seasonal dynamics of carbon and nitrogen pools and fluxes under continuous arable and ley-arable...

Seasonal dynamics of carbon and nitrogen pools andfluxes under continuous arable and ley-arable rotationsin a temperate environment

D. V. MURPHY*, E. A. STOCKDALE, P. R. POULTON, T. W. WILLISON & K. W. T. GOULDING

Soil Science Department, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK

Summary

Improved understanding of the seasonal dynamics of C and N cycling in soils, and the main controls on

these fluctuations, is needed to improve management strategies and to better match soil N supply to crop N

demand. Although the C and N cycles in soil are usually considered to be closely linked, few data exist

where both C and N pools and gross N fluxes have been measured seasonally. Here we present measure-

ments of inorganic N, extracted soluble organic N, microbial biomass C and N, gross N fluxes and CO2

production from soil collected under wheat in a ley-arable and continuous arable rotation within

a long-term experiment. The amounts of inorganic N and extracted soluble organic N were similar

(range 5–35 kg N ha�1; 0–23 cm) but had different seasonal patterns: whilst inorganic N declined dur-

ing wheat growth, extracted soluble organic N peaked after cultivation and also during maximal stem

elongation. The microbial biomass was significantly larger in the ley-arable (964 kg C ha�1; 0–23 cm)

than the continuous arable rotation (518 kg C ha�1; 0–23 cm) but with no clear seasonal pattern. In

contrast, CO2 produced from soil and gross N mineralization showed strong seasonality linked to soil

temperature and moisture content. Normalization of soil CO2 production and gross N mineralization

with respect to these environmental regulators enabled us to study the underlying influence of the

incorporation of fresh plant material into soil on these processes. The average normalized gross rates of

N mineralized during the growing season were 1.74 and 2.55 kg N ha�1 nday�1 in continuous arable

and ley-arable rotations respectively. Production rates (gross N mineralization, gross nitrification) were

similar in both land uses and matched rates of NH4þ and NO3

� consumption, resulting in periods

of net N mineralization and immobilization. There was no simple relationship between soil CO2

production and gross N mineralization, which we attributed to changes in the C : N ratio of the

mineralizing pool(s).

Introduction

In annual cropping systems the main sources of carbon (C) for

soil microorganisms are crop residues and rhizodeposition dur-

ing crop growth; decomposition also provides the bulk of their

nitrogen (N). Neither these inputs nor the climatic variables

that affect microbial activity are temporally constant. It is

therefore to be expected that the C and N pools in soil and

fluxes between them would also vary through the year. We need

a better understanding of the seasonal dynamics of C and N

cycling in soils, and their main controls, to improve manage-

ment strategies that match soil N supply to crop N demand.

Most studies of seasonal dynamics have focussed on crop

growth and N uptake, the fluctuations in soil inorganic N pools

and/or net rates of N mineralization. Such field data have been

widely used for the evaluation of C and N cycle models (e.g.

Shaffer et al., 2001). However, few studies have made detailed

measurements of the seasonal dynamics of microbial biomass,

soluble organic N or the gross rates of C and N fluxes in the

soil-plant system. Research models encapsulate hypotheses

about system dynamics and their controls and a number of

models include microbial biomass and/or calculate gross rates

of C and N cycling implicitly or explicitly (Shaffer et al., 2001).

It is important that field data are made available to test these

hypotheses (models) robustly, before they are developed for

applied use, for example to support fertilizer recommendation.

*Present address: Soil Biology Group, School of Earth and Geo-

graphical Sciences, Faculty of Natural and Agricultural Sciences, The

University of Western Australia, Crawley, WA 6009, Australia

Correspondence: D. V. Murphy. E-mail: [email protected]

Received 20 March 2006; revised version accepted 22 June 2007

1410# 2007 The Authors

Journal compilation # 2007 British Society of Soil Science

European Journal of Soil Science, December 2007, 58, 1410–1424 doi: 10.1111/j.1365-2389.2007.00946.x

Leaching of soluble organic matter from soil has been shown

to cause a significant decline in gross N mineralization rates

(Cookson & Murphy, 2004), suggesting that including the tem-

poral dynamics of soluble organic N pools in studies of C andN

cycling is important for improving our prediction of soil N

supply. It is well known that the dynamics of the soil microbial

biomass in a range of agricultural systems is strongly dependent

on climate, with only small fluctuations in temperate climates

(Wardle, 1992). However, Ross et al. (1995) concluded that the

activity of the microbial biomass, and consequently potential

rates of C and N mineralization, were more susceptible to sea-

sonal effects than the size of the microbial biomass pool itself.

Hoyle & Murphy (2006) found that the size of this pool was

strongly related to historical treatment differences in crop resi-

due retention, whereas microbial activity (CO2 production and

gross N mineralization) and microbial community structure

were strongly linked to seasonal variations in temperature and

soil moisture content. In temperate climates, decomposition

(CO2 production) and gross N mineralization in arable soils

are primarily controlled by temperature (Recous et al., 1999),

while under grassland Jamieson et al. (1998) found soil mois-

ture content was the dominant environmental variable con-

trolling nitrification. In grasslands the dynamics of C and N

fluxes have been measured in situ in cores containing growing

plants (e.g. Corre et al., 2002). Studies in arable soils have usu-

ally been made in fallow soils with and without residue incor-

poration (Jensen et al., 1997; Recous et al., 1999). However, we

know of no previous study that has made detailed measure-

ments of CO2 fluxes and gross N mineralization rates in situ

under winter wheat in a temperate climate.

We therefore quantified the magnitude of C and N pools, CO2

production and gross N fluxes under a crop of winter wheat in

continuous arable and ley-arable rotations under a temperate

climate. Our aim was both to improve our fundamental under-

standing of the seasonal dynamics of key C and N pools in soil

and to generate a data set that would allow more complete eval-

uations of process-based models of soil C and N dynamics.

Materials and methods

Site

Long-term experiments provide sites for monitoring and/or

hypothesis testingwhere previousmanagement is verywell char-

acterized andwhere the impact of the environment (temperature

and moisture), current crop inputs and previous crop residues

canbe resolvedunder field conditions (Poulton, 1996).We there-

fore selected plots in contrasting rotationswithin theLey-Arable

experiment at Woburn (Bedfordshire, UK), a long-term field

experiment established in 1938with awell-documentedmanage-

ment andyield history.The soil is a sandy loamwith 68.4%sand,

18.2% silt and 13.4% clay (Stackyard series; Catt et al., 1979,

Cambic Arenosol; FAO, 2006). Johnston (1973) gave full

details of the rotations and results from the first 32 years of the

experiment. Plots receive adequate lime and fertilizer to target

soil chemical values of soil pH at c. 7, exchangeable Kþ > 250

mg kg�1, exchangeable Mg2þ > 50 mg kg�1 and available P

(Olsen) at c. 45 mg kg�1, which is sufficient for all arable crops.

