C4 Perennial Grass Research Articles

Belowground Carbon Dynamics in Tropical Perennial C4 Grass Agroecosystems

Effective soil management is critical to achieving climate change mitigation in plant-based renewable energy systems, yet limitations exist in our understanding of dynamic belowground responses to the cultivation of energy crops. To better understand the belowground dynamics following cultivation of a grassland in a high-yielding tropical perennial C4 grass in a zero-tillage production system, changes in soil carbon (C) pools were quantified, modeled, and projected and the chemical composition of the aggregate-protected pool was determined. Multiple C pools with different ecosystem functions and turnover increased following cultivation: immediately available microbial substrate (measured as hot water-soluble C) and active C (determined through laboratory incubation) increased by 12% and 30% respectively over time and soil C accumulated significantly in multiple physical fractions. A more rapid and dynamic nature of multiple C pools and transfers between pools existed than is often assumed in belowground models used widely in the field to simulate soil C accumulation. Multiple indicators of fresh roots, including the more easily degraded lignin monomers and root-derived long chain substituted fatty acids, appeared in aggregate-protected pools of cultivated soils over time since planting. This rapid transfer of plant inputs through active and intermediate C pools into mineral-dominated pools is the ultimate outcome required for building soil C stocks. Initial model runs suggested that this is evident, even on a two-year frame, in transfer rates of 0.485 and 0.890 from active to slow and slow to passive pools respectively. The rapid transfer of fresh root-derived input to stable pool suggests that soil C under zero-tillage management may be resilient to disturbances, such as replanting following a kill-harvest, that would otherwise result in losses from unprotected or readily available pools.

Miscanthus × giganteus: Regeneration system with assessment of genetic and epigenetic stability in long-term in vitro culture

Species of Miscanthus Anderss., C4 perennial rhizomatous grasses with low input requirements, have become representative crops for lignocellulosic bioenergy and material resources in temperate climates. Most of the commercial cultivars appropriate for cropping within Europe and North America are based on Miscanthus × giganteus species. Because of their triploid hybrid nature, micropropagation of these species using in vitro cultures is a common method both of plant multiplication on a commercial scale and of long-term preservation of their valuable genotypes. The somaclonal variation that can occur during in vitro culture may be useful for strain improvement during plant breeding, but is undesirable for long-term genotype preservation; therefore, the genetic and epigenetic stability of regenerated plants needs to be assessed. The objectives of our study were (1) to develop an effective system for Miscanthus × giganteus plantlet production, starting from callus induction, and shoot and root regeneration, and ending with plant acclimatization; and (2) to evaluate any genetic and epigenetic instability induced by long periods of in vitro cultivation using ISSR, RAPD and MS-ISSR molecular markers on selected M. × giganteus clones. We present an efficient and stable method of M. × giganteus regeneration and preservation by long-term in vitro culture. Both morphological and molecular marker analyses revealed no major genetic or epigenetic changes among clonally propagated M. × giganteus that had been maintained by long-term shoot culture. However, when screening for genetic variation, particular attention should be paid to genomic regions associated with GACA, GATA, AG and CTC repeats.

VOC emissions and carbon balance of two bioenergy plantations in response to nitrogen fertilization: A comparison of Miscanthus and Salix

Energy crops are an important renewable source for energy production in future. To ensure high yields of crops, N fertilization is a common practice. However, knowledge on environmental impacts of bioenergy plantations, particularly in systems involving trees, and the effects of N fertilization is scarce. We studied the emission of volatile organic compounds (VOC), which negatively affect the environment by contributing to tropospheric ozone and aerosols formation, from Miscanthus and willow plantations. Particularly, we aimed at quantifying the effect of N fertilization on VOC emission. For this purpose, we determined plant traits, photosynthetic gas exchange and VOC emission rates of the two systems as affected by N fertilization (0 and 80 kg ha−1 yr−1). Additionally, we used a modelling approach to simulate (i) the annual VOC emission rates as well as (ii) the OH. reactivity resulting from individual VOC emitted. Total VOC emissions from Salix was 1.5- and 2.5-fold higher compared to Miscanthus in non-fertilized and fertilized plantations, respectively. Isoprene was the dominating VOC in Salix (80–130 μg g−1 DW h−1), whereas it was negligible in Miscanthus. We identified twenty-eight VOC compounds, which were released by Miscanthus with the green leaf volatile hexanal as well as dimethyl benzene, dihydrofuranone, phenol, and decanal as the dominant volatiles. The pattern of VOC released from this species clearly differed to the pattern emitted by Salix. OH. reactivity from VOC released by Salix was ca. 8-times higher than that of Miscanthus. N fertilization enhanced stand level VOC emissions, mainly by promoting the leaf area index and only marginally by enhancing the basal emission capacity of leaves. Considering the higher productivity of fertilized Miscanthus compared to Salix together with the considerably lower OH. reactivity per weight unit of biomass produced, qualified the C4-perennial grass Miscanthus as a superior source of future bioenergy production.

