Impact of no-till and reduced tillage on aggregation

Soil & Tillage Research 150 (2015) 107–113

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Impact of no-till and reduced tillage on aggregation and aggregate-associated carbon in Northern European agroecosystems Jatta Sheehy a,b, * , Kristiina Regina a , Laura Alakukku c , Johan Six b,d a

Natural Resources Institute Finland (Luke), Plant Production Research, FI-31600 Jokioinen, Finland Department of Plant Sciences, University of California, Davis, CA 95616, USA Department of Agricultural Sciences, University of Helsinki, Finland d Department of Environmental Systems Science, Swiss Federal Institute of Technology, ETH-Zürich, CH 8092, Zurich, Switzerland b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 28 January 2014 Received in revised form 29 December 2014 Accepted 30 January 2015

Minimum tillage practices have been shown to enhance soil aggregation and soil organic carbon (SOC) stabilization. Carbon turnover rate slows down when soil aggregation increases and SOC is protected within stable microaggregates (53–250 mm). However, this has not been investigated in boreal soils. Therefore, the objective of this study was to quantify the long-term effects of no-till (NT) and reduced tillage (RT) on SOC stabilization in four soils typical for the boreal region. Distribution of SOC in different soil fractions in a 0–20 cm soil layer was analyzed by wet sieving and further isolation of microaggregates (mM) from large (>2000 mm, LM) and small (250–2000 mm, sM) macroaggregates. Aggregate size decreased in the order of NT > RT > CT at all study sites. In addition to increased mean weight diameter (MWD) under NT, a general trend of redistribution of SOC into these formed macroaggregates was found at all study sites, i.e., the LM fraction gained SOC. However SOC was lost in other fractions under NT compared to CT at some sites and none of the sites showed any significant changes in bulk soil SOC content under NT or RT. Also our hypothesis that there would be more SOC incorporated in mM fraction in NT and RT compared to CT was corroborated only at site 4 under NT. Thus, although the potential to accumulate SOC under NT or RT compared to CT seems to be limited in boreal agroecosystems, the redistribution of SOC to the more stable conditions within the aggregates indicates positive impacts of no-till practice. ã 2015 Elsevier B.V. All rights reserved.

Keywords: No-till Reduced tillage Microaggregates Soil aggregation C sequestration

1. Introduction The increasing CO2 concentration in the atmosphere has led to an increased interest in mitigating greenhouse gas emissions while maintaining sustainable agricultural production. It has been estimated that the world's agricultural sector has a potential as great as 4.5–6.0 Pg CO2 equivalents per year to mitigate greenhouse gas emissions (Smith et al., 2007). The amount of CO2 released to the atmosphere from agricultural soils is mainly dependent on the rate of soil organic carbon (SOC) formation versus decomposition (Van Breemen and Feijtel, 1990). The key role of croplands in the soil-atmosphere carbon cycling makes preservation of SOC an essential component of sustainable cropping systems. Minimum tillage practices, including no-till (NT) and reduced tillage (RT), have received attention due to their ability to both reduce soil erosion and increase C sequestration in agricultural

* Corresponding author. Tel.: +1 5412631470. E-mail address: jatta.sheehy@luke.fi (J. Sheehy). http://dx.doi.org/10.1016/j.still.2015.01.015 0167-1987/ ã 2015 Elsevier B.V. All rights reserved.

surface soils (Cole et al., 1997) by increasing aggregate stability. Aggregates are groups of soil particles that bind more strongly to each other compared to other surrounding soil particles. Increased levels of SOC under NT have been found especially in the tropics (Bayer et al., 2006; Lal, 1997; Maia et al., 2010) and temperate regions of the world (Lal et al., 1999). According to Freibauer et al. (2004) the potential for NT and RT management practices to sequester SOC in European (EU15) agricultural soils is 0.3–0.4 0.1 Mg C ha1 year1 and 2000 mm), small macroaggregates (sM; 250–2000 mm), microaggregates (m; 53–250 mm) and silt and clay (s + c; 250 mm), microaggregates within macroaggregates (mM; 53–250 mm) and silt and clay (s + cM; RT > CT at all sites, but not always significantly (Table 2). Statistically significant differences were found at sites 1 and 4 between CT and NT (p = 0.022 and p = 0.001 respectively). At site 2, the difference Table 2 Mean weight diameter (MWD, mm SE) after wet sieving in conventional tillage (CT), reduced tillage (RT) and no-till (NT), and bulk soil SOC (Mg C ha1 SE) for all study sites. Field

Treatment

MWD (mm)

Bulk soil SOC (Mg C ha1)

Site 1

CT RT NT

0.55 0.07b 0.59 0.06ab 0.84 0.10a

57.3 3.5a 61.2 2.8a 61.2 2.5a

Site 2

CT RT NT

1.36 0.27a 1.36 0.15a 2.10 0.20a

59.7 1.3a 56.9 1.2a 60.8 0.6a

Site 3

CT NT

1.20 0.17a 1.58 0.18a

83.7 3.9a 69.4 4.1a

Site 4

CT NT

0.28 0.0b 0.58 0.03a

53.0 0.3a 49.5 1.5a

Statistically significant differences between management practices are indicated by different lower case letters within each study site.

