ARS | Publication request: Nitrate-nitrogen and oxygen isotope ratios for identification of nitrate sources and dominant nitrogen cycle processes in a tile-drained dryland agricultural field

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 14, 2012
Publication Date: November 8, 2012
Citation: Kelley, C.J., Keller, K.C., Evans, R.D., Orr, C.H., Smith, J.L., Harlow, B.A. 2012. Nitrate-nitrogen and oxygen isotope ratios for identification of nitrate sources and dominant nitrogen cycle processes in a tile-drained dryland agricultural field. Soil Biology and Biochemistry. 57:731-738.

Interpretive Summary: Agricultural systems are a leading source of reactive nitrogen to aquatic and atmospheric ecosystems. The processes that govern the amount of reactive nitrogen reaching these ecosystems was studied over a 5 year period in an agricultural watershed. We found that most of the nitrogen leaving the system from tile drains into waterways was from the conversion of fertilizer nitrogen to nitrate which was leached from the soil. A small amount was from the conversion of soil organic matter nitrogen to nitrate and leached from the soil. Our conclusion was that the hydrology of watersheds is important to the understanding of nitrogen reaching aquatic systems. This information will be useful for scientists investigating the fate of nitrogen in agricultural and natural ecosystems.

Technical Abstract: Agricultural systems are a leading source of reactive nitrogen to aquatic and atmospheric ecosystems. Natural d15Nnitrate and d18Onitrate are used to identify the dominant nitrogen cycle processes and sources of NO3- leached from a tile-drained, dryland agricultural field. Tile-drain water discharge d18Onitrate suggest nitrification is the dominant soil nitrogen cycle process throughout the 5-year study period, since the expected d18Onitrate from nitrification is -2 ‰ and the measured value was -1.3 ± 1.5 ‰. There is no evidence that denitrification was occurring at a large enough scale to influence [NO3-], based on the constant d18Onitrate. d15Nnitrate varied seasonally during the high-discharge season (January through May) and low-discharge season (June through December) with weighted means of 1.0 ± 1.0 ‰ and 4.7 ± 2.3 ‰ respectively. This suggests, that during the high-discharge season NO3- originates from nitrification of NH4+ fertilizer, and during the low-discharge season NO3- originates from nitrification of mineralized soil organic nitrogen. This study suggests that understanding the hydrology of a region is necessary before dominant nitrogen cycling processes can be accurately determined.