In August 1996, we identified plots for detailedmeasurements

where therewas likely to be a very significant difference in the net

N dynamics in an unfertilized wheat crop due to the previous

management history. Plots followed either an 8-year fertilized

cut grass ley (plots 69 and 70, in ley for 16 of the previous 18

years; the ley-arable rotation) or an arable rotation of spring

barley, spring barley, winter beans (plots 67 and 68, which had

been in arable crops since 1938; the continuous arable rotation).

The plots used are in Block V of themain experiment. Plots with

andwithout previous applications of FYM,which had ceased in

1966, were treated as if they were replicate plots of the ley-arable

and continuous arable rotations. Soil characteristics for the

plots immediately before our experiment began anddetailed plot

history for the three previous years cropping are given in

Table 1.

In October 1996, all plots were ploughed, rolled and drilled to

winter wheat, Triticum aestivum, cv. Hereward. During 1997,

the winter wheat crop received no N fertilizer, but was other-

wise managed as a conventional crop of winter wheat. Crop

observations during the experiment indicate that the period of

maximal growth/stem elongation took place during April and

May 1997, with anthesis in mid-June. Grain yields at harvest

(16 August 1997) were 1.91, 1.26, 2.27 and 2.79 t ha�1 for

plots 67 to 70, respectively. Using the measured grain %N, N

uptake above ground was estimated to have been 42, 30, 50

and 57 kg N ha�1 for plots 67 to 70, respectively, during this

season. Plots were ploughed on 23 September 1997.

A meteorological station is maintained approximately 2 km

from the field experiment and meteorological variables are

measured daily, including air and soil temperatures, rainfall

and potential evapotranspiration. Average annual rainfall is

630 mm; the actual rainfall and temperature during the experi-

ment are presented in Figure 1.

Sampling scheme

Within the plots (4.27 � 9.34 m) an area of 4.27 � 2.8 m was

allocated for the collection of soil samples through the season;

this left sufficient area for reliable estimation of crop yield with

a small-plot combine harvester. There were 14 main sampling

dates (Figure 1) between 1 October 1996 (before cultivation)

and 30 September 1997 (after harvest). An additional four sam-

plings were made opportunistically for some measurements in

response to environmental or management changes giving

increased sampling intensity at times when C and/or N cycling

was expected to change rapidly, for example the sampling 3

weeks after cultivation was immediately followed by a major

rainfall event (> 25 mm) and sampling for some pools and pro-

cesses was repeated. Intact soil cores and composite bags of

disturbed soil were collected on each occasion according to the

Seasonal dynamics of C and N fluxes in arable soil 1411

# 2007 The Authors

Journal compilation # 2007 British Society of Soil Science, European Journal of Soil Science, 58, 1410–1424

requirements of the measurements; details are given with each

method below.

Inorganic N and soluble organic N

Triplicate samples from 0–23, 23–60 and 60–90 cm soil layers

were taken in each field plot using a 2.5 cm diameter split barrel

soil auger. Soil was sieved (< 4mm) in the laboratory and gravi-

metric moisture content was determined (105°C, 24 hours). Thecontents of inorganic N (NH4

þ-N and NO3�-N) and total oxi-

dizable organic N were determined for KCl extracts (50 g fresh

soil; 100 ml 2 M KCl; Whatman No 1 filter paper). Soil

extracts were frozen immediately for storage and defrosted at

room temperature prior to analysis. Total oxidizable N was

determined after K4S2O8 oxidation of the extract. Both inor-

ganic N and total oxidizable N were analysed by flow injection

analysis (SANPLUS System, Skalar, Breda, The Netherlands).

Extracted soluble organic N is the difference between total

oxidizable N and inorganic N originally in the KCl extract.

Microbial biomass

A composite sample from the 0–23 cm soil layer (c. 1 kg) was

collected from each field plot using a 2.5 cm diameter auger

and sieved (< 4 mm) in the laboratory. Microbial biomass-C

and N were determined by the chloroform fumigation and

extraction method (Brookes et al., 1985) using three labora-

tory replicates from each field replicate. Briefly, fresh soil

equivalent to 45 g dry weight was fumigated with ethanol-free

chloroform for 24 hours and then extracted for 1 hour (100 ml

0.5 M K2SO4; Whatman No 42 filter paper). Equivalent sam-

ples of soil were extracted without fumigation. Extracted solu-

ble organic N was determined on the extracts as described

above. Organic-C was determined in soil extracts by high-

temperature oxidization using a TOC200 (Analytical Sciences,

Cambridge, UK) after removal of carbonates. Microbial

biomass-C and -N were calculated from the flush of dissolved

organic C (or N) resulting from fumigation using a conversion

factor of 2.22 (Jenkinson et al., 2004).

CO2 production

Fresh soil from the 0–23 cm soil layer (100 g, broken roughly in

the field to < 4 mm) was placed in 1 litre gas tight chambers

(three replicates per field plot) with a vial containing 10 ml of

1.0 M NaOH. Chambers without soil were also incubated in

parallel. The chambers were buried in the field and incubated

for 7 days. After the incubation the alkali was analysed for

CO2 (trapped as CO32�) by titration between pH 8.3 and 3.7

with 0.5 M HCl on a pH titrator (Radiometer, Copenhagen,

Denmark). Production of CO2 from soil was calculated from

the accumulated CO2 adjusted for background.

Table 1 Topsoil characteristics (0–23 cm) measured as part of a routine monitoring programme in spring 1996. Plot management and crop yields for

previous 3 years (1994–96)

Plot No Rotation Manurea Year Crop

Sowing dateb N applied Harvest date Yield Organic C Total N Olsen P pH

(1:5 water)/kg ha�1 /t ha�1 /% /mg g�1

67 Continuous arable FYM

residues

1994 S. barley 21/03/94 80 21/08/94 4.80

1995 S. barley 24/03/95 80 07/08/95 4.80

1996 W. beans 23/10/95 0 13/09/96 4.00 1.01 0.103 50 7.1

68 Continuous arable No FYM 1994 S. barley 21/03/94 80 21/08/94 4.80

1995 S. barley 24/03/95 80 07/08/95 4.80

1996 W. beans 23/10/95 0 13/09/96 4.00 0.94 0.088 38 7.0

69 Ley-arable rotation No FYM 1994 Grass 15/05/89 150d 08/06/94 3.24

02/09/94 1.05

1995 Grass 150d 13/06/95 4.14

19/12/95 0.25

1996 Grass 150d 13/06/96 5.30 1.00 0.098 44 7.0

11/10/96 0.20

70 Ley-arable rotation FYM

residues

1994 Grass 15/05/89 150d 08/06/94 4.51

02/09/94 1.32

1995 Grass 150d 13/06/95 4.99

19/12/95 0.25

1996 Grass 150d 13/06/96 6.05 1.17 0.115 60 7.0

11/10/96 0.28

a38 t ha�1 of farmyard manure (FYM) was applied every fifth year to split plots from 1938–66; no FYM has been applied since 1966.bSowing date for ley-arable plots is the original sowing date for the 8 year ley.cYields of grain for arable crops at 85% DM, farm estimates, total DM yields for grass measured directly.dSplit application (2 � 75 t ha�1).

