No abstract

Shallow mechanical loosening of soil to 22 cm deep (aeration) was investigated as a method for ameliorating soil compaction caused by dairy cattle treading. Soil physical and pasture measurements taken over 46 weeks compared plots grazed under normal grazing practice (non-aerated) with plots under normal grazing practice where soil was mechanically loosened (aerated). Aerated soil initially showed reduced (P < 0.05) penetration resistance, degree of packing, and bulk density, and increased (P < 0.05) hydraulic conductivity, total porosity, macroporosity, and proportion of small aggregates, compared with non-aerated soil. However, after 40 weeks aerated soil showed some reversion back to a non-aerated state, and only the most sensitive measurements (penetration resistance, degree of packing, soil structure, and *Author for correspondence A99038 Received 6 August 1999; accepted 13 April 2000 macroporosity) showed significant (P < 0.05) treatment differences. Pasture herbage yield, botanical composition, and root length were unaffected (P < 0.05) by aeration, but aeration increased (P < 0.05) root dry weight and decreased bare ground. This work suggests that timing of aeration with regard to soil moisture and atmospheric conditions is vital for optimal soil and pasture responses. The need to use methods which sample large volumes of soil and pasture to detect soil physical and pasture changes due to loosening is also stressed.

Like time domain reflectometers, cheaper CS615 water content reflectometers (WCRs) also measure dielectric properties of the soil to determine its volumetric water content (VWC), but are more affected by environmental factors. Quadratic equations described the laboratory data from 12 horizons of 4 soils of vastly differing properties slightly better than linear equations. Root mean squared errors (RMSE) averaged over the 3 horizons increased slightly in the order Gley (av. 1.0%), Pumice (av. 1.3%), Recent (av. 1.6%), and Allophanic soil (av. 1.9%). Using the manufacturer’s standard calibration resulted in significantly higher RMSE (av. 6.2–25.3%) and mean errors (av. –5.5% to +21.5%), with the VWC of the 2 soils of volcanic origin being underestimated. An atypical dielectric response of water stored in volcanic soils has been attributed to their low bulk density, high porosity, and large specific surface area. The in-situ verification was hampered by the variability observed between the data from duplicate WCRs. Measuring the inter-sensor variability in air and water indicated that this could account for a significant part of this variability, while small-scale variation of VWC in-situ was also observed. Nevertheless, the laboratory calibrations were usually better, or at least similarly suited, to describe the in-situ data than the manufacturer’s calibrations.

In a 6-month laboratory incubation study, we compared the net C and N mineralisation of the soil organic matter (SOM) of 3 pasture soils and the mineralisation of glucose-C in intact versus sieved&sol;refilled soil cores. The main questions were whether the net C and N mineralisation differed between intact and sieved&sol;refilled soil cores after a conditioning period of 4 weeks, and how much the C and N mineralisation of SOM differed among the similarly managed pasture soils. Apart from the net nitrogen mineralisation in one soil, there were no significant differences in cumulated mineralisation of C or N from SOM between the core types. In a fine-textured soil, net mineralisation of glucose-C differed significantly between core types, which was attributed to the different distribution of the amended glucose in intact and sieved&sol;refilled cores. Net C and N mineralisation of SOM were closely correlated in the sieved&sol;refilled cores, whereas no significant correlation was found in the intact cores. Expressing net C and N mineralisation as percentages of total soil C and N showed a more than 2-fold maximum difference between the soils in spite of similar long-term organic matter input. Subsequent studies should be done using more replicates and wider diameter, better controllable cores on ceramic plates. CO2, net nitrogen mineralisation (NNM), soil microbial biomass.

In a previous study, a denitrification wall was constructed in a sand aquifer using sawdust as the carbon substrate. Ground water bypassed around this sawdust wall due to reduced hydraulic conductivity. We investigated potential reasons for this by testing two new walls and conducting laboratory studies. The first wall was constructed by mixing aquifer material in situ without substrate addition to investigate the effects of the construction technique (mixed wall). A second, biochip wall, was constructed using coarse wood chips to determine the effect of size of the particles in the amendment on hydraulic conductivity. The aquifer hydraulic conductivity was 35.4 m/d, while in the mixed wall it was 2.8 m/d and in the biochip wall 3.4 m/d. This indicated that the mixing of the aquifer sands below the water table allowed the particles to re‐sort themselves into a matrix with a significantly lower hydraulic conductivity than the process that originally formed the aquifer. The addition of a coarser substrate in the biochip wall significantly increased total porosity and decreased bulk density, but hydraulic conductivity remained low compared to the aquifer. Laboratory cores of aquifer sand mixed under dry and wet conditions mimicked the reduction in hydraulic conductivity observed in the field within the mixed wall. The addition of sawdust to the laboratory cores resulted in a significantly higher hydraulic conductivity when mixed dry compared to cores mixed wet. This reduction in the hydraulic conductivity of the sand/sawdust cores mixed under saturated conditions repeated what occurred in the field in the original sawdust wall. This indicated that laboratory investigations can be a useful tool to highlight potential reductions in field hydraulic conductivities that may occur when differing materials are mixed under field conditions.

No abstract

The Toenepi catchment (15 km 2 ) is dominated by dairying, and ranges in elevation from 40 to 130m above sea level (ASL). Most of the catchment is flat land, with some rolling and steep land occurring on the boundaries. Annual rainfall is 1132 mm and mean annual temperature is 13.3°C. Well-drained Allophanic soils dominate in the catchment in close association with granular soils of moderate permeability. Poorly drained Gley soils occur in the lowest areas adjacent to the stream and require artificial drainage. The average stocking rate is 3.0 cows ha -1 , which graze all year. The catchment export of total nitrogen through the stream had been calculated in an earlier project as 35 kg ha -1 yr -1 . The median total nitrogen concentration in the stream was 3 mg L -1 (1995/97). To better understand nitrogen flowpaths, we initially installed groundwater monitoring transects in seven subcatchments, which reflected the major site and landuse conditions. Monthly sampling indicated that the concentrations of inorganic nitrogen in the shallow groundwater were generally well below the concentrations measured in the stream. This result would not support the hypothesis that the majority of the nitrate in the stream is derived from groundwater. Monitoring of the nitrogen concentrations in drains indicated that artificial drainage may be a major conduit for nitrogen into the stream. Artificial drains bypass the deeper subsoil and riparian zones where denitrification is likely to occur. A mathematical groundwater discharge model is used to quantify the proportion of streamflow that can be explained by groundwater discharge in contrast to near-surface flowpaths (surface runoff, artificial drainage). Understanding the pathways through which nitrogen enters Toenepi Stream is considered a prerequisite for the development of the most effective and efficient measures to reduce the N contamination of the stream.