Soil carbon loss from managed peatlands along a land use gradient - a comparison of three different methods

Carbon (C) loss from managed organic soils is an important flux in the global carbon cycle. Different approaches exist to estimate greenhouse gas emissions and thus the C balance of these soils. Here we compare two methods using soil profiles and a method of greenhouse gas (GHG) flux measurements using closed chambers to quantify the net C loss from managed peatlands. We applied these methods to the well-studied peatland complex Ahlen-Falkenberger Moor near Cuxhaven in northern Germany. The peatland represents a land use gradient from near-natural wetland (NW) to extensively-used grassland (GE) (rewetted in 2003/2004) to intensively-used grassland (GI). The three methods are: (i) the so-called combined method which makes use of differences in bulk density and ash content between the upper and deeper parts of the profile (ii) the C accumulation method which uses peat accumulation rates derived from 14C age-dated samples and their calculated C-stock in a certain depth and (iii) a method which gives the net ecosystem carbon balance (NECB) using closed chambers to quantify the GHG fluxes. Drainage at the Ahlen-Falkenberger Moor commenced at the beginning of the 20th century, and land use was intensified in the middle of the 20th century. For methods (i) and (ii), three peat cores down to approximately 100 cm at each site were taken in November 2012. These two profile-based methods give the C loss since the onset of drainage activities. Compared to this, the NECB represents the C balance (2007-2009) under present climate and management conditions. According to the profile-based methods (i and ii), all three sites have lost C since the onset of drainage in the order NW<GE<GI. Calculated total C losses are, depending on the method, about 12 kg C m−2 for site NW, 19 to 38 kg C m−2 for site GE and 43 to 53 kg C m−2 for site GI. Based on chamber-derived GHG measurements, site NW currently accumulates C, site GE shows a neutral C balance and site GI is a C source. A comparison of these methods demonstrates that the historical C loss can be assessed by the two profile-based methods, but not by the flux measurements. By contrast, present changes in the C balance are captured by the flux measurements but not by the profile-based methods. Taken together, profile-based methods and flux measurements indicate that the C balance of these peatlands, since the beginning of drainage activities, has been changing over time.