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Research Branch > Stand Management > Fertilization > EP 886.15
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.