Jeffrey A. Coulter

Ammonium sorption and ammonia inhibition of nitrite-oxidizing bacteria explain contrasting soil N2O production.

Better understanding of process controls over nitrous oxide (N2O) production in urine-impacted ‘hot spots’ and fertilizer bands is needed to improve mitigation strategies and emission models. Following amendment with bovine (Bos taurus) urine (Bu) or urea (Ur), we measured inorganic N, pH, N2O, and genes associated with nitrification in two soils (‘L’ and ‘W’) having similar texture, pH, C, and C/N ratio. Solution-phase ammonia (slNH3) was also calculated accounting for non-linear ammonium (NH4+) sorption capacities (ASC). Soil W displayed greater nitrification rates and nitrate (NO3−) levels than soil L, but was more resistant to nitrite (NO2−) accumulation and produced two to ten times less N2O than soil L. Genes associated with NO2− oxidation (nxrA) increased substantially in soil W but remained static in soil L. Soil NO2− was strongly correlated with N2O production, and cumulative (c-) slNH3 explained 87% of the variance in c-NO2−. Differences between soils were explained by greater slNH3 in soil L which inhibited NO2− oxidization leading to greater NO2− levels and N2O production. This is the first study to correlate the dynamics of soil slNH3, NO2−, N2O and nitrifier genes, and the first to show how ASC can regulate NO2− levels and N2O production.

Performance of agricultural residue media in laboratory denitrifying bioreactors at low temperatures

Denitrifying bioreactors can be effective for removing nitrate from agricultural tile drainage; however, questions about cold springtime performance persist. The objective of this study was to improve the nitrate removal rate (NRR) of denitrifying bioreactors at warm and cold temperatures using agriculturally derived media rather than wood chips (WC). Corn ( L.) cobs (CC), corn stover (CS), barley ( L.) straw (BS), WC, and CC followed by a compartment of WC (CC+WC) were tested in laboratory columns for 5 mo at a 12-h hydraulic residence time in separate experiments at 15.5 and 1.5°C. Nitrate-N removal rates ranged from 35 to 1.4 at 15.5°C and from 7.4 to 1.6 g N m d at 1.5°C, respectively; NRRs were ranked CC > CC+WC > BS = CS > WC and CC ≥ CC+WC = CS ≥ BS > WC for 15.5 and 1.5°C, respectively. Although NRRs for CC were increased relative to WC, CC released greater amounts of carbon. Greater abundance of nitrous oxide (NO) reductase gene () was supported by crop residues than WC at 15.5°C, and CS and BS supported greater abundance than WC at 1.5°C. Production of NO relative to nitrate removal (NO) was consistently greater at 1.5°C (7.5% of nitrate removed) than at 15.5°C (1.9%). The NO was lowest in CC (1.1%) and CC-WC (0.9%) and greatest in WC (9.7%). Using a compartment of agricultural residue media in series before wood chips has the potential to improve denitrifying bioreactor nitrate removal rates, but field-scale verification is needed.

Planting date and glyphosate timing on soybean

Planting date of soybean may be one factor that affects the crops ability to compete with weeds. Field experiments were conducted over 3 yr at two locations in Illinois to determine whether planting date affects optimal weed management strategies and the critical time of weed removal (CTWR) in glyphosate-resistant soybean. Across planting dates, a PRE application of metolachlor plus metribuzin followed by a single glyphosate application at a 10- or 20-cm weed height at Monmouth or at a 20-cm weed height at Urbana was effective at protecting yield. At Monmouth, higher yields occurred with sequential glyphosate applications than with a single application in the early planting in 2 yr when weeds were removed at a height of 10 cm. Across planting dates, highest yields at Monmouth occurred with a single glyphosate application when weeds were 20 to 30 cm tall. At Urbana, the CTWR was not affected by planting date and occurred between 176 and 290 growing-degree days (GDD) after planting, corresponding to the V1 to V2 stage of soybean development and a weed height of 11 to 19 cm. Applying glyphosate near the CTWR at Urbana (10-cm weed height) required a sequential application to prevent yield loss with early and middle planting dates in 1 of 3 yr. Overall, planting date did not affect optimal weed management strategies at either location. Nomenclature: Glyphosate; metolachlor; metribuzin; paraquat; soybean, Glycine max (L.) Merr.

