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Gross primary production (GPP) is a key driver of the peatland carbon cycle. Although many studies have explored the apparent GPP under natural light conditions, knowledge of the maximum GPP at light-saturation (GPP max ) and its spatio-temporal variation is limited. This information, however, is crucial since GPP max essentially constrains the upper boundary for apparent GPP. Using chamber measurements combined with an external light source across experimental plots where vegetation composition was altered through long-term (20-year) nitrogen addition and artificial warming, we could quantify GPP max in-situ and disentangle its biotic and abiotic controls in a boreal peatland. We found large spatial and temporal variations in the magnitudes of GPP max which were related to vegetation species composition and phenology rather than abiotic factors. Specifically, we identified vegetation phenology as the main driver of the seasonal GPP max trajectory. Abiotic anomalies (i.e. in air temperature and water table level), however, caused species-specific divergence between the trajectories of GPP max and plant development. Our study demonstrates that photosynthetically active biomass constrains the potential peatland photosynthesis while abiotic factors act as secondary modifiers. This further calls for a better representation of species-specific vegetation phenology in process-based peatland models to improve predictions of global change impacts on the peatland carbon cycle.

1. The aim of this study was to assess the feasibility of 100% organic feed for organic broilers by comparing it with 80% organic feed (situation at the time of the experiment; 2004) and 95% organic feed (alternative). 2. Diets were optimised for nutritional value, allowing a maximum 10% increase in feed price when using 100% organic feedstuffs. This could only be achieved at the expense of the methionine content. 3. The birds were reared from 0 to 3 weeks of age in a broiler house in three groups of 500 broilers each on either an 80, a 95 or a 100% organic starter diet. At 3 weeks of age, they were transferred to 15 pens with an outdoor run. Each treatment group of 500 birds was divided into 5 groups of 95 and given an 80, a 95 or a 100% organic finisher diet. 4. Broilers receiving 100% organic feed reached a lower body weight and grew more slowly than those receiving 95% organic feed, mainly because of a lower feed intake. 5. Broilers on 95 or 100% organic feed had a higher incidence of breast blisters than broilers receiving 80% organic feed. 6. The cost price for meat from broilers that received 80% organic feed was euro1.83 per kg live weight. The cost prices for broilers that received 95 and 100% organic feed were euro1.84 (+0.8%) and euro1.93 (+5.4%) per kg live weight, respectively. 7. In conclusion, 95% organic feed led to a better performance than 100% organic feed in this study. Probably, the lower methionine content in the 100% organic feed negatively affected performance. The results for 95% organic feed were similar to 80% organic feed, except for a higher incidence of breast blisters.