Soil microbial biomass along a hydrologic gradient in a subsiding coastal bottomland forest: Implications for future subsidence and sea-level rise

Soil microbial biomass was analyzed in a rapidly subsiding coastal bottomland forest at three sites along an elevation change of ∼1 m and an associated hydrologic gradient of 400 m from rare (ridge site), to occasional (intermediate site), to frequent flooding (swamp site). Given the current rate of relative sea-level rise in this area (subsidence+global mean sea-level rise ∼1.2 cm y⁻¹), this gradient may represent a space-for-time substitution for about one century of future sea-level rise. Along the hydrologic gradient, microbial biomass carbon (MBC) in the upper 20 cm of mineral soil was 157±26 (ridge), 134±14 (intermediate), and 90±20 (swamp) g C m⁻². MBC was positively correlated with soil organic matter (r²=0.76, P=0.002) and the ratio of MBC to soil organic C ranged from 0.008 to 0.017 depending on soil depth and site. Generally, MBC decreased with increasing soil moisture from the ridge to the swamp site. Although MBC was statistically similar overall in the ridge and intermediate sites, the intermediate site had the largest fraction (45%) at 0-5 cm, whereas the ridge site had the largest fraction (40%) below 10-20 cm. Based on a space-for-time substitution model using non-linear regression analysis, we predict that MBC in the upper 20 cm of soil is likely to decrease by about one-third along the transect over the next century as a result of subsidence and sea-level rise.