Compost and microbial biostimulant applications improve plant growth and soil biological fertility of a grass-based phytostabilization system.

In this work, a grass-based phytoremediation system integrated with an organic amendment and biostimulants was evaluated for remediating contaminated sites. Plant growth and biological fertility were monitored to assess the efficacy of a vegetative cap used as a safety measure to reduce sanitary and environmental risks of industrially contaminated soils and soil-washing sludges. Both matrices were potentially contaminated with Pb and Zn with an ecological risk index from low to moderate. According to potentially toxic elements (PTEs) bioaccessibility tests, the exposure to the released fine particulate matter may cause serious risks to human beings, in particular to children. The grass mixture was well adapted to both the substrates and a low PTEs mobility was detected, thus, reducing the leaching risk to ground water sources. Compost addition augmented significantly nitrogenase reductase (nifH) and ammonia monooxygenase (amoA) gene expression abundance in both substrates. Furthermore, a positive interaction between compost fertilization and a Trichoderma-based biostimulant inoculation was recorded in sludges resulting in a significant stimulation of nitrogen-fixing and ammonia-oxidizing bacteria. The application of compost and biostimulant increased soil fertility and plant growth. Furthermore, there was a slight reduction in PTE bioaccessibility, thus, improving the efficiency of the phytostabilization, limiting the resuspension and dispersion of the health-risk soil particulate.

Decomposition and organic amendments chemistry explain contrasting effects on plant growth promotion and suppression of Rhizoctonia solani damping off.

Organic Amendments (OAs) has been used in agroecosystems to promote plant growth and control diseases caused by soilborne pathogens. However, the role of OAs chemistry and decomposition time on plant growth promotion and disease suppression is still poorly explored. In this work, we studied the effect of 14 OAs at four decomposition ages (3, 30, 100, and 300 days) on the plant-pathogen system Lactuca sativa-Rhizoctonia solani. OAs chemistry was characterized via 13C-CPMAS NMR spectroscopy as well as for standard chemical (i.e. N content, pH, EC) and biological parameters (i.e. phytotoxicity and R. solani proliferation bioassay). OAs have shown variable effects, ranging from inhibition to stimulation of Lactuca sativa and Lepidium sativum growth. We recorded that N rich OAs with high decomposability were conducive in the short-term, while converting suppressive in the long term (300 days). On the other hand, cellulose-rich OAs with high C/N ratio impaired L. sativa growth but were more consistent in providing protection from damping-off, although this property has significantly shifted during decomposition time. These results, for the first time, highlight a consistent trade-off between plant growth promotion and disease control capability of OAs. Finally, we found that OAs effects on growth promotion and disease protection can be hardly predictable based on the chemical characteristic, although N content and some 13C CPMAS NMR regions (alkyl C, methoxyl C, and carbonyl C) showed some significant correlations. Therefore, further investigations are needed to identify the mechanism(s) behind the observed suppressive and conducive effects and to identify OAs types and application timing that optimize plant productivity and disease suppression in different agro-ecosystems.