High-solids ethanol fermentation with single-stage methane anaerobic digestion for maximizing bioenergy conversion from a C4 grass (Pennisetum purpereum)

High-solids simultaneous saccharification and fermentation integrating single-stage anaerobic digestion was designed for bioenergy conversion from steam-exploded Pennisetum purpereum, a perennial C4 grass. Three scenarios for ethanol production, methane production and ethanol-methane co-production were investigated and compared in terms of production yield. A terminal ethanol titer of 62.6 ± 1.9 g/L with the highest ethanol yield of 90.9% was achieved when the fed-batch mode was carried out at a final substrate loading of 35% (w/v) dry matter content. The ethanol stillage was used for methane production in a semi-continuous CSTR system operated with different organic loading rates (OLRs) at a hydraulic retention time (HRT) of 30 days. The high performance was achieved at OLR of 6.0 g VS·L−1 d−1, average yielding 358.7L CH4 per kilogram volatile solid (VS) with a total VS removal rate of 73.8%. On the basis of mass balance, the overall production yield achieved in this study was: 121.6 g ethanol + 110.6 g methane per kilogram grass with a finally 89.6% of cellulose recovery + 62.5% hemicellulose recovery. These results indicated that the high-solids ethanol fermentation with single-stage methane anaerobic digestion of lignocellulosic biomass is a valid approach to maximize the holocellulose bioconversion and improve the bioenergy production.

Quantitative trait loci for cell wall composition traits measured using near-infrared spectroscopy in the model C4 perennial grass Panicum hallii

BackgroundBiofuels derived from lignocellulosic plant material are an important component of current renewable energy strategies. Improvement efforts in biofuel feedstock crops have been primarily focused on increasing biomass yield with less consideration for tissue quality or composition. Four primary components found in the plant cell wall contribute to the overall quality of plant tissue and conversion characteristics, cellulose and hemicellulose polysaccharides are the primary targets for fuel conversion, while lignin and ash provide structure and defense. We explore the genetic architecture of tissue characteristics using a quantitative trait loci (QTL) mapping approach in Panicum hallii, a model lignocellulosic grass system. Diversity in the mapping population was generated by crossing xeric and mesic varietals, comparative to northern upland and southern lowland ecotypes in switchgrass. We use near-infrared spectroscopy with a primary analytical method to create a P. hallii specific calibration model to quickly quantify cell wall components.ResultsAsh, lignin, glucan, and xylan comprise 68% of total dry biomass in P. hallii: comparable to other feedstocks. We identified 14 QTL and one epistatic interaction across these four cell wall traits and found almost half of the QTL to localize to a single linkage group.ConclusionsPanicum hallii serves as the genomic model for its close relative and emerging biofuel crop, switchgrass (P. virgatum). We used high throughput phenotyping to map genomic regions that impact natural variation in leaf tissue composition. Understanding the genetic architecture of tissue traits in a tractable model grass system will lead to a better understanding of cell wall structure as well as provide genomic resources for bioenergy crop breeding programs.

First Report of Leaf Spot Caused by Bipolaris victoriae on Switchgrass in Georgia

Switchgrass (Panicum virgatum) is a native North American perennial C4 grass and was selected by the Department of Energy as a model bioenergy crop. However, for the purposes of bioenergy production, switchgrass will be grown on a large number of hectares for multiple years. As a result of extended production periods, foliar diseases may impact biomass yield and quality. In the summer of 2010, light-to-dark brown, irregularly-shaped leaf spots with white necrotic centers and chlorosis were observed on the Southern Switchgrass Diversity Panel (SSDP) maintained at the University of Georgia in Watkinsville. The SSDP consists of 480 switchgrass genotypes, and has been characterized at phenotypic (plant height, plant color) and genotypic levels (Evans et al. 2014). Disease symptoms increased in the field each year and were observed on 30% of the SSDP entries during the summer of 2016. Symptomatic leaf tissue was collected from the field in 2016, surface sterilized, air dried and plated on 2% water agar. After ...

