Nitrogen balance in Iowa and the implications of corn-stover harvesting

• Manure-treated corn fields in Iowa pose an elevated risk to water quality.
• Harvesting corn stover lowers leachable N in the northern tier of Iowa counties.
• Harvesting corn stover depletes soil N in the southern tier of Iowa counties.
• The N balance in manure-treated corn fields allows sustainable corn stover removal.

Increased corn production and removal of corn stover for biofuel production can adversely affect water quality, soil fertility and productivity due to low nitrogen (N) use efficiency. In this study, the average annual county-level N balances in Iowa are calculated for three corn-involved rotations: corn-soybean (C-S), corn-corn-soybean (C-C-S) and continuous corn (C-C), receiving either synthetic-N or manure fertilizer under 0, 30, 50 and 75% corn stover removal scenarios. Geo-referenced data on soil, crop and livestock are used to estimate net changes in total N balance in the mineral form after accounting all soil N inflows and outflows. Under a zero stover removal scenario, a state average for net N was 34 kg ha−1 yr−1. Approximately 86% of the land area in the three corn-involved rotations receives synthetic-N fertilizer, and 24% of total synthetic-N treated land is estimated with net N of 24 kg ha−1 yr−1 (i.e., average net N for synthetic-N treated rotations) or more. Manure-treated rotations are estimated to have 2–6 times higher net N than synthetic-N treated rotations; continuous corn rotations contributing to a higher net N. The northern and central crop districts dominated by animal and corn production have higher net N. Removal of corn stover reduces net N, and synthetic-N treated rotations are estimated to be affected the most. The percentage of total synthetic-N treated rotations estimated with net N of 24 kg ha−1 yr−1 or more lowers from 24% under no stover harvesting to 3% under 75% stover (mass basis) removal scenario. Conversely, the areas with negative net N increase from 1% with no stover harvesting to 10% under 75% harvesting. This study will help prioritize the regions in which management practices that reduce nitrogen loss are most needed, and those regions most suitable from a nutrient balance standpoint for sustainable stover harvesting.