Soil CO2 and N2O fluxes from simple and diversified crop rotations in the Central Corn Belt

Agriculture in the United States has become highly productive but environmental consequences remain. Agriculture makes up a disproportionate share of net US greenhouse gas emissions compared to its contribution to the economy; the issue may be related to the decrease in crop diversity and reliance on synthetic fertilizer. In the US Corn Belt, crop rotational diversity is mostly limited to maize (Zea mays) and soybean (Glycine max). We compared soil carbon dioxide (CO₂) and nitrous oxide (N₂O) fluxes from a long-term experiment comparing the 2-year maize–soybean rotation to two other more diversified rotations: a 3-year maize–soybean–oat (Avena sativa)/red clover (Trifolium pratense) and a 4-year maize–soybean–oat/alfalfa (Medicago sativa]–alfalfa rotations. Both 3- and 4-year rotations also received composted cattle (Bos taurus) manure. We tested whether these more diversified rotations that replace a portion of the synthetic fertilizer with organic sources could decrease CO₂ and N₂O losses. Soil CO₂ fluxes in the 3- or 4-year rotations were 36% and 54% greater than in the 2-year rotation, driven by the maize phase, which might be due to the prior years’ leguminous crops (red cover or alfalfa), tillage, and manure. The crop phases within a rotation had significant effect on soil CO₂ (alfalfa > oat > maize = soybean) and N₂O (maize = alfalfa > soybean = oat) fluxes. Soil temperature–crop phase interactions had more control over soil CO₂ fluxes than soil moisture. In the Central Corn Belt of the United States, replacing fertilizer-N supported maize–soybean rotation with diversified rotation and replacing inorganic N with an organic N source increased soil CO₂ flux but did not affect N₂O flux.