Seasonal CO2 emission under different cropping systems on Histosols in southern Sweden

Geoderma Regional 7 (2016) 338–345

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Seasonal CO2 emission under different cropping systems on Histosols in southern Sweden Lisbet Norberg ⁎, Örjan Berglund, Kerstin Berglund Swedish University of Agricultural Sciences, Department of Soil and Environment, P.O. Box 7014, 750 07 Uppsala, Sweden

a r t i c l e

i n f o

Article history: Received 12 February 2016 Received in revised form 27 June 2016 Accepted 27 June 2016 Available online 1 July 2016 Keywords: Agriculture Carbon dioxide Cropping system Entisols Greenhouse gas Histosols Inceptisols Peat Peaty marl

a b s t r a c t Drained and cultivated organic soils contribute a substantial proportion of estimated anthropogenic greenhouse gas emissions in Sweden. According to rough estimates, different cropping systems give rise to different subsidence rates and, since some of this subsidence originates from oxidation of organic material, soil respiration may also vary with different crops. This field study investigated the possibility of mitigating carbon dioxide (CO2) emissions from cultivated organic soils by using a specific cropping system. The CO2 emission rates from soils under different crops in similar environmental conditions were measured at 11 field sites in southern Sweden representing different types of organic soils. The variation in emissions between the crops tested was low compared with total CO2 emissions from the soil and differences between crops were not consistent. This shows that growing a particular crop cannot be recommended as a mitigation option for limiting CO2 emissions from cultivated organic soils. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Pristine peatlands represent a large store of carbon worldwide. Under natural conditions, most peatlands are accumulators of organic plant material and, at least in their early life, carbon sinks. Drainage and cultivation of peat soils increase soil aeration and reverse the carbon flux into net carbon dioxide (CO2) emissions. During the past two centuries, large peatland areas in Sweden have been drained for agriculture and forestry purposes. Drained peatlands subside due to consolidation, shrinkage, compaction, erosion and oxidation of the organic material (Berglund, 1996). The soil respiration rate, i.e. oxidation of organic material, is mainly determined by temperature and soil moisture (Koizumi et al., 1999; Fang and Moncrieff, 2001). Vegetation type is another important factor. In Sweden, a rule of thumb, based on measurements of the subsidence in long-term field experiments on organic soils, states that different cropping systems give different subsidence rates, e.g. permanent grassland gives a lower rate than row crops (Berglund, 1989). Since part of the subsidence originates from degradation of organic material, i.e. CO2 emissions, row crops are considered to emit more CO2 than grassland (Kasimir-Klemedtsson et al., 1997). One problem with using subsidence as an estimate of organic matter decomposition is how to distinguish between the different processes causing the ⁎ Corresponding author. E-mail addresses: [email protected] (L. Norberg), [email protected] (Ö. Berglund), [email protected] (K. Berglund).

http://dx.doi.org/10.1016/j.geodrs.2016.06.005 2352-0094/© 2016 Elsevier B.V. All rights reserved.

subsidence (Glenn et al., 1993). For example, arable soils are more prone to wind erosion than grasslands, where the peat is protected by the grass (Irwin, 1977; Lucas, 1982; Parent et al., 1982). The windtransported material is deposited in another place (Parent et al., 1982), sometimes the adjacent grassland, but in this case the decomposition rate of the arable land is not increased, nor is it decreased on the grassland. Well-decomposed (sapric) peat soils are often used to produce high-value row crops and are also the most wind erodible (Kohake et al., 2010). Differing subsidence rates between cropping systems of different intensity may be due mainly to a combination of crop cover, differences in soil type and how prone the soil type is to wind erosion. Old data on subsidence measurements are very often used to upscale the effect of changing crops on CO2 emissions from organic soils (Kasimir-Klemedtsson et al., 1997), but it is very important to measure the actual emission rates from a specific soil under different crops in similar environmental conditions in order to evaluate the true effect. In a digital soil survey in 2003 (Berglund and Berglund, 2010), the area of agricultural organic soils in Sweden was estimated to be approx. 301,500 ha, with 202,400 ha of deep peat (N50 cm peat depth), 50,200 ha of shallow peat (b 50 cm peat depth) and 49,000 ha of gyttja soils (gyttja, marl, marl-containing gyttja, clay gyttja and gyttja clay). This was equivalent to about 8% of Sweden's agricultural land at that time. Managed grasslands and pastures dominated the use of these cultivated organic soils and annual crops only occupied 25% (Berglund and Berglund, 2010). It has been estimated that 6–8% of total annual anthropogenic emissions of greenhouse gases in Sweden originate directly

