Brian D. Strahm

NITRATE SORPTION IN A VARIABLE-CHARGE FOREST SOIL OF THE PACIFIC NORTHWEST

Despite the general acceptance that soils of the Pacific Northwest (United States) have a high retention capacity for phosphate (PO4−3), and perhaps even sulfate (SO4−2), few studies in the region have investigated the potential of physicochemical mechanisms to retain nitrate (NO3−). The specific objectives of this study were to (i) determine the capacity of different horizons of a mesic, Typic Fulvudand under intensive forest management in southwestern Washington to sorb NO3−, (ii) determine the point of zero net charge for each horizon of this soil, and (iii) relate specific mineralogical characteristics to the physicochemical soil properties. Five soil pits were excavated to a depth of approximately 150 cm, and soil samples were composited by genetic horizon, including A, AB, 2Bw1, and 2Bw2 horizons. Through batch equilibration, NO3− sorption isotherms were created for each horizon and showed an increase in sorption with both depth and increased NO3-N solution concentrations. The point of zero net charge of the two Bw horizons was determined to exist between a pH range of 3.5 to 3.6. Selective dissolution techniques of the mineral soil were used to determine the presence of crystalline and noncrystalline aluminosilicates. Allophane and imogolite contents of the less than the 2-mm mineral soil fraction ranged from 0.6% to 3.0% across all observations, although noncrystalline forms of Fe and Al comprised a majority of the subsurface horizons, and increased with depth. The presence of variable charge soil components on this site, coupled with the acidic soil pH regime associated with the coniferous forest stand, allows for the concentration-dependent sorption of NO3−, which may serve to retain a significant proportion of an otherwise highly mobile form of an essential plant nutrient.

Dissolved carbon and nitrogen leaching following variable logging-debris retention and competing-vegetation control in Douglas-fir plantations of western Oregon and Washington

We examined the effect of logging-debris retention and competing-vegetation control (CVC, initial or annual applications) on dissolved organic carbon (DOC), dissolved organic nitrogen, and nitrate-N leaching to determine the rela- tive potential of these practices to contribute to soil C and N loss at two contrasting sites. Annual CVC resulted in higher soil water nitrate-N concentration and flux, with the magnitude and duration of the effect greatest at the high-N site. Most of the increase in nitrate-N at the low-N site occurred in treatments where logging debris was retained. Dissolved organic nitrogen increased at the high-N site in March of each year following annual CVC, but the contribution of this increase to total N concentration was small (2%-4% of total N flux). There was no effect of logging-debris retention or CVC treat- ment on soil water DOC concentrations, indicating that DOC inputs from logging debris and competing vegetation were either retained or consumed in the mineral soil. The estimated increase in leaching flux of dissolved C and N associated with the treatments was low relative to total soil pools, making it unlikely that loss of these elements via leaching will negatively affect future soil productivity at these sites.

Effects of harvest frequency and biosolids application on switchgrass yield, feedstock quality, and theoretical ethanol yield.

Sustainable development of a bioenergy industry will require low‐cost, high‐yielding biomass feedstock of desirable quality. Switchgrass (Panicum virgatum L.) is one of the primary feedstock candidates in North America, but the potential to grow this biomass crop using fertility from biosolids has not been fully explored. The objective of this study was to examine the effects of harvest frequency and biosolids application on switchgrass in Virginia, USA. ‘Cave‐in‐Rock’ switchgrass from well‐established plots was cut once (November) or twice (July and November) per year between 2010 and 2012. Class A biosolids were applied once at rates of 0, 153, 306, and 459 kg N ha−1 in May 2010. Biomass yield, neutral and acid detergent fiber, cellulose, hemicellulose, lignin, and ash were determined. Theoretical ethanol potential (TEP, l ethanol Mg−1 biomass) and yield (TEY, l ethanol ha−1) were calculated based on cellulose and hemicellulose concentrations. Cutting twice per season produced greater biomass yields than one cutting (11.7 vs. 9.8 Mg ha−1) in 2011, but no differences were observed in other years. Cutting once produced feedstock with greater TEP (478 vs. 438 l Mg−1), but no differences in TEY between cutting frequencies. Biosolids applied at 153, 306, and 459 kg N ha−1 increased biomass yields by 25%, 37%, and 46%, and TEY by 25%, 34%, and 42%, respectively. Biosolids had inconsistent effects on feedstock quality and TEP. A single, end‐of‐season harvest likely will be preferred based on apparent advantages in feedstock quality. Biosolids can serve as an effective alternative to N fertilizer in switchgrass‐to‐energy systems.

