No-till only increases N 2 O emissions in poorly-aerated soils

Soil & Tillage Research 101 (2008) 97–100Contents lists available at ScienceDirectSoil & Tillage Researchjournal homepage: www.elsevier.com/locate/stillShort communicationNo-till only increases N 2 O emissions in poorly-aerated soilsPhilippe Rochette *Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd, Québec City, QC, Canada G1V 2J3ARTICLEINFOABSTRACTArticle history:Received 10 March 2008Received in revised form 22 May 2008Accepted 21 July 2008Keywords:No-tillN 2 ODrainage classSoil aerationGreenhouse gasesDenitrification rates are often greater in no-till than in tilled soils and net soil-surface greenhouse gasemissions could be increased by enhanced soil N 2 O emissions following adoption of no-till. The objectiveof this study was to summarize published experimental results to assess whether the response of soil N 2 Ofluxes to the adoption of no-till is influenced by soil aeration. A total of 25 field studies presenting directcomparisons between conventional tillage and no-till (approximately 45 site-years of data) werereviewed and grouped according to soil aeration status estimated using drainage class and precipitationduring the growing season. The summary showed that no-till generally increased N 2 O emissions inpoorly-aerated soils but was neutral in soils with good and medium aeration. On average, soil N 2 Oemissions under no-till were 0.06 kg N ha 1 lower, 0.12 kg N ha 1 higher and 2.00 kg N ha 1 higher thanunder tilled soils with good, medium and poor aeration, respectively. Our results therefore suggest thatthe impact of no-till on N 2 O emissions is small in well-aerated soils but most often positive in soils whereaeration is reduced by conditions or properties restricting drainage. Considering typical soil C gainsfollowing adoption of no-till, we conclude that increased N 2 O losses may result in a negative greenhousegas balance for many poorly-drained fine-textured agricultural soils under no-till located in regions witha humid climate.Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved.1. IntroductionNo-till has been proposed to increase stocks of soil organicmatter and mitigate greenhouse gas emissions (Gregorich et al.,2005). However, denitrification is usually greater in soils under notillthan under conventional tillage as a result of higher bulkdensity and water content (Doran, 1980; Groffman, 1984; Arahet al., 1991; Palma et al., 1997). Denitrification is often the mainsource of N 2 O in agricultural soils and the benefits of the adoptionof no-till on atmospheric CO 2 sequestration could be offset byincreased N 2 O emissions (Six et al., 2002).The impact of no-till on soil N 2 O emission is variable. Higher(Ball et al., 1999; Rochette et al., 2008) and lower (Chatskikh andOlesen, 2007; Gregorich et al., 2008) N 2 O–N losses have beenmeasured in no-till compared to tilled soils. Predictions bymathematical models also indicated that the influence of no-tillon N 2 O emissions could be either positive (Mummey et al., 1998; Liet al., 2005) or negative (Li et al., 1996). Six et al. (2004) concludedthat soil N 2 O emissions are increased under no-till but that thisimpact decreases with time. However, an explanation of the high* Tel.: +1 418 210 5042; fax: +1 418 648 2402.E-mail address: rochettep@agr.gc.ca.inter-site variability of the influence of no-till on soil N 2 Oemissions is still lacking. In this study, we hypothesized thatadoption of no-till only increases N 2 O emissions in poorly-aeratedsoils.2. Materials and methodsWe summarized reports of field N 2 O emissions from 25 studies(approximately 45 site-years of data) with same-site comparisonsof no-till and tilled soils. Soil aeration is closely related to watercontent (Linn and Doran, 1984). Therefore, each situation wasclassified under either ‘‘good’’, ‘‘medium’’ or ‘‘poor’’ soil aerationclass based on soil drainage and precipitation during the