Eric B. Sucre

The Challenge of Lignocellulosic Bioenergy in a Water-Limited World

It is hoped that lignocellulosic sources will provide energy security, offset carbon dioxide enrichment of the atmosphere, and stimulate the development of new economic sectors. However, little is known about the productivity and sustainability of plant cell-wall energy industries. In this study, we used 16 global circulation models to project the global distribution of relative water availability in the coming decades and summarized the available data on the water-use efficiency of tree- and grass-based bioenergy systems. The data on bioenergy water use were extremely limited. Productivity was strongly correlated with water-use efficiency, with C4 grasses having a distinct advantage in this regard. Our analysis of agro climatic drivers of bioenergy productivity suggests that relative water availability will be one of the most important climatic changes to consider in the design of bioenergy systems.

Biofuel intercropping effects on soil carbon and microbial activity

Biofuels will help meet rising demands for energy and, ideally, limit climate change associated with carbon losses from the biosphere to atmosphere. Biofuel management must therefore maximize energy production and maintain ecosystem carbon stocks. Increasingly, there is interest in intercropping biofuels with other crops, partly because biofuel production on arable land might reduce availability and increase the price of food. One intercropping approach involves growing biofuel grasses in forest plantations. Grasses differ from trees in both their organic inputs to soils and microbial associations. These differences are associated with losses of soil carbon when grasses become abundant in forests. We investigated how intercropping switchgrass (Panicum virgalum), a major candidate for cellulosic biomass production, in loblolly pine (Pinus taeda) plantations affects soil carbon, nitrogen, and microbial dynamics. Our design involved four treatments: two pine management regimes where harvest residues (i.e., biomass) were left in place or removed, and two switchgrass regimes where the grass was grown with pine under the same two biomass scenarios (left or removed). Soil variables were measured in four 1-ha replicate plots in the first and second year following switchgrass planting. Under switchgrass intercropping, pools of mineralizable and particulate organic matter carbon were 42% and 33% lower, respectively. These declines translated into a 21% decrease in total soil carbon in the upper 15 cm of the soil profile, during early stand development. The switchgrass effect, however, was isolated to the interbed region where switchgrass is planted. In these regions, switchgrass-induced reductions in soil carbon pools with 29%, 43%, and 24% declines in mineralizable, particulate, and total soil carbon, respectively. Our results support the idea that grass inputs to forests can prime the activity of soil organic carbon degrading microbes, leading to net reductions in stocks of soil carbon. Active microbial biomass, however, is higher under switchgrass, and this microbial biomass is a dominant precursor of soil carbon formation. Future studies need to investigate soil carbon dynamics throughout the lifetime of intercropping rotations to evaluate whether increases in microbial biomass can offset initial declines in soil carbon, and hence, maintain ecosystem carbon stocks.

Greenhouse gas emissions in response to nitrogen fertilization in managed forest ecosystems

Nitrogen (N) fertilizer use in managed forest ecosystems is increasing in the United States and worldwide to enhance social, economical and environmental services. However, the effects of N-fertilization on production and consumption of greenhouse gases (GHGs), especially carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in managed forest ecosystems are poorly understood, unlike in agriculture where effects are well documented. Therefore, a review of the available literature was conducted to comprehend the effects of N-fertilization on CO2, CH4 and N2O emissions in managed forest ecosystems to summarize sources, sinks, and controlling factors, as well as potential mitigation strategies and research gaps to reduce GHG emissions. This review clearly identifies the importance of N-fertilizer management practices on CO2, CH4 and N2O emissions. Potential N management practices to mitigate GHG emissions in managed forest ecosystems include improving N uptake efficiency, identifying and managing spatial variation in soil fertility, using the right fertilizer source at the right time, adopting appropriate methods of N-fertilizer application, and introducing nitrification/denitrification inhibitors. Nitrogen-fertilizer response is affected by soil physical (e.g., moisture, drainage, bulk density, and texture), chemical (e.g., nutrient availability, labile carbon, soil pH, and C/N ratio) and local climatic factors (e.g., temperature, relative humidity, and rainfall). Therefore, the interactions of these factors on GHG emissions need to be considered while evaluating N-fertilizer management practices. Existing studies are often limited, focusing primarily on temperate forest ecosystems, lacking estimation of net emissions considering all three predominant soil-derived GHGs, and were often conducted on a small scale, making upscaling challenging. Therefore, large-scale studies conducted in diverse climates, evaluating cumulative net emissions, are needed to better understand N-fertilization effects on GHG emissions and develop mitigation strategies. Mitigation strategies and research gaps have also been identified, which require the collaborative efforts of forest owners, managers, and scientists to increase adoption of N-fertilization best management practices and understand the importance of N-fertilizer management strategies in reducing emissions and enhancing the net GHG sink potential for managed forest ecosystems.

