An ecosystem approach to assess soil quality in organically and conventionally managed farms in Iceland and Austria

Introduction Conclusions References


Introduction
Soil is considered to be one of the most important natural resources for life on Earth.Soil processes govern a wide array of ecosystem services, such as the provision of food, feed and fibre, carbon sequestration, hydrological regulation, and contaminant attenuation (Costanza et al., 1997).
Mostly due to human activities, soil quality, here defined in terms of the soil's ability to deliver ecosystem services, is being drastically reduced in many locations worldwide (Vitousek, 1997).Global loss of soil ecosystem services is due to many different environmental threats, such as climate change, intensive agricultural production, and environmental pollution.
In order to come up with effective strategies to protect and enhance soil quality, the Critical Zone Observatory (CZO) network was established across the USA and Europe (Anderson et al., 2008).The CZO network is an internationally coordinated interdisciplinary research effort to better understand the chemical, physical and biological processes that shape the Earth's surface and support the terrestrial life on the planet.
As part of the CZO research effort, the European Commission has provided funding for a large multi-disciplinary research project: Soil Transformations in European Catchments (SoilTrEC).This project aims to understand and quantify the physical, chemical, and biological processes that are critical to soil ecosystem functions and services in the European CZOs (Bernasconi et al., 2011;Menon et al., 2014).
The European CZO network consists of sites along soil formation gradients (Austria, Switzerland, Iceland), along a soil degradation gradient (Greece), along a pollution gradient (Czech Republic), and of agricultural sites differing in methods of soil management (Austria, Iceland) (Menon et al., 2014;Banwart et al., 2011).
This paper presents the soil quality assessment as carried out for the agricultural CZO sites in Europe.The agricultural sites have been chosen as part of the CZO network because intensive agricultural production is an important driver of loss in soil quality, e.g.due to decreased organic matter contents.Intensive agriculture may also cause environmental problems, e.g.nitrate leaching to nearby natural ecosystems, and pesticide contamination of surface and groundwater (Skinner et al., 1997).The agricultural CZO sites consist, in total, of eight farms: four grassland farms in Iceland, of which two are conventional and two organic, and four arable farms in Austria, of which two are conventional and two organic.The organic farms differed from the conventional farms in that only organic fertilisers were applied and no pesticides were used.On the conventional grassland farms in Iceland, some organic fertilisation was used in addition to the artificial inorganic fertilisers.On the conventional arable farms in Austria, only artificial inorganic fertilisers were applied together with pesticides.The central idea behind the organic farming practice is that the community of soil organisms will become more important in terms of delivering important soil ecosystem functions, especially in terms of soil structure formation, soil carbon dynamics, and nutrient mineralisation, as well as the suppression of soil-borne diseases (Birkhofer et al., 2008).The present study investigates biological, physical, and chemical soil quality parameters, focused on soil structure formation, soil organic matter dynamics and nutrient cycling, and the soil as a habitat for species-rich communities.
Soil structure is an important attribute of soil quality.Soil aggregates and the pores between the aggregates provide space, water, and oxygen, thereby creating habitats for a large diversity in soil organisms (Anderson, 1978;Sulkava and Huhta, 1998).Soil organisms play an important twofold role in determining soil structure formation.Firstly, microorganisms produce exudates (polysaccharides) that enhance aggregation of soil particles, and fungal hyphae also physically bind soil particles (de Gryze et al., 2005;Wright et al., 2007;Tisdall and Oades, 1982).Secondly, the soil fauna plays a role in creating a stable soil pore structure through moving in the soil and the formation of faecal pellets (Oades, 1993;Lee and Foster, 1991;Jastrow and Miller, 1991;Lavelle et al., 2006).Furthermore, soil structure is strongly linked to soil organic matter (SOM) dynamics, as incorporation of SOM into the soil aggregates "protects" it from microbial decomposition, thereby stabilising SOM content and sequestering carbon in the soil, with potentially positive effects on plant productivity (Golchin et al., 1994).
Soil organic matter is an essential component of soil quality, governing processes like carbon sequestration, nutrient cycling, water retention, and soil aggregate turnover.Soil organic matter dynamics are driven by land use through root turnover, deposition of plant residues, and decomposition by the soil microbial populations.Soil organisms are known to play important roles in SOM dynamics (Wardle et al., 2004;de Ruiter et al., 1994;Lavelle et al., 2006) by decomposing SOM.This process mineralises carbon (C) and nutrients like nitrogen (N), making these available for plant uptake.To understand the role of soil organisms in decomposition processes, SOM has been defined in terms of fractions based on decomposability (Golchin et al., 1994).The idea behind this fractioning is that the labile fractions, such as dissolved and particulate organic matter, are better available for biological decomposition, contribute more to soil structure formation, and are more sensitive to soil management than more stable fractions such as lignin (Beare et al., 1994;Tisdall and Oades, 1982).
The soil as habitat for species-rich communities has increasingly received attention for the intrinsic and functional value of soil biodiversity.High levels of biodiversity are thought to enhance stability of soil functions and services against perturbations and disturbances, and aid in the suppression of soil-borne pests and diseases (Griffiths et al., 2000;Altieri, 1999;Barrios, 2007).Soil biodiversity is also recognised as a sensitive biological indicator for effects of environmental change and disturbance (Wardle et al., 1995;Ritz et al., 2009;Pattison et al., 2008;Ponge et al., 2006).One of the key indicator groups is the soil microarthropods, because these are abundant, functionally diverse, and respond to a variety of ecological and environmental factors (Gardi and Parisi, 2002;Parisi et al., 2005).In addition, the area covered during the lifecycle is representative of the examined site, and their life histories permit insights into soil ecological conditions (Gardi et al., 2009).
The results presented in this paper are from a field survey on all agricultural CZO sites, in which soil was analysed in terms of its physical, chemical, and biological properties.Soil physical and chemical measurements included soil aggregate size fractions ( < 20, 20-250, 250-5000 µm); soil organic matter contents and distribution (based on different organic matter fractions); nutrient contents, including nitrogen (N), phosphorus (P), and potassium (K); and soil pH.Soil biological measurements included the presence and abundance of soil microbes (bacteria, fungi) and soil fauna (protozoa, nematodes and microarthropods), representing the main taxonomic groups and trophic levels in the soil food web.In addition we measured the taxonomic richness and diversity within the group of microarthropods, as well as vegetation diversity.

