Soil C and N contents in a paired survey of dairy and dry stock pastures in New Zealand

A previous temporal sampling study of New Zealand soils under different grazing systems indicated that soils on flat land under dairy farming had lost significant amounts of C and N in the last few decades, while soils under drystock farming on flat land had not. This result suggested that dairy farms would have lower soil C stocks than adjacent drystock farms. To test this hypothesis, we sampled 25 adjacent dairy and drystock farms to 0.6 m depth and analysed samples for C, N and soil dry bulk density by horizon. Paired sampling sites were on average 108 m apart, on the same soil with similar slope, aspect and topography and had been in that farming system for at least 10 years prior. The average stocking rate for dairy farms (24 stock units ha−1) was higher (P < 0.01) than drystock farms (14 stock units ha−1). The mean total C and total N stocks for the whole soil profile (0–0.6 m) were 173 t C ha−1 and 15.7 t N ha−1 for the dairy farms and 183 t C ha−1 and 16.1 t N ha−1 for the drystock farms and these were not significantly different. However, when the soil horizons were considered separately, the A horizon of dairy farms had significantly lower C (8 t C ha−1, P < 0.05) than drystock farms. The A horizon thickness under dairy farming was also shallower (P < 0.05) with a greater soil dry bulk density (P < 0.05) than the drystock farms indicating soil compaction, presumably due to higher stocking rates and heavier animals on dairy farms. Changes in soil dry bulk density and A horizon depth offset one another and the total mass of soil sampled from the A horizons was the same (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 as a result of sampling different masses of soil. Lower soil C content of the A horizon in this paired site study is consistent with an earlier sampling using temporal comparisons. We do not know the causes for these differences in C, but they might be linked to the higher stocking rates of dairy systems, where large dairy cows exert greater physical pressure on the soil, consume more above ground biomass, and deposit more intense urine patches that have been linked to solubilisation of soil C.