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Root and Mycorrhizal Respiration at Harvard Forest Soil Warming Experiments 2007-2008

Soil heating typically causes large initial increases in soil respiration, with the enhancement lessening over time. This transient response is attributed to rapid decomposition of labile soil C compounds in the first years of heating, but the potential role of changes in root and mycorrhizal respiration is not well understood. To assess the degree to which root respiration adjusts to warmer soil temperature regimes, specific root respiration rates (nmol CO2/g/s) are being measured in three soil warming experiments at Harvard Forest. Soils in the experiments had been heated since 1991, 2003 and 2006. Respiration rates for fine roots (less than 1 mm) from control and heated plots were measured both at a common reference temperature of 18 deg C and at the ambient soil temperature of the measurement date for the control and heated (+ 5 deg C) treatments. Mycorrhizal respiration is being assessed through the use of hyphal ingrowth bags which allow determination both mycorrhizal hyphal biomass and respiration rate. Changes in mycorrhizal community composition are also being investigated. Specific questions we will address include: Does rapid temperature acclimation occur in roots of large perennial woody plants? How do root biomass, root N concentration, and root respiration rates adjust to long-term changes in soil temperature and moisture and concomitant changes in N availability? How is mycorrhizal biomass and activity influenced by the effects of warmer soil temperatures on host C balance and soil N availability? How do the short- and long-term responses of roots and mycorrhizae to warming and associated changes in soil nutrient cycling affect soil CO2 efflux and C availability for aboveground NPP? Are the interrelationships between warmer soil temperature regimes and C fluxes to and from roots and mycorrhizae adequately described by current ecophysiological models?

openCC0May 2025View details →

Barre Woods Soil Warming Experiment at Harvard Forest since 2001

Two of the major results of our original soil warming study were that: (1) warming stimulated the decay of a labile soil carbon pool; and (2) it also increased the availability of inorganic nitrogen to plants. Because of the small size of the original warming plots an important question we have not been able to answer conclusively is: Has the increase in available nitrogen led to an increase in carbon storage in the vegetation? And if yes, how much? In a systems context an additional question is: What is the balance between the carbon lost from the soil and the carbon stored in the vegetation in response to soil warming? We are now addressing these important questions with a new warming experiment in the Barre Woods area of the Harvard Forest. The Barre Woods site was chosen because of its similarities to the Prospect Hill site - stand age, tree species composition, and soil type. There is no evidence of a plow horizon at the Barre Woods site, though surrounding stone walls indicate past use as pastureland. Historical records indicate the site was destroyed during the 1938 hurricane and then regrew naturally. During the summer and fall of 2001 we buried 3.4 miles of heating cable in a 30 x 30m plot. Cables were buried at a 10cm depth, spaced 20cm apart. A second 30 x 30m area was delineated to serve as the control plot. Results from the original soil warming experiment confirmed that the soil disturbance associated with the installation of heating cables has had no effect on soil temperatures and only minor and variable impacts on soil moisture. In April 2002, we began a one-year period of baseline measurements before turning on the heat in the new megaplot. These measurements included nitrogen mineralization, trace gas fluxes (CO2, CH4, N2O), tree growth (dendrometer bands), an understory species survey, canopy foliage analysis (C:N, NIR), and lysimetry. Thermistors were installed in both plots to begin tracking soil temperatures. The baseline measurements have con

Litter Decomposition in Response to Nitrogen Addition and Soil Warming at Harvard Forest 2010-2012

The purpose of this study is to examine whether two environmental change stressors (warming and nitrogen deposition) differentially impact litter decomposition. We investigated this using a two year litterbag decomposition experiment at the chronic N amendment experiment and the Barre Woods Soil warming experiment, and measured litter decay dynamics, enzyme activities and litter chemistry. In both years mass loss of the mixed litter was suppressed under N addition, with most of the mass loss observed in the first year compared to the second year (70% and 30% of total mass loss, respectively). Both years showed either increased activity for some hydrolytic enzymes (e.g. cellobiohydrolase) or no difference (e.g. ß-N-acetylglucosaminidase) with increased N. The lignolytic enzymes (e.g. peroxidases) showed no difference in activity in the first year, but had a highly reduced activity in year 2 under elevated N conditions. Soil warming did not significantly affect litter mass loss, and only had an effect on the activity of a few enzymes. In the oak reciprocal litterbag study, decay of oak litter originating from the highest N addition plot was negatively affected by simulated N deposition in the first year of decomposition, while after two years, simulated N deposition negatively affected all litter, and litter originating from the highest N addition plot decayed more slowly than control litter even without added N (i.e. in the control plot). In addition, in the first year of decomposition lignolytic enzyme activities were suppressed in litter originating from the N addition treatments, but due to simulated N deposition in year two.

