Abstract
Boreal forests are an important source of wood products, and fertilizers could be used to improve forest yields, especially in nutrient poor regions of the boreal zone. With climate change, fire frequencies may increase, resulting in a larger fraction of the boreal landscape present in early-successional stages. Since most fertilization studies have focused on mature boreal forests, the response of burned boreal ecosystems to increased nutrient availability is unclear. Therefore, we used a nitrogen (N) fertilization experiment to test how C cycling in a recently-burned boreal ecosystem would respond to increased N availability. We hypothesized that fertilization would increase rates of decomposition, soil respiration, and the activity of extracellular enzymes involved in C cycling, thereby reducing soil C stocks. In line with our hypothesis, litter mass loss increased significantly and activities of cellulose- and chitin-degrading enzymes increased by 45–61% with N addition. We also observed a significant decline in C concentrations in the organic soil horizon from 19.5 ± 0.7% to 13.5 ± 0.6%, and there was a trend toward lower total soil C stocks in the fertilized plots. Contrary to our hypothesis, mean soil respiration over three growing seasons declined by 31% from 78.3 ± 6.5 mg CO 2–C m −2 h −1 to 54.4 ± 4.1 mg CO 2–C m −2 h −1. These changes occurred despite a 2.5-fold increase in aboveground net primary productivity with N, and were accompanied by significant shifts in the structure of the fungal community, which was dominated by Ascomycota. Our results show that the C cycle in early-successional boreal ecosystems is highly responsive to N addition. Fertilization results in an initial loss of soil C followed by depletion of soil C substrates and development of a distinct and active fungal community. Total microbial biomass declines and respiration rates do not keep pace with plant inputs. These patterns suggest that N fertilization could transiently reduce but then increase ecosystem C storage in boreal regions experiencing more frequent fires.
Highlights
Nitrogen (N) is known to limit net primary productivity (NPP) (LeBauer and Treseder, 2008), in high-latitude ecosystems with low rates of N fixation and N mineralization (Tamm, 1991; Vitousek and Howarth, 1991)
NPP responds more strongly to N than decomposition, and ecosystem C storage increases with N addition (Pregitzer et al, 2008)
Other studies have observed that the effect of N addition on C storage depends on ecosystem type and soil organic matter quality (Neff et al, 2002; Waldrop et al, 2004)
Summary
Nitrogen (N) is known to limit net primary productivity (NPP) (LeBauer and Treseder, 2008), in high-latitude ecosystems with low rates of N fixation and N mineralization (Tamm, 1991; Vitousek and Howarth, 1991). A number of studies have examined the response of boreal forests to N fertilization and found evidence for N limitation of NPP (Mälkönen and Kukkola, 1991; Tamm, 1991; Moilanen et al, 1996). This finding is important because boreal forests are an important source of wood. Other studies have observed that the effect of N addition on C storage depends on ecosystem type and soil organic matter quality (Neff et al, 2002; Waldrop et al, 2004). In a tundra ecosystem near Toolik Lake, Alaska, 20 years of N addition caused
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