David W. Valentine

Generalized model for N2 and N2O production from nitrification and denitrification

We describe a model of N2 and N2O gas fluxes from nitrification and denitrification. The model was developed using laboratory denitrification gas flux data and field-observed N2O gas fluxes from different sites. Controls over nitrification N2O gas fluxes are soil texture, soil NH4, soil water-filled pore space, soil N turnover rate, soil pH, and soil temperature. Observed data suggest that nitrification N2O gas fluxes are proportional to soil N turnover and that soil NH4 levels only impact N2O gas fluxes with high levels of soil NH4 (>3 μg N g−1). Total denitrification (N2 plus N2O) gas fluxes are a function of soil heterotrophic respiration rates, soil NO3, soil water content, and soil texture. N2:N2O ratio is a function of soil water content, soil NO3, and soil heterotrophic respiration rates. The denitrification model was developed using laboratory data [Weier et al, 1993] where soil water content, soil NO3, and soil C availability were varied using a full factorial design. The Weiers model simulated observed N2 and N2O gas fluxes for different soils quite well with r2 equal to 0.62 and 0.75, respectively. Comparison of simulated model results with field N2O data for several validation sites shows that the model results compare well with the observed data (r2 = 0.62). Winter denitrification events were poorly simulated by the model. This problem could have been caused by spatial and temporal variations in the observed soil water data and N2O fluxes. The model results and observed data suggest that approximately 14% of the N2O fluxes for a shortgrass steppe are a result of denitrification and that this percentage ranged from 0% to 59% for different sites.

Ecosystem and physiological controls over methane production in northern wetlands

Peat chemistry appears to exert primary control over methane production rates in the Canadian Northern Wetlands Study (NOWES) area. We determined laboratory methane production rate potentials in anaerobic slurries of samples collected from a transect of sites through the NOWES study area. We related methane production rates to indicators of resistance to microbial decay (peat C:N and lignin:N ratios) and experimentally manipulated substrate availability for methanogenesis using ethanol (EtOH) and plant litter. We also determined responses of methane production to pH and temperature. Methane production potentials declined along the gradient of sites from high rates in the coastal fens to low rates in the interior bogs and were generally highest in surface layers. Strong relationships between CH 4nproduction potentials and peat chemistry suggested that methanogenesis was limited by fermentation rates. Methane production at ambient pH responded strongly to substrate additions in the circumneutral fens with narrow lignin:N and C:N ratios (pCH 4/pEtOH = 0.9n2.3mg g m1) and weakly in the acidic bogs with wide C:N and lignin:N ratios (pCH 4/pEtOH = m0.04n0.02 mg g m1). Observed Q 10nvalues ranged from 1.7 to 4.7 and generally increased with increasing substrate availability, suggesting that fermentation rates were limiting. Titration experiments generally demonstrated inhibition of methanogenesis by lownpH. Our results suggest that the low rates of methane emission observed in interior bogs during NOWES likely resulted fromnpH and substrate quality limitation of the fermentation step in methane production and thus reflect intrinsically low methane production potentials. Low methane emission rates observed during NOWES will likely be observed in other northern wetland regions with similar vegetation chemistry.

CH4 and N2O fluxes in the Colorado shortgrass steppe: 1. Impact of landscape and nitrogen addition

