Ambus, P. and G.P. Robertson
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
Soils are important sources of nitrous oxide (N2O), and upland soils are important sinks of methane (CH4). Both CH4 and N2O are major greenhouse gases which continue to increase in concentration in the atmosphere. Studies have shown that nitrogen fertilization increases the soil N2O source strength and decreases the CH4 sink strength. Thus, increased N deposition may theoretically contribute to an atmospheric increase of both N2O and CH4. However, in most works high single doses of N have been applied to study the N effects on trace gas fluxes from soils. This study examines fluxes of CH4, N2O, and CO2 from plots in the KBS LTER deciduous forest sites, coniferous forest sites, and successional field sites where N is being applied to simulate enhanced atmospheric depositions. Amended plots receive 10 or 30 kg N ha-1 annually, equivalent to or three-fold higher than the annual N deposition in the study area. The nitrogen is added as NH4NO3 in 5 doses over the growing season, and gas fluxes recorded at bi-monthly to monthly intervals.The results from 1995, the first year of sampling, show that average N2O fluxes from coniferous sites increased up to 5-fold in response to the N fertilization (Table 1). However, the numbers are associated with large variability and are not statistically different. Nitrous oxide fluxes from the the deciduous forest and successional fields were not altered by the N fertilization. The increased N2O activity in the coniferous sites was usually observed within two weeks following the N application (Fig. 1). At this point it is not clear why the N2O fluxes from the coniferous sites and not the deciduous and successional sites responded to the N. The inorganic N pools averaged 6.0±0.6 µg N g-1 dry soil, 6.6±0.8, and 4.5±0.5 in the coniferous, deciduous and successional sites, respectively. The results show a potentail for substantial increases in N2O emissions due to anticipated increases in atmospheric N depositions.Net CH4 uptake occured in all sites (Table 2). The CH4 uptake rates were higher in the old-growth deciduous forest than in the recently disturbed conifer plantations and successional fields. This is in agreement with other works showing decreased CH4 uptake following soil disturbance, and that this depressed CH4 uptake may be mirrored decades after the land use has been reversed. Methane fluxes were not clearly affected by the fertilization, that is in some sites, e.g. 1 DF, the CH4 uptake tended to decrease whereas in other sites, e.g. 2 DF, the activity tended to increase. We found evidence that peak CH4 uptake activity is located at > 5 cm depth (Fig. 2). Since the N was added in low doses of only 2-3 kg N ha-1 on the soil surface it is likely that most nitrogen was utilized by plants and microorganisms in the very top soil and never reached the deeper layers of peak methanotrophic activity. Whether this mechanism is in effect will be examined in controlled experiments in the laboratory and through continued fertilization and gas sampling in 1996. Continued fertilization may create more N saturated conditions in the top soil layers and cause not only higher emissions of N2O, but also affect CH4 uptake on the long term.Table 1. Fluxes of N2O from fertilized forest and successional sites. Numbers are mean daily flux over the season of 1995 (SE; n=11 or *n=4). CF = Coniferous Forest; DF = Deciduous Forest; SF = Successional Field. Nd= not determined.Table 2. Fluxes of CH4 from fertilized forest and successional sites. Numbers are mean daily flux over the season of 1995 (SE; n=11 or *n=4). Negative number indicate net CH4 uptake. CF = Coniferous Forest; DF = Deciduous Forest; SF = Successional Field. Nd= not determined.
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