Preprints
https://doi.org/10.5194/bg-2021-144
https://doi.org/10.5194/bg-2021-144

  14 Jun 2021

14 Jun 2021

Review status: this preprint is currently under review for the journal BG.

The carbon budget of the managed grasslands of Great Britain constrained by earth observations

Vasileios Myrgiotis, Thomas Luke Smallman, and Mathew Williams Vasileios Myrgiotis et al.
  • School of GeoSciences and National Centre for Earth Observation, University of Edinburgh, Edinburgh EH9 3FF, UK

Abstract. Grasslands cover around two thirds of the land area of Great Britain (GB) and are important reservoirs of terrestrial biological carbon (C). Outside a few well-monitored sites the quantification of C dynamics in managed grasslands is made complex by the spatio-temporal variability of weather conditions combined with grazing and cutting patterns. Earth observation (EO) missions produce high-resolution frequently-retrieved proxy data on the state of grassland canopies but synergies between EO data and biogeochemical modelling to estimate grassland C dynamics are under-explored. Here, we show the potential of model-data fusion (MDF) to provide robust near-real time analyses of managed grasslands of GB (England, Wales andScotland). We combine EO data and process-based modelling to estimate grassland C balance and to examine the role of management. We implement a MDF algorithm to (1) infer grassland management from vegetation reduction data (Proba-V), (2) optimise model parameters by assimilating leaf area index (LAI) data (Sentinel-2) and (3) simulate livestock grazing, grass cutting, and C allocation and loss to the atmosphere. The MDF algorithm was applied for 2017 and 2018 at 1855 fields sampled from across GB. The algorithm was able to effectively assimilate the Sentinel-2 based LAI time series (overlap = 80 %, RMSE = 1 gCm−2, bias = 0.35 gCm−2) and predict livestock densities per area that correspond with independent census-based data (r = 0.68). The mean total removed biomass across all simulated fields was 6 (±1.8) tDM ha−1 y−1. The simulated grassland ecosystems were on average C sinks in 2017 and 2018; the GB-average net ecosystem exchange (NEE) and net biome exchange (NBE) for 2017 was −232 ± 94 and for 2018 was −120 ± 103 gCm−2 y−1. The 2018 summer drought reduced C sinks, with a 9-fold increase in the number fields that were C sources (NBE > 0) in 2018 compared to 2017. We conclude that management in the form of sward condition and the timing, intensity and type of defoliation are key determinants of the C balance of managed grasslands. Nevertheless, extreme weather, such as prolonged droughts, can convert grassland C sinks to sources.

Vasileios Myrgiotis et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'RC Comment on bg-2021-144', Katja Klumpp, 08 Jul 2021
    • AC1: 'Reply on CC1', Vasileios Myrgiotis, 19 Aug 2021
  • RC1: 'Comment on bg-2021-144', Anonymous Referee #1, 17 Jul 2021
    • AC3: 'Reply on RC1', Vasileios Myrgiotis, 03 Sep 2021
  • RC2: 'Comment on bg-2021-144', Aiming Qi, 01 Aug 2021
    • AC2: 'Reply on RC2', Vasileios Myrgiotis, 19 Aug 2021

Vasileios Myrgiotis et al.

Model code and software

DALEC-Grass Vasileios Myrgiotis https://github.com/GCEL/DALEC-Grass

Vasileios Myrgiotis et al.

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Short summary
This study shows that grassland vegetation management (livestock grazing, grass cutting) determines whether a grassland is adding (source) or removing (sink) carbon (C) to/from the atmosphere. The annual C balance of 1855 managed grassland fields in Great Britain was quantified for 2017–2018. The examined fields were, on average, small C sinks but the summer drought of 2018 led to a 9-fold increase in the number of fields that became C sources in 2018 compared to 2017.
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