Mark A. Liebig

Appropriateness of Management Zones for Characterizing Spatial Variability of Soil Properties and Irrigated Corn Yields across Years

zones in agricultural fields (Franzen et al., 2002). This approach has been applied in Illinois and Indiana where Recent precision-agriculture research has focused on use of man40% of grain yield variability was explained by topoagement zones (MZ) as a method for variable application of inputs like N. The objectives of this study were to determine (i) if landscape graphical characteristics and selected soil properties attributes could be aggregated into MZ that characterize spatial varia(Kravchenko and Bullock, 2000). Aerial photographs, tion in soil chemical properties and corn yields and (ii) if temporal crop canopy images, and yield maps have also been variability affects expression of yield spatial variability. This work was suggested as approaches to delineate MZ (Schepers et conducted on an irrigated cornfield near Gibbon, NE. Five landscape al., 2000). Remote sensing technology is especially apattributes, including a soil brightness image (red, green, and blue pealing to identify MZ because it is noninvasive and bands), elevation, and apparent electrical conductivity, were acquired low in cost (Mulla and Schepers, 1997). Additionally, for the field. A georeferenced soil-sampling scheme was used to deterscientific evidence for suggesting practical use of remote mine soil chemical properties (soil pH, electrical conductivity, P, and sensing technology to delineate MZ is increasing (Varvel organic matter). Georeferenced yield monitor data were collected for et al., 1999). five (1997–2001) seasons. The five landscape attributes were aggregated into four MZ using principal-component analysis of landscape Another promising noninvasive approach to define attributes and unsupervised classification of principal-component the boundaries of MZ involves the use of electromagscores. All of the soil chemical properties differed among the four netic induction to measure apparent electrical conducMZ. While yields were observed to differ by up to 25% between the tivity (ECa). This approach has been used to effectively highestand lowest-yielding MZ in three of five seasons, receiving map variations in surface soil properties such as salinity, average precipitation, less-pronounced ( 5%) differences were noted water content, and percentage clay (Corwin and Lesch, among the same MZ in the driest and wettest seasons. This illustrates 2003; Kitchen et al., 2003). In a semiarid cropping systhe significant role temporal variability plays in altering yield spatial tem, Johnson et al. (2003) showed that ECa–determined variability, even under irrigation. Use of MZ for variable application MZ could be used to characterize spatial variation in of inputs like N would only have been appropriate for this field in wheat (Triticum aestivum L.) and corn (Zea mays L.) three out of the five seasons, seriously restricting the use of this approach under variable environmental conditions. yields. Magnetic induction has also been used to track soluble nutrient levels in soil (Eigenberg et al., 2002). Caution is necessary when using this approach because of the extreme sensitivity to soil type and management R research in precision agriculture has focused conditions, but its ease of use makes it an attractive tool on use of MZ as a method to more efficiently for precision farming applications (Lund et al., 1998). apply crop inputs such as N across variable agricultural Yield mapping is yet another approach to delineate landscapes (Franzen et al., 2002; Ferguson et al., 2003). MZ. This approach is considered to be the primary form Management zones, in the context of precision agriculof precision-agriculture technology in the USA (Pierce ture, are field areas possessing homogenous attributes and Nowak, 1999). However, practical application of in landscape and soil condition. When homogenous in yield mapping to identify zones has been plagued by a specific area, these attributes should lead to the same spatial and temporal variation in measured yield (Hugresults in crop yield potential, input use efficiency, and gins and Alderfer, 1995; Sadler et al., 1995). Conseenvironmental impact. quently, most efforts in yield map interpretation have Approaches to delineate MZ vary. Topography has focused on identifying generalized zones of low, mebeen suggested as a logical basis to define homogenous dium, and high yield (Stafford et al., 1998). While using MZ to characterize spatial variability in A.R. Schepers, J.F. Shanahan, J.S. Schepers, and S. Johnson, USDAARS, and Dep. of Agron. and Hortic., Univ. of Nebraska, Lincoln, soil and crop properties is important in site-specific studNE 68583; M.A. Liebig, USDA-ARS, Northern Great Plains Res. ies, it is equally important to consider the temporal Lab., Mandan, ND 58554; and A. Luchiari, Jr., Embrapa Meio Ameffects of climate variability on expression of spatial variabiente, Jaguariuna, SP, Brazil. Joint contribution of USDA-ARS and tion in crop yields. For example, Eghball and Varvel Agric. Res. Div. of the Univ. of Nebraska. Published as Journal Ser. (1997) and Lamb et al. (1997) found under rainfed conno. 14176. Mention of commercial products and organizations in this article is solely to provide specific information. It does not constitute ditions that temporal variability of corn yields was more endorsement by USDA-ARS over other products and organizations dominant than spatial variability, indicating that spatial not mentioned. The USDA-ARS is an equal opportunity/affirmative patterns in grain yields were greatly affected by yearly action employer, and all agency services are available without discrimination. Received 18 Aug. 2003. *Corresponding author (jshanahan1@ Abbreviations: CV, coefficient of variation; DGPS, differential global unl.edu). positioning system; DN, digital number; EC, electrical conductivity; ECa, apparent electrical conductivity; GIS, geographical information Published in Agron. J. 96:195–203 (2004).  American Society of Agronomy systems; MZ, management zones; OM, organic matter; PC, principal component; PCA, principal-component analysis. 677 S. Segoe Rd., Madison, WI 53711 USA

