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Recent soil and crop management technologies have potential for mitigating greenhouse gas emissions; however, these management strategies must be profitable if they are to be adopted by producers. The economic feasibility of reducing net greenhouse gas emissions in irrigated cropping systems was evaluated for 5 yr on a Fort Collins clay loam soil (a fine-loamy, mixed, superactive, mesic Aridic Haplustalf). Cropping systems included conventional tillage continuous corn (Zea mays L.) (CT-CC), no-till continuous corn (NT-CC), and no-till corn--bean (NTCB) including 1 yr soybean [Glycine max (L.) Merr.] and 1 yr dry bean (Phaseolus vulgaris L.). The study included six N fertilization rates ranging from 0 to 246 kg [ha.sup.-1]. Results showed highest average net returns for NT-CB, exceeding net returns for NT-CC and CT-CC by US$182 and US$228 [ha.sup.-1], respectively at economically optimum N fertilizer rates. Net global warming potential (GWP) generally increased with increasing N fertilizer rate with the exception of NT-CC, where net GWP initially declined and then increased at higher N rates. Combining economic and net GWP measurements showed that producers have an economic incentive to switch from CT-CC to NT-CB, increasing annual average net returns by US$228 [ha.sup.-1] while reducing annual net GWP by 929 kg C[O.sub.2] equivalents [ha.sup.-1]. The greatest GWP reductions (1463 kg C[O.sub.2] equivalents [ha.sup.-1]) could be achieved by switching from CT-CC to NT-CC while also increasing net returns, but the presence of a more profitable NT-CB alternative means NT-CC is unlikely to be chosen without additional economic incentives. Abbreviations: CB, corn--bean rotation; CC, continuous corn; CT, conventional tillage; GHG, greenhouse gas; GHGI, greenhouse gas intensity; GWP, global warming potential; NT, no-till; SOC, soil organic carbon. doi:10.2136/sssaj2009.0080

Nitrogen fertilization is essential for optimizing crop yields; however, it increases [N.sub.2]O emissions. The study objective was to compare [N.sub.2]O emissions resulting from application of commercially available enhanced-efficiency, N fertilizers with emissions from conventional dry granular urea in irrigated cropping systems. Nitrous oxide emissions were monitored from corn (Zea mays L.) based rotations receiving fertilizer rates of 246 kg N [ha.sup.-1] when in corn, 56 kg N [ha.sup.-1] when in dry bean (Phaseolus vulgaris L.), and 157 kg N [ha.sup.-1] when in barley (Hordeum vulgare L. ssp. vulgare). Cropping systems included conventional-till continuous corn (CT-CC), no-till continuous corn (NT-CC), no-till corn-dry bean (NT-CDb), and no-till corn-barley (NT-CB). In the NT-CC and CT-CC systems, a controlled-release, polymer-coated urea (ESN) and dry granular urea were compared. In the NT-CDb and NT-CB rotations, a stabilized urea source (SuperU) was compared with urea. Nitrous oxide fluxes were measured during two growing seasons using static, vented chambers and a gas chromatograph analyzer. Cumulative growing season [N.sub.2]O emissions from urea and ESN application were not different under CT-CC, but ESN reduced [N.sub.2]O emissions 49% compared with urea under NT-CC. Compared with urea, SuperU reduced [N.sub.2]O emissions by 27% in dry bean and 54% in corn in the NT-CDb rotation and by 19% in barley and 51% in corn in the NT-CB rotation. This work shows that the use of no-till and enhanced-efficiency N fertilizers can potentially reduce [N.sub.2]O emissions from irrigated systems. Abbreviations: CT, conventional-till; CT-CC, conventional-till continuous corn; DOY, day of the year; NT, no-till; NT-CC, no-till continuous corn; NT-CB, no-till corn--barley; NT-CDb, no-till corn--dry bean; WFPS, water-filled pore space. doi:10.2136/sssaj2009.0072

