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An analytical solution for steady-state oxygen transport in soils including 2 sink terms, viz roots and microbes with the corresponding vertical distribution scaling lengths forming a ratio p, showed p governed the critical air-filled porosity, [θ.sub.c], needed by most plants. For low temperature and p, [θ.sub.c] was < 0.1 but at higher temperatures and p = 1, [θ.sub.c] was >0.15 [m.sup.3]/[m.sup.3]. When root length density at the surface was [10.sup.4] m/[m.sup.3] and p > 3, [θ.sup.c] was 0.25 [m. sup.3] [m.sup.3], more than half the pore space. Few combinations of soil and climate regularly meet this condition. However, for sandy soils and seasonally warm, arid regions, the theory is consistent with observation, in that plants may have some deep roots. Critical [θ.sub.c] values are used to formulate theoretical solutions in a forward mode, so different levels of oxygen uptake by roots may be compared to microbial activity. The proportion of respiration by plant roots increases rapidly with p up top ≅ 2. Synthesis of vertical root biomass density, L [= [L.sub.0] exp (-z/[Z.sub.r]), z is the depth positive down (m)] (m/[m.sup.3]), data using an exponential function to represent the distribution suggested that, on average, 70 ± 10% of fine roots in 10 terrestrial biomes were located in the upper 0.1 m of soil. Integrated over the root-zone, [L.sub.T] is given by the product of the function's 2 parameters, the surface value of L, [L.sub.0] (m/[m.sup.3]), and length scale, [Z.sub.r] (m). As postulated, negative correlations were obtained between [L.sub.0] and [Z.sub.r]. For a maize (Zea mays L.) crop, significantly different distributions were measured during relatively dry and wet seasons and predicted by our model. For woody and herbaceous plants, [Z.sub.r] (the value determines the rate of decrease in L with depth) averaged 0.3 and 0.2 m, respectively, while the corresponding averages for R,n0 [= [L.sub.O].Pr, Pr is root density (kg/m)] were 2.7 and 1.1 kg/[m.sub.3]. Additional keywords: oxygen, roots, root respiration, soil respiration, soil aeration., Introduction For most plants to grow in soil, a proportion of the pore space needs to be filled with air (Glifiski and Stepniewski 1985). This allows for the exchange of [...]

Grazing animals excrete urine and create transitorily high pH, nitrogen (N)-replete soil patches. Beneath grazed pasture, we postulated the soil microbial community would be highly responsive to N application. Lesser responses were expected of soils beneath grassland without grazing animals. Soil samples were collected near Lincoln, New Zealand (43.6°S, 172.5°E), beneath pasture regularly grazed by dairy cattle, an adjacent pasture set aside from grazing 20 years ago, and a nearby grassland that has never been grazed. Soil microbial respiration responses to repeated urea (500 kg N/ha) applications were determined by laboratory incubation experiments and the soil pH and water-soluble C content were also measured. The first application induced 0.13 ± 0.04 (dairy farm), 0.15 ± 0.05 (set aside), and 0.20 ± 0.04 (ungrazed) g C/kg increases in microbial respiration over 9 days, excluding carbon dioxide production from carbonate hydrolysis. After a second application, 9-day respiration increased by 0.26 ± 0.04 (dairy farm), 0.41 ± 0.04 (set aside) and 0.20 ± 0.07 (ungrazed) g C/kg. For the dairy farm and ungrazed soils, the microbial communities responded differently to repeated urea addition. The responses included transitory changes in pH and reflected the limited amounts of readily decomposable organic matter. Additional keywords: microbial respiration, urea, pH, organic matter., Introduction Pastoral ecosystems can achieve high rates of net primary productivity, but the ephemeral nature of grass leaves means that they turnover rapidly. Production that is not utilised, potentially representing [...]

Beneath pasture grazed by farmed animals, the soil's nitrogen (N), oxygen, and temperature regimes can be unevenly distributed in time and space. It is difficult to capture spatial and temporal variation of [N.sub.2]O using conventional emission measurement technology based on gas samples taken in chambers that briefly cover a small area of the soil's surface. We report the results from field deployment of alternative, non-intrusive [N.sub.2]O emission measurement technology that uses subsurface measurements incorporating the soil processes controlling the net [N.sub.2]O production and gas diffusion rates. During 100 autumn and winter days after dairy cattle urine was applied (650 kg N/ha) to freely and poorly drained pastoral soils near Hamilton, New Zealand (37.8°S, 175.3°E), [N.sub.2]O emissions were determined. The measured values ranged from 0.024 to 1.55 and 0.048 to 3.33 mg [N.sub.2]O-N/[m.sup.2].h for the freely and poorly drained soils, respectively. Over the 100 days, it was estimated that 0.4 and 1.3% of the applied N was directly emitted to the atmosphere as [N.sub.2]O from the freely and poorly drained soils, respectively. Additional keywords: [N.sub.2]O, Fick's Law, excreta, pastoral soil., Introduction Agricultural soils are the principal source of nitrous oxide ([N.sub.2]O) emissions (Mosier et al. 1998a). Emissions are often measured at the surface using chambers inserted into the soil (e.g. [...]