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Nitrous oxide (N 2 O) is a potent greenhouse gas and a precursor to compounds which deplete stratospheric ozone. Agricultural soils are the principal anthropogenic N 2 O source, which includes urine patches of cattle fed by grazing pasture. While such N 2 O emissions can be measured, involved time and space scales are usually only minutes and metres, respectively. Nevertheless, climate change stakeholders need large-scale national emissions over time to construct annual N 2 O emissions inventories for international treaty compliance requirements. We have developed a mathematical model to bridge the gap between measurement and stakeholder requirements for N 2 O emissions from grazing cattle urine patches. Our methodology begins with the emissions from a patch. Convolution integrals are then used to spatially and temporally scale the emissions from discrete grazing events to paddock, farm and region levels across the year. The model developed shows how the emissions are smoothed out with scaling and the implications from the scaling. [ABSTRACT FROM AUTHOR]

Between 11 May 2000 and 31 January 2013, 185 field trials were conducted across New Zealand to measure the direct nitrous oxide (N2O) emission factors (EF) from nitrogen (N) sources applied to pastoral soils. The log(EF) data were analysed statistically using a restricted maximum likelihood (REML) method. To estimate mean EF values for each N source, best linear unbiased predictors (BLUPs) were calculated. For lowland soils, mean EFs for dairy cattle urine and dung, sheep urine and dung and urea fertiliser were 1.16 ± 0.19% and 0.23 ± 0.05%, 0.55 ± 0.19% and 0.08 ± 0.02% and 0.48 ± 0.13%, respectively, each significantly different from one another (p < 0.05), except for sheep urine and urea fertiliser. For soils in terrain with slopes >12°, mean EFs were significantly lower. Thus, urine and dung EFs should be disaggregated for sheep and cattle as well as accounting for terrain. [Copyright &y& Elsevier]

Abstract: Dicyandiamide (DCD, C2H4N4) is a nitrification inhibitor that has been studied for more than 80 years. However, there are few papers that have examined the use of DCD on dairy farms where cattle graze pasture and where urine is the primary form of nitrogen (N) deposited onto soils. After DCD was applied (10kgDCDha−1) with bovine urine (700–1200kgNha−1) to five soils throughout New Zealand, the reduction in direct nitrous oxide (N2O) emissions was significant and remarkably consistent (71±8%, average±standard error). The application of DCD to these soils occurred in autumn and winter; daily average soil temperature (T) was reported but these data were not further analysed. Perusal of the literature suggested no consensus on the temperature dependence of DCD degradation in soils. Based on published data from controlled-environment studies of soils sampled in four countries, we quantified the relation between T and the time for DCD concentration in soils to decline to half its application value (t ½) as t ½ (T)=168e−0.084T with parameter standard errors of ±16d and ±0.011d−1, respectively (n =16). For example, at 5°C a 1°C increase in T reduced t ½ from 110 to 101d whereas at 25°C the reduction was 20–19d. Analysing T data from the New Zealand trials using our t ½ (T) function, over 43–89d when direct N2O emissions from treated plots became indistinguishable from the controls, the estimated percentage of applied DCD remaining in the soil averaged 43±10%. These calculations suggested the apparently remaining DCD was ineffective with respect to direct N2O emissions. In the absence of measurements, explanations for this interpretation included vertical displacement of the DCD and sorption onto organic matter in soils. The consistent DCD efficacy from these trials corresponded with T generally <10°C, so it is suggested as an application criteria for the reduction of direct N2O emissions from pastoral soils subjected to urine excretion by grazing cattle. [Copyright &y& Elsevier]

Abstract: Nitrous oxide research has generally focused directly on measuring fluxes of N2O from the soil surface. The fate of N2O in the subsoil has often been placed in the ‘too hard’ basket. However, determining the production, fate and movement of N2O in the subsoil is vital in fully understanding the sources of surface fluxes and in compiling accurate inventories for N2O emissions. The aim of this study was to generate and introduce into soil columns 15N labelled N2O, and to try and determine the consumption of the 15N2O and production of ambient N2O. Columns, 100cm long by 15cm diameter, were repacked with sieved soil (sampled from 0 to 5cm depth) and instrumented with silicone rubber gas sampling ports. Nitrous oxide enriched with 15N was generated using a thermal decomposition process at 300°C and then transferred to 2l flasks. After equilibrating with SF6 tracer gas the 15N2O was introduced into the soil columns via passive diffusion. Gas samples from the soil profile and headspace flux were taken over a 12-day period. A watering event was simulated to perturb the 15N2O gas composition in the soil profile. Using the measured 15N enriched fluxes and the rate of decline in 15N in the N2O reservoir, from which the N2O diffused into the soil, we calculated an N2O sink (consumption plus absorption by water) equal to 0.48ng N2O g−1 soil h−1. The decrease in the 15N enrichment between successive soil depths indicated N2O production in the soil profile and we calculated a net N2O production rate of 0.88ng N2O g−1 soil h−1. This pilot study demonstrated the potential for simultaneously measuring both N2O consumption and production rates, using the 15N enrichment of the N2O measured. Further potential refinements of the methodology are discussed. [Copyright &y& Elsevier]