Genome-Wide Identification, Phylogeny, and Expression Analysis of ARF Genes Involved in Vegetative Organs Development in Switchgrass.

Auxin response factors (ARFs) have been reported to play vital roles during plant growth and development. In order to reveal specific functions related to vegetative organs in grasses, an in-depth study of the ARF gene family was carried out in switchgrass (Panicum virgatum L.), a warm-season C4 perennial grass that is mostly used as bioenergy and animal feedstock. A total of 47 putative ARF genes (PvARFs) were identified in the switchgrass genome (2n = 4x = 36), 42 of which were anchored to the seven pairs of chromosomes and found to be unevenly distributed. Sixteen PvARFs were predicted to be potential targets of small RNAs (microRNA160 and 167). Phylogenetically speaking, PvARFs were divided into seven distinct subgroups based on the phylogeny, exon/intron arrangement, and conserved motif distribution. Moreover, 15 pairs of PvARFs have different temporal-spatial expression profiles in vegetative organs (2nd, 3rd, and 4th internode and leaves), which implies that different PvARFs have specific functions in switchgrass growth and development. In addition, at least 14 pairs of PvARFs respond to naphthylacetic acid (NAA) treatment, which might be helpful for us to study on auxin response in switchgrass. The comprehensive analysis, described here, will facilitate the future functional analysis of ARF genes in grasses.

Variability in grass forage quality and quantity in response to elevated CO2 and water limitation

AbstractGlobal climate change is expected to alter carbon dioxide concentration ([CO2]) and water availability, with uncertain impacts on agriculture. Forage quality and quantity in grazing systems are of particular concern because C3 and C4 plants respond differently to altered environmental conditions. In a growth chamber, we compared crude protein content, biomass recovery and total crude protein across a set of perennial C3 and C4 grasses from the northern U.S. Great Plains under elevated [CO2] and simulated drought. Simulated 95% confidence intervals indicate both C3 and C4 grasses increased forage quality and quantity under elevated [CO2]. C4 grasses were generally resistant to water limitation while forage quality and quantity of C3 grasses declined under simulated drought. Our results are consistent with literature on forage quantity responses to elevated [CO2] and drought, but forage quality responses contradict expectations. We suggest measuring plant functional traits might better elucidate response mechanisms and ameliorate methodological differences even if traits are not directly applicable to grazing management.

Exploiting genetic and physiological variation of the native forage grass Trichloris crinita for revegetation in arid and semi‐arid regions: An integrative review

AbstractIn arid, semi‐arid and dry subhumid regions, which represent ~ 41% of the Earth's land surface, desertification and soil degradation are very frequent, leading to low soil fertility and productivity. In these regions, revegetation with locally adapted native species may aid in ameliorating desertification processes. Trichloris crinita is a C4 perennial grass native to arid and semi‐arid regions of the American continent. Its good forage quality, drought tolerance, resistance to trampling and grazing, and rapid growth and competing aggressiveness among other native species warrant its use as forage and for revegetation purposes. In the last decades, many studies have revealed broad intraspecific genetic variation for ecophysiological, morphological, biomass production, nutritional quality (as forage) and adaptive stress response‐related traits. Also, results from field trials evaluating T. crinita genotypes as forage and for restoration of degraded areas suggest great potential for—and have encouraged—its utilization under different habitats and environmental conditions. In this integrative review, we compiled and discussed the most relevant research data regarding T. crinita, focusing on aspects and traits that influence its utilization both as forage and in rehabilitation of degraded lands. Challenges and prospects towards the improvement of this species in breeding programmes with specific goals are discussed.

