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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #300965

Title: Denitrification and N2O emissions in annual croplands, perennial grass buffers, and restored perennial grasslands

item IQBAL, JAVED - Iowa State University
item Parkin, Timothy
item HELMERS, MATTHEW - Iowa State University
item ZHOU, XIAOBO - Monsanto Corporation
item CASTELLANO, MICHAEL - Iowa State University

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2014
Publication Date: N/A
Citation: N/A

Interpretive Summary: Vegetative filter strips next to cropland have been shown to reduce nitrate contamination of surface waters. A primary way this is accomplished is by promoting the bacterial process, denitrification, whereby nitrate is converted to the gasses nitrous oxide (N2O) and dinitrogen (N2). Because N2O is a potent greenhouse gas, there is concern that enhanced denitrification in vegetative filter strips may solve a water polution problem, but may contribute to the global warming problem. This research found that while denitrification was higher in the vegetative filter strips, elevated N2O emissions did not occur. This result can be explained by the more complete denitrification to the final gaseous product, dinitrogen gas. Further, it is speculated that the more complete denitrification process that occurs in the vegetative filter strip soil is due to higher organic availablity to the denitrifying bacteria. This information should be of use to policy makers and other scientists.

Technical Abstract: Inclusion of perennial vegetation filter strips (PFS) in the toeslope of annual cropland watersheds can decrease nitrate (NO3) losses to ground and surface waters. Although PFS are similar to riparian buffers, PFS are a relatively new conservation tool and the processes responsible for NO3 removal from these systems are not well understood. Our objectives were to: i) determine the importance of denitrification as a sink for NO3 loss from PFS, and ii) evaluate how PFS alter the biophysical processes that affect the relative importance of nitrous oxide (N2O) and dinitrogen (N2) emissions. We hypothesized that i) potential denitrification in PFS is greater than cropland, and ii) PFS promote more complete denitrification to N2 (i.e., lower N2O emissions despite higher potential denitrification). To test these hypotheses, we used a coupled field-laboratory approach using experimental watersheds that were in place for 5 years and included the following treatments: (1) PFS covering the bottom 10% of the watershed and a corn-soybean rotation covering the remaining upslope 90% (PFS); (2) 100% corn-soybean rotation (corn); and (3) 100% restored native grassland (RNG). In situ N2O fluxes, and laboratory N2O/(N2+N2O) ratios were highest in corn watersheds followed by PFS and RNG watersheds. In contrast, potentially mineralizable C and denitrification enzyme activity (DEA) were highest in PFS and RNG watersheds. Furthermore, there was a negative correlation between N2O/(N2+N2O) ratio and DEA. In the laboratory, N2 fluxes measured by 15N tracer and by acetylene inhibition (C2H2) were highest in PFS followed by RNG and corn. These results suggest PFS function similar to riparian buffers and have potential to reduce NO3 losses from annual croplands when hydrological connectivity is maintained.