Update on soil carbon balance in Hungarian crop rotation systems

Long term flux measurements are needed to improve our understanding of the carbon balance of arable lands. The objective of our study was to determine the seasonal dynamics of carbon cycling in a Hungarian cropland and to examine the effect of crop rotation on net ecosystem exchange of CO₂ (NEE), furthermore to assess the influences of C outputs and inputs derived from lateral fluxes on soil organic carbon (SOC) stock. In this study we update the results presented in our poster of last year’s conference (EGU21-10977).

The experiment began in 2017 and crop rotation of the measured field consisted of winter wheat (2017-2018 and 2019-2020), rapeseed (2018), sorghum (2019) and sunflower (2021). CO₂ fluxes and annual net ecosystem exchange (NEE) of CO₂ were measured by a field-scale eddy covariance (EC) station at a Central Hungarian cropland site. Both vertical and lateral C fluxes were taken into account when calculating the net ecosystem carbon budget (NECB).

As presented in our previous study the largest sink activity was observed in the sorghum season (-277 g C m^-2 from sowing to harvest). The cropland acted as a source of CO₂ during the rapeseed season (140 g C m^-2) due to incomplete germination caused by extreme autumnal drought.

We found that during the study period both meteorological variables and lateral carbon fluxes such as C inputs derived from seed and crop residues and outputs (harvest) had significant influence on the C dynamics. The higher temperatures and precipitation amount that characterised the fall of 2019 caused large differences in NEE dynamics for winter wheat when compared to 2017. The impact of climatic factors could be seen in the sunflower period since lack of precipitation in 2021 led to remarkably low carbon uptake.

Fallow periods in total covered a relatively long period of time (approximately 1 year out of the 4 year long study period). These fallow periods had a significant effect on NECB values due to immense C loss. During the four years of our experiment cumulative NEE was -222 g C m^-2 and NECB was 726 g C m^-2 as carbon loss during fallow periods (437 g C m^-2 in total) and carbon export through harvest (964 g C m^-2 in total) counterbalanced the crop’s CO₂ uptake.

We can conclude that while this Hungarian cropland was a sink of carbon it could not maintain the soil organic carbon content as it was not able to sequester enough carbon to do so. Cover plants and crop residue retention could be a solution to reduce the risk of soil carbon stock depletion but further studies are needed in the field of soil management practices.