Glasshouse vs field experiments: do they yield ecologically similar results for assessing N impacts on peat mosses?

<br> • Peat bogs have accumulated more atmospheric carbon (C) than any other terrestrial ecosystem today. Most of this C is associated with peat moss (<i>Sphagnum</i>) litter. Atmospheric nitrogen (N) deposition can decrease <i>Sphagnum</i> production, compromising the C sequestration capacity of peat bogs. The mechanisms underlying the reduced production are uncertain, necessitating multifactorial experiments. <br> • We investigated whether glasshouse experiments are reliable proxies for field experiments for assessing interactions between N deposition and environment as controls on <i>Sphagnum</i> N concentration and production. We performed a meta-analysis over 115 glasshouse experiments and 107 field experiments. <br> • We found that glasshouse and field experiments gave similar qualitative and quantitative estimates of changes in <i>Sphagnum</i> N concentration in response to N application. However, glasshouse-based estimates of changes in production – even qualitative assessments – diverged from field experiments owing to a stronger N effect on production response in absence of vascular plants in the glasshouse, and a weaker N effect on production response in presence of vascular plants compared to field experiments. <br> • Thus, although we need glasshouse experiments to study how interacting environmental factors affect the response of <i>Sphagnum</i> to increased N deposition, we need field experiments to properly quantify these effects.