Red Maple Seedling Soil Warming Experiment in Harvard Forest Lath House 2015

Microhabitat environmental conditions are an important filter for seedling establishment, controlling the availability of optimal recruitment sites. Understanding how tree seedlings respond to warming soil temperature is critical for predicting population recruitment in the future hardwood forests of northeastern North America, particularly as environmental conditions and thus optimal microhabitat availabilities change. We examined the effect of 5˚C soil warming during the first growing season on germination, survival, phenology, growth, and stem and root biomass allocation in Acer rubrum (red maple) seedlings. While there was no effect of soil warming on germination or survival, seedlings growing in warmer soils demonstrated significantly accelerated leaf expansion, delayed autumn leaf senescence, and an extended leaf production period. Further, seedlings growing in warmer soils showed larger leaf area, stem and root structures at the end of the first growing season, with no evidence of biomass allocation tradeoffs. Results suggest A. rubrum seedlings can capitalize on soil warming by adjusting leaf phenology and leaf production, resulting in a longer period of carbon uptake and leading to higher overall biomass. The absence of growth allocation tradeoffs suggests A. rubrum will respond positively to increasing soil temperatures in northeastern forests, at least in the early life stages.

Effects of Warming on Soil Microbial Communities at Harvard Forest 2011

As Earth’s climate warms, soil carbon pools and the microbial communities that process them may change, altering the way in which carbon is recycled in soil. In this study, we used a combination of metagenomics and bacterial cultivation to evaluate the hypothesis that experimentally raising soil temperatures by 5°C for 5, 8, or 20 years increased the potential for temperate forest soil microbial communities to degrade carbohydrates. Warming decreased the proportion of carbohydrate-degrading genes in the organic horizon derived from eukaryotes and increased the fraction of genes in the mineral soil associated with Actinobacteria in all studies. Genes associated with carbohydrate degradation increased in the organic horizon after 5 years of warming but had decreased in the organic horizon after warming the soil continuously for 20 years. However, a greater proportion of the 295 bacteria from 6 phyla (10 classes, 14 orders, and 34 families) isolated from heated plots in the 20-year experiment were able to depolymerize cellulose and xylan than bacterial isolates from control soils. Together, these findings indicate that the enrichment of bacteria capable of degrading carbohydrates could be important for accelerated carbon cycling in a warmer world.

Effects of Warming on Soil Biogeochemistry at Harvard Forest 2014-2015

More than two decades of experimental soil warming in the Prospect Hill tract of the Harvard Forest has revealed non-linear soil respiration (HF005) and soil carbon loss patterns (Melillo et al., Science, 2017). The datasets here were collected to evaluate how the quantity and quality of soil carbon has been affected by 5C warming, and to assess the role that altered extracellular enzyme allocation may play in this. We collected soils over the 2014 growing season, after 23 years of soil warming. We confirmed previous results that warming had depleted the soil of organic matter, and this was accompanied by an overall decrease in microbial biomass particularly apparent in the forest floor in the fall. Using pyrolysis-GCMS, we found that the relative abundance of soil organic matter compound classes was unaffected by warming in the upper organic horizon (“forest floor”). However, lipids increased in relative abundance to the detriment of polysaccharides and lignin in the upper mineral soil, where warming had depleted soil mineral surfaces of organic matter. Despite these changes in soil organic matter quantity and quality, the potential extracellular enzyme activity per gram of soil was unaffected by warming treatment under common lab conditions. However, under in-situ temperature conditions, warming increased enzyme activity per unit microbial biomass, indicating heated plot microbes get better return on their enzyme investment than those in control plots.

Phenology and Vegetation Growth in Prospect Hill Soil Warming Experiment at Harvard Forest 1992-1993

