Modelling the effect of active roots on soil organic matter turnover

The aim of this experiment was to study the effect of living roots on soil carbon metabolism at different decomposition stages during a long-term incubation. Plant material labelled with 14C and 15N was incubated in two contrasting soils under controlled laboratory conditions, over two years. Half the samples were cropped with wheat (#Triticum aestivum$) 11 times in succession. At earing time the wheat was harvested, the roots were extracted from the soil and a new crop was started. Thus the soil were continuously occupied by active root systems. The other half of the samples was maintained bare, without plants under the same conditions. Over the 2 years, pairs of cropped and bare soils were analysed at eight sampling occasions (total, plant debris-, and microbial biomass-C and -14C). A five compartment (labile and recalcitrant plant residues, labile microbial metabolites, microbial biomass and stabilised humified compounds) decomposition model was fitted to the labelled and soil native organic matter data of the bare and cropped soils. Two different phases in the decomposition processes showed a different plant effect. (1) During the initial fast decomposition stage, labile 14C-material stimulated microbial activities and N immobilisation, increasing the 14C-microbial biomass. In the presence of living roots, competition between microorganisms and plants for inorganic N weakly lowered the measured and predicted total-14C mineralisation and resulted in a lower plant productivity compared to subsequent growths. (2) In contrast, beyond 3-6 months, when the labile material was exhausted, during the slow decomposition stage, the presence of living roots stimulated the mineralisation of the recalcitrant plant residue-14C in the standy soil and of the humified-14C in the clay soil... (D'après résumé d'auteur