Carbon stocks in sight: high-resolution vertical depth profiles to quantify carbon reservoirs

Peatlands are significant terrestrial carbon reservoirs which play a key role in the global carbon cycle, both as major sinks and significant emitters of carbon dioxide. When studying carbon accumulation and carbon emissions from peatlands it is essential to scale the observed fluxes (net emissions) to the available soil carbon (C) stocks. For many peatland areas in the Netherlands, the average thickness of the peat deposit is known, however, important parameters on peat carbon density and peat substrate quality are poorly documented.

In this study we present high resolution carbon profile data for a wide range of Dutch research locations to quantify their total carbon stock. The locations are part of the NOBV emission monitoring network. We determined the botanical composition, measured the bulk density, organic matter content and composition and the degree of degradation using extraction techniques and stable isotopes of N and C. Based on these data we gained insight in the total carbon pool sizes, the variance in chemical composition of the peat layers and the peat degradation stage along the depth profiles. We combined these C stock data with the site-specific groundwater dynamics and divided the carbon stocks into different risk classes for aerobic decomposition, depending on the number of days that they were above the actual groundwater level.

Average carbon stocks were 87 kg/ m2 based on a usual soil profile depth of 120cm. Strikingly, carbon stocks in a peaty soil were similar to a relative undisturbed peat due to the higher density of the organic matter in degraded soils. C:N ratios are strongly driven by botanical origin of the peat. Degradation proxies largely followed the hydrological gradient with a clear decrease in δ15N with depth and shifts in ratios between acid soluble to acid insoluble organic fractions indicating a specific preservation of lignin type of substrates in anoxic peat layers. This study highlights the variability in peat carbon stocks in the Dutch coastal peatlands and underlines the need to extend the emission control measures to include the peaty soils as they still contain significant amounts of carbon.