Agricultural Management Effects on Nitrogen Cycling and Nitrous Oxide Emissions Across the Soil Profile

Abstract

In many agricultural systems, nitrogen (N) is the most common nutrient limiting plant growth, and widespread application of N fertilizers has contributed to huge gains in productivity across the world. This increase in crop production is necessary to feed a growing population and to mitigate the conversion of wild lands to crop lands. However, in parallel with increased N application and crop yields, N pollution has also increased rapidly. The ease of access and price of N fertilizers in many parts of the world, as well as the expense or time involved in the implementation of improved practices, results in over application of N in relation to plant demand. Excess N is highly susceptible to loss and contributes to a host of environmental problems such as eutrophication of water bodies, contamination of drinking water, loss of biodiversity and global warming. N pollution largely contributes to global warming through the production of nitrous oxide (N2O), since approximately 60% of global N2O emissions come from agriculture. N2O is a powerful greenhouse gas, with a global warming potential 280 times that of CO2. N2O emissions are difficult to manage because they represent a small fraction of applied N, are produced by multiple pathways and are highly heterogeneous in space and time. The case of irrigated rice production exemplifies well the challenges and trade-offs of managing for N2O emissions. For example, the efforts made with alternate water management strategies, aimed at mitigating methane emissions (10% of global emissions) and reducing water consumption (40% of global irrigation water consumption), can also lead to higher N2O emissions, thereby negating any net reduction in greenhouse gas emissions. This thesis has three foci: a) to evaluate agricultural management strategies to mitigate N2O emissions across different scales; b) to improve our mechanistic understanding of the processes underlying N2O emissions or more general N cycling under alternative water management in irrigated rice production and c) to advance natural abundance N2O isotopocule methodologies used to identify the processes involved in N2O production, N2O consumption and their relative contribution to total N2O emissions. In the first study (Chapter 2), I performed a meta-analysis to evaluate the efficacy of biochar to mitigate N2O emissions at the field scale in different cropping systems. Biochar is a carbon (C) rich, charcoal-like product formed during the pyrolysis of organic matter. In this meta-analysis, I found that biochar did decrease N2O emissions by 9.2 to 12.4% across cropping systems. However, the magnitude and significance of this reduction was affected by the weighting function (either by variance or by number of observations). Indeed, currently available field data is biased by a disproportionate number of studies in flooded rice systems (41% of observations), a low number of studies with a comparatively high number of treatments, and an uneven representation of biochars and N fertilizers used. My second, third and fourth studies (Chapters 3-5) were conducted over the course of two growing seasons in the Northern Italian rice-growing region. These studies set out to improve our understanding of how N cycling, N2O production and consumption processes change along a soil profile in relation to flooding and drainage events and ultimately how surface emissions are affected. To this end, I utilized an experimental platform with three water management treatments: water-seeded + conventional flooding (WS-FLD), water-seeded + alternate wetting and drying (WS-AWD), and direct dry seeding (DS-AWD). Soil environmental parameters as well as N and carbon (C) substrates were measured in parallel with N2O concentrations and isotopocule signatures of emitted N2O and pore air N2O. N2O emissions in both years were higher in the AWD treatments. These emissions were more affected by the mineralization of native soil N or availability or pre-season fertilizer N in the top soil rather than by fertilizer applied within the growing season. N2O production was most likely limited by nitrification and the production of NO3- or NO2-, but was ultimately derived from coupled nitrification-denitrification, from denitrification or from nitrifier-denitrification. Results of a dual isotope mapping and mixing model approach with site preference (SP) and δ18O-N2O ratios showed extensive N2O reduction in all treatments regardless of more aerobic conditions in the DS-AWD treatment. The isotope modeling approach allowed me to estimate N2O reduction to N2, a notoriously unconstrained aspect of the N cycle. Mean N2 emissions ranged from 245-333 g N ha-1d-1 (~ 29-40 kg N ha-1 per 120d season), thus representing a considerable loss relative to applied N. My final study (Chapter 5), evaluated the role of interactions between iron (Fe) and N reduction-oxidation reactions on NH4+ availability. NH4+ fixation was highest in the WS-FLD but more dynamic in the WS-AWD treatment, demonstrating an influential role of water management. Fe reduction and the likelihood of Fe-N redox reactions clearly increased in the WS treatments but I could not directly relate these processes to NH4+ concentrations. In summary, this thesis demonstrates the ability to detect significant impacts of management practices such as biochar additions or AWD irrigation on N2O emissions. I also showed that there is a real potential to use the SP-N2O and δ18O-N2O to source-partition N2O production and estimate N2 emissions, thereby better constraining N2O emissions and N2 losses. In irrigated rice systems, nitrification in the top soil appears to be a bottleneck for N2O production and N2 losses. Therefore, management to reduce nitrification such as delaying pre-plant fertilizer application or the use of slow release fertilizers may help to reduce N losses. Future work should investigate if processes such as NH4+ fixation can be beneficially manipulated to further regulate NH4+ availability in relation to plant demand, thereby mitigating N2O emissions and improving the nitrogen use efficiency of cropping systems.