Andrew M. S. McMillan

Age‐dependent variation in the biophysical properties of boreal forests

The changes in boreal forest hydrology, biogeochemistry, and biophysics during succession have critical implications for the sign and magnitude of the vegetation-climate feedbacks that might occur with a change in fire frequency, and also for the identification and attribution of changes in boreal forest to climate. We combined in situ measurements from eddy covariance sites located along an age transect in a Canadian boreal forest with spectral vegetation indices (SVIs) derived from Landsat and MODIS imagery. We found tight spatial relationships between Landsat SVIs and in situ measurements of three important biophysical properties: albedo, maximum daily uptake of CO2 (F CO2-max), and leaf area index (LAI). The tasseled cap indices were particularly well suited for tracking biophysical variation along an age transect. Trends in brightness, greenness, and wetness from 1984 to 2005 indicated how succession drives temporal trends in biophysical properties. Albedo and F CO2-max increased rapidly in the decade following fire and then decreased for the remainder of succession, while LAI continued to increase until ∼135 years and may decrease thereafter. The ratio of greenness to wetness indicated that photosynthesis was limited by leaf area before 10–12 years and by reduced leaf-level photosynthetic rates thereafter, coinciding with the successional replacement of broadleaf deciduous species by evergreen conifer species. The timing of phenological events was also strongly age-dependent, but the normalized difference vegetation index (NDVI) confounded the disappearance of the snowpack in spring for the onset of photosynthesis. Secondary succession was the dominant source of temporal variability in the biophysical properties we examined.

On the performance of SF6 permeation tubes used in determining methane emission from grazing livestock

In a technique for measuring methane emitted by grazing ruminant livestock, a calibrated source of inert tracer sulfur hexafluoride (SF6) is inserted into the rumen of each participating animal prior to collection of “breath” samples for gas analysis. Each source comprises a “permeation tube” from which an SF6 charge slowly escapes through a permeable membrane, to be sampled with the breath. This paper reports analyses of the permeation characteristics of such tubes and provides evidence that the permeation rate slowly changes rather than stays constant as the technique supposes. This feature has been observed routinely over several generations of tube fills in our laboratory. Failure to take account of a changing permeation rate can lead to a systematic error in the inferred methane emission rate of up to about 15%. A quality control strategy is proposed that enables permeation rates to be extrapolated with confidence, based on the monitored performances of control tubes as proxies for inserted tubes.

Stoichiometry of CH4 and CO2 flux in a California Rice Paddy

Rice paddies contribute significantly to the atmospheric burden of CH4 but may also sequester atmospheric CO2. Previous studies based on putative relationships between net CO2 exchange and CH4 emissions have concluded that globally significant amounts of carbon can be stored in rice paddy soils. However, the annual ratio of CH4 emissions to net CO2 exchange has not previously been measured. We simultaneously measured the net exchange of CO2 (F CO2) by eddy covariance and the CH4 emission rate (F CH4) using a combination of a flux gradient technique and weekly chamber sampling. During rice growth, F CH4 was 1.9% to 2.4% of net carbon uptake (mole per mole). F CO2 closely followed biomass accumulation. In contrast, F CH4 increased during vegetative rice growth and decreased over the ripening and reproductive phases of rice growth, suggesting that the plants release substrate for methanogenesis early in the season. CH4 emissions represented 4.8% to 5.6% of the net CO2 uptake when summed over an entire year (including a 20-week period over which the field was unplanted and flooded). Assuming harvested rice is remineralized within 1 year, the remaining 0.67 t C ha−1 that was sequestered by the paddy potentially offsets the radiative forcing of the emitted CH4 by 26% to 31%. The ratio of F CH4 to F CO2 varied widely over the course of a year depending on management practices in a specific field. The results reported here emphasize the importance of year-round measurements to obtain a reliable estimate of CH4/CO2 exchange stoichiometry.

