Mobilization of aged and biolabile soil carbon by tropical deforestation

Abstract

Introductory Paragraph In the mostly pristine Congo Basin, agricultural land-use change has intensified in recent years. One potential and understudied consequence of this deforestation and conversion to agriculture is the mobilization and loss of organic matter from soils to rivers as dissolved organic matter. Here, we quantify and characterize dissolved organic matter sampled from 19 catchments of varying deforestation extent near Lake Kivu over a two-week period during the wet season. Dissolved organic carbon from deforested, agriculturally-dominated catchments was older (14C age: ~1.5kyr) and more biolabile than from pristine forest catchments. Ultrahigh-resolution mass spectrometry revealed that this aged organic matter from deforested catchments was energy-rich and chemodiverse, with higher proportions of nitrogen- and sulfur-containing formulae. Given the molecular composition and biolability, we suggest that organic matter from deforested landscapes is preferentially respired upon disturbance, resulting in elevated in-stream concentrations of carbon dioxide. We estimate that while deforestation reduces the overall flux of dissolved organic carbon by ~56%, it does not significantly change the yield of biolabile dissolved organic carbon. Ultimately, the exposure of deeper soil horizons through deforestation and agricultural expansion releases old, previously stable, and biolabile soil organic carbon into the modern carbon cycle via the aquatic pathway.

Identifying the Molecular Signatures of Agricultural Expansion in Amazonian Headwater Streams

“…Additionally, small water impoundments in the cropland catchments (Riskin et al, ) may also be sources of autochthonous DOM; however, the low DOC concentrations in the cropland streams (Table ) argue against this as a major contributor. Moreover, the shift in molecular signatures observed in this study is similar to that seen in recent work examining DOM compositional changes between forested and agricultural sites in the Democratic Republic of Congo where no impoundments of water are present (Drake et al, ). Therefore, the change in litter layer and surface soils caused by cropping appears likely to be the primary driver of the observed differences in molecular signatures of cropland and forest headwater stream DOM.…”

“…Microbially derived material, likely from extensively worked soil OM, appears as the source of cropland DOM as this has low O/C and high H/C (Figure c), as well as is enriched in nitrogen (Figures c and c and Table ; D'Andrilli et al, ; Gonsior et al, ; Kellerman et al, ). The molecular signature of Amazonian cropland stream DOM is comparable to that observed for agricultural impacts in other recent subtropical and tropical studies (Drake et al, ; Roebuck et al, ), highlighting that as soils in these watersheds all formed in areas with high annual temperatures and rainfall and are deeply weathered, their response to agricultural impacts is similar. The analogous compositional assemblages in van Krevelen space between DOC and percent forest cover in the catchment also strongly suggest that the sources of DOM composition and the higher DOC concentrations (Figure a and Table ) observed in forest streams are the same.…”

An international laboratory comparison of dissolved organic matter composition by high resolution mass spectrometry: Are we getting the same answer?

“…With the exception of the number of assigned peaks, all metric values and proportions of compound classes had higher variability arising from intersample compositional differences than from instrumental differences (Table 2). However, our study covers a more diverse sample set compared to studies that focus on DOM temporal or spatial trends (Kellerman et al 2014;Hertkorn et al 2016;Hawkes et al 2018b;Drake et al 2019;Roth et al 2019). For sample sets with higher DOM compositional similarity, instrument bias may become more important and trends in features such as compound class proportion may not be reproducibly determined among research groups.…”

“…High-resolution mass spectrometry (HRMS) has become a central tool in the analysis of dissolved organic matter (DOM), due to seminal work reported over the last three decades (Fievre et al 1997;Kujawinski et al 2002;Stenson et al 2002;Stenson et al 2003;Dittmar and Koch 2006;Sleighter and Hatcher 2007;Reemtsma et al 2008;Gonsior et al 2009) and the detailed biogeochemical insight afforded by molecular compositional patterns in large and diverse sample sets (Flerus et al 2012;Jaffé et al 2012;Kellerman et al 2014;Lechtenfeld et al 2014;Hertkorn et al 2016;Drake et al 2019). Since Fievre et al collected the first HRMS spectrum of DOM in 1997 (Fievre et al 1997), there have been large increases in both the number of researchers using HRMS for DOM analysis and the variety of instrument types being employed.…”

“…The wide range of potential conditions for analysis, resulting interpretation of the spectra, and the conclusions based on those interpretations poses potential problems with common interpretation across the community using different instruments, laboratories, and practices. As both climate and ecosystems change (Williamson et al 2015;Drake et al 2019), it is critical for the community to understand the degree to which archived data can be compared to newer measurements, and if data generated among various research groups can be robustly compared and related to emerging global trends.…”