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

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, Cantabria y Pais 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 (GA w ) and those of the rest of the study area (Central and Eastern Asturias, Cantabria and Pais Vasco) (A ce CV), although these are neighbouring regions. In forest and/or scrub soils with a udic soil moisture regime, in GA w , 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 A ce CV 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 GA w 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 A ce CV, where the main species of OC are easily biodegradable.