Microwave pretreatment effects on switchgrass and miscanthus solubilization in subcritical water and hydrolysate utilization for hydrogen production

Microwave pretreatment is an energy-efficient and environmentally benign technology that can be used to reduce the recalcitrance of complex biomass structure. Switchgrass (Panicum virgatum L.) and miscanthus (Miscanthus x giganteus) are perennial C4 grasses that are being developed as bioenergy crops because they have high yield potential and desirable agronomic traits. These materials are promising candidates for biofuels, bioproducts and green chemicals production from biomass.In the present study, miscanthus and switchgrass biomass were solubilized in subcritical water after pretreatment by microwave at different processing temperatures. The hydrolysates obtained were evaluated for hydrogen-rich gas production by aqueous-phase reforming (APR).Higher temperature microwave processing reduced the biomass recalcitrance resulting in microwave treated materials having 7–10% higher solubility in subcritical water than untreated materials. However, gasification of pretreated biomass hydrolysates produced less gaseous products compared to untreated biomass for both switchgrass and miscanthus. Miscanthus biomass was more vulnerable to destruction by microwave treatment and recalcitrance of this biomass was achieved at lower temperature compared to switchgrass. Miscanthus biomass that was not microwave treated produced the highest gas yield. Microwave pretreatment caused significant increases in the formation of ungasified solid carbon residue in the APR process.

Salinity tolerance among a large range of bermudagrasses (Cynodon spp.) relative to other halophytic and non-halophytic perennial C4 grasses

The increasing demand on potable water has resulted in a greater reliance on poorer quality water, including saline sources, for maintaining forage and turfgrasses in agricultural and urban landscapes. Consequently, it will be crucial to identify grasses that can tolerate saline irrigation water. This study aimed to determine salinity tolerance among a large range of bermudagrasses relative to other perennial C4 grasses and test the relationship between salt tolerance and drought resistance. We report the salinity tolerance of 70 genotypes of mostly Australian bermudagrass ecotypes that were compared to halophytic cultivars of seashore paspalum (Paspalum vaginatum Swartz) and a non-halophytic cultivar of Queensland blue couch (Digitaria didactyla Willd) using a flood and drain sand culture system with salt treatments 1–40dSm−1. For the first time for C4 grasses, salt tolerance was determined by comparing total biomass of the grasses with and without salt treatment. Large genetic variation in salinity tolerance was identified and six bermudagrasses collected from saline habitats had salinity tolerance equal to that of seashore paspalum under the salinity treatments used in this study. There was no correlation between salt tolerance and drought resistance phenotypes determined from our previous research. Canopy temperature differential during salt stress was negatively correlated (r=−0.71 to −0.91, P<0.001) to salt tolerance and has potential to be used for screening bermudagrasses for salt tolerance using flood and drain sand culture. Salinity levels above 20dSm−1 for 8 weeks appeared to be effective for detecting large variation for salt tolerance in bermudagrass.

Metabolic Pathways Involved in Carbon Dioxide Enhanced Heat Tolerance in Bermudagrass.

Global climate changes involve elevated temperature and CO2 concentration, imposing significant impact on plant growth of various plant species. Elevated temperature exacerbates heat damages, but elevated CO2 has positive effects on promoting plant growth and heat tolerance. The objective of this study was to identify metabolic pathways affected by elevated CO2 conferring the improvement of heat tolerance in a C4 perennial grass species, bermudagrass (Cynodon dactylon Pers.). Plants were planted under either ambient CO2 concentration (400 μmol⋅mol-1) or elevated CO2 concentration (800 μmol⋅mol-1) and subjected to ambient temperature (30/25°C, day/night) or heat stress (45/40°C, day/night). Elevated CO2 concentration suppressed heat-induced damages and improved heat tolerance in bermudagrass. The enhanced heat tolerance under elevated CO2 was attributed to some important metabolic pathways during which proteins and metabolites were up-regulated, including light reaction (ATP synthase subunit and photosystem I reaction center subunit) and carbon fixation [(glyceraldehyde-3-phosphate dehydrogenase, GAPDH), fructose-bisphosphate aldolase, phosphoglycerate kinase, sedoheptulose-1,7-bisphosphatase and sugars) of photosynthesis, glycolysis (GAPDH, glucose, fructose, and galactose) and TCA cycle (pyruvic acid, malic acid and malate dehydrogenase) of respiration, amino acid metabolism (aspartic acid, methionine, threonine, isoleucine, lysine, valine, alanine, and isoleucine) as well as the GABA shunt (GABA, glutamic acid, alanine, proline and 5-oxoproline). The up-regulation of those metabolic processes by elevated CO2 could at least partially contribute to the improvement of heat tolerance in perennial grass species.

