Sulphur Fertilization of Lodgepole Pine: a Stable Isotope Tracer Study (EP 886.15)
Despite well-documented evidence of S deficiencies in interior forests, there is still considerable uncertainty as to which form of S is most effective in stimulating growth response and in sustaining long-term improvement in site S status. Soluble sulphate-S forms (e.g., ammonium sulphate) are rapidly taken up by trees, but may persist in soils and foliage for a relatively short period. Although the initial availability of elemental S is lower, this "slow release" effect may improve the efficiency of S application and may result in long-term improvement in soil S availability. For large-scale fertilizer operations, S requirements are currently satisfied by blending urea (46% N) with ammonium sulphate (21% N, 24% S) – a choice based more on convenience and availability than on well-documented research results.
In 2001, the University of Northern British Columbia (Dr. Paul Sanborn) and the BC Ministry of Forests, Research Branch implemented a cooperative project to evaluate the behaviour of different forms of applied S in two lodgepole pine ecosystems in central British Columbia. One site is southwest of Vanderhoof within the Dry Cool subzone of the SBS biogeoclimatic zone (SBSdk). The other site is southeast of Prince George within the Willow Wet Cool variant of the SBS biogeoclimatic zone (SBSwk1). Using stable isotope methods, the fate of applied S is being followed at both sites by using carefully selected fertilizers that differ slightly from natural background in their content of the less abundant heavy isotope, 34S.
The objectives of this study are threefold:
- How much of the added S is actually taken up by trees, and how much is retained in the soil and/or lost from the site?
- Does a slow-release form of S, such as elemental S, provide more or less long-term improvement in S nutrition than a more readily available form, such as a soluble sulphate salt?
- What is the magnitude of the tree growth response to these treatments?
The treatments at both sites compare either sulphate-S or elemental S (added in combination with N), with N-only and an unfertilized control. The four treatments are replicated four times at each site. At one site, another elemental S source – added alone and in combination with N – was also tested. All S additions were at 100 kg S/ha, and N (as urea) at 300 kg N/ha. All treatments were applied to circular, area-based plots.
- Sanborn, P. and R. Brockley. 2005. Sulphur deficiencies in lodgepole pine: occurrence, diagnosis, and treatment. B.C. Min. For., Victoria, B.C. Exten. Note 71.
- Sanborn, P., R. Brockley, B. Mayer, M. Yan, and J. Prietzel. 2005. Sulphur fertilization of lodgepole pine: a stable isotope tracer study (E.P. 886.15) establishment report. B.C. Min. For., Res. Br., Victoria, B.C. Tech. Rep. 020.