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Dicyandiamide (DCD) is a nitrification inhibitor of variable efficacy. In soils, DCD biodegradation rate is known to be a function of temperature; however, microbial activity can also be affected by soil aeration and substrate availability. Studies determining the effects of soil aeration on DCD degradation are few. We tested the null hypothesis that the rate of degradation of DCD in soil would be the same under aerobic and anaerobic conditions. Soils from two sites with different organic matter concentrations but the same parent material were sampled to the same depth, sieved, and repacked into tubes ('soil cores'). These were saturated with a DCD solution (30 µg [mL.sup.-1]) and placed under controlled aeration conditions by imposing five levels of matric potential (0, -1, -3, -6, and -10kPa) at a constant temperature (22°C). The relative [O.sub.2] diffusivity ([O.sub.2] diffusion coefficient in soil/[O.sub.2] diffusion coefficient in air, Dp/Do) was measured, along with periodic destructive sampling of soil cores over 40 days, to assess the DCD concentrations. Fitting first-order exponential functions to plots of soil DCD concentration v. time showed that the DCD degradation rate was greater (P< 0.05) when the soil was aerobic (Dp/Do ≥ 0.01). Consequently, the null hypothesis was rejected. These results show that soil aeration determines the degradation rate of DCD. Additional keywords: half-life, matric potential, relative [O.sub.2] diffusivity, volumetric water content, water retention curve., Introduction Dicyandiamide (DCD; [C.sub.2][H.sub.4][N.sub.4]) is a nitrification inhibitor. The efficacy of DCD in pasture systems, and of nitrification inhibitors in general, has been reported to vary (Linquist et al. 2012; [...]

A set of equations governing oxygen diffusion and consumption in soils has been developed to include microbial and plant-root sinks. The dependent variable is the transformed oxygen concentration, which is the difference between the gaseous concentration and a scaled value of the aqueous oxygen concentration at the root-soil interface. The results show how, as the air-filled porosity decreases, the reduced oxygen flux causes the depth of extinction to decrease. The results also show how the depth of extinction at a particular value of soil water content decreases with increasing temperature, due to increased microbial respiration. The critical value of water content at which the oxygen concentration goes to extinction at a finite depth was compared with alternative calculations with only a microbial sink. By ignoring the feedback of oxygen concentration on root uptake, the alternative calculations yielded substantially higher critical values of water content at all temperatures. Two soil oxygen diffusion coefficient functions from the literature were compared and shown to give significantly different critical values of water content for fine-textured soils, one more realistic than the other. A single relationship between the extinction depth and the ratio of the water content to the critical value was shown to apply for all temperatures and soil textures. The oxygen profiles were used along with a function relating redox potential to oxygen concentration to generate redox potential profiles. This application of the model could be useful in explaining soil biochemical processes in soils. For one such process, denitrification, the depth at which a critical oxygen concentration is reached was calculated as a function of the air-filled porosity and temperature of the soil. The implications of the critical value of soil water content in terms of water-filled pore space and matric potential are discussed in relation to the diffusion coefficient functions and recent literature. Additional keywords: air-filled pore space, denitrification, diffusion coefficient function, oxygen transport, redox potential, soil respiration., Introduction The transport of oxygen into soil and its distribution within the profile are fundamental to the understanding of aeration, and biochemical and chemical reactions in soil. The oxygen status [...]

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. [...]