ArticlePublisher preview available

A long-term decrease in the persistence of soil carbon caused by ancient Maya land use

Nature Geoscience

September 2018
11(9)

DOI:10.1038/s41561-018-0192-7

Authors:

Peter Douglas

McGill University

Timothy I. Eglinton

ETH Zurich

Mark Brenner

University of Florida

Show all 7 authorsHide

The long-term effects of deforestation on tropical forest soil carbon reservoirs are important for estimating the consequences of land use on the global carbon cycle, but are poorly understood. The Maya Lowlands of Mexico and Guatemala provide a unique opportunity to assess this question, given the widespread deforestation by the ancient Maya that began ~4,000 years ago. Here, we compare radiocarbon ages of plant waxes and macrofossils in sediment cores from three lakes in the Maya Lowlands to record past changes in the mean soil transit time of plant waxes (MTTwax). MTTwax indicates the average age of plant waxes that are transported from soils to lake sediments, and comparison of radiocarbon data from soils and lake sediments within the same catchment indicates that MTTwax reflects the age of carbon in deep soils. All three sediment cores showed a decrease in MTTwax, ranging from 2,300 to 800 years, over the past 3,500 years. This decrease in MTTwax, indicating shorter storage times for carbon in lake catchment soils, is associated with evidence for ancient Maya deforestation. MTTwax never recovered to pre-deforestation values, despite subsequent reforestation, implying that current tropical deforestation will have long-lasting effects on soil carbon sinks.

Relief map of the Maya Lowlands indicating the location of the studied lakes Additional lakes that were the sources of pollen records are also shown. The inset map shows the location of the Maya Lowlands (square) relative to Saanich Inlet, Canada (triangle), Lake Soppensee, Switzerland (circle) and the Congo River Fan (diamond) (see also Fig. 4).

…

Plant wax and bulk SOC radiocarbon results from soils and surficial sediments in the Lake Chichancanab catchment Both plant waxes and bulk SOC generally exhibit greater age with soil depth, a trend that is more consistent for plant waxes. Plant wax Δ¹⁴C values from lake surface sediments are included in the same plot to indicate the depth in the soil profile at which soil plant waxes would have equivalent Δ¹⁴C values, assuming a linear decrease in plant wax Δ¹⁴C with subsoil depth at a given location. Error bars indicate the analytical error of ¹⁴C measurements (see Methods). PW, plant wax.

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Comparison of changes in MTTwaxwith records of environmental change in the Maya Lowlands. a, Late Holocene records of MTTwax and comparison with environmental change in the Maya Lowlands. b, Mid-Holocene MTTwax from Salpeten. MTTwax error bars indicate 95% confidence intervals. Approximate dates of Maya archaeological periods are indicated below a (PostC, postclassic). c, Records of MARwax (plant wax mass accumulation rate). d, Evidence for soil erosion at Salpeten and Itzan. Soil erosion rates from Salpeten (right y axis) are derived from sediment accumulation measurements¹²; magnetic susceptibility at Itzan (left y axis) also records soil erosion. Magnetic susceptibility values (dimensionless) are based on SI units. e, Pollen records from three lake cores reflect regional trends in deforestation13,15,17; disturbance pollen indicates the sum of non-forest pollen taxa. f, A composite palaeohydrological record for the Maya Lowlands calculated from the z-scores of regional palaeolimnological and speleothem palaeoclimate datasets in ref. 36; the grey envelope indicates the 95% confidence interval.

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Comparison of MTTwax data from the Maya Lowlands with three other globally distributed Holocene-age records These data suggest widespread changes in the transit time of plant waxes, particularly during the past 6,000 years, although they probably differ with respect to the causes of these changes. The error bars indicate the 95% confidence intervals for MTTwax values. Data from refs 27–29.

