Impact of management on CO2 fluxes and drivers of regrowth rates in a temperate grassland during 20 years of measurements

Grasslands serve a unique role in the global carbon (C) cycle and cover about 30% of the European and about 70% of the Swiss area used for agriculture. The CO₂ fluxes of managed grasslands are substantially influenced by climate conditions and land management practices. The eddy covariance (EC) technique is the only approach to directly measure the net ecosystem exchange (NEE) of CO₂. NEE represents the balance between two large ecosystem processes: gross primary production (GPP; amount of CO₂ fixation through photosynthesis), and ecosystem respiration (Reco; amount of CO₂ released via plant and soil respiration). Our study aimed to (1) investigate intra- and inter-annual changes in grassland NEE as well as regrowth after mowing/grazing events, (2) understand key drivers of GPP regrowth rates, and (3) examine grassland responses to sward renewal.

 

In our study, we measured EC fluxes and meteorological variables at the temperate grassland site Chamau (CH-Cha as part of FLUXNET) in Switzerland. This grassland is intensively managed, with 4-6 mowing/grazing events per year, accompanied by organic fertilization (on average 271 kg N ha^-1 yr^-1) and sward renewal every 7-10 years. We applied machine learning approaches such as Extreme Gradient Boosting (XGBoost) and Shapley Additive exPlenations (SHAP) analysis to address our aims, using 20 years (2005-2024) of EC flux, meteorological, and detailed management data.

 

Over the 20 years, a pronounced intra-seasonal course of NEE was found due to mowing and grazing, with the maximum CO₂ uptake in early spring (March-April) and the largest CO₂ loss in early winter (December-January). During the main growing season (April-September), the average GPP regrowth rate was 10 g C m^-2 day^-1. We did not find a significant trend for GPP regrowth rates over the 20 years. The most important drivers of GPP regrowth rates were air temperature and light, while water-related drivers dominated regrowth rates during summer droughts (e.g., 2015 and 2018). Nitrogen fertilization did not play a key role in GPP regrowth rates. Moreover, sward renewal years resulted in either very large CO₂ losses (in 2012) or in reduced CO₂ uptake rates (in 2021), most likely caused by the different timing of the renewal, i.e., February vs. August, respectively. Thus, our study provides novel insights into climate-smart management options and helps to develop mitigation strategies for current and future climate risks.