1412 D. V. Murphy et al.

# 2007 The Authors


Net N mineralization (intact cores)

Eight intact soil cores were collected from each field plot using

steel sleeves (5.5 cm diameter, 30 cm length). The intact cores

were buried in a sand bed at field temperature for 7 days starting

on the day of soil sampling. The upper 0–23 cm of four replicate

cores was removed on days 0 and 7 and thoroughly mixed prior

to extraction and measurement for NH4þ-N and NO3

�-N

(above). Rates of net N mineralization were calculated as the

difference between the amount of inorganic N (NH4þ-N þ

NO3�-N) on days 0 and 7.

Gross N mineralization (intact cores)

Twelve intact coreswere collected from each field plot using steel

sleeves (5.5 cm diameter, 30 cm length). The cores were uni-

formly labelled with 15NH3 (0–24 cm) in the laboratory with

an injector system, which had been validated on this soil by

Murphy et al. (1999). The injector delivers a mixture of NH3

gas (8%) and air (92%) into the soil through seven needles.

We modified the system slightly to deliver gas into the bottom

section of the core (24 to 12 cm deep) and then with a second

set of needles to the top section (12 to 0 cm deep). The 12 cores

collected allowed 4 replicate cores for extraction at each time

interval and 4 additional cores to enable some to be discarded

if the injection of 15NH3 was disrupted by stones. The mixture

of 1.5% 15NH3 at 99.9 atom%, 6.5% 14NH3 at natural abun-

dance and air (92%) was prepared in a 2-litre Teflon gas bag

attached to the injector manifold. After NH3 loss during

the injection process, approximately 2–3.5 mg 15N labelled

NH4þ-N kg�1 was retained in the soil at the beginning of the

incubation depending on the soil moisture content. Cores were

incubated in a sand bed at field temperature and sampled

destructively after 24 hours and then after either 48 or 72

hours to enable calculation of gross N mineralization rates

(the longer incubation period was used when the soil was cold

or dry).

The 0–23 cm layer of soil was removed in the laboratory and

the remainder discarded. Soil was immediately sieved (< 4mm),

mixed and sub-sampled. We extracted NH4þ-N and NO3

�-N

(80 g fresh soil; 160 ml 2 M KCl; Whatman No 1 filter paper)

and determined NH4þ-N and NO3

�-N concentrations by flow

injection analysis (above). The 15N : 14N isotopic ratio of the

inorganic N in the extracts was obtained after diffusion of

NH4þ-N and NO3

�-N on to separate acidified glass fibre

disks, by a modified procedure of Brooks et al. (1989). Where

necessary a 14N spike was used to reduce 15N enrichments to

within suitable detection limits for the mass spectrometer and

reference materials. We determined the 15N : 14N ratio by mass

spectrometry (Europa Tracermass in series with a Europa

Roboprep C-N analyser: Europa Scientific, Crewe, UK).

Gross N mineralization and ammonium consumption rates

were calculated from the change in size, and decline in atom%15N excess, of the 15N-labelled NH4

þ pool as a result of the

0

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Date

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nfal

l /m

m

-10

-5

0

5

10

15

20

25

30

Tem

pera

ture

/°C

5 131211109876432 141

Figure 1 Rainfall (mm; vertical bars) and air temperature (°C; continuous line) for Woburn Experimental Farm for the period of the experiment.

Main sampling dates are indicated by arrows.


# 2007 The Authors


mineralization of soil derived organic-N at natural abundance

(Kirkham & Bartholomew, 1954; Murphy et al., 2003).

Gross nitrification (intact cores)

Soil cores were collected as described for gross Nmineralization

with four replicate cores for each extraction time. A secondNH3

gas injector system was modified to deliver 15N labelled KNO3

solution into the 0–24 cm layer of the soil cores by a two-layer

injection process as described above. The bore diameter of

each syringe was reduced by stainless steel inserts and new

plungers so that a smaller volume could be delivered through

each needle. We applied 15N labelled KNO3 solution at a rate

of 2.5 mg N kg�1 at 10.3 atom% 15N. Cores were injected in

the laboratory, incubated in a sand bed at field temperature

and sampled destructively (above) after 24 and 168 hours. We

extracted NH4þ-N and NO3

�-N and determined atom% 15N

excess as above. Gross nitrification and nitrate consumption

rates were calculated from the change in size, and decline in

atom % 15N excess, of the 15N-labelled NO3� pool as a result

of the oxidation of NH4þ at natural abundance (Kirkham &

Bartholomew, 1954; Murphy et al., 2003).

Calculation of the climatic factor

Soil temperature (at 20 cm under grass) was recorded at the

meteorological station on the farm. The average soil tempera-

ture from October 1996 to September 1997 was 10.5°C. Soiltemperature (Figure 2) depended strongly on air temperature

(Figure 1) and showed a similar seasonal pattern. Gravimetric

soil moisture content wasmeasured on the samples taken for the

determination of inorganic and extracted soluble organic N.We

converted measured soil moisture contents to water potential

using a moisture potential curve constructed from data given

for the plots by Catt et al. (1979).

We used the climatic factor model (Andren & Paustian, 1987)

to estimate the effect of soil temperature and water potential on

soil CO2 production and gross N mineralization rates. In this

model, rates of decomposition and mineralization are stan-

dardized by reference to optimal soil moisture (;0.01 MPa)

and a reference soil temperature and expressed as normalized

days (nday) so that underlying treatment effects can be investi-

gated (Table 2). We used a reference temperature of 10°C,which is close to the mean annual soil temperature, and also

permits direct comparison with the C and N transformation

rates normalized by Recous et al. (1999). The data were nor-

malized for temperature, assuming an Arrhenius relationship

describing the temperature dependence of the rate constant,

and taking a Q10 value of 3, as found by several authors for

decomposition and/or mineralization over the temperature

range covered in this experiment (Fierer et al., 2003; Janssens

& Pilegaard, 2003).