There are concerns that restored grasslands currently under the Conservation Reserve Program (CRP) will experience increased soil erosion when they are returned to crop production. Our objective was to compare runoff, erosion, and erodibility on CRP land converted to annual hay production (permanent hayed, PH) and crop production under conventional-till (preplant disk tillage, CT) and no-till (NT) management. Erosion study was conducted in central North Dakota on Typic Argiustoll soil 6 yr after the CRP land had been converted to hay production and crop production with a spring wheat (Triticum aestivum L.)--winter wheat-dry pea (Pisum sativum L.) rotation. Runoff volumes and soil loss (by alum precipitation of sediment) were measured on 1.5 by 5 m bordered plots on 4% slope under a rainfall simulator delivering 1-to 3-h rains at 50 or 75 mm [h.sup.-1], followed by three rains of 20 min or less at rates from 25 to 125 mm [h.sup.-1.] Erodibility was calculated from ratios and regressions of soil loss rates versus runoff rates measured at relative steady state. Runoff rates from 50 and 75 mm [h.sup.-1] rains for CT, NT, and PH averaged 9, 12, and 21 mm [h.sup.-1], respectively, and supported soil loss rates of 20, 7, and 8 g [m.sup.-2] [h.sup.-1]. Erodibility of undisturbed CT, NT, and PH was 1.65, 0.29, and 0.28 g [m.sup.-2] [mm.sup.-1], respectively, showing NT did not differ from PH and that CT management increased erodibility six-fold above PH. Thorough disk tillage increased erodibility three-fold over CT, 15-fold over NT, and nine-fold over PH. Complete, nondisturbing residue removal increased erodibility less than tillage, from 1.2 times for CT to 2.5 times for NT. Chemically weeded NT exhibited the same low erodibility as the grassland PH treatment under the conditions of study--4% land slope, above average precipitation, and a residue-productive crop rotation. However, erodibility of tilled NT was significantly higher than that of tilled PH, showing the higher inherent stability of grassland surface soil with its perennial plant rant structures.

Soil C sequestration can improve soil quality and reduce agriculture's contribution to C[O.sub.2] emissions. The long-term (12 yr) effects of tillage system and N fertilization on crop residue production and soil organic C (SOC) sequestration in two dryland cropping systems in North Dakota on a loam soil were evaluated. An annual cropping (AC) rotation [spring wheat (SW) (Triticum aestivum L.)-winter wheat (WW)-sunflower (SF) (Helianthus annuus L.)] and a spring wheat-fallow (SW-F) rotation were studied. Tillage systems included conventional-till (CT), minimum-till (MT), and no-till (NT). Nitrogen rates were 34, 67, and 101 kg N [ha.sup.-1] for the AC system and 0, 22, and 45 kg N [ha.sup.-1] for the SW-F system. Total crop residue returned to the soil was greater with AC than with SW-F. As tillage intensity decreased, SOC sequestration increased (NT > MT > CT) in the AC system but not in the SW-F system. Fertilizer N increased crop residue quantity returned to the soil, but generally did not increase SOC sequestration in either cropping system. Soil bulk density decreased with increasing tillage intensity in both systems. The results suggest that continued use of a crop-fallow farming system, even with NT, may result in loss of SOC. With NT, an estimated 233 kg C [ha.sup.-1] was sequestered each year in AC system, compared with 25 kg C [ha.sup.-1] MT and a loss of 141 kg C [ha.sup.-1] with CT. Conversion from crop-fallow to more intensive cropping systems utilizing NT will be needed to have a positive impact on reducing C[O.sub.2] loss from croplands in the northern Great Plains.

No-till (NT) increases the potential to crop more frequently in the Great Plains than with the conventional-till (CT) crop-fallow farming system. More frequent cropping requires N input to maintain economical yields. We evaluated the effects of N fertilization on crop residue production and its subsequent effects on soil organic C (SOC) and total soil N (TSN) in a dryland NT annual cropping system. Six N rates (0, 22, 45, 67, 90, and 134 kg N ha(super -1) were applied to the same plots from 1984 through 1994, except 1988 when rates were reduced 50%, on a Weld silt loam (fine, smectitic, mesic Aridic Argiustoll). Spring barley (Hordeum vulgare L.), corn (Zea mays L.), winter wheat (Triticum aestivum L.), and oat (Avena sativa L.)-pea (Lathyrus tingitanus L.) hay were grown in rotation. Crop residue production varied with crop and year. Estimated average annual aboveground residue returned to the soil (excluding hay years) was 2925, 3845, 4354, 4365, 4371, and 4615 kg ha(super -1), while estimated annual contributions to belowground (root) residue C were 1060, 1397, 1729, 1992, 1952, and 2031 ka C ha (super -1) for the above N rates, respectively. The increased amount of crop residue returned to the soil with increasing N rate resulted in increased SOC and TSN levels in the 0- to 7.5-cm soil depth after 11 crops. The fraction of applied N fertilizer in the crop residue decreased with increasing N rate. Soil bulk density (D(sub b) in the 0- to 7.5-cm soil depth decreased as SOC increased. The increase in SOC with N fertilization contributes to improved soil quality and productivity, and increased efficiency of C sequestration into the soil. Carbon sequestration can be enhanced by increasing crop residue production through adequate N fertility.

