Comparison of soil carbon and nitrogen stocks of adjacent dairy and drystock pastures

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