Dissolved organic carbon and total dissolved nitrogen production by boreal soils and litter: the role of flooding, oxygen concentration, and temperature

Abstract

About 0.3 million km2 of boreal forests and peatlands and their soils have been flooded through the creation of hydroelectric reservoirs. To establish the effect of flooding on dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) production, we incubated six sample types of boreal forest and peatland soils and litters for 15 weeks under flooded (anoxic and suboxic) and non-flooded (oxic) conditions at three temperatures (4, 12, and 22 °C). Water was sampled at 3-week intervals and analyzed for DOC and TDN concentration. Cumulative DOC and TDN production ranged widely: from 4.4 to 146.2 mg C g−1 C for DOC and 4.4 to 112.2 mg N g−1 N for TDN under flooded conditions, and from 1.3 to 16.6 mg C g−1 C for DOC and 0.7 to 8.5 mg N g−1 N for TDN under non-flooded conditions. Cumulative DOC and TDN production was greater under flooded than non-flooded conditions for all sample types with flooded:non-flooded ratios averaging 10.6 for DOC and 9.5 for TDN. Within the flooded treatments, anoxic conditions produced more cumulative DOC and TDN than suboxic conditions, with average anoxic:suboxic ratios of 1.3 and 5.8 for DOC and TDN, respectively. Raising temperatures increased cumulative DOC and TDN production marginally under both flooded and non-flooded conditions as the average ratios for 22:12 and 12:4 °C were 1.1 for DOC and 1.2 for TDN. Generally, more DOC and TDN were released from litters than soils under both the flooded and non-flooded incubations, with litter:soil ratios averaging 8.5 for DOC and 2.1 for TDN. DOC:TDN ratio was generally higher in the solutions than in the original sample, particularly in the litters. This study thus indicated that DOC and TDN production was largely affected by flooding regime, O2 concentration, and substrate characteristics and less affected by temperature.