CONCLUSIONES Y RECOMENDACIONES

Climate station records highlight the importance for land management and environmental protection of understanding infiltration and conductivity behaviour for the Gorge soil, because these records show that there is a regular soil-water ‘drainage season’. The flat topography and low slope mean that any surface ponding of surplus water is not likely to generate runoff. Climate records also show that rainfalls are typically of low intensity and amount, further reducing the risk of runoff. Soil colour indicates that periods of saturation are minimal and of short duration. This is further evidence of good drainage and thereby supports the contention that the water surplus moves as drainage rather than runoff. Soil morphology shows distinct soil layers, where a topsoil, worm-mixed, and subsoil are easily identifiable. Whilst the exact boundary between layers is difficult to define, the layers do approximately correlate with the 20 cm increments that are used for the lysimeter experiments. The measured physical attributes also correlate with the defined soil layers, particularly the pattern in bulk density, soil carbon, and θv(Ψm). It is clear from

the physical attributes that the 0 – 5 cm depth should be defined as a separate soil layer, even though this was not at first apparent in the soil morphology.

The measurements show a consistent decline in soil porosity with depth, which appears to be mostly due to a reduction in the pore network responsible for drainage. Measurement of θv(Ψm) over the drainage-pore range highlights distinct differences between the soil

layers, particularly in relative abundance of pores over a particular Ψm range. Below 25

cm soil depth the θv(Ψm) measurements for each lysimeter detected little or no decline in

porosity over the range of -0.5 to -2 kPa. This range is of particular importance for water infiltration and drainage because it includes the macropore network often associated with preferential flow (Jarvis, 2007), and is the pore network of most interest in this study. Whilst these results appear to highlight a sharp reduction in macropore abundance below the topsoil layer, it may also partly reflect sampling error. This error could arise if macropores occur in spatially distinct regions such as between large aggregates, and the volume and number of cores was too small to ‘capture’ these regions. It is also possible that the subsoil pore network has an air-entry value such that it resists the applied suction until a critical suction is reached, at which drainage commences. This would be at odds with the morphology which shows good drainage, and also the rapid drainage behaviour

that was observed during the lysimeter experiments. Therefore, if a critical air-entry value is present, it is possibly an artefact of the measurement method.

Sampling error may also arise if the artificially wet conditions during measurement induced atypical wetting of the soil. It was observed that it was difficult to obtain stable weights at these low suctions, with a number of cores appearing to even slightly gain weight as the suction increased from -0.5 to -2 kPa (refer to layer B θv(Ψm) pattern, Figure

4-10). The prolonged artificially wet conditions could induce swelling of clays and removal of entrapped air, which would draw water up from the suction plate, and offset water that may have drained at the applied suction. These conditions would not arise in the field situation because of the natural good drainage of the soil.

Another key feature is the remarkable uniformity in soil texture. The measurements show no variability in texture among lysimeters and soil layers. This is important because texture is often used to explain differences in infiltration and drainage behaviour. Texture is also a key parameter in most models that are used to predict soil hydraulic attributes. The uniform texture of this soil indicates that there should be minimal variation in the infiltration and conductivity attributes both among the lysimeters, and between the individual soil layers. This soil does show a consistency between the θv(Ψm) pattern, and

changes in bulk density and carbon. These results indicate that the reduction in the drainage pore network with soil depth is strongly related to decreasing levels of soil carbon, which is reflected in aggregates increasing in both density and size. The relationship with carbon is not purely causative. There is a correlation between carbon and drainage pores because both in turn are related to biological activity.

Overall the soil morphology and physical attributes indicate that this soil shows high physical fertility. Soil water storage and aeration are seen as key indicators of physical fertility (Webb et al., 1995). The physical measurements show that the soil has high water storage capacity (AWC), and the volume of drainage pores indicate that at field capacity there is adequate air-filled porosity (AFP). This is reflected in the soil colour which indicates good drainage and minimal periods of anaerobic conditions. McKenzie et al. (2004) rate a soil’s physical fertility on the relationship between AFP and AWC. The profile-averaged AFP (10%) and AWC (22%) of the Gorge soil would rate as good physical fertility. Physical fertility declines with depth, with good to very good in Layer S decreasing to moderate in the subsoil.