In the current context, there is a growing interest in reducing the use of chemical fertilizers and pesticides to promote ecological agriculture. The use of biochar and plant growth-promoting rhizobacteria (PGPR) is an environmentally friendly alternative that can improve soil conditions and increase ecosystem productivity. However, the effects of biochar and PGPR amendments on forest plantations are not well known. The aim of this study is to investigate the effects of biochar and PGPR applications on soil nutrients and bacterial community. To achieve this goal, we applied amendments of (i) biochar at 20 t hm⁻², (ii) PGPR at 5 × 10¹⁰ CFU mL⁻¹, and (iii) biochar at 20 t hm⁻² + PGPR at 5 × 10¹⁰ CFU mL⁻¹ in a eucalyptus seedling plantation in Guangxi, China. Three months after applying the amendments, we collected six soil samples from each treatment and from control plots. From each soil sample, we analyzed several physicochemical properties (pH, electrical conductivity, total N, inorganic N, NO₃⁻-N, NH₄⁺-N, total P, total K, and soil water content), and we determined the bacterial community composition by sequencing the ribosomal 16S rRNA. Results indicated that co-application of biochar and PGPR amendments significantly decreased concentrations of soil total P and NH₄⁺-N, whereas they increased NO₃-N, total K, and soil water content. Biochar and PGPR treatments increased the richness and diversity of soil bacteria and the relative abundance of specific bacterial taxa such as Actinobacteria, Gemmatimonadetes, and Cyanobacteria. In general, the microbial composition was similar in the two treatments with PGPR. We also found that soil physicochemical properties had no significant influence on the soil composition of bacterial phyla, but soil NH₄⁺-N was significantly related to the soil community composition of dominant bacterial genus. Thus, our findings suggest that biochar and PGPR amendments could be useful to maintain soil sustainability in eucalyptus plantations.

(1) Background: Biochar and plant growth-promoting rhizobacteria (PGPR) are widely used as amendments to increase the availability of nutrients and the diversity of the bacterial community within soil. (2) Methods: In this study, we investigated the effects of biochar and PGPR amendments on plant performance, soil physicochemical property, and soil microbial diversity, as well as their relationship in a Eucalyptus (clone DH32−29) plantation in Guangxi, China. We determined the microbial AWCD, Simpson, Shannon, and McIntosh indices, and soil inorganic nitrogen (NH₄⁺, NO₃⁻), total phosphorus (TP), total potassium (TK), total nitrogen (TN), and plant growth and nutrient concentrations; (3) Results: Biochar-only had a significant impact on soil microbial community function, although the effects on plant performance were limited. PGPR plus biochar was found to significantly increase the diversity indices of soil microbes, as well as soil TK and TP. Besides, soil microbes displayed a preference for carbohydrates rather than other carbon sources. (4) Conclusion: Soil microbial functional diversity responded to changes in plant performance and, therefore, it could indicate soil ecological stability and ecosystem productivity. These findings may suggest that biochar and PGPR could potentially maintain ecological sustainability in the soil and improve plant performance through altering soil physicochemical properties in a eucalyptus plantation.

Forest leaf litter is an important source of soil nutrients, but how its decomposition products affect the availability of soil heavy metals is not totally clear. In order to understand the effects of leaf litter decomposition on soil heavy metal availability in a forest ecosystem, leaf litter samples were collected from Daluoshan in Wenzhou and the Wuyanling National Nature Reserve of Zhejiang and subjected to analysis. The leaf litter was produced by the forests’ dominant tree species, such as Myrica ruba, Bambusa cerosissima, Pinus sylvestris, Machilus thunbergia, Cunninghamia lanceolata, and Quercus acutissima. Decomposition of the leaf litter samples at room temperature and leaching decomposition of the samples were carried out to analyze the acid production characteristics of the litter samples and their impact on soil heavy metal availabilities. Oxalic acid, lactic acid, fumaric acid, succinic acid, malic acid, and citric acid were the dominant organic acids in the leaf litter decomposer, and the sum of these six organic acids accounted for more than 50% of the total organic acid. During a 30-day litter decomposition, the levels of organic acids basically increased, with significant increases appearing in the early stage. After leaching, the available contents of Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb increased by a maximum of 11.95, 2.33, 12.00, 0.80, 0.83, 0.54, 51.50, and 0.28-fold, respectively, compared with those of the original soil sample. During leaching, the higher the content of low molecular weight organic acids in the leaf litter, the longer the leaching time required and the more obvious the influence on the level of available heavy metals. A stronger leaching effect was found for the soil samples taken from a depth of 0–20 and 20–40 cm accompanied by a more obvious increase in the available heavy metals in the soil. PLSPM results showed that there were significant differences between the soil acidity index and the available heavy metals in Daluoshan (p < 0.05) and that there were significant differences between the soil acidity index and the available heavy metals in Wuyanling (p < 0.01). The decomposition of litter produced low molecular weight organic acids, which caused a decrease in soil pH and an increase in exchangeable H⁺ and Al³⁺. Both changes had an impact on soil organic matter and further led to an increase in the activity of heavy metals in the soil. This will further lead to the increase of ecological and environmental risks to forest soil.