Summer drought is a feature of the semi-arid region of central Oregon, USA, where vegetation naturally develops into ponderosa pine (Pinus ponderosa var. Laws) forest. Forest management consists of clearcut harvest and natural regeneration. Soil microbial activity is interconnected with forest processes because substrate quality and availability can be important driving variables. Stand development influences the soil water regime, and water availability may also limit microbial activity. We determined factors limiting litter and mineral soil carbon (C) mineralisation rates in undisturbed old growth and regenerating (hereafter, young) ponderosa pine stands under a semi-arid climate. Mass of litter and dead fine roots did not differ significantly between the stands, but litter substrate quality was different. Young stand litter had significantly higher concentrations of total nitrogen (N), extractable organic N, extractable C, and microbial C and N than that from the old stand, probably because of litter fall from the broadleaved shrub understorey, including the N-fixing species Purshia tridentata (Pursch) DC, that comprised 40% of the young stand’s leaf area. The old stand contained no understorey. For litter samples from the two stands, wetted to 60% of water-holding capacity (WHC), net mineral-N and CO2–C mineralisation rates were similar despite the substrate quality differences. Mineral soil properties at 0–0.1 m depth were similar in the two stands, except for lower CO2–C production in samples from the young stand; at 0.1–0.5 m depth, total C and N and microbial N concentrations were higher in the young stand. Net mineral-N production in field-moist soil, sampled during a typical summer drought and incubated at 25 °C for 56 days, was generally 3–6 mg kg−1 soil at both sites, but increased up to 29 mg kg−1 upon wetting to 60% of water-holding capacity. Over 56-day-long incubations, wetting also increased litter and soil microbial respiration rates by factors of about 500 and 3, respectively. The incubations yielded a proportionality between respiration rate and water content that was supported by in situ measurements of soil respiration in the young stand, before and after irrigation. A hypothetically wet year without soil water deficit caused a 2.5-fold increase in a modelled estimate of the young stand’s annual soil respiration rate. Litter and soil C mineralisation rates in these ponderosa pine forests thus appeared to be limited much more by the availability of water than by a lack of available C or N substrates. [Copyright &y& Elsevier]

Two 8-year-old Pinus radiata trees growing in a dry-land plantation forest were subjected to a partial shading treatment for 2 years in order to investigate the responses of photosynthesis and tree growth to a long-term reduction in illuminated leaf area. The lower 60% of the crown of shaded trees was surrounded by 50% neutral density shade cloth. This reduced radiation absorption by 13% for the trees as a whole compared to the control trees during the first year, while modelled annual whole-canopy photosynthesis was 9% lower, and was strongly regulated by seasonal soil water availability, temperature and irradiance. The shaded trees responded by increasing carbon partitioning to the branches, at the expense of the stem. As a result, basal area growth of the shaded trees was almost 50% lower than that for control trees.In the second year, the effects of the shade cloth on absorbed radiation, photosynthesis and growth were reduced. This occurred because of needle abscission below the shade cloth and rapid branch and leaf area growth above the shade cloth, while the lower foliage on the control trees became increasingly self-shaded. Allocation to stem growth increased in the shaded trees, resulting in similar relative basal area increment between shaded and control trees. The difference in modelled whole-canopy photosynthesis was smaller in the second year. This was partly due to the reduced effect of the shade cloth, but also to a significant summer drought, which severely reduced stomatal conductance, photosynthesis and stem growth in both shaded and control trees. Seasonal root-zone water deficits at this site reduce annual carbon assimilation by around 40%, and increase the proportions of carbon assimilation occurring in spring, autumn and winter. [Copyright &y& Elsevier]

Following advances with methods for 13C/12C isotopic analysis of methane in small (&les;4 L) air samples, new isotope dilution techniques are proposed for measurement of methane emissions at the paddock scale from grazing ruminant animals. These techniques combine measurement of the isotopic δ13CH4 composition of air samples with a non-intrusive mass balance method applied in the nocturnal boundary layer. Flux estimates from trials using the isotope dilution techniques are compared with estimates based on scaling up individual animal emission measurements using a rumen gas tracer technique. The methane flux assessed by the latter technique ranged from 35 to 70 mg (CH4) m−2 d−1 with a stocking density between 10 and 20 sheep ha−1. The isotope dilution based nocturnal boundary-layer estimates generally agreed to better than a factor of 2 and usually to within 20% of the average of individual animal emission rate per unit area of paddock. Both static and advecting mass balance methods are developed. In the advecting case, the upwind/downwind contrast in δ13C was typically 0.2–0.5‰. Care was necessary with air sampling to avoid error in this small contrast contributing to error in the flux. Agreement between concentration- and isotope-based nocturnal boundary layer methods and the sheep breath measurements indicated that sample representativeness was generally good. Factors which affect the accuracy of the method are examined and include variability in nocturnal mixing height, the assumed δ13CH4 composition of the source sheep breath and diurnal patterns in sheep emission. This paper establishes new techniques useful in the paddock to landscape scale although widespread application awaits further development of technology for rapid and repeatable field analysis of δ13CH4 in small samples. [Copyright &y& Elsevier]