A comparison of carbon storage potential in corn- and prairie-based agroecosystems

Increasing carbon (C) in the soil is important both for removing harmful C from the atmosphere and improving the health of the soil. In this dissertation, I set out to examine how planting and harvesting prairies on soils suitable for corn affected soil C storage potential when compared to corn-based systems. I addressed three questions designed to a) improve our understanding of the fundamental differences between how prairies and corn grow throughout the year, b) test how prairies and corn add C to the soil after establishment, and c) use our current understanding of prairie and corn growth and C and N dynamics to predict how SOC will change over the next 50 years. Measurements of fundamental differences showed corn produced more aboveground and overall biomass with faster growth rates that peaked later in the season than prairie. Duration of growth was shorter in corn than prairie. Corn allocated a much smaller proportion of its biomass belowground than prairie and produced much less root biomass than prairie. Corn biomass had higher N concentrations, but less efficient growth relative to these concentrations. Six years after establishment of the experiment, I found prairies had more root mass that was more recalcitrant and located at depths unfavorable to decomposition, but did not have greater amounts of labile C (POXC) or total SOC than corn-based treatments, nor greater amounts of total SOC than in the establishment year. However, it was important to note that prairies placed ~5 times more C belowground as root C than corn-based treatments. Simulations made over 50 years using the APSIM model showed that prairies had much larger increases in root C, fresh organic matter, and microbial biomass pools, while a corn-based system with a winter cover crop maintained these pools, and continuous corn and corn-soybean rotations lost C in these pools. However, all treatments lost C from the more stable C pool, resulting in an overall loss of SOC after 50 years. The lack of gain in soil C was attributed to a combination of C-saturated soils and rhizosphere-induced priming. However, the validity of these mechanisms needs more investigation

The Influence Of Winter Field Cover On Spring Nitrous Oxide Emissions

Agriculture is responsible for 58% of the anthropogenic emissions of nitrous oxide (N2O), a source of stratospheric ozone depletion and a greenhouse gas that contributes to global climate change. In temperate regions, a majority of this N2O is emitted during freeze-thaw cycles (FTC) in the spring. Climate change models predict that warming trends in the northeast will result in less snow cover, potentially leading to colder soils that may in turn lead to higher N2O emissions. A winter soil management strategy is needed to mitigate spring N2O emissions. In this study, I examined the influence of two winter field covers, snow and winter rye, on soil temperature and nitrogen (N) content and subsequent spring N2O emissions from a NY corn field over two years. A 2 x 2 factorial of rye (+/-) by snow cover (+/- ) was established in a randomized complete block design. Nitrous oxide emissions were measured bi-weekly using a static chamber method. The first season (2006-07) was a cold winter (2309 h below 0 degrees C at 8 cm soil depth), historically typical for the region. The snow removal treatment resulted in colder soils and higher N2O fluxes (73.3 vs. 57.9 ng N2O-N cm-2 h-1). The rye cover had no effect on N2O emissions. The second season (2007-08) was a much milder winter (1271 h below freezing at 8 cm soil depth), with lower N2O fluxes overall. Winter rye cover resulted in lower N2O fluxes (5.9 vs. 33.7 ng N2O-N cm-2 h-1), but snow removal had no effect. These results suggest that if winters remain typically cold in the Northeastern U.S., but snowfall is reduced, we may expect higher N2O emissions, with winter rye cover unlikely to mitigate this. If, however, less snow cover is due to warmer temperatures as predicted, we may be trending towards lower spring N2O emissions where winter rye cover cropping may be a useful mitigation tool. The field experiment showed that temperature buffering created by an insulating soil cover during the winter may lead to lower N2O emissions in the spring. Insulation may result in higher minimum soil temperatures, shorter freeze duration, fewer FTC, and slower rates of freezing and thawing. One of these temperature variables, slower thawing, was examined by measuring N2O fluxes in a laboratory-simulated FTC. Slower thawing led to higher N2O emissions (1200 vs. 750 ng N2O-N cm-2 h-1). This suggests that slower thawing is not the mechanism responsible for lower N2O emissions observed in agricultural fields with soil cover. Rather, one of the other variables mentioned may be more important. It is unclear if high spring emissions result from a decrease in the efficiency of N2O reduction to N2. A laboratory-simulated FTC was also used to investigate the ratio of N2O to total gaseous N emitted (rN2O) during periods of high N2O emissions. Results showed that rN2O decreased (0.64 0.0) over time after thawing. This suggests that a lack of reduction of N2O to N2 may contribute to high N2O emissions measured during soil thawing. Gaining an understanding of why N2O emissions are high during spring thawing and how these emissions are affected by snow cover, rye cover cropping and the rate of soil thawing will aid researchers and land owners in designing useful N2O emission mitigation strategies

