Cutting peatland CO 2 emissions with water management practices

[摘要] Peat decomposition in managed peatlands is responsiblefor a decrease of 0.52 GtC yr −1 in global carbon stock and is stronglylinked to drainage to improve the agricultural bearing capacity, whichincreases aeration of the soil. Microbial aerobic decomposition isresponsible for the bulk of the net CO 2 emission from the soil andcould be reduced by wetting efforts or minimizing drainage. However, theeffects of rewetting efforts on microbial respiration rate are largelyunknown. In this study, we aimed to obtain more process-based understandingof these rewetting effects on peat decomposition by integrating high-qualityfield measurements and literature relationships with an advancedhydrological modelling approach where soil moisture and temperature arecentralized as the main drivers for peat decomposition. In 2020 and 2021,two dairy farming peatlands, where subsoil irrigation and drainage (SSI) wastested against a control situation, were continuously monitored for CO 2 fluxes, groundwater table, soil moisture and soil temperature. After successfully representing field hydrology and carbon dynamic measurementswithin our process-based model, we further explored the effects of rewettingunder different weather conditions, water management strategies (raisingditchwater levels and SSI) and hydrological seepage settings. To represent peat carbon dynamics we introduced a methodology to estimate potentialaerobic microbial respiration rate, based on potential respiration ratecurves for soil temperature and water-filled pore space (WFPS). Measurementsshow 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 CO 2  m −2  yr −1 (83  ±  25 %) and 0.66  ±  0.62 kg CO 2  m −2  yr −1 (28  ±  15 %) for Assendelft and Vlist respectively in 2020. SSI had a negligibleeffect in 2021 for both research locations, due to more precipitation, lowertemperatures and different SSI management (in Assendelft) as compared to2020. Simulated rewetting effects were in agreement with measured rewettingeffects. Model simulations indicate that raising ditchwater levels alwaysreduces peat respiration rates. Furthermore, we found that the applicationof SSI (i) reduces yearly peat respiration rates in a dry yearand/or with downward hydrological fluxes and (ii) increases peat respirationrates in a wet year and/or when upward groundwater seepage is present.Moreover, combining SSI with high ditchwater levels or pressurizing SSIsystems will further reduce peat respiration rates. We show that ourprocess-based approach based on temperature and WFPS soil conditions todetermine NECB represents observed variance to a greater extent thanempirical relationships that involve average groundwater level observationsor 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 meangroundwater levels and NECB are prone to underestimating NECB for SSIparcels.

[学科分类] 大气科学