Long-term loblolly pine land use reduces methane and net greenhouse gas emissions in a subtropical Cambisol, despite increasing nitrous oxide

Abstract

Key message: In loblolly pine land use of 17–32 years following forest clearing, CH4consumption and N2O emission diminished by 17 years, due to high soil moisture (~ 80% WFPS, N2O into N2), but increased by 32 years, where medium moisture favoured methanotrophy and denitrification into N2O. Soil greenhouse gases (GHG) emission was positive by 17 years, but negative by 32, when soil sequestered carbon. Context: Much of the role of planted forests in the gaseous soil-atmosphere exchanges in the subtropics remains to be evaluated. Aims: To assess the impacts of loblolly pine (Pinus taeda L.) on soil nitrous oxide (N2O) and methane (CH4) fluxes in a subtropical Cambisol. Methods: Fluxes were monitored over 1 year with static chambers, in forest stands under natural forest (NF) and pine plantation for 17 (P17) and 32 years (P32). Results: The NF soil showed the lowest N2O emission and the highest CH4 consumption, because of the lowest water-filled pore space (WFPS, < 40%) and highest soil macroporosity. In P17, N2O emission was still low, but CH4 consumption diminished sixfold, possibly because of the predominance of methanotrophy, favoured by the highest WFPS (~ 80%) and lowest macroporosity that together with low mineral N concentration also did not favour the formation of N2O. In P32, the improved soil mineral N, macroporosity and intermediate WFPS (~ 60%) increased CH4 consumption and also N2O emission, in an environment supposedly favourable to methanotrophy and also to N2O production. Considering soil organic carbon (SOC) from a concurrent study, the net GHG emission (Mg Ceq ha−1 year−1) was 2.8 in P17 and − 1.1 in P32. Conclusion: Soil under pine plantation is the source of GHG in the first rotation (17 years), because of the low CH4 consumption and SOC losses, but soil becomes C sink in the second rotation (32 years), by sequestering SOC and consuming more CH4, despite emitting more N2O.