The kinetic characteristics of gross nitrification rates are

known to be distinct from those measured for decomposition

(Hoyle et al., 2006). In addition, studies of the effects of tem-

perature and water potential on nitrification have shown sig-

nificant differences between soils and sites (e.g. Russell et al.,

2002) and also highlighted the overriding impact of NH4þ

availability (Willison et al., 1998). Consequently, we have not

attempted to normalize gross nitrification rates.

0.00

0.05

0.10

0.15

0.20

0.25

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

Days since cultivation

Gra

vim

etric

moi

stur

e co

nten

t /g

g-1

0

10

20

30

Tem

pera

ture

/°C

Continuous arableLey–arableSoil temperature

Figure 2 Gravimetric moisture content (g g�1) for continuous arable

(plots 67, 68) and ley-arable (plots 69, 70) rotations and soil tempera-

ture (°C; 10 cm depth) measured at the adjacent weather station.

Table 2 Separate soil moisture content (H2O) and temperature (T)

factors used in a climatic factors model (f (H2O, T)) to normalize soil

CO2 production and gross N mineralization. Soil moisture was scaled

between 0 (permanent wilting point) and 1 (field capacity and above;

assuming greater soil moisture contents did not have a negative effect

on decomposition). Temperature was adjusted to a normalized day of

10°C using a Q10 ¼ 3

Days since

cultivation

Continuous arable Ley-arable

H2O T f (H2O, T) H2O T f (H2O, T)

�11 0.53 1.19 0.63 0.19 1.19 0.22

�5 0.63 1.20 0.75 0.26 1.20 0.31

0 0.52 1.12 0.58 0.33 1.12 0.38

1 0.44 1.11 0.49 0.19 1.11 0.21

7 0.81 1.12 0.90 0.64 1.12 0.71

21 1.00 1.11 1.11 1.00 1.11 1.11

28 1.00 0.65 0.65 1.00 0.65 0.65

38 1.00 0.51 0.51 1.00 0.51 0.51

49 1.00 0.54 0.54 1.00 0.54 0.54

83 1.00 0.50 0.50 1.00 0.50 0.50

118 1.00 0.75 0.75 1.00 0.75 0.75

146 0.56 0.95 0.53 0.67 0.95 0.64

182 0.88 1.12 0.98 0.77 1.12 0.86

202 0.52 2.12 1.10 0.44 2.12 0.92

230 1.00 1.47 1.47 1.00 1.47 1.47

251 0.54 2.61 1.40 0.52 2.61 1.35

279 0.23 3.76 0.88 0.40 3.76 1.49

314 0.78 1.61 1.25 0.78 1.61 1.25


# 2007 The Authors


Statistical analyses

All variables were tested for normality of the distribution and

transformed when necessary (CO2 production, inorganic N,

extracted soluble organic N) to minimize deviance. Because we

sampled repeatedly from small plots, analysis of variance

(ANOVA) with a repeated measures design was used to account

for the possible auto-correlation within the resulting time

series (Webster & Payne, 2002). However, in this case repeated

measures ANOVA is an incomplete analysis as the residual

degrees of freedom between subjects are fewer than the num-

ber of samplings so that we could not test for symmetry of the

covariance. Consequently, we also made a two-way ANOVA

analysis taking account of treatment, sampling time and their

interactions.

Results and discussion

Inorganic N and soluble organic N

At the beginning of the experiment, the topsoil (0–23 cm) con-

tained 17.7 kg inorganic N ha�1 following beans in the continu-

ous arable rotation, significantly more than in the, as yet,

uncultivated ley in the ley-arable rotation (7.4 kg N ha�1). After

cultivation (day 0) there was a rapid increase of inorganic N in

the topsoil in both rotations (Figure 3a,e). The measured inor-

ganic N profiles (Figure 3) suggest that N was leached: sequen-

tial peaks of inorganic N were measured in the 0–23 cm layer of

the continuous arable rotations around day 25, in the 23–60 cm

layer around day 50 and in the inorganic N content of the 60–

90 cm layer between days 50 and 75. A much broader peak is

seen in the ley-arable rotation after the cultivation of the ley,

which suggests greater losses of N by leaching in this treatment,

as commonly found after the ploughing of grassland (Lloyd,

1992). The movement of inorganic N by leaching is strongly

linked to the rainfall pattern (Figure 1).

During cropgrowth in springand early summer, inorganicN in

all soil layers is depleted.Minimumconcentrationsof inorganicN

were measured in all soil layers between days 225 and 250, that is

around anthesis (Figure 3), after which inorganic N increased in

all soil layers. During crop senescence nutrient uptake slows and,

as the soils remain relatively dry, any N released through miner-

alization accumulates as inorganicN. By the time leaching begins

in autumn, soils can contain tens of kilograms of inorganic N.

More inorganic N was measured in the ley-arable (60 kg N ha�1,

0–90 cm) than in the continuous arable rotation (29 kg N ha�1)

at the end of the experiment. This supports the finding that

leaching losses remain large for more than one season after

grassland is ploughed out to arable cultivation (Lloyd, 1992).

Extracted soluble organicN represents a pool ofNof the same

magnitude as inorganic N in these (Figure 3) and similar arable,

grassland andwoodland soils (e.g. Jones &Willett, 2006). There

was significantly more soluble organic N extracted from the

topsoil of the ley-arable rotation than the continuous arable

plots during the season (Figure 3). However, there was no sig-

nificant difference in the size of the soluble organic N pool

extracted from the 23–60 and 60–90 cm layers. After cultivation

and residue incorporation (around day 25) a peak of extractable

soluble organic N is seen in the 23–60 and 60–90 cm soil layers.

Large flushes of extractable soluble organic N, contributing up

to 90% of the total dissolved N pool, have previously been

reported after cultivation and reseeding of grasslands (Bhogal

et al., 2000). This peak of extractable soluble organic N is tran-

sitory and had disappeared by day 75. A second peak occurred

simultaneously in all soil layers in May (day 175–200) at the

same time as rapid stem elongation and might be associated

with below-ground inputs during root elongation (Swinnen

et al., 1995). Soil CO2 production (below) suggests that root

exudation rather than mineralization of organic matter is the

source of this peak. We could not determine whether the

extracted soluble organic N had been mineralized and taken

up by the crop, whether the crop used a component of the sol-

uble organic N directly, or indeed whether other microbial

consumptive processes caused the decline. The seasonal dy-

namics of the extracted soluble organic N (Figure 3) suggest

that it is composed of at least two pools: (i) stabilized and/or

slowly decomposable soluble organic matter resulting in a con-

stant background concentration and (ii) more mobile and/or

labile soluble organic matter produced through the season by

residue decomposition and root exudation. Others (e.g. Jones

et al., 2004) have also proposed that the soluble organic matter

pool is comprised of a rapidly decomposable fraction that

turns over in < 1–5 days, and a slowly decomposable fraction

that may persist for several years.