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Articles

https://doi.org/10.1038/s41561-018-0192-7

1Department of Geology and Geophysics, Yale University, New Haven, CT, USA. 2Department of Earth and Planetary Sciences, McGill University, Montreal,

Quebec, Canada. 3Geological Institute, ETH Zürich, Zurich, Switzerland. 4Department of Geological Sciences and Land Use and Environmental Change

Institute, University of Florida, Gainesville, FL, USA. 5Natural Sciences Department, University of Wisconsin–Superior, Superior, WI, USA. 6Independent

Scholar, Columbus, OH, USA. 7Deceased: Mark Pagani. *e-mail: peter.douglas@mcgill.ca

Soil carbon is the largest terrestrial carbon reservoir (approxi-

mately 1,500 PgC)1,2, and there is concern that it is being desta-

bilized by climate and land-use change2–4. Tropical forests

contain about 30% of global soil carbon (~470 Pg), of which about

half is contained in subsoils below organic-rich topsoils (typically

> 20–30 cm depth)1,2,5. Radiocarbon data indicate that tropical for-

est subsoils contain a sizeable proportion of slow-cycling carbon

that persists for thousands of years6–8. Tropical forest soil carbon is

at an especially high risk of destabilization because of widespread

deforestation over the past 50 years3,9, but the long-term impact of

land-cover change on the persistence of carbon in subsoils remains

poorly constrained10.

Ancient Maya land use provides an opportunity to evaluate the

long-term effects of deforestation on carbon cycling in tropical

forests. The low-elevation tropical forests of southeastern Mexico

and northern Central America—the Maya Lowlands (Fig. 1)—

sustained large human populations between approximately 2,500

and 1,000 yr 11, and ancient Maya urbanization and agriculture

led to widespread deforestation and soil erosion12–15. Furthermore,

because the Lowland Maya population declined substantially dur-

ing the Terminal Classic period (roughly 1,250 to 1,100 yr ), and

following the Spanish conquest (~450 to 350 yr )11, the Maya

Lowlands also experienced lengthy periods of reduced land-use

intensity. Previous studies applied palynological and sedimentologi-

cal methods to infer the response of forests in the Maya Lowlands to

land-use change12,13,15–17, but did not evaluate soil carbon dynamics.

In this study we measured radiocarbon (14C) in long-chain

(C26, C28, C30 and C32) n-alkanoic acids, which are derived from the

cuticular waxes of terrestrial vascular plants18 (referred to as plant

waxes), in sediment cores from three lakes in the Maya Lowlands:

Chichancanab, Salpeten and Itzan (Fig. 1; see Methods). Stable iso-

tope data confirm that the plant waxes in sediments from these

lakes are derived from terrestrial plants19,20 (see Methods), and wax

production does not vary substantially among the angiosperm

trees and grasses present in these catchments21. We defined the

mean soil transit time of plant waxes (MTTwax) as the mean age

of plant waxes that are transported from soils to lake sediments

at a given point in time22, which we calculated as the difference

between the age of plant waxes in sediments and the age of the sedi-

ment horizon in which they were buried. Sediment horizon ages

were estimated by 14C analysis of plant macrofossils (see Methods).

Previous studies of plant wax 14C values indicated that these mol-

ecules are representative of slow-cycling soil carbon pools in ter-

restrial ecosystems23–26, and that MTTwax values in sediment cores

serve as an indicator of changes in the persistence of soil carbon

across a catchment through time27–29.

