Saltwater intrusion induces shifts in soil microbial diversity and carbon use efficiency in a coastal grassland ecosystem

Salt accumulation and salinisation of coastal soils is a global issue. Further, climate change is likely to increase the amount of land affected by salinity due to the increasing 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 lowering 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 subjected to sea and brackish water flooding, to explore the relationship between plant/microbial growth and CUE along a natural salinity gradient. To reflect steady state conditions, we traced the turnover and partitioning of a low (ambient) dose and high (growth stimulation) dose of 14C-labelled glucose into microbial anabolic and catabolic pools, from which microbial CUE was calculated. This was supported by measurements of the diversity of the bacterial community across the salinity gradient using 16S metabarcoding. Our results showed that coastal flooding significantly reduced plant growth (p < 0.005), increased soil C content (p < 0.05) and induced an increase in microbial CUE under low glucose-C conditions (p < 0.05). Conversely, no significant difference in CUE or microbial growth was apparent when a high glucose-C dose was used. Soil bacterial community alpha (α) diversity increased with soil salinity while beta (β) diversity also shifted in response to the higher saline conditions. Our analysis suggests that the largest impact of coastal flooding on soil C cycling was the inability of the plant community to adapt, leading to higher plant residue inputs as well as the 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.