The role of sorption in control of riverine dissolved organic carbon concentrations by riparian zone soils in the Amazon basin

Terrestrially derived dissolved organic carbon (DOC) is an important component of biogeochemical cycling in river channels. Despite this, the processes controlling its export from terrestrial ecosystems to river channels are not well known. Sorption is thought to be an important process in controlling riverine DOC concentrations. We describe the sorption of litter-derived DOC by soils of the Barreiras sediment formation in the Amazon basin. Soils were collected along a single transect of a soil toposequence. Clay-rich soils dominate on plateaus and slopes, whereas sandy soils dominate in valleys that compose riparian zones of the region. Soils from each topographic position were subjected to sorption experiments, and soil properties were analyzed. Based on our results, the toposequence was divided into two sorption regions. Plateau and slope soils sorbed 60 ± 5% of initial DOC, whereas valley soils sorbed 34 ± 4%. Plateau and slope soils sorbed DOC twice as quickly (t½ ≤ 1440 min) as valley soils (t½ = 2880 min). A regression of sorption experiment results and soil properties showed that sorption correlates with both soil organic C content and mineral surface area. Our results suggest that control of riverine DOC concentrations by riparian zones is the result of the sorption mechanism operating in soils of this region of the Amazon River basin. In conjunction with hydrologic models and more detailed soil data, it may be possible to apply results from similar replicated studies to the landscapes of the Amazon basin in an effort to better understand C dynamics in tropical river basins.

The Fall River Long-Term Site Productivity study in coastal Washington: site characteristics, methods, and biomass and carbon and nitrogen stores before and after harvest.

The Fall River research site in coastal Washington is an affiliate installation of the North American Long-Term Soil Productivity (LTSP) network, which constitutes one of the world’s largest coordinated research programs addressing forest management impacts on sustained productivity. Overall goals of the Fall River study are to assess effects of biomass removals, soil compaction, tillage, and vegetation control on site properties and growth of planted Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Biomass-removal treatments included removal of commercial bole (BO), bole to 5-cm top diameter (BO5), total tree (TT), and total tree plus all legacy woody debris (TT+). Vegetation control (VC) effects were tested in BO, while soil compaction and compaction plus tillage were imposed in BO+VC treatment. All treatments were imposed in 1999. The preharvest stand contained similar amounts of carbon (C) above the mineral soil (292 Mg/ha) as within the mineral soil to 80-cm depth including roots (298 Mg/ha). Carbon stores above the mineral soil ordered by size were live trees (193 Mg/ha), old-growth logs (37 Mg/ha), forest floor (27 Mg/ha), old-growth stumps and snags (17 Mg/ha), coarse woody debris (11 Mg/ha), dead trees/snags (7 Mg/ha), and understory vegetation (0.1 Mg/ha). The mineral soil to 80-cm depth contained 248 Mg C/ha, and roots added 41 Mg/ha. Total nitrogen (N) in mineral soil and roots (13 349 kg/ha) was more than 10 times the N store above the mineral soil (1323 kg/ha). Postharvest C above mineral soil decreased to 129, 120, 63, and 50 Mg/ha in BO, BO5, TT, and TT+, respectively. Total N above the mineral soil decreased to 722, 747, 414, and 353 Mg/ha in BO, BO5, TT, and TT+, respectively. The ratio of total C above the mineral soil to total C within the mineral soil was markedly altered by biomass removal, but proportions of total N stores were reduced only 3 to 6 percent owing to the large soil N reservoir on site.

Changes in soil nitrogen cycling in a northern temperate forest ecosystem during succession

Nitrogen (N) transformations in forest soils are fundamentally important to plant and microbial N nutrition and the N balance of forest ecosystems, but changes in the patterns and rates of N transformations during forest succession are poorly understood. In order to better understand how soil N cycling changes during ecosystem succession, we analyzed four years of soil N cycling measurements in a 90-year-old secondary forest undergoing dieback of early-successional, dominant canopy trees. We expected that tree mortality would decrease root biomass, leading to increased soil NH4+ availability, and that these changes would prompt fundamental shifts in the N cycle such as the initiation of significant nitrification and increased cycling of oxidized N compounds in gas phase and soil solution. As expected, indices of soil NH4+ and NO3− availability increased with successional stage (defined as the proportion of dead trees), and were negatively correlated with the amount of fine root biomass. However, the standing amount of fine root biomass was not affected by tree mortality; increased soil NH4+ and NO3− availability therefore more likely resulted from successional increases in N-mineralization than decreases in root N uptake. Nitrification (as indicated by NO efflux as a proxy) increased due to elevated substrate (NH4+) availability, and the soil solution NO3− concentration increased as a result. Soil N2O efflux was not affected by succession, nor was it related to other N cycling parameters. Collectively, these results indicate that recent successional advancement has accelerated soil N cycling and shifted the N economy of this ecosystem towards greater importance of NO3−.

