The largest terrestrial store of carbon (C) is in soil and research has shown that anthropogenic land use change and management practices can alter soil C stocks. A concern is that small losses of soil C can contribute to large increases in atmospheric CO2. Research has focused on identifying which land use conversions modify soil C dynamics and more recently, how management practices influence soil C stocks, with particular emphasis on croplands and forests but less on grazed pasture systems. The soil nitrogen (N) cycle has also been modified with increased N inputs, especially under agriculture where N fertilisers and N-fixing plants are used.
About 33% of New Zealand’s total land area is used for grazing. A previous study observed that between the 1980s and 2000s soils on flat land under dairy farming had lost significant amounts of C and N, while soils under drystock farming on flat land had not. A conclusion drawn from the previous study was that a dairy farm was likely have a lower soil C stock than an adjacent drystock farm on the same soil, on flat land. The reasons for the reported soil C and N losses from dairy farm soils are not well understood and require further testing and verification using other approaches.
The objectives of this thesis were to firstly, determine if there was a difference in soil C and N between adjacent dairy and drystock farms on the same soil and secondly, if differences were detected whether they were dependant on differences in farming intensity, as defined by stocking rate.
A synthesis of recent literature showed that when differences in soil C have been observed under various grazing intensities, soil C was generally always lower under higher stocking rates. However, many of the grazing intensity studies were based in semi-arid regions and not particularly applicable to New Zealand’s pastoral grazing systems.
I sampled 25 adjacent dairy and drystock farms (paired sites) on flat land in the Waikato Region to 0.6 m depth and analysed samples for C, N and soil dry bulk density by horizon. Sampling sites at each paired site were an average of 108 m apart and located on the same soil with a similar slope, aspect and topography. The estimated average stocking rate for dairy farms (24 ± 0.8 SU ha-¹) was higher (P<0.01) than drystock farms (14 ± 2.0 SU ha-¹). The mean soil C and N stocks for the whole soil profile (0–0.6 m) were 173.1 ± 12.4 t C ha-¹ and 18.5 ± 0.9 t N ha-¹ for the dairy farms and 182.7 ± 15.0 t C ha-¹ and 19.1 ± 5.7 t N ha-¹ for the drystock farms. The soil C and N stocks for the whole soil profile were not significantly different between dairy and drystock farms. However, when soil horizons were considered separately there was a significant difference in C stocks of the A horizon (P<0.05). The mean soil C in the A horizon under dairying was 94.7 ± 5.7 t C ha-¹ and 103.3 ± 6.1 t C ha-¹ under drystock, with dairy farms having an average of 8.6 ± 4.1 t C ha-¹ less than the drystock farms (P<0.05). No significant difference in soil N stock of the A horizons was detected. The increased variability of soil C and N with depth meant that the significant difference in soil C of the A horizon was not evident when the whole soil profile was considered. The A horizon thickness under dairy farming was shallower (P<0.05) and the soil dry bulk density was higher (P<0.05) than the drystock farms, indicating soil compaction. The total mass of soil sampled from the A horizons was similar for both types of grazing (0.14 ± 0.01 t m-²). Therefore, the significant difference in soil C of the A horizon was likely to be a consequence of land management rather than sampling different masses of soil.
My result that dairy farms had less topsoil C than adjacent drystock farms aligned with the conclusion drawn from a previous study of New Zealand pastoral grazing systems. The result also supported the general trend of less soil C under higher stocking rates than lower stocking rates observed in the literature synthesis. Further work is required to understand what has driven the difference in topsoil C under dairy and drystock farming on flat land in New Zealand. Future research should include exploring how important stocking rates and the type of livestock being grazed are on soil C and N dynamics, as this may be useful information for future farming management decision making
