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Title: Maximum soil organic carbon storage in Midwest U.S. cropping systems when crops are optimally nitrogen-fertilized

item POFFENBARGER, HANNA - Iowa State University
item BARKER, DANIEL - Iowa State University
item HELMERS, MATTHEW - Iowa State University
item MIGUEZ, FERNANDO - Iowa State University
item Olk, Daniel - Dan
item SAWYER, JOHN - Iowa State University
item SIX, JOHAN - Swiss Federal Institute Of Technology Zurich
item CASTELLANO, MICHAEL - Iowa State University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2017
Publication Date: 3/1/2017
Citation: Poffenbarger, H.J., Barker, D.W., Helmers, M.J., Miguez, F.E., Olk, D.C., Sawyer, J.E., Six, J., Castellano, M.J. 2017. Maximum soil organic carbon storage in Midwest U.S. cropping systems when crops are optimally nitrogen-fertilized. PLoS One. doi: 10.1371/journal.pone.0172293.

Interpretive Summary: Natural organic matter plays important roles in soil performance and water quality. Its effectiveness is partially controlled by its quantity in soil. There is controversy whether the amount of natural organic matter in soil increases or instead decreases when nitrogen fertilizer is applied. Here we demonstrate that increasing application of nitrogen fertilizer in Iowa corn fields led to increased amounts of natural organic matter, as long as the fertilizer amount did not surpass the amount needed for maximum crop grain yield. These results will improve management of both nitrogen and natural organic matter in crop production fields. They are of interest to farmers and other land managers, soil conservationists, and researchers of natural organic matter, fertilizers, and soil ecology.

Technical Abstract: Nitrogen fertilizer is critical to optimize short-term crop yield, but its long-term effect on soil organic C (SOC) is actively debated. Using 60 site-years of maize (Zea mays L.) yield response to a wide range of N fertilizer rates in continuous maize and annually rotated maize-soybean [Glycine max (L.) Merr.] cropping systems of the U.S Midwest, we show that an increase in N fertilizer input from zero to the empirically determined Agronomic Optimum N Rate (AONR; the N rate that maximizes grain yield) does not reduce SOC within the 0-15 cm depth. In contrast, N fertilizer was required to maintain or increase SOC. Across locations spanning a broad range of climates and soil types, SOC changes over time at the AONR were positive in continuous maize and neutral in the maize-soybean rotation. However, in both systems, zero N input resulted in negative SOC balances. The positive response of SOC change to N rate was minor for rotated maize-soybean. Conversely, for continuous maize, the rate of SOC gain over time increased significantly with N rate, reaching a maximum near the AONR, and then declining as N rates exceeded the AONR. Our results demonstrate that uncertainty about the effect of N fertilizer on SOC can be resolved by relating N fertilizer application to the yield-optimizing N rate. When managed for optimum crop yield, N fertilizer has no negative effect on SOC and is critical to maintain or increase SOC.