Simulation of boreal black spruce chronosequences: Comparison to field measurements and model evaluation

This study used the Biome Biogeochemical Cycles (Biome-BGC) process model to simulate boreal forest dynamics, compared the results with a variety of measured carbon content and flux data from two boreal chronosequences in northern Manitoba, Canada, and examined how model output was affected by water and nitrogen limitations on simulated plant production and decomposition. Vascular and nonvascular plant growth were modeled over 151 years in well-drained and poorly drained forests, using as many site-specific model parameters as possible. Measured data included (1) leaf area and carbon content from site-specific allometry data, (2) aboveground and belowground net primary production from allometry and root cores, and (3) flux data, including biometry-based net ecosystem production and tower-based net ecosystem exchange. The simulation used three vegetation types or functional groups (evergreen needleaf trees, deciduous broadleaf trees, and bryophytes). Model output matched some of the observed data well, with net primary production, biomass, and net ecosystem production (NEP) values usually (50–80% of data) within the errors of observed values. Leaf area was generally underpredicted. In the simulation, nitrogen limitation increased with stand age, while soil anoxia limited vascular plant growth in the poorly drained simulation. NEP was most sensitive to climate variability in the poorly drained stands. Simulation results are discussed with respect to conceptual issues in, and parameterization of, the Biome-BGC model.

Chambers, micrometeorological measurements, and the New Zealand Denitrification–Decomposition model for nitrous oxide emission estimates from an irrigated dairy-grazed pasture

Nitrous oxide (N2O) emissions from soils are notoriously variable in space and time. Measuring and understanding variance in these emissions is imperative for improving the accuracy of the greenhouse gas inventory and assessing the viability of mitigation options; but data for N2O emissions are rather limited. Farm-scale emissions data are also required for developing and verifying predictive model estimates. A measurement campaign was undertaken from 12 October to 1 November 2006 at a highly productive grass-clover irrigated dairy farm on a stony silt loam soil in North Canterbury, South Island, New Zealand. The ∼7 ha experimental field, grazed in two morning 6-h grazing sessions (21–22 October 2006) by 718 milking dairy cattle, received two irrigations during the measurements, one before the grazing event and the other during grazing period. We first compare the emission measurements using a chamber technique against those made using a micrometeorological technique with tuneable diode-laser technology. We then compare the measured emissions against emissions predicted by a process-based model (New Zealand Denitrification–Decomposition (NZ-DNDC)). Daily averaged micrometeorological measurements gave a pre-grazing emission of 35 g N2O N/ha/day that increased to >60 g N2O N/ha/day following grazing by the dairy herd. The average pre-grazing emission of 10 g N2O N/ha/day from the chambers increased to 25 g N2O N ha−1 day−1 following grazing. The emissions were simulated with NZ-DNDC model, which gave average daily emissions of 15 ± 9 g N2O N ha−1 day−1 for the pre-grazing period and 22 ± 6 g N2O N ha−1 day−1 for the post-grazing period. Here we describe these measurement approaches, compare their emission estimates and discuss the advantages of combining them for verification of emissions.

Chemical formation of hybrid di-nitrogen calls fungal codenitrification into question.

Removal of excess nitrogen (N) can best be achieved through denitrification processes that transform N in water and terrestrial ecosystems to di-nitrogen (N2) gas. The greenhouse gas nitrous oxide (N2O) is considered an intermediate or end-product in denitrification pathways. Both abiotic and biotic denitrification processes use a single N source to form N2O. However, N2 can be formed from two distinct N sources (known as hybrid N2) through biologically mediated processes of anammox and codenitrification. We questioned if hybrid N2 produced during fungal incubation at neutral pH could be attributed to abiotic nitrosation and if N2O was consumed during N2 formation. Experiments with gas chromatography indicated N2 was formed in the presence of live and dead fungi and in the absence of fungi, while N2O steadily increased. We used isotope pairing techniques and confirmed abiotic production of hybrid N2 under both anoxic and 20% O2 atmosphere conditions. Our findings question the assumptions that (1) N2O is an intermediate required for N2 formation, (2) production of N2 and N2O requires anaerobiosis, and (3) hybrid N2 is evidence of codenitrification and/or anammox. The N cycle framework should include abiotic production of N2.

Field-scale verification of nitrous oxide emission reduction with DCD in dairy-grazed pasture using measurements and modelling

