Agriculture, Ecosystems and Environment 199 (2014) 77–84

Comparing N2O fluxes from recently created extensive grasslands andsites remaining under intensive agricultural management

C.A. Horrocks a,*, J.A.J. Dungait b, K.V. Heal a, L.M. Cardenas b

a School of GeoSciences, The University of Edinburgh, Crew Building, West Mains Road, Edinburgh EH9 3JN, UKb Sustainable Soil and Grassland Systems Department, Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK

A R T I C L E I N F O

Article history:Received 26 June 2014Received in revised form 21 August 2014Accepted 25 August 2014Available online xxx

Keywords:Nitrous oxideExtensificationAgricultural managementEcosystem servicesLand-use

A B S T R A C T

The creation of extensive grasslands is encouraged under the current European Union CommonAgricultural Policy (CAP), with the aim of enhancing the provision of ecosystem services, including thereduction of greenhouse gas (GHG) emissions. Nitrous oxide (N2O) is a potent GHG strongly associatedwith the use of nitrogen (N) fertiliser application in agriculture, but soil conditions (temperature andmoisture) are also considered to be important drivers of N2O fluxes. Management strategies to reduceN2O emissions are urgently required to inform the sustainable agricultural practice. N2O fluxes fromrecently created (<10 years) extensive grassland on former intensive arable land in South East Scotlandand on former intensive grassland in South West England were compared with paired sites remainingunder long-term intensive management. N2O fluxes, soil temperature (Ts), water filled pore space (WFPS)and available N were measured in a range of grassland soils over three years. N2O emissions weregenerally <50 g N ha�1 day�1 under all types of management. Maximum emissions were not explained bychanges in Ts or WFPS. Lower N2O emissions were not observed in the extensified grasslands compared tolong-term intensively managed sites. These findings suggest that the extensification of grasslands maynot reduce GHG emissions for at least 10 years following extensification. This enables cost-benefitanalyses of agri-environment schemes promoting enhanced ecosystem service provision from recentlyestablished species-rich grasslands (SRG) to be made.

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1. Introduction

The substantial increase in global food production since themid-20th century is largely due to advances in fertiliser, pesticideand plant breeding technologies (Dungait et al., 2012). However,management to maximise production has been shown to cause adecline in other Ecosystem Services (ES; MA, 2003) including theenhancement of nitrous oxide (N2O) emissions (Mosier et al.,1991), N2O is a potent greenhouse gas (GHG) with a globalwarming potential 296 times that of carbon dioxide (CO2;mass-for-mass) and an atmospheric residency time of �120 years(Van Groenigen et al., 2010). Approximately 42% of global N2Oemissions are derived from agriculture through the use ofnitrogenous fertiliser (IPCC, 2007). The percentage of total N2Oemissions from agriculture varies by region and may be muchgreater than the global average: it is 78% and 63% in the USA andEuropean Union (EU), respectively (Johnson et al., 2007).

* Corresponding author. Tel.: +44 1873883500.E-mail address: claire.horrocks@rothamsted.ac.uk (C.A. Horrocks).

http://dx.doi.org/10.1016/j.agee.2014.08.0180167-8809/ã 2014 Elsevier B.V. All rights reserved.

In the EU, the common agricultural policy (CAP) providessubsidies to farmers through the European Agricultural Fund forRural Development (EAFRD) to support management changes thatenhance the provision of non-production ES (DEFRA, 2011).Included in the activities funded by the EAFRD are the creationand restoration of extensively managed species-rich grasslands(SRGs) on former intensively managed (IM) sites (EC, 2009). UKfarmers entering into these agri-environment schemes must agreeto manage land as extensive grassland for between 5 and 10 years,during which time limited or no fertilisers, herbicides or pesticidesmay be applied. On entering the scheme the existing crop orgrassland sward is usually ploughed up and the area re-sown witha species-rich seed mix. A recent review of the ES provision byextensified grasslands in the UK suggested that the GHG emissions(including N2O) may remain elevated for many years as a legacyeffect of intensive management (Horrocks et al., 2014). Such alegacy effect has been observed in a short grass steppe ecosystem(Colorado, USA), where the elevated N2O emissions observedunder a wheat-fallow cropping system remained 8 years afterreversion back to native grassland (Mosier et al., 1997). To ourknowledge, no research has been conducted into the potential for alegacy effect to maintain enhanced N2O emissions from recently

78 C.A. Horrocks et al. / Agriculture, Ecosystems and Environment 199 (2014) 77–84

created semi-natural temperate grasslands (<10 years). Herein, thehypothesis was tested that extensification of intensively managedagricultural land reduces N2O fluxes using paired sites in SEScotland and SW England.

2. Materials and methods

2.1. Location and experimental design

Eight paired field plots were established on adjacent SRG andIM land on arable and grassland farms in SE Scotland (fouranonymous sites near to Greenlaw; 55� 420 2300N, 2� 2704900W) andSW England (Rothamsted Research, North Wyke, Devon, UK;50� 4601000N, 3� 5400500W). Soil characteristics are given in Table 1.Measurements were carried out in Scotland over a 13 monthperiod from May 2010 until July 2011, and in England an initialsampling period of 18 months from March 2010 was followed by aperiod of high frequency sampling from February–June 2012 tomore closely observe contributing processes.

