Changes in the bacterial community across a saline gradient

Salt accumulation and salinisation of coastal soils is a global issue. Further, climate change is likely to increase the amount of land affected through increasing the frequency and severity of coastal flooding and brackish water ingress. The impact of this on the ability of soils to deliver ecosystem services, particularly carbon (C) storage, however, remains unclear. We hypothesized that coastal inundation would negatively affect C storage by reducing plant C inputs and by placing greater osmotic stress on the microbial community leading to a reduced C use efficiency (CUE). Here, we use a coastal grassland ecosystem, which is becoming increasingly subject to sea and brackish water flooding, to explore the relationship between plant/microbial growth and CUE along a natural salinity gradient. Measurements of the diversity of the bacterial community across the salinity gradient using 16S metabarcoding also was carried. Our results showed that coastal flooding reduced plant growth, increased soil C content and induced a small increase in microbial CUE under low glucose-C conditions. Soil bacterial community alpha diversity increased with soil salinity while beta diversity also shifted in response to the higher saline conditions. Our analysis suggests that the biggest impact of coastal flooding on soil C cycling was the inability of the plant community to adapt, leading to higher plant residue inputs and also a decline in soil structure. Conversely, the microbial community had adapted to the increased salinity, resulting in only small changes in the uptake and metabolic partitioning of C.