Compositional dynamics of dissolved lignin in watersheds: small temperate streams to large tropical rivers
Abstract
The most abundant biochemicals on land are cellulose, hemicellulose, and lignin. Lignin, alone, composes roughly 30% of the organic carbon (OC) in the terrestrial biosphere (Boerjan et al., 2003) and a significant portion of the OC mobilized into stream and river networks worldwide. Here we present a synthesis of several studies examining (i) the mobilization dynamics/compositional changes in dissolved lignin during rapid storm events in small temperate streams (Hood Canal, WA, USA), and (ii) the respiration dynamics/biological overturning of dissolved (and particulate) lignin in the Amazon River mainstem. Rapid sampling (3 hour intervals) during short-term rainfall events has revealed that the concentration of dissolved lignin phenols (as well as DOC) in small temperate streams is strongly correlated with river discharge (Ward et al., 2012). Additionally, rapid discharge increases resulted in an increase in Ad/Al and C/V ratios and decrease in the S/V ratio of dissolved lignin phenols, indicating a mobilization of relatively degraded non-woody/gymnosperm-derived material in the dissolved phase during storms occurring after a long dry period. We hypothesize that sorption to soil surfaces imparts an additional control on lignin mobilization: degraded phenols are relatively more soluble than their non-degraded counterparts and are easily mobilized by rapid flow, whereas non-degraded phenols are slowly mobilized by base flow and continuously degraded in soils. Once lignin is mobilized into the aquatic setting it is often assumed to be refractory. However, evidence in the Amazon River mainstem suggests the contrary. We have assessed the biodegradability of dissolved (and particulate) lignin, as well as a vast suite (~120) of similar phenolic compounds with a series of incubation experiments performed on four Amazon River cruises. We estimate that on average the degradation of lignin and similar phenolic compounds supports 30-50% of bulk respiration rates in the river, implying that lignin may be a highly important organic matter fuel for CO2 outgassing from the world's inland waters. Contrary to bulk respiration rates, which poorly correlate with bulk OC concentrations, the respiration of lignin appears to be almost entirely controlled by substrate composition. Lignin (and OC) concentrations and lignin degradation rates both correlate well with the seasonal hydrograph in the Amazon (Ward et al., submitted). Based on the measurements made here, and by others (Hedges et al., 1988; Field et al., 1998; Houghton et al., 2001; Malhi et al., 2008; Bose et al., 2009) we estimate that roughly 80 Tg C of lignin is sequestered in Amazonian soils annually, roughly 40% of this lignin is respired in soils, 55% is respired within the river continuum, and less than 5% is delivered to the ocean.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B13I..07W
- Keywords:
-
- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 0424 BIOGEOSCIENCES / Biosignatures and proxies;
- 0428 BIOGEOSCIENCES / Carbon cycling;
- 0458 BIOGEOSCIENCES / Limnology