2.1.1. South East ScotlandPaired field plots (11 m � 11 m) were established on 4 farms in

fields where a SRG seed mix had been sown 3, 5, 8 or 9 yearspreviously, in a portion (at least 1 ha) of an IM field (arablerotation) so that each pair were matched for soil type, slope andaspect. The plots will be referred to hereafter as S3, S5, S8 and S9,respectively. The SRG plots received no fertiliser or biocides andwere maintained by light grazing or cutting no earlier than the lastweek in August each year. The 30 year (1971–2000) annual meanprecipitation for the region was 400–900 mm. The annual meanwinter temperature for the region was 1.5–3.5 �C and the annualmean summer temperature 13.5–15.5 �C (Met Office, 2011).

2.1.2. South West EnglandFour paired plots (25 m � 35 m) were established in a field

where a SRG seed mix had been sown 2 years previously, and anadjacent field under IM (grassland), so that each pair were matchedfor soil type, slope and aspect. The plots will be referred tohereafter as NWA, NWB, NWC, NWD, respectively. The SRG plotsreceived no fertiliser or biocides and were maintained by cutting inon 4 June and 16 August 2010 and 26 July 2011. The 30 year(1971–2000) annual mean precipitation for the region was

Table 1Soil properties including, texture, bulk density (BD), pH, total organic carbon (TOC)and total nitrogen (TN) for paired species-rich grassland (SRG) and intensivelymanaged (IM) study plots, ‘S’ denotes Scottish study plots, NW denotes plotsRothamsted Research, North Wyke (England).

Site Plot Soil texture BD pH TOC TN

S3 SRG Sandy loam 1.6 5.72 1.01 0.1IM Sandy loam 1.5 5.8 0.02 0.95

S5 SRG Silty loam 1.1 5.8 2.53 0.23IM Silty loam 1.2 5.78 2 0.2

S8 SRG Silty laom 1.1 5.66 2.1 0.2IM Silty loam 1.4 5.7 1.97 0.18

S9 SRG Silty loam 1.1 5.82 2.97 0.28IM Silty loam 1.4 6 2.74 0.26

NWA SRG Silty clay 0.98 5.84 3.18 0.35IM Silty clay 0.74 5.68 3.08 0.32

NWB SRG Silty clay 0.95 5.9 5.68 0.54IM Silty clay 0.7 5.48 6.18 0.6

NWC SRG Silty clay 0.92 6.3 3.68 0.38IM Silty clay 0.7 5.87 3.08 0.32

NWD SRG Silty clay 0.93 5.82 3.13 0.29IM Silty clay 0.71 5.86 6 0.58

1400–1800 mm. The annual mean winter temperature for theregion was 2.5–5 �C and the annual mean summer temperature12–15 �C (Met Office, 2011). The 40 year (1961–2000) annual meanprecipitation for the site is 1056 mm and the 40 year monthly meantemperatures are 4.5 �C (February) and 15.5 �C (August).

2.2. Static chamber design and operation

A static chamber method was used to measure N2O fluxes fromthe soils (Rochette and Erisken-Hamel, 2008). Different chamberdesigns were used at the Scottish and English sites. In Scotland,three plastic chambers (50 cm diameter, 50 cm height) were placedalong a transect 7.5 m apart inserted to a depth of 10 cm. Thechambers were removed during grazing periods and replacedbefore at least 7 days before sampling. The chambers weresealed with gas tight metal lids fitted with a three way tap. In

Fig. 1. Meteorological data summary for region around Scottish field sites in 2010and 2011, including (a) mean daily maximum temperature and (b) mean dailyminimum temperature in 2010 and 2011 recorded at Greenlaw met. station(Borders Weather, 2012) alongside the corresponding 30-year (1971–2000)temperature averages recorded at Edinburgh and Boulmer met. stations. Alsoshowing (c) monthly rainfall totals in 2010 and 2011 averaged for the NE Englandregion, alongside the 30-year mean monthly rainfall total for the same region,(rainfall data obtained from UK Met. Office).

C.A. Horrocks et al. / Agriculture, Ecosystems and Environment 199 (2014) 77–84 79

England, 4 rectangular metal bases (0.3 m � 0.4 m) were inserted todepth of 10 cm along a transect 5 m apart and remained in placefor the duration of the experiment. Sealed box chambers(0.3 m � 0.4 m base, 23 cm height) fitted with a three way tapwere placed onto the bases. Gases were extracted from anevacuated 20 ml vial using a 5 ml syringe. Samples at all siteswere standardised by sampling between 10:00 and 14:00 h as thisis the recommended time to measure N2O emissions thatis representative of the daily mean (Yamulki et al., 1995; Cardenaset al., 2010). During sampling the chambers were closed for 40 minand a gas sample taken at the start (T0) and end of the closureperiod (T40) to determine the change in gas concentration in thechamber headspace during the closure period. Gas accumulationwithin the headspace has previously been shown to occur at aconstant rate over this time period for agricultural soils in bothScotland and at NW (Smith and Dobbie, 2001; Cardenas et al.,2010). In Scotland sampling was conducted on four to sevenoccasions at each site from May 2010 to July 2011. In EnglandSampling was carried out on 10 occasions between March 2010 andSeptember 2011 and then at a greater intensity for 4 months fromFebruary to June 2012.