Jones & Willett (2006) found that there was little difference

between the amount of N and C recovered in soluble organic

matter where either 0.5 M K2SO4 or 2 M KCl were used as

extractants. In our study the topsoil soluble organic N concen-

tration in non-fumigated 0.5 M K2SO4 extracts (data not

shown) was not strongly correlated with the soluble organic N

concentration extracted with 2 M KCl (r ¼ 0.28). However, as

topsoil soluble C and soluble N were both determined in the 0.5

M K2SO4 extract, we have used these data to estimate the C : N

ratio of the extracted soluble organic pool (Figure 4). These

C : N ratios averaged 7.6 in the continuous arable and 6.7 in the

ley-arable plots, with an overall minimum of 4 and maximum

of 11. Jones & Willett (2006) report a similar range of soluble

organic C : N ratios (7–17) in grassland and woodland soils

from Wales. Our data indicate that there is significant fluctua-

tion in the C : N ratio of the soluble organic pool between sam-

plings, with a smaller C : N ratio in the period 100–175 days

immediately before the peak in the soluble organic N pool in all

the soil layers. While attention has rightly been focussed on the

dynamics of inorganic N in soil, these data on the amount and

quality of the soluble organic pool in soil confirm that it is

highly dynamic; further assessment of both the composition

and the dynamics of the soluble organic N pool and its contri-

bution to soil N supply are warranted.


# 2007 The Authors


0

5

10

15

20

25

30

35

40

45

50

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325 -25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325 -25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

N /k

g ha

-1

0

5

10

15

20

25

30

35

40

45

50

0

5

10

15

20

25

30

35

40

45

50

N /k

g ha

-1

0

5

10

15

20

25

30

35

40

45

50

05

101520253035404550

N /k

g ha

-1N

/kg

ha-1

05

101520253035404550

67 - Inorganic N 68 - Inorganic N

67 - SON 68 - SON

69 - Inorganic N 70 - Inorganic N

69- SON 70 - SON

(a) (e)

(b) (f)

(c)

(d) (h)

(g)

Continuous arable 0 - 23 cm Ley arable 0 - 23 cm

Continuous arable 23 - 60 cm Ley arable 23 - 60 cm

Continuous arable 60 - 90 cm

Continuous arable 0 - 90 cm

Ley arable 60 - 90 cm

Ley arable 0 - 90 cm

0

20

40

60

80

100

0

20

40

60

80

100


-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325


Figure 3 Inorganic N (kg N ha�1) and extractable soluble organic N (SON; kg N ha�1) for the 0–23, 23–60, 60–90 and 0–90 cm soil layers for

continuous arable (a–d; plots 67, 68) and ley-arable (e–h; plots 69, 70) rotations. Symbols represent field plot means, bars represent standard

errors of the plot mean. The solid line represents the treatment mean for inorganic N and dashed lines represent the treatment mean for SON.

Numbers in legends are field plot numbers.


# 2007 The Authors


Microbial biomass

The microbial biomass pool is significantly larger in the ley-

arable (average 964 kg C ha�1 and 122 kg N ha�1) than in the

continuous arable rotation (average 518 kg C ha�1 and 92 kg

N ha�1; Figure 5). In both rotations the seasonal fluctuation

in microbial biomass N was approximately 60 kg N ha�1

(Figure 5). On day 118 (late winter) microbial biomass C was

at a minimum and subsequently doubled during crop growth

(Figure 5). Joergensen et al. (1994) and Kaiser et al. (1995) also

showed increases in microbial biomass during the growing sea-

son under wheat in a temperate climate. Campbell et al. (1999)

linked changes in microbial biomass C to both precipitation

and inputs from crop residues and rhizodeposition. In contrast

Patra et al. (1990) showed almost constant microbial biomass

pools through the year under wheat.

Before cultivation the C : N ratio of the microbial biomass in

both plots was between 6 and 8. During the winter wheat phase

of the rotations the average C : N ratio for all plots was 4.9, with

no significant difference between the continuous arable and ley-

arable rotations. Joergensen et al. (1994) recorded a similar

biomass C : N ratio (mean 5.5) under winter wheat in a sea-

sonal study.

CO2 production

Prior to cultivation, rates of CO2 production were significantly

greater in the ley (20 kg CO2-C ha�1 day�1) than in the arable

plots (12 kg CO2-C ha�1 day�1, Figure 6a,b). Normalization

of the CO2 production data (Figure 6c,d) highlights the very

significant difference between CO2 production in an estab-

lished ley and under arable cultivation. During the winter

wheat phase of the rotation (from cultivation to harvest) there

were also significantly greater rates of total CO2 production in

0

2

4

6

8

10

12

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325Days since cultivation

Plot 67Plot 68

67-68

(a)

C:N

rat

io

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325


Plot 69Plot 70

69-70

C:N

rat

io

0

2

4

6

8

10

12(b)

Figure 4 Carbon : nitrogen ratio (C : N) of the extractable soluble

organic pool (0.5 M K2SO4) in the topsoil (0–23 cm) of (a) continu-

ous arable (plots 67, 68) and (b) ley-arable (plots 69, 70) rotations.

Symbols represent field plot means whilst lines represent treatment

means. Numbers in legends are field plot numbers.

0

200

400

600

800

1000

1200

1400

Days after cultivation

Mic

robi

al b

iom

ass

/kg

ha-1

67 - MBC

67 - MBN

68 - MBC

68 - MBN

67-68 MBC

67-68 MBN

69 - MBC

69 - MBN

70 - MBC

70 - MBN

69-70 MBC

69-70 MBN

(a)

0

200

400

600

800

1000

1200

1400

Mic

robi

al b

iom

ass

/kg

ha-1

(b)

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

Days after cultivation-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

Figure 5 Microbial biomass-C (MBC; kg C ha�1) and microbial bio-

mass-N (MBN; kg N ha�1) in the topsoil (0–23 cm) of (a) continuous

arable (plots 67, 68) and (b) ley-arable (plots 69, 70) rotations. Sym-

bols represent field plot means, bars represent standard errors of the

plot mean. The solid line represents the treatment mean for MBC

and dashed lines represent the treatment mean for MBN. Numbers in

legends are field plot numbers.


# 2007 The Authors


the ley-arable (mean 17 kg CO2-C ha�1 day�1) than in the

continuous arable rotation (mean 9 kg CO2-C ha�1 day�1).

Tillage is often associated with a temporary increase in CO2

production. We observed that inversion tillage (0–23 cm) on

14 October 1996 caused an immediate reduction in the rates of

CO2 production and strongly increased variability in the meas-

urements made in both rotations for a short time (Figure 6).