L. Norberg et al. / Geoderma Regional 7 (2016) 338–345

from farmed organic soils (Berglund and Berglund, 2010). When greenhouse gas emissions from cultivated organic soils are converted into CO2-equivalents, it has been estimated that CO2 contributes 85–95% of the Global Warming Potential (GWP), nitrous oxide (N2O) contributes 5–15% and methane (CH4) b1% (Maljanen et al., 2004; Grönlund et al., 2006). Therefore CO2 should be the most important gas for which to find mitigation options. There is a great need for development of management strategies on peat and other organic soils to reduce subsidence and emission rates. In many countries legislation or subsidy systems to regulate the management of cultivated organic soils, e.g. choice of crop, are currently being discussed. Previous studies regarding the effect of choice of cropping system on CO2 emissions show contradictory results. For example, Maljanen et al. (2002) measured CO2 emissions from drained organic soil with different vegetation types and found that barley (Hordeum vulgare) emitted slightly more than grassland, while bare soil and forest emitted less than agricultural crops. In other studies the opposite trend has been observed, with barley emitting less than grassland (Martikainen et al., 2002; Maljanen et al., 2004). Lohila et al. (2003) and Elsgaard et al. (2012) reported lower CO2 emissions from potato fields than from grassland and arable fields. In a Nordic inventory of greenhouse gas emissions from peat soil, the emissions from perennial grassland were about the same as from barley (Maljanen et al., 2010). Kandel et al. (2013) concluded that a shift from annual spring barley to perennial reed canary grass (Phalaris arundinacea) did not change the greenhouse gas emission rates. These discrepancies between studies indicate that the crops grown on a particular field are not the decisive factor for CO2 emissions. Other factors that can be of great importance include ground-water level (Petersen et al., 2012; Karki et al., 2014), soil properties (Maljanen et al., 2004; Berglund and Berglund, 2011) and spatial and temporal differences (Rochette et al., 1991; Camporese et al., 2008). For instance, the date/time of sampling are commonly reported to have significant effects on CO2 emissions (Elder and Lal, 2008b; Regina and Alakukku, 2010). Moreover, crops require different kinds of management inputs, such as fertiliser, soil tillage, irrigation etc., leading to variations in soil physical and chemical conditions, creating different environments for greenhouse gas production. One method for measuring the respiration originating from soil organic matter (SOM) and the influence of plants is to compare measurements on plots with a crop and plots with the crop removed (Berglund et al., 2011; Karki et al., 2015). Plots with a crop represent the total CO2 emissions from soil, which can be divided into plant-derived CO2 and SOM-derived CO2. Kuzyakov (2006) suggests three different sources of plant-derived CO2, namely root respiration, rhizomicrobial respiration and microbial respiration of dead plant residues, and two different sources of SOM-derived CO2, namely microbial decomposition of SOM in root-free soil and microbial decomposition of SOM in root-affected soil, i.e. the priming effect. The priming effect can be described as a change in SOM decomposition caused by rhizodeposition. Plots with the crop removed only represent SOM-derived CO2 in root-free soil. Our interest in this study was SOM (i.e. peat) decomposition in soil covered with different crops. The difference in CO2 emissions between plots with a crop and plots with the crop removed represents the plant-derived CO2, including the priming effect. The priming effect in nutrientrich soils (e.g. peats and peaty marls) is most likely low (Fontaine et al., 2003). However, different crops can generate different levels of priming effect, both positive and negative (Kuzyakov, 2002; Cheng et al., 2003). In the present study, soil CO2 emissions were measured from peat and peaty marls under different cropping systems in similar conditions. Peaty marls defined as marls with a topsoil with organic matter content mainly originating from peat. Study sites on farms distributed throughout the southern half of Sweden were selected, in order to give a diverse set of soil types. Two cropping systems were compared at each site, cropping system being defined as the crop and the management practices associated with that crop. The variation in soil CO2 emissions

