Metabolic capacities of microorganisms from a long-term bare fallow

Soil degradation due to erosion or to soil organic matter (SOM) depletion is known to reduce the ecosystems services performed by soils. Amendments of organic matter are often proposed to restore the soil quality as they may increase water holding capacity, microbial activity and decrease erosion. In this work, we tested the ability of microbial communities from a soil very depleted in organic matter to mineralize fresh organic matter. We used a soil deprived of organic matter inputs since 1929 as a model of degraded soil. We amended it with 13C-cellulose or 13C-straw at two different levels (0.5 g C kg−1 of soil vs. 5 g C kg−1). The same treatments were performed on an arable soil as control. The soils were incubated at 20 °C during 209 days and the amount of CO2 released and δ13C–CO2 were monitored during incubation. We measured the microbial biomass by fumigation extraction and determined the metabolic profiles of microorganisms by using Biolog® ECOPlates. The straw and cellulose were mineralized without lag-time in both soils. It suggested that the bare fallow soil had kept its metabolic capacities to degrade straw and cellulose, but at a lower rate when compared to the arable control soil. The Shannon index (H) calculated from Biolog® ECOPlates was very high in the two types of soils, which suggest that the two microbial communities were ubiquist. Moreover, the bare fallow microbial community was efficient to mineralize the amino acids and the mannitol in the Biolog® ECOPlates. Consequently, we assumed that the bare fallow soil microbial community had kept its metabolic capacities because of the microbial turnover. The dead microbial biomass might be an important substrate for the microbial community explaining why it may easily mineralize labile organic matter such as straw or cellulose.

► Soil microbial communities rely mainly on fresh organic matter (OM) inputs.
► A long term bare fallow soil depleted in OM because of no inputs for 80 year is compared to a cropped soil.
► It can mineralize plant material, abeilt slowly.
► It has kept metabolic capacities (biolog) quite similar to the cropped soil.