Seasonal Soil Respiration and Methane Emissions in a SE Freshwater Tidal Riparian System

Monthly measurements of gross soil respiration (Rs), soil microbial respiration (Rsm) and CH4 emissions were conducted along a tidally influenced freshwater bottomland riparian transect at the Santee Experimental Forest, South Carolina from May 2015 through August 2016. Three sites were monitored: Full Tidal (FT); Intermediate (I) and Terrestrial (T) with multiple sites representing the wetland microtopography (hummocks and hollows) being measured at each site. Replicate soil respiration measurements were made on trenched and non-trenched collars using a portable PP Systems EGM-4 SRC-1 Soil Respiration Chamber. Methane samples were collected via syringe from static chambers and subsequently analyzed on a SRI8610C gas chromatograph. Continuous measurements of soil temperature, soil moisture and soil redox conditions were also conducted. Rs and Rsm display significant seasonal trends positively related to soil temperature at all sites except for the FT Hollow sites where little or no seasonal variation was shown and soil respiration was almost always < 2 mmol/m2/sec. Soil CH4 emission rates in FT Hollow sites increased dramatically beginning in May and were significantly higher than all other sites measured with average CH4 emissions as high as 703 mg/m2/day for July, 2016 as compared to 2 mg/m2/day, -2 mg/m2/day and 2 mg/m2/day for FT Hummock, I Hummock and I Hollow sites, respectively. Our results indicate that there is a complex interaction between riparian microtopgraphy, temperature, tidal hydrology and carbon cycling in freshwater tidal bottomland forests that will be significantly altered by predicted sea level rise.