Soil Use andManagement (2000) 16, 36^41

Nitrous oxide and carbon dioxide emissions fromgrassland amendedwith sewage sludge

A. Scott1*, B.C. Ball1, I.J. Crichton1 &M.N. Aitken2

Abstract.Land disposal of sewage sludge in theUK is set to increase markedly in the next few years and much ofthis will be applied to grassland.Here we applied high rates of digested sludge cake (1^1.56 103 kg total N ha71)to grassland and incorporated it prior to reseeding. Using automated chambers, nitrous oxide (N2O) and carbondioxide (CO2) fluxes from the soil were monitored 2^4 times per day, for 6 months after sludge incorporation.Peaks of N2O emission were up to 1.4 kgN ha71d71 soon after incorporation, and thereafter were regularlydetected following significant rainfalls. Gas emissions reflected diurnal temperature variations, though N2Oemissions were also strongly affected by rainfall. Although emissions decreased in the winter, temperaturesbelow 4 �C stimulated short, sharp fluxes of both CO2 and N2O as temperature increased.The aggregate loss ofnitrogen and carbon over the measurement period was up to 23 kgN ha71 and 5.1 t C ha71. Losses of N2O in thesludge-amended soil were associated with good microbial conditions for N mineralization, and with highcarbon and water contents. Since grassland is an important source of greenhouse gases, application of sewagesludge can be at least as significant as fertilizer in enhancing these emissions.

Keywords: Nitrous oxide, carbon dioxide, emission, grasslands, sewage sludge, soil

INTRODUCTION

N itrous oxide (N2O) is a radiatively active gas 280 times aseffective as carbon dioxide in causing global warming

(Houghton et al., 1996). Being chemically inert in the loweratmosphere, N2O slowly diffuses into the stratosphere whereit participates in photochemical reactions. This may lead tothe destruction of the earth-protecting ozone layer (Crutzen,1981), causing increased incidences of skin cancer (Peopleset al., 1995).

The main source of N2O emissions from agricultural soilsis from the microbial processes of nitrification and denitrifi-cation (Firestone &Davidson,1989) which are commonly sti-mulated by fertilizer applications (Clayton et al., 1994). Inaddition, the land spreading of farmyard manure has alsobeen identified as a significant contributor ofN2O (Goulding&Webster, 1989).

Soil respiration can also provide a net emission of CO2 andthis can be stimulated by the addition of manure (Gregorichet al., 1998), though it is a minor source compared with fossilfuel burning (Houghton et al., 1996).

Little research appears to have been done on the effects ofsludge applications to soils of fine or medium texture(Smith, 1996), although Mosier et al. (1982) found rather lowN2O fluxes from coarse textured soils. Digested sewagesludge contains a similar amount of total nitrogen to farm-yard manure (Aitken, 1997), and is considered to stimulatethe production of N2O and CO2 in the soil. Although a sub-stantial amount of N is available from this source for

recycling, as yet the amount emitted as N2O is unknown(IPCC,1996).

Rates of application of sewage sludge to agricultural land inthe UK are expected to rise from 0.6 million tonnes dry solidper annum (MtDSyr71) to around 1MtDSyr71, now thatsea dumping is no longer permitted. Additional treatmentrequired to comply with the EC Directive on urban wastewater treatment (CEC, 1991), could increase this further to1.5MtDSyr71 by 2005 (Houghton, 1996). Grassland is anattractive route for sludge disposal as it is readily trafficable.

Our objectivewas to measureN2O andCO2 emissions froma medium-textured grassland soil amended with digestedsludge cake in an area considered suitable to receive suchsludge. We also relate these emissions to weather and soilconditions in the 7-month monitoring period after sewagesludge amendment. We reported some measurements ofN2O fluxes in the 7-week period immediately after amend-ment in a short communication (Scott et al., 1998). Here wepresent a more comprehensive report of the experimentalwork.

MATERIALS AND METHODS

The experimentwas located nearAuchincruive, Ayr on a levelsite on Peebles series, an alluvial sandy clay loam of imperfectdrainage. The primary objective of the experiment was tostudy long-term heavy metal accumulation in soils treatedwith sludge and was one of several replicated throughout theUK (Chambers et al., 1999). Digested sewage sludge cake hadbeen applied to a short term Italian ryegrass (Lolium spp.)sward at a rate of 185 t DS ha71yr71 for three years prior to afinal application on 5th July 1997.The total N applied to thetrial plots (S1 and S2) was 2500 kgN ha71yr71, 30% consid-ered as available. For comparison, a high rate of N

1Land Management Department, SAC, West Mains Road, Edinburgh,EH93JG, UK.2Environmental Science Department, SAC, Auchincruive, Ayr, UK.*Corresponding author.