Site description
The soils analysed were sampled from the eight agricultural CZO research sites; of these, four are under sub-arctic (Iceland) and four under continental (Austria) climatic conditions.The four farms in Iceland were grassland farms, and the four in Austria were arable farms practicing crop rotations (Tables 1, 2).In each country, two farms applied "organic" practices and two farms applied "conventional" practices.The organic farms differed from the conventional farms in that only organic fertilisers were applied and no pesticides.On the conventional grassland farms in Iceland, some organic fertilisation was used in addition to the artificial inorganic fertilisers.The organic fields in Iceland were ploughed the first three consecutive years when grasslands were renewed to apply green manure, whereas conventional fields were ploughed only once.On the conventional arable farms in Austria only artificial inorganic fertilisers were applied together with pesticides.In Iceland, one pair of organic and conventional farms (in the southwest) was on Histic Andosols; the other pair (southern Iceland) was on Haplic (Brown) Andosols.In Austria, one pair of organic and conventional farms grew potatoes as the current crop; the other pair grew winter wheat.All Austrian farms were situated in the Marchfeld, southeast of Vienna, on Haplic Chernozems.Farm properties are listed in Tables 1 and 2.

Sampling scheme
Samples were taken in May-June 2011 (0-10 cm in Iceland, 0-15 cm in Austria).On each farm, three plots were selected at which all measurements were carried out; the plots were approximately 30-40 m apart.At each plot, mixed soil samples (ca. 1 kg, from 10 to 15 cores) were taken by use of a 8 cm diameter corer for microbial (bacteria, fungi), microfaunal (protozoa, nematodes), soil chemical and physical measurements, and a 5 cm diameter corer for the mesofauna (enchytraeids and microarthropods).In the grasslands on Iceland, vegetation diversity was estimated by application of four 2 m line transects at all farms, except for the conventional farm in southern Iceland, for which the vegetation data were supplied by the farmer.A line-intercept method was applied and four 2 m length tapes were laid out from the sampling point, each tape separated by 90 • .Species were recorded each time a plant species intercepted the tape, or when a group of equally mixed plant species occurred (e.g.Kent and Coker, 1992).Vegetation richness was calculated as the total number of plant species present on the transects.

