621.

Svendsen, H., Hansen, S., Jensen, H.E., 1995. Simulation of crop

production, water and nitrogen balances in 2 German agroecosystems

using the daisy model. Ecological Modelling 81, 197–212.

ARTICLE IN PRESSF.J. Cook, V.A. Orchard / Soil Biology & Biochemistry 40 (2008) 1013–10181018

Thornton, P.E., Rosenbloom, N.A., 2005. Ecosystem model spin-up:

estimating steady state conditions in a coupled terrestrial carbon and

nitrogen cycle model 189, 25–48.

Wang, K.Y., Kellomaki, S., Zha, T.S., Peltola, H., 2004. Component

carbon fluxes and their contribution to ecosystem carbon dioxide

exchange in a pine forest an assessment based on eddy covari-

ance measurements and an integrated model. Tree Physiology 24,

19–34.

West, A.W., Sparling, G.P., Speir, T.W., Wood, J.M., 1988a. Dynamics of

microbial C, N-flush and ATP and enzyme activities of different

textured soils subject to gradual drying. Australian Journal of Soil

Research 26, 217–229.

West, A.W., Sparling, G.P., Speir, T.W., Wood, J.M., 1988b. Dynamics of

microbial C, N-flush and ATP and enzyme activities of gradually dried soils

from a climosequence. Australian Journal of Soil Research 26, 519–530.

Whitmore, A.P., 1996. Alternative kinetic laws to describe the turnover of

the microbial biomass. Plant and Soil 181, 169–173.

Whitmore, A.P., Gunnewick, H.K., Crocker, G.J., Klir, J., Korschens,

M., Poulton, P.R., 1997. Modelling the turnover of carbon in soil

using the Verberne/MOTOR model. Geoderma 81, 137–151.