The effect of increased land use intensification on the physical properties of a silt loam topsoil

Since settlers first arrived land use has been continually changing across New Zealand. In the Canterbury region, recent years have seen widespread conversion of dryland sheep and cattle grazing, to a more intensive irrigated dairy farming. To determine the effects of these land uses on soil physical properties sampling was carried out at 0-10 cm, 10-20 cm and 20-30 cm depths on the Lincoln University Dairy Farm (DF), a nearby dryland sheep grazed site (SF) and a neighbouring control site (CS), (n= 45 for each site). The three sites were located on the same Templeton soil, with the same climate, except for irrigation input. Soil properties measured were: macroporosity, bulk density, water holding capacity (WHC) at -10 kPa, -40 kPa and -100 kPa, soil particle size and soil carbon. Macroporosity was significantly affected by irrigation and treading. Values for the 0-30 cm increment were significantly lower (p < 0.05) for the DF (8.8 ± 0.6%) than both SF (19.3 ± 0.6%) and CS (14.8 ± 0.9%). Within each site there was also a significant increase with each 10cm depth increment, apart from on the SF for the 10-20 cm and 20-30 cm increments and notably on the DF for the 0-10 cm and 10-20 cm increments where values were similar. This indicated effects of compaction to a greater depth on the DF. These differences in macroporosity meant that soil water content at -10 kPa, -40 kPa and -100 kPa was higher at the DF than the SF and CS. Differences were solely from the differences in macroporosity between sites and not the result of changes in the quantity of the storage pores between these matric potentials. Therfore, no significant difference was measured in plant readily available water (RAW, -10 kPa to -40 kPa, and RAW, -10 kPa to -100 kPa) between the sites. However, at -100 kPa the DF was found to have a significantly higher (p < 0.05) volumetric water content (θ) for the 0-30 cm increment (31.7 ± 1.1%) than both the SF (23.7 ± 1.3%) and CS (25.6 ± 1.4%). This indicated an increase in the number of pores ≤ 3 μm in diameter (at suctions greater than -100 kPa) which were not tested in this study. These findings were in agreement with other studies comparing the effect of irrigation and grazing. In these studies irrigated dairy treatments also had similar values for RAW as dryland and sheep grazed sites but had significantly lower values for macroporosity and higher amounts of water held in pores ≤ 3 μm in diameter. Results for soil carbon were compared as total C, C density and C storage and there were no significant differences (p > 0.05) between sites for any of these measurements. This finding was similar to that of another study, carried out on the LUDF and a control site in 2012, where the C storage was slightly higher on the LUDF than the control site. Further research is suggested in an additional 5-10 years to determine whether a stable state of C storage has been reached. The results for the macroporosity, bulk density and soil carbon were analysed using target ranges for soil quality and using the soil natural capital framework. Macroporosity was found to be a more sensitive indicator for the effects of compaction on soil than bulk density. Furthermore, the soil natural capital framework was found to be a more holistic method for evaluating the state of the soil physical resource than the target ranges, established for soil quality assessment, alone. Use of the natural capital framework allowed changes in soil properties with time to be taken into account. It also allowed changes to these soil properties to be considered in terms of the ecosystem services that might be affected and human physiological needs that might not be met.