Drought and salt stress in Chrysopogon zizanioides leads to common and specific transcriptomic responses and may affect essential oil composition and benzylisoquinoline alkaloids metabolism

Abiotic stresses affect crop productivity worldwide. Understanding molecular mechanisms of plant abiotic stress tolerance is important for developing stress tolerant crop plants for sustaining crop productivity in future. Chrysopogon zizanioides (vetiver) is a perennial C4 grass reported to be tolerant to water, salinity and submergence stress. Here, we subjected C. zizanioides seedlings to salt and drought stress and carried out whole transcriptome profiling of leaf and root tissues. Assessing the global transcriptome changes under drought and salt stress resulted in the identification of several genes contributing to stress response in this species. Overall, more transcriptomic changes were observed in leaf tissue compared to root tissue. The response to either stress manifested primarily as upregulation of gene expression in both leaf and root. The study identified stress responsive genes commonly and differently regulated under stress/tissue conditions. Several DEGs in our data were identified as enzymes involved in biosynthesis of essential oil components. The differential expression of these genes under drought and salt stress may affect the vetiver essential oil composition under these stresses. Similarly, several genes involved in the biosynthesis of benzylisoquinoline alkaloids including morphin were found to be differentially expressing in our data. Our data may facilitate further molecular studies on stress tolerance of C. zizanioides. The DEGs from our results are potential candidates for understanding and engineering abiotic stress tolerance in plants.

An oxygen isotope chronometer for cellulose deposition: the successive leaves formed by tillers of a C4 perennial grass.

Multiannual time series of (palaeo)hydrological information can be reconstructed from the oxygen isotope composition of cellulose (δ18 OCel ) in biological archives, for example, tree rings, but our ability to temporally resolve information at subannual scale is limited. We capitalized on the short and predictable leaf appearance interval (2.4d) of a perennial C4 grass (Cleistogenes squarrosa), to assess its potential for providing highly time-resolved δ18 OCel records of vapour pressure deficit (VPD). Plants grown at low (0.63kPa) or high (1.58kPa) VPD were swapped between VPD environments and exposed to the new environment for 7d with simultaneous 13 CO2 labelling. Then, leaves were sampled by age/position along individual tillers. Five leaves at different developmental stages were growing simultaneously. The period of most-active leaf elongation, from 10 to 90% of final length, lasted 6.6d, and ~80% of all carbon and oxygen incorporation in whole-leaf cellulose occurred within 7d. Cellulose deposition stopped at (or shortly after) full leaf expansion. The direction of change, low-to-high or high-to-low VPD, had no differential effect on new oxygen and carbon incorporation in cellulose. Successive leaves produced by tillers of C.squarrosa provide a δ18 OCel record useful for reconstructions of short-term hydrological dynamics.

A flexible method to determine the sensitive physical parameter combination for soil carbon under five plant types

AbstractModel parameter errors are a critical source of uncertainty in simulations of soil carbon. It is important to determine which parameter errors should be reduced to improve the simulation ability of soil carbon. It is essential to determine the most sensitive parameter combination. In this study, a new flexible approach is employed to identify the most sensitive parameter combination based on the conditional nonlinear optimal perturbation related to parameter (CNOP‐P) using the Lund‐Potsdam‐Jena model. The CNOP‐P is a type of parameter error that triggers the upper bound of uncertainty in a numerical simulation and prediction, for which a nonlinear effect can be reported. To compare the sensitivity of parameter combinations, the one‐at‐a‐time (OAT) approach is also applied to determine the sensitivity of each parameter. Using the CNOP‐P approach, parameter errors can really cause large simulation errors for soil carbon throughout an entire study region (2674.69 g C/m2). Results show the most sensitive parameter combinations of soil carbon are different for five plant functional types (PFTs). For all cases, fair (fraction of decomposed litter emitted as CO2 to the atmosphere) and (fraction of soil‐bound decomposed litter entering intermediate soil carbon pool) comprise the most sensitive parameter combination for boreal needle‐leaved evergreen trees, boreal needle‐leaved summergreen trees, temperate broadleaved summergreen trees, temperate broadleaved evergreen trees, and C3 perennial grass. For C3 perennial grass under semi‐arid conditions, the hydrology process is also critical in the uncertainty of simulated soil carbon. The most sensitive parameter combination using the new flexible approach is different to that of the top rank of sensitivity of each parameter when employing the CNOP‐P and OAT methods, which implies that a nonlinear effect of the parameter combination is crucial for determining the sensitive parameter combination. The sensitivity of the parameter combination is dependent on the PFTs and climate condition. Among factors causing the uncertainty of simulated soil carbon, the fast‐decomposing soil carbon pool plays the main contribution. It is considered that the physical process involved in the fast‐decomposing soil carbon pool needs to be improved as a priority compared to other physical processes to enhance the simulation ability of soil carbon.