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between sites and over growing seasons was determined. The main aim was to determine whether it is possible for farmers to mitigate the CO2 emissions from organic soils by changing the crop grown. 2. Materials and methods In 2009 the study started with CO2 measurements at four sites on four different farms, in 2010 it was extended to seven sites and in 2011 it was reduced to three sites (Table 1). The sites cover a wide range of soil types and cropping system intensities (Tables 1–4). The soil types (Tables 2–4) at the sites can be divided into two main groups; peat soils (sites 1–7) and peaty marls (sites 8–11). The crops grown (Table 1) can be divided into three main groups; cereals (oats, barley, rape seed, spring wheat and spring triticale), row crops (potato, carrot and parsnip) and grassland (for fodder production and for lawn grass production). Every second year the lawn grass was tilled and resown, in year 1 the lawn grass was in its second year and in year 2 in its first year. Mowing was performed weekly. Grassland was renewed every third or fourth year and cut 2–3 times a year for fodder production. In this study they were all in their first or second year of growth. At site 1 the grassland was used for grazing in year 1. Row crops and cereals were spring sown annual crops with tillage every year. 2.1. Climate The climate at the different sites varies slightly. At sites 1–2, mean annual rainfall is 539 mm and mean annual temperature 6.0 °C. At sites 3–4, mean annual rainfall is 625 mm and mean annual temperature 5.6 °C. At sites 5–11, mean annual rainfall is 513 mm and mean annual temperature 6.6 °C. All these values are 30-year averages,1961–90 (SMHI, 2015). Spring 2009 was warmer and drier than normal, followed by normal weather until after midsummer, when a period of warm, dry weather occurred. July was rainy throughout, followed by a warm, dry August. The spring of 2010 and 2011 was dominated by the previous long, snow-rich winter. Although April was warmer and drier than normal, there was still much water on the fields from snow melt. In 2010, May and June were rainy, which led to difficulties with tillage at some sites. July was dry and warm, with exceptionally hot weather at sites 5–11. The last part of the season had normal rainfall and temperature and finished with an early autumn at the end of September. In 2011, May and June were less rainy than in 2010, which gave farmers a better start to the growing season. July was dominated by changeable weather, followed by a rainy August and September. The growing season of 2011 was colder throughout than that of 2009 and 2010. 2.2. Experimental design A comparison was made between two different crops grown in the same field or adjacent fields (Fig. 1). The soil type (parent material of the organic soil), peat depth, drainage intensity (same distance from drainage ditch) and weather conditions were the same for both crops, and only the crop and its associated management differed. Ten study plots (approx. 1 m2) were laid out in a transect along the crop border, about 5 m inward from the border at most sites (maximum 15 m). For the row crops, the study plots (2 m long) were placed along one row. The study plots consisted of five subplots with a crop and five with the crop manually removed (bare soil) in the beginning of the season and the surface was kept bare thereafter by manual weeding once a month. For grassland plots a spade was initially used to remove the vegetation. Measurements of CO2 emissions were made on plots both with and without crop. Within 1 h prior to gas measurement, 0.25 m2 of the plot with crop (0.5 m row length for row crops) was cut to facilitate measurements and to reduce the above-ground biomass part of the CO2 flux. This area was then used for the gas flux measurements. On each measuring occasion, a new square/area of the study plot was

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L. Norberg et al. / Geoderma Regional 7 (2016) 338–345

Table 1 Location, crops grown and information on management practice at the sites. Site and year

Coordinates

Crop type

Site 1 2010

59°18′N 16°32′ E 59°18′N 16°32′ E 59°18′N 16°33′ E

Oats Grassland Barley Grassland Grassland Lawn grass

Site 2 2011

59°18′N 16°33′ E

Site 3 2010

Site 1 2011 Site 2 2010

Site 4 2011 Site 5 2009 Site 5 2010 Site 6 2009 Site 7 2010

Site 8 2009 Site 9 2010 Site 10 2009 Site 11 2010

Sowing (time)

Harvest (time)

Important notes

Avena sativa Mixture e.g. Phleum pratense Hordeum vulgare Mixture e.g. Phleum pratense Mixture e.g. Phleum pratense Poa pratensis + Festuca rubra

May Spring 2008 April Spring 2008 Spring 2008 Autumn 2008

Treated with glyphosate after harvest, before measurement Aug. the plots were influenced by grazing cows

Grassland Lawn grass

Mixture e.g. Phleum pratense Poa pratensis + Festuca rubra

Spring 2008 Autumn 2010

59°12′N 15°34 E 59°11′N 15°35′ E 57°34′N 18°38′ E 57°34′N 18°38′ E 57°42′N 18°29′ E 57°42′N 18°29′ E

Carrots Spring rape Carrot Spring wheat Carrot Grassland Barley Grassland Potato Barley Potato Barley

Daucus carota Brassica napus Daucus carota Triticum aestivum Daucus carota Mixture e.g. Phleum pratense Hordeum vulgare Mixture e.g. Phleum pratense Solanum tuberosum Hordeum vulgare Solanum tuberosum Hordeum vulgare

May May June May May Spring 2007 May Spring 2007 May May June May

September 2 times September 2 times 3 times Cut once a week 3 times Cut once a week October September October September September 2 times September 3 times September September September August

57°44′N 18°27′ E 57°44 N 18°27′ E 57°43′N 18°28′ E 57°43′N 18°28′ E

Carrots Spring wheat Parsnip Spring wheat Potato Barley Potato Spring triticale

Daucus carota Triticum aestivum Pastinaca sativa Triticum aestivum Solanum tuberosum Hordeum vulgare Solanum tuberosum Triticum aestivum/Secale cereale

May April May April May April May May

October September October September September August October September

used. The measuring area was also rotated in plots with bare soil in order to get the same soil disturbance in both plot types. The set-up was the same at all study sites. The study plots were managed by the respective farmers in the same way as the rest of the field. 2.3. Measurements CO2 measurements were performed once a month during the growing season (May–September) in 2010 and 2011. In 2009, measurements were only performed in July and August. All measurements were made during daytime. CO2 was measured with the closed dark chamber method. Within 1 h before measurement round PVC collars with a base area of 0.07 m2 were inserted 3 cm into the soil (carefully, to minimise disturbance). PVC chambers, 28 cm high in 2009–2010 and 13 cm high in 2011, were placed over the collars during incubation and sealed with a rubber seal. Two chambers were used for CO2 measurement, one in

