Canopy soil nutrient cycling and response to elevated nutrient levels along an elevation gradient of tropical montane forests

Although canopy soils can contribute significantly to aboveground labile biomass, they are often overlooked in nutrient cycling studies. In forests with large accumulations of canopy soil, such as those found in tropical montane regions, this could contribute to an incomplete understanding of nutrient cycling in the overall forest. Canopy soils are collections of organic material commonly found on the branches of trees in humid forests; they are primarily made up of decomposed epiphytic material but also include intercepted litter, dust, invertebrates, fungi and microorganisms. There are only a handful of studies that have looked at nitrogen (N) cycling and/or greenhouse gas (GHG) flux in canopy soils and none have tried to assess the actual field rates or investigated how these soils - which are particularly sensitive to atmospheric processes - could change with nutrient deposition. This dissertation presents the results of a research study that quantified rates of canopy soil N cycling and GHG flux and assessed how these rates were affected by increased levels of N and phosphorus (P) in the soil. In areas of atmospheric N and P deposition, canopy soils receive both direct inputs and indirect enrichment via enriched throughfall and plant litter. We measured rates of (1) free-living N2 fixation, (2) net and gross mineral N cycling, and (3) carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) exchange, in canopy soils of tropical montane forests along an elevation gradient (1000 m, 2000 m and 3000 m) and assessed the indirect effects of N and P addition to the forest floor. Rates of N2 fixation, N cycling and GHG flux measured in canopy soil were compared with those measured on the forest floor (either as a part of this work or in parallel studies by two other members of our working group), to put canopy soil activity in the context of the total forest. N2 fixation was determined using the acetylene reduction assay, net N cycling rates were determined using the buried bag method and gross N cycling rates were determined using 15N pool dilution techniques. Gas fluxes were measured using static chambers with permanent bases in the soil, as well as intact soil cores sealed in jars. N2 fixation and N cycling measurements took place in the field, in the wet and dry seasons, using intact cores of soil. GHG measurements were done five times during a one-year period. The forest floor of our study sites had been fertilized biannually with moderate amounts of N (50 kg N ha-1 yr-1) and P (10 kg P ha-1 yr-1) for 4 years; treatments included control, N, P and N+P. The canopy contributed 7-13 % of total (canopy + forest floor) soil N2 fixation, which ranged from 0.8 to 1.5 kg N ha-1 yr-1. N2 fixation rates exhibited little variation with elevation but were much higher in the dry season than the wet season. N2 fixation was inhibited in forest floor N plots compared to control and P plots, and stimulated in canopy P plots compared to control. Canopy soils contributed up to 23% of total (canopy + forest floor) mineral N production; gross N mineralization in canopy soils ranged from 22.7 to 45.8 mg N kg-1 d-1 and gross nitrification ranged from 1.2 to 2.0 mg N kg-1 d-1. In control plots, gross rates of ammonium (NH4+) transformations decreased with increasing elevation, whereas gross rates of nitrate (NO3-) transformations did not exhibit a clear elevation trend but were significantly affected by season. Nutrient-addition effects were different at each elevation, but combined N+P generally increased N cycling rates at all elevations. Rates of canopy CO2 emissions based on chamber area (10.5 to 109.5 mg CO2-C m-2 h-1) were similar to those measured on the forest floor and decreased with increasing elevation. However, canopy emissions based on forest area (0.15 to 0.51 Mg CO2-C ha-1 yr-1) made up only 5-11% of total (canopy + forest floor) soil CO2 emissions. Canopy soil CH4 fluxes (-0.07 to 0.02 kg CH4-C ha-1 yr-1) and N2O fluxes (0.00 to 0.01 kg N2O-N ha-1 yr-1) made up less than 5% of the total soil fluxes. P addition decreased net CH4 emissions at all elevations, so that canopy soils acted as a slight sink for CH4 (-10.8 to -2.94 μg CH4-C m-2 h-1). At 2000 m only, canopy soils with N addition became a slight N2O source (2.43 ± 3.72 μg N2O-N m-2 h-1), whereas P addition decreased CO2 emissions by approximately 50%. Results show that canopy soils have active microbial communities, which provide valuable nutrient cycling services to the canopy ecosystem. Additionally, in contrast to theories that canopy soil is decoupled from nutrient cycling in forest floor soil, nutrient cycling in the canopy soils of our forests was clearly linked to forest floor nutrient availability. We observed that canopy soils at higher elevations were more likely to make up a significant percentage of total forest nutrient cycling; this should be considered in nutrient cycling studies carried out in such areas. Long-term atmospheric N and P deposition has the potential to significantly change the dynamics of nutrient cycling in these canopies. N deposition may lead to inhibition of N2 fixation, with hotspots still occurring in areas with higher amounts of P. Internal N cycling in canopy soils will likely be stimulated by N and P deposition, but chronic nutrient addition may also lead to increased mineral N losses from the canopy soil. GHG-related processes in canopy soils will likely also respond to N and P deposition, but with the exception of CO2 emissions, fluxes in canopy soils are unlikely to significantly contribute to total forest GHG budgets.