Thomas C. Kaspar

Relationship Between Six Years of Corn Yields and Terrain Attributes

Crop yield, soil properties, and erosion are strongly related to terrain attributes. The objectives of our study were to examine the relationship between six years of corn (Zea mays L.) yield data and relative elevation, slope, and curvature, and to develop a linear regression model to describe the spatial patterns of corn yield for a 16 ha field in central Iowa, USA. Corn grain yield was measured in six crop years, and relative elevation was measured using a kinematic global positioning system. Slope and curvature were then determined using digital terrain analysis. Our data showed that in the four years with less than normal growing season precipitation, corn yield was negatively correlated with relative elevation, slope, and curvature. In the two years with greater than normal precipitation, yield was positively correlated with relative elevation and slope. A multiple linear regression model based on relative elevation, slope, and curvature was developed that predicted 78% of the spatial variability of the average yield of the transect plots for the four dry years. This model also adequately identified the spatial patterns within the entire field for yield monitor data from 1997, which was one of the dry years. The relationship between terrain attributes and corn yield spatial patterns may provide opportunities for implementing site-specific management.

Denitrification in Wood Chip Bioreactors at Different Water Flows

Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO(3)-N) and concentrations frequently exceed 10 mg L(-1). A laboratory column study was conducted to investigate the ability of a wood chip bioreactor to promote denitrification under mean water flow rates of 2.9, 6.6, 8.7 and 13.6 cm d(-1) which are representative of flows entering subsurface drainage tiles. Columns were packed with wood chips and inoculated with a small amount of oxidized till and incubated at 10 degrees C. Silicone sampling cells at the effluent ports were used for N(2)O sampling. (15)Nitrate was added to dosing water at 50 mg L(-1) and effluent was collected and analyzed for NO(3)-N, NH(4)-N, and dissolved organic carbon. Mean NO(3)-N concentrations in the effluent were 0.0, 18.5, 24.2, and 35.3 mg L(-1) for the flow rates 2.9, 6.6, 8.7, and 13.6 cm d(-1), respectively, which correspond to 100, 64, 52, and 30% efficiency of removal. The NO(3)-N removal rates per gram of wood increased with increasing flow rates. Denitrification was found to be the dominant NO(3)-N removal mechanism as immobilization of (15)NO(3)-N was negligible compared with the quantity of (15)NO(3)-N removed. Nitrous oxide production from the columns ranged from 0.003 to 0.028% of the N denitrified, indicating that complete denitrification generally occurred. Based on these observations, wood chip bioreactors may be successful at removing significant quantities of NO(3)-N, and reducing NO(3)-N concentration from water moving to subsurface drainage at flow rates observed in central Iowa subsoil.

Do cover crops increase or decrease nitrous oxide emissions? A meta-analysis

There are many environmental benefits to incorporating cover crops into crop rotations, such as their potential to decrease soil erosion, reduce nitrate (NO3) leaching, and increase soil organic matter. Some of these benefits impact other agroecosystem processes, such as greenhouse gas emissions. In particular, there is not a consensus in the literature regarding the effect of cover crops on nitrous oxide (N2O) emissions. Compared to site-specific studies, meta-analysis can provide a more general investigation into these effects. Twenty-six peer-reviewed articles including 106 observations of cover crop effects on N2O emissions from the soil surface were analyzed according to their response ratio, the natural log of the N2O flux with a cover crop divided by the N2O flux without a cover crop (LRR). Forty percent of the observations had negative LRRs, indicating a cover crop treatment which decreased N2O, while 60% had positive LRRs indicating a cover crop treatment which increased N2O. There was a significant interaction between N rate and the type of cover crop where legumes had higher LRRs at lower N rates than nonlegume species. When cover crop residues were incorporated into the soil, LRRs were significantly higher than those where residue was not incorporated. Geographies with higher total precipitation and variability in precipitation tended to produce higher LRRs. Finally, data points measured during cover crop decomposition had large positive LRRs and were larger than those measured when the cover crop was alive. In contrast, those data points measuring for a full year had LRRs close to zero, indicating that there was a balance between periods when cover crops increased N2O and periods when cover crops decreased emissions. Therefore, N2O measurements over the entire year may be needed to determine the net effect of cover crops on N2O. The data included in this meta-analysis indicate some overarching crop management practices that reduce direct N2O emissions from the soil surface, such as no soil incorporation of residues and use of non-legume cover crop species. However, our results demonstrate that cover crops do not always reduce direct N2O emissions from the soil surface in the short term and that more work is needed to understand the full global warming potential of cover crop management.