Nitrogen leaching from Douglas-fir forests after urea fertilization.

Leaching of nitrogen (N) after forest fertilization has the potential to pollute ground and surface water. The purpose of this study was to quantify N leaching through the primary rooting zone of N-limited Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] forests the year after fertilization (224 kg N ha(-1) as urea) and to calculate changes in the N pools of the overstory trees, understory vegetation, and soil. At six sites on production forests in the Hood Canal watershed, Washington, tension lysimeters and estimates of the soil water flux were used to quantify the mobilization and leaching of NO(3)-N, NH(4)-N, and dissolved organic nitrogen below the observed rooting depth. Soil and vegetation samples were collected before fertilization and 1 and 6 mo after fertilization. In the year after fertilization, the total leaching beyond the primary rooting zone in excess of control plots was 4.2 kg N ha(-1) (p = 0.03), which was equal to 2% of the total N applied. The peak NO(3)-N concentration that leached beyond the rooting zone of fertilized plots was 0.2 mg NO(3)-N L(-1). Six months after fertilization, 26% of the applied N was accounted for in the overstory, and 27% was accounted for in the O+A horizon of the soil. The results of this study indicate that forest fertilization can lead to small N leaching fluxes out of the primary rooting zone during the first year after urea application.

Soil bacterial and fungal communities show distinct recovery patterns during forest ecosystem restoration

ABSTRACT Bacteria and fungi are important mediators of biogeochemical processes and play essential roles in the establishment of plant communities, which makes knowledge about their recovery after extreme disturbances valuable for understanding ecosystem development. However, broad ecological differences between bacterial and fungal organisms, such as growth rates, stress tolerance, and substrate utilization, suggest they could follow distinct trajectories and show contrasting dynamics during recovery. In this study, we analyzed both the intra-annual variability and decade-scale recovery of bacterial and fungal communities in a chronosequence of reclaimed mined soils using next-generation sequencing to quantify their abundance, richness, β-diversity, taxonomic composition, and cooccurrence network properties. Bacterial communities shifted gradually, with overlapping β-diversity patterns across chronosequence ages, while shifts in fungal communities were more distinct among different ages. In addition, the magnitude of intra-annual variability in bacterial β-diversity was comparable to the changes across decades of chronosequence age, while fungal communities changed minimally across months. Finally, the complexity of bacterial cooccurrence networks increased with chronosequence age, while fungal networks did not show clear age-related trends. We hypothesize that these contrasting dynamics of bacteria and fungi in the chronosequence result from (i) higher growth rates for bacteria, leading to higher intra-annual variability; (ii) higher tolerance to environmental changes for fungi; and (iii) stronger influence of vegetation on fungal communities. IMPORTANCE Both bacteria and fungi play essential roles in ecosystem functions, and information about their recovery after extreme disturbances is important for understanding whole-ecosystem development. Given their many differences in phenotype, phylogeny, and life history, a comparison of different bacterial and fungal recovery patterns improves the understanding of how different components of the soil microbiota respond to ecosystem recovery. In this study, we highlight key differences between soil bacteria and fungi during the restoration of reclaimed mine soils in the form of long-term diversity patterns, intra-annual variability, and potential interaction networks. Cooccurrence networks revealed increasingly complex bacterial community interactions during recovery, in contrast to much simpler and more isolated fungal network patterns. This study compares bacterial and fungal cooccurrence networks and reveals cooccurrences persisting through successional ages.

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.

Predicting macronutrient concentrations from loblolly pine leaf reflectance across local and regional scales

Given the economic importance of loblolly pine (Pinus taeda) in the southeastern US, there is a need to establish efficient methods of detecting potential nutrient deficiencies that may limit productivity. This study evaluated the use of remote sensing for macronutrient assessment in loblolly pine. Reflectance-based models were developed at two spatial scales: (1) a natural nutrient gradient across the species’ range, and (2) localized fertilization and genotype treatments in North Carolina and Virginia. Fascicles were collected regionally from 237 samples of 3 flushes at 18 sites, and locally from 72 trees with 2 fertilization treatments and 6 genotypes. Sample spectral reflectance was calculated using a spectroradiometer, and nutrient concentrations were measured with dry combustion and wet chemical digestion. Results were analyzed statistically using nutrient correlations with reflectance and common vegetation indices, and partial least squares regression (PLSR). PLSR performed well at the regional scale, with R2 values for nitrogen, phosphorus, potassium, calcium, and magnesium of 0.81, 0.70, 0.68, 0.42, and 0.51, respectively. No model successfully predicted nutrients at local sites for any treatment or canopy stratum. This discrepancy implies that a large nutrient range and/or spatial scale may be necessary to model loblolly pine nutrients with spectral reflectance.