Soil quality and organic carbon ratios in mountain agroecosystems of South-east Spain

Soil physical and chemical characteristics as well as climatic and geomorphological factors have been determined in 68 sites of a mountain calcimorphic ecosystem (Sierra María-Los Vélez Natural Park, Almería) in Southeastern Spain. Land use and vegetation were natural pine forest, evergreen oak forest, reforested pine forest of different ages, bush, juniper forest, and olive, almond and cereal crops under conventional tillage. By using multivariate data treatments, 17 soil variables were processed. A large part of the total variability was controlled by local topographical features through their effect on moisture retention and vegetation. Most characteristics were significantly correlated with total organic C (mean = 28.5 ± 4.6 g kg− 1), which demonstrates the central role of the organic matter in the functioning of the whole ecosystem. New soil quality descriptors consisting of ratios to soil organic carbon were obtained, informing about the specific activity (per C unit) or performance of the organic matter, independently of its total content. When soil data are directly processed by using principal component analysis, we found a set of high quality soils under natural and old reforested forests, where environmental services provided by soil depend on the high levels of quality descriptors related to organic carbon, e.g. cation exchange capacity (CEC), total porosity, or aggregate stability. When variables such as CEC, porosity and aggregate stability are calculated as ratios to the total organic carbon, a new classification pattern is obtained, allowing to detect soils with organic matter of high maturity which in general do not coincide with soils with high organic matter content. The results suggest the assessment of soil quality based on ratios informing on the organic matter performance should be emphasized as an alternative to direct descriptors based on the total organic carbon content.