A weekly, year-round nitrous oxide (N2O) and methane (CH4) flux measurement program was initiated in nine sites within the Central Plains Experimental Range in the Colorado shortgrass steppe in 1990 and continued through 1994. This paper reports the observed intersite, interannual, and seasonal variation of these fluxes along with the measured variation in soil and air temperature and soil water and mineral nitrogen content. We found that wintertime fluxes contribute 20–40% of the annual N2O emissions and 15–30% of CH4 consumption at all of the measurement sites. Nitrous oxide emission maxima were frequently observed during the winter and appeared to result from denitrification when surface soils thawed. Interannual variation of N2O maximum annual mean fluxes was 2.5 times the minimum during the 4-year measurement period, while maximum annual mean CH4 uptake rates were 2.1 times the minimum annual mean uptake rates observed within sites. Generally, site mean annual flux maxima for CH4 uptake corresponded to minimum N2O fluxes and vice versa, which supports the general concept of water control of diffusion of gases in the soil and limitations of soil water content on microbial activity. We also observed that pastures that have similar use history and soil texture show similar N2O and CH4 fluxes, as well as similar seasonal and annual variations. Sandy loam soils fertilized with nitrogen 5–13 years earlier consumed 30–40% less CH4 and produced more N2O than unfertilized soils. In contrast, the N addition 13 years ago does not affect CH4 uptake but continues to increase N2O emissions in a finer-textured soil. Our long-term data also show that soil mineral N concentration is not a reliable predictor of observed changes, or lack of changes, in either N2O efflux or CH4 uptake. Finally, from our data we estimate that annual global N2O emission rates for native, temperate grasslands are about 0.16 Tg N2O-N yr−1, while CH4 consumption totals about 3.2 Tg CH4-C yr−1.

Effect of land use change on methane oxidation in temperate forest and grassland soils

Evidence is accumulating that land use changes and other human activity during the past 100 to 200 years have contributed to decreased CH4 oxidation in the soil. Recent studies have documented the effect of land use change on CH4 oxidation in a variety of ecosystems. Increased N additions to temperate forest soils in the northeastern United States decreased CH4 uptake by 30 to 60%, and increased N fertilization and conversion to cropland in temperate grasslands decreased CH4 uptake by 30 to 75%. Using these data, we made a series of calculations to estimate the impact of land use and management changes which have altered soil the CH4 sink in temperate forest and grassland ecosystems. Our study indicates that as the atmospheric mixing ratio of CH4 has increased during the past 150 y, the temperate CH4 sink has risen from approximately 8 Tg y−1 to 27 Tg y−1, assuming no loss of land cover to cropland conversion. The net effect of intensive land cover changes and extensive chronic disturbance (i.e., increased atmospheric N deposition) to these ecosystems have resulted in about 30% reduction in the CH4 sink relative to the soil sink assuming no disturbance to any of the temperate ecosystems. This will impact the global CH4 budget even more as atmospheric CH4 concentrations increase and as a result of further disturbance to other biomes. Determining the reasons for the decreased CH4 uptake due to land disturbance is necessary to understand the role of CH4 uptake in conjunction with the increasing atmospheric CH4 concentrations. Without accounting for this approximately 20 Tg y−1 temperate soil sink, the atmospheric CH4 concentration would be increasing about 1.5 times the current rate.

Fifty-year biogeochemical effects of green ash, white pine, and Norway spruce in a replicated experiment

Binkley, D. and Valentine, D., 1991. Fifty-year biogeochemical effects of green ash, white pine, and Norway spruce in a replicated experiment. For. Ecol. Manage., 40:13-25. Few long-term, replicated experiments are available to provide information on the effects of tree species on soil chemistry and ecosystem biogeochemistry. We examined replicated, 50-year-old plots of green ash (Fraxinus pennsylvanica Marsh), white pine (Pinus strobus L.), and Norway spruce [ Picea abies ( L. ) Karst. ] that had been planted in an abandoned agricultural field. The pHwater of the 0-5-cm soil layer under green ash was 4.6, compared with 4.2 under white pine and 3.8 under Norway spruce. The Norway spruce soil was substantially less-well buffered against further acidification than the soil under green ash, and contained less than half the quantity of exchangeable Ca 2 + + Mg 2 + + K + in the 0-15-cm depth. The decline in these cations under Norway spruce was accompanied by higher concentrations of exchangeable A13 +. The most important factor in the lower pH under Norway spruce was the greater acid strength of the soil organic matter, with a secondary role played by the higher saturation of the exchange complex with aluminum. Nitrogen mineralization in resin cores averaged 40 kg/ha under green ash, 84 kg/ha under white pine, and 56 kg/ha under Norway spruce. The only significant difference in litterfall biomass and chemistry was a greater content of aluminum and lower content of magnesium in litterfall in the Norway spruce plots relative to green ash plots. These major biogeochemical differences between tree species demonstrate the need for replicated experiments for assessing the mechanisms that drive long-term changes in ecosystems relative to differing species, management regimes, and atmospheric deposition.

