Tropical Systems
Tropical forest and agricultural soils are the largest source of the greenhouse gas nitrous oxide. Planning for future climate change will require an understanding of how management and global change effects will influence greenhouse gas emissions from these soils. As a doctoral student at Brown University, Maya's dissertation focused on exploring ecosystem nitrogen status and losses from tropical agriculture and tropical forests. In Kenya, she looked at whether tropical soil characteristics might buffer the region from fertilizer pollution experienced in temperate agricultural regions. In Puerto Rico, she examined how global change effects, like changes in nitrogen deposition and rainfall, affect greenhouse gas emissions from tropical forest soils.
Abstracts
Farm Management.
Tropical smallholder agriculture is undergoing rapid transformation in nutrient cycling pathways as international development efforts strongly promote greater use of mineral fertilizers to increase crop yields. These changes in nutrient availability may alter the composition of microbial communities with consequences for rates of biogeochemical processes that control nutrient losses to the environment. Ecological theory suggests that altered microbial diversity will strongly influence processes performed by relatively few microbial taxa, such as denitrification and hence nitrogen losses as nitrous oxide, a powerful greenhouse gas. Whether this theory helps predict nutrient losses from agriculture depends on the relative effects of microbial community change and increased nutrient availability on ecosystem processes. We find that mineral and organic nutrient addition to smallholder farms in Kenya alters the taxonomic and functional diversity of soil microbes. However, we find that the direct effects of farm management on both denitrification and carbon mineralization are greater than indirect effects through changes in the taxonomic and functional diversity of microbial communities. Changes in functional diversity are strongly coupled to changes in specific functional genes involved in denitrification, suggesting that it is the expression, rather than abundance, of key functional genes that can serve as an indicator of ecosystem process rates. Our results thus suggest that widely used broad summary statistics of microbial diversity based on DNA may be inappropriate for linking microbial communities to ecosystem processes in certain applied settings. Our results also raise doubts about the relative control of microbial composition compared to direct effects of management on nutrient losses in applied settings such as tropical agriculture.
Nitrogen Deposition.
Atmospheric nitrogen (N) deposition in tropical forests may increase substantially in coming decades, stimulating a concomitant increase of soil N gas emissions. At the same time, climate change may increase the prevalence of drought, altering the processes that produce these gases (dominantly aerobic nitrification and anaerobic denitrification). This alteration is of particular concern if global changes increase the fraction of N gas released as nitrous oxide (N2O; a greenhouse gas) relative to dinitrogen (N2). To simulate the effects of atmospheric N deposition on the amount and species of soil N gas emissions, we installed fertilized ion exchange resin bags across a hillslope in the Luquillo Experimental Forest of Puerto Rico. Our experiment took place during a severe drought, providing opportunities to consider how N addition and dry soil conditions interact. After 2 months of fertilization, we measured denitrification potential using a denitrification enzyme assay, which utilizes anoxic incubations where N and carbon limitation are relieved and nitrification is inhibited. We also measured N2 and N2O emissions using a Nitrogen Free Air Recirculation Method (NFARM), which quantifies emissions from both nitrification and denitrification. Data from these two methods suggest that N inputs stimulated N2O emissions associated with aerobic nitrification. Our data suggest that soil drying during the drought decreased N2 emissions associated with anaerobic denitrification and changed the spatial patterns of emissions in the landscape. Together these results suggest that, at least during a drought, N inputs increase N2O emissions associated with nitrification, which may represent a positive feedback to climate change.
Soil Oxygen.
Tropical soils are the largest global source of N2O (a powerful greenhouse gas) to the atmosphere, yet little is known about the magnitude and spatial distribution of N2 (an inert gas that can serve as an alternate fate for N2O) production across tropical landscapes. We collected soils from across macro- and micro-topographic gradients that have previously been shown to differ in O2 availability and trace gas emissions. We then incubated these cores under oxic and anoxic headspaces to explore the relative effect of soil location versus transient redox conditions. No matter where the soils came from, or what headspace O2 was used in the incubation, N2 emissions dominated the flux of N gas losses. In the macrotopography plots, production of N2 and N2O was higher in low O2 valleys than on more aerated ridges and slopes. In the microtopography plots, N2 emissions from plots with lower mean soil O2 concentrations (5-10%) were greater than in plots with higher mean soil O2 (10-20%). Across all conditions, the N2:N2O ratio was highest from microtopography plots with low soil O2 (5-10%). Similar results were also found across headspace O2 concentrations, as the N2:N2O ratio was the highest under anoxic incubations (0% O2). We estimate an N gas flux of ~37 kg N ha-1 yr-1 from this forest, 99% as N2. These results suggest that N2 fluxes may have been systematically underestimated in these landscapes, and that the measurements we present call for a reevaluation of the N budgets in lowland tropical forest ecosystems.