Modelling of carbon dioxide net ecosystem exchange of hayfield on drained peat soil: land use scenario analysis

The data of episodic field measurements of carbon dioxide balance components (soil respiration — $R_{soil}$, ecosystem respiration — $R_{eco}$, net ecosystem exchange — NEE) of hayfields under use and abandoned one are interpreted by modelling. The field measurements were carried within five field campaigns in 2018 and 2019 on the drained part of the Dubna Peatland in Taldom District, Moscow Oblast, Russia. The territory is within humid continental climate zone. Peatland drainage was done out for milled peat extraction. After extraction was stopped, the residual peat deposit (1–1.5 m) was ploughed and grassed (Poa pratensis L.) for hay production. The current ground water level (GWL) varies from 0.3–0.5 m below the surface during wet and up to 1.0 m during dry periods. Daily dynamics of CO$_2$ fluxes was measured using dynamic chamber method in 2018 (August) and 2019 (May, June, August) for abandoned ditch spacing only with sanitary mowing once in 5 years and the ditch spacing with annual mowing. NEE and $R_{eco}$ were measured on the sites with original vegetation, and $R_{soil}$ — after vegetation removal. To model a seasonal dynamics of NEE, the dependence of its components ($R_{eco}$, $R_{soil}$, and Gross ecosystem — atmosphere exchange of carbon dioxide — GEE) from soil and air temperature, GWL, photosynthetically active radiation, underground and aboveground plant biomass were used. The parametrization of the models has been carried out considering the stability of coefficients estimated by the bootstrap method. $R^2$ ($\alpha=0.05$) between simulated and measured $R_{eco}$ was 0.44 ($p<0.0003$) on abandoned and 0.59 ($p<0.04$) on under use hayfield, and GEE was 0.57 ($p<0.0002$) and 0.77 ($p<0.00001$), respectively. Numerical experiments were carried out to assess the influence of different haymaking regime on NEE. It was found that NEE for the season (May 15 – September 30) did not differ much between the hayfield without mowing ($4.5\pm 1.0$ tC$\cdot$ha$^{-1}\cdot$season$^{-1}$) and the abandoned one ($6.2\pm 1.4$). Single mowing during the season leads to increase of NEE up to $6.5\pm 0.9$, and double mowing — up to $7.5\pm 1.4$ tC$\cdot$ha$^{-1}\cdot$season$^{-1}$. This means increase of carbon losses and CO$_2$ emission into the atmosphere. Carbon loss on hayfield for both single and double mowing scenario was comparable with abandoned hayfield. The value of removed phytomass for single and double mowing was $0.8\pm 0.1$ tC$\cdot$ha$^{-1}\cdot$season$^{-1}$ and $1.4\pm 0.1$ (45% carbon content in dry phytomass) or 3.0 and 4.4 tC$\cdot$ha$^{-1}\cdot$season$^{-1}$ of hay (17% moisture content).
In comparison with the fallow, the removal of biomass of $0.8\pm 0.1$ at single and $1.4\pm 0.1$ tC$\cdot$ha$^{-1}\cdot$season$^{-1}$ double mowing is accompanied by an increase in carbon loss due to CO$_2$ emissions, i.e., the growth of NEE by $0.3\pm 0.1$ and $1.3\pm 0.6$ tC$\cdot$ha$^{-1}\cdot$season$^{-1}$, respectively. This corresponds to the growth of NEE for each ton of withdrawn phytomass per hectare of $0.4\pm 0.2$ tC$\cdot$ha$^{-1}\cdot$season$^{-1}$ at single mowing, and $0.9\pm 0.7$ tC$\cdot$ha$^{-1}\cdot$season$^{-1}$ at double mowing. Therefore, single mowing is more justified in terms of carbon loss than double mowing. Extensive mowing does not increase CO$_2$ emissions into the atmosphere and allows, in addition, to “replace” part of the carbon loss by agricultural production.