Plant wax 14C in sediments reflects subsoil carbon age

We compared 14C measurements of plant waxes and bulk soil

organic carbon (SOC) in soils from the Lake Chichancanab catch-

ment to inform our interpretation of changes in MTTwax in the sedi-

ment cores: Δ

14C values for plant waxes and bulk SOC are within

error for three of seven samples, while for the remainder they differ

by as much as 51‰ (Fig. 2). Negative Δ

14C values in subsoil samples

(≥ 20 cm depth) indicate that subsoil carbon is, on average, hun-

dreds of years old. In subsoil samples, plant wax Δ

14C is either

within error or higher than that of bulk SOC, with 14C ages of plant

waxes as much as 380 years younger than bulk SOC ages. Plant

waxes are generally considered a recalcitrant fraction of soil car-

bon26, and soils in which bulk SOC is older than plant waxes proba-

bly contain other forms of recalcitrant carbon, such as black carbon

or petrogenic carbon30. Plant waxes exhibit progressively lower

14C in deeper soils (Fig. 2), whereas bulk SOC Δ

14C exhibits a less

consistent pattern with soil depth. This probably reflects local-scale

A long-term decrease in the persistence of soil

carbon caused by ancient Maya land use

Peter M. J. Douglas 1,2*, Mark Pagani1,7, Timothy I. Eglinton 3, Mark Brenner4, Jason H. Curtis4,

Andy Breckenridge5 and Kevin Johnston6

The long-term effects of deforestation on tropical forest soil carbon reservoirs are important for estimating the consequences

of land use on the global carbon cycle, but are poorly understood. The Maya Lowlands of Mexico and Guatemala provide a unique

opportunity to assess this question, given the widespread deforestation by the ancient Maya that began ~4,000 years ago.

Here, we compare radiocarbon ages of plant waxes and macrofossils in sediment cores from three lakes in the Maya Lowlands

to record past changes in the mean soil transit time of plant waxes (MTTwax). MTTwax indicates the average age of plant waxes

that are transported from soils to lake sediments, and comparison of radiocarbon data from soils and lake sediments within

the same catchment indicates that MTTwax reflects the age of carbon in deep soils. All three sediment cores showed a decrease

in MTTwax, ranging from 2,300 to 800 years, over the past 3,500 years. This decrease in MTTwax, indicating shorter storage

times for carbon in lake catchment soils, is associated with evidence for ancient Maya deforestation. MTTwax never recovered to

pre-deforestation values, despite subsequent reforestation, implying that current tropical deforestation will have long-lasting

effects on soil carbon sinks.

NATURE GEOSCIENCE | VOL 11 | SEPTEMBER 2018 | 645–649 | www.nature.com/naturegeoscience 645

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The ¹⁴C content of sedimentary organic matter (OM) and specific organic molecules provide valuable information on the source and age of OM stored in sediments, but these data are limited for tropical fluvial and lake sediments. We analyzed ¹⁴C in bulk OM, palmitic acid (C16), and long‐chain n‐alkanoic acids (C24, C26, and C28), within fluvial and lake sediments in the catchment of Lake Izabal, a large tectonic lake basin in Guatemala. We combined these measurements with bulk and compound‐specific δ¹³C measurements, as well as sediment organic carbon to nitrogen (OC:N) ratios, to understand the source and age of sedimentary OM in different regions of the lake catchment. Most fatty acid and bulk OM samples were characterized by pre‐modern carbon, indicating important input of aged carbon with residence times of hundreds to thousands of years into sediments. We identified two mechanisms leading to aged carbon export to sediments. In the high‐relief and deforested Polochic catchment, older OM and fatty acids are associated with low % total organic carbon (TOC) and low OC:N, indicating aged OM associated with eroded mineral soil. In the smaller, low‐relief, and largely forested Oscuro catchment, old OM and fatty acids are associated with high %TOC and high OC:N ratios, indicating export of undegraded aged plant biomass from swamp peat. The age of bulk OM and fatty acids in Lake Izabal sediments is similar to the ages observed in fluvial sediments, implying that fluvial input of aged soil carbon makes an important contribution to lake sediment carbon reservoirs in this large tropical lake.