Giant Ragweed (Ambrosia trifida) Seed Production and Retention in Soybean and Field Margins

As herbicide-resistant weed populations become increasingly problematic in crop production, alternative strategies of weed control are necessary. Giant ragweed, one of the most competitive agricultural weeds in row crops, has evolved resistance to multiple herbicide biochemical sites of action within the plant, necessitating the development of new and integrated methods of weed control. This study assessed the quantity and duration of seed retention of giant ragweed grown in soybean fields and adjacent field margins. Seed retention of giant ragweed was monitored weekly during the 2012 to 2014 harvest seasons using seed collection traps. Giant ragweed plants produced an average of 1,818 seeds per plant, with 66% being potentially viable. Giant ragweed on average began shattering hard (potentially viable) and soft (nonviable) seeds September 12 and continued through October at an average rate of 0.75 and 0.44% of total seeds per day during September and October, respectively. Giant ragweed seeds remained on the plants well into the Minnesota soybean harvest season, with an average of 80% of the total seeds being retained on October 11, when Minnesota soybean harvest was approximately 75% completed in the years of the study. These results suggest that there is a sufficient amount of time to remove escaped giant ragweed from production fields and field margins before the seeds shatter by managing weed seed dispersal before or at crop harvest. Controlling weed seed dispersal has potential to manage herbicide-resistant giant ragweed by limiting replenishment of the weed seed bank. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR; soybean, Glycine max (L.) Merr. Conforme las poblaciones de malezas resistentes a herbicidas se hacen incrementalmente más problemáticas en la producción de cultivos, estrategias alternativas de control de malezas se hacen cada vez más necesarias. Ambrosia trifida, una de las malezas agrícolas más competitivas en cultivos en hileras, ha evolucionado resistencia a múltiples sitios bioquímicos de acción de herbicidas dentro de la planta, lo que ha hecho necesario el desarrollo de métodos nuevos e integrados de control de malezas. Este estudio evaluó la cantidad y duración de la retención de semilla de A. trifida creciendo en campos de soja y márgenes de campos adyacentes. La retención de semilla de A. trifida fue monitoreada semanalmente durante las temporadas de cosecha desde 2012 a 2014 usando trampas de colección de semilla. Las plantas de A. trifida produjeron un promedio de 1,818 semillas por planta, con una viabilidad potencial de 66%. En promedio, A. trifida inició la dispersión de semilla dura (potencialmente viable) y suave (no-viable) el 12 de Septiembre y continuó durante Octubre, con una tasa promedio de 0.75 y 0.44% del total de semillas por día, durante Septiembre y Octubre, respectivamente. Las semillas de A. trifida permanecieron en las plantas hasta la temporada de cosecha de soja en Minnesota, con un promedio de 80% del total de las semillas estando retenidas al 11 de Octubre, cuando la cosecha de soja en Minnesota había sido completada al 75%, en los años de este estudio. Estos resultados sugieren que existe una cantidad de tiempo suficiente para remover A. trifida que haya escapado al control en campos de producción y en márgenes de campos antes de que la semilla sea liberada de la planta, mediante el manejo de la dispersión de semilla de malezas antes o durante la cosecha. El controlar la dispersión de semillas de malezas tiene el potencial de manejar A. trifida resistente a herbicidas al limitar el suministro de nuevas semillas al banco de semillas de malezas.