36 Gas emissions from sludge treated soil

(480 kgN ha71)was broadcast as ammonium nitrate fertilizer(F plot) in 120 kg increments from 29th July to 23rd Septem-ber 1997.This control plot had not received any fertilizer forseveral years. All sludge-treated plots had been rotaryspaded to a target depth of 250mm 26 days after sludgespreading.

Following the final sewage sludge application in July 1997,gas fluxes were measured using one automated and twomanually closed chamber systems per treatment.These wereinstalled in the S1, S2 and F plots five days after sludge in-corporation.The delay in installationwas to enable site prep-aration prior to re-seeding. The chamber encloses theatmosphere immediately above the soil surface and issampled one hour after closure. For a constant net emissionof N2O or CO2, we have found that the increase in concentra-tion within closed chambers is linear over a period of up to3 h (Scott et al., 1999). This change in concentration is aresult of net emission from the soil and enables gas flux to bedetermined.

We have developed separate gas sampling techniqueswhich allow N2O and CO2 to be measured on a sample fromeach type of chamber (Scott et al., 1999). The manual cham-bers (Clayton et al., 1994) were sampled twice per week for thefirst seven weeks then once per week using portable evacu-ated aluminium tubes fitted with an integral needle valve.The automatic chambers were programmed to close for 1hand remain open for 3 or 7 h, thereby giving 6 or 3 flux assess-

Fig. 1.N2O flux and rainfall during the first period, high activity.

Fig. 2.CO2 flux and soil surface temperature during the first period, high activity.

Table1. Mean chemical composition of digested sewage sludge cake appliedbetween1994 and1997.

Dry matter (g100 g71) 66.4Organic carbon (g100 g71DS) 12.2Total N (g100 g71DS) 1.37NH4-N (g100 g71DS) 0.05Total P (g100 g71DS) 2.42Total K (g100 g71DS) 0.27

DS� dry solids.

A. Scott et al. 37

ments per day. At the end of each closure period, accumulatedgases in each chamber were collected in one of 24 evacuatedaluminium tubes (each of volume 30 cm3) and isolated by arotary switching valve. The contents of the tubes were ana-lysed in the laboratory by gas chromatography.The frequencyof sampling was reduced after 7 weeks because the temporalvariability of emission decreased. Fluxes were monitoredfrom 5 August 1997 (5 days after incorporation) to 7 February1997 (208 days after incorporation). Monitoring ceased atthis stage because of other demands on the equipment.

Rainfall was measured using a tipping bucket, and soil sur-face temperature was measured using a thermistor probe at5 cm. Values were recorded at hourly intervals during theperiod of flux measurement. Soil ammonium, and nitrateconcentrations were measured at frequent intervals aftersludge incorporation, using continuous flow colorimetricanalysis of 1M KCl extracts prepared from field-moist soilusing a soil : solution ratio of 1 : 5.Volumetric soil water con-

tent was also determined regularly using a `Theta-probe'(Delta Devices, Cambridge). Dry bulk density, pH, electricalconductivity, organic carbon and available P, K and Mg weremeasured in 1994 before the experimental treatments wereapplied. pH, organic carbon, total N, P, K and ammonium-Nwere determined on the sewage sludge cake prior to applica-tion (Table 1).

RESULTS AND DISCUSSION

N2O and CO2 fluxes are divided into three periods corre-sponding to an initial period of high activity, a middle periodof episodic activity and a late period of diminished activity.

First period, 5 August to17 September, high activityThe N2O and CO2 fluxes from S1 and S2 showed a similar,complex pattern so that automatic chamber results will begiven only for the more active treatment (S1). N2O and CO2

Fig. 4.N2O flux and rainfall during the second period, episodic activity.

Fig. 3.Volumetric moisture content for the sludge (S1) and fertilizer (F) treatments.

