Due to high annual temperatures and precipitation amounts tropical ecosystems rapidly cycle carbon and nitrogen, which leads to high soil CO2 and N2O emissions. These emissions are highly variable in space and time, mainly due to their coupling to soil moisture content and tree species (van Haren et al., 2010). Soil moisture influences the microbial activity and whether nitrification and denitrification are prevalent, whereas tree species are the main source of labile carbon for denitrifiers and compete with the microbes for nutrients. Both potentially drive feedbacks to climate change with the precipitation and forest composition most likely changing dramatically in the near future.
Our work has focused on the questions:
1) what is the influence of tree species and overall forest dynamics on soil trace gas fluxes, and
2) how does artificial drought affect soil trace gas fluxes?
To address the first question we have measured soil gas fluxes in the Tapajós National Forest and a nearby plantation in Belterra. There we have established that tree species can affect soil gas fluxes in diverse tropical forests and that this potentially is related to tree growth rates (van Haren et al., 2010). We also found a strong correlation between overall forest growth rates -as measured in large (~20 ha) plots- and soil N2O fluxes, which suggests that similar factors (precipitation and soil fertility) influence both (van Haren et al., to be submitted to PNAS). Native tree plantations hold great potential to offset greenhouse gas (GHG) emissions when planted on abandoned agricultural lands, but our work demonstrates that tree species effects measured on plantations have little predictive capability for diverse forest (van Haren et al. submitted to Biotropica).
The second question we are currently addressing at Caxiuanã and Biosphere 2. In Caxiuanã National Forest in Brazil, we have measured the soil trace gas fluxes and in particular N2O fluxes at the control and treatment sites of the EsecaFlor dry-down experiment during the summer of 2009 and 2010. Fluxes were also measured at nearby clayey and fertile “Terra Preta” sites to compare the effects of soil texture and fertility on the magnitude of nitrous oxide production. Our 2009 results were surprising, in the sense that we found higher N2O fluxes in the drier treatment plot. These results could not be repeated during our 2010 campaign, but our more detailed and repeated measurements did reveal that N2O production deeper in the soil might explain the higher flux in 2009. This finding indicates the importance of considering soil physical properties like texture and porosity when determining surface emissions of greenhouse gases such as nitrous oxide.
van Haren J, de Oliveira RC, Restrepo-Coupe N, Hutyra L, de Camargo PB, Keller M, Saleska S (2010) Do plant species influence soil CO2 and N2O fluxes in a diverse tropical forest? JGR-biogeosciences, 115, G03010, 10.1029/2009JG001231
van Haren J, Oliveira RC, Keller M, Saleska S (Submitted) Tree Species effects on soil CO2 and N2O production in a tropical monoculture and diverse forest. Biotropica.
van Haren J, Oliveira RC, de Camargo P, Keller M, Weintraub S, Townsend A, Saleska S, (Submitted) Low N2O fluxes associated with legume tree species on monoculture plantations. Soil Biology and Biochemistry.
van Haren J, Saleska S, Oliveira RC, Li C, Frolking, S, Varner R, Keller M, Crill P (To Be submitted Fall 2011) Forest growth predicts tropical soil N2O fluxes. PNAS.
Joost van Haren (graduated with Ph.D, University of Arizona), Lindsey Hovland (graduated with MSc., University of Arizona), Rosa Paes (graduated Ph.D., UFPA, Belém, Brazil), Veber Souza de Mouro (INPA, Manaus, Brazil)
Hinsby Cadillo-Quiroz (ASU, Tempe, USA), Plinio de Camargo (CENA, Piracicaba, Brazil), Rachel Gallery (University of Arizona, USA), Michael Keller (Unicamp, Campinas, Brazil), Dan Metcalf (Swedish University of Agricultural Research, Umea, Sweden), Raimundo Cosme de Oliveira Jr. (EMBRAPA, Santarém, Brazil)