Soil physicochemical measurements
Particle size distribution (clay content) was determined with a combined sieve and pipette method after removal of organic matter with hydrogen peroxide and dispersion by reciprocal shaking with sodium metaphosphate solution for 12 h (Burt, 1992).Soil pH was measured electrochemically (microprocessor pH meter pH196 WTW, Weilheim, Germany) in H 2 O at a soil : solution ratio of 1 : 2.5 (Burt, 1992).Calcium (Ca) content was measured by flame atomic absorption spectrophotometry (Perkin-Elmer 2100).Plant available P and K were determined by calcium acetate-lactate (CAL) extraction (ÖNORM L1087).
A three-step procedure was carried out to fractionate soil aggregates and organic matter.Free particulate organic matter (fPOM, 20-5000 µm) was separated using sodium polytungstate solution (density of 1.8 g cm −3 ).To obtain particulate organic matter occluded in aggregates (oPOM, 20-5000 µm), the heavy fraction of soil aggregates (> 1.8 g cm −3 ) was treated by ultrasound (8 J mL −1 ), which disrupted the macroaggregates and protected the microaggregates (Lehtinen et al., 2014).With a subsequent density fractionation step (sodium polytungstate solution, 1.8 g cm −3 ), the oPOM floating on the suspension was obtained after centrifugation (10 min at 4350 rpm).POM fractions were washed with deionised water until the electric conductivity dropped below 5 µS cm −1 (Steffens et al., 2009).The residue of the density fractionation procedure -mineral particles and organo-mineral associations -was sieved at 250 and 20 µm to obtain macroaggregates (250-5000 µm) and microaggregates (20-250 and < 20 µm).All aggregate fractions were washed with deionised water until the electronic conductivity dropped below 5 µS cm −1 ; subsequently they were oven- dried at 100 • C and weighed.The weights of aggregates were corrected for the sand content of the same size (for aggregates 20-250, and > 250 µm) in order to exclude a sand particle from being weighed as an aggregate (Six et al., 2000;Lehtinen et al., 2014).Mean weight diameter (MWD) of the sand-corrected aggregates was calculated according to Kemper and Rosenau (1986) as the sum of the geometric means of aggregate sizes multiplied by the respective fraction.Total carbon (TC) and nitrogen (TN) in bulk soil, aggregates, and POM fractions were quantified by dry combustion using an elemental analyser (Carlo Erba NA 1500 analyser).For the analysis, 5 g of sieved (< 2 mm) soil without visible roots and litter was ground to a size < 63 µm for homogenisation and 1-1.5 mg soil was used for the analysis.Total organic carbon (TOC) was calculated as the difference of total and inorganic C, measured as carbonate C by treating 0.5-2 g of fine-ground soil material with 10 % HCl acid and quantifying the evolved CO 2 .Hot-water-extractable carbon (HWC) was measured as the C present in solution after 16 h at 80 • C, while water-soluble carbon (WSC) was measured after 30 min at 20 • C (Ghani et al., 2003).Labile carbon was defined as HWC, while recalcitrant carbon was determined as the difference between TOC and labile carbon.Potentially mineralisable nitrogen (PMN) was measured as the increase in NH 4 during 1 week of anoxic incubation in slurry at 40 • C  (Canali and Benedetti, 2006).Potential carbon and nitrogen mineralisation were measured by incubation of 200 g of homogenised and sieved soil for 6 weeks at 20 • C (Bloem et al., 1994).Results of the first week (disturbance) were not used.N mineralisation was calculated from the increase in mineral N (nitrate and ammonium) between week 1 and week 6.Total concentrations of O 2 and CO 2 were measured weekly using a gas chromatograph (Carlo Erba GC 6000) equipped with a hotwire detector (HWD 430) and helium as carrier gas, and weekly rates were calculated from that.Only bottles in which O 2 concentration dropped below 15 % within the 6-week period were flushed and reset to environmental concentrations to prevent O 2 limitation.For the statistical analyses, we took the average of weekly rates over the 5-week period after the first week.

