Richard D. Bowden

Factors controlling atmospheric methane consumption by temperate forest soils

Over the past 6 years (1988–1993), we have examined the effects of soil temperature, soil moisture, site fertility, and nitrogen fertilization on the consumption of atmospheric CH4 by temperate forest soils located at the Harvard Forest in Petersham, Massachusetts. We found that soil temperature is an important controller of CH4 consumption at temperatures between −5° and 10°C but had no effect on CH4 consumption at temperatures between 10° and 20°C. Soil moisture exerts strong control on CH4 consumption over a range of 60 to 100% water-filled pore space (% WFPS). As moisture increased from 60 to 100% WFPS, CH4 consumption decreased from 0.1 to 0 mg CH4-C m−2 h−1 because of gas transport limitations. At 20 to 60% WFPS, site fertility was a strong controller of CH4 consumption. High-fertility sites had 2 to 3 times greater CH4 consumption rates than low-fertility sites. Nitrogen-fertilized soils (50 and 150 kg NH4NO3-N ha−1 yr−1 ) had annually averaged CH4 consumption rates that were 15 to 64% lower than annually averaged CH4 consumption by control soils. The decrease in CH4 consumption was related to both the years of application and quantity of nitrogen fertilizer added to these soils.

Carbon dioxide and methane fluxes by a forest soil under laboratory-controlled moisture and temperature conditions

Abstract Carbon dioxide and methane are important greenhouse gases whose exchange rates between soils and the atmosphere are controlled strongly by soil temperature and moisture. We made a laboratory investigation to quantify the relative importance of soil moisture and temperature on fluxes of CO2 and CH4 between forest soils and the atmosphere. Forest floor and mineral soil material were collected from a mixed hardwood forest at the Harvard Forest Long-Term Ecological Research Site (MA) and were incubated in the laboratory under a range of moisture (air-dry to nearly saturated) and temperature conditions (5–25°C). Carbon dioxide emissions increased exponentially with increasing temperature in forest floor material, with emissions reduced at the lowest and highest soil moisture contents. The forest floor Q10 of 2.03 (from 15–25°C) suggests that CO2 emissions were controlled primarily by soil biological activity. Forest floor CO2 emissions were predicted with a multiple polynomial regression model (r2=0.88) of temperature and moisture, but the fit predicting mineral soil respiration was weaker (r2=0.59). Methane uptake was controlled strongly by soil moisture, with reduced fluxes under conditions of very low or very high soil moisture contents. A multiple polynomial model accurately described CH4 uptake by mineral soil material (r2=0.81), but only weakly (r2=0.45) predicted uptake by forest floor material. The mineral soil Q10 of 1.11 for CH4 uptake indicates that methane uptake is controlled primarily by physical processes. Our work suggests that inclusion of both moisture and temperature can improve predictions of soil CO2 and CH4 exchanges between soils and the atmosphere. Additionally, global change models need to consider interactions of temperature and moisture in evaluating effects of global climate change on trace gas fluxes.

Forest Response to Disturbance and Anthropogenic Stress Rethinking the 1938 Hurricane and the impact of physical disturbance vs. chemical and climate stress on forest ecosystems

emperate forests of the northeastern United States are dynamic ecosystems that have been shaped through geological and historical time by climate change, natural disturbance, and human activity (Davis 1986, Peterken 1993, Whitney 1994). Particularly noticeable through a retrospective view is the remarkable resiliency of these forests to a wide range of physical disturbances, including windthrow, fire, and land clearance (Boose et al. 1993, Foster 1994, Motzkin et al. 1996, Raup 1964). Despite this resiliency, novel environmental stresses may surpass the ability of these forests to control important ecosystem processes (Likens et al. 1996). For instance, changes in the global earthatmosphere system resulting from industrial and human land-use ac-

Effects of nitrogen additions on annual nitrous oxide fluxes from temperate forest soils in the northeastern United States

