Hydrological and biogeochemical controls on the timing and magnitude of nitrous oxide flux across an agricultural landscape

Anticipated increases in precipitation intensity due to climate change may affect hydrological controls on soil N₂O fluxes, resulting in a feedback between climate change and soil greenhouse gas emissions. We evaluated soil hydrologic controls on N₂O emissions during experimental water table fluctuations in large, intact soil columns amended with 100 kg ha^?1 KNO₃‐N. Soil columns were collected from three landscape positions that vary in hydrological and biogeochemical properties (N= 12 columns). We flooded columns from bottom to surface to simulate water table fluctuations that are typical for this site, and expected to increase given future climate change scenarios. After the soil was saturated to the surface, we allowed the columns to drain freely while monitoring volumetric soil water content, matric potential and N₂O emissions over 96 h. Across all landscape positions and replicate soil columns, there was a positive linear relationship between total soil N and the log of cumulative N₂O emissions (r²= 0.47; P= 0.013). Within individual soil columns, N₂O flux was a Gaussian function of water‐filled pore space (WFPS) during drainage (mean r²= 0.90). However, instantaneous maximum N₂O flux rates did not occur at a consistent WFPS, ranging from 63% to 98% WFPS across landscape positions and replicate soil columns. In contrast, instantaneous maximum N₂O flux rates occurred within a narrow range (?1.88 to ?4.48 kPa) of soil matric potential that approximated field capacity. The relatively consistent relationship between maximum N₂O flux rates and matric potential indicates that water filled pore size is an important factor affecting soil N₂O fluxes. These data demonstrate that matric potential is the strongest predictor of the timing of N₂O fluxes across soils that differ in texture, structure and bulk density.     

Competition between Australian native and introduced grasses along a nutrient gradient

Abstract Seven grass species were grown in monocultures and in multispecies mixtures along a gradient of total nutrient levels that ranged from 1/64 to 16× the normal level of nutrient solution. The seven grasses represented three ecological groups: (i) three perennial species native to Australia (Themeda triandra, Poa labillardieri and Danthonia carphoides); (ii) two introduced annuals (Vulpia bromoides and Hordeum leporinum); and (iii) two introduced perennials (Lolium perenne and Dactylis glomerata). We hypothesized that the native grasses would prove less competitive when grown at increased nutrient levels than those introduced from Europe. Results supported the hypothesis. The native species were unable to compete in mixtures even at the lowest nutrient level, where T. triandra was the most productive species in monoculture. Lolium perenne and Dactylis glomerata dominated mixtures at intermediate nutrient levels. The responses of the annual introduced grasses differed in that Vulpia bromoides showed an optimum at intermediate nutrient levels in both monoculture and in mixtures, whereas Hordeum leporinum dominated at the highest nutrient levels in mixture but was suppressed by V. bromoides, L. perenne and D. glomerata at intermediate levels. The results are discussed in terms of predicting species responses in mixtures from their performance in monocultures as well as in terms of previous observations on the sequential changes in botanical composition of south‐eastern Australian grasslands after 150 years of continuous grazing by sheep.     

Territorial Sea Baselines along Ice-Covered Coasts: International Practice and Limits of the Law of the Sea

This article considers the relevant international law pertaining to territorial sea baselines and reviews the application of that law to ice-covered coasts. The international literature concerning the status of ice in international law is examined and state practice for both the Arctic and Antarctic is reviewed. The Law of the Sea Convention contains virtually no provisions pertaining to ice, as during its negotiation, in an effort to reach a consensus, all discussion of Antarctica was avoided. International lawyers appear to favour the notion that where ice persists for many years and is fixed to land or at least is connected to ice that is connected to land, it may be able to generate territorial sea baselines. Neither the International Court of Justice nor any other international tribunal has had the opportunity to consider the status of ice, except in the most general terms. This article considers some alternatives and difficulties in their application. The impact of the Antarctic Treaty on any system is also considered, as Articles IV and VI of the Treaty may be affected by any international action by claimants in proclaiming baselines.-