A Nitrogen budget for latesuccessional hillslope tabonuco forest, Puerto Rico

1
A Nitrogen budget for late-successional hillslope
tabonuco forest, Puerto Rico
2
Idealized Illustration
3
Two objectives for developing nitrogen budget

• To quantify imbalances in the nitrogen budget of
  HTF stands
• To explicitly identify gaps for future
  investigations of the N cycle in the LEF
• HTF hillslope tabonuco forest
• LEF Luquillo Experimental forest

4
External inputs

• Principal potential sources atmospheric
  deposition, geological weathering, and
  nitrogen-fixation
• Inputs of DON has not been quantified, estimated
  using DOCDON ratios for precipitation in Costa
  Rica
• Bedrock N, geological weathering has not been
  measured

5
Outflows

• Aqueous export and gaseous losses
• Assume nitrogen entering system through lateral
  subterranean flow equals leaving
• Use groundwater vs. streamwater N to estimate
  inorganic aqueous transport
• Avoid effects of riparian and in-stream processing

6
Biomass accumulation

• Principal reservoirs aboveground biomass, root
  biomass, forest floor, and soil organic matter
  (0-60 cm)
• Estimated using Crow (1980) estimated during an
  inter-hurricane period over 30 years
• Estimated coarse root biomass from assuming equal
  ratio of aboveground woody biomass and coarse
  root biomass to calculate a net belowground
  accumulation estimate

7
Internal fluxes

• Principal internal N-fluxes above- and
  belowground litterfall, litter decomposition, net
  SOM mineralization, and net throughfall (total
  throughfall minus precipitation)
• N-flux from leaf litter placed in decomposition
• N-flux from SOM is estimated by determining
  potential mineralization rates
• Aboveground leaves, fine woodlt1 cm, coarse woody
  debris 1-6 cm, whole tree mortality
• Belowground fine root turnover (estimated)

8
(No Transcript)
9
Estimates potential range based on dry deposition
rates at other sites
Estimates based on basin geologic characteristics
Biomass accumulation internal flux
10
Decomposition and mineralization data
11
Results, abbreviated

• No published research on spatially-integrated
  N-fixation rates in canopy or forest floor of HTF
  or others in Puerto Rico
• PON losses included for erosion and runoff
• Following catastrophic disturbances, aboveground
  biomass N returns to pre-disturbance levels
  faster than the actual aboveground biomass
• During succession, CN ratio increases as ratio
  of woody to non-woody tissue increases, implies
  that N curve reaches asymptote before biomass
  asymptote
• Fine litter largest aboveground input of N to
  floor

12
Conclusions

• Measured inputs and outputs are unbalanced
• Precipitation weathering 4 kg N/ha/yr
• Groundwater output 5-11 kg N/ha/yr
• PON gas loss 1-5 kg N/ha.yr
• Long-term accretion of abovebelow biomass
  sequesters up to 8 kg N/ha/yr
• must be supplied from additional unmeasured
  inputs or a change in N storage in SOM
• Unmeasured N-fixation likely to account for some
  portion of imbalance
• Ignores topographic temporal variability

13
Directions for future research

• What is the magnitude, distribution, and spatial
  and temporal variability in N-fixation?
• Are the SOM and forest floor pools really in
  steady state?
• How are the impacts of large and small
  disturbances on these N processes distributed
  within affected HTF stands?
• What spatial and temporal scales of analysis are
  required to accurately represent these processes
  in stand and catchment-level nutrient budget
  analyses?