Productivity, Economics, and Soil Quality in the Minnesota Variable-Input Cropping Systems Trial

Organic input (OI) and low external input (LEI) cropping systems with extended crop rotations have potential to maintain crop yields while enhancing net return and soil quality. From 1992 to 2007, contrasting cropping systems were evaluated in a 2-year soybean [Glycine max (L.) Merr.]-corn (Zea mays L.) rotation and a 4year oat (Avena sativa L.)/alfalfa (Medicago sativa L.)-alfalfa-corn-soybean rotation in southwestern Minnesota. When compared to the high external input (HEI) 2-year rotation, corn grain yield was not reduced with LEI and OI 4-year rotations, and soybean yield was not reduced with the LEI 4-year rotation over all 16 years or with the LEI 2-year rotation in the last 4 years. Across years and crops, net return was 88% greater with the OI 4-year rotation than the HEI 2year rotation, but was 19 and 15% lower with the LEI 2and 4-year rotation, respectively. Particulate organic matter and potentially mineralizable C in 2001 were higher with the OI system than the other systems in both rotations. These results demonstrate that with diversified rotations, organic systems can produce high and profitable crop yields while enhancing soil quality, and that corn and soybean yields can be maintained in LEI systems. However, OI and LEI systems are constrained by greater management and labor requirements and pest management challenges than HEI systems. Introduction Long-term agronomic studies integrate environmental and cropping effects to provide a realistic picture of the impact of production practices on crop yield and yield stability, net return, economic risk, pest populations, and soil properties. Long-term research is especially important when evaluating rotations of several crops which may be grown only once within a 4to 5-year cycle. The Morrow Plots, established in 1876 in Illinois, are the oldest example of cropping systems research in the United States (34). This experiment has shown the value of crop rotations with a forage legume in maintaining longterm corn yield compared to continuous corn. The Morrow Plots were also among the first to show the value of manure and fertilizer inputs for sustaining crop yields and soil organic C (2,34). More recently, long-term experiments in the Midwest have re-confirmed the value of forage legumes in crop rotations and have demonstrated the economic and agronomic potential of OI and LEI systems in rotations (7,8,10,13,17,37). However, with the availability of inexpensive fertilizers and pesticides and the decline of livestock on farms beginning in the mid-1900s, many farmers in the Midwest made the transition from diversified cropping systems containing forages to more specialized grain cropping systems (19). Risks associated with these simpler cropping systems were supported by strong commodity markets for grain, federal price supports, and crop insurance programs. 29 April 2013 Crop Management Published June 13, 2014

Seedbank Depletion and Emergence Patterns of Giant Ragweed (Ambrosia trifida) in Minnesota Cropping Systems

In the midwestern United States, biotypes of giant ragweed resistant to multiple herbicide biochemical sites of action have been identified. Weeds with resistance to multiple herbicides reduce the utility of existing herbicides and necessitate the development of alternative weed control strategies. In two experiments in southeastern Minnesota, we determined the effect of six 3 yr crop-rotation systems containing corn, soybean, wheat, and alfalfa on giant ragweed seedbank depletion and emergence patterns. The six crop-rotation systems included continuous corn, soybean—corn—corn, corn—soybean—corn, soybean—wheat—corn, soybean—alfalfa—corn, and alfalfa—alfalfa—corn. The crop-rotation system had no effect on the amount of seedbank depletion when a zero-weed threshold was maintained, with an average of 96% of the giant ragweed seedbank being depleted within 2 yr. Seedbank depletion occurred primarily through seedling emergence in all crop-rotation systems. However, seedling emergence tended to account for more of the seedbank depletion in rotations containing only corn or soybean compared with rotations with wheat or alfalfa. Giant ragweed emerged early across all treatments, with on average 90% emergence occurring by June 4. Duration of emergence was slightly longer in established alfalfa compared with other cropping systems. These results indicate that corn and soybean rotations are more conducive to giant ragweed emergence than rotations including wheat and alfalfa, and that adopting a zero-weed threshold is a viable approach to depleting the weed seedbank in all crop-rotation systems. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR, alfalfa, Medicago sativa L., corn, Zea mays L., soybean, Glycine max (L.) Merr., wheat, Triticum aestivum L.

