Decomposition of super(14)C- and super(15)N-labelled plant material, under controlled conditions, in coniferous forest soils from a north-south climatic sequence in western Europe

The aim of this work was to clarify how decomposition kinetics and microbial biomass size and activity are controlled by humus and soil properties. The organic horizons (including Oh and Al horizons) of seven coniferous forest soils (on siliceous and limestone parent material) from a north-south climatic sequence in western Europe from Boreal to Mediterranean climate, were incubated in laboratory under controlled temperature and moisture conditions, for 150 d, with super(14)C- and super(15)N-labelled mature low N wheat straw material. The soils represented a wide range of the major humus types. The basal respiration (soil-native CO sub(2) release) increased significantly in soils sampled from north to south and as the soil pH increased. super(14)C remaining in soils at the end of the experiment ranged from 52 to 80% of initial super(14)C. Significant differences were observed between humus types. A regression procedure was used to fit the sum of two exponential functions to the super(14)C decay curves. The mineralisation rate constant (k sub(a)) of the labile compartment (A) varied within a narrow range (0.033 to 0.05 d super(-1)), indicating that this fraction was essentially controlled by the quality of the added labelled plant material. The mineralisation rate constant (k sub(b)) of the stabilised compartment (B) varied significantly over a much wider range (0.0005 to 0.0026 d super(-1)), indicating that humus types exert strong controls on the decomposition of the recalcitrant fractions. Within each humus group, k sub(b) increased in soils sampled from north to south and was essentially controlled by the pH gradient through the climatic sequence. After 150 d, the labelled N mineralisation rates ranged from 10% in the Boreal soils to 30% in the Mediterranean soils. The size of the labelled microbial biomass (MB- super(14)C) was related to the humus types and the soil pH. The lowest values were obtained for Oh and acid Al horizons from the high latitude sites and the highest for basic or neutral Mediterranean Al horizons. A range of differently textured Al horizons was used to examine the effect of clay and sand. The metabolically labelled respiration quotient (qCO sub(2)- super(14)C) indicated (1) an initial active phase characterised by the use of labile organic compounds; (2) a lower activity phase indicating the exhaustion of available resources and the use of recalcitrant material. (qCO sub(2)- super(14)C) was related to the humus types. The calcula