Moisture dynamics and carbon sequestration in boreal soils

The soils of boreal forests store large amounts of carbon (C), more than three times the amount in living plants and more than double the amount of C in the atmosphere. Therefore, carbon fixation in soils is an important ecosystem service which affects global climate. In wet soils, there is more C than in dry soils. However, exactly how much more, and why, is not very well understood. Most models existing today underestimate the stocks of C in wetlands and other poorly drained areas, which are very common in Norway. This also implies that the consequences of climate change are not represented properly in the models. The project Moisture dynamics and carbon sequestration in boreal soils (Mocabors) has investigated the effect of soil moisture on carbon dynamics in boreal forest soils. We have set up automatic measurement systems for soil temperature, moisture and respiration (CO2) at locations in forests close to Hurdal, to Løten and to Tingvoll municipalities. The temporal resolution is 10 min for each of these parameters. This ensures that every precipitation event is covered by observations; often, rainfall has profound albeit short-term consequences for the carbon flux out of the soil (heavy increases). This is what we observed multiple times. The new data and knowledge are used to improve the models. Time series obtained from our sites provide insights into the dynamics and interaction between temperature and moisture and the resulting soil respiration. A modeling approach was followed that assumes different processes for Soil Organic Matter (SOM) decomposition depending on soil moisture and oxygen availability, the Dual-Arrhenius Michaellis-Menten (DAMM) model. The time series obtained from our field sites where used to calibrate DAMM at the site level (Pallandt et al. 2019a,b and 2021 (submitted to JGR-Biosciences)). As another set of sites from Spain were also available, this could be used to model the decomposition rate at a larger spatial scale and in different climate zone, using climate data and soil properties taken from a database. Roughly, including soil moisture led to faster decomposition at wet sites and to slower decomposition at dry sites. The effect seems to be largest in semi-arid and in boreal areas, respectively, and predicted respiration rates deviate by as much as 20% compared to a ?temperature only? approach which is the standard so far. The inclusion of moisture control has also implications for the impacts of hydrometeorological extremes (combinations of drought and heat) on the carbon cycle (Sippel et al. 2018). The detectability of extreme events within measuring networks (like the Integrated Carbon Observation System) is also improved when soil moisture can be added as an observable. The difference in carbon dynamics was pronounced when site-level simulations were carried out with the Earth System Model CLM4.5 (Lee et al. 2020). Summarizing, Mocabors concludes that soil moisture is an important driver for carbon storage and decomposition, second most relevant after temperature, and even more important when the soil is either very wet or very dry.

Jordfuktighet er en veldig viktig variabel som styrer karbonlagring og -nedbrytning i jorda, rett etter temperaturen generelt og enda viktigere hvis det er enten veldig våt eller veldig tørr. Jordsystem- og klimamodeller bør ta hensikt til jordfuktighet som dynamisk variabel for å simulere karbonomsetningen i jorda. Så langt er karbondynamikk i boreale (skogs-)områder trolig underestimert hvis det er kun temperatur som er tatt i betraktning.

The project addresses carbon sequestration and storage in boreal forest soils, a globally important ecosystem service. It contributes with new data for Earth System Modelling used in IPCC assessments of global climate change effects. Specifically, the effect of soil moisture on forest soil C dynamics in the boreal zone is addressed. So far, ESMs have simplistic representations of hydrology as driver of soil C dynamics. The project will improve our understanding of this key process and generate new data on soil C stocks and moisture conditions, which will be shared with the international research community. We hypothesize: - Soil moisture conditions are decisive for the dynamics of soil respiration. - Models parameterized at measurement sites can be upscaled to produce landscape soil C stocks and the resulting decomposition representation is an improvement when compared to ESMs. - Local variables can be combined in models to predict soil moisture classes, with validation against moisture measurements and/or vegetation or soil type known to reflect site hydrology. There are higher soil C stocks in poorly drained boreal forest soils than in well-drained soils. Poor drainage is very common in boreal forests and current soil carbon models underestimate soil C stocks in such wet conditions. Modelled feedback due to climate change is biased as well for these systems. Work package 1 gathers new data on soil C from the ICOS site and poorly drained soils. WP2 modifies soil C models relative to soil moisture. WP3 develops routines for calculating soil moisture at high spatial resolution, and soil moisture index maps as input to WP4. The latter integrates the Norwegian data and process knowledge in Nor-ESM. We expect this work to decrease the uncertainty in the prediction of global soil C dynamics. Furthermore, as forests play a major role in the Norwegian greenhouse gas inventory, key results will be communicated to policymakers when revising the Norwegian climate policy.