Soil organic carbon in northern Spain (Galicia, Asturias, Cantabria and País Vasco)

SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2015 VOLUME 5 ISSUE 1 41 Soil organic carbon in northern Spain (Galicia, Asturias, Cantabria and País Vasco) Carbono orgánico en los suelos del norte de España (Galicia, Asturias, Cantabria y País Vasco) Carbono orgânico nos solos do norte de Espanha (Galiza, Astúrias, Cantabria e País Basco) ABSTRACT ABSTRACT The soil organic carbon content was analyzed in more than 7,000 soil samples under different land uses, climates and lithologies from northern Spain (Galicia, Asturias, Cantábria y País Vasco). GIS maps (1:50 000) were made of the % SOC and SOC stocks. The % SOC varies according to land use (higher in forest and scrub soils and lower in agricultural soils) and climate, and there is a highly significant correlation between SOC content and mean annual precipitation. There are significant differences between the soils of Galicia/Western Asturias (GAw) and those of the rest of the study area (Central and Eastern Asturias, Cantabria and País Vasco) (AceCV), although these are neighbouring regions. In forest and/or scrub soils with a udic soil moisture regime, in GAw, the SOC is usually > 7% and the average stocks 260 t ha-1 (0-30 cm), and >340 t ha-1 (0-50 cm) in soils with thick organic matter rich horizons (> 40 cm); these values greatly exceed the average contents observed in forest soils from temperate zones. Under similar conditions of vegetation and climate in soils of AceCV the SOC average is 3% and the mean stocks 90-100 t ha-1 (0-30 cm). The andic character of acid forest soils in GAw and the formation of C-Al,Fe complexes are pointed out as the SOC stabilization mechanism, in contrast to the neutral and calcareous soils that predominate in AceCV, where the main species of OC are easily biodegradable. RESUMEN Se analiza el contenido de carbono orgánico (CO) en más de 7.000 muestras de suelos del norte de España (Galicia, Asturias, Cantábria y País Vasco) bajo diferentes tipos de ocupación, condiciones climáticas y litología, y se elaboran mapas SIG (1:50 000) del porcentaje y stock de carbono en los suelos. El porcentaje de CO varía de acuerdo al uso del suelo (mayor en suelos forestales y con matorral, y menor en suelos de cultivo) y al clima, reconociéndose una correlación altamente significativa entre el porcentaje de CO y la precipitación media anual. En cualquier caso, aún tratándose de regiones próximas, se establecen diferencias importantes entre los suelos de Galicia-oeste de Asturias (G/Ao) y los del resto del área de estudio (Asturias central y oriental, Cantabria y País Vasco) (AceCV). En suelos forestales y/o con matorral y régimen de humedad údico, en G/Ao, el porcentaje de CO es habitualmente > 7% y el stock medio 260 tC ha-1 (0-30 cm) y puede ser > 340 tC ha-1 (0-50 cm), teniendo en cuenta la abundancia de suelos con rasgos cumúlicos y horizontes humíferos con > 40 cm de espesor; los valores obtenidos superan ampliamente los contenidos medios señalados por diferentes autores para suelos forestales de áreas templadas. En similares condiciones de vegetación y clima, el contenido medio de CO en suelos de (AceCV) es de 3% y el stock medio 90-100 tC ha-1 (0-30 cm). El carácter ándico de los suelos forestales ácidos de G/Ao, y la formación de complejos C-Al,Fe se señala como mecanismo de estabilización del CO, en contraste con los suelos neutros y calcáreos que predominan en AceCV, en los que la especie principal es el CO fácilmente biodegradable. Received: 12.11.2014 Revised: 29.01.2015 Accepted: 02.02.2015 DOI: 10.3232/SJSS.2015.V5.N1.04 Calvo de Anta R.@, 111 [email protected] Luís Calvo E.1 Casás Sabarís F.1 Galiñanes Costa J.M.1 Matilla Mosquera N.1 Macías Vázquez F.1 Camps Arbestain M.2 Vázquez García N.1 @ Corresponding Author 1 Departamento de Edafología y Química Agrícola. Facultad de Biología. Universidad de Santiago de Compostela. Avda. Lope Gómez de Marzoa s/n. Santiago de Compostela. Spain. 