Jensen et al. (1997) and Recous et al. (1999) measured increa-

ses in CO2 production where crop residues (oilseed rape and

wheat straw, respectively) were mixed into fallow soil in

autumn in similar climates. Residue quality, the extent of con-

tact between crop residue and soil, and the size of crop residue

fragments are critical factors in controlling the kinetics of resi-

due decomposition (Sparling et al., 1995).

The rate of CO2 production increased from day 83 to reach

a maximum in mid-summer (day 230; Figure 6a,b). At day 280

slow CO2 production in both treatments was associated with

dry soil (Figure 2). In fallow soil, Recous et al. (1999) did not

measure any increased CO2 production in spring and summer,

but Jensen et al. (1997) did measure a second peak of CO2 pro-

duction in mid-summer. Rochette et al. (1999) also found

greater CO2 production during summer in fallow soil and

under maize, and highlighted the importance of temperature in

controlling CO2 production. Their measurement of d13C abun-

dance in the CO2 produced indicated that plant-derived C con-

tributed significantly to CO2 production, with the pattern

coincidental with the seasonal variation in root density. Nor-

malization of the CO2 production data during the period of

crop growth (Figure 6c,d) reduces, but does not completely

remove, the increase in CO2 production during the period of

root elongation and hence increased exudation. Our data

therefore highlight the importance of: (i) temperature and soil

moisture content in controlling the increase in CO2 production

in spring in temperate climates and (ii) the contribution of

root and root exudate derived CO2 as opposed to CO2 pro-

duction from decomposition of native soil organic matter.

0

10

20

30

40



CO

2-C

/kg

ha-1

day

-1

Plot 67

Plot 68

67-68

Plot 69

Plot 70

69-70

Plot 67

Plot 68

67-68

(a)

0

10

20

30

40



Plot 69

Plot 70

69-70

(b)

0

20

40

60

80

100

0

20

40

60

80

100

CO

2-C

/kg

ha-1

nda

y-1

Normalized Normalized(c) (d)

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325 -25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

Figure 6 CO2 production (kg C ha�1 day�1) in the topsoil (0–23 cm) of (a) continuous arable (plots 67, 68) and (b) ley-arable (plots 69, 70) rota-

tions. Normalized CO2 production rates (kg C ha�1 nday�1) are also presented for (c) continuous arable and (d) ley-arable rotations. Symbols

represent field plot means whilst lines represent treatment means. Numbers in legends are field plot numbers. Bars represent standard errors of the

plot mean.


# 2007 The Authors


Net N mineralization (in situ)

Both net N mineralization and net immobilization rates (indi-

cated by a negative net N mineralization rate) were measured

during the experiment, but the rates were small, indicating that

mineralization-immobilization turnoverwas balanced.Therewas

no significant difference in net Nmineralization between the con-

tinuous (range: �0.60 to 0.60 kg N ha�1 day�1; average 0.08 kg

N ha�1 day�1) and ley-arable (range: �0.84 to 0.94 kg N ha�1

day�1; average 0.21 kg N ha�1 day�1) rotations nor was there

any clear seasonal pattern (Figure 7a,b). This is not entirely

consistent with the actual inorganic N profiles (Figure 3) and

suggests that the 7-day incubation period was too short to

quantify changes in net N mineralization rates accurately.

Gross N mineralization and NH4þ consumption

Approximately 54% of the total 15N labelled NH3 gas retained in

soil was recovered as NH4þ-N after 24 hours; 10–48% was

recovered as NH4þ-N at the end of the incubation (Table 3 gives

a typical data set). 15N enrichment of the NH4þ pool did not

decline below 2.8 atom%; markedly above natural abundance,

thus enabling the robust calculation of rates of gross N minerali-

zation and NH4þ consumption (Wessel & Tietema, 1992).

Prior to cultivation, gross N mineralization rates were not

significantly different in the ley (1.76 kg N ha�1 day�1) and

arable plots following beans (2.69 kg N ha�1 day�1, Figure 7

c,d). However, there was an increase in gross N mineralization

rates following tillage and incorporation of the ley (days 0–49),

and gross N mineralization rates were significantly faster

during this period in the ley-arable (6.56 kg N ha�1 day�1)

than continuous arable rotations (0.94 kg N ha�1 day�1).

Recous et al. (1999) measured increased rates of gross N min-

eralization immediately following incorporation of straw: in

contrast, we did not observe faster gross N mineralization

rates following cultivation of the bean residues (Figure 7c,d).

GrossNmineralization rates showed significant seasonal fluc-

tuation (P ¼ 0.009). Gross mineralization rates were generally

slow throughout the winter with an increase in summer

between days 200 and 230 (Figure 7c,d). In arable soils under

similar climates, both Nishio & Fujimoto (1989) and Recous

et al. (1999) found increased rates of gross N mineralization

in summer. In contrast, in grassland Jamieson et al. (1998)

observed slower rates of gross N mineralization in summer

due to moisture limitation, while Corre et al. (2002) found no

clear seasonal trend.

We used the climatic factor to take out the effects of soil tem-

peratureandmoisture content (Figure 7e,f), and foundnochange

in the rateof grossNmineralization throughout the experiment in

the continuous arable rotation, and in the ley-arable rotation

from day 83 onwards (after the peak associated with ley incorpo-

ration). Normalization of these data therefore confirms the

importance of the incorporation of readily decomposable crop

residues (here ley) as a controlling variable for soil N dynamics.

The average normalized gross rate of N mineralization (between

days 83 and 279) is 1.74 kg N ha�1 nday�1 in the continuous

arable rotation and 2.55 kg N ha�1 nday�1 in the ley-arable

rotation. The rates of N mineralization measured here are faster

than those found by Recous et al. (1999) in fallow soils with and

without straw incorporation. Murphy et al. (1999) previously

measured similar gross N mineralization rates equivalent to

1.14 kg N ha�1 nday�1 in the continuous arable rotation (under

barley) at this site and a rate equivalent to 4.09 kg N ha�1

nday�1 in the sixth year of a grass-clover ley.

Normalization of the grossNmineralization data (Figure 7e,f)

also confirms that changes in gross N mineralization rates

can be linked to patterns of soil temperature and moisture con-

tent. For example, the fast rate on day 230 (Figure 7c,d) is linked

to the soil reaching optimumwater potential for biological activ-

ity, while the soil was warm (Table 2). Interactions between soil

temperature andmoisture content during the season can be used

to explain much of the temporal variation in the rates of gross N

mineralization in a range of climates (e.g. Nishio & Fujimoto,

1989; Hoyle & Murphy, 2006).

