Tropical forests as drivers of lake carbon burial

Abstract

A significant proportion of carbon (C) captured by terrestrial primary production is buried in lacustrine ecosystems, which have been substantially affected by anthropogenic activities globally. However, there is a scarcity of sedimentary organic carbon (OC) accumulation information for lakes surrounded by highly productive rainforests at warm tropical latitudes, or in response to land cover and climate change. Here, we combine new data from intensive campaigns spanning 13 lakes across remote Amazonian regions with a broad literature compilation, to produce the first spatially-weighted global analysis of recent OC burial in lakes (over ~50-100-years) that integrates both biome type and forest cover. We find that humid tropical forest lake sediments are a disproportionately important global OC sink of ~80 Tg C yr^-1 with implications for climate change. Further, we demonstrate that temperature and forest conservation are key factors in maintaining massive organic carbon pools in tropical lacustrine sediments.

Fig. 1. Global distribution of OC burial rates (average ± SE) in lakes across biomes.

The red color indicates human-altered areas, while black stars the locations of the study lakes. No data were available for subtropical, other tropical latitudes, and deserts.

Fig. 2. Annual air temperature and recent OC burial in lake sediments among classes of natural and human-altered biomes around the world.

Classes of lake biomes were ordered from right to left along the X-axis to represent the increasing trend in the annual air temperature. The bars and error bars represent the average and standard error, respectively, and equal letters represent no statistical difference (one-way ANOVA, post hoc Tukey test, p < 0.05).

Fig. 3. The role of forests on lake OC accumulation rates with increasing temperature.

Panel A shows the relationships between forest OC accumulation (green triangles), lake OC burial in natural (black circles), and human-altered (red crosses) biomes with annual air temperatures along the global gradient. The number 1 represents the subpolar and polar biomes; 2 the boreal forest; 3 temperate forest; 4 the other medium latitudes; and 5 the humid tropical forest. In turn, numbers 6, 7, and 8 represent the human-altered biomes in cold, moderate-temperature, and warm anthropic environments, respectively >15.0, 7.5–15.0, and <7.5 °C. The solid green line indicates the fitted second-order polynomial model of forest OC accumulation (g C m⁻² yr⁻¹) = 82.93 ± 15.10 + 4.01 ± 1.02 × Temperature (°C) + 0.08 ± 0.02 × Temperature (°C)²; p < 0.01, while the solid black line represents the linear model of lake OC burial in natural biomes (g C m⁻² yr⁻¹) = 4.23 ± 0.08 × Temperature (°C) + 15.15 ± 1.01; p < 0.0001. All error bars in Panel A represent the SE. Panel B shows the relationship between OC burial in Amazon floodplain lakes and the relative area of non-flooded forests in their surroundings (see more details in Methods). The solid black line in this panel represents the exponential regression of Amazon lake OC burial (g C m^-² yr^-¹) = 56.21 ± 4.943 × e^{0.01608 ±0.008 × Non-flooded forest area (%)}; p < 0.01 (note the logarithmic scale on the y-axis in panel B) for 6 km² buffer (See Methods for more details). Panel C shows the scatter plots of the δ¹³C values from the organic material fraction vs C:N molar ratios of the Amazon lake sediments based on data from our own survey (black circles, N = 54) and a compilation from the literature (red circles N = 18). The boundaries of this panel C are based on endmember values reported in the literature (see Methods and Supplementary Material for further details) and the relative contribution to the Amazon lake sediments are as follows: C₃ soil (light brown rectangle, 41%), C₄ (yellow rectangle, 0%), C₃ (dark brown rectangle, 20%), and phytoplankton (lime green rectangle, 7%).