Soil food web measurements
The soils were analysed for the presence and abundances of the major taxonomic groups of soil organisms: bacteria, fungi, protozoa, nematodes, enchytraeids, and microarthropods.Within these taxonomic groups we defined "trophic groups" based on diet and life-history traits, following the method of Moore et al. (1988).Abundances were transformed into estimates of biomass based on body-size information, and expressed in units of kilograms of carbon per hectare for the 0-10 cm top soil layer.Bacterial biomass, fungal biomass, leucine incorporation, and protozoa were measured after a pre-incubation period of 2 weeks at 20 • C. Bacterial numbers and cell volumes, and fungal hyphal lengths were measured in microscopic slides (Bloem and Vos, 2004).Bacterial cell numbers and volumes were determined using confocal laser scanning microscopy combined with an image analysis system.The data were transformed into bacterial biomass, taking a specific carbon content of 3.20 × 10 −13 g C µm −3 (Bloem et al., 1995).For the transformation of fungal hyphal lengths to fungal biomass we described fungal volume as a cylinder with spherical ends (V = (π/4)W 2 (L − W/3), where V = volume in µm 3 , L = length in µm, and W = diameter in µm), with a mean hyphal diameter of 2.5 µm and a specific carbon content of 1.30 × 10 −13 g C µm −3 .Bacterial growth activity was estimated by measuring incorporation rates of [ 14 C]leucine (Bloem and Bolhuis, 2006).
Two trophic groups of protozoa (flagellates and amoebae) were measured using the most-probable-number method (Bloem et al., 1994).Numbers were converted to biomass assuming a spherical shape with diameters of 4.6 and 9.1 µm for flagellates and amoebae, respectively, and a volume-to-C conversion factor of 1 × 10 −13 C µm −3 (Bloem et al., 1994).
Soil nematodes were counted in 9 mL of soil solution extracted by Oostenbrink elutriators from 100 g of soil.Numbers per trophic group (bacterivore, fungivore, herbivore, omnivore, predaceous) were derived from species composition in the samples (Bongers, 1988).Nematode biomasses were calculated using fresh weight data from Didden et al. (1994) and taking a moisture content of 75 % and a carbon content of 40 % (Didden et al., 1994).
Enchytraeid numbers were obtained through a (wet) extraction using Baermann funnels with increasing light and heat each 30 min after the start of the extraction over a total extraction time of 3 h.Enchytraeid numbers were converted into biomass C by measuring the average fresh weight and taking a moisture content of 85 % and a carbon content of 50 % of the dry weight (Didden et al., 1994).
Microarthropods were extracted from four soil cores of 196 mL per replicate, during a 1-week period with Tullgren funnels, and processed using the gel-based sub-sample methodology (Jagers op Akkerhuis et al., 2008).Total numbers were recorded, while species composition was assessed in sub-samples of 100 individuals following Jagers op Akkerhuis et al. (2008), and references therein.Microarthropod biomass C was calculated based on individual weights, moisture contents, and C contents from Didden et al. (1994).
Microarthropod diversity was quantified in three ways: by absolute number of taxa present, by the Shannon diversity index (H ), and by the Pielou evenness index.For the Shannon diversity index (H ) we used the following formula: in which p i is the proportion of the total biomass (S), i.e. the relative biomass, of species i.For the Pielou evenness index (J ) we used the formula in which H represents the Shannon diversity index and N the total number of taxa present.

Statistics
The data were from eight farms that differed in various ways: climate, soil type, soil management, and crop.There were no real replicates, as the triplicate measurements for all variables were from plots on the same farm.Hence, we performed a nested two-way ANOVA with two factors: country (Iceland-Austria) and farm management (organic-conventional), and farm as a random nested factor.By taking country as a factor, we separated the grassland (Iceland) from the cropland (Austria) farms.By including farm as a random nested factor, we accounted for the variation among farms.We tested the differences between soil types separately using a one-way ANOVA with soil as a factor.All data were log-transformed to obtain homogeneity of variances.Statistical analyses were carried out using SPSS (20.0.0) and R (2.15.2).

