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Management practices such as fertilizer or tillage regime may affect nitrous oxide (N₂O) emissions and crop yields, each of which is commonly expressed with respect to area (e.g., kg N ha or Mg grain ha). Expressing N₂O emissions per unit of yield can account for both of these management impacts and might provide a useful metric for greenhouse gas inventories by relating N₂O emissions to grain production rates. The objective of this study was to examine the effects of long-term (>17 yr) tillage treatments and N fertilizer source on area- and yield-scaled N₂O emissions, soil N intensity, and nitrogen use efficiency for rainfed corn ( L.) in Minnesota over three growing seasons. Two different controlled-release fertilizers (CRFs) and conventional urea (CU) were surface-applied at 146 kg N ha(-1) several weeks after planting to conventional tillage (CT) and no-till (NT) treatments. Yield-scaled emissions across all treatments represented 0.4 to 1.1% of the N harvested in the grain. Both CRFs reduced soil nitrate intensity, but not N₂O emissions, compared with CU. One CRF, consisting of nitrification and urease inhibitors added to urea, decreased N₂O emissions compared with a polymer-coated urea (PCU). The PCU tended to have lower yields during the drier years of the study, which increased its yield-scaled N₂O emissions. The overall effectiveness of CRFs compared with CU in this study may have been reduced because they were applied several weeks after corn was planted. Across all N treatments, area-scaled N₂O emissions were not significantly affected by tillage. However, when expressed per unit yield of grain, grain N, or total aboveground N, N₂O emissions with NT were 52, 66, and 69% greater, respectively, compared with CT. Thus, in this cropping system and climate regime, production of an equivalent amount of grain using NT would generate substantially more N₂O compared with CT.

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Irrigated potato (Solanum tuberosum L.) production requires large inputs of N, and therefore has high potential for N loss including emissions of N2O. Two strategies for reducing N loss include split applications of conventional fertilizers, and single applications of polymer‐coated urea (PCU), both of which aim to better match the timing of N availability with plant demand. The objective of this 3‐yr study was to compare N2O emissions and potato yields following a conventional split application (CSA) using multiple additions of soluble fertilizers with single preplant applications of two different PCUs (PCU‐1 and PCU‐2) in a loamy sand in Minnesota. Each treatment received 270 kg of fertilizer N ha−1 per season. An unfertilized control treatment was included in 2 of 3 yr. Tuber yields did not vary among fertilizer treatments, but N2O emissions were significantly higher with CSA than PCU‐1. During 3 consecutive yr, mean growing season emissions were 1.36, 0.83, and 1.13 kg N2O‐N ha−1 with CSA, PCU‐1, and PCU‐2, respectively, compared with emissions of 0.79 and 0.42 kg N2O‐N ha−1 in the control. The PCU‐1 released N more slowly during in situ incubation than PCU‐2, although differences in N2O emitted by the two PCUs were not generally significant. Fertilizer‐induced emissions were relatively low, ranging from 0.10 to 0.15% of applied N with PCU‐1 up to 0.25 to 0.49% with CSA. These results show that N application strategies utilizing PCUs can maintain yields, reduce costs associated with split applications, and also reduce N2O emissions.

Quantifying N2O emissions from corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] fields under different fertilizer regimes is essential to developing national inventories of greenhouse gas emissions. The objective of this study was to compare N2O emissions in plots managed for more than 15 yr under continuous corn (C/C) vs. a corn–soybean (C/S) rotation that were fertilized during the corn phase with either anhydrous NH3 (AA) or urea (U). During three growing seasons, N2O emissions from corn following corn were nearly identical to corn following soybean. In both systems, however, N2O emissions with AA were twice the emissions with U. After accounting for N2O emissions during the soybean phase, it was estimated that a shift from C/S to C/C would result in an increase in annual emissions of 0.78 kg N ha−1 (equivalent to 0.11 Mg CO2–C ha−1) when AA was used, compared with only 0.21 kg N ha−1 (0.03 Mg CO2–C ha−1) with U. In light of trends toward increased use of U, these results suggest that fertilizer‐induced soil N2O emissions may decline in the future, at least per unit of applied N, although further study is needed in different soils and cropping systems. While soil CO2 emissions were 20% higher under C/C, crop residue from the prior year did not affect soil inorganic N or dissolved organic C during the subsequent season. We also compared different flux‐calculation schemes, including a new method for correcting chamber‐induced errors, and found that selection of a calculation method altered N2O emissions estimates by as much as 35%.

