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The long-term effects of conservation management practices on greenhouse gas fluxes from tropical/subtropical croplands remain to be uncertain. Using both manual and automatic sampling chambers, we measured N2O and CH4 fluxes at a long-term experimental site (1968-present) in Queensland, Australia from 2006 to 2009. Annual net greenhouse gas fluxes (NGGF) were calculated from the 3-year mean N2O and CH4 fluxes and the long-term soil organic carbon changes. N2O emissions exhibited clear daily, seasonal and interannual variations, highlighting the importance of whole-year measurement over multiple years for obtaining temporally representative annual emissions. Averaged over 3 years, annual N2O emissions from the unfertilized and fertilized soils (90 kg N ha−1 yr−1 as urea) amounted to 138 and 902 g N ha−1, respectively. The average annual N2O emissions from the fertilized soil were 388 g N ha−1 lower under no-till (NT) than under conventional tillage (CT) and 259 g N ha−1 higher under stubble retention (SR) than under stubble burning (SB). Annual N2O emissions from the unfertilized soil were similar between the contrasting tillage and stubble management practices. The average emission factors of fertilizer N were 0.91%, 1.20%, 0.52% and 0.77% for the CT-SB, CT-SR, NT-SB and NT-SR treatments, respectively. Annual CH4 fluxes from the soil were very small (−200-300 g CH4 ha−1 yr−1) with no significant difference between treatments. The NGGF were 277-350 kg CO2-e ha−1 yr−1 for the unfertilized treatments and 401-710 kg CO2-e ha−1 yr−1 for the fertilized treatments. Among the fertilized treatments, N2O emissions accounted for 52-97% of NGGF and NT-SR resulted in the lowest NGGF (401 kg CO2-e ha−1 yr−1 or 140 kg CO2-e t−1 grain). Therefore, NT-SR with improved N fertilizer management practices was considered the most promising management regime for simultaneously achieving maximal yield and minimal NGGF. [ABSTRACT FROM AUTHOR]

Understanding nitrous oxide (NO) fluxes from grain-legume crops in semiarid and arid regions is necessary if we are to improve our knowledge of global terrestrial NO losses resulting from biological N fixation. NO fluxes were measured from a rain-fed soil, cropped to a grain-legume in a semiarid region of southwestern Australia for 1 year on a subdaily basis. The site included plots planted to narrow-leafed lupin ( Lupinus angustifolius; 'lupin') and plots left bare (no lupin). Fluxes were measured using soil chambers connected to a fully automated system that measured NO by gas chromatography. Daily NO fluxes were low (−0.5 to 24 g NO-N ha day) and not different between treatments, culminating in an annual loss of 127 g NO-N ha. Greatest daily NO fluxes occurred from both treatments in the postharvest period, and following a series of summer and autumn rainfall events. At this time of the year, soil conditions were conducive to soil microbial NO production: elevated soil water contents, increased inorganic nitrogen (N) and dissolved organic carbon concentrations, and soil temperatures generally > 25 °C; furthermore, there was no active plant growth to compete for mineralized N. NO emissions from the decomposition of legume crop residue were low, and approximately half that predicted using the currently recommended IPCC methodology. Furthermore, the contribution of the biological N fixation process to NO emissions appeared negligible in the present study, supporting its omission as a source of NO from the IPCC methodology for preparing national greenhouse gas inventories. [ABSTRACT FROM AUTHOR]

In contrast to the significant importance of tropical rainforest ecosystems as one of the major sources within the global atmospheric N2O budget (2.2–3.7 Tg N yr−1), regional estimates of their N2O source strength are still limited and highly uncertain. To contribute toward more reliable estimates of the N2O source strength of tropical rainforest ecosystems on a regional scale, we modified a process-oriented biogeochemical model, PnET-N-DNDC, and parameterized it to simulate C and N turnover and associated N2O emissions in and from tropical rainforest ecosystems. Model modifications included: (1) new parameterizations associated with plant physiology and soil hydrology and the addition of algorithms relating daily leaf litterfall to water stress as well as to daily rainfall to account for the effects of heavy rainfall damage; (2) the development of a denitrifier activity index that depends on soil moisture conditions and influences N turnover by denitrification; and (3) the addition of a biological N fixation algorithm. Daily simulated N2O emissions based on site data were in good agreement (model efficiencies up to 0.83) with field observations in the Wet Tropics of Australia and Costa Rica. The model was even able to reproduce the highly dynamic pattern of N2O emissions with short-term increases during the wet season. Sensitivity analyses demonstrated that the PnET-N-DNDC model was sensitive to changes in soil properties such as pH, clay content, soil organic carbon and climatic factors such as rainfall and temperature. By linking the PnET-N-DNDC model to a geographic information systems database, tropical rainforests in a 9000 km2 area of the Wet Tropics of Australia are estimated to emit 962 t N2O-N yr−1 (2.4 kg N2O-N ha−1 yr−1) between July 1997 and June 1998. [ABSTRACT FROM AUTHOR]

Abstract: Semi-arid lands represent one fifth of the global land area but our understanding of greenhouse gas fluxes from these regions is poor. We investigated if inclusion of a grain legume and/or lime in a crop rotation altered greenhouse gas emissions from an acidic soil. Nitrous oxide (N2O) and methane (CH4) fluxes were measured from a rain-fed, cropped soil in a semi-arid region of Australia for two years on a sub-daily basis. The randomised-block design included two cropping rotations (lupin–wheat, wheat–wheat) by two liming treatments (0, 3.5tha−1) by three replicates. The lupin–wheat rotation only received N fertilizer during the wheat phase (20kgNha−1), while the wheat–wheat received 125kgNha−1 during the two year study. Fluxes were measured using soil chambers connected to a fully automated system that measured N2O and CH4 by gas chromatography. Nitrous oxide fluxes were low (−1.4 to 9.2gN2O-Nha−1 day−1), and less than those reported for arable soils in temperate climates. Including a grain legume in the cropping rotation did not enhance soil N2O; total N2O losses were approximately 0.1kgN2O-Nha−1 after two years for both lupin–wheat and wheat–wheat rotations when averaged across liming treatment. Liming decreased cumulative N2O emissions from the wheat–wheat rotation by 30% by lowering the contribution of N2O emissions following summer–autumn rainfall events, but had no effect on N2O emissions from the lupin–wheat rotation. Daily CH4 fluxes ranged from −14 to 5g CH4-Cha−1 day−1. Methane uptake after two years was lower from the wheat–wheat rotation (601gCH4-Cha−1) than from either lupin–wheat rotations (967gCH4-Cha−1), however liming the wheat–wheat rotation increased CH4 uptake (1078gCH4-Cha−1) to a value similar to the lupin–wheat rotation. Liming provides a strategy for lowering on-farm greenhouse gas emissions from N fertilised soils in semi-arid environments via decreased N2O fluxes and increased CH4 uptake. [Copyright &y& Elsevier]