A vertical representation of soil carbon in the JULES land surface scheme (vn4.3_permafrost) with a focus on permafrost regions

Abstract

Abstract. An improved representation of the carbon cycle in permafrost regions will enable more realistic projections of the future climate-carbon system. Currently JULES (the Joint UK Land Environment Simulator) -the land surface model of the UK Earth System Model (UKESM) -uses the standard four-pool RothC soil carbon model. This paper describes a new version of JULES (vn4.3_permafrost) in which the soil vertical dimension is added to the soil carbon model, with a set of four pools in every soil layer. The respiration rate in each soil layer depends on the temperature and moisture conditions in that layer. Cryoturbation/bioturbation processes, which transfer soil carbon between layers, are represented by diffusive mixing. The litter inputs and the soil respiration are both parametrized to decrease with increasing depth. The model now includes a tracer so that selected soil carbon can be labelled and tracked through a simulation. Simulations show an improvement in the large-scale horizontal and vertical distribution of soil carbon over the standard version of JULES (vn4.3). Like the standard version of JULES, the vertically discretized model is still unable to simulate enough soil carbon in the tundra regions. This is in part because JULES underestimates the plant productivity over the tundra, but also because not all of the processes relevant for the accumulation of permafrost carbon, such as peat development, are included in the model. In comparison with the standard model, the vertically discretized model shows a delay in the onset of soil respiration in the spring, resulting in an increased net uptake of carbon during this time. In order to provide a more suitable representation of permafrost carbon for quantifying the permafrost carbon feedback within UKESM, the deep soil carbon in the permafrost region (below 1 m) was initialized using the observed soil carbon. There is now a slight drift in the soil carbon ( < 0.018 % decade −1 ), but the change in simulated soil carbon over the 20th century, when there is little climate change, is comparable to the original vertically discretized model and significantly larger than the drift.

ORCHIDEE-MICT (v8.4.1), a land surface model for the high latitudes: model description and validation

“…In order to reproduce the burial of SOC below the active layer, the strategy followed in ORCHIDEE-MICT is inspired by Koven et al (2009), who used a diffusion equation to move carbon in the permafrost where no decomposition occurs. Other studies (Koven et al, 2013;Burke et al, 2017a) further limited the rate of decomposition of SOC at depth, to reproduce the lack of oxygen inhibiting decomposition. It should be noted also that the decomposition scheme of SOC is still based on Parton et al (1988) as classically done in land surface models (Friedlingstein et al, 2006).…”

“…At the regional level, model-observed agreement still holds. Arguably, the main variables that should be well simulated for a high-latitude land ecosystem model regarding future carbon-cycle climate feedbacks (Koven et al, 2013;Burke et al, 2017a) are the carbon stocks in biomass, litter and soils. In this respect, ORCHIDEE-MICT performs generally well for biomass with GSWP3 forcing, including the latitudinal profile, showing a peak between 50 and 60 • N, despite the simple constant background mortality used for forest (Krinner et al, 2005) in a region where harsh climate is known to induce mortality events.…”

Representation of dissolved organic carbon in the JULES land surface model (vn4.4_JULES-DOCM)

“…All units are given in Table 2. The moisture and temperature rate modifiers are based on the RothC formulations (Coleman and Jenkinson, 2014); τ z is the empirical factor for a decrease in C decomposition rates with soil depth, as recently introduced in JULES (Burke et al, 2017). The DOC production rate is further modified by D f , which considers the decrease in the SOC decomposition rate as an increase in silt plus clay content given in a fraction (Parton et al, 1987):…”

“…Two days prior to sample collection a tension of 600 hPa was applied to each suction cup. Samples were collected at three locations and pooled Koven et al (2013), Burke et al (2017). c .…”

Representation of dissolved organic carbon in the JULES land surface model (vn4.4_JULES-DOCM)

“…The moisture and temperature rate modifiers are based on the RothC formulations. t z is the empirical factor for decrease of C decomposition rates with soil depth, as recently introduced in JULES (Burke et al 2016). …”