2.3. Analysis of gas samples

All gas samples were analysed within 14 days of collection using agas chromatograph (PerkinElmer Clarus 500 GC with a TurboMatrix110 auto headspace sampler). The GC was equipped with an ECD(300 �C) and a PerkinElmer EliteQ PLOT megabore capillary (30m� 0.53 mm i.d.) column, and operated at 35 �C with N2 as the carriergas. Gas fluxes (g N ha�1 day�1) were calculated from the rate of gasaccumulation in the chamber head space (Eq. (1)).

2.4. Soil parameter measurements

In addition to N2O flux measurements, soil temperature(0–2 cm depth; (Ts), gravimetric soil water content and availableN, i.e. KCL extractable N (Pansu and Gautheyrou, 2006), were also

Table 2Nitrous oxide (N2O) fluxes (g nitrogen (N) ha�1 day�1) measured in 2010 and 2011 from

surface temperature (�C) during the sampling period along with soil available N (mg�

means � standard errors.

Available soil N

Site IM Mar2010

SRG Mar2010

IM Jul2010

SRG Jul2010

IM Apr2011

SRG Apr2011

Date

S3 5.1(�2.1)

5.5(�3.1)

9.8(�0.3)

4.8(�0.6)*

12.4(� 1.8)

14.2(�10.8)

07-May-10

06-Jun-10

09-Jun-10

07-Jul-10

24-Oct-10

S5 3.3(�0.7)

5.9(�1.8)*

7.9(�1.3)

12.9(�2.5)*

NA NA 03-May-10

08-Jun-10

12-Jul-10

24-Sep-10

11-Oct-10

03-Nov-10

S8 20.8(�2.9)

17.5(�2.8)

9.3(�0.12)

10.4(�2.9)

33.7(� 5.6)

21.4(� 6.8)*

06-Jun-10

09-Jun-10

09-Jul-10

15-Oct-10

28-Oct-10

11-May-11

06-Jul-11

S9 9.3(�3.9)

20.8(�2.6)*

42.2(�27.2)

9.5(�1.5)

79.1(� 14.3)

50.1(�15.5)*

15-Oct-10

28-Oct-10

11-May-11

06-Jul-11

* Indicates significant differences (P < 0.05) between paired intensively managed (IM

measured to aid interpretation of N2O flux data. The % soil waterfilled pore space (WFPS) was calculated according to the function:

%WFPS ¼Soilwatercontentðgg�1ÞSoil porosity

� Bulk densityDensity of water

� 100 (2)

where

Particle density ¼ ðOM � 1:3Þ þ ðð1 � OMÞ � 2:65Þ (3)

assuming particle densities of 2.65 g cm�3 and 1.3 g cm�3 for soilmineral and organic particles, respectively (Ilstedt et al., 2000), and

Soil porosity ¼ 1 � Bulk densityParticle density

� �(4)

using soil bulk density (Table 1) and organic matter content(loss on ignition at 450 �C).

Ammonium (NH4+) and nitrate (NO3

�) concentrations in the KClextract were measured according to Keeney and Nelson (1982)using a Bran & Luebbe Auto Analyser III running methodsG-102-93 and G-109-94, respectively.

Soil samples for determination of available N and soil water werecollected using a soil corer (5 cm diameter,10 cm length). Three coresfrom each plot (n = 4 at each site) were bulked for analysis.

2.5. Statistical analysis

Statistical analysis was carried using GENSTAT 14. Measure-ments taken at paired IM and SRG plots were comparedseparately for each Scottish farm site using either a t-test or aheteroscedastic t-test if an F-test for homogeneity of variancessuggested the population variances were significantly different.Mean (n = 4) Ts, WFPS and N2O fluxes were compared using atwo-factor ANOVA with site (SRG and IM) and plot pair number(1–4) as the factors. Regular N2O flux sampling at NW in2012 allowed a time series analysis to be undertaken on theN2O flux data. A repeated-measures ANOVA was carried out on themean N2O fluxes from each plot. P values � 0.05 were consideredsignificant.

the paired plots at the Scottish sites, along with soil water filled pore space (%) and1 kg�1 dry soil) measured in March and July 2010, and April 2011. All values are

Soil surface temperature(Ts) (n = 3)

WFPS (n = 3) N2O flux (n = 3)

IM SRG IM SRG IM SRG

13.3 (�0.1) 11.7 (�0.2)* 56.7 (�0.5) 69.0 (�7.7) �0.8 (�1.7) �0.002 (�0.08)14.4 (�0.6) 14.9 (�0.4) 51.7 (�1.2) 52.6 (�8.1) 3.1 (�0.7) 2.4 (�0.2)13.4 (�0.3) 13.8 (�0.2) 53.5 (�1.7) 80.8 (�4.8)* 0.9 (�0.5) (2.6 � 2.5)19.3 (�0.4) 18.8 (�0.7) 42.3 (�1.3) 26.8(�4.3)* 0.6 (�0.6) 0.2 (�0.2)6.0 (�0.3) 7.4 (�0.3)* 66.0 (�0.64) 76.5 (�0.87)* 1.1 (��0.2) �18.7 (�1.1)*

