Additional carbon sequestration following repeated urea fertilization of second-growth Douglas-fir stands in western Washington

We examined whether N fertilization of Douglas-fir (Psuedotsuga menziesii (Mirb.)) plantations in western Washington State could affect C sequestration in both the vegetation and soils, Three sites, which received a total of 896-1120 kg ha(-1) over a 16-year-period, were compared with adjacent unfertilized control sites. Carbon contained in the soil, understory vegetation and snags was measured in 1993, Since the tree biomass started at different levels (treated versus controls), C biomass and increment was analyzed dynamically from 1969-1993. There was 6.2% more C (8 Mg ha(-1)) stored in the sum of non-tree components, but the difference was not statistically significant. The N treatment added an average of 26.7 Mg ha(-1) (significant at 0.01 level) to the live tree component. On average, the entire forest system of the fertilized plots added 34.7 Mg C ha(-1). When considered individually, there was no significant difference between the amount of C stored in the snags, understory vegetation and all sampled soil horizons and depths to 85 cm in the control and fertilized plots. However, the urea-treated plots did contain on average 34% more C in the O horizon (20 versus 15 Mg of C ha(-1)). A large portion of the soil C was found in deeper horizons. When sampling to a depth of 85 cm, 75% of soil C was found below the A horizon and 40% below 25 cm. This result illustrates how failing to sample below surface horizons can cause soil C to be underestimated in these forest ecosystems. However, these soil layers changed little in C concentration due to the treatment, This study suggests that N fertilization of commercial forests in western Washington could increase C stored in these forest ecosystems, though the absolute effect of widespread fertilization on stands of varying properties was not evaluated in this study. (C) 2000 Elsevier Science B.V. All rights reserved.