Management practices can influence soil CO(2) emission and C content in cropland, which can effect global warming. We examined the effects of combinations of irrigation, tillage, cropping systems, and N fertilization on soil CO(2) flux, temperature, water, and C content at the 0- to 20-cm depth from May to November 2005 at two sites in the northern Great Plains. Treatments were two irrigation systems (irrigated vs. non-irrigated) and six management practices that contained tilled and no-tilled malt barley (Hordeum vulgaris L.) with 0 to 134 kg N ha(-1), no-tilled pea (Pisum sativum L.), and a conservation reserve program (CRP) planting applied in Lihen sandy loam (sandy, mixed, frigid, Entic Haplustolls) in western North Dakota. In eastern Montana, treatments were no-tilled malt barley with 78 kg N ha(-1), no-tilled rye (Secale cereale L.), no-tilled Austrian winter pea, no-tilled fallow, and tilled fallow applied in dryland Williams loam (fine-loamy, mixed Typic Argiborolls). Irrigation increased CO(2) flux by 13% compared with non-irrigation by increasing soil water content in North Dakota. Tillage increased CO(2) flux by 62 to 118% compared with no-tillage at both places. The flux was 1.5- to 2.5-fold greater with tilled than with non-tilled treatments following heavy rain or irrigation in North Dakota and 1.5- to 2.0-fold greater with crops than with fallow following substantial rain in Montana. Nitrogen fertilization increased CO(2) flux by 14% compared with no N fertilization in North Dakota and cropping increased the flux by 79% compared with fallow in no-till and 0 kg N ha(-1) in Montana. The CO(2) flux in undisturbed CRP was similar to that in no-tilled crops. Although soil C content was not altered, management practices influenced CO(2) flux within a short period due to changes in soil temperature, water, and nutrient contents. Regardless of irrigation, CO(2) flux can be reduced from croplands to a level similar to that in CRP planting using no-tilled crops with or without N fertilization compared with other management practices.

Little information exists about how global warming potential (GWP) is affected by management practices in agroecosystems. We evaluated the effects of irrigation, tillage, crop rotation, and N fertilization on net GWP and greenhouse gas intensity (GHGI or GWP per unit crop yield) calculated by soil respiration (GWP and GHGI) and organic C (SOC) (GWP and GHGI) methods after accounting for CO emissions from all sources (irrigation, farm operations, N fertilization, and greenhouse gas [GHG] fluxes) and sinks (crop residue and SOC) in a Lihen sandy loam from 2008 to 2011 in western North Dakota. Treatments were two irrigation practices (irrigated vs. nonirrigated) and five cropping systems (conventional-till malt barley [ L.] with N fertilizer [CTBN], conventional-till malt barley with no N fertilizer [CTBO], no-till malt barley-pea [ L.] with N fertilizer [NTB-P], no-till malt barley with N fertilizer, and no-till malt barley with no N fertilizer [NTBO]). While CO equivalents were greater with irrigation, tillage, and N fertilization than without, NO and CH fluxes were 2 to 218 kg CO eq. ha greater in nonirrigated NTBN and irrigated CTBN than in other treatments. Previous year's crop residue and C sequestration rate were 202 to 9316 kg CO eq. ha greater in irrigated NTB-P than in other treatments. Compared with other treatments, GWP and GWP were 160 to 9052 kg CO eq. ha lower in irrigated and nonirrigated NTB-P. Similarly, GHGI and GHGI were lower in nonirrigated NTB-P than in other treatments. Regardless of irrigation practices, NTB-P may lower net GHG emissions more than other treatments in the northern Great Plains.

by reducing postharvest NO 3 levels (Karlen et al., 1996). However, N application rates are difficult to optimize, A better understanding of how N management practices affect owing to the uncertainty inherent in predicting net mintransformations and movement of fertilizer N may lead to more effieralization of soil organic N, which may vary greatly cient N management. The objectives of this work were to determine how long-term N fertilizer history in a continuous corn (Zea mays L.) regarding the effects of rate and time of N application Published in Agron.

and Pierre, 1977;Di and Cameron, 2000). The effectiveness of this strategy was demonstrated by data presented Increased fertilizer N uptake efficiency (FNUE) leads to more in a companion paper (Stevens et al., 2005) where posteconomical corn (Zea mays L.) production and lower environmental impact. Excessive N application reduces FNUE and may affect subse-harvest profile concentrations of mineral N were requent crop response through its influence on NO 3 -N carryover and ported to have increased by 195 to 373% when N applithe amount of readily mineralizable organic N in the soil. Our objeccations exceeded the long-term average optimum N rate. tive was to determine how prior fertilizer N application rate affectsUnfortunately, optimum N rates are difficult to pre-(i) grain yield and agronomic optimum N rate, (ii) contributions of dict for a particular site and year because of variability fertilizer-and soil-derived N to N uptake, and (iii) FNUE. Labeled in soil moisture content and temperature, which greatly 15 NH 4 15 NO 3 was applied at 0, 67, 134, 201, or 268 kg N ha Ϫ1 to subplots affect microbial N transformations (Franzluebbers et within a continuous corn long-term N rate study. Estimates of FNUE al. , 1995;Weinhold and Halvorson, 1999). This difficulty were higher by the difference method (49-69%) than with the isotope is particularly serious in the UMRB, owing to a highly ( 15 N) method (31-37%), and different trends were observed with each variable climate and the widespread occurrence of soils method as N application rate increased. The disparity between methods is consistent with a differential effect of long-term N application having high organic matter content. A yield-based aprate on mineralization-immobilization. Recovery of labeled N from proach is often used in this region to make fertilizer N the plant-soil system ranged from 71% at the 67 kg ha Ϫ1 N application recommendations for corn although an economic yield rate to 64% at the 201 kg ha Ϫ1 application rate. Fertilizer N accounted response is not always obtained. This was the case, for for an increasing proportion of crop N uptake as the N rate was in-

No abstract