Soil physicochemical measurements
Many physicochemical soil characteristics varied strongly over farms, as a consequence of different soil types, soil management, and climatic conditions (countries) (Table 3).The most pronounced differences were found between the soils from the two different countries.Clay content was lowest in the Haplic Andosols in Iceland (p = 0.001).Soils in Austria were alkaline (pH 8) as a result of the much higher calcium content of the Chernozems, whereas the Andosols in Iceland had a lower pH (pH 5-6).Plant available nutrients (P, K) were much higher on the farms in Austria than in Iceland, due to the strong nutrient retention in Andosols (p = 0.001 and p = 0.026, Table 3).
For the MWD of soil aggregates, we found a difference between farm management: on the organic farms in Iceland the MWD was more than twice as high as on the conventional farms, although the difference was not statistically significant (p = 0.173).The opposite was found in Austria, although here the differences were relatively small (Table 3).Mean weight diameter was positively correlated to fungal (Pearson test, r = 0.739, p = 0.006) and bacterial biomass (Pearson test, r = 0.664, p = 0.019), whereas  no significant correlations were found with organic matter parameters.The content of free particulate organic matter (fPOM) and occluded particulate organic matter (oPOM) varied strongly between the different countries and between soil types within countries.The fPOM content in the Icelandic Histic Andosols (358-444 g kg −1 ) was higher than in the Icelandic Haplic Andosols (23-33 g kg −1 ) and all Austrian soils (2-3 g kg −1 , p < 0.001).The oPOM content showed a similar pattern.The high contents of particulate organic matter in Iceland, especially in the Histic Andosols, reflect the very high content of organic carbon (contents of TOC, HWC, and WSC) and nitrogen (both total N and PMN) in these soils: TOC (p = 0.010), HWC (p = 0.072), total N (p = 0.020), and PMN (p = 0.022) were all higher in Iceland compared to Austria.The farms on Histic Andosols in Iceland had a lower C mineralisation rate (2157-2654 kg ha −1 yr −1 ), but a much higher potential N mineralisation rate (746-1010 kg ha −1 yr −1 ), than the farms on Haplic Andosols in Iceland; these differences were even more pronounced compared to the farms in Austria (p = 0.032).
The way organic carbon (OC) and nitrogen (N) were distributed over aggregate sizes and organic matter fractions, was also different between farms.On the organic farm on Haplic Andosol in Iceland, macroaggregates > 250 µm contributed the greatest quantities of OC and N to bulk soil (65 % OC, 65 % for N).On both farms on Histic Andosols in Iceland the fPOM fraction contributed the largest quantities of OC and N to bulk soil (61 and 69 % for OC and 56 and 62 % for N, respectively).On the winter wheat farms in Austria, microaggregates of 20-250 µm contributed the greatest quantities of OC and N to bulk soil (46 and 50 % for OC and 45 and 45 % for N, respectively), while on the potato farms in Austria the microaggregates < 20 µm contributed the greatest quantities of OC and N to bulk soil (51 and 46 % for OC and 51 and 47 % for N, respectively).

Soil food web measurements
Based on presence-absence data of the soil organisms, we constructed soil food web diagrams for all farms (Fig. 1).These diagrams were very similar; despite differences in climatic conditions, crop type, soil type, and soil management, most of the trophic groups were present on all farms.Some of the trophic groups were only present at some farms, including predaceous nematodes, bacterivore mites, herbofungivore mites, and Diplura (Fig. 1, Table 4).
Trophic groups showed differences in abundances (Table 3) and species composition (see microarthropod diversity).Bacterial biomass was consistently higher on organic farms in both countries, although the differences were not statistically significant.Bacterial activity, measured as the incorporation rate of [ 14 C]leucine, did not differ significantly between farms.Fungal biomass did not show a consistent pattern over all farms, although fungal biomass tended to be lower on the farms on Histic Andosols.Protozoa ) of the trophic and taxonomic groups in the soil food webs, bacterial activity, and microarthropod diversity.Numbers represent mean and standard deviation (between brackets), measured in the topsoil (0-10 cm); nd: not detected.Significance values of the factors farming (organic vs. conventional), country (Iceland vs. Austria), and the interaction effect are shown.(amoebae, flagellates) and enchytraeids showed no clear pattern in biomass (Table 4).
Nematode biomass was consistently higher on organic farms than in conventional farms, regarding all trophic groups, although differences were only statistically significant for herbivorous nematodes (p = 0.035) and total nematode biomass (p = 0.015, Fig. 2b).
Microarthropod abundance varied strongly from just over 12 000 m −2 to over 200 000 m −2 .We did not find systematic differences between country or management type.Total microarthropod biomass was much higher on the conventional farms in Iceland compared to all other farms (Fig. 2c).Total Acari biomass was significantly higher on conventional farms compared to organic farms (p = 0.023, Table 4).The higher biomass of omnivorous mites (p = 0.012) and, to a lesser extent, also the consistently higher Acari biomass (p = 0.023) on conventional farms was fully accounted for by the high biomass of the astigmatid mite Tyrophagus similis.T. similis accounted for 98.1 and 99.7 % of the total omnivorous mite biomass, and 59.8 and 69.7 % of the total microarthropod biomass in the conventional grasslands in Iceland, while this species was (nearly) absent at all other farms (Appendix A).In Iceland, collembolan biomass was higher on conventional farms compared to organic farms (Table 4).

