Increases in the concentrations of greenhouse gases, carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), and halocarbons in the atmosphere due to human activities are associated with global climate change. The concentration of N 2 O has increased by 16% since 1750. Although the atmospheric concentration of N 2 O is much smaller (314 ppb in 1998) than of CO 2 (365 ppm), its global warming potential (cumulative radiative forcing) is 296 times that of the latter in a 100-year time horizon. Currently, it contributes about 6% of the overall global warming effect but its contribution from the agricultural sector is about 16%. Of that, almost 80% of N 2 O is emitted from Australian agricultural lands, originating from N fertilisers (32%), soil disturbance (38%), and animal waste (30%).Nitrous oxide is primarily produced in soil by the activities of microorganisms during nitrification, and denitrification processes. The ratio of N 2 O to N 2 production depends on oxygen supply or water-filled pore space, decomposable organic carbon, N substrate supply, temperature, and pH and salinity. N 2 O production from soil is sporadic both in time and space, and therefore, it is a challenge to scale up the measurements of N 2 O emission from a given location and time to regional and national levels.Estimates of N 2 O emissions from various agricultural systems vary widely. For example, in flooded rice in the Riverina Plains, N 2 O emissions ranged from 0.02% to 1.4% of fertiliser N applied, whereas in irrigated sugarcane crops, 15.4% of fertiliser was lost over a 4-day period. Nitrous oxide emissions from fertilised dairy pasture soils in Victoria range from 6 to 11 kg N 2 O-N/ha, whereas in arable cereal cropping, N 2 O emissions range from <0.01% to 9.9% of N fertiliser applications. Nitrous oxide emissions from soil nitrite and nitrates resulting from residual fertiliser and legumes are rarely studied but probably exceed those from fertilisers, due to frequent wetting and drying cycles over a longer period and larger area. In ley cropping systems, significant N 2 O losses could occur, from the accumulation of mainly nitrate-N, following mineralisation of organic N from legume-based pastures. Extensive grazed pastures and rangelands contribute annually about 0.2 kg N/ha as N 2 O (93 kg/ha per year CO 2 -equivalent). Tropical savannas probably contribute an order of magnitude more, including that from frequent fires. Unfertilised forestry systems may emit less but the fertilised plantations emit more N 2 O than the extensive grazed pastures. However, currently there are limited data to quantify N 2 O losses in systems under ley cropping, tropical savannas, and forestry in Australia. Overall, there is a need to examine the emission factors used in estimating national N 2 O emissions; for example, 1.25% of fertiliser or animal-excreted N appearing as N 2 O (IPCC 1996).The primary consideration for mitigating N 2 O emissions from agricultural lands is to match the supply of mineral N (from fertiliser applications, legume-...

APSIM (Agricultural Production Systemsof both the total amounts in the whole projle and their distribution with depth. Since neither of these datasets included measurements of the runof component of the water balance, this aspect of model performance was evaluated, and shown to be generally good, using data from a third source where runoff had been measured from contour bay catchments. 0 1997

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Near infrared diffuse reflectance spectrophotometry, within the wavelength range 1100 to 2500 nm, was investigated for use in the simultaneous prediction of the moisture, organic C, and total N contents of air‐dried soils. An infraAlyzer 500 C (Technicon Instruments Corp.) scanning spectrophotometer was used to obtain near infrared reflectance of soils at 2‐nm intervals. Calibration equations for each of the soil constituents studied were based upon selection of the best combination of three wavelengths in a multiple regression analysis. The wavelengths selected for moisture, organic C, and total N, respectively, were 1926, 1954, and 2150 nm, 1744, 1870 and 2052 nm, and 1702, 1870 and 2052 nm. The standard errors of prediction for finely ground samples (<0.25 mm) from the top layers (0‐0.1, 0.1‐0.2, 0.2‐0.3, 0.3‐0.6 m) were 0.58, 0.16, and 0.014% for moisture, organic C, and total N, respectively. The standard errors of prediction, however, were much larger for coarsely ground soils (<2 mm), soils containing low amounts of organic C (<0.3%) and total N (<0.03%), and for those with a wide range in colors. Within a narrow range in soil color and at moderate amounts of organic matter (0.3–2.5%C), the near infrared reflectance technique provides a rapid, nondestructive, and simultaneous measurement of moisture, organic C and total N in soils