9.7 (�0.5) 12.3 (�0.3)* 51.3 (�5.5) 50.1 (�4.2) �2.4 (� 2.4) �3.2 (�0.3)11.4 (�0.1) 13.2 (�0.2)* 62.9 (�8.4) 63.6 (�5.9) 1.3 (� 0.2) 1.7 (�0.3)16.9 (�0.2) 18.6 (�0.3)* 61.1 (�7.6) 33.6 (�3.7)* 1.4 (�0.3) �0.8 (�0.2)*

12.6 (�0.1) 12.3 (�0.9) 76.9 (�2.03) 72.5 (�5.3) �2.6 (�11.3) 9.1 (�7.2)10.5 (�0.1) 10.7 (�0.1) 64.5 (�25.1) 110 (�15.6) 14.7 (�1.7) 16.1 (�0.5)6.9 (�0.9) 7.0 (�0.1) 72.6 (�5.8) 73.7 (�12.9) 0.2 (�0.5) 0.7 (�0.1)

12.8 (�0.1) 12.8 (�0.5) 46.9 (�11.8) 37.9 (�4.2) 2.3 (�1.4) 1.6 (�0.6)12.1 (�.2) 12.0 (�0.1) 65.8 (�8.9) 65.1 (�9.1) 3.0 (�3.5) 2.3 (�1.8)18.6 (�0.6) 15.3 (�1.0)* 40.6 (�2.6) 25.2 (�2.9)* 1.5 (�0.3) 2.3 (�0.1)9.9 (�0.2) 10.3 (�0.1)* 68.5 (�6.3) 52.4 (�13.1) 12.1 (�7.5) 4.2 (�0.2)7.8 (�0.3) 8.3 (�0.1) 72.2 (�4.4) 58.2 (�0.3)* 2.0 (�0.1) 1.2 (�0.1)*

13 (�0.2) 13 (�0.1) 59.0 (�4.8) 56.8 (�8.8) 70.3 (�46.3) 0.007 (�0.01)14.9 (�0.4) 10.4 (�0.2)* 40.9 (�4.6) 25.5 (�1.4)* 0.63 (�0.3) 0.8 (�0.1)

10.4 (�0.2) 10.6 (�0.1) 79.5 (�1.4) 67.9 (�1.5)* 8.7 (�0.5) 5.7 (�0.6)*

9.4 (�0.1) 8.8 (�0.1)* 87.1 (�2.7) 80.6 (�17.6) 1.2 (�0.3) �0.3 (�1.2)12.1 (�0.1) 12.0 (�0.0) 78.9 (�0.9) 45.4 (�0.9)* 33.2 (�22.8) �0.5 (�1.5)15.2 (�0.2) 15.2 (�0.1) 95.6 (�12.1) 80.0 (�7.1) 0.85 (�0.4) 0.5 (�0.1)

) and species-rich grassland (SRG) plots on each sample occasion.

Fig. 2. (a) Monthly rainfall totals for 2010–2012 alongside 30-year (1982–2011)mean monthly rainfall totals recorded at the North Wyke (NW) automated weatherstation (AWS). Monthly mean maximum daily temperature (b) and mean minimumdaily temperature (c), for 2010–2012 alongside 30-year mean recorded at the NWAWS. 2012 data only included up until June, when the study ended, errorbars � 1standard deviation.

80 C.A. Horrocks et al. / Agriculture, Ecosystems and Environment 199 (2014) 77–84

3. Results

3.1. Results from Scottish sites

3.1.1. Meteorological data2010 was generally cooler than the 30-year mean in the winter

months with lower mean daily maximum temperature (MDmax)(Fig. 1a) and mean daily minimum temperature (MDmin) (Fig. 1b),and warmer than the 30-year average through the rest of the year.Temperatures in 2011 generally exceeded the 30-year mean. N2Osampling was conducted in May, June, July, September, Octoberand November 2010 and May and July 2011. In May June andOctober 2010, the MDmax was greater than the 30-year mean by4.3, 3.3, 5.0 and 5.7 �C, respectively. In July, September andOctober 2010, the MDmax was within 3 �C of the 30-year mean.The MDmin was greater than the 30-year mean by 3.5 �C in June2010 and within 3 �C of the 30-year mean during the othermonths.

The rainfall total in January 2010 was near average whilstFebruary and March were slightly wetter than average, withrainfall totals of 148% and 152% respectively of the 30-year mean(Fig. 1c). The first part of the 2010 growing season (April–June) wasdrier than average, with monthly rainfall totals 39%, 49% and 53%of the 30-year mean. The monthly mean rainfall totals inJuly–October 2010 were within 50% of the 30-year mean, whilstNovember 2010 was wetter than average with total rainfall of 156%of the 30-year mean. The mean rainfall total for the region in2010 was 803 mm, very similar to the 30-year annual mean of805 mm. In 2011, the months just before sampling commencedwere drier than average with rainfall totals for March and April of42 and 16% of the 30-year mean respectively. Rainfall totals for Mayand July were within 50% of the 30 year mean. The total rainfall forthe region in 2011 was 719 mm, substantially lower than the 30-year annual mean.

3.1.2. N2O fluxes and soil properties at Scottish sitesThe mean N2O fluxes measured from all plots were generally

<20 g N ha�1 day�1, with occasional negative fluxes being recorded(Table 2) There was a very slight increase in the background N2Oflux in September and October 2010 from both plots at site S5, butthe only substantial peaks, i.e. >20 g N ha�1 day�1, of mean N2O fluxwere measured in the IM plots of site pairs S8 and S9 on 11 May2011. On these occasions the standard deviation in the measure-ments from the three chambers in the IM plot was large, so the fluxwas not significantly different from the SRG plot for either siteS8 or S9.