Sept. grass were in early growth after the field was resown

Field waterlogged during parts of the summer

Aug. above-ground green parts were cut at soil surface

Sept. field was stubble cultivated, plant residues in soil surface Field waterlogged during parts of the summer

Aug. above-ground green parts were cut at soil surface

each of the crops compared. The CO2 measurements were performed at the same time in both crops, a pair of plots at a time, following the transect of plots (starting at plot 1 and ending at plot 10), including bare soil plots (Fig. 1). This procedure was performed twice on each measuring occasion and a mean value of these was used in calculations. The air in the closed chamber was circulated for 3–5 min through a portable infrared gas analyser (Carbocap CO2 Probe GMP343, Vaisala Ltd., Vantaa, Finland) where measurements were taken every 5 s. The device was calibrated with standard procedures by Vaisala (Vaisala Instruments Service, Vantaa, Finland). The emission rate was calculated from the linear increase in gas concentration in the chamber headspace (Berglund and Berglund, 2011). Measurements with poor quality i.e. those that departed significantly from linearity (R2 b 0.85), were omitted, since this indicates problems with the measurements. However, 98% of the measurements were of sufficiently high quality for inclusion in the dataset. It was mainly on two occasions that some measurements

Table 2 Description of the soil type and peat depth at the study sites. Site

0–20 cm

20–40 cm

40–60 cm

Peat depth (cm)

1 2 3 4 5 6 7 8 9 10 11

Fen peat Fen peat Fen peat Fen peat Fen peat Fen peat Fen peat Peaty marl Peaty marl Peaty marl Peaty marl

Peat with tree remains Peat with tree remains Peat with plant remains Peat with plant remains Marl/lime gyttja Algae gyttja/cracks with peat Algae gyttja Lime gyttja layered Lime gyttja with sand layer Lime gyttja layered Layered marl

Peat mixed with gyttja Peat mixed with gyttja Gyttja clay Peat mixed with gyttja Clay gyttja Lime gyttja Lime gyttja/cracks with topsoil Lime gyttja/cracks with topsoil Gyttja clay Clay with sand layer Gyttja

55 50 50 55 27 30 20

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Table 3 Soil properties of the topsoil (0–20 cm) at the sites. Sites 1–7 are peat soils and sites 8–11 peaty marls.

Humification degree Loss on ignition % pH (H2O) Tot-C % Carb.-C % Org.-C % Tot-N % C:N

Site 1

Site 2

Site 3

Site 4

Site 5

Site 6

Site 7

Site 8

Site 9

Site 10

Site 11

H 9–10 53.5 5.6 27.2 0.0 27.2 1.6 17

H 9–10 83.0 5.7 39.6 0.1 39.5 2.0 19

H 9–10 78.3 5.4 39.2 0.1 39.1 2.6 15

H 9–10 86.1 6.0 41.6 0.5 41.1 2.1 20

H 10 65.4 7.7 35.0 2.5 32.5 2.3 14

H 10 83.4 6.3 39.9 0.9 38.9 2.7 14

H 9–10 83.0 5.7 42.8 0.7 42.1 2.7 16

16.8 7.6 12.2 4.6 7.5 0.6 12

10.1 8.1 9.5 5.2 4.3 0.3 12

15.1 7.8 11.6 4.6 7.0 0.6 12

15.6 7.7 11.4 5.1 6.3 0.5 12

were excluded, at site 4 in June 2011 and site 9 in September 2010. High wind speed caused unsteady measurements, resulting in lower calculated emissions (as also reported by Conen and Smith, 1998). In parallel to gas measurements, soil temperature and soil water content were determined. Soil temperature was measured with a thermometer at approx. 10 cm depth next to the collars to avoid disturbances. Volumetric soil moisture content in the upper 6 cm was determined with a WET sensor (Delta-T Devices Ltd., Cambridge, UK) in each collar immediately after gas measurement. An average of four measurements was used.

Joseph, MI, USA). Soil sampling with undisturbed soil cores (Ø7.2 cm, 10 cm high, 4 replicates) was performed in the topsoil of each crop at all sites (plots 1, 2, 9 and 10 in each crop) (Fig. 1) and dry bulk density and water content at matric potential −0.05, −0.3, −0.5, −0.7, −1.0 and − 6.0 m water column (− 0.5, − 3, − 5, − 7, − 10 and − 60 kPa) were determined. Porosity was calculated from particle density (ethanol method) and dry bulk density, while air-filled pore space at matric potential −1 m was calculated from water retention data.

