Sorption of Pure N2O to Biochars and Other Organic and Inorganic Materials under Anhydrous Conditions

Suppression of nitrous oxide (N2O) emissions from soil is commonly observed after amendment with biochar. The mechanisms accounting for this suppression are not yet understood. One possible contributing mechanism is N2O sorption to biochar. The sorption of N2O and carbon dioxide (CO2) to four biochars was measured in an anhydrous system with pure N2O. The biochar data were compared to those for two activated carbons and other components potentially present in soils-uncharred pine wood and peat-and five inorganic metal oxides with variable surface areas. Langmuir maximum sorption capacities (Q(max)) for N2O on the pine wood biochars (generated between 250 and 500 degrees C) and activated carbons were 17-73 cm(3) g(-1) at 20 degrees C (median 51 cm(3) g(-1)), with Langmuir affinities (b) of 2-5 atm(-1) (median 3.4 atm(-1)). Both Q(max) and b of the charred materials were substantially higher than those for peat, uncharred wood, and metal oxides [Q(max) 1-34 cm(3) g(-1) (median 7 cm(3) g(-1)); b 0.4-1.7 atm(-1) (median 0.7 atm(-1))]. This indicates that biochar can bind N2O more strongly than both mineral and organic soil materials. Q(max) and b for CO2 were comparable to those for N2O. Modeled sorption coefficients obtained with an independent polyparameterlinear free-energy relationship matched measured data within a factor 2 for mineral surfaces but underestimated by a factor of 5-24 for biochar and carbonaceous surfaces. Isosteric enthalpies of sorption of N2O were mostly between -20 and -30 kJ mol(-1), slightly more exothermic than enthalpies of condensation (-16.1 kJ mol(-1)). Q(max) of N2O on biochar (50000-130000 mu g g(-1) biochar at 20 degrees C) exceeded the N2O emission suppressions observed in the literature (range 0.5-960 mu g g(-1) biochar; median 16 mu g g(-1)) by several orders of magnitude. Thus, the hypothesis could not be falsified that sorption of N2O to biochar is a mechanism of N2O emission suppression.