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

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 (GA_w) and those of the rest of the study area (Central and Eastern Asturias, Cantabria and País Vasco) (A_ceCV), although these are neighbouring regions. In forest and/or scrub soils with a <em>udic</em> soil moisture regime, in GA_w, the SOC is usually &gt; 7% and the average stocks 260 t ha^-1 (0-30 cm), and &gt;340 t ha^-1 (0-50 cm) in soils with thick organic matter rich horizons (&gt; 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_ceCV the SOC average is 3% and the mean stocks 90-100 t ha^-1 (0-30 cm). The <em>andic</em> 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_ceCV, where the main species of OC are easily biodegradable.