Wrap up and Validation of the Yield Forecast Project for 2015

During the 2015 growing season, a group of scientists from the Department of Agronomy ran a pilot project with the objective of forecasting end-of-season yields and in-season water and nitrogen dynamics (crop demand and soil supply). In-season updates were put in past ICM News articles (June 17th, July 31st, and August 14th). Briefly, this project combined the use of a cropping systems model (APSIM), a climate model (WRF), and high-resolution, in-season measurements to create the forecasts. The project focused on eight cropping systems in 2015: two sites (Ames and Sutherland), two crops (corn and soybean), and two planting dates of each crop. More details can be found in a previous ICM News article with plot management details from June 17, 2015. In this article we’ll present validation results of our last forecast on September 12

Nitrogen myths and realities

More than 1/3 of the nitrogen in human bodies is derived from industrially synthesized nitrogen fertilizer (Smil, 2004). Without nitrogen fertilizer, agriculture could not sustain the global population of 7 billion people living today, much less the 11 billion people expected to be living by 2100. There is no doubt: nitrogen fertilizer is critical to sustain human health and grow developing economies

Above- and Below-ground Biomass Production in Corn and Prairie Bioenergy Cropping Systems

The Comparison of Biofuel Systems (COBS) project is a long-term, 20-acre field experiment designed to provide quantitative, side-by-side comparisons of corn- and prairie-based biofuel feedstock production systems with respect to biomass yields, liquid fuel potential, and multiple environmental impacts. Here, we report on above- and below-ground biomass production from selected treatments

Plant litter quality affects the accumulation rate, composition, and stability of mineral-associated soil organic matter

Mineral-associated organic matter (MAOM) is a relatively large and stable fraction of soil organic matter (SOM). Plant litters with high rates of mineralization (high quality litters) are hypothesized to promote the accumulation of MAOM with greater efficiency than plant litters with low rates of mineralization (low-quality litters) because litters with high rates of mineralization maximize the synthesis of microbial products and most MAOM is microbial-derived. However, the effect of litter quality on MAOM is inconsistent. We conducted four repeated short-term incubations (46-d each) of four plant litters (alfalfa, oats, maize and soybean) in two low-carbon subsoils (sandy loam and silty loam) with and without nutrient addition. Our short-term incubations focused on the initial stage of litter decompositionduring the time when litter quality has a measureable effect on mineralization rates. Plant litter quality had a much greater effect on litter-C mineralization rate and MAOM-C accumulation than did soil type or nutrient addition. Soils amended with high-quality oat and alfalfa litters had greater MAOM-C accumulation than soils amended with low-quality maize and soybean litters. However, soils amended with high-quality litters also had greater litter-C mineralization than soils amended with low-quality litters. As a result, the accumulation of MAOM-C per unit of litter-C mineralization was lower in soils amended with high-vs. low-quality litters (0.65 vs. 1.39 g MAOM-C accumulated g−1 C mineralized). Cellulose and hemicelluose indices of accumulated MAOM were greater for maize and soybean than oats and alfalfa, however, most carbohydrates in MAOM were plant-derived regardless of litter quality. At the end of the incubations, more of the accumulated MAOM-N was potentially mineralizable in soils amended with high quality litters. Nevertheless, most of the litter-C remained as residual litter; just 12% was mineralized to CO2 and 13% was transferred to MAOM. Our results demonstrate several unexpected effects of litter quality on MAOM stabilization including the direct stabilization of plant-derived carbohydrates

Maize and Prairie Root Contributions to Soil CO2 Emissions in the Field

Increasing soil carbon content via agricultural practices not only enhances the production potential of the land, but also counteracts rising atmospheric CO2 levels. When predicting production systems’ effects on soil carbon, quantifying CO2 efflux derived from live roots is of particular importance as it is a through-flux and does not signify depletion of soil carbon. This field study aimed to measure and compare soil CO2 emissions derived from roots in annual and perennial agroecosystems. We used periodic 48-hour shading over two growing seasons to estimate root growth-derived CO2 in continuously grown maize (CC) with grain and 50% stover harvested each year, unfertilized reconstructed tallgrass prairie (P), and the same prairie grown with spring nitrogen fertilization (PF), both which had biomass harvested post-frost. In CC, P, and PF root-derived CO2 contributed to 28, 31, and 30% of each crop’s respective growing season cumulative CO2 emissions in 2012, and 19, 24, and 28% in 2013, respectively. Season-cumulative root-derived CO2 was not proportional to end-of-season belowground biomass (BGB): P had nearly twice the BGB of PF, but their cumulative root-derived fluxes were not significantly different in either year. A significant proportion of soil CO2 emissions is derived from roots, making it a critical process to consider when comparing or modeling soil emissions of cropped or prairie soils. Using BGB alone may not be a useful proxy for estimating root contributions