The "4R" approach of using the right rate, right source, right timing, and right placement is an accepted framework for increasing crop N use efficiency. However, modifying only one 4R component does not consistently reduce nitrous oxide (N 2 O) emissions. Our objective was to determine if N fertilizer applied in three split applications (Sp), by itself or combined with changes in N source and rate, could improve N recovery efficiency (NRE) and N surplus (NS) and decrease N 2 O emissions. Over two corn (Zea mays L.) growing seasons in Minnesota, N 2 O emissions ranged from 0.6 to 0.9 kg N ha -1. None of the treatment combinations affected grain yield. Compared with urea applied in a single application at the recommended N rate, Sp by itself did not improve NRE or NS and did not decrease N 2 O. Combining Sp with urease and nitrification inhibitors and/or a 15% reduction in N rate increased NRE from 57 to >73% and decreased NS by >20 kg N ha . The only treatment that decreased N 2 O (by 20-53%) was Sp combined with inhibitors and reduced N rate. Emissions of N 2 O were more strongly correlated with NS calculated from grain N uptake (R 2 = 0.61) compared with whole-plant N uptake (r 2 = 0.39), possibly because most N losses occurred before grain filling. Optimizing both application timing and N source can allow for a moderate reduction in N rate that does not affect grain yield but decreases N 2 O. Grain-based NS may be a more useful indicator of N 2 O emissions than whole-plant-based NS. Evaluation of Intensive "4R" Strategies for Decreasing Nitrous Oxide Emissions and Nitrogen Surplus in Rainfed CornRodney T. Venterea,* Jeffrey A. Coulter, and Michael S. Dolan O ptimizing the four basic aspects of N fertilizer management-the "4R" approach of using the right rate, right source, right timing, and right placement-is often recommended for increasing crop N use efficiency and decreasing soil N 2 O emissions (Snyder et al., 2009). However, modifying one of the 4R components by itself may not be reliable in reducing N 2 O emissions, particularly in rainfed cropping systems (Decock, 2014). For example, the use of delayed application and/or split application (Sp) while maintaining N rate, source, and placement (Phillips et al., 2009;Zebarth et al., 2008Zebarth et al., , 2012 or the use of specialized N fertilizer sources (e.g., urea containing microbial inhibitors [UI]) while maintaining N rate, timing, and placement (Parkin and Hatfield, 2014;Sistani et al., 2011) have been inconsistent in reducing N 2 O emissions.The inconsistency of single-modification strategies is likely due to interactions of crop, soil, and weather factors. Recent studies in Minnesota corn systems using broadcast urea (U) showed no effectiveness of inhibitors alone over 5 site-years (Maharjan and Venterea, 2013;Venterea et al., 2011a) or timing alone over 2 site-years . Few studies have attempted to optimize combinations of timing, source, and rate to maintain corn yield and decrease N 2 O. Burzaco et al. (2013Burzaco et al. ( , 2014) meas...

Few studies have examined the impacts of rotational tillage regimes on soil carbon (C) and nitrogen (N). We measured the C and N content of soils managed under corn (Zea mays L.)-soybean (Glycine max L.) rotation following 10 and 15 yr of treatments. A conventional tillage (CT) regime employing moldboard and chisel plowing in alternate years was compared with both continuous no-till (NT) and biennial tillage (BT), which employed chisel plowing before soybean only. While masses of C and N in the upper 0.3 m under both BT and NT were higher than CT, only the BT treatment differed from CT when the entire sampled depth (0.6 m) was considered. Decreased C inputs, as indicated by reduced grain yields, may have limited C storage in the NT system. Thus, while more C was apparently retained under NT per unit of C input, some tillage appears necessary in this climate and cropping system to maximize C storage. Soil carbon dioxide (CO 2) fluxes under NT were greater than CT during a drier than normal year, suggesting that C storage may also be partly constrained under NT due to wetter conditions that promote increased soil respiration. Increased temperature sensitivity of soil respiration with increasing soil moisture was also observed. These findings indicate that long-term biennial chisel plowing for corn-soybean in the upper mid-west USA can enhance C storage, reduce tillage-related fuel costs, and maintain yields compared with more intensive annual tillage.

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