The effects of excessive moisture on soil C and N mineralization and site productivity

1. The effects of excessive moisture on soil C and N mineralization and site productivity Toktam Sajedi Faculty of Forestry

2. An ecological enigma HA CH Poor conifer regeneration and low nutrient supply in both old-growth and clearcut cedar-hemlock (CH) forests but not in adjacent hemlock- amabilis fir (HA) sites.

3. Role of moisture • Evidence indicated that excessive moisture may contribute to low nutrient supply in CH sites: • Frequent hydromors • Lower fauna abundance • Presence of copepods • > 80% water in organic matter • Frequent gleyed horizons • Slightly lower slope position

4. Three approaches were taken to test the “excessive moisture” theory: Field evidence of excessive moisture (field research) Effects of excessive moisture on C and N mineralization (laboratory research) Evaluating drainage effects on forest productivity and nutrient cycling (drainage trial)

5. Field research - Objectives and hypotheses Do CH and HA differ in soil and vegetation properties consistent with excessive moisture? Hypothesis: Compared with HA sites, CH sites will have: • Higher moisture content • Lower redox potential (especially in lower soil horizons) • Shallower aerated depth • Greater frequency of anoxic conditions in the forest floor and mineral soil

6. Sampling Port McNeill CH HA F Clearcut Forest H A B

7. Field research - Measurements Moisture (volumetric) Redox potential (Eh-combination electrode) pH Aerated depth (iron rods) Mottling Bulk density (excavation method) Rooting depth Layer thickness Mass loss (cellulose filter paper) Field capacity (pressure plate method) Soil particle size Total C and N (total combustion method) 13. Composition of plant species 14. Plant species coverage (%)

8. Field research - Soil moisture and aeration CH forests were wetter and less aerated than HA forests Anaerobic condition More than 40% of the sampling plots in CH forests had an average redox value less than +300 mV

9. Field research - Aerated depth Soil surface 37 cm 56 cm 72% of the rods put in the CH sites indicate presence of anoxic conditions, compared to 46% in HA sites. CH HA Oxic Anoxic FF FF CH HA

10. Field research-Results Principal component analysis (PCA) of understory plant species

11. Field research - Soil variability in CH/HA sites PCA-soil variables Sand Forest type CH HA OXD EhB OMTH EhHU pHB Axis 2 pHHU MCHU Silt Clay Axis 1 Group 3: well-drained soils moist soils, highest Eh, deep oxic layer with sandy texture CH-Group 3 Group 1: anaerobic soils with high moisture, lowest Eh and shallow oxic layer caused by pan layers HA CH-Group 1 Group 2: poorly drained soils with high amounts of silt and clay and signs of mottling CH-Group 2

12. Field research - Results Canonical correspondence analysis of soil variables in relation to plant species

13. Field research-Harvesting effects HA clearcuts were less aerated and likely to be anaerobic compared with HA forests. b b a ab a a a a aB a a a Also, compared with HA forests HA clearcuts had shallower aerated depth, rooting depth and slower decomposition rate

14. Incubation study- Objectives and hypotheses How does excessive moisture affect mineralization of carbon and nitrogen? Hypothesis 1: Microbial biomass and CO2 evolution will decrease under saturated conditions in humus and soil from both site types. Hypothesis 2: NO3 concentrations and the proportion of N in the form of SIN (i.e SIN:SON) will decrease under saturated conditions in humus and soil from both site types.

15. Incubation study-Methods Site 1 Site 2 Site 5 CH …… HA H humus B mineral 200% (field capacity) 60% (field capacity) 350% 90% 500% 110% 600% (saturation) 130% (saturation) • Measurements: • C mineralization (CO2 efflux) • Microbial biomass C and N • N mineralization -NH4 & NO3 • Soluble organic N

16. Incubation study-Methods Incubated samples at 20 οC and dark condition CO2 measurement with LI-COR auto-analyzer

17. Incubation study-Results Redox potential declined sharply above 500% moisture threshold

18. Incubation study-Results Mineralization of carbon declined after reaching maxima at intermediate moisture levels

19. Incubation study-Results Microbial biomass C declined with increasing moisture H horizon B horizon

20. Incubation study-Differences in N forms in humus

21. Incubation study-Summary Excessive moisture resulted in anaerobic conditions, less microbial biomass and activity and lower SIN:SON in cedar-hemlock forest soils

22. Drainage study- Study site

23. Drainage study- Study site Drainage significantly improved growth of regenerating trees. b a Average tree height (m)

24. Drainage study- Results a Lower microbial biomass and mineralization of C in drained areas a a b

25. Drainage study-Results Soils were better aerated but still anaerobic in drained areas a b a a Control Drained

26. Overall Conclusions • Excessive moisture and resulting anaerobic conditions in soil play a fundamental role in the low nutrient supply in cedar-hemlock forests in coastal B.C. • The effect of moisture on nutrient supply appears to be indirect, and arises through interactions with vegetation and litter quality

27. High precipitation HA CH Upper slope Lower slope Silt and clay Sand Well drained Poorly drained Pan Well aerated Poorly aerated Soil chemistry Low O2 Reduced form of elements & SON, NO3 Soil chemistry More O2 Oxidized form of elements & SON, NO3 Vegetation (Vaccinium, fireweed) More easily decomposable material Vegetation (Cedar, salal, deer fern) More phenolic compounds Decomposition Humification Binding of N Decomposition Humification Binding of N Aerobic microbes More microbial biomass Greater enzyme activity Facultative anaerobes Less microbial biomass Lower enzyme activity High nutrient supply Low nutrient supply High productivity Low productivity High litter N High N mineralization Low litter N Low N mineralization

28. Management implications • Clearcut harvesting may shift HA ecosystems towards more anaerobic conditions similar to CH forests. • Drainage could be a useful silvicultural practice for improving the productivity of cedar-swamp ecosystems in coastal B.C. • It may be possible to improve tree growth without stimulating loss of soil C using drainage. This requires maintaining redox levels within a range below +300 mV in the bulk soil.