Switchgrass Cultivar and Intraspecific Diversity Impacts on Nitrogen Use Efficiency

Bioenergy feedstock production is an important component of the national renewable energy strategy, which is based on biomass supply. Biofuels for ethanol production may be produced in high-input crop production systems, but the efficacy of these systems for increasing net energy yields over its full life-cycle compared to traditional fuels is under debate, because it is now evident that the benefits of feedstock production are maximized only when biofuels are derived from feedstocks produced with much lower life-cycle greenhouse-gas emissions than traditional fossil fuels. To this end, the reduction of agricultural inputs is key to developing an effective biofuel feedstock crop. Native prairie grasses have low-input production requirements, and upon land conversion for biofuel production they have positive impacts on belowground carbon (C) sequestration, a measure of soil quality. Specifically, Panicum virgatum (hereafter switchgrass), a perennial C4 grass native to the mid-west of the United States, is a promising bioenergy crop. It has large root systems, which allow it to produce large amounts of biomass with less water and nutrient requirements than traditional bioenergy crops, such as corn. To produce switchgrass feedstock in an environmentally sustainably manner (i.e., with the least amount of fertilizer inputs), we will need to adopt agricultural practices that promote N cycling efficiency in the system. Previous studies have found that different cultivars of switchgrass vary significantly in specific root length (SRL), and greater SRL may be linked to greater N acquisition owing to the root systems’ greater surface area. In addition, it has been found that growing switchgrass in genotypically diverse mixtures enhanced biomass production, which may result from belowground niche differentiation and complementarity effects that enhance N acquisition. With this study, I aimed to evaluate (1) whether differences in the architecture among root systems of switchgrass cultivars led to differences in the efficiency of nitrogen uptake, and (2) whether growing switchgrass cultivars in diverse mixtures would enhance the efficiency of nitrogen cycling though niche differentiation and complementarity effects. Our experiment was conducted at the Sustainable Bioenergy Crop Research Facility at the Fermilab National Environmental Research Park, where experimental field plots consisted of seven switchgrass cultivars, planted either in monoculture or in diverse mixtures of 2, 4, or 6 randomly selected cultivars. To evaluate differences in nitrogen use efficiency (NUE) among cultivars in monocultures and among diversity treatments, I applied a stable isotope 15N tracer at the beginning of the growing season. Following senescence, the switchgrass was harvested and the percent of 15N recovered was measured in the aboveground biomass to determine NUE. I found that switchgrass cultivars differed in NUE and these differences could potentially be linked to germplasm origin in relation to the field site. I also found that NUE was not influenced by increases in cultivar diversity. Our results suggest that NUE is not the sole mechanism behind greater biomass production associated with enhanced diversity

Precipitation Impacts on Root Morphology and Decomposition Processes in the Sagebrush

Increased carbon dioxide concentrations due to the burning of fossil fuels and other anthropogenic activities has led to climate change, which may lead to changes in precipitation. Changes in precipitation may affect plant root morphology and chemistry. Since plant roots intermingle with the soil matrix, any changes in these characteristics can affect the soil microbial community and root decomposition. Understanding how precipitation changes affect root decomposition is important, because increased root decomposition enhances the flux of carbon from soil back to the atmosphere. Since soil is the largest terrestrial reservoir for carbon, a small increase in carbon respiration through decomposition will exacerbate climate change. With this study we aimed to assess (1) if changes in precipitation impact root architecture and (2) if fine and coarse roots decompose differently. Roots were collected from a site that has had ongoing precipitation treatments for 20 years including ambient (i.e. natural conditions), summer (+200 mm), or winter (+200 mm) additions of water. The roots were scanned and separated into fine and coarse diameter size classes. They were then decomposed in soil in airtight microcosms for approximately 6 months. During the incubation we measured carbon respiration, a measure of root decomposition. We found that (1) precipitation treatments significantly affected root morphology, and (2) these changes did not affect root decomposition rates. Our results can contribute to making predictions for the impact of climate change on decomposition processes

