Ingeborg Callesen: Transfer functions for carbon sequestration, nitrogen retention and nutrient release capability in forest soils based on soil texture classification. Ph.D. thesis. Skov & Landskab, Danish Centre for Forest, Landscape and Planning. Danish Forest and Landscape Research Institute. KVL The Royal Veterinary and Agricultural University

Abstract: Quantification of environmental benefits related to the state and function of forest soils are lacking. Quantitative soil data from Danish and Nordic soil surveys were used to establish empirical relationships between environmental functions and soil properties using soil texture classification as a transfer function. Investigated states and functions were tree growth, accumulation of carbon (C) and nitrogen (N) in forest floors and mineral soil, nitrogen mineralisation and net nitrification, nitrate leaching, and release of phosphorus and base cations by acid hydrolysis. A classification based on subsoil texture into fine, medium, and coarse textured soils separated soils in respect of nutrient status, carbon and nitrogen storage, nitrate availability, and capability for nutrient release by weathering. The volume growth of beech and oak was correlated with availability of soil nutrients. Texture class was seen as a driving variable for pedological development of soils, illustrated by texture-class-related differences in short-term and long-term release of aluminium and base cations in repeated dilute nitric acid extracts. The proportion of base cations (BC) to BC+Aluminium was termed based index. The base index of long-term release indicated that coarse textured soils had a more acid mineralogy than medium and fine textured soils. In a proposed classification of soil nutrient regimes, the long-term base index was used to distinguish podsolised soils, i.e. soils with accumulation of reactive Al-oxides, from not podsolised soils. Subsoil pH was less than 5 in podsolised soils and 5-7 in not podsolised soils. Subsoil pH was therefore suggested as an indicator of soil nutrient regime.

The texture classification was used in a climosequence study of C and N storage in forest soils ranging from dry cold boreal forest to humid cool temperate forest. C and N pools in forest floors and in mineral soil increased with mean annual temperature and mean annual precipitation. Soil texture classes were differently related to carbon and nitrogen storage. High carbon pools were observed in B horizons of podsolised soils in nutrient poor coarse and medium textured soils in the cool humid climate that characterises Denmark. Organic horizons accounted for high carbon pools in nutrient poor soils as compared to low carbon pools in nutrient rich soils. C:N ratios were distinguished by texture class and genetic soil horizons. Low C:N ratios indicated a higher degree of decomposition in fine textured soils. Coarse textured soils had the highest C:N ratios.

Changes in soil organic matter storage or nitrogen retention in response to a changing climate and atmospheric N deposition depend on both biomass production and mineralisation of organic matter. Here, analyses of accumulated C and N pools in soils indicated that these processes behave differently according to the inherent soil nutrient status that may be characterised by soil texture class and subsoil pH. These parameters are therefore suggested as indicators of forest soil qualities like growth, N-storage, -transformation and -retention, and long-term nutrient release.