Microarthropod species identity and diversity
In total, 82 taxa of microarthropods were found in our study sites, with an overall larger diversity in Austria than Iceland.All farms showed striking differences in the microarthropod species composition: only 3 out of the 82 taxa were present on all farms (the mesostigmatid Arctoseius cetratus and the prostigmatids Eupodes sp. and Pygmephorus sp.).In Iceland, 27 taxa were found that did not occur in Austria, and 37 taxa were found only in Austria, while only 18 taxa were found in both countries.The number of taxa only occurring on organic farms amounted to a total of 33, either in Iceland (14 taxa) or in Austria (18 taxa), while 1 taxon (Tyrophagus sp.) was found on organic farms in both Iceland and Austria.Moreover, 12 taxa were found only on conventional farms, of which 5 were in Iceland and 7 in Austria.The organic wheat farm in Austria had a remarkably high microarthropod taxonomic richness, with 34 taxa present, of which 12 were unique to that farm.The conventional grasslands in Iceland in particular had low taxonomic richness of only 18 taxa (Hi-Acon) and 17 taxa (HaAcon).
Organic farms had a significantly higher microarthropod diversity measured according to all diversity measures; for the Shannon index (p = 0.027, Fig. 3a) and the Pielou index for evenness (p = 0.008, Fig. 3b), differences were statistically significant; for taxonomic richness it was not statistically significant (p = 0.122, Fig. 3c).

Discussion
In this study we investigated soil quality parameters (physical, chemical, and biological) on the organically and conventionally managed farms that are part of the European CZO network.
4.1 Soil aggregate formation, soil organic matter, and soil nutrient cycling Regarding soil structure formation and soil organic matter, the different farming practices, organic versus conventional, did not reveal systematic differences in many physical and chemical soil properties.The soil aggregate size distributions were consistently higher on organic than on conventional farms in Iceland, but no differences were found in Austria.Other management practices such as tillage (Beare et al., 1994) or crop rotation history may have obscured effects of organic amendments.For example, the arable farms in Austria applied a crop rotation with a yearly tillage.As soil aggregates are sensitive to soil tillage (Beare et al., 1994(Beare et al., , 1997;;Six et al., 2000), it could be expected that the differences between organic and conventional arable farms are comparably small.In contrast, the Icelandic grasslands had not been tilled for 8-16 years (Table 1).Also, the addition of higher quantities of organic amendments was expected to have a positive effect through enhanced soil biological activity, in terms of aggregate-forming substances.However, the observed higher mean weight diameters on the organic farms on Iceland could not be linked to higher organic matter contents, e.g. in terms of total carbon, or a difference in organic matter composition.However, mean weight diameter of aggregates was significantly correlated with fungal and bacterial biomass.Both bacteria and fungi produce soil-binding compounds like polysaccharides, which are important for production of relatively small aggregates (de Gryze et al., 2005;Wright et al., 2007).Soil fungi are assumed to be especially more important for the formation of larger soil aggregates through entanglement by hyphae (Tisdall and Oades, 1982).
Regarding the soil carbon and nitrogen, we also did not detect systematic differences between organic and conventional farming.C and N mineralisation rates as well as the measured C and N pools (TOC, HWC, total N, PMN; Table 3) were quite similar on organic and conventional farms.Furthermore, bacterial activity was similar on organic and conventional farms.The present results partly confirm the results reported from earlier studies (van Diepeningen et al., 2006;Bloem et al., 2006).
In summary, C and N contents and dynamics between organic and conventional farms have been studied in three different ways: factorial field experiments on a single farm, pairwise comparisons of farms (as in our study), and comparisons across a larger number of farms (n = 10-20).In a factorial field experiment on an arable farm, the Lovinkhoeve in the Netherlands, Bloem et al. (1994) found a higher C and  N mineralisation in an integrated field compared to a conventional field, probably as a result of organic amendments.Similarly, on a grassland farm in the Netherlands, a higher N mineralisation and potentially mineralisable N has been measured when organic fertiliser was applied, while no difference has been found in C mineralisation (van Eekeren et al., 2009).Also, Poudel et al. (2002) found a higher potential N mineralisation in organically managed crop rotation fields than in conventional fields in California, but there the organic fields also grew legumes between growing seasons, enhancing N availability.In Switzerland, Birkhofer et al. ( 2008) observed a lower N mineralisation when only mineral fertiliser was used, while C mineralisation did not show differences between the fields.Also, in this study, no differences were found between organic fields and fields that received both artificial fertilisers and organic manure, similar to the Icelandic grasslands in the present study.Thus, in factorial experiments on a single farm, the effects of organic management on soil N dynamics are quite clear, while the effects on C dynamics are not consistent.
In an example of a pairwise comparison between organic and conventional arable farms in the Netherlands, van Diepeningen et al. ( 2006) observed lower nitrate levels on organic farms, with no differences in total organic C, organic N, or total N. Conventional farms in that study also applied organic manure in addition to artificial fertilisers, which is comparable to the grasslands in Iceland, where we also did not find differences in total organic C and total N.In an example of a comparison across larger number of farms in the Netherlands (n = 10-20), Bloem et al. (2006) showed higher C and N mineralisation rates in organic grasslands compared to conventional grasslands, but not in the comparison between organic and conventional arable farms.Thus, our study confirms the notion that more factors are variable and differences between organic and conventional farming are less prominent when C and N dynamics are studied on a larger scale with more farms involved.