Nitrous oxide (N2O) fluxes between aerobic soils and the atmosphere were measured each month for 2 years (except during the period of snow cover) in control and N-fertilized plots in a red pine plantation and a mixed hardwood forest in the northeastern United States. Nitrogen was added as NH4NO3; application rates were 0 (control), 37 (low-N), and 120 (high-N) kg N/ha/yr in year 1. In year 2, application rates were 0 (control), 50 (low-N), and 150 (high-N) kg N/ha/yr. A total of 2520 individual flux measurements were made over the 2 years. Mean emission rates (μg N/m2/h) from the control plots were low (pine: year 1 = 0.17, year 2 = 1.10; hardwood: year 1 = 0.27, year 2 = 0.29). Fertilization resulted in only small increases in N2O effluxes even in the second year, with low rates in the low-N plots (pine: year 1 = 2.19, year 2 = 3.87; hardwood: year 1 = 1.16, year 2 = 0.77) and also in the high-N plots (pine: year 1 = 0.71, year 2 = 5.24; hardwood: year 1 = 1.12, year 2 = 0.90). The maximum N2O loss was 0.350 kg N/ha/yr from the red pine high-N plot. Weak seasonal flux trends were noted in both stands, with highest efflux rates in spring and mid to late summer. No statistically significant relationships between fluxes and soil temperatures, soil moisture, or soil NO3− or NH4+ were detected. Low rates of net nitrification in both the control plots and the fertilized plots are thought to be responsible for the low N2O emissions. Small increases in net nitrification were measured in the forest floor of the the pine stand, with rates in the control, low-N, and high-N plots of 1.5, 4.5, and 8.5 kg N/ha/yr, respectively. Net nitrification rates in the hardwood plots were 0.1, 0.9, and 0.6 kg N/ha/yr in the control, low-N, and high-N plots, respectively. Low NH4+ availability in the forest floor and low NH4+ concentrations in soil solution below the rooting zone (0.5–0.7 m) suggest that NH4+ availability for nitrification is limited by competition with plant uptake and microbial immobilization, and that this competition is not alleviated by 2 years (50–150 kg N/ha/yr) of N addition.

Chronic nitrogen additions suppress decomposition and sequester soil carbon in temperate forests

The terrestrial biosphere sequesters up to a third of annual anthropogenic carbon dioxide emissions, offsetting a substantial portion of greenhouse gas forcing of the climate system. Although a number of factors are responsible for this terrestrial carbon sink, atmospheric nitrogen deposition contributes by enhancing tree productivity and promoting carbon storage in tree biomass. Forest soils also represent an important, but understudied carbon sink. Here, we examine the contribution of trees versus soil to total ecosystem carbon storage in a temperate forest and investigate the mechanisms by which soils accumulate carbon in response to two decades of elevated nitrogen inputs. We find that nitrogen-induced soil carbon accumulation is of equal or greater magnitude to carbon stored in trees, with the degree of response being dependent on stand type (hardwood versus pine) and level of N addition. Nitrogen enrichment resulted in a shift in organic matter chemistry and the microbial community such that unfertilized soils had a higher relative abundance of fungi and lipid, phenolic, and N-bearing compounds; whereas, N-amended plots were associated with reduced fungal biomass and activity and higher rates of lignin accumulation. We conclude that soil carbon accumulation in response to N enrichment was largely due to a suppression of organic matter decomposition rather than enhanced carbon inputs to soil via litter fall and root production.

Fluxes of greenhouse gases between soils and the atmosphere in a temperate forest following a simulated hurricane blowdown

Fluxes of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) between soils and the atmosphere were measured monthly for one year in a 77-year-old temperate hardwood forest following a simulated hurricane blowdown. Emissions of CO2 and uptake of CH4 for the control plot were 4.92 MT C ha−1 y−1 and 3.87 kg C ha−1 y−1, respectively, and were not significantly different from the blowdown plot. Annual N2O emissions in the control plot (0.23 kg N ha−1 y−1) were low and were reduced 78% by the blowdown. Net N mineralization was not affected by the blowdown. Net nitrification was greater in the blowdown than in the control, however, the absolute rate of net nitrification, as well as the proportion of mineralized N that was nitrified, remained low. Fluxes of CO2 and CH4 were correlated positively to soil temperature, and CH, uptake showed a negative relationship to soil moisture. Substantial resprouting and leafing out of downed or damaged trees, and increased growth of understory vegetation following the blowdown, were probably responsible for the relatively small differences in soil temperature, moisture, N availability, and net N mineralization and net nitrification between the control and blowdown plots, thus resulting in no change in CO2 or CH4 fluxes, and no increase in N2O emissions.