2 Institute of Natural Resources, Private Bag 11222, Massey University. Palmerton North 4442, New Zealand. AUTHORS SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2015 VOLUME 5 ISSUE 1 42 KEY WORDS Digital map of SOC, soil-C stocks PALABRAS CLAVE Mapa digital de COS, stocks de C-suelo PALAVRASCHAVE Mapa digital de COS, stocks de C-solo RESUMO Analisou-se o teor de carbono orgânico en mais de 7.000 amostras de solo do norte de Espanha (Galiza, Astúrias, Cantábria e País Basco), sob diferentes usos, condições climáticas e litologias, e procedeu-se ao mapeamento SIG (1:50 000) da percentagem e stock de carbono nos solos. A percentagem de CO varia de acordo com o uso do solo (maior em solos de floresta e mata e menor em solos agrícolas) e condições climáticas, observando-se uma correlação altamente significativa entre a percentagem de CO e a precipitação média anual. Em qualquer caso, mesmo para regiões vizinhas, registam-se diferenças significativas entre os solos da Galiza-oeste e das Astúrias (G/Ao) e o resto da área em estudo (Astúrias central e oriental, Cantabria e País Basco) (AceCV). Em solos de floresta e/ou de mata e regime de humidade údico, em G/Ao, a percentagem de CO é geralmente > 7% e o stock médio 260 t C ha-1 (0-30 cm), e > 340 t C ha-1 (0-50 cm) em solos com espessos horizontes ricos em matéria orgânica (> 40 cm); estes valores excedem largamente os teores médios observados em solos florestais de zonas temperadas. Em condições semelhantes de vegetação e clima, em solos de AceCV, o teor médio de CO é de 3% e o stock médio de 90-100 t C ha-1 (0-30 cm). O carácter andico dos solos florestais ácidos da Galiza, e a formação do complexos C-Al,Fe são apontados como o mecanismo de estabilização do CO, em contraste com os solos neutros e calcários prevalecentes em AceCV, onde a espécie predominante é o CO facilmente biodegradável. 1. Introduction From the United Nations Framework Convention on Climate Change (1994), the Kyoto Protocol 1997-2005 (United Nations 1998) has established targets for reductions in emissions of greenhouse gases (GHG), with mandatory compliance for industrialized countries. Policy measures include the possibility of using carbon sinks (plant biomass and soil), taking into account the net fixation/emissions due to certain activities (Land use, Land Use Change and Forestry) as outlined in articles 3.3 and 3.4 of the Protocol. The assessment of biomass as a sink generated additional rights in some EU member states of between 1% and 4% above the emission targets, for the period 2008-2012. Soil was not considered in these calculations because of the lack of sufficient data for modelling the net fixation on a temporal scale for each geographical region and land use, despite the abundance of information about the importance of soil as a carbon sink. The global C content of soils throughout the world is estimated to be close to 1,600 Pg C (0-100 cm) (Batjes 1992; Eswaran et al. 1993; Sombroek et al. 1993), which is twice the amount estimated to exist in vegetation biomass (Lal et al. 1995; IPCC 2000; Saugier et al. 2001). Taking into account data reported by different authors (Bohn 1982; Post et al. 1982; Eswaran et al. 1993), the United States Department of Agriculture constructed maps of the organic C content of the world’s soils (scale 1: 100,000,000) (USDA 2000). The maps show the zonal nature of the distribution, with values > 800 tC ha-1 (0-100 cm depth) in peatland areas of frigid and cryic environments, and of < 20 tC ha-1 in aridic environments. In Europe, the European Soil Bureau has made several maps (scale 1:1,000,000) by applying a calculation model to the available data included in the “European Soil Database” and harmonized in accordance with the