Quantifying NO and N2O fluxes in the Amazon’s Agricultural Frontier
The largest area of recent agricultural expansion on earth is on the boundary between the Amazon and Cerrado biomes in Brazil (Neill et al. 2017). The field work in this project is located at a commercial farm in the Cerrado-Amazon transition region, Tanguro Ranch. Tanguro is representative in terms of the physical and climate environment of the expanding soy and soy-maize cropland industry in the Cerrado-Amazon region. I measured NO emissions in this region, relate them to N2O emissions, and explore how these vary across land use types (soy, maize, and intact primary tropical forest). I experimentally raised nitrogen (N) fertilizer amounts and quantify NO and N2O responses.
Tanguro deep soil nitrate controls and adsorption thresholds
Several studies have documented nitrate (NO3–) accumulation below 1 meter in croplands, intact and disturbed forests, and plantations in Amazonia and the Cerrado. At Tanguro, there is a large pool of nitrate ranging from ~50-300 kg N ha-1 adsorbed to the soil approximately 3-4m deep, and the pool increases from the forest to soy to soy-maize double cropping land uses (Jankowski et al. 2018. Deep Soils Modify Environmental Consequences of Increased Nitrogen Fertilizer Use in Intensifying Amazon Agriculture.) The high anion exchange capacity of these soils may be the mechanism explaining this deep soil N pool, but it has not yet been measured at Tanguro. It is also unclear how much nitrate may be adsorbed to these soils before the exchange sites are all filled and how this threshold may vary by depth or land use. I will quantify soil ammonium and nitrate concentrations, anion exchange capacity, and maximum N sorption capability at different depths.
Are agricultural N losses significantly different between tropical and temperate croplands?
Increasing agricultural production over the coming decades will likely drive increased N fertilizer use in tropical regions with below-average yields or where cropland is likely to expand. However, N fertilizer use is very inefficient; only roughly half of N inputs reach crop biomass in a given year. Excess N inputs result in nitrate, nitric oxides, and nitrous oxide losses, which degrade water and air quality and contribute to climate change. This inefficiency has largely contributed to the anthropogenically-driven doubling of the global reactive N cycle. This project explores whether the relationship of N losses to N inputs differs between temperate and tropical regions and which environmental and management factors control N losses.
N losses from Perennial and Annual Wheat
Perennial grains have much deeper roots than annual gains which enables them to access deeper water and nutrient sources. Lower soil moisture and nitrate concentrations in perennial systems will likely reduce N leaching. Reduced N leaching losses coupled with a more robust soil organic pool may enable perennial grain systems to use N fertilizer more efficiently than annual systems. However, more research is needed to test the potential N efficiency of perennial wheat systems. In this study, we aim to: track the total fertilizer recovery and fate of 15N-labeled fertilizer to various pools, quantify annual NO3–-N leaching losses, and monitor soil N dynamics through one season.