growingseason. Soil aeration was estimated to be ‘‘poor’’ if drainage was‘‘poor’’ irrespective of precipitation. When not present in the citeddocument, information on drainage class for a given soil series wasobtained from soil survey publications. Situations with poordrainage and aeration were also mostly characterised by finetexturedsoils under cool humid climates (Table 1). Soils with goodor medium drainage were assigned to a ‘‘good’’ or ‘‘medium’’aeration class depending if precipitation (including irrigation)during the growing season was 400 mm, respectively.At most sites characterized by a semi-arid climate,precipitation was

98P. Rochette / Soil & Tillage Research 101 (2008) 97–100Table 1Cumulative field N 2 O emissions and other ancillary parameters in tilled (T) and no-till (NT) agricultural soilsAerationstatusDrainageGrowing seasonprecipitations a(mm)Soil texture Climate Tillage btype(depth)Measurementperiod (d yr 1 )Cumulated N 2 O emissions(kg N 2 O–N ha 1 )ReferenceNT T NT–TGood High 384 Loam Cool MP (30) 365 1.32 0.80 0.52 Oorts et al., 2007High 305 Loam Semi-arid MP (15) 365 0.25 0.31 0.06 Kessavalou et al. (1998)Medium 271 Sandy cl. loam Semi-arid R (10) 110 0.12 0.25 0.12 Malhi et al. (2006)Medium 270 Sandy cl. loam Semi-arid R (10) 130 0.34 0.40 0.06 Malhi and Lemke (2007)Medium 320 Loam Semi-arid R (10) 170 0.30 0.29 0.01 Lemke et al. (1999)Medium 242 Clay loam Semi-arid R (10) 170 0.97 1.46 0.49 Lemke et al. (1999)Medium 291 Silt loam Semi-arid D (10) 365 0.38 0.38 0.0 Dusenbury et al. (2008)Mean 298 239 0.39 c 0.45 c 0.06Medium ns 694 Volcanic ash Cool humid MP (25) 365 0.83 d 0.27 0.56 Koga et al. (2004)High 640 Sandy loam Cool humid MP (20) 215 1.11 0.99 0.12 Rochette et al. (2008)High 622 Loam Cool humid MP (ns) 365 1.32 1.20 0.12 Grandy et al. (2006)High 429 Loamy sand Cool MP (20) 113 0.43 0.89 0.46 Chatskikh and Olesen(2007)Medium 410 Loam Cool humid ns (25) 65–79 1.97 0.46 1.51 Baggs et al. (2003)High 620 Loam Cool humid C + RT 150 1.84 1.88 0.04 Jacinthe and Dick (1997)High 560 Loam Cool humid MP (20) 180 1.00 1.34 0.34 Gregorich et al. (2008)Medium 552 Clay loam Semi-arid + Ir. MP (ns) 365 1.19 1.51 0.32 Liu et al. (2005)Medium 552 Clay loam Semi-arid + Ir. MP (ns) 365 0.99 1.29 0.30 Mosier et al. (2006)High 574 Loam Cool humid MP (20) 225 2.10 1.80 0.30 Larouche (2006)High 1200 Clay Sub-tropical D (20) 365 0.87 0.70 0.17 Jantilia et al. (2008)High 1000 Clay loam Sub-tropical D (15) 180 0.31 0.35 0.04 Metay et al. (2007)Mean 654 247 1.02 c 0.90 c 0.12Poor Poor 430 Clay loam Cool humid MP (15) 215 3.35 3.82 0.47 Drury et al. (2006)Poor 430 Clay loam Cool humid MP (15) 215 1.00 1.15 0.15 Kaharabata et al. (2003)Poor 590 Loam Cool humid C (18) 365 7.74 7.58 0.16 Parkin and Kaspar (2006)Poor 400 Heavy clay Cool humid MP (ns) 200 4.40 2.00 2.40 Burford et al. (1981)Poor e 377 Clay loam Cool humid MP (20) 77 13.0 f 3.80 f 9.20 Ball et al. (1999)Poor 680 Silty loam Cool humid MP (25) 260 12.0 9.20 2.80 Choudhary et al. (2002)Poor 574 Heavy clay Cool humid MP (20) 240 2.79 1.99 0.80 MacKenzie et al. (1997)Poor 640 Heavy clay Cool humid MP (20) 215 32.7 13.3 19.34 Rochette et al. (2008)Mean 515 223 5.97 c 3.97 c 2.00a Including irrigation.b MP = moldboard plowing; R = rotovator; C = Chizel; RT = ridge tillage; D = disking; ns = not specified.c Geometric mean.d Reduced tillage.e Gleysol.fEstimated using values in Fig. 3 of Ball et al. (1999).threshold above which soil water content can restrict aeration.When emission data were reported for several years in a givenstudy, all cumulative N 2 O emissions during the measurementperiod were averaged. Finally, to account for the skeweddistribution of soil N 2 O fluxes, the mean cumulated N 2 O for eachsoil aeration class was estimated as the geometric mean of valuesreported for each tillage practice.3. Results and discussionMost studies conducted in poorly-aerated soils had greater N 2 Oemissions under no-till than under conventional