Apolline Auclerc

Litter chemistry prevails over litter consumers in mediating effects of past steel industry activities on leaf litter decomposition

Soil pollution has adverse effects on the performance and life history traits of microorganisms, plants, and animals, yet evidence indicates that even the most polluted sites can support structurally-complex and dynamic ecosystems. The present study aims at determining whether and how litter decomposition, one of the most important soil ecological processes leaf, is affected in a highly trace-metal polluted site. We postulated that past steel mill activities resulting in soil pollution and associated changes in soil characteristics would influence the rate of litter decomposition through two non-exclusive pathways: altered litter chemistry and responses of decomposers to lethal and sub-lethal toxic stress. We carried out a litter-bag experiment using Populus tremula L. leaf litter collected at, and allowed to decompose in, a trace metal polluted site and in three unpolluted sites used as controls. We designed a fully-factorial transplant experimental design to assess effects of litter origin and exposure site on the rate of litter decomposition. We further determined initial litter chemistry, fungal biomass, mesofauna abundance in litter bags, and the soil macrofauna community. Irrespective of the site of litter exposure, litter originating from the polluted site had a two-fold faster decomposition than litter from the unpolluted sites. Litter chemistry, notably the lignin content, seemed most important in explaining the degradation rate of the leaf litter. Abundance of meso and macro-detritivores was higher at the polluted site than at the unpolluted sites. However, litter decomposition proceeded at similar rates in polluted and unpolluted sites. Our results show that trace metal pollution and associated soil and litter changes do not necessarily weaken consumer control on litter decomposition through lethal and sub-lethal toxic stress.

Assessment of derelict soil quality: Abiotic, biotic and functional approaches

The intensification and subsequent closing down of industrial activities during the last century has left behind large surfaces of derelict lands. Derelict soils have low fertility, can be contaminated, and many of them remain unused. However, with the increasing demand of soil surfaces, they might be considered as a resource, for example for non-food biomass production. The study of their physico-chemical properties and of their biodiversity and biological activity may provide indications for their potential re-use. The objective of our study was to investigate the quality of six derelict soils, considering abiotic, biotic, and functional parameters. We studied (i) the soil bacteria, fungi, meso- and macro-fauna and plant communities of six different derelict soils (two from coking plants, one from a settling pond, two constructed ones made from different substrates and remediated soil, and an inert waste storage one), and (ii) their decomposition function based on the decomposer trophic network, enzyme activities, mineralization activity, and organic pollutant degradation. Biodiversity levels in these soils were high, but all biotic parameters, except the mycorrhizal colonization level, discriminated them. Multivariate analysis showed that biotic parameters co-varied more with fertility proxies than with soil contamination parameters. Similarly, functional parameters significantly co-varied with abiotic parameters. Among functional parameters, macro-decomposer proportion, enzyme activity, average mineralization capacity, and microbial polycyclic aromatic hydrocarbon degraders were useful to discriminate the soils. We assessed their quality by combining abiotic, biotic, and functional parameters: the compost-amended constructed soil displayed the highest quality, while the settling pond soil and the contaminated constructed soil displayed the lowest. Although differences among the soils were highlighted, this study shows that derelict soils may provide a biodiversity ecosystem service and are functional for decomposition.