Besides water vapour, greenhouse gases CO2, CH4, O3 and N2O contribute ~60%, 20%, 10% and 6% to global warming, respectively; minor contribution is made by chlorofluorocarbons and volatile organic compounds (VOC). We present CO2, CH4 and N2O fluxes from natural and relatively unmanaged soil–plant ecosystems (the ecosystems minimally disturbed by direct human or human-induced activities). All natural ecosystems are net sinks for CO2, although tundra and wetlands (including peatlands) are large sources of CH4, whereas significant N2O emissions occur mainly from tropical and temperate forests. Most natural ecosystems decrease net global warming potential (GWP) from –0.03 ± 0.35 t CO2-e ha–1 y–1 (tropical forests) to –0.90 ± 0.42 t CO2-e ha–1 y–1 (temperate forests) and –1.18 ± 0.44 t CO2-e ha–1 y–1 (boreal forests), mostly as CO2 sinks in phytobiomass, microbial biomass and soil C. But net GWP contributions from wetlands are very large, which is primarily due to CH4 emissions. Although the tropical forest system provides a large carbon sink, the negligible capacity of tropical forests to reduce GWP is entirely due to N2O emissions, possibly from rapid N mineralisation under favourable temperature and moisture conditions. It is estimated that the natural ecosystems reduce the net atmospheric greenhouse gas (GHG) emissions by 3.55 ± 0.44 Gt CO2-e y–1 or ~0.5 ppmv CO2-e y–1, hence, the significant role of natural and relatively unmanaged ecosystems in slowing global warming and climate change. However, the impact of increasing N deposition on natural ecosystems is poorly understood, and further understanding is required regarding the use of drainage as a management tool, to reduce CH4 emissions from wetlands and to increase GHG sink from the restoration of degraded lands, including saline and sodic soils. Data on GHG fluxes from natural and relatively unmanaged ecosystems are further compounded by large spatial and temporal heterogeneity, limited sensitivity of current instruments, few and poor global distribution of monitoring sites and limited capacity of models that could integrate GHG fluxes across ecosystems, atmosphere and oceans and include feedbacks from biophysical variables governing these fluxes.

Soil salinity (high levels of water-soluble salt) and sodicity (high levels of exchangeable sodium), called collectively salt-affected soils, affect approximately 932 million ha of land globally. Saline and sodic landscapes are subjected to modified hydrologic processes which can impact upon soil chemistry, carbon and nutrient cycling, and organic matter decomposition. The soil organic carbon (SOC) pool is the largest terrestrial carbon pool, with the level of SOC an important measure of a soil's health. Because the SOC pool is dependent on inputs from vegetation, the effects of salinity and sodicity on plant health adversely impacts upon SOC stocks in salt-affected areas, generally leading to less SOC. Saline and sodic soils are subjected to a number of opposing processes which affect the soil microbial biomass and microbial activity, changing CO 2 fluxes and the nature and delivery of nutrients to vegetation. Sodic soils compound SOC loss by increasing dispersion of aggregates, which increases SOC mineralisation, and increasing bulk density which restricts access to substrate for mineralisation. Saline conditions can increase the decomposability of soil organic matter but also restrict access to substrates due to flocculation of aggregates as a result of high concentrations of soluble salts. Saline and sodic soils usually contain carbonates, which complicates the carbon (C) dynamics. This paper reviews soil processes that commonly occur in saline and sodic soils, and their effect on C stocks and fluxes to identify the key issues involved in the decomposition of soil organic matter and soil aggregation processes which need to be addressed to fully understand C dynamics in salt-affected soils.

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Increases in the concentrations of atmospheric greenhouse gases, carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O) due to human activities are associated with global climate change. CO 2 concentration in the atmosphere has increased by 33% (to 380 ppm) since 1750 AD, whilst CH 4 concentration has increased by 75% (to 1,750 ppb), and as the global warming potential (GWP) of CH 4 is 25 fold greater than CO 2 it represents about 20% of the global warming effect. The purpose of this review is to: (a) address recent findings regarding biophysical factors governing production and consumption of CH 4 , (b) identify the current level of knowledge regarding the main sources and sinks of CH 4