In March 2010 the mean (n = 5) available soil N was significantlygreater in the SRG plot than the IM plot at sites S5 and S9 (Table 2).In July 2010 the available soil N was significantly greater in the IMplot at site S3 and in the SRG plot at site S5. In 2011 the available Nin the IM plots at sites S8 and S9 was significantly greater than inthe paired SRG plot. Otherwise available soil N did not differsignificantly between paired IM and SRG sites.

The mean (n = 3) soil WFPS in the plots at the time of N2O gasflux measurement varied between 25 and 110% across all plots. Onsome sampling occasions there was large within plot variation, butmean WFPS was often similar between paired IM and SRG plots.

The mean (n = 3) Ts measured across all four site pairs rangedbetween 6.0 and 19.3 �C. At site S5 the mean Ts was significantlygreater in the SRG plot on three occasions by >1 �C. At site S8 themean Ts of the SRG plot was significantly and substantially (>1 �C)lower than that of the IM plot on two occasions, whilst at site S3 themean Ts in the SRG plot was significantly lower than in the IM ploton 11 May 2010 and significantly greater on 24 October 2010. On allother sampling dates Ts was either not significantly differentbetween management types or differed by <1 �C.

3.2. Results from North Wyke

3.2.1. Meteorological dataDuring the study period the MDmax tended to be lower than the

30-year means. In 2010 the MDmax in February, April, June, July andAugust was more than 1 standard deviation (SD) below the 30-yearmean, whilst in the other seven months of 2010 the MDmaxwas morethan twicethe SD below the 30-year mean (Fig. 2). In 2011 the MDmax

in January, March, May June, July, August and September was morethan 1 SD lower than the 30-year mean. 2012 was generally warmerthan 2010 and 2011; in March 2012 the MDmaxwas more than twicethe SD greater than the 30-year mean, although in April and June theMDmax was more than 1 SD less than the 30-year mean.

The MDmin for the study period tended to be closer to the30-year mean, in January, November and December 2010 theMDmin was more than 1 SD lower than the 30-year mean, whilst inJuly it was more than 1 SD greater than the 30-year mean. InFebruary, April, September, October, November and December

Table 3Mean (n = 4) nitrous oxide (N2O) fluxes (g nitrogen (N) ha�1 day�1) and soil surfacetemperatures (�C) measured during 2010–2011 from four paired intensivelymanaged (IM) and species-rich grassland (SRG) plots at North Wyke. P valueobtained from a two factor ANOVA.

N2O flux Temp

Date IM SRG P IM SRG P

23 Mar-10 39.5 (�9.6) 29.7 (�10.5) 0.061 9.1 (�0.25) 7.8 (�0.6)* 0.00913 May-10 47.3 (�12.3) 108 (�20.0) <0.001 10.9 (�0.2) 12.7 (�0.8)* 0.01222 Jun-10 12.2 (�25.8) 0.2 (�13.5) 0.52 20.8 (�0.7) 19.9 (�0.3) 0.1526 Aug-10 10 (�9.1) �0.4 (�8.8) 0.13 17.0 (�0.7) 17.7 (�0.5) 0.114 Sep 2010 �3.6 (�3.01) �2 (�3.2) 0.59 16.6 (�0.1) 17.3 (�0.4) 0.08416 Nov 2010 1.9 (�0.7) 1.9 (�1.5) 0.99 8.4 (�0.4) 6.6 (�0.4)* <0.00117 Nov-10 �1.7 (�1.4) �3.1 (�1.5) 0.1 9.4 (�0.3) 9.1 (�0.1) 0.1915 Dec 2010 �0.3 (�2.1) �2.6 (�1.1) 0.078 2.7 (�1.2) 1.8 (�0.3) 0.164 Apr 2011 �1.3 (�2.0) �3 (�4.6) 0.33 10.7 (�0.2) 10.5 (�0.1) 0.5516 Sep 2011 2.8 (�2.0) 0.2 (�1.1) 0.024 17.2 (�0.3) 17.1 (�0.3) 0.75

* indicates significant differences (P < 0.05) between paired intensively managed(IM) and species-rich grassland (SRG) plots on each sample occasion.

a)

b)

c)

-20 -10

0 10 20 30 40 50 60 70 80 90

100

1-Feb 21-Feb 12-Mar 1-Apr 21-Apr 11-May 31-May 20-Jun

N2O

flux

(g N

ha-

1 da

y-1)

-20

-10

0

10

20

30

40

50

60

70

80

90

100

1-Feb 21-Feb 12-Mar 1-Apr 21-Apr 11-May 31-May 20-Jun

N2O

flux

(g N

ha-

1 da

y-1)

-20 -10

0 10 20 30 40 50 60 70 80 90

100

1-Feb 21-Feb 12-Mar 1-Apr 21-Apr 11-May 31-May 20-Jun

N2O

flux

(g N

ha-

1 da

y-1)

SRG IM

Fig. 3. Mean (n = 4 �1standard deviation) nitrous oxide (N2O) fluxes measuredFebruary–June 2012 from the paired intensively managed (IM)-and species-richgrassland (SRG) plots at North Wyke (NW); for plot pairs (a) NWA, (b) NWB (c) NWC.Arrows indicate time of fertiliser application (solid line) and grass cutting (dashedline) at IM plots. First fertiliser application ‘A’ on 9 March 2012 was 80 kg N ha�1

ammonium nitrate, 2nd application ‘B’ on 17 April was 39 kg N ha�1 ammoniumnitrate.

