DECOMPOSITION OF C-14-LABELED AND N-15-LABELED 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 mic robial biomass size and activity are controlled by humus and soil prop erties. The organic horizons (including Oh and Al horizons) of seven c oniferous forest soils ton siliceous and limestone parent material) fr om a north-south climatic sequence in western Europe from Boreal to Me diterranean climate, were incubated in laboratory under controlled tem perature and moisture conditions, for 150 d, with C-14- and N-15-label led mature low N wheat straw material. The soils represented a wide ra nge of the major humus types. The basal respiration (soil-native CO2 r elease) increased significantly in soils sampled from north to south a nd as the soil pH increased. C-14 remaining in soils at the end of the experiment ranged from 52 to 80% of initial C-14. Significant differe nces were observed between humus types. A regression procedure was use d to fit the sum of two exponential functions to the C-14 decay curves . The mineralisation rate constant (k(a)) 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 l abelled plant material. The mineralisation rate constant (k(b)) of the stabilised compartment (B) varied significantly over a much wider ran ge (0.0005 to 0.0026 d(-1)), indicating that humus types exert strong controls on the decomposition of the recalcitrant fractions. Within ea ch humus group, k(b) increased in soils sampled from north to south an d was essentially controlled by the pH gradient through the climatic s equence. 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-C-14) was related to the humus typ es 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 ne utral Mediterranean Al horizons. A range of differently textured Al ho rizons was used to examine the effect of clay and sand. The metabolica lly labelled respiration quotient (qCO(2)-C-14) indicated (1) an initi al active phase characterised by the use of labile organic compounds; (2) a lower activity phase indicating the exhaustion of available reso urces and the use of recalcitrant material. (qCO(2)-C-14) was related to the humus types. The calculated metabolic efficiencies were general ly high (0.2 to 0.5) when labile material was used as substrate. Decom position of more resistant compounds reduced the efficiency to values <0.1, indicating higher energy requirements per unit of synthesised bi omass. Eleven decomposition variables were analysed by means of corres pondence analysis. The descriptors of soil properties were used as add itional variables. The seven humus types were clearly separated. The o rdination of the major decomposition variables were highly related to soil pH, suggesting that pH is the best soil related predictor of deco mposition parameters. The discussion is focused on the ecological sign ificance of decomposition parameters in relation to the quality of soi l humus and soil physico-chemical characteristics. (C) 1998 Elsevier S cience Ltd. All rights reserved.