The bulk soil SOC stocks in the 0–20 cm soil layer did not significantly differ between management practices (Table 2). The largest difference between management practices was found at site 3 with bulk soil SOC being 17% lower in NT compared to CT, but the difference was only marginally statistically different (p = 0.066). Enrichment of SOC under NT management was observed in the LM fraction (at site 4 LM + sM) with statistically significant differences at sites 1, 2 and 4 (p = 0.009; p = 0.025; p = 0.001, respectively) (Fig. 1). On the other hand, this effect was diluted at sites 2 and 4 by a loss of SOC in both m and s + c fraction (p = 0.008; p = 0.036; p = 0.001; p = 0.001), which highlights a redistribution of SOC across the different fractions rather than an additional stabilization. A statistically significant depletion of SOC under NT was found at site 3 where the SOC decreased in the sM fraction (p = 0.023). RT had no significant effect on the SOC content compared to CT. The amount of SOC in the mM fraction (microaggregates within macroaggregates) increased significantly only at site 4 in NT compared to CT (Fig 2). No significant differences were found between RT and the other two treatments. Statistically significant changes in SOC content were found in macroaggregate-occluded silt and clay fractions at sites 1, 2 and 4 (p = 0.046; p = 0.026; p = 0.011) where SOC increased in NT in comparison to CT. There were no statistically significant changes at site 3. However, site 3 had relatively the largest amount of mM fraction within macroaggregates (g mM fraction gM1) with 63% in CT and 64% in NT treatment; however it was also the only site with a decreasing trend in the overall SOC content when transitioning from CT to NT. Site 4 had statistically significant differences between management practices in all macroaggregate-occluded fractions (cPOM, mM and s + cM) which highlights an enrichment of SOC in macroaggregate-occluded fractions under NT compared to CT. 4. Discussion A general trend of redistribution of SOC within the different fractions was found at all study sites while no significant changes were present in the bulk soil SOC content under NT or RT compared

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Table 3 Proportional aggregate weight (%) of different soil fractions acquired from wet sieving of whole soil (Part A) and from fractions isolated from macroaggregates (Part B) in the 0–20 cm soil layer in conventional tillage (CT), reduced tillage (RT) and no-till (NT) systems (mean SE). Part A LM1

Part B sM2,8

m3

s + c4

cPOM5

mM6

s + cM7

Site 1

CT RT NT

1.1 0.2a 2.1 0.5a 5.6 1.5a

31.3 3.3a 36.9 3.9a 44.2 2.8a

50.5 3.3a 45.4 3.5a 37.5 3.0a

17.1 0.2a 15.6 1.3a 12.7 1.2a

27.1 2.7a 24.0 0.7a 19.0 1.4a

44.8 2.8a 45.6 2.0a 50.2 1.9a

28.1 3.8a 30.4 1.8a 30.8 1.9a

Site 2

CT RT NT

9.7 5.1a 14.2 3.5a 26.1 1.9a

54.9 1.7a 54.7 2.5a 51.2 1.2a

25.8 2.3a 22.6 1.0a 16.7 1.2a

9.6 1.2a 8.5 0.7a 6.0 0.5a

14.0 1.1a 12.5 0.8a 11.6 1.2a

57.6 1.7a 57.8 1.3a 60.8 1.8a

28.4 2.2a 29.7 1.8a 27.6 2.0a

Site 3

CT NT

10.5 3.6a 16.3 2.7a

56.3 0.9a 50.3 0.6a

22.6 2.6a 23.1 2.4a

10.6 0.7a 10.3 0.9a

3.0 0.4a 4.2 0.8a

63.2 1.6a 63.6 1.5a

33.8 1.6a 32.2 1.1a

Site 4

CT NT

14.5 1.3b 30.5 1.8a

66.0 1.5a 52.5 1.8a

19.7 0.5a 17.0 1.3a

14.7 1.1a 8.5 0.8a

47.4 1.5a 59.1 1.7a

37.9 1.1a 32.4 1.2a

Statistically significant differences between different treatments within a study site are denoted by different lower case letters. 1 LM: large macroaggregates. 2 sM: small macroaggregates. 3 m: microaggregates. 4 s + c: silt and clay. 5 cPOM: coarse particulate organic matter. 6 mM: microaggregates (within macroaggregates). 7 s + cM: silt and clay (within macroaggregates).