38 Gas emissions from sludge treated soil

fluxes during 5 August to 17 September (5^47 days aftersludge incorporation) are given in Figures 1 and 2.Very highdiurnal fluxes (Fig. 1) of N2O (>4mgNm72 h71) wereemitted during the first 4 days, from the plots amended withsludge. These emissions are typical of fluxes resulting fromdenitrification of a mixture of moist organic substrate con-taining available nitrogen and carbon (Table1), and a relativelywarm (Fig. 2), moist soil (Fig. 3). A further large peak(>5.8 mgNm72 h71) was measured on day 10, possibly sti-mulated by the rain which fell on the previous day.

Thereafter, over the next 20 days, N2O emissions (Fig. 1)decreased as soil water (Fig. 3), surface temperature (Fig. 2),and ammonium and nitrate contents declined (Table 3).Theammonium-N to nitrate-N ratio (1 : 7.5) during this periodindicated that nitrogen was likely to have been mineralized,providing a substrate for nitrification. The diurnal fluxes ofN2O (between 0.4 and 1.3 mgNm72 h71) throughout thisphase were possibly as a result of this nitrification. On day33, further high N2O fluxes (>2mgNm72 h71) were mea-sured, typical of rainfall induced denitrification associatedwith partly waterlogged soil (Granli & BÖckman, 1994). Thispattern of response was repeated after further heavy rainfallsup to the end of this period.

CO2 fluxes in the sludge and fertilizer treatments (Fig. 2)showed a marked diurnal pattern which reflected soil surfacetemperature. N2O emission was also influenced by surfacetemperature, the marked decrease in temperature on day 10giving a reduction in fluxes of both gases. The averageweekly N2O emissions from the manual chambers were1.3 kgfrom S1 and 2.0 kg from S2, reflecting slight variation inslurry application. However, the N2O flux from autochamberS1was constantly greater than that from S2.These effects arelikely to be associated with the placement of the chamberson areas of contracting microbial activity. The fertilizedcontrol plot gave low fluxes throughout this period(0.06 kg wk), possibly due to rapid uptake by the grass ofthe fertilizer N71.

Second period, 17 September to 6 December, episodic activityGas fluxes during the period17 September to 6December (48to 128 days after sewage sludge incorporation) are given forS1 in Figures 4 and 5 (values from F were consistently below0.15mgNm72 h71 in this and the third period and are notshown). Rainfall-induced denitrification is likely to haveresulted in the very high N2O fluxes on day 69, 72 and 79(Fig. 4). CO2 emission in the sludge treatment during thisperiod again followed a diurnal pattern with variation asso-ciated with surface temperature (Fig. 5). However, the mark-edly greater CO2 emissions from the sludge treatment thanfrom the fertilized treatment indicate the importance oforganic substrate in maintaining respiration. This would be

Fig. 5.CO2 flux and soil surface temperature during the second period, episodic activity.

Table 2. Soil chemical properties and bulk density (0^15 cm) before (1994)and after (1997) sludge application. S1, S2 andFcorrespond to sludge amend-ments of185 tds ha year71 and fertilizer amendments 480 kgN ha71.

Before (1994) After (1997)

All plots S1 S2 F

Dry bulk density (t m73) 1.33 0.86 0.88 0.98pH 6.0 5.9 6.2 5.4Conductivity (mS) 0.17 0.65 0.49 0.40Organic carbon (g100 g71) 2.5 5.5 5.2 2.6Organic matter (g100 g71) 5.0 13.4 13.1 4.7Total N (g100 g71) 0.22 0.57 0.58 0.29Available P (mg kg71) 28 128 122 27Available K (mg kg71) 45 47 42 34AvailableMg (mg kg71) 264 142 112 210

Table 3. Soil ammonium and nitrate contents at 2.5^15 cm depth. See cap-tion of Table 2 for explanation of S1, S2 and F.

Days after sludge NH4 (mg kg71) NO3 (mg kg71)incorporation (Date)

S1 S2 F S1 S2 F

15 (15 August) 34 78 46 284 533 7329 (29 August) 4 2 2 28 20 336 (5 September) 2 3 9 13 10 647 (16 September) 0 0 10 4 9 361 (30 September) 11 11 134 40 59 1782 (21October) 4 2 301 48 27 24103 (11November) 1 2 1 5 6 2124 (2 December) 2 1 1 6 5 3138 (16 December) 1 1 1 2 2 1

A. Scott et al. 39

expected because respiratory activity responsible for CO2production during decomposition of organic material isstrongly influenced by soil temperatures (Kirschbaum,1995).After this period (days 97 to 116), lower rainfall intensity andsoil temperatures reduced N2O flux and the CO2 flux patternbecame less diurnal.