Seed-row residue management for corn establishment in the northern US Corn Belt

In the northern US Corn Belt, plant residue retained on the soil surface increases risk of poor stand establishment and growth of corn (Zea mays, L.). This limits adoption of no-tillage and other conservation tillage systems which are effective in reducing soil erosion. Field and laboratory research has shown that surface residue reduces soil heat unit accumulation by reducing soil heat flux, and conserves soil water by reducing evaporation rate. Surface residue also hinders planter operation and uniformity of seed placement. Removing excessive or non-uniform plant residue from the seed row increases germination and emergence rate by improving seed depth uniformity and by increasing soil heat unit accumulation. Appropriate use of planter attachments to manage surface plant residue has been shown to improve conditions in the seed zone for reliable corn establishment in the northern US Corn Belt.

Cover crops in the upper midwestern United States: Potential adoption and reduction of nitrate leaching in the Mississippi River Basin

Nitrate (NO3) losses from agricultural lands in the Midwest flow into the Mississippi River Basin (MRB) and contribute significantly to hypoxia in the Gulf of Mexico. Previous work has shown that cover crops can reduce loadings, but adoption rates are low, and the potential impact if cover crops were widely adopted is currently unknown. This paper provides an analysis of potential cover crop adoption and relative benefits to water quality across the five-state region of Ohio, Indiana, Illinois, Iowa, and Minnesota in the upper midwestern MRB. Two agricultural counties were selected in each of the five states, and the potential for fall-planted cover crop adoption was estimated based on cash crop rotation and tillage systems. In these 10 counties, an estimated 34% to 81% of the agricultural land could have cover crops integrated into their corn (Zea mays L.) and soybean (Glycine max L.) cropping systems. These adoption rates would in some cases require shifts of current tillage practices from fall to spring, but could be even higher with increased adoption of no-till and mulch-till. Nitrate reduction simulated with the Root Zone Water Quality Model for the tile drained portion of the corn–soybean and continuous corn cropping systems in the five-state area, under the assumed management systems and uniform soil properties, showed that cover crops have the potential to reduce NO3 loadings to the Mississippi River by approximately 20%. These predictions suggest that cover crop adoption would have a beneficial impact on water quality in the MRB and would contribute greatly towards meeting the national goal of significant reduction in NO3-nitrogen (N) load entering the Gulf.

Cover crops in the upper midwestern United States: Simulated effect on nitrate leaching with artificial drainage

A fall-planted winter cover crop is an agricultural management practice with multiple benefits that may include reducing nitrate (NO3) losses from artificial drained agricultural fields. While the practice is commonly used in the southern and eastern United States, little is known about its efficacy in midwestern states where winters are longer and colder, and artificial subsurface drainage is widely used in corn–soybean systems (Zea mays L.–Glycine max L.). We used a field-tested version of the Root Zone Water Quality Model (RZWQM) to simulate the adoption of cereal rye (Secale cereale L.) as a winter cover crop and estimate its impact on NO3 losses from drained fields at 41 sites across the Midwest from 1961 to 2005. The average annual nitrogen (N) loss reduction from adding winter rye ranged from 11.7 to 31.8 kg N ha−1 (10.4 to 28.4 lb N ac−1) among four simulated systems. One of the simulated treatments was winter rye overseeded (aerial seeded) into a no-till corn–soybean rotation at simulated main crop maturity (CC2). On average, this treatment reduced simulated N loss in drainage by 20.1 kg N ha−1 (17.9 lb N ac−1) over the sites compared to systems without winter rye (NCC2), from 47.3 to 27.2 kg N ha−1 (42.2 to 24.3 lb N ac−1). Adding spring tillage to this treatment and killing the rye earlier (CC3) reduced simulated N loss from 57.3 (NCC3) to 34.4 kg N ha−1 (30.7 lb N ac−1). Replacing the corn–soybean rotation with continuous corn and spring tillage reduced simulated N loss from 106 (NCC4) to 74.2 kg N ha−1 (CC4) (94.6 to 66.2 lb N ac−1). Adding a winter rye cover crop reduced N loss more in the continuous corn system despite earlier spring termination of the winter rye and slightly less N uptake by the rye possibly because of more denitrification. Regression analysis of the RZWQM variables from these sites showed that temperature and precipitation during winter rye growth, N fertilizer application rates to corn, and simulated corn yield account for greater than 95% of the simulated site-to-site variability in NO3 loss reductions in tile flow due to winter rye. Our results suggest that on average winter rye can reduce N loss in drainage 42.5% across the Midwest. Greater N loss reductions were estimated from adding winter rye at sites with warmer temperatures and less precipitation because of more cover crop growth and more soil N available for cover crop uptake.