Microbial nitrogen cycling response to forest-based bioenergy production.

Concern over rising atmospheric CO2 and other greenhouse gases due to fossil fuel combustion has intensified research into carbon-neutral energy production. Approximately 15.8 million ha of pine plantations exist across the southeastern United States, representing a vast land area advantageous for bioenergy production without significant landuse change or diversion of agricultural resources from food production. Furthermore, intercropping of pine with bioenergy grasses could provide annually harvestable, lignocellulosic biomass feedstocks along with production of traditional wood products. Viability of such a system hinges in part on soil nitrogen (N) availability and effects of N competition between pines and grasses on ecosystem productivity. We investigated effects of intercropping loblolly pine (Pinus taeda) with switchgrass (Panicum virgatum) on microbial N cycling processes in the Lower Coastal Plain of North Carolina, USA. Soil samples were collected from bedded rows of pine and interbed space of two treatments, composed of either volunteer native woody and herbaceous vegetation (pine-native) or pure switchgrass (pine-switchgrass) in interbeds. An in vitro 15N pool-dilution technique was employed to quantify gross N transformations at two soil depths (0-5 and 5-15 cm) on four dates in 2012-2013. At the 0-5 cm depth in beds of the pine-switchgrass treatment, gross N mineralization was two to three times higher in November and February compared to the pine-native treatment, resulting in increased NH4(+) availability. Gross and net nitrification were also significantly higher in February in the same pine beds. In interbeds of the pine-switchgrass treatment, gross N mineralization was lower from April to November, but higher in February, potentially reflecting positive effects of switchgrass root-derived C inputs during dormancy on microbial activity. These findings indicate soil N cycling and availability has increased in pine beds of the pine-switchgrass treatment compared to those of the pine-native treatment, potentially alleviating any negative effects of N competition between pine and switchgrass. We expect that reduced soil C in the pine-switchgrass treatment, effects of pine and switchgrass rooting on soil C availability, and plant N demand are major factors influencing soil N transformations. Future research should examine rooting architecture in-intercropped systems and the effects on soil microbial communities and function.

Nitrous Oxide Fluxes in Fertilized L. Plantations across a Gradient of Soil Drainage Classes.

The effect of fertilizer management on nitrous oxide (NO) fluxes in agricultural ecosystems is well documented; however, our knowledge of these effects in managed forests is minimal. We established a comprehensive research study to address this knowledge gap across a range of soil drainage classes (poorly, moderately, and well drained) common in southern pine plantation management. Fertilizer treatments in each drainage class comprised of control (no fertilizer), urea + phosphorus (P), and P-coated urea fertilizer (CUF). Fertilization (168 kg N ha) occurred independently during the spring, summer, and fall to assess the effects of application timing. Nitrous oxide sampling, using vented static chambers, started immediately after seasonal fertilizer application and was performed every 6 wk for more than 1 yr. Time-integrated net annual NO emissions increased with urea (1.15 kg NO-N ha) and CUF (0.88 kg NO-N ha) application compared with unfertilized control (0.22 kg NO-N ha). Mean annual NO flux was significantly increased with fall fertilization (1.17 kg NO-N ha) relative to spring (0.73 kg NO-N ha) or summer (0.33 kg NO-N ha). Similarly, average annual NO flux was higher in poorly drained soils (1.40 kg NO-N ha) than in moderately drained (0.46 kg NO-N ha) and well-drained soils (0.39 kg NO-N ha). This study suggests that NO emissions after fertilization can be minimized by avoiding fall fertilization and poorly drained soils and by selecting enhanced-efficiency N fertilizers over urea.