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Aug 2017
ORG GEOCHEM

Karst areas are widespread landforms present on all continents, formed by the dissolution of carbonate or evaporite host rock. Little is known about the composition and nature of dissolved organic matter (DOM) as it moves through karst systems, although karst DOM has been recognized as important for a range of natural processes. Microbial communities living in karst systems are some of the most diverse and intriguing on the planet, and their metabolism and life cycle can give clues related to the development of a host of different life forms. Karst areas are also of interest due to their mostly subterranean hydrology, and the repercussions of these processes on local carbon cycles. We illustrate some of the processes acting on DOM in karst waters through the analysis of soil, drip and cave pool waters at the tropical site of Yok Balum Cave, in southern Belize. Water samples were analyzed using ultra-high resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS), a technique that enables the resolution of single molecular formulae within a DOM spectrum. We perform multivariate statistics to detect trends in the data and identify provenance of detected molecular components. In addition to karst waters, four aliquots of a powdered stalagmite sample from the same cave system are analyzed. Our results show a clear gradient between the soil and the cave system. We hypothesize that both sorption on mineral surfaces and microbial reworking are responsible for the observed trend in DOM composition. The stalagmite extracts show an anomalous DOM pattern, which may be due to a variety of factors, including microbial activity on the stalagmite surface and different affinities of compounds to incorporation in the carbonate. The goal of this study was to follow the molecular transformations of DOM on its journey from the surface to the cave, and to provide a molecular basis for the establishment of stalagmite DOM proxies in karst systems.

Stability and instability on Maya Lowlands tropical hillslope soils

Article

Feb 2018
GEOMORPHOLOGY

Substantial lake core and other evidence shows accelerated soil erosion occurred in the Maya Lowlands of Central America over ancient Maya history from 3000 to 1000 years ago. But we have little evidence of the wider network of the sources and sinks of that eroded sediment cascade. This study begins to solve the mystery of missing soil with new research and a synthesis of existing studies of tropical forest soils along slopes in NW Belize. The research aim is to understand soil formation, long-term human impacts on slopes, and slope stability over time and to explore ecological implications. We studied soils on seven slopes in tropical forest areas that have experienced intensive ancient human impacts and those with little ancient impacts. All of our soil catenas, except for one deforested from old growth two years before, contain evidence for about 1000 years of stable, tropical forest cover since Maya abandonment. We characterized the physical, chemical, and taxonomic characteristics of soils at crest-shoulder, backslopes, footslopes, and depression locations, analyzing typical soil parameters, chemical elements, and carbon isotopes (δ¹³C) in dated and undated sequences. Four footslopes or depressions in areas of high ancient occupation preserved evidence of buried, clay-textured soils covered by coarser sediment dating from the Maya Classic period. Three footslopes from areas with scant evidence of ancient occupation had little discernable deposition. These findings add to a growing corpus of soil toposequences with similar facies changes in footslopes and depressions that date to the Maya period. Using major elemental concentrations across a range of catenas, we derived a measure (Ca + Mg) / (Al + Fe + Mn) of the relative contributions of autochthonous and allochthonous materials and the relative age of soil catenas. We found very low ratios in clearly older, buried soils in footslopes and depressions and on slopes that had not undergone ancient Maya erosion. We found high (Ca + Mg) / (Al + Fe + Mn) values on slopes with several lines of evidence that suggest relative youth, soils possibly formed since Maya abandonment. Carbon isotopes (δ¹³C) also provide some evidence of past vegetation change on slopes. We found strong evidence for maize or other alien C4 species in an ancient terrace soil and additional evidence in buried footslopes but only evidence for C3 species (like tropical trees) on the backslopes and other crest-shoulders. The fact that steep slopes preserved no evidence of C4 species inputs may mean that the ancient Maya maintained forests here. Alternatively, ancient Maya land uses eroded slopes, with the δ¹³C signatures detected today being the result of more recent soil development under forest over the last millennium. Additional evidence that these soils are recent in age includes elevated (Ca + Mg) / (Al + Fe + Mn) values, skeletal soil profiles, and low soil magnetic susceptibility. Besides the evidence for truncating backslopes and aggrading footslopes, the ancient Maya built agricultural terraces that accumulated soils and altered drainage. All these ancient Maya slope alterations would have influenced modern tree distributions, because many tree species in the modern forest show strong preferences for different soil types and topographic situations that the ancient Maya changed.

