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.

沿海土壤的盐分积累和盐渍化是一个全球性问题。此外，由于沿海洪水和微咸水进入的频率和严重程度增加，气候变化可能会增加受盐度影响的土地数量。然而，这对土壤提供生态系统服务，特别是碳（C）储存能力的影响仍不清楚。我们假设沿海洪水会通过降低植物碳输入和对微生物群落施加更大的渗透压力导致碳利用效率 (CUE) 降低，从而对碳储存产生负面影响。在这里，我们使用越来越多地遭受海水和微咸水泛滥的沿海草原生态系统，探索植物/微生物生长与自然盐度梯度 CUE 之间的关系。为了反映稳态条件，¹⁴ C标记的葡萄糖进入微生物合成代谢和分解代谢池，从中计算微生物CUE。这得到了使用 16S 元条形码测量盐度梯度中细菌群落多样性的支持。我们的研究结果表明， 在低葡萄糖-C 条件下，沿海洪水显着降低了植物生长 ( p  < 0.005)、增加了土壤 C 含量 ( p < 0.05) 并诱导了微生物 CUE 的增加 ( p  < 0.05)。相反，当使用高葡萄糖-C 剂量时，CUE 或微生物生长没有明显差异。土壤细菌群落α（α）多样性随土壤盐分增加而β（β ）多样性也随着更高的盐分条件而发生变化。我们的分析表明，沿海洪水对土壤碳循环的最大影响是植物群落无法适应，导致植物残留物投入增加以及土壤结构下降。相反，微生物群落已经适应了盐度的增加，导致 C 的摄取和代谢分配只有很小的变化。