Home and community composts in Nantes city (France): quality and safety regarding trace metals and metalloids.

Home and community composting are key strategies for local organic waste management. The quality and safety of industrial composts are controlled, but those of home and community composts are not, and this could make them unsafe for use in kitchen gardens. Home (n = 20) and community (n = 41) composts, from urban and suburban areas including mildly Pb-contaminated allotment gardens, were analyzed for quality and safety regarding trace metals and metalloids (TMM) using mid-infrared Fourier transform spectrometry (FT-MIR) and portable X-ray fluorescence spectrometry, respectively. Home composts had a significantly higher Pb content (98 mg.kg-1 ± 10 mg.kg-1) than community composts (21 mg.kg-1 ± 2 mg.kg-1). Numerous home composts (85%) and a few community composts (17%) exceeded the organic farming thresholds for Pb (45 mg.kg-1) and Zn (100 mg.kg-1). The high mineral matter content and the relative abundance of chemical functions attributable to silicates (up to 35%) highly paralleled with TMM contents, mostly concentrated in the fine fraction. Co-inertia analysis highlighted strong and significant links between TMM contents and the whole chemical signature delivered by FT-MIR spectrometry. Pb-contaminated soil could be carried into home compost by green waste or by voluntary addition. Covariance analyses indicated that mineral matter and chemical functions only partly explained the variability in Pb content, suggesting a more complex combination of drivers. Community composting appears as a suitable local solution resulting in high-quality compost that complies with European organic farming regulations, while home composting from allotment gardens should be seriously evaluated to comply with such safety requirements.

Geographical Variability of Mineral Elements and Stability of Restrictive Mineral Elements in Terrestrial Cyanobacteria Across Gradients of Climate, Soil, and Atmospheric Wet Deposition Mineral Concentration.

Terrestrial cyanobacteria Nostoc commune is an ideal species to study the geographical variation of mineral elements of soil cyanobacteria at the species level. Here, we first address the following questions: (1) from where are these mineral elements, (2) are there geographical variations for these mineral elements, and if so, (3) which environmental factors drive the geographical variation of these mineral elements? Second, we tested whether the soil cyanobacterial mineral elements followed the "restrictive element stability hypothesis" of higher plants. Finally, we explored the effect of mineral geographic variation on ecological adaptation of soil cyanobacteria. We collected N. commune samples across gradients of climate, soil, and atmospheric wet deposition mineral concentration in mainland China. We measured fifteen minerals, including five macroelements (N, Ca, K, Fe, P), five microelements (Mn, Zn, Cu, Co, Se), and five heavy metals (Pb, Cr, As, Cd, Hg). We found that five elements (P, Cu, Zn, Co, Pb) had significant geographical variation. They increased as the distance from the equator increased and decreased as the distance from the prime meridian increased. Mean annual precipitation and mean annual temperature explained most of the variation. We did not find any significant correlations between the mineral element contents in N. commune and the minerals in soil and rainfall, except for P. There was no significant correlation between the variation coefficients of different elements and their actual detected contents and their potential physiological required contents. The statistical results of our experiment did not support the "restrictive element stability hypothesis." We speculated that net accumulation of mineral elements in cyanobacterial cells and extracellular polysaccharides (EPS) might play an important role for terrestrial cyanobacteria in the adaptation to dry and cold conditions.

Desert leaf litter decay: Coupling of microbial respiration, water-soluble fractions and photodegradation.

The mechanisms of plant litter decay in drylands are poorly understood, limiting the accuracy of nutrient-cycling models for these systems. We monitored the decay of 12 leaf litter types on the soil surface of the Sonoran Desert for 34 months and assessed what traits predicted mass loss and how exposure to different wavebands of sunlight influenced mass loss. Mass loss varied considerably among litter types, ranging from 42%-96% after 34 months in full sunlight. Traditional indices of litter quality (e.g., initial C:N or lignin:N ratios) failed to predict differences in mass loss among litter types. The strongest predictor of mass loss was the microbial respiration rate of initial litter, which explained 45%-54% of the variation in loss among litter types. Microbial respiration rates were not correlated with traditional indices of litter quality, but were positively correlated with the water-soluble fraction in litter and concentrations of dissolved organic C in this fraction. Traditional indices of litter quality failed to predict decay likely because they did a poor job of predicting microbial degradability of litter, not because microbial degradation was a minor driver of decay. In all radiation-exposure treatments, water-soluble fractions and respiration rates increased through decay and were several times higher after 34 months than initially. Hence, labile pools and microbial degradability of litter increased through decay in contrast to traditional views that labile pools decline and constrain microbes. Litter exposed to UV or UV through blue radiation wavelengths, lost on average 1.3 times or 1.5 times more mass, respectively, than litter not exposed to these wavebands. The magnitude of this photodegradation was greater in litter types that had higher initial concentrations of hemicellulose and cellulose per unit surface area. Litter exposed to full sun had higher water-soluble fractions and usually had higher respiration rates, illustrating that sunlight accelerated microbial degradation by increasing labile pools. The processes driving litter decay appeared to differ appreciably from mesic systems and involved strong couplings between abiotic and biotic drivers.