Nitrogen-mineralization rates are needed to accurately determine N fertilization requirements to meet plant needs while minimizing environmental contamination. A spring wheat (Triticum aestivum L.)-fallow (SW-F) system was compared with a spring wheat-winter wheat-sunflower (Helianthus annuus L.) (SW-WW-SF) system on a Temvik-Wilton silt loam (fine-silty, mixed Typic and Pachic Haploborolls) at three N rates (0, 22, and 45 kg ha J for SW-F and 34, 67, and 101 kg [ha.sup.-1] for SW-WW-SF) under conventional, minimum, and no-tillage. After 10 yr, soil samples were incubated to determine N-mineralization rates. Cropping intensity, N rate, and tillage intensity interacted to affect N-mineralization rates. Within the SW-F system N-mineralization rates in 0- to 0.05-m depth were 8.2 [+ or -] 0.8 kg [ha.sup.-1] [wk.sup.-1] in the fallow phase vs. 5.0 [+ or -] 0.7 kg [ha.sup.-1] [wk.sup.-1] in the crop phase under conventional tillage and were 6.2 [+ or -] 0.3 kg [ha.sup.-1] [wk.sup.-1] under minimum and no-tillage in both phases. The N-mineralization rates were 2.3 [+ or -] 0.4 kg [ha.sup.-1] [wk.sup.-1] in 0.05- to 0.15-m depth soils of the SW-F system. in spring wheat, N-mineralization rates in 0- to 0.05-m depth soil were 9.9 [+ or -] 0.8 kg [ha.sup.-1] [wk.sup.-1] in the SW-WW-SF system vs. 5.6 [+ or -] 0.4 kg [ha.sup.-1] [wk.sup.-1] in the SW-F system and in the 0.05- to 0.15-m depth were 3.6 [+ or -] 0.1 kg [ha.sup.-1] [wk.sup.-1] in the SW-WW-SF system vs. 2.4 [+ or -] 0.2 kg [ha.sup.-1] [wk.sup.-1] in the SW-F system Within the SW-WW-SF system, N-mineralization rates in the 0- to 0.05-m soil layer were 6.8 [+ or -] 0.5 kg [ha.sup.-1] [wk.sup.-1] under winter wheat vs. 9.9 [+ or -] 0.8 kg [ha.sup.-1] [wk .sup.-1] under spring wheat and 9.2 [+ or -] 0.6 kg [ha.sup.-1] [wk.sup.-1] under sunflower. In the 0.05- to 0.15-m soil layer, N-mineralization rates were 3.3 [+ or -] 1.0 kg [ha.sup.-1][wk.sup.-1]. More intensive cropping and conservation tillage increased N-mineralization rates in this soil and may ameliorate the decline in N fertility associated with crop-fallow systems.

We hypothesized that drought accelerates wind erosion by increasing plant and soil factors of erodibility together, compounding the erosion hazard. Erodibility factors measured in biennial spring wheat--fallow on Pachic and Typic Haploborolls soil were (i) soil-inherent wind erodibility (SIWE) by rotary sieving, (ii) surface roughness by pin meter and chain methods, (iii) standing residue profile, and (iv) residue coverage photographically. Four tillage treatments ranged from low residue (LR) to no-till (NT). The erodible fraction of surface soil (a SIWE measure) changed from 53% during a dry period (1989-1990) to a less erodible 26% during a wet period (1992-1994). Median erosion protection values calculated from flat and standing residue measurements made after seeding were, respectively, 16 and 43% in 1989 to 1990, and 80 and 76% in 1992 to 1994. Soil losses estimated by RWEQ model equations were 11 to 6100 times greater during 1989 to 1990, compared with 1992 to 1994. No-till was protective, and estimated soil losses on LR were up to 3000 times greater than those on NT. However, low residue yields in 1988 (930 vs. 3640 kg/ha avg.) resulted in inadequate protection after seeding in 1990, even in NT; and soil losses in LR and NT were 13 and 8 Mg/ha, respectively. Results indicate biennial small grain-fallow is nonsustainable in the long term from a soil-erosion perspective.