How does deforestation for cropland affect moisture, temperature, and trace gas concentrations at depth in Southeastern Amazonian soils?

Background/Question/Methods

Tropical soils contain large stocks of carbon (C) and nitrogen (N), but it remains poorly documented how C and N in these deep, weathered soils are affected by land use change.  Evidence from the top 30 centimeters (cm) of soil indicates that land use change from forest to agriculture in the Amazon depletes C and N stocks, depresses carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions and reduces methane (CH₄) uptake; how CO₂, N₂O and CH₄ change below 30 cm soil depth after deforestation remains poorly understood.  Characterizing how trace gas fluxes vary down the soil profile can provide information about C and N availability below the rooting zone as well as the origin of greenhouse gases within the soil profile between land uses.  In this study, we measured concentrations of CO₂, N₂O and CH₄in soil air at equilibrium from 15 cm depth to 450 cm depth, in combination with soil temperature and volumetric water content, in 10-meter soil pits located in mature forest and monoculture soybean/maize cultivation at a research site in southeastern Amazonia.  

Results/Conclusions

Deep agricultural soils had higher temperature and volumetric water content than forest soils (p < 0.001), likely due to shifts in sensible and latent heat fluxes between land uses.  We found that CO₂ concentration differed significantly between land uses, with lower CO₂ concentration at depth (> 250 cm) in agriculture than forest.  Similarly, N₂O concentration at depth (> 40 cm) was lower in agriculture than in forest, while CH₄ concentrations were higher in relatively shallow and relatively deep agricultural soils (< 75 cm, >350 cm), suggesting that these soil depths have lower CH₄ uptake rates.  In all cases, concentrations of trace gases, temperature and volumetric water content differed significantly between land uses (p < 0.001).  We estimated trace gas flux at depth using a diffusion model that showed higher variability in trace gas fluxes in deep forest soils than in deep agricultural soils.  Because deep agricultural soils had higher temperature and volumetric water content than forest soils, but lower N₂O and CO₂ concentrations, our results suggest that C and N availability differed between land uses. Trace gas patterns throughout the soil profile support indications that intensive agriculture in this system does not substantially contribute to higher landscape-level GHG emissions.