Fig. 1. Soil organic C content (kg C m2 standard error) of large macroaggregates (LM), small macroaggregates (sM) (M = LM + sM), microaggregates (m) and silt and clay (s + c) in conventional tillage (CT), no-till (NT) and reduced tillage (RT) systems.

to CT. This is in contrast to most studies finding reduced tillage and no-till practices to stabilize SOC compared to CT. Stabilization of SOC under NT compared to CT has not only been realized in agricultural soils including Oxisols (Denef et al., 2007), Alfisols (Chung et al., 2008) and integrated crop-livestock systems after intensifying agricultural land use (Franzluebbers and Stuedemann, 2008) but also in medium input tropical cropping systems where NT has been shown to increase C stock when compared to native soils (Maia et al., 2010).

Soil processes controlling C sequestration are strongly affected by climate but there are conflicting results for the C sequestration rates for humid and arid zones of the world. Some studies in temperate regions of the world have shown that SOC stabilization is slower in arid regions compared to humid climatic conditions (Six et al., 2004). On the other hand a data compilation done in Canada showed increased soil C stocks under NT only in the arid western parts of the country (VandenBygaart et al., 2003). It has been observed that the CT practice may not decrease the SOC


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4

Site 1

Site 2

SOC (kg C m-2)

a

a

a

2

a

a

a 1

CT NT RT

a

3

a

a a a

b

ab

b

ab

a a a

0

SOC (kg C m-2)

4

a

Site 3

a

Site 4

3 a

2 1

a

a

a

a

b

a

b

b

a

0 cPOM

mM

s+cM

cPOM

mM

s+cM

Aggregate size fractions Fig. 2. Soil organic C content (kg C m2 standard error) of coarse particulate organic matter (cPOM), microaggregates within macroaggregates (mM) and silt and clay within macroaggregates (s + cM) in conventional tillage (CT), no-till (NT) and reduced tillage (RT) systems.

stocks as quickly in cool and humid compared to arid regions (Gregorich et al., 2005; Hermle et al., 2008). One reason for this could be that the decomposition rate of crop residues incorporated deep into the soil slows down under CT due to limited aeration under these climatic conditions (Angers et al., 1997). In cold climate, the physical disruption of aggregates during the winter soil frosting in all management practices may reduce the differences between treatments and counteract the macroaggregate formation occurring during the warmer seasons (Le Guillou et al., 2012) Agroecosystems under NT may experience reduced yields which in turn may negatively affect C sequestration rates (BlancoCanqui and Lal, 2008; Venterea et al., 2006; West and Post, 2002). Reduced yields have been found at sites 1 and 2 where the average yield since transition has been 4% and 12% lower, respectively, under NT (Site 1: 4276 kg ha1; site 2: 3872 kg ha1) compared to CT (Site 1: 4447 kg ha1; site 2: 4390 kg ha1). This was also the case at site 3 where the average yield of the last decade in CT (4029 kg ha1) was 22% higher compared to NT (3290 kg ha1). This might be one of the key factors affecting the SOC depletion that was found at site 3 in NT compared to CT. No long-term yield data was available for site 4. Similar results have been reported by Follett (2001) and Lal et al. (1999) on Bojac soil with lower yields and reduced amounts of residue returned to the field each season resulting in lower SOC content in NT compared to CT. Our results show a low bulk soil SOC content for site 4 compared to the other study sites. This might be explained by the difference in soil type. High SOC content of clayey soils is generally acknowledged (Heikkinen et al., 2013; Nichols, 1984; Wiseman and Puttmann, 2006). Clay soils have more sites for strong adsorption of SOC on the soil minerals and thus higher potential for C sequestration (Chan et al., 2008; West and Six, 2007). Long-term studies have shown that soil C stock does not necessarily increase with higher C inputs (Paustian et al., 1997b; Reicosky et al., 2002;