Switchgrass Root Decomposition Impacts on Soil Carbon Sequestration

The use of fossil fuels has many negative environmental consequences associated with it, including high carbon-dioxide (CO2) emissions. Carbon dioxide is a greenhouse gas, that contributes to global climate change. The negative impacts of fossil fuel use has led to increased interest in the development environmentally sustainable biofuels. Cellulosic-derived ethanol is one such biofuel, and the perennial prairie-grass Pancium virgatum L. (hereafter: switchgrass) has been identified as a viable ethanol source. Ethanol not only offers reduced emissions compared to fossil fuel use, but some ethanol crops, including switchgrass, may actively reduce atmospheric CO2 by sequestering carbon (C) in the soil, while not competing with food production. The plant characteristics that promote increased soil C sequestration are not currently well understood. Switchgrass cultivars display variable root morphologies, and we hypothesized that decomposition of different root types would impact soil C sequestration differently. Specifically, we predicted that decomposition of coarse root systems would lead to greater C stabilization than would the decomposition of fine root systems. In order to test this hypothesis, we decomposed either fine or coarse roots from 3 different switchgrass cultivars in soil microcosms for 60 or 120 days and then determined the amount and stability of the soil C. Our results support our hypothesis that coarse root decomposition leads to increased soil C sequestration

The Effects of Specific Root Length on Soil Decomposition Processes and Carbon Respiration

Soils store more carbon (C) than vegetation and the atmosphere combined, thus small changes in the amount of C stored in soil can alter atmospheric CO2 concentrations. Since CO2 is a greenhouse gas, it is important that we understand the biological processes that drive soil C storage for accurate predictions of future global temperatures. Soil C storage is a function of the balance between soil C input via plants and soil C output via microbial decomposition. Plant roots contribute to regulating both processes, making them an important component of the global C cycle. Plant root turnover is a major contributor to soil organic carbon (SOC) input, which regulates soil microbial activity and thus the rate of C cycles among plants, soils and the atmosphere. The majority of root-C input to soil is derived from the finest roots in a system, but it is currently uncertain how specific root length (i.e. the relative abundance of fine versus coarse roots) affects decomposition processes and thus loss of soil C to the atmosphere. The objective of this experiment is to determine if and how differences in specific root length (cm/g) affect decomposition processes in soil. We predicted that increased surface area from fibrous root systems increases decomposition rates in soil due to enhanced root derived C inputs. To determine how root architecture affects decomposition and C respiration, we collected soil from six different switchgrass cultivars grown at Argonne National Lab in Illinois. The samples were collected at different depths (0-60cm), and then incubated for 60 days. The roots were extracted and analyzed using WinRhizo (Regent Instruments Inc., Quebec, Canada) for fine (0-.5mm), medium (0.5-1mm) and coarse (1-2.5mm) architecture. Following analysis, root samples were dried and weighed. We found significant differences in root architecture among switchgrass cultivars and are currently evaluating how those differences affect decomposition processes

Differential Priming of Soil Carbon Driven by Soil Depth and Root Impacts on Carbon Availability

Enhanced root-exudate inputs can stimulate decomposition of soil carbon (C) by priming soil microbial activity, but the mechanisms controlling the magnitude and direction of the priming effect remain poorly understood. With this study we evaluated how differences in soil C availability affect the impact of simulated root exudate inputs on priming. We conducted a 60-day laboratory incubation with soils collected (60 cm depth) from under six switchgrass (Panicum virgatum) cultivars. Differences in specific root length (SRL) among cultivars were expected to result in small differences in soil C inputs and thereby create small differences in the availability of recent labile soil C; whereas soil depth was expected to create large overall differences in soil C availability. Soil cores from under each cultivar (roots removed) were divided into depth increments of 0–10, 20–30, and 40–60 cm and incubated with addition of either: (1) water or (2) 13C-labeled synthetic root exudates (0.7 mg C/g soil). We measured CO2 respiration throughout the experiment. The natural difference in 13C signature between C3 soils and C4 plants was used to quantify cultivar-induced differences in soil C availability. Amendment with 13C-labeled synthetic root-exudate enabled evaluation of SOC priming. Our experiment produced three main results: (1) switchgrass cultivars differentially influenced soil C availability across the soil profile; (2) small differences in soil C availability derived from recent root C inputs did not affect the impact of exudate-C additions on priming; but (3) priming was greater in soils from shallow depths (relatively high total soil C and high ratio of labile-to-stable C) compared to soils from deep depths (relatively low total soil C and low ratio of labile-to-stable C). These findings suggest that the magnitude of the priming effect is affected, in part, by the ratio of root exudate C inputs to total soil C and that the impact of changes in exudate inputs on the priming of SOC is regulated differently in surface soil compared to subsoil