tillage while bothpositive and negative responses of emissions to no-till wereobserved on soils with good or medium aeration (Table 1).Cumulative emissions in poorly-aerated soils were greater underno-till in 6 out of 8 studies and differences were >2kgNha 1 onfour occasions. In addition, the distribution of differences in N 2 Oemissions between tilled and no-till situations was skewedtowards positive values, indicating that the impact of no-till onN 2 O emissions can be exceptionally high at a few poorly-drainedlocations. In contrast, in soils with good and medium aeration,differences in emissions between tillage practices were small andequally distributed between positive and negative values. Onaverage, soil N 2 O emissions under no-till were 0.06 kg N ha 1lower, 0.12 kg N ha 1 higher and 2.00 kg N ha 1 higher than undertilled soils with good, medium and poor aeration, respectively. Theratio of mean cumulative emissions from no-till to tilled soils in thesame three aeration classes was 0.87, 1.13 and 1.50 (Fig. 1). Thissummary therefore suggests that the mean impact of no-till onN 2 O emissions is small in well-aerated soils but most often positiveon soils where aeration is restricted.The response of N 2 O emissions to conversion from conventionaltillage to no-till was variable in all soil aeration classes butvariability was greatest in poorly-aerated soils (Table 1). Thegrouping of situations based on soil drainage class may explainpart of this variability. Soil drainage classes in soil classificationsystems do not necessarily reflect the current drainage level butrather reflect the conditions that prevailed during soil development.Especially, several poorly-drained agricultural soils areartificially drained and therefore have a better aeration status thanindicated by their drainage class. Also, the structure of surface soilwas shown to improve with time after conversion to no-till andthis evolution of soil properties likely impacts on N 2 O productionand emission (Six et al., 2004). Therefore, grouping soils withdifferent tillage history has likely contributed to increasevariability.Increased emissions associated with poorly-aerated conditionsstrongly suggest that the additional N 2 O originates from enhanceddenitrification in no-till soils. Oxygen often limits denitrification inagricultural soils (Smith and Tiedje, 1979) and higher soil watercontent in no-till soils usually results in lower aeration and greaterdenitrification rates than in tilled soils (Doran, 1980; Groffman,

P. Rochette / Soil & Tillage Research 101 (2008) 97–100 99ReferencesFig. 1. Mean ratio of cumulated N 2 O emissions from no-till (NT) to tilled (T) soilswith poor, medium and good aeration.1984; Arah et al., 1991; Palma et al., 1997). Accordingly, Rochetteet al. (2008) observed that N 2 O emissions were increased in aheavy clay under no-till only when water-filled pore space (WFPS)was 0.6 m 3 m 3 , the threshold above which denitrification isfavoured (Linn and Doran, 1984). In a well-drained gravely loam,WFPS remained below 0.6 m 3 m 3 and emissions were similar inno-till and moldboard plowed soils (Rochette et al., 2008). Wehypothesize that the influence of soil aeration on the response ofN 2 O emissions to no-till (Table 1) is in part explained by the factthat, even though no-till increases soil density and water contentin most soils, WFPS values reach 0.6 m 3 m 3 more often in poorlyaeratedsoils than in well-aerated soils.A relationship between N 2 O emissions and soil aeration suggeststhat disaggregating agricultural land into sub-categories based onsoil and climate characteristics may provide an opportunity forimproving our estimates of the response of soil N 2 O emission to notill.Soil aeration level and water content are highly variable in spaceand time. However, their mean value under given climaticconditions are closely related to soil texture. 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