BayCEER Workshop 2015

Depletion and recovery of dissolved organic carbon and nitrogen pools in forest floors

Introduction

The release of DOC and DON from soil organic matter is a complicated process driven by biological, physicochemical and hydrological processes. Heavy precipitation events might deplete the pools of mobilizable DOC and DON and the question of the recovery of these pools arises.

Material and Methods

Here we tested the following hypotheses: 1) Depletion and recovery of the DOC and DON pools behaves similar. 2) The recovery rate of the DOM pools depends on tree species, microbial activity and temperature. The depletion and recovery of DOC and DON pools was studied in two experimental approaches: i) batch extraction and ii) percolation extraction. Forest floor samples were taken from spruce, larch, beech, and mixed deciduous (oak, birch, maple) sites, and incubated at 5 or 15^oC. In the batch extraction experiment, soil samples were extracted for 2 hours with artificial solution (50 or 200 ml). After the initial extraction, soil was incubated for 1, 3, 7, and 14 days and then subsequently extracted again. In the percolation experiment, forest floor samples in syringe columns were percolated continuously with low (3 ml d^-1) and high (15 ml d^-1) percolation rates for 25 days.

Results

DON and DOC behaved similar as indicated by stable DOC/DON ratio over time. Following the depletion, the DOM pools recovered within 14 days, but some was not recovered in larix and mixed deciduous samples. DOC recovery rate was significantly correlated to CO₂ evolution (r²=0.76), DON recovery rate (r²=0.48), DOC/DON ratio (r²=0.56). The recovery rate was in the order; deciduous>coniferous (mixed deciduous>beech>spruce>larix), Oi>Oe≥Oa, high>low temperature, large>small extraction volume/percolation rate. Average Q₁₀ for DOM recovery (2.1) was less than for soil respiration (3.3).

Conclusions

The results suggest that the recovery of DOM pools in forest floor following depletion by heavy precipitation events depends on substrate quality and soil respiration.