Challenges and opportunities for mitigating nitrous oxide emissions from fertilized cropping systems

Nitrous oxide (N2O) is often the largest single component of the greenhouse-gas budget of individual cropping systems, as well as for the US agricultural sector as a whole. Here, we highlight the factors that make mitigating N2O emissions from fertilized agroecosystems such a difficult challenge, and discuss how these factors limit the effectiveness of existing practices and therefore require new technologies and fresh ideas. Modification of the rate, source, placement, and/or timing of nitrogen fertilizer application has in some cases been an effective way to reduce N2O emissions. However, the efficacy of existing approaches to reducing N2O emissions while maintaining crop yields across locations and growing seasons is uncertain because of the interaction of multiple factors that regulate several different N2O-producing processes in soil. Although these processes have been well studied, our understanding of key aspects and our ability to manage them to mitigate N2O emissions remain limited.

Cropping system effects on soil biological characteristics in the Great Plains

Soil biological quality can affect key soil functions that support food production and environmental quality. The objective of this study was to determine the effects of management and time on soil biological quality in contrasting dryland cropping systems at eight locations in the North American Great Plains. Alternative (ALT) cropping systems were characterized by greater cropping intensity (less fallow), more diverse crop sequences, and/or reduced tillage than conventional (CON) cropping systems. Soil biological properties were assessed at depths of 0-7.5, 7.5-15, and 15-30 cm from 1999 to 2002 up to three times per year. Compared to CON, ALT cropping systems had greater microbial biomass and potentially mineralizable N. ALT cropping systems also had greater water stable aggregates in the surface 7.5 cm, but only at four locations. Total glomalin (TG), an organic fraction produced by fungi associated with aggregate stability, differed only at one location (Mandan), where the ALT cropping system had 27% more TG than the CON cropping system. Fatty acid methyl ester (FAME) profiles were highly location dependent, but total extracted FAME tended to be higher in ALT cropping systems. Soil biological properties fluctuated over time at all locations, possibly in response to weather, apparent changes in soil condition at sampling, and the presence or absence of fallow and/or legumes in rotation. Consequently, preplant and post-harvest sampling, when weather and soil conditions are most stable, is recommended for comparison of soil biological properties among management practices. Overall, ALT cropping systems enhanced soil function through: (1) improved retention and cycling of nutrients and (2) maintenance of biodiversity and habitat, implying improved agro-ecosystem performance over time.