Third period,7 December to 24 February, low activityN2O andCO2 fluxes during 7December to 24 February (128^208 days after sewage sludge incorporation) are given for S1in Figs. 6 and 7. Precipitation (Fig. 6) was greatest during thisperiod but, unlike earlier periods, had minimal influence onN2O flux, partly due to the lower soil temperatures (Fig. 7).

Short term peaks in N2O and CO2 emissions occurred fol-lowing periods when the temperature was below 4 �C (Figs 6and 7). Although a number ofworkers have reported stimula-tion of N2O flux by freezing (Cates &Keeney, 1987; Christen-sen & Tiedje, 1990; RÎver et al., 1998) following a cold winter,similar temperature related peaks also occurred over a mild

winter. The low background N2O emission throughout thisperiod indicated continued mineralization.

Cumulative fluxThe cumulative fluxes throughout the entire monitoringperiod are given in Table 4. The manual chamber results forboth gases reflect the treatment effects better than thosefrom the automated chambers. However, the spread ofresults, indicated by the high CVs, highlight the problem ofspatial variation even in relatively small plots.These cumula-tive fluxes for sewage sludge are large, typically 2 to 3 timeshigher than those from conserved grass amendedwith fertili-zer or animal slurry plus fertilizer mixtures (McTaggart et al.,1997). The cumulative fluxes over a 7 month period providean N2O emission factor of up to 1% of the applied N. Thislies within the range of N2O emission factors for mineral fer-tilizer (Dobbie et al., 1999). The initial very high emissionsreflect the high level of sludge application and its burial inconditions that promote microbial activity. The sustained

Fig. 6.N2O flux in sludge (S1) and fertilizer (F) treatments and rainfall during the third period, low activity. Arrows indicate when soil surface temperatureswere below 4 �C.

Fig. 7.CO2 flux in sludge (S1) and fertilizer (F) treatments and soil surface temperature during the third period, low activity.

40 Gas emissions from sludge treated soil

emissions from sewage amendment are likely to result fromthe soil microflora adaptation, their ability to mobiliseorganicN fixed in proteinaceous material and mineralizationof earlier incorporated material.

Throughout this experimentN2O andCO2 fluxes from theF (fertilized) plot were consistently low. This was attributedto the soil being severely deficient in nitrogen before treat-ment application (Table 2) such that the repeated applicationsof fertilizer up to 480 kg ha71 did not result in the expectedhigh N2O emissions.The soil of this fertilized plot had a lowpH (Table 2), nitrate (Table 3) and water content (Fig. 3), fac-tors likely to reduce microbial activity and production ofN2O (Granli & BÖckman, 1994).

CONCLUSIONS

High cumulative emissions of N2O and CO2 from sewagesludge amended soil were detected both by automated andmanual sampling systems. The automated system was espe-cially useful in detecting the marked temporal variability ofthese gases, associated with the diurnal temperature changeand rainfall.Warm soil surface temperatures (10^25 �C) com-bined with moderate to high rainfall (4^12mm) to stimulateN2O and CO2 fluxes. But surface temperatures below 4 �Calso stimulated short sharp fluxes.The high cumulative emis-sions from sewage sludge amended soil were attributed tothe high N, C and moisture levels in the soil. These effectsand the adaptation of the soil microbial population to mobi-lise organic N in wastes require further study. Sustainedmixing of high levels of moist digested sludge over severalyears may not be the best means of disposal for minimisingatmospheric pollution. Further investigation is required toassess the suitability of other sludge treatment options andland management practices for improved nutrient conserva-tion and crop uptake.

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Received April 1999, accepted September1999.

# British Society of Soil Science 2000

Table 4. CumulativeN2O (kgN ha71) andCO2 (tonneC ha71) flux measure-ments for the period 5August1997 to 25February1998 from individual cham-bers. See caption of Table 2 for explanation of S1, S2 and F.

Chamber S1 S2 F

N2O (Automatic) 20.6 9.5 1.3N2O (Manual1) 14.0 14.7 1.9N2O (Manual 2) 12.5 23.3 1.8CV (%) (Manual) 27.4 44.0 19.3CO2 (Automatic) 5.1 3.7 1.9CO2 (Manual) 5.3 5.0 3.5CO2 (Manual) 4.6 6.0 3.9CV (%) (Manual) 7.2 23.5 29.9

A. Scott et al. 41