international classification scheme (FAO-UNESCO-ISRIC 1990) and the CORINE report on land cover and land use (Rusco et al. 2001; Jones et al. 2003, 2004). The maps show the % SOC (0-30 cm depth), but not the SOC stocks (tC ha-1) because there are insufficient data available for bulk density. In their final recommendations, the authors indicated SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2015 VOLUME 5 ISSUE 1 43 [ SOIL ORGANIC CARBON IN NORTHERN SPAIN (GALICIA, ASTURIAS, CANTABRIA AND PAÍS VASCO) ] the main deficiencies and urged Member States to validate the information provided or, where appropriate, to apply sampling programmes and analysis aimed at amplifying the database and thus to improve the model fit (Jones et al. 2004). Subsequently, Hiederer et al. (2011) performed an estimation of Global SOC and the Harmonized World Soil Database. In their conclusions they recognized that the various sources of spatial data for estimating global SOC stocks need further proccesing or data amendments in order to provide coherent and comparable results, and indicated again the scarcity of some necessary parameters for calculation (bulk density, coarse fragments and soil depth). The objective of this study is to create a semidetailed map (1:50,000) of the organic carbon content of the soils of some regions in northern Spain (Galicia, Asturias, Cantabria and the País Vasco), amplifying the existing information on geo-referenced samples in environments with different climate, land cover, soil type, lithological substrate, etc. 2. Material and Methods The study area includes the territories of the autonomous communities of Galicia (29 574 km2), Asturias (10 604 km2), Cantabria (5 321 km2) and the País Vasco (7 234 km2) in northern Spain. Overall, the soil temperature regime in the whole territory is mesic and the soil moisture regime is udic , with an average annual rainfall between 800 and > 2000 mm; in some inland areas of Orense (Galicia) and south of Álava (País Vasco), the soil moisture regime is xeric . The lithology is varied. In Galicia and western Asturias there is a predominance of acid rocks (granite, quartz schists and shales) and a lower proportion of basic (gabbros and amphibolites) and ultramafic rocks (serpentinites); the soils are mainly classified as haplic and andic Umbrisols on acid rocks, umbric Andosols on basic rocks and haplic Phaeozems on serpentinites (IUSS-WRB 2014). In the Cantabrian coast the lithology is mainly calcareous sedimentary materials (marl, limestone, sandstone and other) and soils are classified as calcaric Phaeozems ( haplic Umbrisols when decalcified), or calcaric (or eutric ) Cambisols if not satisfied the requirements for umbric or mollic epipedon (IUSS-WRB 2014). In total, 7 140 soil samples were collected under different types of land use (forests, shrubs, crops, meadows, pastures, marshes, dunes, peats, etc.), 3 752 in Galicia and 3 388 in Asturias, Cantabria and the País Vasco (hereafter ACV). Most of the samples (4 505) correspond to the surface layer with higher organic matter content (A horizon: 0-10/15 cm or 0-30 cm, as the case) and the remaining (2 635) to the underlying layer (AB, B or C horizons), up to a depth of 50 cm. Air-dried soil samples (< 2 mm) were analyzed for total carbon content (LECO-TruSpec CHN analyzer), total inorganic carbon (carbonate-C) (by Bernard calcimeter method), total N (LECO), pH-H2O (1:2.5) and pH-KCl; furthermore, the dry bulk density was determined (by using cylindrical cores of 5 x 5 cm) as well as the volume of coarse fragments (> 2 mm). The organic C content (% OC) was determined by difference between total C and inorganic C. Soil organic carbon stocks (OCS) (tC ha-1) were computed separately for both layers from OC content, coarse fragments content, layer thickness and bulk density, as: OCSL = % OC x BD x [1- (VG/100)] x LT where, OCSL: total