Soil food web structure
The trophic structure of the soil food webs showed a high similarity; nearly all trophic groups were present on all farms.This indicates that the trophic structure of the soil food webs was neither very sensitive to management nor to climate, soil type, and farm type.Biomass of the different organisms, however, differed between farms.
Microbial biomass, as the sum of bacteria and fungi, was consistently higher on organic farms, although not statistically significant.The higher microbial biomass, especially bacterial biomass, is in line with previous studies that have compared organic and conventional farms (Bloem et al., 2006;Hole et al., 2005;Haubert et al., 2009;Mäder et al., 2002;Birkhofer et al., 2008;van Diepeningen et al., 2006;Gunapala and Scow, 1998).Other studies also have reported a higher microbial activity (Bloem et al., 2006;Hole et al., 2005), which we did not find in our study.We did not find differences in fungal biomass, in contrast with some previous results (Yeates et al., 1997;de Vries et al., 2006), but in line with others (Shannon et al., 2002).These results might be due to the fact that added organic amendments in organic farming are generally easily degradable and therefore enhance mainly bacterial biomass and activity (Hole et al., 2005).
We observed a significantly higher total nematode biomass on organic farms.Although a higher biomass was observed for all trophic groups of nematodes, the difference was mostly accounted for by herbivorous nematodes.This is in agreement with the higher nematode abundance that was found after addition of organic manure to wheat fields in Switzerland, where herbivorous nematodes were also the dominant group (Birkhofer et al., 2008).It is also in agreement with the higher nematode abundance (although dominated by fungivores) found in organic grasslands in Wales (Yeates et al., 1997).Hence, our results support the notion that nematodes are sensitive to farming type and that they profit from the addition of organic amendments.
Microarthropod biomass measurements did not reveal systematic differences between farm types, although total microarthropod biomass was highest on the conventional farms within Iceland.We also did not find a difference between the grassland farms in Iceland and the arable farms in Austria.This is a bit unexpected because it is frequently observed that microarthropod biomass is higher in grasslands compared to arable farms, because ploughing decreases microarthropod biomass, which is more intense for root/tuber crops such as potato (Vreeken-Buijs et al., 1998).In our study, the organic grasslands in Iceland were, however, ploughed in the three consecutive years when the field was renewed, which, together with the colder climatic conditions, may explain why biomass of microarthropods was not higher in the grasslands than in the arable fields (Sjursen et al., 2005).
We found a statistically higher biomass of mites (Acari) on the conventional farms compared to the organic farms.
We lack an explanation for this somewhat unexpected result.For example, it is contrary to the results from an earlier study, showing higher abundances of Acari in organic grasslands compared to conventional grasslands in Wales (Yeates et al., 1997).The similar collembolan biomass on organic and conventional farms is in line with the results of Birkhofer et al. (2008) in Switzerland, but in contrast with the results of Bardgett et al. (1993), who reported higher collembolan biomass in organic fields.The two species of collembolans that are by far the most abundant in the study of Bardgett et al. (1993) were much less abundant (Onychiurus procampatus) or even absent (Folsomia quadrioculata) in our data, which may explain the difference between the studies.

