The use of irrigation has become important for sustaining the production of food, fibre and biofuel. In New Zealand (NZ), irrigation of pastures is increasing in areas with seasonal water deficits, such as the Central North Island. Nationally, there was an estimated 794,443 ha of land under irrigation in 2017. A synthesis of the literature showed that soil C and N stocks response to irrigation was dependent on the initial soil organic matter content and climatic zone. However, most of the literature focused on arid/semiarid climate regimes where cereals and other crops were grown, with few studies conducted on pastoral land in temperate climates. However Mudge et al. (2017), measured a decrease in soil carbon (C) (6.99 t C ha⁻¹) and nitrogen (N) (0.58 t N ha⁻¹) to 0.3 m depth in irrigated grazed pastures relative to adjacent non-irrigated grazed pastures. Of their 34 paired sites across NZ, only seven were sampled from the Pumice Soil order, which was insufficient to determine whether this soil order was vulnerable to soil C and N loss.
The main objective of this thesis was to compare the total soil C and N stocks of irrigated and non-irrigated Pumice Soils. A second objective was to determine where there were any trends in changes in soil C and N in relation to irrigation duration.
Fourteen paired irrigated and adjacent non-irrigated sites were sampled in the Reporoa Basin, north of Taupo and soil C and N stocks were measured accounting for differences in soil bulk density. Most sites where sampled to 0.6 m depth, however, due to high water tables and buried soils, some sites were only sampled to 0.4 m or 0.5 m depth. One site was omitted from statistical analysis due to high residuals (F11-35) and was considered an outlier that potentially represented a poorly matched pair of sites. On average, for the remaining 13 paired sites, irrigated soils had significantly (P < 0.05) lower soil C stocks (5.93 t C ha⁻¹) to 0.2 m depth and lower N stocks (0.24 t N ha⁻¹) to 0.1 m depth in comparison to adjacent non-irrigated soils. When considering the full depth of the sampled soils (0-0.6 m), there were no significant changes in soil C and N stocks (P > 0.05). Using this coring method, I was able to detect a change of about 6.9 % in soil C stocks and a 4.8 % in soil N stocks. This study also found no evidence of a relationship between soil response to irrigation and duration of irrigation.
With the loss of soil C and N under irrigation comes potential release of CO₂ to the atmosphere and N leaching into surrounding water bodies. Based on the findings in this thesis, there is a need for further investigation on the effects of irrigation for different Soil Orders. This research will aid farmers in understanding their land management on-farm and inform them of their potential effects on greenhouse gas emissions on a national scale.