As the mean annual temperature of northeast North America rises as a component of global climatic change, it is important to understand how the predominant vegetation of the region will be affected. Existing experimental and correlative evidence from field sites suggests that temperature rise will significantly modify soil processes, nutrient availability, and plant growth. We investigated the responses of temperate deciduous forest vegetation to artificial soil warming at 20 sampling dates during the 1992 and 1993 growing season. We explored whether soil warming measurably altered growth and the temporal dynamics of leaf and fruit production in 26 species of three contrasting plant growth forms (herbaceous perennials, shrubs, and canopy trees). We hypothesized that soil warming would exert differential effects on emergence, phenology, leaf expansion rates, growth, photosynthesis, and vegetative and sexual reproduction among species, with implications for changing community structure in these forests. Timing of leaf emergence and flower production was not affected by treatment in saplings; however, mature trees and shrubs leafed out slightly earlier and in larger numbers in heated plots. Soil warming significantly enhanced relative growth in stem diameters of woody plants, especially shrubs, in 1992. This effect was less pronounced in 1993. Species richness was lower in heated plots than in intact control plots in both years; disturbed but unheated control plots showed the lowest species richness of all plots. Changes in relative abundance of herbaceous species from 1992 to 1993 were not significantly affected by treatment. Rank abundances of species were more stable between years in the heated and disturbance-control plots than in the intact plots. Total density of herbaceous species was highest in heated plots during April and May of both years, reflecting greatly accelerated emergence of two dominant species, Maianthemum canadense and Uvularia sessilifolia, due t

Effects of Soil Warming on Bacterial Degradation of Carbohydrates at Harvard Forest 2011

As Earth’s climate warms, soil carbon pools and the microbes that process them may change, altering the way in which carbon is recycled in soil. We used bacterial cultivation to evaluate the hypothesis that experimentally raising soil temperatures by 5°C for 20 years increased the potential for temperate forest soil microbial communities to degrade carbohydrates. A greater proportion of the 295 bacteria from 6 phyla (10 classes, 14 orders, and 34 families) isolated from heated plots in the 20-year experiment were able to depolymerize cellulose and xylan than bacterial isolates from control soils. These findings indicate that the enrichment of bacteria capable of degrading carbohydrates could be important for accelerated carbon cycling in a warmer world. Data for the isolates from the Harvard Forest culturing project is archived at https://osf.io/ahb2v/.

Effects of Soil Warming and Substrate Complexity on Microbial Carbon Use Efficiency at Harvard Forest 2017

Soil microbial carbon use efficiency (CUE) is a combination of growth and respiration, which may respond differently to climate change depending on physical protection of soil carbon (C) and its availability to microbes. In a mid-latitude hardwood forest in central Massachusetts, 27 years of soil warming (+5 ◦C) has resulted in C loss and altered soil organic matter (SOM) quality, yet the underlying mechanisms remain unclear. Here, we hypothesized that long-term warming reduces physical aggregate protection of SOM, microbial CUE, and its temperature sensitivity. Soil was separated into macroaggregate (250–2000 μm) and microaggregate (less than 250 μm) fractions, and CUE was measured with 18O-enriched water in samples incubated at 15 and 25 ◦C for 24 h. We found that long-term warming reduced soil C and nitrogen concentrations and extracellular enzyme activity in macroaggregates, but did not affect physical protection of SOM. Long-term warming showed little effect on CUE or microbial biomass turnover time because it reduced both growth and respiration. However, CUE was less temperature sensitive in macroaggregates from the warmed compared to the control plots. Our findings suggest that microbial thermal responses to long-term warming occur mostly in soil compartments where SOM is less physically protected and thus more vulnerable to microbial degradation.

Carbon Cycle Dynamics in Soil Warming Experiments at Harvard Forest 2019

Microbes are responsible for cycling carbon (C) through soils, and predicted changes in soil C stocks under climate change are highly sensitive to shifts in the mechanisms assumed to control the microbial physiological response to warming. Two mechanisms have been suggested to explain the long-term warming impact on microbial physiology: microbial thermal acclimation and changes in the quantity and quality of substrates available for microbial metabolism. Yet studies disentangling these two mechanisms are lacking. To resolve the drivers of changes in microbial physiology in response to long-term warming, we sampled soils from 13- and 28-year-old soil warming experiments in different seasons. We performed short-term laboratory incubations across a range of temperatures to measure the relationships between temperature sensitivity of physiology (growth, respiration, carbon use efficiency, and extracellular enzyme activity) and the chemical composition of soil organic matter. We observed apparent thermal acclimation of microbial respiration, but only in summer, when warming had exacerbated the seasonally-induced, already small dissolved organic matter pools. Irrespective of warming, greater quantity and quality of soil carbon increased the extracellular enzymatic pool and its temperature sensitivity. We propose that fresh litter input into the system seasonally cancels apparent thermal acclimation of C-cycling processes to decadal warming. Our findings reveal that long-term warming has indirectly affected microbial physiology via reduced C availability in this system, implying that earth system models including these negative feedbacks may be best suited to describe long-term warming effects on these soils.