Toward Genomics-Based Breeding in C3 Cool-Season Perennial Grasses.

Most important food and feed crops in the world belong to the C3 grass family. The future of food security is highly reliant on achieving genetic gains of those grasses. Conventional breeding methods have already reached a plateau for improving major crops. Genomics tools and resources have opened an avenue to explore genome-wide variability and make use of the variation for enhancing genetic gains in breeding programs. Major C3 annual cereal breeding programs are well equipped with genomic tools; however, genomic research of C3 cool-season perennial grasses is lagging behind. In this review, we discuss the currently available genomics tools and approaches useful for C3 cool-season perennial grass breeding. Along with a general review, we emphasize the discussion focusing on forage grasses that were considered orphan and have little or no genetic information available. Transcriptome sequencing and genotype-by-sequencing technology for genome-wide marker detection using next-generation sequencing (NGS) are very promising as genomics tools. Most C3 cool-season perennial grass members have no prior genetic information; thus NGS technology will enhance collinear study with other C3 model grasses like Brachypodium and rice. Transcriptomics data can be used for identification of functional genes and molecular markers, i.e., polymorphism markers and simple sequence repeats (SSRs). Genome-wide association study with NGS-based markers will facilitate marker identification for marker-assisted selection. With limited genetic information, genomic selection holds great promise to breeders for attaining maximum genetic gain of the cool-season C3 perennial grasses. Application of all these tools can ensure better genetic gains, reduce length of selection cycles, and facilitate cultivar development to meet the future demand for food and fodder.

Effects of the frequency and the rate of N enrichment on community structure in a temperate grassland

Nitrogen (N) enrichment caused by human activities threatens biodiversity and alters plant community composition and structure. It has been found that heavy and infrequent N inputs may over-estimate species extinction, but it remains unclear whether plant community structure will equally respond to frequent reactive N enriched conditions. We independently manipulated the rates and the frequencies of N addition in a temperate steppe, northern China, between 2008 and 2013. We found that plant community structure changes, measured by ‘Euclidean distance’ involving species richness, composition and productivity, were significantly positively related to increasing N enrichment rates rather than frequencies. Changes in aboveground net primary productivity (ANPP), plant species richness and shifts in dominant species were observed. Community ANPP increased with N enrichment, whereas species richness reduced. The frequency of N enrichment increased species richness but had no impacts on community ANPP and the relative ANPP of the two dominant species, C₃ perennial bunchgrass Stipa grandis and C₃ perennial rhizome grass Leymus chinensis. The ANPP and relative ANPP of the two dominant species were significantly negatively correlated with each other. Moreover, changes in the relative ANPP of S. grandis was negatively associated with the changes in community structure. After 5 years’ treatment, direct influence of the frequency of N enrichment on plant community structure was not observed, but the effects of the rate of N enrichment were apparent. Our results suggested that further study in various ecosystems and with long-term and well-controlled comparisons the frequency vs. the rate of N enrichment may still be needed.

Soil particulate organic matter increases under perennial bioenergy crop agriculture

Annual row crop agriculture contributes to carbon (C) losses from Midwest soils, while the establishment of perennial crops for food and fuel has the potential to increase soil C stocks. Perennial grasses eliminate the need for tillage and increase belowground biomass, both critical to the accumulation and conservation of soil organic matter and to soil C sequestration. The effect of C4 perennial grasses on particulate organic matter carbon (POM-C), consisting primarily of partially decomposed plant material, was evaluated in Illinois, where native switchgrass (Panicum virgatum L.) and a sterile hybrid of the Asian grass Miscanthus (Miscanthus x giganteus) were planted as bioenergy feedstocks at the University of Illinois Energy Farm in 2008. Six years after establishment of perennial crops, POM-C was compared with a maize-maize-soybean (Zea mays L., Glycine max L.) rotation typical of the area and a 28-species restored prairie. POM-C concentrations increased for all crops between 31 and 71% over 6 years, with the greatest increases in prairie and M. x giganteus soils. POM-C concentrations were highest at the 0–10 cm depth. Isotopic analyses showed 23–44% of POM-C was new C4 material under perennial bioenergy crops after 6 years. As soil organic matter is primarily plant-derived, increases in POM-C reflect increased organic matter inputs or decreases in the rate of decomposition from the cessation of tillage. Increases in POM-C under annual row crops may result from the incorporation of aboveground organic matter by tillage, while POM-C increases in untilled perennial crops mirror increases in belowground biomass. As soil aggregation protects POM-C from microbial degradation, untilled soils under long-term perennial crop production increase the residence time for soil C.