Effects of forest-based bioenergy feedstock production on shallow groundwater quality of a drained forest soil

Managed forests in southern U.S. are a potential source of lignocellulosic biomass for biofuel production. Changes in management practices to optimize biomass production may impact the quality of waters draining to nutrient-sensitive waters in coastal plain regions. We investigated shallow groundwater quality effects of intercropping switchgrass (Panicum virgatum L.) with managed loblolly pine (Pinus taeda L.) to produce bioenergy feedstock and quality sawtimber in a poorly drained soil of eastern North Carolina, U.S.A. Treatments included PINE (traditional pine production), PSWITCH (pine-switchgrass intercropped), SWITCH (switchgrass monoculture) and REF (mature loblolly pine stand). Each treatment was replicated three times on 0.8ha plots drained by parallel-open ditches, 1.0-1.2m deep and 100m apart. Water samples were collected monthly or more frequently after fertilizer application. Water samples were analyzed for organic nitrogen (ON), ammonium N (NH4+- N), and nitrite+nitrate N (NO3-+ NO2-- N), ortohophosphate phosphorus (OP), and total organic carbon (TOC). Overall, PSWITCH did not significantly affect shallow groundwater quality relative to PINE and SWITCH. ON, NO3-+ NO2-- N, and TOC concentrations in PSWITCH, PINE and SWITCH were substantially elevated during the two years after tree harvest and site establishment. The elevated nutrient concentrations at the beginning of the study were likely caused by a combination of rapid organic matter decomposition of the abundant supply of post-harvest residues, warming of exposed soil surfaces and reduction of plant nutrient uptake that can occur after harvesting, and pre-plant fertilization. Nutrient concentrations returned to background levels observed in REF during the third year after harvest.

Soil and Aggregate-Associated Carbon in a Young Loblolly Pine Plantation: Influence of Bioenergy Intercropping

ABSTRACT In order to assess the carbon (C) footprint of forest-based bioenergy systems, it is necessary to quantify soil C storage. This study addressed effects of intercropping loblolly pine (Pinus taeda L.) with switchgrass (Panicum virgatum L.) for wood and bioenergy production on soil C storage in coastal North Carolina, USA. Spaces between rows of bedded pine were intercropped with switchgrass or contained native vegetative regrowth after site preparation. Two years after switchgrass establishment, soils were collected from beds and interbeds of each treatment, and C concentration and &dgr;13C were measured in bulk soils and aggregate fractions. Soil C concentration, soil C density (Mg ha−1), and aggregate-associated C were lower in pine beds adjacent to switchgrass compared with pines adjacent to native regrowth. In the greater than 2,000-&mgr;m aggregate size class, 11% of C was derived from new pine inputs in beds of the pine-switchgrass treatment compared to the pine-native treatment. These results indicate that increased belowground C flow in pine beds adjacent to switchgrass may be driving breakdown soil C. In the pine-switchgrass intercropping treatment, a greater percentage of aggregates (by weight and C content) was found in the 2,000- to 250-&mgr;m size class of both beds and interbeds, suggesting that this aggregate size class is sensitive to management. This study provides a baseline analysis of C storage under different management scenarios in pine forests and for investigating long-term (10+ years) impacts. Although presence of switchgrass reduced soil C over the short term, bioenergy intercropping may still be pragmatic from an economical and land-use diversification view point.