Net greenhouse gas global warming potential of a 57-year-old west coast Douglas-fir stand following nitrogen fertilization

Simulated nitrogen (N) deposition has been shown to increase carbon (C) storage in northern temperate forests (Pregitzer et al., 2007). A meta-analysis has shown that N fertilization was the only forest management practice that had a clearly positive effect on the soil C pool (Johnson and Curtis, 2001). In BC's coastal regions, soils are commonly deficient in N (Hanley et al., 1996) so that tree growth is typically N-limited (Nadelhoffer et al., 1999). Thus Douglas-fir stands generally benefit from N fertilization. Fertilization is one of the eligible management practices for C sequestering and hence reducing CO₂ emissions under Article 3.4 of the Kyoto Protocol. However, there have been conflicting reports on the effect of N fertilization on soil respiration (R_s) and its heterotrophic (R_h) and rhizospheric (R_r) components. Another major concern with N fertilization is the potential loss of gaseous N, especially soil N₂O emissions. However, little is known about such losses in N-fertilized forest soils.

Here we report effects of N fertilization of a 57-year-old Douglas-fir stand with 200 kg N ha^-1 on R_s, R_h, R_r, forest-floor N₂O emissions, and eddy covariance (EC)-measured stand NEP. An area of 80 ha (which included the 80% cumulative flux footprint of the flux tower) was fertilized with urea by helicopter on Jan 13, 2007. A non-fertilized area of about 17 ha served as a control for comparing differences in tree growth, C stocks and greenhouse gas emissions. Every 2-3 weeks, soil CO₂ effluxes were measured in non-fertilized and fertilized as well as trenched and non-trenched plots using a portable dynamic chamber while soil N₂O emissions were measured using a portable static chamber. Increment in stand NEP in 2007 due to fertilization was computed using an empirical model fitted to pre-fertilization (1998-2006) NEP and climate data. Effect of fertilization on net greenhouse gas global warming potential (GWP) was calculated considering changes in soil N₂O emissions and NEP, and energy requirements of fertilizer production, transport and aerial spreading.

Results showed that annual NEP increased from 335 g C m^-2 calculated assuming the stand was not fertilized to the measured value of 488 g C m^-2 with fertilization, thus resulting in a 46% increase in the first year after fertilization. Fertilization decreased soil CO₂ efflux in the trenched plots but increased it in the non-trenched plots indicating small decreases in R_h but increases in R_r. We found significant N₂O losses in the fertilized plots compared to a small uptake in the non-fertilized plots. N loss in the fertilized plots was about 16 kg N₂O ha^-1 in the first year, which is equivalent to 10 kg N ha^-1 or 5% of the applied fertilizer N. The latter contrasts to 1.25% assumed by the IPCC. At the end of first year after N fertilization, the net greenhouse gas GWP was -0.06 t CO₂ ha^-1, thereby indicating a small favourable effect of fertilization even in the first year after fertilization. Judging by a substantial increase in NEP in the first year, and since the effect of fertilization is expected to last over several years, it appears that increases in NEP in the following years will result in a significant decrease in greenhouse gas GWP.