Nitrous oxide and methane emissions from a Ferralsol as affected by loblolly pine cultivation time in subtropical...

We hypothesized that the age of loblolly pine stands influences soil methane (CH₄) and nitrous oxide (N₂O) emissions. This is a relevant topic to be studied in subtropical Brazil, where the pine plantation area is increasing considerably. We evaluated N₂O and CH₄ emissions for two years in a Ferralsol under loblolly pine (Pinus taeda L.) stands of 1, 9 and 18 year-olds and a native forest (NF). We calculated the net CO₂eq emission by considering the N₂O and CH₄ emissions from soil and the carbon (C) accumulation as litter in the forest floor. The soil N₂O emission reduced gradually over the loblolly pine cultivation years, whereas CH₄ uptake rates showed no clear pattern. Soil N₂O emission showed a positive relationship with soil temperature in NF, and with soil ammonium and nitrate intensities in the pine stands. Soil CH₄ uptake was inversely related to water-filled pore space in the pine stands, but this relationship was not observed in NF. The soil CH₄ uptake rate was 4.6 times higher (p < 0.10) in NF than the average uptake in loblolly pine stands. On the other hand, soil N₂O emissions in 9 and 18-year-old stands were similar (p > 0.10) to those in NF (1.3 kg N ha^-1 yr^-1). Our results suggest that cultivation with loblolly pine for 18 years can reduce soil N₂O emission, and the uptake of CH₄ in this system offsets 17 % of N₂O emissions. Furthermore, the C accumulation as litter in the forest floor of the mature pine stands (9- and 18-year-old) generated a net emission of -1.6 Mg CO₂eq ha^-1 yr^-1, showing to be an expressive offsetting mechanism. Therefore, we conclude that aged loblolly forests can reach N₂O emissions levels comparable to those of NF, and the C sequestration in these forests floor can significantly contribute to offset N₂O emissions and act as sink for net atmospheric CO₂eq.