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

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