The effects of excessive moisture on soil carbon and nitrogen mineralization and forest productivity

Conifers of regenerating cedar-hemlock (CH) forests exhibit slow growth and nutrient deficiencies (N and P), which are not observed on adjacent cutovers of hemlock-amabilis fir (HA) forests. I test the theory that excessive moisture and resulting low oxygen availability in CH sites create the low N supply and poor growth in these ecosystems. A field experiment determined: 1) whether CH and HA forests differ in soil moisture and aeration, 2) whether decomposition rate and soil C stores differ in CH and HA forests, 3) whether composition of plant communities are related to soil moisture and aeration, and 4) the impact of harvesting CH and HA forests on moisture and aeration conditions. A laboratory experiment investigated the effects of moisture levels, from field capacity to saturation level, on C and N mineralization rates. Lastly, a field trial was carried out to assess drainage as a potential forest management solution in wetland forests by comparing C dynamics in drained and un-drained sites. As hypothesized, CH forests were wetter, less aerated, had shallower aerated depth and higher frequency of anaerobic conditions compared with HA forests. Composition of plant species was related to soil moisture and aeration, however plant diversity was not. Soil aeration was the most important factor, explaining 25% of the variability of species within plant communities. Compared with HA forests with well-aerated soils, soils in HA clearcuts were anaerobic, had slower decomposition rate and shallower rooting depth. Microbial biomass, C mineralization and the soluble inorganic N: soluble organic N (SIN:SON) ratio all declined under water-saturated conditions. Concentrations of SIN increased with increasing moisture in HA soils; whereas in CH humus and soil, the SIN pool was small and decreased with increasing moisture. The results indicate that the low N availability on CH sites results from synergistic effects of litter quality and greater frequency of waterlogging. Drainage could be a useful silvicultural practice for improving the productivity of cedar-swamp ecosystems without stimulating loss of soil C, provided that redox levels are maintained at less than +300 mV, at which level oxygen is sufficient for plant growth but not for aerobic microbial decomposition.Forestry, Faculty ofGraduat