The calculated rates of NH4þ consumption (in the presence

of added NH4þ) were significantly faster in the ley-arable than

the continuous arable rotation (P < 0.001) and were season-

ally variable (P ¼ 0.003, Figure 7c,d). Ammonium consump-

tion rates were greater or equal to the rates of gross N

mineralization throughout the experiment (P < 0.001), con-

firming both large microbial demand for NH4þ (Davidson

et al., 1990) and that this demand is rarely met. Consequently,

residence times for the NH4þ pool are very short, for example

before cultivation in the grass ley 2.6 days, after beans 0.6 days

and under winter wheat in both plots it was < 1 day. These

residence times are in line with those measured by Murphy

et al. (1999) in a range of temperate arable and grassland rota-

tions and Hatch et al. (2000) in temperate grassland systems.

Gross nitrification and nitrate consumption

Across all sampling dates, recovery of 15N from added K15NO3

solution in the soil NO3� pool after 24 hours was, on average,

72% and at 7 days had declined to 48% on average (Table 3

gives a typical data set). No NH4þ substrate was added to the

incubations and consequently the measured rates represent

actual gross nitrification rates (Willison et al., 1998) and are

slower than the corresponding rates of gross N mineralization.

Cultivation caused a significant but short-term increase in

gross nitrification rates (P < 0.001, Figure 8a,b). Significantly

faster gross nitrification rates were measured on day 279 after

the harvest of winter wheat (P < 0.001, Figure 8a,b) and were

associated with the rewetting of the soil (Figure 2) and resulted

in large NO3� pools (Figure 3). The average rate of gross nitri-

fication for the experimental period was significantly slower

(0.62 kg N ha�1 day�1) in continuous arable than in the ley-

arable plots (0.97 kg N ha�1 day�1, P < 0.001). Recous et al.

(1999) measured much faster rates, equivalent to the rate of


# 2007 The Authors


gross mineralization measured in their study, and larger seasonal

variation in gross nitrification, but because their incubations

added NH4þ they measured potential rather than actual rates.

Calculated NO3� consumption rates (in the presence of

added NO3�, 0.5–1 kg N ha�1 day�1) are not significantly dif-

ferent to the gross nitrification rates (t ¼ 1.09 with 43 degrees

of freedom, P ¼ 0.282), which suggests that the microbial

demand for NO3� could be met by gross nitrification. Conse-

quently, residence times for NO3� are longer than for NH4

þ,

that is 7 days before cultivation in the grass ley, 20 days after

beans and an average of 13 days under winter wheat in both

plots, excluding the impact of plant demand. These are longer

-4.0

-2.0

0.0

2.0

4.0

-4.0

-2.0

0.0

2.0

4.0


N /k

g ha

-1 d

ay-1

(a)67 - Net mineralization68 - Net mineralization67-68

69 - Net mineralization70 - Net mineralization69-70

(b)

-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

0

2

4

6

8

10

N /k

g ha

-1 d

ay-1

(c)

68 - Mineralization

67 - Mineralization

67-68 Mineralization

67 - Consumption

68 - Consumption

67-68 Consumption

70- Mineralization 69 - Mineralization

69-70 Mineralization

69 - Consumption70 - Consumption69-70 Consumption

0

2

4

6

8

10

(d)

Days since cultivation-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

67 - Mineralization68 - Mineralization67-68

69 - Mineralization70 - Mineralization69-70

0

5

10

15

20

25

30

35

N /k

g ha

-1 n

day-1

Normalized(e)

0

5

10

15

20

25

30

35Normalized

(f)


-25 0 25 50 75 100 125 150 175 200 225 250 275 300325


-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325



Figure 7 Net N mineralization (kg N ha�1 day�1) rates in the topsoil (0–23 cm) of (a) continuous arable (plots 67, 68) and (b) ley-arable (plots

69, 70) rotations. Gross N mineralization (kg N ha�1 day�1) and NH4þ consumption (kg N ha�1 day�1) rates in the topsoil (0–23 cm) of (c) con-

tinuous arable and (d) ley-arable rotations. Normalized gross N mineralization rates (kg N ha�1 nday�1) are also presented for (e) continuous

arable and (f) ley-arable rotations. Symbols represent field plot means whilst lines represent treatment means. Numbers in legends are field plot

numbers. Bars represent standard errors of the plot mean.


# 2007 The Authors


residence times than previously reported by Hatch et al. (2000)

for temperate grassland systems (< 4 days).

Relationships between C and N fluxes

Strong linear relationships have been found between soil CO2

production and gross N mineralization, which indicates that C

availability is an important control on N cycling rates in soil

(Schimel, 1986; Hart et al., 1994). We found no simple linear

relationship between soil CO2 production and the N released

during mineralization (R2 ¼ 0.10; all data) or for continuous

arable and ley-arable plots separately (R2 ¼ 0.16 and 0.05

respectively). Recous et al. (1999) also found no relationship

between gross N mineralization and CO2 produced in straw-

amended soils, although they did find a strong relationship in

unamended soils. Variations in both the C : N ratio of the

mineralizing substrate and/or a change in microbial C use effi-

ciency will affect the relationship, and only if both are con-

stant (or their product is constant) will soil CO2 production

and gross N mineralized show a simple linear relationship

(Hart et al., 1994). Murphy et al. (2003) proposed that the rela-

tionship between soil CO2 production and gross N mineraliza-

tion should be represented by:

Gross mineralization

ðrate or amount of N mineralizedÞ ¼ 1

1 � ESoil CO2 productionðrate or amount of C producedÞ

C : N ratio of mineralizing pool;

ð1Þ

where E is the microbial C efficiency.

Luxhøi et al. (2006) showed that this relationship held during

the early phase of decomposition of plant residues with a wide

range of C : N ratios. Microbial C efficiency is affected by soil

texture (Schimel, 1986), mineralizing substrate/residue type (Hart

et al., 1994; Mueller et al., 1997) and temperature (Henriksen

& Breland, 1999). Assuming a typical range for microbial C

efficiency (40–60%), Equation (1) can be used to estimate the

average C : N ratio of the pools being mineralized in our

experiment (Hart et al., 1994). As expected the C : N ratio of

mineralizing material is not constant throughout the season;

more N is mineralized per unit C after the incorporation of

crop residues in autumn than during spring and summer under

the growing winter wheat (Figure 9). Continuous arable and

ley-arable rotations show similar temporal patterns (Figure 9).

The average C : N ratio of the mineralizing material in all plots

assuming a microbial C efficiency of 40% is 9.9 (assuming

60%, 14.8) and lies between the C : N ratios typically measured

for particulate organic matter (17–20, Campbell et al., 1999) and

those for soluble organic matter (7–17, Jones & Willett, 2006;

4–11, this study) and the microbial biomass (4–6, this study).