2.4. Soil classification and soil analysis

A two-sample t-test was used to test for differences in CO2 emissions between I) crops at each site, both for seasonal average (n = 10–25) and individual measuring occasions (n = 5), and II) soil types (peat soil and peaty marl). A two-way ANOVA with a general linear model procedure was used to test for differences between crops (divided into the main groups cereals, row crops and grassland) and sites for total CO2 emissions (plots with crop) and for bare soil CO2 emissions (plot with bare soil). For the general linear model and seasonal average t-test, the CO2 emissions data were ln-transformed to meet normality and homoscedasticity requirements. Possible relationships between CO2 emissions and soil properties were tested with different regression

To characterise the soil type at each site, soil sampling and soil profile description were performed at one representative spot within the study site. Classified according to Soil Taxonomy (Soil Survey Staff, 2014), sites 1–4 were Euic coprogenous Limnic Haplosaprists, sites 5– 7 Calcareous cracked Histic Humaquepts and sites 8–11 Calcareous cracked Typic Fluvaquents. Soil properties of the topsoil (5–15 cm) at all sites are shown in Tables 3 and 4. The humification degree was determined according to von Post (1922). Soil pH was measured at a soil–solution ratio of 1:2 with deionised water. Organic matter content (loss on ignition) was measured by dry combustion at 550 °C for 24 h after predrying at 105 °C for 24 h. Total nitrogen (tot-N), total carbon (tot-C) and carbonate-C content were determined on a LECO CN-2000 analyser (St.

2.5. Statistical analysis

Table 4 Porosity, dry bulk density and air-filled pore space at matric potential −1 m in topsoil (5– 15 cm) under the crops compared at sites 1–11. Sites 1–7 are peat soils and sites 8–11 peaty marls. Standard deviations are in brackets. Site and year

Crop type

Site 1 2010 Site 2 2010 Site 3 2010

Oats Grassland Grassland Lawn grass Carrot Spring rape Carrot Grassland Potato Barley Potato Barley Carrot Spring wheat Parsnip Spring wheat Potato Barley Potato Spring triticale

Site 5 2009 Site 6 2009 Site 7 2010 Site 8 2009 Site 9 2010 Site 10 2009 Site 11 2010

Porosity (vol.%)

Dry bulk density (g cm−3)

Air filled pore space (vol.%)

75⁎ (2.0) 69 (3.0) 77⁎ (1.0) 74 (0.4) n.d. n.d.

0.42 (0.04) 0.52⁎(0.04) 0.36 (0.02) 0.41⁎(0.01) 0.36 (0.02) 0.34 (0.00)

12⁎ (2.9) 5 (1.7) 8 (2.3) 7 (2.5) n.d. n.d.

82⁎ (0.4) 78 (0.5) 83⁎ (0.7) 80 (0.3) 80 (0.2) 79 (0.5) 66⁎ (0.4) 61 (1.8)

0.33 (0.01) 0.42⁎(0.01) 0.29 (0.01) 0.34⁎(0.00) 0.34 (0.01) 0.34 (0.01) 0.82 (0.01) 0.93⁎(0.04)

32⁎ (2.2) 13 (2.6) 38⁎ (2.3) 20 (3.7) 23⁎ (1.0) 16 (1.8) 20⁎ (1.0) 8 (3.5)

58 (0.4) 59 (0.3)

1.05 (0.01) 1.05 (0.01)

14 (0.8) 16⁎ (0.7)

68⁎ (0.9) 63 (0.8) 69⁎ (0.7) 62 (0.8)

0.78 (0.02) 0.91⁎(0.02) 0.72 (0.02) 0.91⁎(0.02)

26⁎ (1.6) 12 (0.8) 25⁎ (1.4) 11 (1.6)

⁎ Values marked with * are significantly higher (pb0.05) than those for the comparison crops.

Fig. 1. Schematic diagram of the field set-up. Five replicate plots with a crop (a) and five replicate plots with bare soil (b) for the two crops compared (crop 1 and crop 2).

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models. Mean values are presented with standard deviation. All statistical analyses were carried out in Minitab 17 (Minitab Inc., USA). 3. Results and discussion This study examined whether it is possible for farmers to mitigate the CO2 emissions from organic soils by changing the crop grown. For this purpose, the study was set up according to the following principles. First, the two crops compared were grown on the same soil type with the same weather conditions and drainage intensity. Only the crop and its associated management differed. Second, the same experimental set-up was used on different sites with a range of soil properties within the groups peat and peaty marl. Third, the study was run over several years and during the whole growing season (May–September). Moreover, the CO2 emissions were always measured at the same time from plots with a crop and from plots with bare soil, which can give an indication of the peat oxidation without the impact of plants. The CO2 emission rates in this study were in the same range as in other studies in the Nordic countries reviewed by Maljanen et al. (2010). 3.1. Impact of cropping system on CO2 emissions The measured CO2 emissions data were evaluated on three different scales: I) all sites stacked together, II) seasonal averages from individual sites and III) averages from single measuring occasions from individual sites. The results varied depending on the scale at which the CO2 data were evaluated. The general linear models with all data for total CO2 emissions showed grassland to be a greater emitter of CO2 than cereals and row crops (Table 5). It is problematic to analyse the data stacked together in groups since sites/soils have diverse properties and locations are possibly of greater importance for the CO2 emissions than the crop grown. However, the higher CO2 emissions from soil with grassland are probably partly due to the larger root mass and longer vegetation period under grass compared with cereals and row crops (Haynes and Francis, 1993; Lohila et al., 2003). In addition, the microbial activity rises when rhizodeposition by living roots increases (Kuzyakov, 2002), which further increases the CO2 production from grassland. On the other hand, the longer vegetation period for grassland also promotes greater photosynthetic CO2 uptake (Lohila et al., 2004), which can make the net ecosystem exchange of CO2 for the three groups of crop more similar. Most of the seasonal average total CO2 emissions (plots with crop) showed no difference between the crops compared (Table 5), but with six exceptions. Grassland emitted more than barley (site 1, 2011), carrots emitted more than spring rape-seed and spring wheat (sites 3, 2010 and 4, 2011), grassland emitted more than carrots and barley (site 5, 2009 and 2010) and barley emitted more than potato (site 10, 2009). For the seasonal bare soil CO2 emissions, three sites deviated from the main trend of no difference between plots: carrots emitted more than spring wheat at site 4 in 2011, spring wheat emitted more than parsnip at site 9 in 2010 and barley emitted more than potato at site 10 in 2009 (Table 5). Many of the individual CO2 measuring occasions showed significant differences between the crops compared but the trend sometimes changed over the season, e.g. site 1 in 2010 and site 5 in 2009, and it was not possible to detect a significant seasonal difference (Fig. 2). Plots with bare soil often displayed the same trend as the corresponding plots with a crop, e.g. site 1 in 2011, or smaller differences, e.g. site 2 in 2011. The evaluation of seasonal averages from each site separately did not show any clear trend (Table 5), nor did the individual measuring occasions at each site (Fig. 2). The seasonal averages sometimes indicated higher CO2 emissions from one of the paired crops compared at one site, but not at another site or in the next year (Table 5). Differences also varied during the season within the same crop comparison at the

