Denitrification Patterns Across a Dryland Agroecosystem in the Northern Great Plains
Abstract
Nitrogen loss from cultivated soils threatens the economic and environmental sustainability of agriculture. Nitrate accumulated from fertilization or mineralization/nitrification of soil organic matter (SOM) may be lost from soils to denitrification, producing the greenhouse gas N2O. Nitrate accumulated in soils and not taken up by crops is also subject to leaching loss, which can degrade water quality and is subject to denitrification downstream. Here we use patterns in the isotopic compositions of nitrate and water to characterize the influence of nitrogen loss to denitrification within soils, groundwater, and streams (process domains) of a non-irrigated agroecosystem in the northern Great Plains. Within a relatively simple conceptual framework of water and dissolved organic carbon (DOC) availability, the isotopic character of nitrate shows a remarkably clear influence of denitrification across process domains, expressed as positive correlation between δ15N and δ18O in nitrate. The apparent δ15N fractionation factor (ɛ) with decreasing nitrate concentration - interpreted as further evidence of denitrification - is lowest in soils, where it may be suppressed by heterogeneous water and carbon distribution in the soil environment as well as by inmixing of nitrate from fertilization and mineralization/nitrification of SOM in-situ. Even so, a clear denitrification signal was observed during fallow (no crop growing) periods in soils with architecture characterized by thicker fine textured surface horizons. The clearest denitrification signals and largest ɛ occurred at riparian sites where saturation limits oxygen supply, DOC is abundant, and in-situ nitrate sources are minimal compared to inputs from incoming groundwater. This study reveals how soil water content and groundwater recharge, affected by crop rotation, influence landscape scale patterns of denitrification. Understanding these controls on patterns in denitrification will be critical for managing the role of agricultural ecosystems in the global N cycle and the climate system.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H51G..04S
- Keywords:
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- 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES;
- 1630 Impacts of global change;
- GLOBAL CHANGE;
- 1831 Groundwater quality;
- HYDROLOGY;
- 1871 Surface water quality;
- HYDROLOGY