Daniel Liptzin

Global patterns in belowground communities

Although belowground ecosystems have been studied extensively and soil biota play integral roles in biogeochemical processes, surprisingly we have a limited understanding of global patterns in belowground biomass and community structure. To address this critical gap, we conducted a meta-analysis of published data (> 1300 datapoints) to compare belowground plant, microbial and faunal biomass across seven of the major biomes on Earth. We also assembled data to assess biome-level patterns in belowground microbial community composition. Our analysis suggests that variation in microbial biomass is predictable across biomes, with microbial biomass carbon representing 0.6-1.1% of soil organic carbon (r(2) = 0.91) and 1-20% of total plant biomass carbon (r(2) = 0.42). Approximately 50% of total animal biomass can be found belowground and soil faunal biomass represents < 4% of microbial biomass across all biomes. The structure of belowground microbial communities is also predictable: bacterial community composition and fungal : bacterial gene ratios can be predicted reasonably well from soil pH and soil C : N ratios respectively. Together these results identify robust patterns in the structure of belowground microbial and faunal communities at broad scales which may be explained by universal mechanisms that regulate belowground biota across biomes.

Alpine treeline of western North America; linking organism-to-landscape dynamics

Although the ecological dynamics of the alpine treeline ecotone are influenced by climate, it is an imperfect indicator of climate change. Mechanistic processes that shape the ecotone—seed rain, seed germination, seedling establishment and subsequent tree growth form, or, conversely tree dieback—depend on microsite patterns. Growth forms affect wind and snow, and so develop positive and negative feedback loops that create these microsites. As a result, complex landscape patterns are generated at multiple spatial scales. Although these mechanistic processes are fundamentally the same for all forest-tundra ecotones across western North America, factors such as prior climate, underlying geology and geomorphology, and genetic constraints of dominant tree species lead to geographic differences in the responses of particular ecotones to climate change.

Early-successional dynamics of single-aged mixed hardwood stands in a southern New England forest, USA

Abstract The pattern of stand development was studied in two mixed-species single-aged stands that originated after true clearcutting at the Great Mountain Forest in northwestern Connecticut. One stand was located on a mesic swale-till site and the other on a more xeric thin-till site. At the time of cutting all sprout growth and advanced regeneration was eradicated, except for 1-year-old red oak ( Quercus rubra ) seedlings. Twenty-eight years after the stands originated trees were stratified by shade tolerance such that the canopies were dominated by the intolerant species (paper birch Betula papyrifera , gray birch B. populifolia , pin cherry Prunus pensylvanica ) with mid-tolerant species (black birch B. lenta , black cherry P. serotina ) becoming prevalent. The number of stems was decreasing but the basal area was steadily increasing. The pattern in species-specific growth rates and crown position were common to both sites; but there were also differences between the sites in the sizes of trees and positioning of the mid-tolerant trees in the canopy. At age 28 the more mesic swale-till site had fewer, taller trees, with pioneer species more typical of northern hardwood climates. On this site red oak was doing poorly, all gray birch and eastern white pine ( Pinus strobus ) had died, black birch and black cherry were beginning to dominate the canopy of the stand, and a significant understory of beech ( Fagus grandifolia ) had developed through the encroachment of root suckers from the stand edge. Sugar maple ( Acer saccharum ), white ash ( Fraxinus americana ) and eastern hemlock ( Tsuga canadensis ), all significant components of the swale-till site before clearcutting, were noticeably absent. On the thin-till site red oak had not attained the canopy of the stand but was still a significant component of the mid-story with red maple ( A. rubrum ). However, black birch was self-thinning more rapidly on the thin-till site than that of the swale-till suggesting that red oak and red maple might well dominate the canopy within another 20 years. In general, the diameter growth rate of the thin-till site currently lags behind the swale-till by approximately 10 years.

