Do peat accumulation and loss rates within soil profiles vary across depths in floating marshes?

Background/Question/Methods

Buoyancy in Louisiana floating marshes results from underlying peat mats, which trap gases and reduce bulk density.  These marshes occur in warm climes and rise and fall with the water table, possibly resulting in rapid rates of decomposition relative to other peatlands.  If decomposition rates are high, marshes must be either extremely productive to maintain the peat that enables buoyancy or become submerged.  Although these marshes are buoyant and the vegetation and uppermost layers of soil are rarely flooded, most of the soil mat actually occurs below the water table throughout most of the year.  A relationship likely exists between depth within the peat mat and decomposition rate.  Although reduced buoyancy and continued submergence of floating marshes could lead to losses of coastal wetlands, this relationship has not been studied.  To determine effects of depth on net peat accumulation rate, 40 peat cores were collected, frozen and sectioned in 2008.  The sections of these cores were dried, weighed, wrapped in mesh, reformed into cores based on corresponding depths, and reburied based on initial locations.  After 12 months, cores were reexcavated and sections were dried and reweighed.  Non-linear regression was performed to determine the relationship between depth and net peat accumulation.

Results/Conclusions

Non-linear regression indicated a significant relationship between depth and rate of peat loss (R²=0.32, p<0.0001).  T-tests were performed to determine if changes in peat mass within depth profiles were significantly different from zero; significant peat loss occurs in the 0-3 cm (t = 9.35, p<0.0001) and 3-6 cm (t=5.65, p<0.0001) depth classes, but loss rates were not significant within other depths.  Non-significant peat accumulation occurred at depths 9-12 cm (mean loss rate = -1.41 x 10^-4 g yr^-1) and 12-15 cm (mean loss rate = -1.29 x 10^-3 g yr^-1).  These results suggest that peat loss likely occurs in the root zone, which is at least sometimes aerobic, but some accumulation may occur in the frequently flooded mat zone.  Given the high productivity of the system, it is also possible that periodic weather patterns inhibit decomposition, leading to rapid peat accumulation during short time intervals, but resulting peat is slowly lost between these short bursts of accumulation.  Thus, the system may be self-regulating; if peat loss decreases buoyancy, oxygen availability may also decrease, creating anaerobic conditions that favor peat accumulation.