C.A. Horrocks et al. / Agriculture, Ecosystems and Environment 199 (2014) 77–84 81

2011 the MDminwas more than 1 SD greater than the 30-year mean,whilst in July it was more than 1 SD lower than the 30-year mean.

During the study period, rainfall was substantially below the30-year mean, apart from two very wet months in April and June2012. There was less rainfall than average for the 30-year period inevery month of 2010 and 2011 (Fig. 2). November and June werethe only months in 2010 to receive more than 50% of the 30-yearmean total rainfall. The total rainfall for 2010 was 389 mm, 37% ofthe 30-year mean (1042 mm). In 2011 June and September werethe only months to receive more than 50% of the 30-year meanrainfall total. The total rainfall in 2011 was 379 mm, 36% of the30-year mean. In 2012 January, February and March receiving 77,28 and 33% respectively of the 30-year mean monthly rainfalltotals. April and June were wetter than average, receiving 169 mmand 157 mm rainfall, respectively, corresponding to 259% and 276%of the 30-year mean monthly rainfall.

3.2.2. N2O fluxes and soil properties from North Wyke in 2010–2011The N2O fluxes measured in 2010 and 2011 were frequently

<5 g N ha�1 day�1 from both the IM and SRG plots; fluxes >5 g Nha�1 day�1 were only recorded during the growing season(March–August) in 2010 (Table 3). The greatest N2O fluxes weremeasured on 13 May 2010 when the mean N2O flux from the SRGplots (108 � 20.0 g N ha�1 day�1), was significantly greater than themean flux from the IM plots (47.3 � 12.3 g N ha�1 day�1). The IMplots had received 80 kg N ha�1 ammonium nitrate fertiliser20 days prior to the peak N2O flux whilst the SRG plots did notreceive any fertiliser. The only other statistically significantdifference in N2O flux between the IM and the SRG plots in the2010–2011 was measured on 16 September 2011, when the meanflux from the IM plots (2.8 � 2.0 g N ha�1 day�1) was significantlygreater than the mean flux from the SRG plot (0.2 �1.1 g Nha�1 day�1). Soil temperature showed no consistent differencebetween the two managements during 2010–2011.

3.2.3. N2O fluxes and soil properties from North Wyke in 2012The N2O fluxes recorded from the NW plots in 2012 were

generally <10 g N ha�1 day�1 (Fig. 3) and often varied greatlybetween the four chambers within each plot. The mean fluxes alsovaried between plots, although similar patterns were seen over timeacross the three plots ateach site. Repeated measures ANOVAcarriedout on the 2012 flux data showed mean fluxes over both sites variedsignificantly with sampling date, but there was no significantinteraction between site (IM vs SRG) and sampling date. Emissionspeaks (>33 g N ha�1day�1) were observed from all three plots at boththe IM and SRG sites at the start of the sampling period on16 February, prior to any fertiliser applications at the IM site. The

greatest fluxes single fluxes were recorded on 12 March from the IMplots, three days after the first fertiliser application to the site.

On every sampling occasion in 2012 the mean WFPS was lowerin the IM plots than the SRG plots (Fig. 4) and on nine of the18 occasions this difference was statistically significant. The meandifference between the WFPS of the IM and SRG plots across allsampling occasions was 12.0 � 5.77%. WFPS measured at both sitesfollowed a similar trend over time, increasing and decreasing inresponse to periods of higher or lower rainfall, respectively.

Soil temperatures were usually closer to the MDmax than theMDmin (Fig. 4) There was no consistent difference in Ts between thetwo management types; Ts was significantly greater in the IM plotson five occasions and in the SRG plots on six occasions.

The mean soil NO3� N concentration was greater than the mean

NH4+-N on all sampling occasions except for 16 May (Fig. 5). The

greatest mean NO3� N concentration for both plot types was on

23 March,14 days after the first fertiliser application at the IM plots.Soil NH4

+-N concentrations were similar at both sites on all sample

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Fig. 4. (a) Daily rainfall totals with mean (n = 3 � 1standard deviation (SD)) water filled pore space (WFPS) and b) daily minimum and maximum air temperature with mean(n = 3 �1SD) soil temperature for North Wyke intensively managed (NWIM)/species-rich grassland (NWSRG) plots from 1 February–16 June 2012. * indicates significantdifference in soil WFPS or soil temperature between NWIM and NWSRG plots.

82 C.A. Horrocks et al. / Agriculture, Ecosystems and Environment 199 (2014) 77–84

occasions and were always <9 mg N kg�1 of dry soil, apart from on16 May (29 days after the second fertiliser application at the IMplots), when the mean concentrations in the SRG and IM plots were32.7 � 12.3 mg N kg�1 dry soil and 125 � 7.14 mg N kg�1 dry soilrespectively. On the five sampling dates in March and April, the meansoil NO3

� N concentration in the IM plots was significantly greaterthan in the SRG plots, in May and June there was no significantdifference in NO3

– N concentration between the two plot types.