Plastic Biofilm Carrier after Corn Cobs Reduces Nitrate Loading in Laboratory Denitrifying Bioreactors

Nitrate-nitrogen (nitrate-N) removal rates can be increased substantially in denitrifying bioreactors with a corn ( L.) cob bed medium compared with woodchips; however, additional organic carbon (C) is released into the effluent. This laboratory column experiment was conducted to test the performance of a postbed chamber of inert plastic biofilm carrier (PBC) after corn cobs (CC) to extend the area of biofilm colonization, enhance nitrate-N removal, lower total organic C losses, and reduce nitrous oxide (NO) production at warm (15.5°C) and cold (1.5°C) temperatures. Treatments were CC only and CC plus PBC in series (CC-PBC). Across the two temperatures, nitrate-N load removal was 21% greater with CC-PBC than CC, with 54 and 44% of total nitrate N load, respectively. However, total organic C concentrations and loads were not significantly different between treatments. Colonization of the PBC by denitrifiers occurred, although gene abundance at the outlet (PBC) was less than at the inlet (CC). The PBC chamber increased nitrate-N removal rate and reduced cumulative NO production at 15.5°C, but not at 1.5°C. Across temperatures and treatments, NO production was 0.9% of nitrate-N removed. Including an additional chamber filled with PBC downstream from the CC bioreactor provided benefits in terms nitrate-N removal but did not achieve C removal. The presence of excess C, as well as available nitrate, in the PBC chamber suggests another unidentified limiting factor for nitrate removal.

Erratum: Ammonium sorption and ammonia inhibition of nitrite-oxidizing bacteria explain contrasting soil N2O production (Scientific Reports (2015) 5 (12153))

Scientific Reports 5: Article number: 12153; published online: 16 July 2015; updated: 08 February 2016. Ping Wang and Michael J. Sadowsky were omitted from the author list in the original version of this Article. This has been corrected in the PDF and HTML versions of the Article. The Author Contributions section now reads:

Giant Ragweed (Ambrosia trifida) Emergence Model Performance Evaluated in Diverse Cropping Systems

Accurate weed emergence models are valuable tools for scheduling planting, cultivation, and herbicide applications. Multiple models predicting giant ragweed emergence have been developed, but none have been validated in diverse crop rotation and tillage systems, which have the potential to influence weed emergence patterns. This study evaluated the performance of published giant ragweed emergence models across various crop rotations and spring tillage dates in southern Minnesota. Across experiments, the most robust model was a mixed-effects Weibull (flexible sigmoidal function) model predicting emergence in relation to hydrothermal time accumulation with a base temperature of 4.4 C, a base soil matric potential of -2.5 MPa, and two random effects determined by overwinter growing degree days (GDD) (10 C) and precipitation accumulated during seedling recruitment. The deviations in emergence between individual plots and the fixed-effects model were distinguished by the positive association between the lower horizontal asymptote (Drop) and maximum daily soil temperature during seedling recruitment. This finding indicates that crops and management practices that increase soil temperature will have a shorter lag phase at the start of giant ragweed emergence compared with practices promoting cool soil temperatures. Thus, crops with earlyseason crop canopies such as perennial crops and crops planted in early spring and in narrow rows will likely have a slower progression of giant ragweed emergence. This research provides a valuable assessment of published giant ragweed emergence models and illustrates that accurate emergence models can be used to time field operations and improve giant ragweed control across diverse cropping systems. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR.

Erratum: Evaluation of intensive "4R" strategies for decreasing nitrous oxide emissions and nitrogen surplus in rainfed corn [J. Environ. Qual., 45, 4, (1186-1195)] DOI: 10.2134/jeq2016.01.0024

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 (N2O) 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 N2O emissions. Over two corn (Zea mays L.) growing seasons in Minnesota, N2O 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 N2O. 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-1. The only treatment that decreased N2O (by 20–53%) was Sp combined with inhibitors and reduced N rate. Emissions of N2O were more strongly correlated with NS calculated from grain N uptake (R2 = 0.61) compared with whole-plant N uptake (r2 = 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 N2O. Grain-based NS may be a more useful indicator of N2O emissions than whole-plant-based NS. Evaluation of Intensive “4R” Strategies for Decreasing Nitrous Oxide Emissions and Nitrogen Surplus in Rainfed Corn