CH4 and N2O fluxes in the Colorado shortgrass steppe: 2. Long‐term impact of land use change

As part of a weekly, year-round program to measure the soil-atmosphere exchange of nitrous oxide (N2O) and methane (CH4) in a shortgrass steppe, we examined the impact of land use change on these fluxes from 1992 through 1995. We found that conversion of grassland to croplands typically decreased the soil consumption of atmospheric CH4 and increased the emission of N2O. Mean annual CH4 consumption and N2O efflux over 3 years in native grasslands were 35 µg C m−2 hr−1 and 1.9 µg N m−2 hr−1, respectively. Immediately after tilling a native grassland site, CH4 consumption decreased by about 35% and remained at these lower rates for the next 3 years. Although N2O fluxes were about 8 times higher for 18 months following plowing, the relative rates declined to 25–50% higher than the native site after 3 years. Grasslands converted to a winter wheat-fallow production system about 70 years ago consumed about 25% less CH4 than a newly plowed site, while N2O emissions 2 years after plowing were similar to the wheat fields. During the fallow periods when soils were typically wetter and mineralized N accumulated, CH4 uptake rates were lower and N2O emissions were higher than the correspondingly active wheat fields. A wheat field that was reverted back to grassland in 1987 through the conservation reserve program (CRP) continued to exhibit annual CH4 uptake and N2O emission rates similar to the wheat fields. Winter N2O emissions were, however, much higher in the CRP because of greater snow accumulation and winter denitrification events. Another wheat field that was returned to grassland in 1939 exhibited the same CH4 and N2O flux rates as comparable native pastures.

Impact of agriculture on soil consumption of atmospheric CH4 and a comparison of CH4 and N2O flux in subarctic, temperate and tropical grasslands

Increasing concentrations of methane (CH4) in the atmosphere are projected to account for about 25% of the net radiative forcing. Biospheric emissions of CH4 to the atmosphere total approximately 400 Tg C y-1. An estimated 300 Tg of CH4-C y-1 is oxidized in the atmosphere by hydroxyl radicals while about 40 Tg y-1 remains in the atmosphere. Approximately 40 Tg y-1 of the atmospheric burden is oxidized in aerobic soils. Research efforts during the past several years have focused on quantifying CH4 sources while relatively less effort has been directed toward quantifying and understanding the soil sink for atmospheric CH4.Recent research has demonstrated that land use change, including agricultural use of native forest and grassland systems has decreased the soil sink for atmospheric methane. Some agricultural systems consume atmospheric CH4 at rates less than 10% of those found in comparable undisturbed soils.While it has been necessary to change land use practices over the past centuries to meet the required production of food and fiber, we need to recognize and account for impacts of land use change on the biogeochemical nutrient cycles in the biosphere. Changes that have ensued in these cycles have and will impact the atmospheric concentrations of CH4 and N2O. Since CH4 and N2O production and consumption are accomplished by a variety of soil microorganisms, the influence of changing agricultural, forest, and, demographic patterns has been large. Existing management and technological practices may already exist to limit the effect of land use change and agriculture on trace gas fluxes. It is therefore important to understand how management and land use affect trace gas fluxes and to observe the effect of new technology on them.This paper describes the role of aerobic soils in the global CH4 budget and the impact of agriculture on this soil CH4 sink. Examples from field studies made across subarctic, temperate and tropical climate gradients in grasslands are used to demonstrate the influence of nutrient cycle perturbations on the soil consumption of atmospheric CH4 and in increased N2O emissions.