4. Discussion

4.1. Limited difference in N2flux between paired IM and SRG plots

The N2O emissions measured in this study were similar for theSRG and IM plots, even at the SRG plot that had been taken out ofintensive management for 9 years. This supports the theory thatpast intensive management can have a long lasting legacy on soilproperties and function, as demonstrated in a forest ecosystem byCompton and Boone (2000) and steppe grassland by Mosier et al.,(1997) who found the soil N cycle was still influenced by former

management after 50 and 8 years of agricultural abandonmentrespectively. The lack of a significant difference in N2O emissionsbetween the two management types in this study occurred despiteoccasional significant differences in available soil N, WFPS and Tsbetween paired plots. Large intra-plot variation in soil WFPS andavailable N, highlights the large spatial variability in soil propertiesand could partly explain the high variability in N2O fluxes.

The very low, and occasionally negative, N2O fluxes measured atthe Scottish sites were similar to other studies, which reported thatN2O emissions peaks can be infrequent and short lived fromagricultural sites (Neftel et al., 2007), with negative fluxescommonly measured (Chapius-Lardy et al., 2007). Net N2Oconsumption may be a response to the presence of anaerobicmicrosites, leading to N2O reduction to N2, which can occur evenunder low WFPS within aggregates where oxygen consumptionexceeds diffusion (Neftel et al., 2007).

The greatest difference in mean N2O flux between the pairedScottish IM/SRG plots was recorded on 11 May 2011, when the fluxesfrom the IM plots at sites S8 and S9 were >20 g N ha�1 day�1. Both IMplots received 230 kg ha�1 of ammonium nitrate fertiliser on 19 April

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Fig. 5. Mean (n = 3 � 1standard deviation) soil available nitrogen (N) as nitrate N (NO3N) and ammonium N (NH4+-N) at North Wyke intensively managed (IM) and species-

rich grassland (SRG) plots in March–June 2012, showing dates of fertiliser application (solid arrows) and grass cutting (dashed arrow) at IM plots. The first fertiliser application‘A’ on 9 March 2012 was 80 kg N ha�1 ammonium nitrate, 2nd application ‘B’ on 17 April was 39 kg N ha�1 ammonium nitrate.

C.A. Horrocks et al. / Agriculture, Ecosystems and Environment 199 (2014) 77–84 83

2011. This increase in available soil N coupled with the rainfall eventthat occurred on 8 May 2011 would have contributed to this peak inN2O emissions from the IM plots (Clayton et al., 1997).

In SW England the only date when the mean N2O flux was bothlarger than 5 g N ha�1 day�1 and differed significantly betweenmanagement types was 13 May 2010, when the mean flux from theSRG plots was greater than from the IM plots although an increasein N2O flux relative to other sampling dates was observed for boththe IM and SRG plots. The IM plots had received fertiliser on23 April, which could explain the increase in N2O emissions fromthese plots, compared to other sampling dates but does not explainthe significantly greater N2O emissions from the SRG plots, whereno fertiliser was applied. The inter- and intra-plot variation in N2Oflux from the SRG plots was small, suggesting the cause of theemissions spike was affecting the whole field. One explanation isthat the higher temperatures at the end of April 2010 increasedrates of N mineralisation and nitrification in the soil, increasingNO3

� concentrations. Availability of the NO3� to denitrifiers may

have been greater in the SRG plots due to the nature of the plantcommunity. The sward in the NW SRG plots included speciestypically found in less fertile environments (C Horrocks, PhDthesis) that are slow growing and less able to increase rates of NO3

�

uptake in response to elevated soil N than the rapidly growingLolium perenne, which dominated at the IM sites (Hobbie, 1992;Glass et al., 2002). The higher soil WFPS at the SRG on 13 May2010 compared to the IM plots, as was frequently found during2012, may have also contributed to the greater N2O fluxes.

4.2. N2O fluxes show inconsistent response to soil WFPS and fertilizerapplications at English sites throughout 2012

Substantial N2O emissions are generally reported when soilWFPS is >60% (Dobbie et al., 1999; Bateman and Baggs, 2005; Ruseret al., 2006). However, in this study, even when the WFPS was>60%, the N2O emissions from all plots were generally small(<20 g N ha�1 day�1).

On 23 March 2010 and 16 February 2012 mean N2O fluxes fromboth the NWIM and NWSRG plots were above 30 g N ha�1 day�1.The WFPS at the SRG site on 16 February was >80%, which wouldhave favoured denitrification. The mean WFPS at the IM site was<60%, suggesting the N2O emissions peak from the IM siteoriginated partly from nitrifier-denitrification or nitrificationcoupled denitrification, which have been found to contribute toN2O emissions at WFPS of 50–60% (Bateman and Baggs, 2005; Koolet al., 2011). The N2O emissions from the NW plots on 23 March2010 and 16 February 2012 occurred prior to any fertiliseradditions, thus the GHG was likely derived from organic ormineral N already present in the soil. One explanation for the

fluxes is that they are in response to the onset of warmertemperatures in spring, leading to increased rates of nitrification.This could explain why the peak was observed earlier in 2012 thanin 2010, as the February MDmax was over 3 �C warmer in 2012than in 2010 and similar to the March temperatures in 2010.

