An evaluation of nitrogen indicators for soil health in long-term agricultural experiments

Authors: LIPTZIN, DANIEL - Soil Health Institute; RIEKE, ELIZABETH - Soil Health Institute; CAPPELLAZZI, SHANNON - Soil Health Institute; MAC BEAN, G - Soil Health Institute; COPE, MICHAEL - Soil Health Institute; GREUB, KELSEY L. - Soil Health Institute; NORRIS, CHARLOTTE - Soil Health Institute; TRACY, PAUL - Soil Health Institute; ABERLE, PAUL - North Dakota State University; Ashworth, Amanda; Baumhardt, Roland - Louis; Dell, Curtis; Derner, Justin; Ducey, Thomas; Dungan, Robert - Rob; Fortuna, Ann Marie; Franzluebbers, Alan; Kautz, Mark; Kitchen, Newell; Leytem, April; Liebig, Mark; Moore, Philip; Osborne, Shannon; Owens, Phillip; Sainju, Upendra; Sherrod, Lucretia; Watts, Dexter

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2023
Publication Date: 7/12/2023
Citation: Liptzin, D., Rieke, E.L., Cappellazzi, S.B., Mac Bean, G., Cope, M., Greub, K.H., Norris, C.E., Tracy, P.W., Aberle, P.W., Ashworth, A.J., Baumhardt, R.L., Dell, C.J., Derner, J.D., Ducey, T.F., Dungan, R.S., Fortuna, A., Franzluebbers, A.J., Kautz, M.A., Kitchen, N.R., Leytem, A.B., Liebig, M.A., Moore Jr, P.A., Osborne, S.L., Owens, P.R., Sainju, U.M., Sherrod, L.A., Watts, D.B. 2023. An evaluation of nitrogen indicators for soil health in long-term agricultural experiments. Soil Science Society of America Journal. 87(4):868-884. https://doi.org/10.1002/saj2.20558.
DOI: https://doi.org/10.1002/saj2.20558

Interpretive Summary: Understanding a soils' nitrogen status is key to identifying soil function, nutrient cycling, and potential carbon storage, all of which are related to soil health or a soils ability to function as a vital living ecosystem and sustain crop production. Therefore, researchers across North America set out to evaluate five soil nitrogen lab methods as a proxy for soil health on 124 locations across Canada, Mexico, and the United States. First, researchers explored the spatial differences among sites and evaluated the relationships between indicators and inherent site factors (climate, soil texture, and pH). Second, they tested if the indicators were responsive to long-term conservation agriculture practices. Finally, they explored the relationships among the nitrogen soil health indicators. Study results found that each of the five soil nitrogen associated soil health indicators responded to long-term conservative agriculture practices. Specifically, all soil nitrogen measurements increased in response to decreased tillage, cover cropping, retaining residue, and using organic nutrients such as animal manures. Across all sites, values decreased with temperature and increased with precipitation and soil clay content. Nitrogen measurements and carbon indicators were related, suggesting that measuring nitrogen in addition to carbon indicators would be redundant for evaluating soil health in most producers’ fields. Study results allow for a continental-level assessment of soil nitrogen statuses and provides guidance on appropriate nitrogen indicators for improved soil health management across North America.

Technical Abstract: The cycling of nitrogen (N) to support crop production is a critical soil functions. Various soil health indicators have been proposed to quantify the potential to supply N to crops that measure a chemically defined fraction of N or a process related to N cycling. We compared five indicators (total N, autoclavable citrate extractable N, water extractable organic N, potential N mineralization, and N-Acetyl-'-D-glucosaminidase activity) at 124 sites across North America with long term experiments with a variety of treatments related to soil health. We found that the variability in N indicators was predominantly at the site level with indicator values decreasing with temperature and increasing with precipitation and clay content. The indicators generally increased in response to decreasing tillage, cover cropping, retaining residue, and using organic nutrients. The indicators were also higher in plots receiving optimal fertilization compared to unfertilized controls. On average, the indicators declined in response to more diverse rotations, typically corn-soy compared to continuous corn. The N indicators were highly correlated to C indicators and the CN ratio of the total soil pools and water extractable pools never responded to management. The most consistent result was that increasing the amount of organic inputs whether from residue retention, cover cropping, or rotations with higher residue, resulted in higher values of the N indicators. The N indicators responded to management in similar ways, but they vary in their cost and availability in commercial labs. However, given the strong relationships of these N indicators with C indicators, measuring soil organic carbon (or total N) along with a 24 hour potential carbon mineralization assay could be used as a proxy for N supply instead of measuring potential N mineralization or any other N indicator directly.