Juvenile Tree Responses to Soil Warming at Harvard Forest since 1995

The main goal of this study is to assess eastern tree species’ growth, survivorship, and phenological responses to soil warming in order to forecast future changes in forest succession and carbon dynamics. From 2003 through 2010, we determined that shade-tolerant, normally slower-growing species benefitted most from warming. We are continuing to study species-species responses to investigate the duration of and mechanisms behind species- and functional group responses to climate warming. Monitoring long-term demographic and physiological responses of juvenile trees with and without soil warming will allow us to model future eastern tree species successional shifts under warmer climate conditions.

Effect of Warming on Thermal Adaptation of Soil Microbial Growth Traits at Harvard Forest 2013-2023

Adaptation of soil microbes due to warming from climate change has been observed, but it remains unknown what microbial growth traits are adaptive to warming. We studied bacterial isolates from the Harvard Forest Long-Term Ecological Research site, where field soils have been experimentally heated to 5ºC above ambient temperature with unheated controls for thirty years. We hypothesized that Alphaproteobacteria from warmed plots have (1) less temperature sensitive growth rates; (2) higher optimum growth temperatures; and (3) higher maximum growth temperatures compared to isolates from control plots. We made high-throughput measurements of bacterial growth in liquid cultures over time and across temperatures from 22-37ºC in 2-3ºC increments. We estimated growth rates by fitting Gompertz models to the growth data. Temperature sensitivity of growth rate, optimum growth temperature, and maximum growth temperature were estimated by the Ratkowsky 1983 model and a modified Macromolecular Rate Theory (MMRT) model. To determine evidence of adaptation, we ran phylogenetic generalized least squares tests on isolates from warmed and control soils. Our results showed evidence of adaptation of higher optimum growth temperature of bacterial isolates from heated soils. However, we observed no evidence of adaptation of temperature sensitivity of growth and maximum growth temperature. Our project begins to capture the shape of the temperature response curves, but illustrates that the relationship between growth and temperature is complex and cannot be limited to a single point in the biokinetic range.

openCC0Jan 2026View details →

Soil Warming Plus Nitrogen Addition Experiment at Harvard Forest since 2006

Climate warming and N deposition are occurring on a global scale with unknown long-term effects on soil microbial communities and the biogeochemical processes they perform. Few studies have examined the interactive effects of elevated temperatures and N additions on soil microbial community structure and function. The overall objective of this study is to investigate whether warming and N additions restructure microbial communities and alter the response of soil C pools to these two stressors. A related study is examining the interactive effects of warming and N additions on plant and ant diversity. This research is being carried out at the Soil Warming x Nitrogen Addition Study at the Harvard Forest which includes four treatments: control, warming (heating to 5 deg C above ambient), warming x N, and N additions only (addition of 50 kg N/ha/yr). Soil respiration measurements have been made monthly since the beginning of the experiment in 2006. In 2010 and 2011, two different methods were compared: static chamber measurements and instantaneous field IRGA assessments. Soil samples (~0-10 cm) have been sampled annually for total C and N, N mineralization, and microbial community composition. Most recently, soils were collected in October 2011 from across the entire profile (0-50 cm) to access potential changes in soil C and N pools with depth. First, 20 x 20 cm forest floor samples were collected. Mineral soils were then collected in 10 cm depth increments to ~50 cm. Samples are currently being analyzed for total C and N, microbial biomass and community composition and fungal gene expression (transcriptomics). Additionally, long-term incubations are being conducted to measure labile and recalcitrant C fractions. Additional soil physical (texture) and chemical (pH, inorganic N) are being measured. Field season measurements of soil respiration indicate that both warming and N additions continue to stimulate CO2 flux, with warming treatments having a stronger effect on re

openCC0Jan 2024View details →

Prospect Hill Soil Warming Experiment at Harvard Forest since 1991

The soil warming experiment was installed on the Prospect Hill tract in 1991 to allow us to investigate the effects of a 5 deg C temperature increase on soil processes fundamental to the global cycling of carbon and nitrogen. The experiment is located in an even-aged mixed hardwood forest. Six replicates of three treatments, Heated (resistance heating cables buried at 10cm and maintained at a 5 deg C differential from the control plots), Disturbance Control (cables installed but not powered) and Control treatments make up the randomized block design. The temperature differential is maintained with monitoring at five minute increments by an automated thermistor network in the plots, wired to a multiplexer and a datalogger in the control shed. In the plots, measurements of trace gasses (CO2, N2O and CH4), nitrogen mineralization, soil moisture and soil water chemistry have allowed us to quantify changes in the soil system. Ten years of elevated soil temperatures at the Harvard Forest soil warming experiment suggest that there are limits to a positive feedback to the global warming cycle. After many early years of increased CO2 fluxes from the warmed plots, years nine and ten have revealed no significant differences in releases of CO2 between the heated and control plots. Nitrogen mineralization has shown a large response to warming as well, with twice the rate of N mineralized in years 1-4, followed by a gradual decrease in rates to about the 40% level in 1998. Resumption of mineralization measurements in 2001 reveals a continued decrease in mineralization rates. Field results from the soil warming experiment indicate that only a small fraction of the soil carbon in this mid-latitude forest ecosystem will be lost to the atmosphere in response to warming. We find that a 5 deg C warming of the soil for a decade results in a loss of about 11% of the carbon stored in the top 60 cm of soil, with most of this loss occurring in the first four to five years. By the end of the