Table 5 Seasonal average of total soil CO2 emissions (plot with crop) and bare soil CO2 emissions (plot with bare soil) from the individual sites. Sites 1–7 are peat soils and sites 8–11 peaty marls. Included are also the average CO2 emissions for the three main groups of crop, including all data from the 11 sites. Different letters denote significantly different mean for the three groups of crop (p b 0.05). Standard deviations are indicated in brackets. Total CO2 emissions

Bare soil CO2 emissions

Site and year

Crop type

(mg CO2 m−2 h−1)

(mg CO2 m−2 h−1)

Site 1 20101

Oats Grassland Barley Grassland Grassland Grass lawn Grassland Grass lawn Carrots Spring rape Carrots Spring wheat Carrots Grassland Barley Grassland Potato Barley Potato Barley Carrots Spring wheat Parsnip Spring wheat Potato Barley Potato Spring triticale Cereals6 Row crops7 Grassland8

838 (438) 977 (320) 543 (230) 846⁎ (257) 1253 (399) 1421 (434) 826 (209) 1012 (380) 1200⁎ (368) 930 (222) 907⁎ (490) 554 (350) 1232 (314) 2060⁎ (889) 964 (619) 1454⁎ (880) 607 (68) 873 (407) 703 (331) 848 (398) 754 (122) 1280 (742) 506 (286) 631 (280) 541 (472) 1005⁎ (135) 705 (307) 947 (382) 808a (447) 803a (404) 1170b (573)

613 (293) 623 (306) 392 (139) 471 (190) 904 (290) 1067 (845) 445 (130) 439 (208) 1026 (425) 856 (231) 629⁎ (275) 417 (178) 800 (69) 1261 (739) 875 (452) 1022 (496) 534 (87) 524 (147) 568 (213) 664 (299) 691 (152) 700 (301) 286 (65) 610⁎ (519) 393 (234) 711⁎ (78) 585 (167) 768 (330) 633ab (362) 624a (312) 749b (524)

Site 1 20111 Site 2 20101 Site 2 20113 Site 3 20102 Site 4 20111 Site 5 20094 Site 5 20102 Site 6 20094 Site 7 20102 Site 8 20094 Site 9 20102 Site 10 20094 Site 11 20105

1

n = 25, measured in May, June, July, August, September. n = 20, measured in June, July, August, September. n = 20, measured in May, June, July, August. 4 n = 10, measured in July, August. 5 n = 15, measured in June, July, August. 6 n = 200. 7 n = 137. 8 n = 170. ⁎ Values marked with * are significantly higher (p b 0.05) than those for the comparison crop. 2 3

same site (Fig. 2). These results show the strengths of the study with its comparison of the same crops at different locations and on many measuring occasions. If the experiment had only been performed at one site in one year, the conclusion could be different, since some of the sites displayed significant differences between the compared crops in some years but not in others. Emissions from the bare soil plots are an indicator of peat oxidation without the impact of plants. In this study the average seasonal plant-derived CO2 (plots with crop minus plots with bare soil) was 27% of the average total CO2 emissions, which is at the low end of the range (27–63%) reported in other studies (Silvola et al., 1996; Berglund et al., 2011). This may depend on the type of crop and time of year of measurement. It possibly also depends on the contribution from maintained activity and decomposition of root remains in the soil in bare soil plots (Shurpali et al., 2008). This bias could have been reduced by removing the crop even earlier e.g. a year before, to make sure that the roots were completely dead at the time of CO2 measurement (Alm et al., 2007). There was no significant difference in soil temperature between plots with a crop and plots with bare soil, but the soil moisture tended to be higher on the plots with bare soil, which could have affected the SOM decomposition compared with plots with a crop.