Microarthropod diversity
The most systematic difference we found in the comparison between organic and conventional farming was the higher microarthropod diversity on the organically managed farms.This difference was found across countries, farm types (grassland versus arable), and crop and soil type.This finding is in agreement with Doles et al. (2001) and Macfadyen et al. (2009).
Factors known to enhance soil microarthropod diversity include plant litter diversity leading to a higher microhabitat and resource diversity (Hansen and Coleman, 1998) and plant species identity (Wardle et al., 2005).In Iceland, organic grasslands had a higher plant diversity than conventional grasslands, which supports the hypothesis that plant diversity enhances belowground microarthropod diversity.On the arable farms in Austria, where plant diversity does not play a role, the application of artificial fertilisers may have reduced the microarthropod diversity (Siepel and Van de Bund, 1988).
Soil microarthropod diversity is described as a sensitive biological indicator for effects of environmental change and disturbance on soil quality (Gardi and Parisi, 2002;Parisi et al., 2005;Gardi et al., 2009).Our results confirm that the taxonomic diversity of the soil microarthropods was sensitive to differences in farm type and management system.
If we look at these findings in terms of the role of biodiversity in ecosystem functioning, we see that the higher microarthropod diversity on organic farms did not result in differences in the food web structure, nor did it yield higher ecosystem services, such as soil fertility or C sequestration.This is in agreement with Setälä et al. (2005), who argue that the functional importance of individual groups is rather high at coarse (trophic group) level but low at species level, and that effects of species diversity on ecosystem functioning are most likely found in studies with a very low species richness and therefore a low functional redundancy.Nevertheless, in our study microarthropod diversity was found to be a sensitive and consistent indicator for land management.At present, determining microarthropod diversity is a relatively intensive activity, but when the current progresses in method-ology lead to faster and cheaper analyses, such as barcoding extracted microarthropods, soil microarthropod diversity will become more cost-effective and an even more valuable indicator for soil quality.

Conclusions
In this study we investigated soil biological, chemical, and physical parameters for soil quality on organically and conventionally managed farms.The chosen farms were part of the European Critical Zone Observatory network.Factors that vary across farms -such as climate, soil type, and farm type, and the limited number of replicates taken -have made it difficult to find clear patterns or draw general conclusions.On the other hand, we did observe that the organic farms showed higher biological parameters, in particular the diversity in soil microarthropod diversity, despite these limitations.Physical and chemical parameters showed no clear differences between the organic and conventional farms.Our results therefore do support the use of microarthropod diversity as a soil quality indicator, although physical and chemical soil properties are indispensable for a complete assessment and understanding of soil quality.

Figure 1 .
Figure 1.Soil food web diagram representative for all eight farms.Boxes represent the presence of trophic groups in the soil food web, and arrows represent feeding interactions based on diet information.Solid groups were present at all farms, and dashed groups were only present at some farms.
Biological parameters on the farms studied in Iceland (conventional farms IceHaAcon and IceHiAcon, organic farms IceHaAorg and IceHiAorg) and Austria (conventional farms AusPOTcon and AusWWcon, organic farms AusPOTcon and AusWWorg): biomasses (kg C ha −1

Figure 2 .
Figure 2. Biomass in kg C ha −1 of microbes (bacteria+fungi) (a), nematodes (b), and microarthropods (c) on organic and conventional farms in Austria and Iceland.Bars are means ± standard deviation (n = 6), measured in the topsoil (0-10 cm).P values are the results of a nested univariate analysis of variance (ANOVA), with type (conventional (white bars) or organic (grey bars)) and country (Austria or Iceland) as factors.

Figure 3 .
Figure 3. Shannon diversity index on microarthropod taxa (a), Pielou evenness on microarthropod taxa (b), and absolute microarthropod taxa richness (c) on organic and conventional farms in Austria and Iceland.Bars are means ± standard deviation (n = 6), measured in the topsoil (0-10 cm).P values are the results of a nested univariate analysis of variance (ANOVA), with type (conventional (white bars) or organic (grey bars)) and country (Austria or Iceland) as factors.

Table 1 .
Characteristics of the farms studied in Iceland (conventional farms IceHaAcon and IceHiAcon, organic farms IceHaAorg and IceHiAorg), including vegetation richness (values represent mean and standard deviation (between brackets)).

Table 2 .
Characteristics of the farms studied in Austria (organic farms AusPOTcon and AusWWorg, conventional farms AusPOTcon and AusWWcon).Crop rotation before 2001 was similar to the crop rotation presented in the table.

Table 3 .
Soil physicochemical properties and biologically mediated processes on the farms studied in Iceland (conventional farms IceHaAcon and IceHiAcon, organic farms IceHaAorg and IceHiAorg) and Austria (conventional farms AusPOTcon and AusWWcon, organic farms AusPOTcon and AusWWorg).Values represent mean and standard deviation (between brackets) per farm, measured in the topsoil (0-10 cm).Significance values of the factors farming (organic vs. conventional), country (Iceland vs. Austria), and the interaction effect are shown.