Land-use change, not climate, controls organic carbon burial in lakes

Abstract

Lakes are a central component of the carbon cycle, both mineralizing terrestrially derived organic matter and storing substantial amounts of organic carbon (OC) in their sediments. However, the rates and controls on OC burial by lakes remain uncertain, as do the possible effects of future global change processes. To address these issues, we derived OC burial rates in (210)Pb-dated sediment cores from 116 small Minnesota lakes that cover major climate and land-use gradients. Rates for individual lakes presently range from 7 to 127 g C m(-2) yr(-1) and have increased by up to a factor of 8 since Euro-American settlement (mean increase: 2.8×). Mean pre-disturbance OC burial rates were similar (14-22 g C m(-2) yr(-1)) across all land-cover categories (prairie, mixed deciduous and boreal forest), indicating minimal effect of the regional temperature gradient (approx. 4 °C) on background carbon burial. The relationship between modern OC burial rates and temperature was also not significant after removal of the effect of total phosphorus. Contemporary burial rates were strongly correlated with lake-water nutrients and the extent of agricultural land cover in the catchment. Increased OC burial, documented even in relatively undisturbed boreal lake ecosystems, indicates a possible role for atmospheric nitrogen deposition. Our results suggest that globally, future land-cover change, intensification of agriculture and associated nutrient loading together with atmospheric N-deposition will enhance OC sequestration by lakes.

Keywords: deforestation; disturbance; eutrophication; land-cover; nitrogen; phosphorus.

Figure 1.

Location of the study lakes in Minnesota and their associated ecoregion. WCBP and NGP were formerly tallgrass prairie and have fertile soils; today they are primarily cultivated (approx. 80% of land use) but contain some pasture and open land. NCHF is a transition zone consisting of mixed woodlands (eastern broadleaf trees) and savannah vegetation. The NLF is extensively forested and largely removed from direct human influences, and has less fertile soils. NLF, northern lakes and forests; NCHF, north central hardwood forests, NCHF; WCBP, western corn belt plains; NGP, northern glaciated plains; LAP, lake Agassiz plain; DA, driftless area and NMW, northern Minnesota wetlands [11]. (Online version in colour.)

Figure 2.

(a) Organic carbon accumulation rates (g C m^–2 yr^–1) by ecoregion and time period. (b) Carbon accumulation ratios (selected time period relative to ca 1850) by ecoregion (see Material and methods). Boxes represent the inter-quartile range, midlines indicate median values, solid squares are the means, and whiskers denote 90% CIs.

Figure 3.

Relationships between contemporary C accumulation rates (g C m^–2 yr^–1) and selected limnological variables (total phosphorus, total nitrogen and chlorophyll a; n = 97, 74 (after three extreme outliers being removed) and 95, respectively; see Material and methods for data sources) and percentage agricultural land-use in catchment (n = 52; see [12]).

Figure 4.

Contemporary and pre-disturbance (i.e. pre-Euro-American settlement, pre-1850) C accumulation rates (g C m^–2 yr^–1) plotted against latitude (used here as a surrogate for climate). The 1850-C AR : latitude relationship (grey; r² = 0.14; p < 0.0001) is significant and suggests a possible weak climate control on C burial rates or the effect of more nutrient rich prairie soils on background lake productivity. Removal of the nutrient effect (TP; see figure 3) for the contemporary C AR : latitude relationship (black; r² = 0.40, p < 0.0001) results in a non-significant effect of latitude (i.e. temperature).