Field‐scale soil property changes under switchgrass managed for bioenergy

The capacity of perennial grasses to affect change in soil properties is well documented but information on switchgrass (Panicum virgatum L.) managed for bioenergy is limited. An on‐farm study (10 fields) in North Dakota, South Dakota, and Nebraska was sampled before switchgrass establishment and after 5 years to determine changes in soil bulk density (SBD), pH, soil phosphorus (P), and equivalent mass soil organic carbon (SOC). Changes in SBD were largely constrained to near‐surface depths (0–0.05 m). SBD increased (0–0.05 m) at the Nebraska locations (mean=0.16 Mg m−3), while most South Dakota and North Dakota locations showed declines in SBD (mean=−0.18 Mg m−3; range=−0.42–0.07 Mg m−3). Soil pH change was significant at five of the 10 locations at near surface depths (0–0.05 m), but absolute changes were modest (range=−0.67–0.44 pH units). Available P declined at all sites where it was measured (North Dakota and South Dakota locations). When summed across the surface 0.3 m depth, annual decreases in available P averaged 1.5 kg P ha−1 yr−1 (range=0.5–2.8 kg P ha−1 yr−1). Averaged across locations, equivalent mass SOC increased by 0.5 and 2.4 Mg C ha−1 yr−1 for the 2500 and 10 000 Mg ha−1 soil masses, respectively. Results from this study underscore the contribution of switchgrass to affect soil property changes, though considerable variation in soil properties exists within and across locations.

Cropping system influences on soil chemical properties and soil quality in the Great Plains

Soil management and cropping systems have long-term effects on agronomic and environmental functions. This study examined the influence of contrasting management practices on selected soil chemical properties in eight long-term cropping system studies throughout the Great Plains and the western Corn Belt. For each study, soil organic C (SOC), total N (TN), particulate organic matter (POM), inorganic N, electrical conductivity (EC), and soil pH were evaluated at 0‐7.5, 7.5‐15, and 15‐30 cm within conventional (CON) and alternative (ALT) cropping systems for 4 years (1999‐2002). Treatment effects were primarily limited to the surface 7.5 cm of soil. No-tillage (NT) and/or elimination of fallow in ALT cropping systems resulted in significantly (P < 0.05) greater SOC and TN at 0‐7.5 cm within five of the eight study sites [Akron, Colorado (CO); Bushland, Texas (TX); Fargo, North Dakota (ND); Mandan, ND; and Swift Current, Saskatchewan (SK), Canada]. The same pattern was observed with POM, where POM was significantly (P < 0.05) greater at four of the eight study sites [Bushland, TX, Mandan, ND, Sidney, Montana (MT), and Swift Current, SK]. No consistent pattern was observed with soil EC and pH due to management, although soil EC explained almost 60% of the variability in soil NO3-N at 0‐7.5 cm across all locations and sampling times. In general, chemical soil properties measured in this study consistently exhibited values more conducive to crop production and environmental quality in ALT cropping systems relative to CON cropping systems.

Diversification and ecosystem services for conservation agriculture: Outcomes from pastures and integrated crop-livestock systems

Conservation agriculturalsystems relyon threeprinciplesto enhance ecosystem services: (1) minimizing soil disturbance, (2) maximizing soil surface cover and (3) stimulating biological activity. In this paper, we explore the concept of diversity and its role in maximizing ecosystem services from managed grasslands and integrated agricultural systems (i.e., integrated crop–livestock–forage systems) at the field and farm level. We also examine trade-offs that may be involved in realizing greater ecosystem services. Previous research on livestock production systems, particularly in pastureland, has shown improvements in herbage productivity and reduced weed invasion with increased forage diversity but little response in terms of animal production. Managing forage diversity in pastureland requires new tools to guide the selection and placement of plant mixtures across a farm according to site suitability and the goals of the producer. Integrated agricultural systems embrace the concept of dynamic cropping systems, which incorporates a long-term strategy of annual crop sequencing that optimizes crop and soil use options to attain production, economic and resource conservation goals by using sound ecological management principles. Integrating dynamic cropping systems with livestock production increases the complexity of management, but also creates synergies among system components that may improve resilience and sustainability while fulfilling multiple ecosystem functions. Diversified conservation agricultural systems can sustain crop and livestock production and provide additional ecosystem services such as soil C storage, efficient nutrient cycling and conservation of biodiversity.