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Fig. 2. Total soil respiration from the peat at sites 1–7 and peaty marl at sites 8–11. Bars marked with * are significantly higher (p b 0.05) than those for the comparison crops. Separate comparisons were made for plots with a crop and plots with bare soil.

3.2. Impact of soil properties and environmental factors on CO2 emissions There was no linear correlation between CO2 emissions and any of the soil parameters (Tables 3 and 4) when all sites were used in the same regression or only sites with peat. The four peaty marl sites showed a negative linear correlation between total CO2 emissions and pH (p b 0.034, R2 = 0.93), and an indication of a positive linear correlation between total CO2 emissions and total carbon content (p = 0.064, R2 = 0.87). The average differences in total CO2 emissions between sites (different soil properties) were in the same range as the differences between crops (Table 5). Although the CO2 emissions did not show any clear trends between the crops compared, there were indications of differences in soil properties. The porosity and air-filled pore space at 1 m drainage were generally greater, with more coarser pores and lower dry bulk density in the topsoil under row crops than under the comparison crop (Table 4). Ploughing compared with a no-till regime can increase the porosity and decrease the dry bulk density in the short term (Elder and Lal, 2008b). This increases the aeration in the top few cm of the ridges, possibly enhancing drying of the soil and decreasing the biological activity. There were differences (p b 0.001, n = 507) in average total CO2 emissions between the sites, with site 5 in 2009 having the highest emissions (2060 ± 889 mg CO2 m−2 h−1) and site 9 in 2010 the lowest emissions (506 ± 286 mg CO2 m−2 h−1). The differences in CO2 emissions between sites with different soil types were within the same range as the CO2 emissions from the crops compared at each site (Table 5).

There was a significant difference in total CO2 emissions between the groups of soils (peats and peaty marls), with lower emissions from peaty marls (sites 8–11), 757 ± 425 mg CO2 m−2 h−1, with their low total carbon content (9.5–12.2%) than from peats (sites 1–7), 975 ± 524 mg CO2 m− 2 h− 1, with their much higher total carbon content (27.2–42.8%) (Tables 2, 3 and 5, Fig. 2). Within the group of sites with peat (sites 1–7), there was no correlation between total CO2 emissions and carbon content, but within the group with peaty marl (sites 8–11) an increase in total-C gave higher total CO2 emissions. The CO2 emissions from peaty marl (sites 8–11) are the result of a combination of two processes: 1) a non-biological process where the CaCO3 in the marl reacts with soil particles, forming CO2, and 2) a biological process where the environmental conditions for microbial activity are promoted by favorable pH and substrate availability (Fuentes et al., 2006). Within the small group of sites with peaty marl studied here, higher pH gave lower CO2 emissions, indicating that a limit was exceeded so that an increase in pH was no longer favorable for microbial activity. However, this could also have been due to a lower organic carbon content in soils with a higher pH. If the calcium carbonate in marl is similar to the limestone added to agricultural soil to improve soil fertility, there may be high CO2 emissions from calcareous soil. Liming with CaCO3 on agricultural soil has been reported to increase microbial basal respiration, microbial CO2 flux and soil microbial biomass C in laboratory experiments (Fuentes et al., 2006; Kemmitt et al., 2006), but liming has been found not to increase biotic respiration in other studies (Biasi et al., 2008). According to Eggleston et al. (2006), all the carbon in

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Fig. 2 (continued).

limestone added to agricultural soils is released as CO2, but this is probably an overestimate and instead about 50% (West and McBride, 2005) or one-sixth (Biasi et al., 2008) has been suggested. The range in soil temperature observed during the study period was 6–25 °C and the range in soil water content was 11–75 vol.%. On individual measuring occasions, the soil temperature was occasionally significantly higher for one crop, but generally over the season there were no significant differences between crops. Furthermore, there were no differences in soil temperature between plots with a crop and plots with bare soil. In general, the soil water content was higher in plots with bare soil than in the corresponding plots with a crop. The pattern was the same for all crops. The soil water content in the top 6 cm of the soil was often significantly different between crops on individual measuring occasions. The soil moisture in the top of the ridges of row crops was usually lower than in the soil of cereals, but the CO2 emissions were not lower for the row crops. Elsgaard et al. (2012) reported lower

CO2 emissions from a potato field compared with grassland and cereals, possibly due to drier soil in the ridges. Since the data in the present study were restricted to gas measurements during the growing season and from individual occasions (not continuous measurements), they are not suitable for annual gas flux calculations. In addition, possible short-term changes in gas fluxes due to soil and crop management might have been overlooked. The time of measurement can have a great influence on estimated emissions (Elder and Lal, 2008a). Furthermore, rainfall immediately before measuring can greatly increase soil CO2 emissions (Rochette et al., 1991), changing the seasonal average and complicating comparison with other sites. 4. Conclusions There were differences in CO2 emissions between the crops tested, but the results were not consistent. The variation in CO2 emissions

