Multiple environmental factors interact to affect wet grassland ecosystem functions

Wet grasslands are highly productive wetland ecosystems that provide many important ecosystem services, including nutrient removal for water purification, carbon (C) sequestration, local climate regulation, water retention, and flood attenuation. Like other ecosystems, wet grasslands are affected by multiple biotic and abiotic factors, the interactions of which may affect their functionality. We conducted a fully factorial mesocosm experiment to disentangle the importance of soil type (mineral or organic), water (high vs low) and nutrient level (NPK fertilized or unfertilized) effects on plant and soil parameters and how these affect ecosystem respiration (R_ECO) and greenhouse gas (GHG) emissions. In the fifth year of the experiment, we measured plant biomass and production, soil biological and chemical parameters, and GHG fluxes four times during the year, once before the start of the growing season and then in the beginning, peak and end of the growing season. We found that plant, soil and GHG parameters showed distinct seasonality and were influenced by all tested factors, both singly and interactively, affecting many aspects of wet grassland ecosystems by acting through several plant-soil feedbacks. Soil type, both primarily and in several interactions with water and nutrient levels, controlled soil properties, microbial biomass, and bacterial and fungal abundances. Plant presence, the productivity of which was stimulated by nutrient addition, together with some plant-soil feedbacks, were the main drivers of R_ECO and GHG emissions (relevant only for CH₄ because N₂O was not emitted in any sampling occasion). As a result, CO₂ and CH₄ emissions were 12 and 3 times greater, respectively, in vegetated compared to un-vegetated samples. In addition, water level and nutrient addition interactions influenced gas emissions, with CO₂ emissions being greater in low water, fertilized conditions, while CH₄ emissions increased under high water, unfertilized conditions. When correcting for the greater global warming potential (GWP) of CH_4, it still accounted for only a maximum of 18 % of the GHG fluxes. We showed that multiple environmental factors interact to impact wet grassland functions. Managers should focus their activities on managing the factors that most allow for wet grasslands to maintain their structure and functions to future disturbances.