NITROGEN IN LODGEPOLE PINE FORESTS IN RELATION TO DECOMPOSITION AND SOIL SOLUTION CHEMISTRY.

摘要

Charge-balance chemistry of forest floor and soil solutions and studies of long-term organic matter decomposition were used to identify controls of N accumulation and flux in lodgepole pine (Pinus contorta ssp. latifolia) ecosystems in southeastern Wyoming.;Decomposition of leaf litter was slow with a mean dry mass loss of 65% during 8 years of decay. Nitrogen content increased to 170% of the original amount before mineralization occurred. Much of this immobilization probably was the result of uptake by decomposers because N storage in microbial biomass accounted for 13% and 7.5% of the total N storage in the 01 and 02 horizons of the forest floor, respectively. Measurement of woody root decay indicated that root wood persists for many years (> 110 yr) following death of the aboveground portion of the tree, but N immobilization in decaying root wood (10 mg N(.)m('-2)(.)yr('-1)) was much less than the amount immobilized in aboveground detritus (500 Mg N(.)m('-2)(.)yr('-1)).;An input/output budget indicated that the ecosystem accumulates about 0.4 g N(.)m('-2)(.)yr('-1), with most of this accretion occurring in aboveground detritus. The results suggest an annual depletion of soil organic-N between periodic fires that occur in the lodgepole pine ecosystem.;With the exception of bulk precipitation, organic-N accounted for > 90% of the dissolved N flux in ecosystem solutions. Total-N flux from the forest floor was greater during spring snowmelt than in response to summer rains (0.46 g N(.)m('-2)(.)yr('-1) vs. 0.20 g N(.)m('-2)(.)yr('-1)), with most of this soluble N being retained in the soil rooting zone and being a potential contribution to soil organic matter formation. Dissolved organic carbon flux appeared to be a primary factor controlling soluble N flux in the ecosystem, as well as in the transport of metallic cations. Chemical characterization of organics indicated that: (1) most were acidic compounds, (2) undissociated and dissociated compounds were the dominant forms in forest floor and soil solutions, respectively, and (3) the C/N ratio increased from 20:1 to 75:1 during the spring snowmelt period, probably as a result of changes in the quality of organics. Dissolved humics became more common late in the snowmelt period.