Stefania Mattana

Ecological risk assessment of organic waste amendments using the species sensitivity distribution from a soil organisms test battery.

Safe amendment rates (the predicted no-effect concentration or PNEC) of seven organic wastes were estimated from the species sensitivity distribution of a battery of soil biota tests and compared with different realistic amendment scenarios (different predicted environmental concentrations or PEC). None of the wastes was expected to exert noxious effects on soil biota if applied according either to the usual maximum amendment rates in Europe or phosphorus demands of crops (below 2 tonnes DM ha(-1)). However, some of the wastes might be problematic if applied according to nitrogen demands of crops (above 2 tonnes DM ha(-1)). Ammonium content and organic matter stability of the studied wastes are the most influential determinants of the maximum amendment rates derived in this study, but not pollutant burden. This finding indicates the need to stabilize wastes prior to their reuse in soils in order to avoid short-term impacts on soil communities.

Changes in soil bacterial community triggered by drought-induced gap succession preceded changes in soil C stocks and quality

The aim of this study was to understand how drought-induced tree mortality and subsequent secondary succession would affect soil bacterial taxonomic composition as well as soil organic matter (SOM) quantity and quality in a mixed Mediterranean forest where the Scots pine (Pinus sylvestris) population, affected by climatic drought-induced die-off, is being replaced by Holm-oaks (HO; Quercus ilex). We apply a high throughput DNA pyrosequencing technique and 13C solid-state Nuclear Magnetic Resonance (CP-MAS 13C NMR) to soils within areas of influence (defined as an surface with 2-m radius around the trunk) of different trees: healthy and affected (defoliated) pines, pines that died a decade ago and healthy HOs. Soil respiration was also measured in the same spots during a spring campaign using a static close-chamber method (soda lime). A decade after death, and before aerial colonization by the more competitive HOs have even taken place, we could not find changes in soil C pools (quantity and/or quality) associated with tree mortality and secondary succession. Unlike C pools, bacterial diversity and community structure were strongly determined by tree mortality. Convergence between the most abundant taxa of soil bacterial communities under dead pines and colonizer trees (HOs) further suggests that physical gap colonization was occurring below-ground before above-ground colonization was taken place. Significantly higher soil respiration rates under dead trees, together with higher bacterial diversity and anomalously high representation of bacteria commonly associated with copiotrophic environments (r-strategic bacteria) further gives indications of how drought-induced tree mortality and secondary succession were influencing the structure of microbial communities and the metabolic activity of soils.

Climate-induced die-off affects plant-soil-microbe ecological relationship and functioning

This study reports the relationship between the diversity and functioning of fungal and bacterial soil communities with vegetation in Mediterranean woodland that experienced severe die-off after a drought episode. Terminal restriction fragment length polymorfism (TRFLP) was used to describe microbial community structure and diversity five years after the episode in different habitats (Juniperus woodland, shrubland, grassland), when the vegetation had not yet recovered. Vegetation diversity was positively related to TRF bacterial richness under unaffected canopies and was higher in diverse grassland. Fungal TRF richness correlated with vegetation type, being greater in Juniperus woodland. Microbial respiration increased in grassland, whereas microbial biomass, estimated from soil substrate-induced respiration (SIR), decreased with bacterial diversity. Die-off increased bacterial richness and changed bacterial composition, particularly in Juniperus woodland, where herbaceous species increased, while fungal diversity was reduced in Juniperus woodland. Die-off increased microbial respiration rates. The impact on vegetation from extreme weather episodes spread to microbial communities by modifying vegetation composition and litter quantity and quality, particularly as a result of the increase in herbaceous species. Our results suggest that climate-induced die-off triggers significant cascade effects on soil microbial communities, which may in turn further influence ecosystem C dynamics.