Stewart et al., 2007) and the efficiency to stabilize SOM decreases in soils with high C levels compared to soils with lower C levels under the same management practices (Campbell et al., 1991). There is a possibility that the clayey soils of sites 1–3 which had significant C redistribution or depletion but no added SOC under NT versus CT have most of the binding sites saturated and therefore less potential to accumulate SOC upon changes in management. The redistribution of SOC was especially observed in the LM fraction, which gained SOC under NT compared to CT at sites 1, 2 and 4. On the other hand, the gains in LM fractions were counteracted by loss of SOC in the other fractions at sites 1 and 4. This suggests, however, that most of the dynamics after transitioning to NT occurs in the LM. Having more large aggregates increases the stability of the soil and could potentially lead to SOC stabilization when carbon inputs to the soil are high enough. On the other hand it is also possible that high amounts of crop residues stratified in the topsoil of NT systems accelerates microbial activity (Hungria et al., 2009) and decomposition which leads to SOC losses (Fontaine et al., 2004). According to Six and Paustian (2014), the mM fraction can serve as a robust indicator for the changes in SOC under different management practices over decadal time scales. Significantly more SOC in the mM fraction of NT soils has been found in several experiments in varying soil types and environments indicating that a substantial proportion of the difference in NT versus CT could be accounted for by C associated with the mM fraction (Six and Paustian, 2014). In contrast, our hypothesis that there would be more incorporated SOC in the mM fraction in NT compared to CT was validated only at site 4 which also had the highest proportion of mM per gram of macroaggregates in comparison to the other study sites. A higher amount of earthworm middens was found in NT at site 4 compared to CT (Regina and Alakukku, 2010). Earthworms are known to change soil aggregation dynamics and significantly increase N incorporation into macroaggregates

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(Giannopoulos et al., 2010). Thus it is possible that increased amount of earthworms also contributes to higher amount of SOC within macroaggregates under NT. On the other hand, our results do not show an increase in the bulk SOC content at this site even though aggregation increased as did the SOC content within macroaggregates. It is possible that the advantage of this process is offset by increased decomposition rate of organic material resulting in higher CO2 emissions from the soil. The increased amount of SOC in macroaggregate-occluded microaggregates in addition to the changes in mean weight diameter, however, indicate higher potential for future soil aggregation and accumulation of SOC under NT compared to CT at this site. 5. Conclusions The results emphasize that aggregate size and aggregateassociated C play an important role in determining the long-term C sequestration potential of agroecosystems in Northern European boreal climate. According to these results the mean weight diameter is enhanced in NT cropping systems compared to CT or RT. However, the bulk soil SOC did not increase under NT or RT management compared to CT in a decade. Most possible reason for this is the lower crop yields found on three of the studied sites in NT resulting in decreased amount of C inputs. Furthermore, these sites might have a low saturation deficit due to their relatively high C content. These factors may have hampered the ability of NT management to increase SOC accumulation. Also, slow decomposition rates in CT soils in the cold climate and breaking of aggregates during the winter in all management practices may reduce the differences between treatments. However, NT increased the amount of SOC in the mM fraction in the coarse soil underlining a potential for future carbon accumulation in this soil type especially if the C inputs improve. Thus, although the potential to accumulate SOC under NT or RT compared to CT seems to be limited in boreal agroecosystems the redistribution of SOC to the more stable conditions within the aggregates indicates positive impact of NT practice. Acknowledgements This study was funded by Maj and Tor Nessling Foundation, Finland, Emil Aaltonen Foundation, Finland, Häme Cultural Foundation, Finland and MTT Agrifood Research Finland and done in cooperation with University of Helsinki, Finland and University of California, Davis, USA. Many thanks to the staff of MTT and Sirkku Manninen for all the support and advice throughout the process. Special thanks to Pauline Chivenge and Robert Rousseau for valuable advice in the laboratory and also thanks to all the people in the Agroecology lab at UC Davis. The staff of the research center of Yara Finland, and the farmers Timo Rouhiainen and Ilmari Seppälä are greatly appreciated for giving us the chance to use their experimental fields as a part of this study. References Angers, D.A., Bolinder, M.A., Carter, M.R., Gregorich, E.G., Drury, C.F., Liang, B.C., Voroney, R.P., Simard, R.R., Donald, R.G., Beyaert, R.P., Martel, J., 1997. Impact of tillage practices on organic carbon and nitrogen in cool, humid soils of eastern Canada. Soil Till. Res. 41, 191–200. Bayer, C., Martin-Neto, L., Mielniczuk, J., Pavinato, A., Dieckow, J., 2006. Carbon sequestration in two Brazilian Cerrado soils under notill. Soil Til. Res. 86, 237–245. Blanco-Canqui, H., Lal, R., 2008. No-tillage and soil-profile carbon sequestration: an on-farm assessment. Soil Sci. Soc. Am. J. 72, 693–701. Campbell, C.A., Lafond, G.P., Zentner, R.P., Biederbeck, V.O., 1991. Influence of fertilizer and straw baling on soil organic matter in a thin black Chernozem in western Canada. Soil Biol. Biochem. 23, 443–446.

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