Johannes Laubach

Methane emissions from beef cattle – a comparison of paddock- and animal-scale measurements

Methane (CH4) emissions from a herd of 58 grazing cattle were determined in a field experiment using paddock-scale (micrometeorological) methods. The emissions were also measured daily from each animal, using the sulfur hexafluoride tracer method. The paddock-scale methods exploit how the gas, once emitted from the cattle, is transported and dispersed by the wind. Hence, the emission rate may be calculated from measurements of windspeed, wind direction and turbulence statistics, as well as CH4 concentration upwind and downwind of the herd. The paddock-scale methods include a mass-budget approach, flux-gradient method and gas dispersion model. Accuracy can be assessed in unprecedented detail because the animal-scale (reference) method included all individuals in the herd, and the measurement site was ideal for micrometeorological methods (flat, usually windy and free of obstructions that alter the turbulent airflow). The cattle were hand-reared steers of average weight 325 ± 20 kg. Based on the animal-scale method, the average CH4 emission rate over 9 days was 161 ± 20 g/steer.day. The gas dispersion model, when utilising vertical concentration profiles, yielded on average 27% greater emissions. The other paddock-scale methods agreed with the animal-scale method, provided the cattle were at least 22 m away from the location of the downwind concentration measurements. When the cattle were allowed to graze as closely as 5 m from the instruments, the paddock-scale methods gave greater emissions than the animal-scale method; reasons for this are discussed.

Ammonia emissions from cattle urine and dung excreted on pasture

Twelve cattle were kept for three days in a circular area of 16 m radius on short pasture and fed with freshlycut pasture. Ammonia (NH 3) emissions from the urine and dung excreted by the cattle were measured with a micrometeorological mass-balance method, during the cattle presence and for 10 subsequent days. Daily-integrated emission rates peaked on Day 3 of the experiment (last day of cattle presence) and declined steadily for five days thereafter. Urine patches were the dominant sources for these emissions. On Day 9, a secondary emissions peak occurred, with dung pats likely to be the main sources. This interpretation is based on simultaneous observations of the pH evolution in urine patches and dung pats created next to the circular plot. Feed and dung samples were analysed to estimate the amounts of nitrogen (N) ingested and excreted. Total N volatilised as NH3 was 19.8 ( ± 0.9) % of N intake and 22.4 ( ± 1.3) % of N excreted. The bimodal shape of the emissions time series allowed to infer separate estimates for volatilisation from urine and dung, respectively, with the result that urine accounted for 88.6 ( ± 2.6) % of the total NH3 emissions. The emissions from urine represented 25.5 ( ± 2.0) % of the excreted urine-N, while the emissions from dung amounted to 11.6 (± 2.7) % of the deposited dung-N. Emissions from dung may have continued after Day 13 but were not resolved by the measurement technique. A simple resistance model shows that the magnitude of the emissions from dung is controlled by the resistance of the dung crust.

Rapid changes in δ13C of ecosystem‐respired CO2 after sunset are consistent with transient 13C enrichment of leaf respired CO2

The CO₂ respired by darkened, light-adapted, leaves is enriched in ¹³C during the first minutes, and this effect may be related to rapid changes in leaf respiratory biochemistry upon darkening. We hypothesized that this effect would be evident at the ecosystem scale. High temporal resolution measurements of the carbon isotope composition of ecosystem respiration were made over 28 diel periods in an abandoned temperate pasture, and were compared with leaf-level measurements at differing levels of pre-illumination. At the leaf level, CO₂ respired by darkened leaves that had been preadapted to high light was strongly enriched in ¹³C, but such a ¹³C-enrichment rapidly declined over 60-100 min. The ¹³C-enrichment was less pronounced when leaves were preadapted to low light. These leaf-level responses were mirrored at the ecosystem scale; after sunset following clear, sunny days respired CO₂ was first ¹³C enriched, but the ¹³C-enrichment rapidly declined over 60-100 min. Further, this response was less pronounced following cloudy days. We conclude that the dynamics of leaf respiratory isotopic signal caused variations in ecosystem-scale ¹²CO₂/¹³) CO₂ exchange. Such rapid isotope kinetics should be considered when applying ¹³C-based techniques to elucidate ecosystem carbon cycling.

Convective boundary layer budgets derived from aircraft data

Convective boundary layer (CBL) budgetting is regarded as a technique with the potential to estimate regionally integrated surface fluxes of trace gases, sensible and latent heat. We present two practical approaches to this technique and apply them to the experimental data, collected by flying vertical profiles over non-homogeneous terrain consisting mainly of beech forest and agricultural fields. Both approaches are quasi-one-dimensional. The first is to evaluate the budget for an air column whose top is set to CBL height and thus varies considerably in the course of a day. The second, presented here for the first time, evaluates the budget for an air column of fixed mass. We show that the latter approach requires fewer assumptions than the former, while producing results of at least comparable, and often better, accuracy. Error estimates typically yield 10–20% for fluxes of CO2 and sensible heat, and 20–30% for latent heat. These estimates, however, do not include the uncertainty due to horizontal advection. We address this issue by showing how profile information above the CBL top can be exploited to estimate bounds for large-scale advection within the CBL. We test this idea on our data, with the ambiguous result that on some occasions the obtained bounds place realistic constraints on the magnitude of the advection, while on others they are too wide to be useful.

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%.

Nitrous oxide fluxes, soil oxygen, and denitrification potential of urine- and non-urine-treated soil under different irrigation frequencies

Despite increased use of irrigation to improve forage quality and quantity for grazing cattle ( Linnaeus), there is a lack of data that assess how irrigation practices influence nitrous oxide (NO) emissions from urine-affected soils. Irrigation effects on soil oxygen (O) availability, a primary controller of NO fluxes, is poorly understood. It was hypothesized that increased irrigation frequency would result in lower NO emissions by increasing soil moisture and decreasing soil O concentrations. This would favor more NO reduction to dinitrogen (N). We examined effects of high (3-d) versus low (6-d) irrigation frequency with and without bovine urine addition to pasture. Nitrous oxide fluxes were measured daily for 35 d. Soil O, temperature, and water content were continuously measured at multiple depths. Inorganic nitrogen, organic carbon, and soil pH were measured at 6-d intervals. Measurements of denitrification enzyme activity with and without acetylene inhibition were used to infer the NO/(NO + N) ratio. The NO/(NO + N) ratio was lower under high- compared with low-frequency irrigation, suggesting greater potential for NO reduction to N with more frequent irrigation. Although NO fluxes were increased by urine addition, they were not affected by irrigation frequency. Soil O decreased temporarily after urine deposition, but O dynamics did not explain NO dynamics. Relative soil gas diffusivity (/) was a better predictor of NO fluxes than O concentration. On a free-draining soil, increasing irrigation frequency while providing the same total water volume did not enhance NO emissions under ruminant urine patches in a grazed pasture.