Decomposition of 14C- and 15N-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 A1 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 14C- and 15N-labelled mature low N wheat straw material. The soils represented a wide range of the major humus types. The basal respiration (soil-native CO2 release) increased significantly in soils sampled from north to south and as the soil pH increased. 14C remaining in soils at the end of the experiment ranged from 52 to 80% of initial 14C. Significant differences were observed between humus types. A regression procedure was used to fit the sum of two exponential functions to the 14C decay curves. The mineralisation rate constant (ka) of the labile compartment (A) varied within a narrow range (0.033 to 0.05 d−1), indicating that this fraction was essentially controlled by the quality of the added labelled plant material. The mineralisation rate constant (kb) of the stabilised compartment (B) varied significantly over a much wider range (0.0005 to 0.0026 d−1), indicating that humus types exert strong controls on the decomposition of the recalcitrant fractions. Within each humus group, kb 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-14C) was related to the humus types and the soil pH. The lowest values were obtained for Oh and acid A1 horizons from the high latitude sites and the highest for basic or neutral Mediterranean A1 horizons. A range of differently textured A1 horizons was used to examine the effect of clay and sand. The metabolically labelled respiration quotient (qCO2-14C) 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. (qCO2-14C) was related to the humus types. The calculated metabolic efficiencies were generally high (0.2 to 0.5) when labile material was used as substrate. Decomposition of more resistant compounds reduced the efficiency to values <0.1, indicating higher energy requirements per unit of synthesised biomass. Eleven decomposition variables were analysed by means of correspondence analysis. The descriptors of soil properties were used as additional variables. The seven humus types were clearly separated. The ordination of the major decomposition variables were highly related to soil pH, suggesting that pH is the best soil related predictor of decomposition parameters. The discussion is focused on the ecological significance of decomposition parameters in relation to the quality of soil humus and soil physico-chemical characteristics.