Additions of sugar and nitrogenous fertiliser affect plant nitrogen status and soil microbial communities

Soil microorganisms are essential to the functioning of soil ecosystems, and this study aimed to examine the effects of additions of carbon and nitrogen (N) to the soil on soil microflora, and whether these amendments could be used to both increase and reduce plant N concentrations in an orchard crop, thereby facilitating studies on the multitrophic interactions among the components of the orchard ecosystem. Sucrose or nitrogenous fertiliser was applied to the soil under sweet orange (Citrus x aurantium L.) trees; and respectively, reduced or increased plant N and chlorophyll concentrations. Next generation sequencing was used to examine bacterial and fungal populations within the test soils. Bacteria from 39 phyla or candidate phyla were found, together with fungi from 18 classes of seven different phyla. The relative abundances of these taxa were similar to those found in other studies with the exception of the Dormibacteraeota (AD3); this candidate phylum contained the fourth most abundant number of reads. For the bacterial populations, measures of alpha diversity in the soil given the fertiliser treatment were significantly lower than in the control; there was also a similar trend in the sugar treatment. The fungal data showed less difference among the treatments with no differences in the indices of alpha diversity. Measures of similarity/dissimilarity showed significant differences between treatments suggesting that both had reduced microbial biodiversity. The soil additions proved to be effective in manipulating tree N status thereby facilitating studies on other components of the orchard ecosystem. The consequences of the reductions in biodiversity due to the additions, and of the relative abundance of Dormibacteraeota on ecosystem functioning need to be determined.