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between different sites were in the same range as the variations between crops. Results from this study showed that cropping system could not explain the variations in soil CO2 emissions and the conclusion is therefore that the choice of crop cannot be recommended as a mitigation option for limiting CO2 emissions from cultivated organic soils. Acknowledgements This study was funded by the Swedish Farmers' Foundation for Agricultural Research and the Nordic Joint Committee for Agricultural and Food Research (NKJ) through the Swedish Research Council (Formas). We thank the farmers at the field sites, especially Magnus Larsson, Per-Erik Karlsson and Sten Wikström. Many thanks to the people assisting in the field and in the laboratory at the Department of Soil and Environment, Swedish University of Agricultural Sciences. Great thanks to the reviewers whose comments helped to improve the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.geodrs.2016.06.005. References Alm, J., Shurpali, N.J., Tuittila, E.-S., Laurila, T., Maljanen, M., Saarnio, S., Minkkinen, K., 2007. Methods for determining emission factors for the use of peat and peatlands — flux measurements and modelling. Boreal Environ Res 12, 85–100. Berglund, K., 1989. Ytsänkning på Mosstorvjord. Avdelningsmeddelande 89:3. SLU, Institutionen för markvetenskap, avdelningen för lantbrukets hydroteknik. Berglund, K., 1996. Cultivated Organic Soils in Sweden: Properties and Amelioration (PhD thesis) Swedish University of Agricultural Sciences, Uppsala (39 pp.). Berglund, Ö., Berglund, K., 2010. Distribution and cultivation intensity of agricultural peat and gyttja soils in Sweden and estimation of greenhouse gas emissions from cultivated peat soils. Geoderma 154, 173–180. Berglund, Ö., Berglund, K., 2011. Influence of water table level and soil properties on emissions of greenhouse gases from cultivated peat soil. Soil Biol. Biochem. 43, 923–931. Berglund, Ö., Berglund, K., Klemedtsson, L., 2011. Plant-derived CO2 flux from cultivated peat soils. Acta Agric Scand Sect B Soil Plant Sci 61, 508–513. Biasi, C., Lind, S.E., Pekkarinen, N.M., Huttunen, J.T., Shurpali, N.J., 2008. Direct experimental evidence for the contribution of lime to CO2 release from managed peat soil. Soil Biol. Biochem. 40, 2660–2669. Camporese, M., Putti, M., Salandin, P., Teatini, P., 2008. Spatial variability of CO2 efflux in a drained cropped peatland south of Venice, Italy. J Geophys Res Biogeosci 113. Cheng, W.X., Johnson, D.W., Fu, S.L., 2003. Rhizosphere effects on decomposition: controls of plant species, phenology, and fertilization. Soil Sci Soc Am J 67, 1418–1427. Conen, F., Smith, K.A., 1998. A re-examination of closed flux chamber methods for the measurement of trace gas emissions from soils to the atmosphere. Eur J Soil Sci 49, 701–707. Eggleston, S., Buendia, L., Miwa, K., Ngara, T., Tanabe, K., 2006. IPCC Guidelines for National Greenhouse Gas Inventories. Inst. Glob. Environ. Strateg, Hayama, Japan. Elder, J.W., Lal, R., 2008a. Tillage effects on physical properties of agricultural organic soils of north central Ohio. Soil Tillage Res. 98, 208–210. Elder, J.W., Lal, R., 2008b. Tillage effects on gaseous emissions from an intensively farmed organic soil in north Central Ohio. Soil Tillage Res. 98, 45–55. Elsgaard, L., Gorres, C.-M., Hoffmann, C.C., Blicher-Mathiesen, G., Schelde, K., Petersen, S.O., 2012. Net ecosystem exchange of CO2 and carbon balance for eight temperate organic soils under agricultural management. Agric Ecosyst Environ 162, 52–67. Fang, C., Moncrieff, J.B., 2001. The dependence of soil CO2 efflux on temperature. Soil Biol. Biochem. 33, 155–165. Fontaine, S., Mariotti, A., Abbadie, L., 2003. The priming effect of organic matter: a question of microbial competition? Soil Biol. Biochem. 35, 837–843. Fuentes, J.P., Bezdicek, D.F., Flury, M., Albrecht, S., Smith, J.L., 2006. Microbial activity affected by lime in a long-term no-till soil. Soil Tillage Res 88, 123–131. Glenn, S., Heyes, H., Moore, T., 1993. Carbon dioxide and methane fluxes from drained peat soils, Southern Quebec. Global Biogeochem Cycles 72, 247–257. Grönlund, A., Sveistrup, T.E., Søvik, A.K., Rasse, D.P., Klöve, B., 2006. Degradation of cultivated peat soils in Northern Norway based on field scale CO2, N2O and CH4 emission measurements. Arch Agron Soil Sci 52, 149–159.

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