Altitude effect on leaf wax carbon isotopic composition in humid tropical forests

Article

Feb 2017
GEOCHIM COSMOCHIM AC

The carbon isotopic composition of plant leaf wax biomarkers is commonly used to reconstruct paleoenvironmental conditions. Adding to the limited calibration information available for modern tropical forests, we analyzed plant leaf and leaf wax carbon isotopic compositions in forest canopy trees across a highly biodiverse, 3.3 km elevation gradient on the eastern flank of the Andes Mountains. We sampled the dominant tree species and assessed their relative abundance in each tree community. In total, 405 sunlit canopy leaves were sampled across 129 species and nine forest plots along the elevation profile for bulk leaf and leaf wax n-alkane (C27 – C33) concentration and carbon isotopic analyses (δ¹³C); a subset (76 individuals, 29 species, five forest plots) were additionally analyzed for n-alkanoic acid (C22 – C32) concentrations and δ¹³C. δ¹³C values display trends of +0.87 ± 0.16 ‰ km⁻¹ (95% CI, r² = 0.96, p < 0.01) for bulk leaves and +1.45 ± 0.33 ‰ km⁻¹ (95% CI, r² = 0.94, p < 0.01) for C29n-alkane, the dominant chain length. These carbon isotopic gradients are defined in multi-species sample sets and corroborated in a widespread genus and several families, suggesting the biochemical response to environment is robust to taxonomic turnover. We calculate fractionations and compare to adiabatic gradients, environmental variables, leaf wax n-alkane concentrations, and sun/shade position to assess factors influencing foliar chemical response. For the 4 km forested elevation range of the Andes, 4–6‰ higher δ¹³C values are expected for upland versus lowland C3 plant bulk leaves and their n-alkyl lipids, and we expect this pattern to be a systematic feature of very wet tropical montane environments. This elevation dependency of δ¹³C values should inform interpretations of sedimentary archives, as ¹³C-enriched values may derive from C4 grasses, petrogenic inputs or upland C3 plants. Finally, we outline the potential for leaf wax carbon isotopes to trace biomarker sourcing within catchments and for paleoaltimetry.

The muddle of ages, turnover, transit, and residence times in the carbon cycle

Article

Nov 2016
GLOBAL CHANGE BIOL

Comparisons among ecosystem models or ecosystem dynamics along environmental gradients commonly rely on metrics that integrate different processes into a useful diagnostic. Terms such as age, turnover, residence, and transit times are often used for this purpose; however, these terms are variably defined in the literature, and in many cases calculations ignore assumptions implicit in their formulas. The aim of this opinion piece is i) to make evident these discrepancies and the incorrect use of formulas, ii) highlight recent results that simplify calculations and may help to avoid confusion, and iii) propose the adoption of simple and less ambiguous terms. This article is protected by copyright. All rights reserved.

Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century

Article

Sep 2016

Soil is the largest terrestrial carbon reservoir and may influence the sign and magnitude of carbon cycle–climate feedbacks. Many Earth system models (ESMs) estimate a significant soil carbon sink by 2100, yet the underlying carbon dynamics determining this response have not been systematically tested against observations. We used 14C data from 157 globally distributed soil profiles sampled to 1-meter depth to show that ESMs underestimated the mean age of soil carbon by a factor of more than six (430 ± 50 years versus 3100 ± 1800 years). Consequently, ESMs overestimated the carbon sequestration potential of soils by a factor of nearly two (40 ± 27%). These inconsistencies suggest that ESMs must better represent carbon stabilization processes and the turnover time of slow and passive reservoirs when simulating future atmospheric carbon dioxide dynamics.