Allometric Models for Predicting Aboveground Biomass and Carbon Stock of Tropical Perennial C4 Grasses in Hawaii.

Biomass is a promising renewable energy option that provides a more environmentally sustainable alternative to fossil resources by reducing the net flux of greenhouse gasses to the atmosphere. Yet, allometric models that allow the prediction of aboveground biomass (AGB), biomass carbon (C) stock non-destructively have not yet been developed for tropical perennial C4 grasses currently under consideration as potential bioenergy feedstock in Hawaii and other subtropical and tropical locations. The objectives of this study were to develop optimal allometric relationships and site-specific models to predict AGB, biomass C stock of napiergrass, energycane, and sugarcane under cultivation practices for renewable energy and validate these site-specific models against independent data sets generated from sites with widely different environments. Several allometric models were developed for each species from data at a low elevation field on the island of Maui, Hawaii. A simple power model with stalk diameter (D) was best related to AGB and biomass C stock for napiergrass, energycane, and sugarcane, (R2 = 0.98, 0.96, and 0.97, respectively). The models were then tested against data collected from independent fields across an environmental gradient. For all crops, the models over-predicted AGB in plants with lower stalk D, but AGB was under-predicted in plants with higher stalk D. The models using stalk D were better for biomass prediction compared to dewlap H (Height from the base cut to most recently exposed leaf dewlap) models, which showed weak validation performance. Although stalk D model performed better, however, the mean square error (MSE)-systematic was ranged from 23 to 43 % of MSE for all crops. A strong relationship between model coefficient and rainfall was existed, although these were irrigated systems; suggesting a simple site-specific coefficient modulator for rainfall to reduce systematic errors in water-limited areas. These allometric equations provide a tool for farmers in the tropics to estimate perennial C4 grass biomass and C stock during decision-making for land management and as an environmental sustainability indicator within a renewable energy system.

Pollen-mediated gene flow from transgenic to non-transgenic switchgrass (Panicum virgatum L.) in the field

BackgroundSwitchgrass is C4 perennial grass species that is being developed as a cellulosic bioenergy feedstock. It is wind-pollinated and considered to be an obligate outcrosser. Genetic engineering has been used to alter cell walls for more facile bioprocessing and biofuel yield. Gene flow from transgenic cultivars would likely be of regulatory concern. In this study we investigated pollen-mediated gene flow from transgenic to nontransgenic switchgrass in a 3-year field experiment performed in Oliver Springs, Tennessee, U.S.A. using a modified Nelder wheel design. The planted area (0.6 ha) contained sexually compatible pollen source and pollen receptor switchgrass plants. One hundred clonal switchgrass ‘Alamo’ plants transgenic for an orange-fluorescent protein (OFP) and hygromycin resistance were used as the pollen source; whole plants, including pollen, were orange-fluorescent. To assess pollen movement, pollen traps were placed at 10 m intervals from the pollen-source plot in the four cardinal directions extending to 20 m, 30 m, 30 m, and 100 m to the north, south, west, and east, respectively. To assess pollination rates, nontransgenic ‘Alamo 2’ switchgrass clones were planted in pairs adjacent to pollen traps.ResultsIn the eastward direction there was a 98% decrease in OFP pollen grains from 10 to 100 m from the pollen-source plot (Poisson regression, F1,8 = 288.38, P < 0.0001). At the end of the second and third year, 1,820 F1 seeds were collected from pollen recipient-plots of which 962 (52.9%) germinated and analyzed for their transgenic status. Transgenic progeny production detected in each pollen-recipient plot decreased with increased distance from the edge of the transgenic plot (Poisson regression, F1,15 = 12.98, P < 0.003). The frequency of transgenic progeny detected in the eastward plots (the direction of the prevailing wind) ranged from 79.2% at 10 m to 9.3% at 100 m.ConclusionsIn these experiments we found transgenic pollen movement and hybridization rates to be inversely associated with distance. However, these data suggest pollen-mediated gene flow is likely to occur up to, at least, 100 m. This study gives baseline data useful to determine isolation distances and other management practices should transgenic switchgrass be grown commercially in relevant environments.