amount of soil organic carbon to given depth (tC ha-1); OC: soil organic carbon content for given depth; BD: dry bulk density (g cm-3); VG: volume of gravels; LT: thickness of soil layer (cm). The OCS stocks thus computed for the two layers were then combined to estimate the OCS stocks for 0-30 and 0-50 cm. Where the soil depth was less than 30 (or 50) cm, stocks were computed to that depth. The analytical results were analyzed by ANOVA, taking into account land use, soil type, lithology, climate, altitude and proximity to the sea. SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2015 VOLUME 5 ISSUE 1 44 [ CALVO DE ANTA R., LUÍS CALVO E., CASÁS SABARÍS F., GALIÑANES COSTA J.M., MATILLA MOSQUERA N., MACÍAS VÁZQUEZ F., CAMPS ARBESTAIN M. & VÁZQUEZ GARCÍA N. ] Moreover, the following thematic maps (GIS) were constructed: (A) Land use maps, using information from CORINE Land Cover and PNOA (National Institute of Geography) and the Forest Map of Spain (Ministry of Agriculture); the Information System of Land Occupation of Spain (SIOSE-2009) (Xunta de Galicia) was used for the territory of Galicia. The combined information was standardized and adapted to the study objectives, and the following units were finally considered for cartography: 1. Marine and coastal environments, differentiating beaches and dunes, coastal salt marshes, estuaries and cliffs; 2. Rocky outcrops; 3. Peatlands; 4. Shrublands; 5. Forests (natural or reforested); 6. Grasslands and pastures; 7. Cropland of cereals or vegetables; 8. Vineyards; 9. Urban areas and infrastructures, including quarries and mines; 10. Water surfaces. (B) Soil depth maps (by field descriptions and photointerpretation), considering the following units: < 10, 10-20, 2030, 30-40, 40-60, 60-100 and > 100 cm thick. Finally, maps of the soil organic carbon content (% OC) and COS (tC ha-1) for 0-30 and 0-50 cm soil thickness) were elaborated at 1:50,000 scale. Treatment and georeferenced analysis of the data was implemented with the GIS software tools (ArcGis Desktop 9.3). 3. Results 3.1. Organic carbon contents of Galician soils The OC content of Galician soils varies widely, from < 0.2% in beaches and coastal dunes, to > 35% in areas with peat, both of which are scarcely represented. The cartographic distribution is strongly dependent on the type of land use. More than 65% of the territory was found to be covered by shrubs and/or trees (mainly reforested areas with understory of shrub); crops and pasture occupy 27% of the land, and the remaining 10% has very diverse coverage (urban, infrastructures, rock, marshes, etc.) (Figure 1). On a local scale, considering the main types of land use, the % OC of the soil followed the order: vineyards<cereal and vegetable crops<grassland<forest≤shrub (Figure 2). This is a common sequence in different parts of the world and is related to the different amounts and/or recalcitrance of organic debris; it should be noted that the management of agricultural soils in Galicia often involves the application of organic amendments. Overall, for the whole region, the mean values obtained follow this sequence according to land use, although a wide variation of the values indicating the influence of other factors are observed (Table 1). Figure 1. Map of land use in northern Spain (made at scale 1:50 000, Calvo de Anta et al. 2013). SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2015 VOLUME 5 ISSUE 1 45 [ SOIL ORGANIC CARBON IN NORTHERN SPAIN (GALICIA, ASTURIAS, CANTABRIA AND PAÍS VASCO) ] On reducing the scale, the climate factors become more important, more specifically the hydric regime since the temperature regime is relatively homogeneous throughout the territory. The data obtained for 1,384 surface horizons of forest and/or shrubland soils (F-SH), in areas with different rates of annual mean precipitation, revealed a highly significant correlation (p < 0.01), described by the following equation: % OCF-SH= 0.005 P – 0.27 ± 1.74 (R= 0.75) where, OCF-SH is the soil organic carbon (0-30 cm) and P is the annual mean precipitation (mm) (Figure 3a). The equation describes a similar trend to that observed in forest soils developed from granites in Portugal (Madeira et al. 2004), although the OC contents are greater in Galicia (0.6% higher) (Figure 3c). The cartographic variability, depending on the weighting of the different factors (land use, climate, altitude, etc.), is illustrated in Figure 4. Ignoring the aforementioned extreme values, the most common interval of variation ranged between 3-4% OC, for cropped soil close to the coast at an elevation <100 m and in inland areas of Lugo and Orense, and 8-10% in shrubland and/or forest areas at an elevation of > 400 m, in the provinces of La Coruña and Pontevedra. In global terms, 63% of the surface of Galicia has > 6% of OC (6-10%), which coincides with the presence of forest soils (F-SH) in the wettest areas (> 1200 mm of annual rainfall). Soil organic carbon stocks (tC ha-1) were determined considering the % OC and the bulk density of the material. The BD is also related to the % OC, resulting a highly significant negative correlation between these two parameters (Figure 5); the BD was not correlated with texture (values ≥ 1.5 g cm-3 were recorded both in soils derived from marls, with > 40% clay, and in coastal dunes containing 100% sand). As result, the BD of the soils varied according to an inverse sequence to % OC, lower in the F-SH soils and higher in cropped soils, and increased from the surface to subsurface horizons (Table 1). Figure 2. Variation in the soil organic carbon content in different land use types (0-30 cm depth) (ACV: Asturias, Cantabria and País Vasco). SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2015 VOLUME 5 ISSUE 1 46 [ CALVO DE ANTA R., LUÍS CALVO E., CASÁS SABARÍS F., GALIÑANES COSTA J.M., MATILLA MOSQUERA N., MACÍAS VÁZQUEZ F., CAMPS ARBESTAIN M. & VÁZQUEZ GARCÍA N. ] Table 1. Organic Carbon (OC), nitrogen, bulk density (BD) and pH in soils in different land use (median and range) (Sh: shrublands; F: forests; G: grasslands; C: cereal/vegetable crops; V: vineyards; GR: soils on granitic rocks; S: schists; SL: slates; Q: sandstones and quartzites; B: basic rocks; UB: ultrabasic rocks; LM: limestones and marls) (c,e: Central and Eastern) GALICIA Sh F G C V Soil layer (cm) % OC 0-30/40 6.03 0.4-22 n=658 5.67 0.6-20 n=901 4.54 0.2-21 n=605 3.6 0.1-16 n=428 2.26 0.4-6 n=107 30/40-50 1.21 0.1-5 n=220 0.98 0.1-8 n=301 0.67 0.1-5 n=220 1.15 0.2-4 n=150 1.10 0.1-3 n=50 % N 0-30/40 0.33 <0.01-5.8 n=658 0.29 <0.01-1.5 n=901 0.33 <0.01-3.0 n=605 0.29 <0.01-1.6 n=428 0.17 <0.01-1.9 n=107 BD (g cm-3) 0-30/40 0.95 0.6-1.3 n=336 0.91 0.5-1.4 n=491 1.06 0.7-1.4 n=271 1.08 0.6-1.5 n=104 1.08 0.9-1.5 n=33 30/40-50 1.18 1.0-1.6 n=117 1.15 0.7-1.7 n=103 1.11 0.8-1.2 n=117 1.29 0.7-1.6 n=130 1.29 0.8-1.6 n=33 pHH2O(F-Sh) GR S SL/Q B UB 0-30/40 4.8 3.6-5.5 n=658 4.9 4.2-6.8 n=901 4.4 3.3-5.4 n=605 5.1 4.0-6.5 n=428 5.9 4.4-6.7 n=107 30/40-50 4.9 3.8-5.9 n=220 5.0 4.7 3.7-6.6 n=220 5.0 3.2-6.8 n=150 6.5 4.7-6.8 n=50 3.3-6.8 n=301 ASTURIAS(c,e), CANTABRIA, P.VASCO Sh F G C V % OC 0-10/15 4.76 3.59 4.12 1.85 1.20 10/15-30 1.85 2.11 1.49 1.85 1.20 0-30 2.82 0.5-16 n=150 2.60 0.20-23 n=1520 2.81 0.5-15 n=734 1.85 0.2-16 n=834 1.20 0.2-10 120 % N 0-30 0.25 <0.01-1.6 n=150 0.17 <0.01-1.3 n=1520 0.39 <0.01-3.8 n=734 0.2 0.03-1.9 n=834 0.15 0.03-1.0 n=120 BD (g cm-3) 0-10/15 1.04 0.95 1.03 1.29 1.38 10/15-30 1.05 1.07 1.13 1.29 1.38 0-30 1.05 0.7-1.6 n=84 1.07 0.5-1.6 n=1410 0.89 0.7-1.5 n=619 1.29 0.4-1.6 n=282 1.38 0.5-1.6 n=66 pHH2O(F&Sh) LM SL Q 0-30 6.94 5.5-8.7 n=1501 4.54 3.5-5.6 n=648 4.25 3.3-5.3 n=209 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2015 VOLUME 5 ISSUE 1 47 [ SOIL ORGANIC CARBON IN NORTHERN