Soil respiration in a northeastern US temperate forest: a 22‐year synthesis

To better understand how forest management, phenology, vegetation type, and actual and simulated climatic change affect seasonal and inter-annual variations in soil respiration (R-s), we analyzed m ...

Long-term litter manipulation alters soil organic matter turnover in a temperate deciduous forest

Understanding soil organic matter (OM) biogeochemistry at the molecular-level is essential for assessing potential impacts from management practices and climate change on shifts in soil carbon storage. Biomarker analyses and nuclear magnetic resonance (NMR) spectroscopy were used in an ongoing detrital input and removal treatment experiment in a temperate deciduous forest in Pennsylvania, USA, to examine how above- and below-ground plant inputs control soil OM quantity and quality at the molecular-level. From plant material to surface soils, the free acyclic lipids and cutin, suberin, and lignin biomarkers were preferentially retained over free sugars and free cyclic lipids. After 20years of above-ground litter addition (Double Litter) or exclusion (No Litter) treatments, soil OM composition was relatively more degraded, as revealed by solid-state 13C NMR spectroscopy. Under Doubled Litter inputs, soil carbon and phospholipid fatty acid (PLFA) concentrations were unchanged, suggesting that the current OM degradation status is a reflection of microbial-mediated degradation that occurred prior to the 20-year sampling campaign. Soil OM degradation was higher in the No Litter treatments, likely due to the decline in fresh, above-ground litter inputs over time. Furthermore, root and root and litter exclusion treatments (No Roots and No Inputs, respectively) both significantly reduced free sugars and PLFAs and increased preservation of suberin-derived compounds. PLFA stress ratios and the low N-acetyl resonances from diffusion edited 1H NMR also indicate substrate limitations and reduced microbial biomass with these treatments. Overall, we highlight that storage of soil carbon and its biochemical composition do not linearly increase with plant inputs because the microbial processing of soil OM is also likely altered in the studied forest.

The detrital input and removal treatment (DIRT) network: Insights into soil carbon stabilization

Ecological research networks functioning across climatic and edaphic gradients are critical for improving predictive understanding of biogeochemical cycles at local through global scales. One international network, the Detrital Input and Removal Treatment (DIRT) Project, was established to assess how rates and sources of plant litter inputs influence accumulations or losses of organic matter in forest soils. DIRT employs chronic additions and exclusions of aboveground litter inputs and exclusion of root ingrowth to permanent plots at eight forested and two shrub/grass sites to investigate how soil organic matter (SOM) dynamics are influenced by plant detrital inputs across ecosystem and soil types. Across the DIRT network described here, SOM pools responded only slightly, or not at all, to chronic doubling of aboveground litter inputs. Explanations for the slow or even negative response of SOM to litter additions include increased decomposition of new inputs and priming of old SOM. Evidence of priming includes increased soil respiration in litter addition plots, decreased dissolved organic carbon (DOC) output from increased microbial activity, and biochemical markers in soil indicating enhanced SOM degradation. SOM pools decreased in response to chronic exclusion of aboveground litter, which had a greater effect on soil C than did excluding roots, providing evidence that root-derived C is not more critical than aboveground litter C to soil C sequestration. Partitioning of belowground contributions to total soil respiration were predictable based on site-level soil C and N as estimates of site fertility; contributions to soil respiration from root respiration were negatively related to soil fertility and inversely, contributions from decomposing aboveground litter in soil were positively related to site fertility. The commonality of approaches and manipulations across the DIRT network has provided greater insights into soil C cycling than could have been revealed at a single site.

The Path From Litter to Soil: Insights Into Soil C Cycling From Long‐Term Input Manipulation and High‐Resolution Mass Spectrometry

U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, DE-FG02-09ER604719. Office of Biological and Environmental Research, 130367. Allegheny College.