Nitrous oxide (N2O) from agricultural soils is a major source of greenhouse gas emissions in New Zealand. Nitrification inhibitors are seen as a potential technology to reduce these N2O emissions from agricultural soils. In previous studies on the effect of dicyandiamide (DCD) on N2O emissions from animal excreta, DCD was directly applied to urine. However, farmers apply DCD to grazed pastures shortly before or after grazing rather than applying it specifically to the urine patches. Accordingly, the objectives of this study were: (1) to test, using chamber measurements, whether the same level of N2O reduction is achieved under grazed conditions where excretal N is non-uniformly deposited, (2) to apply the process-based NZ-DNDC model to simulate the effect of DCD on emission reductions, and (3) to perform a sensitivity analysis on the NZ-DNDC model to investigate how uncertainties in the input parameters affect the modelled N2O emissions. Two circular 1260-m2 treatment plots were grazed simultaneously for 5 h, by 20 cattle on each plot. The following day, DCD was applied in 800 L of water to one of the plots at 10 kg/ha and N2O emissions were measured periodically for 20 days. The cumulative N2O emissions were 220 ± 90 and 110 ± 20 g N2O-N/ha for the untreated and DCD-treated plots, respectively (based on the arithmetic mean and standard error of the chambers). This suggests a reduction in N2O emission from DCD application of ~50 ± 40% from a single grazing event. However, this result should be treated with caution because the possibility of sampling error due to the chamber distribution cannot be excluded. NZ-DNDC simulated N2O emissions of 169 and 68 g N2O-N/ha for the untreated and DCD-treated areas, respectively, corresponding to a reduction of 60% in N2O emissions from DCD application. This level of reduction is consistent with that found in experiments with individual urine patches. N2O emissions found through use of NZ-DNDC were sensitive to uncertainties in the input parameters. The combined effect of varying the initial soil NO3– and NH4+, soil moisture, soil organic carbon, bulk density, clay content, pH, and water-filled pore-space at field capacity inputs within plausible ranges was to change the simulated N2O emissions by –87% to +150%.

Fungal denitrification: Bipolaris sorokiniana exclusively denitrifies inorganic nitrogen in the presence and absence of oxygen

Fungi may play an important role in the production of the greenhouse gas nitrous oxide (N2O). Bipolaris sorokiniana is a ubiquitous saprobe found in soils worldwide, yet denitrification by this fungal strain has not previously been reported. We aimed to test if B. sorokiniana would produce N2O and CO2 in the presence of organic and inorganic forms of nitrogen (N) under microaerobic and anaerobic conditions. Nitrogen source (organic-N, inorganic-N, no-N control) significantly affected N2O and CO2 production both in the presence and absence of oxygen, which contrasts with bacterial denitrification. Inorganic N addition increased denitrification of N2O (from 0 to 0.3 μg N20-N h(-1) g(-1) biomass) and reduced respiration of CO2 (from 0.1 to 0.02 mg CO2 h(-1) g(-1) biomass). Isotope analyses indicated that nitrite, rather than ammonium or glutamine, was transformed to N2O. Results suggest the source of N may play a larger role in fungal N2O production than oxygen status.

Warming and Elevated CO2 Have Opposing Influences on Transpiration. Which is more Important

Plant transpiration is a key component of the terrestrial water cycle, and it is important to understand whether rates are likely to increase or decrease in the future. Plant transpiration rates are affected by biophysical factors, such as air temperature, vapour pressure deficits and net radiation, and by plant factors, such as canopy leaf area and stomatal conductance. Under future climate change, global temperature increases, and associated increases in vapour pressure deficits, will act to increase canopy transpiration rates. Increasing atmospheric CO2 concentrations, however, is likely to lead to some reduction in stomatal conductance, which will reduce canopy transpiration rates. The objective of the present paper was to quantitatively compare the importance of these opposing driving forces. First, we reviewed the existing literature and list a large range of observations of the extent of decreasing stomatal conductance with increasing CO2 concentrations. We considered observations ranging from short-term laboratory-based experiments with plants grown under different CO2 concentrations to studies of plants exposed to the naturally increasing atmospheric CO2 concentrations. Using these empirical observations of plant responses, and a set of well-tested biophysical relationships, we then estimated the net effect of the opposing influences of warming and CO2 concentration on transpiration rates. As specific cases studies, we explored expected changes in greater detail for six specific representative locations, covering the range from tropical to boreal forests. For most locations investigated, we calculated reductions in daily transpiration rates over the twenty-first century that became stronger under higher atmospheric CO2 concentrations. It showed that the effect of CO2-induced reduction of stomatal conductance would have a stronger transpiration-depressing effect than the stimulatory effect of future warming. For currently cold regions, global warming would, however, lengthen the growing seasons so that annual sums of transpiration could increase in those regions despite reductions in daily transpiration rates over the summer months.

Corrigendum: Chemical formation of hybrid di-nitrogen calls fungal codenitrification into question

This corrects the article DOI: 10.1038/srep39077.