Oat plant effects on net nitrogen mineralization

Living plants have been reported to stimulate, inhibit, or have no effect on net nitrogen mineralization in soil. A series of experiments were conducted to evaluate the influence of living oat plants Avena sativa on net N mineralization. Oat plants were grown in plastic cylinders containing soil, and net N mineralization was assessed by determining the N balance in these microcosms. Measured N inputs included N contained in the oat seeds and N2 fixation. N losses by NH3 volatilization and denitrification were also measured. We observed that in some soils net N mineralization was stimulated by as much as 81%, but in other soils there was no effect of living oat plants on net N mineralization. N mineralization responses are related to past cropping histories of the soils.

Isolation of Cultivation-Resistant Oomycetes, First Detected as Amplicon Sequences, from Roots of Herbicide-Terminated Winter Rye

The dynamics of microbial communities associated with dying cover crops are of interest because of potential impacts on disease in a subsequent crop, and because of the importance of microbial activity on plant residue to soil organic matter dynamics and nutrient cycling. We used high throughput amplicon sequencing to characterize the composition and structure of oomycete and fungal communities associated with a rye cover crop, and to track their community dynamics in the first several weeks after herbicide was applied to terminate the cover crop. The dominant oomycetes associated with cereal rye roots were Pythium volutum, Pythium sp. F86 (an unknown species within clade B), and Lagena radicicola. Because P. volutum is sensitive to common additives in isolation media, and L. radicicola is an obligate intracellular parasite, a unique aspect of this work is to reveal the dominance of oomycete taxa that would have been missed entirely under a traditional cultivation-based approach. Based on first detection ...

The Potential for Cereal Rye Cover Crops to Host Corn Seedling Pathogens

Cover cropping is a prevalent conservation practice that offers substantial benefits to soil and water quality. However, winter cereal cover crops preceding corn may diminish beneficial rotation effects because two grass species are grown in succession. Here, we show that rye cover crops host pathogens capable of causing corn seedling disease. We isolated Fusarium graminearum, F. oxysporum, Pythium sylvaticum, and P. torulosum from roots of rye and demonstrate their pathogenicity on corn seedlings. Over 2 years, we quantified the densities of these organisms in rye roots from several field experiments and at various intervals of time after rye cover crops were terminated. Pathogen load in rye roots differed among fields and among years for particular fields. Each of the four pathogen species increased in density over time on roots of herbicide-terminated rye in at least one field site, suggesting the broad potential for rye cover crops to elevate corn seedling pathogen densities. The radicles of corn seedlings planted following a rye cover crop had higher pathogen densities compared with seedlings following a winter fallow. Management practices that limit seedling disease may be required to allow corn yields to respond positively to improvements in soil quality brought about by cover cropping.

Time Interval Between Cover Crop Termination and Planting Influences Corn Seedling Disease, Plant Growth, and Yield

Experiments were established in a controlled-growth chamber and in the field to evaluate the effect of the length of time intervals between winter rye cover crop termination and corn planting on corn seedling disease, corn growth, and grain yield in 2014 and 2015. Rye termination dates ranged from 25 days before planting (DBP) to 2 days after planting (DAP) corn in the field and from 21 DBP to 1 DAP in controlled studies. Results were similar in both environments. In general, shorter intervals increased seedling disease and reduced corn emergence, shoot growth, and grain yield of corn following winter rye compared with corn planted 10 or more days after rye termination or without rye. Incidence of Pythium spp. increased with shorter intervals (less than 8 DBP); incidence of Fusarium spp. was not consistent between runs and experiments. In 2014, in the 1-DAP treatment, number of ears and grain yield were reduced (P = 0.05 and 0.02, respectively). In 2015, all termination intervals reduced plant population, number of ears, and yield (P = 0.01), with the 2-DBP treatment causing the biggest decrease. A 10- to 14-day interval between rye termination and corn planting should be followed to improve corn yield following a rye cover crop.