Investigating nitrate attenuation capacity and processes in pastoral hill country landscapes : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Massey University, Palmerston North, New Zealand
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The presence of agricultural nutrients, particularly nitrate, in ground and surface waters is an issue of increasing concern for the degradation of water quality in New Zealand. Thus, several studies have focused on the management of pastoral agricultural systems to limit the leaching and availability of nitrate in receiving waters. Such studies, however, are rare for pastoral hill country landscapes which occupy more than 60% of New Zealand agricultural area and hence have the potential to impact on water quality. Therefore, this thesis aims to assist in filling this knowledge gap by investigating the influence of hill country landscape features on nitrate attenuation in pastoral hill country. Denitrification has, for decades, been identified as an important nitrate attenuation process in soil-water systems. Its effectiveness below the topsoil is, however, limited by the supply of dissolved organic carbon (DOC). Pastoral hill country landscape features such as soil type, topography, wet areas, land use, and climate have the capacity to impact on the availability of DOC for denitrification in both the topsoil and subsoil. This study examined the contribution of these landscape features on the dynamics of DOC, and the effect on denitrification in the soil profile (100 cm). The Massey University’s Agricultural Experiment Station (Tuapaka farm) was the case study farm used in this thesis. In order to investigate the effect of soil type and slope on DOC concentration and denitrification capacity, soil samples were collected (from three depths down to 100 cm) from the lowest to the highest elevation (50-360 m) in the farm. The sampled locations comprised of three slope classes (low, medium and high) and eight soil types (Tokomaru, Ohakea, Shannon, Tuapaka, Halcombe, Korokoro, Ramiha and Makara), grouped into three drainage classes (poorly-, imperfectly-, and welldrained). The results of the study indicated that compared to slope, soil type had a greater effect on denitrification capacity within the farm. This effect of soil type was mainly associated with soil parent material, as the Ramiha soil which had a higher carbon (C) storage capacity (due to its high content of short-range order constituents), also had the highest amount of DOC (105 mg kg⁻¹, within the 30-60 cm soil depth) and thus the highest denitrification capacity (10 μg kg⁻¹ h⁻¹). The findings of this experiment imply that farms or catchments with soil types similar to the Ramiha soil may have a greater capacity to attenuate nitrate losses to receiving waters. The contribution of hill country wet areas (seepage wetland and hillside seeps) to nitrate attenuation was assessed by first comparing the DOC concentration of the wet areas to that of an adjacent dry area. This showed that mean DOC concentration of the surface 30 cm soil depth was in the following order: seepage wetland (498 mg kg⁻¹) > hillside seep (172 mg kg⁻¹) > dry area (109 mg kg⁻¹). A subsequent more detailed examination of the seepage wetland and dry area showed that the denitrification capacity of the seepage wetland within the 0-30 and 30-60 cm soil depths was 7 and 69 times higher, respectively, than that of the dry area. The higher DOC concentration and the presence of readily-decomposable DOC in the seepage wetland contributed to its higher denitrification capacity. This contrasting nitrate attenuation capacity of the seepage wetland versus that of the dry area highlights the potential contribution of seepage wetlands to nitrate attenuation for improved water quality in pastoral hill country landscapes. Land use change from pasture to forage cropping, which is increasingly being adopted in New Zealand hill country, has the potential to influence the dynamics and leaching of DOC for subsurface denitrification. However, there is limited research understanding on the effect of land use change (forage crop establishment) on DOC dynamics and leaching in pastoral hill country. Therefore, a study was designed to investigate soil DOC dynamics and denitrification capacity as influenced by the establishment of a forage crop (swede, Brassica napobrassica Mill.) via the surface sowing technique (no cultivation). This experiment was carried out in two stages. The first stage monitored the short-term changes in DOC concentration and chemistry immediately after spraying out pasture with selected agrochemicals (active ingredients: glyphosate, dicamba, diazinon and organomodified polydimethyl siloxane). The results showed that the agrochemicals increased DOC concentration only within the surface 5 cm soil depth (by ~ 20 mg kg⁻¹) on days 1 and 6 after the agrochemicals were applied. This increase in topsoil DOC concentration was most likely due to a direct contribution of C from the agrochemical, an indirect C contribution through the displacement of adsorbed organic molecules, and the decomposition of root necromass. DOC chemistry was, however, not altered by the applied agrochemicals. These findings were further confirmed by a follow-up experiment which used δ¹³C isotope technique to measure leached DOC from an organic material (plant residue) added to the topsoil. This showed that one week after the application of organic material, only a negligible amount (≤ 5%) of C derived from the organic material was detected in the subsoil (20-60 cm depth) DOC, compared to > 20% detected in the surface 20 cm soil depth, suggesting the limited leaching of exogenous DOC (under the experimental condition studied) due to its rapid turnover in the topsoil. The second stage of the forage crop establishment experiment monitored temporal changes in DOC concentration and denitrification capacity within a year of forage crop establishment. The results indicated that DOC concentration and denitrification capacity of both topsoil and subsoil layers were generally not affected by the establishment of the forage crop. However, an increase in rainfall and soil moisture, after periods of soil water deficit, increased the DOC concentration of the soil. Forage crop establishment resulted in an initial increase (by > 55%) in the nitrate concentration of the surface 20 cm soil depth, most likely due to poor nitrogen (N) utilisation by the growing brassica forage crop. However, the higher nitrate concentrations were only detected in the topsoil and thus the risk of increased nitrate leaching was assumed to be negligible. This thesis has highlighted the variations that exist in the DOC concentration and denitrification capacity of the different soils within hill country landscapes and thus suggests that these soils require contrasting management practices for effective water quality outcomes. In addition, the potential contribution of hill country seepage wetlands to nitrate attenuation shown in this thesis suggests that management strategies that preserve and enhance these pastoral hill country landscape features should be promoted to attenuate the losses of nitrate to receiving waters. Furthermore, this thesis has demonstrated that the common land use change from pasture to a forage crop, to supplement animal feed production in New Zealand hill country, is unlikely to have any significant impact on the DOC concentration and denitrification capacity of the soil profile (100 cm), within a one-year period. The observed results suggest that this practice is also not likely to negatively impact on water quality via nitrate leaching. However, larger-scale forage crop trials would be required to validate these findings. The findings of this thesis suggest that some hill country landscape features have the potential to attenuate nitrate losses to receiving waters. This information is critical for improving hill country N management for better water quality outcomes, which could potentially credit farmers under possible N loss regulations.
Denitrification, Soils, Carbon content, Hill farming, New Zealand