SPAIN (GALICIA, ASTURIAS, CANTABRIA AND PAÍS VASCO) ] Figure 3. Representation of % OC vs. annual mean precipitation in forest and shrubland soils (0-30 cm depth): (a) Galicia and western Asturias; (b) central and eastern Asturias, Cantabria and the País Vasco; (c) forest soils of Portugal (Madeira et al. 2004). Figure 4. Map of soil organic carbon content in northern Spain (Galicia, Asturias, Cantabria and País Vasco) (topsoil: 0-30 cm) (made at scale 1:50 000, Calvo de Anta et al. 2013). Figure 5. Relationship between organic carbon content (OC) and bulk density (BD): (a) soils of Galicia; (b) soils of Asturias, Cantabria and the País Vasco (including topsoil and subsoil). SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2015 VOLUME 5 ISSUE 1 48 [ CALVO DE ANTA R., LUÍS CALVO E., CASÁS SABARÍS F., GALIÑANES COSTA J.M., MATILLA MOSQUERA N., MACÍAS VÁZQUEZ F., CAMPS ARBESTAIN M. & VÁZQUEZ GARCÍA N. ] The mean OC stocks (OCS) in the upper layer of the Galician soils ranged between 90 tC ha-1 (030 cm), in cropped soils in the coastal area and inland areas of the provinces of Orense and Lugo ( xeric conditions), and 260 tC ha-1, in forest soils with shrub understory at an elevation of > 400 m in the provinces of La Coruña and Pontevedra (Table 2 and Figure 6). Sandy areas, leptic soils and areas with part of the surface sealed contain < 20 tC ha-1. In topsoil (0-30 cm), the peat soils contain less OCS than well-drained forest soils at the same latitudes, due to the low BD of the organic materials (approximately 0.2 g cm-3). In general, the OCS in Galician soils is higher than that observed in nearby areas of Europe, in temperate-humid environments. As reference, in forest soils of France, the mean values ranged between 70 and 80 tC ha-1 (0-30 cm), with maximum values of 100 tC ha-1 in alpine zones (Arrouays and Deslay 2001), and in forest soils of humid zones in northern Portugal, the mean stocks are 215 tC ha-1 (Madeira et al. 2004). The geographical distribution of the OCS in Galician soils is shown in Figure 7. In summary, in 92.5% of the territory, the OCS surpassed 100 tC ha-1 and was distributed as follows: 29.3% in the range 100-160 tC ha-1; 34.9% in the range 160-200 tC ha-1; 18.4 % in the range 200-240 tC ha-1; and 10% in the range 240-280 tC ha-1. OCS < 100 tC ha-1 are found in only 1.5 % of the territory (< 20 tC ha-1 in 0.7% and 20-100 tC ha-1 in 0.9%). Sealed (or without soil) surfaces such as urban areas, roads, infrastructures, water surfaces, etc., occupy the remaining 6% of the territory (see Figure 6). Taking into account the data obtained for topsoil and subsoil layers, the total OCS was estimated for the upper 50 cm of the soil; a profile-type comprising a humiferous horizon of 30 cm thick and the underlying layer (20 cm) was considered (A30 B, C20). The mean values obtained range between 125-130 and 270-280 tC ha-1 (0-50 cm) (Table 2), within the range indicated above, for agricultural soils of coastal and/or xeric areas and for forest soils (F-SH) in the wettest zones, respectively. In mountain peatland, the mean stocks are ≥ 350 tC ha-1 (0-50 cm). The actual OCS is even higher if one considers the frecuent occurrence of soils with cumulic characteristics in F-SH areas with slopes > 13%. In the mid-lower slopes, the humiferous horizons usually reach a thickness of more than 40 cm (and even 60 cm), i.e. umbric-pachic characteristics (IUSS-WRB 2014), with evidence of several K cycles from different phases of biostasy-rhexistasy that have occurred during the Quaternary. Considering type-profiles with humiferous horizons of thickness 40 cm (A40 B, C10), the OCS in the upper 50 cm of the soil may surpass 340 tC ha-1 (Table 2) in large forest (F-SH) hillsides with > 1200 mm of annual precipitation. Analysis of this type of soil shows a slight variation in the % OC with depth (age) of the material, even at > 100 