Life in the Main Channel: Long-Term Hydrologic Control of Microbial Mat Abundance in McMurdo Dry Valley Streams, Antarctica

Given alterations in global hydrologic regime, we examine the role of hydrology in regulating stream microbial mat abundance in the McMurdo Dry Valleys, Antarctica. Here, perennial mats persist as a desiccated crust until revived by summer streamflow, which varies inter-annually, and has increased since the 1990s. We predicted high flows to scour mats, and intra-seasonal drying to slow growth. Responses were hypothesized to differ based on mat location within streams, along with geomorphology, which may promote (high coverage) or discourage (low coverage) accrual. We compared hydrologic trends with the biomass of green and orange mats, which grow in the channel, and black mats growing at stream margins for 16 diverse stream transects over two decades. We found mat biomass collectively decreased during first decade coinciding with low flows, and increased following elevated discharges. Green mat biomass showed the greatest correlations with hydrology and was stimulated by discharge in high coverage transects, but negatively correlated in low coverage due to habitat scour. In contrast, orange mat biomass was negatively related to flow in high coverage transects, but positively correlated in low coverage because of side-channel expansion. Black mats were weakly correlated with all hydrologic variables regardless of coverage. Lastly, model selection indicated the best combination of predictive hydrologic variables for biomass differed between mat types, but also high and low coverage transects. These results demonstrate the importance of geomorphology and species composition to modeling primary production, and will be useful in predicting ecological responses of benthic habitats to altered hydrologic regimes.

Agriculture's Contribution to Nitrate Contamination of Californian Groundwater (1945-2005).

Nitrogen (N) use in intensive agriculture can degrade groundwater resources. However, considerable time lags between groundwater recharge and extraction complicate source attribution and remedial responses. We construct a historic N mass balance of two agricultural regions of California to understand trends and drivers of past and present N loading to groundwater (1945-2005). Changes in groundwater N loading result from historic changes in three factors: the extent of agriculture (cropland area and livestock herd increased 120 and 800%, respectively), the intensity of agriculture (synthetic and manure waste effluent N input rates increased by 525 and 1500%, respectively), and the efficiency of agriculture (crop and milk production per unit of N input increased by 25 and 19%, respectively). The net consequence has been a greater-than-order-of-magnitude increase in nitrate (NO) loading over the time period, with 163 Gg N yr now being leached to groundwater from approximately 1.3 million ha of farmland (not including alfalfa [ L.]). Meeting safe drinking water standards would require NO leaching reductions of over 70% from current levels through reductions in excess manure applications, which accounts for nearly half of all groundwater N loading, and through synthetic N management improvements. This represents a broad challenge given current economic and technical conditions of California farming if farm productivity is to be maintained. The findings illustrate the growing tension-characteristic of agricultural regions globally-between intensifying food, feed, fiber, and biofuel production and preserving clean water.

Spatial patterns of total and available N and P at alpine treeline

Background and aimsVegetation can have direct and indirect effects on soil nutrients. To test the effects of trees on soils, we examined the patterns of soil nutrients and nutrient ratios at two spatial scales: at sites spanning the alpine tundra/subalpine forest ecotone (ecotone scale), and beneath and beyond individual tree canopies within the transitional krummholz zone (tree scale).MethodsSoils were collected and analyzed for total carbon (C), nitrogen (N), and phosphorus (P) as well as available N and P on Niwot Ridge in the Colorado Rocky Mountains.ResultsTotal C, N, and P were higher in the krummholz zone than the forest or tundra. Available P was also greatest in the krummholz zone while available N increased from the forest to the tundra. Throughout the krummholz zone, total soil nutrients and available P were higher downwind compared to upwind of trees.ConclusionsThe krummholz zone in general, and downwind of krummholz trees in particular, are zones of nutrient accumulation. This pattern indicates that the indirect effects of trees on soils are more important than the direct effects. The higher N:P ratios in the tundra suggest nutrient dynamics differ from the lower elevation sites. We propose that evaluating soil N and P simultaneously in soils may provide a robust assay of ecosystem nutrient limitation.

Response of the Phytoplankton Community in an Alpine Lake to Drought Conditions: Colorado Rocky Mountain Front Range, U.S.A.

Abstract Lakes may serve as sentinels for the impacts of changing climate in alpine areas. In the Rocky Mountain region, 2002 was a year with extremely low snowpack. We examined the summer phytoplankton community in Green Lake 4 for a 6-year period that included the summer of 2002. The phytoplankton community variation was examined in the context of the changes in physical and chemical properties of Green Lake 4. The physical changes associated with the 2002 drought included warmer surface water temperatures and greater hydraulic residence times; whereas the chemical changes included higher concentrations of acid neutralizing capacity (ANC) and major ions. During the summer of 2002 the phytoplankton community was dominated by Synedra sp. and Ankyra sp.; two previously rare species. The growth of Synedra sp. was sufficient to cause a decrease in silica concentrations, which has not been observed in other summers in the water quality monitoring record. The results of a redundancy analysis (RDA) indicated that concentrations of major ions and ANC were aligned with Synedra sp. and Ankyra sp. during the 2002 drought year. Following the 2002 drought year, Chrysococcus sp. and Chlorococcum sp., which became abundant, were aligned with nitrate in the RDA. These results indicate that the response of the phytoplankton community to the extreme drought was most strongly correlated with water quality changes that occurred, rather than temperature and hydraulic residence time. The dominant species in the post-drought phytoplankton community were found to be associated with nitrate, which is brought to the watershed by atmospheric deposition and may represent an anthropogenic driver of phytoplankton community composition.

