Professor Cornell University Ithaca, New York, United States
Despite longstanding efforts to understand how anthropogenic nitrogen (N) inputs and acidification affect forest ecosystems, there is still considerable uncertainty about how N and pH independently or interactively affect these systems. Much prior research has been unable to separate N fertilization from pH effects, because elevated N inputs often co-occur with acidification along deposition gradients and most N fertilization studies apply N in forms that acidify. Delineating between influences of N and soil pH is important because geologic and edaphic factors result in variation in ecosystems’ susceptibilities to acidify during times of high deposition or to recover once deposition is curtailed. Here, we investigated the effects of N additions and acidification on aboveground tree growth, N availability, and litterfall chemistry in a ten-year, replicated, full factorial N and pH manipulation study in mixed temperate forests in central New York. Because N often limits production in this region, we anticipated that N addition would stimulate tree growth regardless of concurrent acidification. However, we expected this effect to attenuate with time in stands treated with an acidifying N form, due to reduced phosphorus and base cation availability. We anticipated that acidification alone would reduce tree growth by the same mechanism.
N additions stimulated tree biomass increment in stands treated with both acidifying and non-acidifying N forms relative to controls during the first four years of treatments. However, growth stimulation diminished between years six and nine of fertilization in stands treated with both N forms, resulting in no differences between N-treated stands and controls. After nine years of fertilization, productivity in acidification-only stands was lower relative to N-treated stands, but not controls. Litter chemistry measurements suggest that attenuation of growth responses to N addition may be due in part to nutrient limitation shifting over time from N to other nutrients. In contrast, resin-available N was 4-fold lower in acidified stands than N-treated stands and positively associated with biomass increment, suggesting that acidification reduced growth by decreasing N availability, rather than reducing P or base cation availability. Overall, our results are consistent with regional analyses showing that N deposition can stimulate aboveground growth. However, acidification in the absence of N inputs may reduce aboveground production by decreasing N availability, an effect that has presumably been historically offset by coupled N and acid inputs.