cm depth, indicating the existence of organic matter stabilization processes. Radiocarbon dating analysis (by accelerator mass spectrometry: AMS) of the deep layers of some representative profiles supported this hypothesis, with the result that most of the samples analyzed date from more than 1000 +/- 30 years BP (before present-1950), reaching 6150 +/- 30 years BP in the bottom of the thickest umbric-pachic profile analyzed, at 165170 cm (Calvo de Anta et al. 2014). These are similar to the dates pointed by other researchers for the same type of soil (Martínez Cortizas, pers comm). According to this, the high organic matter content in Galician soils does not seem to depend only on current vegetation or climate, but also on stabilization processes that take place over long periods of time. Densimetric and physicochemical analysis of carbon speciation performed on a selection of forest soils have shown the dominance of the C-Al forms in the soil dense fraction (that represents > 50% of the total OC in the 0-20 cm top layer, and > 90% in the 20-40 cm underlying layer) (Calvo de Anta et al. 2014), i.e. it reveals the importance of the formation of organo-aluminium complexes in the stabilization of organic matter, what has been previously noted (García-Rodeja et al. 1987; Calvo de Anta and Álvarez Rodriguez 1992; Macías and Calvo de Anta 1992; Macías et al. 2004; Verde 2009). The results obtained demonstrate the significant role of forest soils in Galicia as carbon sinks, particularly in the wettest zones in the provinces of La Coruña and Pontevedra where there is a predominance of andic (pachic) Umbrisols . The mean values differ substantially from those indicated for temperate forest soils (150 tC ha-1, 0-100 cm) and are similar to or higher than those reported for forest soils in boreal regions (250 tC ha-1, 0-100 cm) (Lal et al. 1995; Saugier et al. 2001; Robert and Saugier 2004). SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2015 VOLUME 5 ISSUE 1 49 [ SOIL ORGANIC CARBON IN NORTHERN SPAIN (GALICIA, ASTURIAS, CANTABRIA AND PAÍS VASCO) ] CORUÑA, LUGO (n), ASTURIAS(w) PONTEVEDRA P-t (cm):A30 (B,C)20 A40 (B,C)10 A30 (B,C)20 A40 (B,C)10 m.a. 0-30 0-50 0-40 0-50 0-30 0-50 0-40 0-50 tC ha-1 tC ha-1 Sh <100 100-400 >400 133 219 257 156 242 279 177 292 342 189 304 353 131 214 247 150 179 184 111 175 223 87 122 100 - 154 236 269 171 199 204 151 215 263 124 158 141 - 175 285 329 200 238 245 148 233 297 117 163 133 - 186 296 340 210 248 255 168 253 317 135 181 154 - F <100 100-400 >400 162 190 205 182 211 225 216 253 273 226 264 283 G <100 100-400 >400 114 191 239 154 231 278 153 254 318 173 274 338 C <100 >100 89 145 125 181 118 193 136 211 V- - - - P(H) 210 350 280 350 Inner LUGO ORENSE P-t (cm):A30 (B,C)20 A30 (B,C)20 A30 (B,C)20 A40 (B,C)10 0-30 0-50 0-40 0-50 0-30 0-50 0-40 0-50 Sh <400 400-600 600-800 >800 128 144 185 191 151 166 208 214 171 192 247 255 182 203 258 266 103 103 111 146 125 125 134 168 137 137 148 194 148 148 159 205 F <400 400-600 600-800 >800 124 141 169 202 144 162 190 223 165 189 225 270 175 199 236 280 97 97 103 111 117 117 123 132 129 129 137 148 139 139 147 158 G <400 400-600 600-800 >800 111 121 151 162 151 161 191 202 148 161 201 216 168 181 221 236 102 102 110 128 142 142 150 168 137 137 147 171 157 157 167 191 C <600 600-800 >800 95 97 97 131 133 133 127 129 129 145 147 147 92 94 103 134 136 145 123 126 138 144 146 158 V- - - - 55 96 73 94 P(H) 210 350 280 350 210 350 280 350 ASTURIAS (c,e), CANTABRIA, P.VASCO P-t (cm):A10 (BA)20 0-10 0-30 Sh <600 600-800 >800 48 55 66 86 99 118 F <600 600-800 >800 32 36 44 74 85 203 G <600 >600 39 57 60 102 C <600 >600 23 24 68 72 V17 50 P(H) - 210 Table 2. Estimates of soil organic carbon stocks in northern Spain. Mean values for different soil depth, according to land use and altitude (m.a.) (nomenclature as in Table 1) (P: peat; H: horizon; P-t: Profile-type)