High potential for iron reduction in upland soils

Changes in the redox state of iron (Fe) can be coupled to the biogeochemical cycling of carbon (C), nitrogen, and phosphorus, and thus regulate soil C, ecosystem nutrient availability, and greenhouse gas production. However, its importance broadly in non-flooded upland terrestrial ecosystems is unknown. We measured Fe reduction in soil samples from an annual grassland, a drained peatland, and a humid tropical forest We incubated soil slurries in an anoxic glovebox for 5.5 days and added sodium acetate daily at rates up to 0.4 mg C x (g soil)(-1) x d(-1). Soil moisture, poorly crystalline Fe oxide concentrations, and Fe(II) concentrations differed among study sites in the following order: annual grassland < drained peatland < tropical forest (P < 0.001 for all characteristics). All of the soil samples demonstrated high Fe reduction potential with maximum rates over the course of the incubation averaging 1706 ± 66, 2016 ± 12, and 2973 ± 115 μg Fe x (g soil)(-1) x d(-1) (mean ± SE) for the tropical forest, annual grassland, and drained peatland, respectively. Our results suggest that upland soils from diverse ecosystems have the potential to exhibit high short-term rates of Fe reduction that may play an important role in driving soil biogeochemical processes during periods of anaerobiosis.

Spatial patterns in oxygen and redox sensitive biogeochemistry in tropical forest soils

Humid tropical forest soils are characterized by warm temperatures, abundant rainfall, and high rates of biological activity that vary considerably in both space and time. These conditions, together with finely textured soils typical of humid tropical forests lead to periodic low redox conditions, even in well-drained upland environments. The relationship between redox and biogeochemical processes has been studied for decades in saturated environments like wetlands and sediments, but much less is known about redox dynamics in upland soils. The goal of this study was to understand the spatial variability of redox sensitive biogeochemistry within and across two forest types at the ends of a high rainfall gradient (3500 to 5000 mm y−1) in the Luquillo Experimental Forest, Puerto Rico. The two sites differed significantly in average soil chemical and physical properties, but the scale of variability was similar across sites, with greater variability in soil gas concentrations than extractable Fe and P. Soil P and Fe pools and trace gas concentrations were more strongly correlated with each other and exhibited more spatial structure at the wetter site. While the within-site relationships among these redox sensitive variables were typically weak, the relationships across sites were much stronger. We provide a conceptual model that elucidates how the strength of the relationships between indicators of redox-sensitive biogeochemical processes depends on the spatial scale of analysis.

Winter gas exchange between the atmosphere and snow-covered soils on Niwot Ridge, Colorado, USA

Background: There is a growing interest in understanding the gas exchange between the atmosphere and seasonally snow-covered regions, especially in light of projections that climate change will alter the timing and extent of seasonal snow cover. In snow-covered ecosystems, gas fluxes are due both to microbial activity in the snow-covered soils and to chemical and physical reactions with the various gases and/or dissolved constituents in the snowpack. Niwot Ridge, in the Colorado Rocky Mountains, has one of the most extensive sets of measurements of winter gas exchange globally. Aims: Our goal was to examine the temporal patterns and environmental controls on Niwot Ridge of gas fluxes for gases with different sources and sinks. Methods: Here, we review the concentrations and fluxes that have been measured for carbon dioxide, nitrous oxide, methane, nitrogen oxides, ozone, gaseous elemental mercury and volatile organic carbon compounds. Results and Conclusions: We looked for similarities and differences among the gases, but in many cases, the origin, fate and controls of these fluxes still need to be determined. However, we believe that many of the biologically driven reactions are the result of exponential growth of a winter microbial community during the long period of stable environmental conditions under the seasonal snowpack.