Nitrous oxide (N2O) fluxes from soils under different land use in Brazil – Overview, measurements, and modeling

Abstract:

Cropland area has been expanded in Brazil and by now the agricultural sector has been calculated as the major emitter of nitrous oxide (N₂O). This state was preceded by the conversion of natural ecosystems, which inevitably causes changes in the carbon and nitrogen cycle of the soil. However, detailed model and measurement approaches are lacking for sound national estimates. Here we present data from model driven measurement campaigns from different ecosystems and the results of a review of available data on annual N₂O fluxes throughout Brazil. Contrary to expectations, emissions from agricultural land (0.80 kg N ha^-1 yr^-1) tended to be lower than from rainforest (2.29 kg N ha^-1­­ yr^-1). At the same time, N-fertilization did not lead to a relevant increase in annual N­­₂O emissions. Moreover, pastures showed an age-related decrease in emissions: median annual emissions from young pastures (≤ 10 years) were 2.52 kg N ha^-1, whereas old pastures (> 10 years) emitted 0.90 kg N ha^-1 yr^-1. Since N₂O is known to react very sensitively to changes (for example dry-wet changes), it is likely that high emissions occur during actual land conversions. Consequently, our own measurement campaigns concentrated on the transitional period from dry to wet season, as already predicted by previously performed model simulations. Results from these short-term measurements supported the findings of the review, regarding land use-specific differences in N₂O-N fluxes, but did not support our assumptions of emission pulses after rewetting of the soil. Applied process-oriented models performed quite well on the studied sites, but better adaptation is inevitable with regard to the possibility of more precise predictions and spatial extrapolation. This requires spatial and temporal higher resolved monitoring, in order to involve soil types, which have so far not been considered sufficiently, and capture short-term effects from precipitation and fertilizer events.