Changes in soil organic matter under pasture over the last 27 years
Roger Parfitt and colleagues from Landcare Research, GNS and University of Waikato have measured soil C under the same dairy and sheep/beef pasture sites on three occasions over the last 27 years.
They previously reported on the changes between approximately 1982 and 2004 for 30 sites where they found soil C was decreasing by 0.8 tonnes C/ha/year for the whole soil profile. International reporting (IPCC) requires reporting for a soil depth of 0–30 cm. Data have now been obtained for 83 sites, and sheep and beef farms, and tussock soils, show no measurable loss of soil C between 1982 and 2004 (Figure 1). However, there was a significant loss of 0.7 t C/ha/year for 29 dairy sites. The average losses were similar for Non- Allophanic soils and Allophanic soils, although the Non-Allophanic soils contain less total C (122 t/ha to 105 t/ha) compared with Allophanic soils (194 t/ha to 176 t/ha) (Figure 1).
Preliminary soil C data for 2009 from 22 farms (Figure 2) indicate no further measurable loss of C between 2004 and 2009, but it may be that insufficient time has passed to be able to detect ongoing change in soil C stocks due to within site variability. As mentioned on page 8 there is considerable variability of soil C within a paddock, and we are investigating how many sites need to be measured, over how many years, in order to get a robust measure of change in soil C.
The possible reasons for the loss of soil C under dairy are also being investigated. The use of N fertiliser increased from 1990 to 2004 (Figure 3) mainly on dairy farms, and over this time pasture has been farmed more intensively. For dairy pastures, the soil C:N ratio appeared to decrease from 11.4 to 11.1, suggesting soil N fertility was increasing. With increased N input there may be less C input from roots to soil, and there will be greater additions of cow urine-N to the soil. Preliminary work suggests cow urine can accelerate decomposition of soil organic matter. The intensive use of pasture under dairying may therefore contribute to the loss of C. Since 2004 the use of N fertiliser has not increased but it is probably too early to say whether soil C has arrived at a new level.
Roger Parfitt
Figure 1. Soil C for 0–30 cm depth at sheep/beef farms, dairy farms on Non-Allophanic Soils (NonA) and dairy farms on Allophanic soils in Taranaki and Waikato (A), and Tussock. Error bars are standard error of the mean.
Figure 2. Soil carbon at sheep/beef farms, and dairy farms on Non-Allophanic Soils (NonA) and dairy farms on Allophanic soils in Taranaki and Waikato (A), for 0–30 cm depth for 22 sites. Error bars are standard error of the mean.
Figure 3. N Fertiliser consumed in NZ (H. Furness, pers. comm.).