Conclusions

We expected that the C and N pools in soil and fluxes between

them would vary through the year in response to changes in

weather and management. Under a crop of winter wheat in

continuous arable and ley-arable rotations in a temperate

climate we have shown that:

1 The pool of extracted soluble organic N, which was of simi-

lar size to inorganic N, showed marked seasonal dynamics.

These indicate that extracted soluble organic N includes both

Table 3 A typical set of 15N data for a sampling period to show the change in 15N enrichment, and size of the unlabelled (AT) and 15N-labelled (AL)

NH4þ and unlabelled (NT) and 15N-labelled (NL) NO3

� pools after application of either 15NH3 to determine gross N mineralization or 15N-labelled

KNO3 to determine gross nitrification rates in soil. 15N-labelled (NH4)2SO4 was applied at 4.6 mg N kg�1 at 19.0 atom% 15N and the recovery of15N in the NH4

þ pool is reported. 15N-labelled KNO3� was applied at 2.5 mg N kg�1 at 10.3 atom% 15N and the recovery of 15N in the NO3

� pool

is reported. Means of four replicates with SE in brackets

Plot Timea

Applied 15N labelled NH3 Applied 15N labelled NO3�-N

15N enrichment AT AL

Recovery of

applied 15N in

NH4þ pool 15N enrichment NT NL

Recovery of

applied 15N in

NO3� pool

/atom % excess /mg N kg�1 /% /atom% excess /mg N kg�1 /%

67 To 12.86 (0.51) 3.65 (0.18) 2.40 (0.13) 52.25 (2.81) 4.00 (0.16) 4.89 (0.37) 1.96 (0.11) 78.3 (4.4)

Tt 10.67 (0.97) 2.73 (0.27) 1.46 (0.03) 31.63 (0.75) 2.87 (0.15) 6.51 (0.27) 1.87 (0.07) 74.8 (2.7)

68 To 14.27 (0.20) 3.57 (0.12) 2.61 (0.07) 56.74 (1.42) 3.41 (0.29) 6.34 (0.86) 2.10 (0.15) 83.9 (6.1)

Tt 13.34 (0.26) 3.05 (0.06) 2.08 (0.04) 45.31 (0.90) 2.28 (0.25) 6.87 (0.64) 1.58 (0.21) 63.2 (8.5)

69 To 12.20 (0.92) 4.29 (0.22) 2.65 (0.06) 57.64 (1.41) 6.55 (0.05) 2.38 (0.14) 1.57 (0.10) 62.7 (4.1)

Tt 9.34 (0.30) 4.30 (0.53) 2.04 (0.20) 44.35 (4.41) 2.16 (0.35) 2.31 (0.50) 0.51 (0.15) 20.3 (6.0)

70 To 12.22 (0.59) 5.44 (1.33) 3.30 (0.62) 56.37 (2.90) 6.67 (0.22) 2.48 (0.23) 1.66 (0.10) 66.2 (4.1)

Tt 8.56 (0.73) 4.53 (0.25) 2.00 (0.22) 43.87 (4.87) 2.30 (0.63) 1.88 (0.59) 0.27 (0.12) 10.8 (4.7)

aTo ¼ initial soil extraction period, which was 24 hours after 15N application to soil; Tt ¼ final soil extraction period, which, in this example, was 72

hours for soil where 15N labelled (NH4)2SO4 was applied and 168 hours for soil where 15N labelled KNO3 was applied.


# 2007 The Authors


labile soluble organic matter, which is added through the sea-

son by residue decomposition and root exudation, and more

slowly decomposing fraction(s). Experiments such as this may

allow the further development of C and N cycle models to

include pools of soluble organic C and N.

2 The size of the microbial biomass pool is relatively stable

in these temperate cropping systems. Significantly larger mi-

crobial biomass pools were maintained in the soil under ley-

arable rotation than under continuous arable production.

3 Microbial activity exhibited large within-season variation.

The strong dependence of C and N fluxes on both soil temper-

ature and soil moisture content, and their interaction in this

environment, was clearly demonstrated.

4 Normalization using soil temperature and moisture content

reveals the importance of microbial substrate availability as

a controlling factor for decomposition and mineralization pro-

cesses. This arises from historical differences as a result of crop

rotation and as a result of within-season variability (associated

with root elongation). We have also shown that the C : N ratio

of the mineralizing pool varies through the season. Improved

understanding of the relationship between C and N cycling

(which will show a simple linear relationship in limited circum-

stances only) therefore requires a better understanding of how

both microbial C efficiency and the quality of microbial sub-

strates vary throughout the year.

5 Long-term experiments, such as the Woburn Ley-Arable

Experiment used here, are unique and invaluable platform

sites for examining soil processes under stable, well-managed

and documented conditions.

Acknowledgements

Wewould like to acknowledge the analytical andfield support of

James Wakefield, Julie Baker, Sharon Fortune, Mirjam Pulle-

man, Ann Bhogal, Warwick Dunn, Wendy Wilmer, Maureen

Birdsey and JeanneDay. This research was partly funded by the

UK Ministry of Agriculture, Fisheries and Food. Rothamsted

Research receives grant-in-aid from the Biotechnology and Bio-

logical Sciences Research Council of the UK.

0

1

2

3

4

N /k

g ha

-1 d

ay-1

67 - Nitrification

68 - Nitrification

67-68 Nitrification

68 - Consumption

67 - Consumption

67-68 Consumption

69 - Nitrification

70 - Nitrification

69-70 Nitrification

69 - Consumption

70 - Consumption

69-70 Consumption

(a)

0

1

2

3

4

N /k

g ha

-1 d

ay-1

(b)



-25 0 25 50 75 100 125 150 175 200 225 250 275 300 325

Figure 8 Gross nitrification (kg N ha�1 day�1) and NO3� consump-

tion (kg N ha�1 day�1) rates in the topsoil (0–23 cm) of (a) continu-


Symbols represent field plot means whilst lines represent treatment

means. Numbers in legends are field plot numbers. Bars represent

standard errors of the plot mean.

0

5

10

15

20

25

30

-25 250 50 75 100 125 150 175 200 225 250 275 300 325


C:N

rat

io

(a)

40%

60%

0

5

10

15

20

25

30

C:N

rat

io

(b)

60%

40%

-25 250 50 75 100 125 150 175 200 225 250 275 300 325

Days after cultivation

Figure 9 Calculated range of the C : N ratio of the mineralizing

material assuming a microbial C efficiency of either 40 or 60% and

using the equation reported by Murphy et al. (2003) for (a) continu-



# 2007 The Authors


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seasonal dynamics of carbon and nitrogen pools and fluxes under continuous arable and ley-arable...

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