Cutting peatland CO2 emissions with water management practices

Abstract. Peat decomposition in managed peatlands is responsible
for a decrease of 0.52 GtC yr−1 in global carbon stock and is strongly
linked to drainage to improve the agricultural bearing capacity, which
increases aeration of the soil. Microbial aerobic decomposition is
responsible for the bulk of the net CO2 emission from the soil and
could be reduced by wetting efforts or minimizing drainage. However, the
effects of rewetting efforts on microbial respiration rate are largely
unknown. In this study, we aimed to obtain more process-based understanding
of these rewetting effects on peat decomposition by integrating high-quality
field measurements and literature relationships with an advanced
hydrological modelling approach where soil moisture and temperature are
centralized as the main drivers for peat decomposition. In 2020 and 2021,
two dairy farming peatlands, where subsoil irrigation and drainage (SSI) was
tested against a control situation, were continuously monitored for CO2
fluxes, groundwater table, soil moisture and soil temperature. After successfully representing field hydrology and carbon dynamic measurements
within our process-based model, we further explored the effects of rewetting
under different weather conditions, water management strategies (raising
ditchwater levels and SSI) and hydrological seepage settings. To represent peat carbon dynamics we introduced a methodology to estimate potential
aerobic microbial respiration rate, based on potential respiration rate
curves for soil temperature and water-filled pore space (WFPS). Measurements
show that rewetting with SSI resulted in higher summer groundwater levels,
soil temperatures and WFPS. SSI reduced the net ecosystem carbon balance (NECB) by 1.58 ± 0.56 kg CO2 m−2 yr−1 (83 ± 25 %) and 0.66 ± 0.62 kg CO2 m−2 yr−1 (28 ± 15 %) for Assendelft and Vlist respectively in 2020. SSI had a negligible
effect in 2021 for both research locations, due to more precipitation, lower
temperatures and different SSI management (in Assendelft) as compared to
2020. Simulated rewetting effects were in agreement with measured rewetting
effects. Model simulations indicate that raising ditchwater levels always
reduces peat respiration rates. Furthermore, we found that the application
of SSI (i) reduces yearly peat respiration rates in a dry year
and/or with downward hydrological fluxes and (ii) increases peat respiration
rates in a wet year and/or when upward groundwater seepage is present.
Moreover, combining SSI with high ditchwater levels or pressurizing SSI
systems will further reduce peat respiration rates. We show that our
process-based approach based on temperature and WFPS soil conditions to
determine NECB represents observed variance to a greater extent than
empirical relationships that involve average groundwater level observations
or simulations. Therefore, we recommend using this kind of approach to estimate the effectiveness of rewetting. When this is not possible, we recommend using mean summer groundwater level instead of mean annual groundwater level as a proxy to estimate NECB. Such relations between mean
groundwater levels and NECB are prone to underestimating NECB for SSI
parcels.
