Soil amendment alters soil physicochemical properties and bacterial community structure of a replanted apple orchard

Compost amendment reportedly improved apple tree growth in replant soils. However, its effects should be evaluated at different soil depths and locations. This study investigated the impact of soil improvement with compost on soil physicochemical properties and bacterial community structure of a replanted apple orchard in comparison with the original orchard without compost improvement. The V1-V3 region of the bacterial 16S rRNA gene was subjected to high-throughput 454 pyrosequencing, and data were analyzed using the Mothur pipeline. The results showed that the soil improvement benefited tree growth and fruit quality during the study period. The compost amendment markedly increased tree height and stem diameter by a range of 6.1%-21.0% and 4.0%-14.0%, respectively. Fruit yield (9.5%), average weight (9.6%), and soluble solid content (5.6%) were also increased by compost amendment compared to those of the unimproved treatment. The pH, organic matter, and available N, P, and K contents were significantly increased by 5.7%-21.9%, 0.2%-62.9%, 9.3%-29.3%, 36.7%-64.5%, and 17.2%-100.3% in the compost improved soil. The pyrosequencing data showed that the soil improvement changed the bacterial community structure at all soil depths (0-20 cm and 20-40 cm) and locations (in-row and inter-row) considered; e.g., the relative abundance of Proteobacteria (20.2%), Bacteroidetes (2.5%), and Cyanobacteria (1.0%) was increased while that of Chloroflexi (5.5%), Acidobacteria (5.2%), Nitrospirae (4.5%), Gemmatimonadetes (3.8%), and Actinobacteria (1.8%) was decreased. The relative abundance of some dominant genera Burkholderia (2.3%), Pseudomonas (1.0%), and Paenibacillus (0.5%) were enhanced in the compost improved soil. Moreover, other dominant genera such as Nitrospira (6.4%), Gemmatimonas (2.2%), and Phenylobacterium (0.3%) were reduced by the application of compost. Our results indicate that soil improvement benefits the growth of tree and fruit quality, and is likely mediated by increased soil pH, organic matter, and available nutrient contents and beneficial bacterial community composition.