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

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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.
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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 change24. 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 years68. 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 erosion1215. 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 change12,13,1517, 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 ecosystems2326, and that MTTwax values in sediment cores
serve as an indicator of changes in the persistence of soil carbon
across a catchment through time2729.
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
Δ
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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... Consequently, land use has been, and still is, subject to transformations in response to environmental, climatic and socioeconomic factors. Therefore, land use history over the last several hundred or thousands of years, especially from the beginning of the Anthropocene (Ruddiman, 2013;Ruddiman et al., 2015), is an important topic in environmental research (Huang and O'Connell, 2000;Poirier et al., 2011;Lechterbeck et al., 2014;Goldewijk et al., 2017;Douglas et al., 2018;Haas et al., 2020). ...
... At the same time, the soil erosion has been altering aquatic ecosystem of the lakes (Foster et al., 2003;Jenny et al., 2019;Haas et al., 2019Haas et al., , 2020, making more autochthonous organic carbon present in the lakes. Besides, the 14 C content of different organic matter is a stronger tracer of carbon mobilization and storage (Feng et al., 2013), and the 14 C content can be used to identify the time of accelerated erosion and export of 'old' soil organic carbon from the surrounding watershed to the lacustrine sediments (Edwards and Whittington, 2001;Gierga et al., 2016;Douglas et al., 2018, Haas et al., 2019, 2020. Therefore, the 14 C age offsets between bulk sediment organic carbon and the corresponding depositional age can be used to detect the contribution of 'old' soil organic carbon in the lake sediments, enabling time series of soil erosion intensity to be produced which can potentially enable the driving mechanisms to be determined. ...
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... Moreover, the terrestrial n-alkanes might have time lags between their timing of formation and their deposition within the lake because of certain residence times in and transfer times through the catchment. Those transfer times can be in the order of hundreds to thousands of years and have been reported from different lacustrine systems (Aichner et al., 2021;Douglas et al., 2018;Freimuth et al., 2021;Gierga et al., 2016). However, it has been suggested to especially investigate lakes with small hydrological catchments to reduce potential time lags of terrestrial n-alkanes Gierga et al., 2016). ...
... Those possibly relocated n-alkanes further might question if δ 2 H C31 of the surface sediment samples in Lake Khar Nuur represent a contemporary signal that become deposited shortly after the n-alkane formation in the catchment or if those n-alkanes are pre-aged due to residence times in the catchment soils and/or transfer times through the catchment. Such a pre-aging of terrestrial biomarkers has been previously reported from different lakes around the world, and time lags can be in the order of hundreds to thousands years, and often strongly increase with increased anthropogenic activity in the lake catchment (Douglas et al., 2018;Freimuth et al., 2021;Gierga et al., 2016). In this context, compound-specific radiocarbon dating provide the opportunity to directly date single terrestrial n-alkanes. ...
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... Soil is fundamentally important for sustaining human societies; it underpins ecosystem services including food production, and by storing most terrestrial carbon, soil plays a key role in regulating Earth's atmosphere (Hiederer and Köchy, 2011;Douglas et al., 2018). Soil sustainability is reflected in the long-term balance between soil production and erosion (Montgomery, 2007), a balance threatened across large areas of the world due to climate change and human activities, such as deforestation, and agricultural practices that accelerate soil depletion (FAO, 1996;Hippe et al., 2021). ...
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Sediment fingerprinting is widely used in drainage basin analysis to identify the provenance and source contributions of sediments (or other material) in transit from source-to-sink. By enabling source areas of sediment supply to be targeted, the method has become an integral part of sustainable landscape management. The precision and accuracy of sediment fingerprinting is contingent on the choice of mixing model, which quantifies the contribution of potential sediment sources by minimizing the difference between observed properties of sink samples and characteristic properties of the sources. Here, we apply a set of frequentist and Bayesian mixing models with the aim of identifying the optimum composite fingerprint of four sediment sources (viz., agricultural land, rangeland, gullies, and landslides) in a small catchment draining the Iranian Loess Plateau in the Golestan province of northeastern Iran. Forty-four soil samples were collected from the four potential source zones. Based on seven synthetic mixtures with known source contributions we compared the performance of a frequentist Monte Carlo model, GLUE model, a Bayesian end-member model (BEMMA), MixSIAR Bayesian model, and a Brewer Bayesian model. We found that, in terms of uncertainty estimation, the best results were obtained with GLUE and BEMMA. Applying GLUE to our study catchment, we estimated the following source contributions to an earth dam reservoir: agricultural land (55.8 %), rangeland (33.7 %), gullies (15.7 %), and landslides (14.2 %), confirming the view that agriculture is the main cause of reservoir sedimentation. All source contributions exhibited high variability, which we attribute to storm frequency, sediment delivery due to hillslope-sink connectivity, and human activities involving removal of vegetation.
... Bulk organic carbon has often been used in semi-arid regions since terrestrial macrofossils, which are assumed to be rapidly transported into the lake and thus are ideal for dating (Hajdas et al., 1995), are often absent. However, bulk organic carbon can be "pre-aged" because organic material accumulates in the catchment over hundreds to thousands of years and possibly overestimates the "true" deposition age when ending up in the lake (Gierga et al., 2016;Douglas et al., 2018;Haas et al., 2019). ...
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... However, as SOM is a complex mixture of compounds with various origins and decomposability Lehmann and Kleber 2015; Schmidt et al. 2011). Thus, radiocarbon analysis of specific compounds or compound classes with known sources or physical status (such as aggregation and mineral association) can provide key information on the stability and dynamics of SOM constituents at the molecular level (Eglinton et al. , 1997Gleixner 2013;Douglas et al. 2018). Until now, compound-specific 14 C analysis has mainly focused on free extractable lipids (Bol et al. 1996;Eglinton et al. 1997;Huang et al. 1999;Pearson et al. 2001;Rethemeyer et al. 2004;Douglas et al. 2014;van der Voort et al. 2017), cutin-and suberin-derived bound lipids (Feng et al. 2015b), lignin phenols (Hou et al. 2010;Feng et al. 2013Feng et al. , 2015, BPCAs (Ziolkowski and Druffel 2010;Coppola et al. 2014Coppola et al. , 2018 and some microbial membrane lipids (Petsch et al. 2001;Rethemeyer et al. 2005;Ingalls et al. 2006;Kramer and Gleixner 2006;Shah et al. 2008;Kramer et al. 2010;Ziolkowski et al. 2013;Brady et al. 2018) in soils and sediments. ...
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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.
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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.
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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.
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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.