Short-Term Effects on Vegetation in Soil Warming Plus Nitrogen Addition Experiment at Harvard Forest 2006

The purpose of this experiment is to examine the interactive affects of warming and N additions on plant diversity. The experiment consists of four treatments (control, heated +N, heated -N, +N only) with six replicates per treatment in a completely randomized design. Average soil temperature in the heated plots will be elevated 5 deg C above ambient by the use of buried heating cables placed at 10 cm depth in the soil and spaced 20 cm apart. The heating cables will be controlled by a data logger that monitors thermistors (5 per plot) every 10 min. Plots will automatically turn on and off to maintain a 5 deg C temperature difference between the heated and control plots. The N addition plots (heated +N, +N only) will be fertilized following the protocol of the Chronic Nitrogen Addition Study. An aqueous solution of NH4NO3 will be applied at a rate equivalent to the low N plots at the chronic N study (5 g m-2 yr-1). Fertilizer will be applied in equal monthly doses during the growing season (Apr-Oct). The control plots and unfertilized, heated plots (heated -N) will receive water only.

Effects of Warming on Soil Microbial Oxidative Activity at Harvard Forest 2013

As Earth’s climate warms, the massive stores of carbon found in soil are predicted to become depleted, and leave behind a smaller carbon pool that is less accessible to microbes. At a long-term forest soil-warming experiment in central Massachusetts, soil respiration and bacterial diversity have increased, while fungal biomass and microbially-accessible soil carbon have decreased. Here, we evaluate how warming has affected the microbial community’s capability to degrade chemically-complex soil carbon using lignin-amended BioSep beads. We profiled the bacterial and fungal communities using PCR-based methods and completed extracellular enzyme assays as a proxy for potential community function. We found that lignin-amended beads selected for a distinct community containing bacterial taxa closely related to known lignin degraders, as well as members of many genera not previously noted as capable of degrading lignin. Warming tended to drive bacterial community structure more strongly in the lignin beads, while the effect on the fungal community was limited to unamended beads. Of those bacterial operational taxonomic units (OTUs) enriched by the warming treatment, many were enriched uniquely on lignin-amended beads. These taxa may be contributing to enhanced soil respiration under warming despite reduced readily available C availability. In aggregate, these results suggest that there is genetic potential for chemically complex soil carbon degradation that may lead to extended elevated soil respiration with long-term warming.

Soil Carbon Dioxide and Oxygen at the Soil Warming Plus Nitrogen Experiment at Harvard Forest since 2018

This dataset includes soil air CO2 and O2 concentrations measured from the organic/mineral horizon interface and 10, 30, and 50 cm depths of the mineral soil at SWaN using permanently installed stainless steel gas wells. Measurements were made 4-8 times throughout the year in 2018, 2019, and 2020.

openCC0Aug 2024View details →

Effects of Long-Term Soil Warming on Ecosystem Function at Harvard Forest 2019

Across biomes, soil biodiversity promotes ecosystem functions. However, whether this relationship will be maintained within ecosystems under climate change is uncertain. Here, using two long-term soil warming experiments, we investigated how warming affects the relationship between ecosystem functions and bacterial diversity across seasons, soil horizons, and warming duration. Soils were sampled from these warming experiments located at the Harvard Forest Long-Term Ecological Research (LTER) site, where soils had been heated +5°C above ambient for 13 or 28 years at the time of sampling. We assessed seven measurements representative of different ecosystem functions and nutrient pools. We also surveyed bacterial community diversity. We found that ecosystem function was significantly affected by season, with autumn samples having a higher intercept than summer samples in our model, suggesting a higher overall baseline of ecosystem function in the fall. The effect of warming on bacterial diversity was similarly affected by season, where warming in the summer was associated with decreased bacterial evenness in the organic horizon. Despite the decreased bacterial evenness in the warmed plots, we found that the relationship between ecosystem function and bacterial diversity was unaffected by warming or warming duration. Our findings highlight that season is a consistent driver of ecosystem function as well as a modulator of climate change effects on bacterial community evenness.