N2O fluxes and CO2 exchange at different N doses under elevated CO2 concentration in boreal agricultural mineral soil under Phleum pratense

The effects of elevated atmospheric CO2 concentration on N2O fluxes, instant CO2 exchange and the biomass production of timothy (Phleum pratense) were studied in the laboratory. Three sets of 12 farmed sandy soil mesocosms sown with Phleum pratense were fertilised with a commercial fertiliser in order to add 5, 10 and 15 g N m−2, and equally distributed in four thermo-controlled greenhouses. In two of the greenhouses, the CO2 concentration was kept at atmospheric concentration (360 μmol mol−1), and in the other two at double the ambient concentration (720 μmol mol−1). Forage was harvested and the plants fertilised twice during the N2O measurements. This was followed by an extra fertilisation and harvesting. After the third harvest, the growth of P. pratense was maintained at a height of 18 cm for measurements of instant CO2 exchange, performed in two growth chambers. N2O exchange was monitored using a closed chamber technique and a gas chromatograph. Instant CO2 exchange was monitored using an infrared gas analyser. N2O was emitted from the soil in the low, moderate and high N treatments at both CO2 concentrations when the moisture content was low, the N2O probably being mainly derived from nitrification. The highest flux (3303 μg N2O m−2 h−1) occurred in the highest N treatment before thinning the stand of P. pratense under elevated CO2 concentration. P. pratense was acclimated to the elevated CO2 concentration: the NEE and PG of the elevated growth of P. pratense decreased, in contrast to the fluxes of the normal ambient growth, when measured at the changed CO2 concentration (ambient). The rate of respiration (RTOT) in the agroecosystem did not increase due to the elevated CO2 concentration, but instead the results indicated decreased RTOT (on average 2049 and 1808 mg CO2 m−2 h−1 at ambient and elevated CO2 concentration, respectively) when there was an abundant N supply. This infers the possibility of enhanced C accumulation in agriculture mineral soil via P. pratense under an increased atmospheric CO2 supply.