Do forests receive occult inputs of nitrogen

The nitrogen (N) cycle of forest ecosystems is understood relatively well, and few scientists expect that major revisions will be necessary; most current work on N cycling focuses on improving the precision estimates of pools and fluxes, or measuring the magnitudes of well-known pools in response to management or disturbances. However, in the past few decades more than a dozen articles in refereed journals have claimed very high rates of N input, far beyond the rates expected for known sources of N. In this review, we summarize the literature on N accretion rates in forests that lack substantial contributions from symbiotic N-fixing plants. We critique each study for the strength of the experimental design behind the estimate of N accretion and consider whether unexpectedly large inputs of N really occur in forests. Only 6 of 24 estimates of N accretion had strong experimental designs, and only 2 of these 6 yielded estimates of >5 kgN ha-1 y-1. The high accretion estimates with a strong experimental design come from repeated sampling at the Walker Branch watersheds in Tennessee, where N accretion rates ranged from 50 to 80 kg N ha-1 y-1 over 15 years after harvesting. At the same location, an unharvested stand showed no significant change. We conclude that there is no widespread evidence of high rates of occult N input in forests. Too few studies have carefully tested for balanced N budgets in forests (inputs minus outputs plus change in storage), and we recommend that at least a few of these studies be undertaken on soils that permit high precision sampling.

Resin-core and buried-bag estimates of nitrogen transformations in Costa Rican lowland rainforests

We compared the resin-core and buried-bag incubation methods for estimating nitrogen (N) transformation rates using the 15N pool dilution technique in alluvial soils of an early successional forest (ESF) and an old-growth forest (OGF) at the La Selva Biological Station in Costa Rica. Soil cores (38×100-mm) from both forests were incubated in situ for 7 days. The two methods gave generally similar estimates of net N mineralization rates for the two forests. Estimates of ammonium production by the resin-core method were higher than those by the buried-bag method in ESF, but did not differ significantly in OGF (p<0.05). Estimates of nitrate production by the two methods did not differ significantly. Nitrate averaged 74% and 81% of the total inorganic N production in ESF and OGF, respectively. Net N mineralization in ESF (6.6 mmol m-2d-1) did not differ significantly from that in OGF (5.0 mmol m-2d-1). Fluxes of ammonium and nitrate were high for both forests, but the OGF tended to have higher gross mineralization and nitrification rates than ESF. Approximately 60% of the gross nitrate production and less than 30% of the ammonium were immobilized by microorganisms.

Long term 15N studies in a catena of the shortgrass steppe

A set of long term15N studies was initiated during the summers of 1981 and 1982 on the backslope and footslope, respectively, of a catena in the shortgrass steppe of northeastern Colorado. Microplots labeled with15N urea were sampled for15N and total N content in 1981 and 1982 and again in 1992. In November, 1982, 100% of the added N was recovered in the soil-plant system of the finer-textured footslope, compared to 39% in the coarser-textured backslope microplots. Ten years later,15N recovery of the applied N decreased at both topographic positions to 85% in the footslope and 29% in the backslope. Average losses since the time of application were 3.5 g N m−2yr−1 in the backslope and 0.8 g N m−2yr−1 in the footslope. In 1992, soil organic matter was physically fractionated into particulate (POM) and mineral associated (MAON) fractions and 21-day mineralization incubations were conducted to assess the relative amounts of15N that were in the slow, passive and active soil organic matter pools, respectively, of the two soils. Our findings confirm the assumptions that POM represents a large portion of the slow organic compartment and that the MAON represents a large fraction of the passive compartment defined in the Century model. The N located in the MAON had the lowest availability for plant uptake. Isotopic data were consistent with textural effects and with the Century model compartmentalization of soil organic N based on the residence time of the organic N.