Explicitly Synchronizing Soil Water and Carbon Nitrogen Reactive Transport Using CLM-PFLOTRAN: Does Sequential or Synchronized Implementing of Soil Processes Matter to Soil C Stocks?

Abstract:

In nature soil biophysical and biogeochemical processes are coupled spatially and temporally. However due to constrain of both understanding of complexity of process interactions and computing ability, it still remains a challenge to represent fully coupled system of soil hydrological-thermal dynamics and biogeochemical processes in land surface models (LSMs). In the Community Land Model (CLM), the land component of the Community Earth System Model (CESM), soil C-N processes are not only implemented sequentially but also asynchronously coupled to thermal and hydrological processes. PFLOTRAN is an open source, state-of-the-art massively parallel 3-D subsurface flow and reactive transport code. In this study, we extend the subsurface hydrological-thermal process coupling between CLM and PFLOTRAN to include explicitly synchronized soil biogeochemical processes. The resulting coupled CLM-PFLOTRAN model is a LSM capable of resolving 3-D soil hydrological-thermal-biogeochemical processes.

The classic CLM-CN reaction networks, degassing-dissolving of C-N relevant greenhouse gases between soil solution and air, soil N absorption and transportation processes are implemented in PFLOTRAN’s reactive-transport framework. We compare soil C stock estimates from CLM alone and coupled CLM-PFLOTRAN simulations at the Next Generation Ecosystem Experiment-Arctic field sites at the Barrow Environmental Observatory (BEO), AK. Both simulations are compared against available soil C dataset to assess importance of representing this synchronization in LSMs. Contributions of various factors to spatial variance of simulated variations from the two modeling approaches are evaluated across this polygonal coastal tundra landscape. Results indicate that two modeling approaches could produce very contrasting results, especially in the N-limit ecosystem. The developed CLM-PFLOTRAN framework will be used for regional evaluation of climate change caused ecosystem process responses and their feedbacks to climate system.

Key word: Soil C stocks, soil biogeochemistry, soil thermal-hydrology, synchronization, CLM-CN model, PFLOTRAN model, polygonal coastal tundra