Soil greenhouse gas emissions affected by irrigation, tillage, crop rotation, and nitrogen fertilization.

Management practices, such as irrigation, tillage, cropping system, and N fertilization, may influence soil greenhouse gas (GHG) emissions. We quantified the effects of irrigation, tillage, crop rotation, and N fertilization on soil CO, NO, and CH emissions from March to November, 2008 to 2011 in a Lihen sandy loam in western North Dakota. Treatments were two irrigation practices (irrigated and nonirrigated) and five cropping systems (conventional-tilled malt barley [ L.] with N fertilizer [CT-N], conventional-tilled malt barley with no N fertilizer [CT-C], no-tilled malt barley-pea [ L.] with N fertilizer [NT-PN], no-tilled malt barley with N fertilizer [NT-N], and no-tilled malt barley with no N fertilizer [NT-C]). The GHG fluxes varied with date of sampling and peaked immediately after precipitation, irrigation, and/or N fertilization events during increased soil temperature. Both CO and NO fluxes were greater in CT-N under the irrigated condition, but CH uptake was greater in NT-PN under the nonirrigated condition than in other treatments. Although tillage and N fertilization increased CO and NO fluxes by 8 to 30%, N fertilization and monocropping reduced CH uptake by 39 to 40%. The NT-PN, regardless of irrigation, might mitigate GHG emissions by reducing CO and NO emissions and increasing CH uptake relative to other treatments. To account for global warming potential for such a practice, information on productions associated with CO emissions along with NO and CH fluxes is needed.

Management opportunities for enhancing terrestrial carbon dioxide sinks.

The potential for mitigating increasing atmospheric carbon dioxide concentrations through the use of terrestrial biological carbon (C) sequestration is substantial. Here, we estimate the amount of C being sequestered by natural processes at global, North American, and national US scales. We present and quantify, where possible, the potential for deliberate human actions – through forestry, agriculture, and use of biomass-based fuels – to augment these natural sinks. Carbon sequestration may potentially be achieved through some of these activities but at the expense of substantial changes in land-use management. Some practices (eg reduced tillage, improved silviculture, woody bioenergy crops) are already being implemented because of their economic benefits and associated ecosystem services. Given their cumulative greenhouse-gas impacts, other strategies (eg the use of biochar and cellulosic bioenergy crops) require further evaluation to determine whether widespread implementation is warranted.

US agricultural nitrous oxide emissions: context, status, and trends

The use of commercial nitrogen (N) fertilizers has led to enormous increases in US agricultural productivity. However, N losses from agricultural systems have resulted in numerous deleterious environmental impacts, including a continuing increase in atmospheric nitrous oxide (N2O), a greenhouse gas (GHG) and an important catalyst of stratospheric ozone depletion. Although associated with about 7% of total US GHG emissions, agricultural systems account for 75% of total US N2O emissions. Increased productivity in the crop and livestock sectors during the past 30 to 70 years has resulted in decreased N2O emissions per unit of production, but N2O emissions from US agriculture continue to increase at a rate of approximately 0.46 teragrams of carbon dioxide equivalents per year (2002–2009). This rate is lower than that during the late 20th century. Improvements in agricultural productivity alone may be insufficient to lead to reduced emissions; implementing strategies specifically targeted at reducing N2O emiss...

Dynamic Cropping Systems

Cropping systems need to be inherently flexible to take advantage of economic opportunities and/or adapt to environmental realities. A dynamic cropping systems concept-characterized by a management approach whereby crop sequencing decisions are made on an annual basis-has been proposed to improve the adaptability of cropping practices to externalities. A symposium on dynamic cropping systems was held at the 2005 ASA-CSSA-SSSA annual meetings in Salt Lake City, UT. Presentations at the symposium reviewed research results from a recent experiment near Mandan, ND, investigating short-term crop sequence effects on crop production, plant diseases, soil residue coverage, and soil water depletion. This paper briefly reviews each of the presentations at the symposium. Future research opportunities on dynamic cropping systems abound, and may have increased impact if emerging issues in agriculture (e.g., increased use of biofuels; livestock integration in cropping systems) are incorporated in evaluations.