For most of the rest of the February to June 2012 measurementperiod the WFPS was >60% across all the English plots. In the SRGplots, soil available N (NO3

� and NH4+) was typically 20 mg N kg�1

dry soil and never more than 60 mg N kg�1 dry soil. These lowavailable N concentrations could explain the limited N2O fluxesmeasured. The occasional larger fluxes from individual chambersat the SRG site, such as occurred in plot 1 on 5 April 2012, are mostlikely to be due to natural soil heterogeneity, and indicative of hotspots of higher mineral N content. Localised peaks in available soilN can occur from a urine patch from wild fauna, for example(Anger et al., 2003).

Although not statistically different from the mean fluxesobserved at the SRG plots, mean N2O fluxes were greater from theIM plots on some sampling occasions in 2012, including 12 Marchfollowing the first application of ammonium nitrate fertiliser of80 kg N ha�1. This peak in mean N2O flux coincided with an increasein soil NO3

� concentration in the IM plots. However, whilst elevatedsoil NO3

� concentrations were found in all plots, the emission peakswere only observed from IM plots 2 and 3 and fluxes varied greatlybetween the chambers in each plot, indicating that other soilenvironmental factors, such as the distribution of WFPS and possiblythe structure of the microbialcommunity, were highly variable at thesmaller scale leading to the observed variation in N2O emissions.

Following the second fertilizer application to the NW IM plots in2012 (39 kg N ha�1 ammonium nitrate) on 17 April, no increase insoil NO3

� or NH4+ concentration was observed and there was no

peak in N2O emissions. There was a slight increase in soil NH4+

concentration a month later (16 May) at both the SRG and IM plots.This peak in NH4

+ concentration coincided with a slight peak inN2O emissions from NWA SRG and all three plots to a varyingextent at the IM site. N2O can be produced directly from NH4

+ in thesoil, either through nitrifier denitrification by ammonia oxidisingbacteria, or through nitrification-coupled denitrification (reduc-tion of the products of nitrification; Kool et al., 2011).

Other management activities undertaken in the IM field,including grazing in 2010 and grazing and fertiliser additions in2011, did not result in N2O emissions peaks. The soil WFPSmeasured at the IM site during 2012 was frequently found to beboth significantly lower than at the SRG site and <60%. Values forsoil WFPS of <60% suggest a lack of anaerobic pore spaces, whichcan limit N2O fluxes, and could result from greater soil wateruptake from the IM site due to higher rates of evapotranspirationfrom the faster growing IM sward. Even the largest N2O fluxes

84 C.A. Horrocks et al. / Agriculture, Ecosystems and Environment 199 (2014) 77–84

observed from the NW plots throughout the experiment werelower than the peak fluxes recorded by other authors using thestatic chamber technique in UK grasslands. Fluxes of up to243 g N ha�1 day�1 have been reported from an IM grassland10 km south of Edinburgh, Scotland (Conen and Smith, 1998).Peak N2O fluxes of around 100 g N ha�1 day�1 have been reportedat a grazed grassland site adjacent to the SRG plots at NWreceiving fertiliser application of 200 kg N ha�1 yr�1 (Cardenaset al., 2010). However other authors have found fluxes to varygreatly between years, depending on climate. In one wet yearpeak fluxes of up to 340 g N ha�1 day�1 were measured from a UKgrassland receiving three applications of 100 kg N ha�1 yr�1,whilst during a drier year at the same site peak fluxes wereonly 19 g N ha�1 day�1 (Jones et al., 2005). The unusual weatherpatterns throughout the sampling period, including the cooltemperatures, particularly in 2010, and the little rainfall, exceptfor two unusually wet months in April and June 2012, are likely tohave affected key soil environmental factors and plant andmicrobial communities. This could mean the results from thisstudy may differ from those that would have been observed in ayear of more typical weather.

5. Conclusions

The observation that substantial N2O emissions peaks can occurfrom SRG sites in the absence of fertiliser application is importantas it highlights that the soils of newly created SRGs can still be asignificant source of N2O. Compared to the SRG plots the IM plotsshowed a slight tendency for greater total yearly N2O emissions,however the high spatial and temporal variability in N2O fluxesmeant that the difference was rarely significant. Even followingfertiliser N application, N2O fluxes did not peak significantly fromall chambers. Differences in soil WFPS, temperature and availableN were observed between IM and SRG sites and in some casesprovided an explanation for observed N2O fluxes, although onother occasions N2O did not respond as may be predicted tochanges in soil properties. The regulation of greenhouse gasemissions is a vital ecosystem service. Understanding the impact ofagri-environment schemes such as the creation of semi-naturalgrasslands on soil greenhouse gas emissions is vital in determiningthe benefit of policy-driven land use change. In this study we foundno significant difference in N2O fluxes between sites remainingunder intensive management and recently created semi-naturalgrassland. We conclude that the effect of the decrease inmanagement intensity on emissions of N2O from arable rotationand grassland soils is not detectable in the first 10 years of the landuse change.

Acknowledgements

We would like to thank Rab Howard for technical assistance inEdinburgh and Neil Donovan for analysing gas samples at NorthWyke. We acknowledge financial support from the NaturalEnvironment Research Council (NERC) and the Scottish Environ-ment Protection Agency (SEPA). This work represents part of theBBSRC-funded programmes at Rothamsted Research on Sustain-able Soil Function and Climate Change.

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