ArticlePublisher preview available

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

To read the full-text of this research, you can request a copy directly from the authors.

Abstract and Figures

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.
This content is subject to copyright. Terms and conditions apply.
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:
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
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 | 645
The Nature trademark is a registered trademark of Springer Nature Limited.
... 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. ...
Knowledge of the past interactions between climate and human land use is essential for understanding the possible future relationships between global change and human societies. In this study, we used pollen and other multi-proxy analyses of the sediments from Xingyun Lake in central Yunnan Plateau to reconstruct the history of land use and ecosystem dynamics during the last 3,200 years. The pollen results indicate that the vegetation became more open gradually and many secondary forests began to grow during the last 100 years. During the Medieval Climatic Anomaly (MCA), the vegetation is dominated by broad-leaved trees, while during the Little Ice Age (LIA) the broad-leaved trees declined but Picea/Abies expanded. Human land use has impacted on the regional vegetation and landscape since 50 CE; large-scale human land use occurred after 750 CE. The sediment accumulation rate (SAR), pollen record, and 14 C age offsets reveal that soil erosion is well correlated with human activity and changes in vegetation cover. Before 750 CE, soil erosion was probably limited, but after 750 CE it increased drastically as a result of human activity, and then decreased during the last 100 years. According to the results of the redundancy analysis (RDA), we conclude that climate had an important impact on the regional vegetation, and the late Holocene vegetation changes of Xingyun Lake catchment were mainly influenced by temperature.
... 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. ...
Full-text available
The compound-specific hydrogen isotopic composition (δ2H) of n alkanes is a valuable proxy to investigate hydrological conditions in lake sediments. While terrestrial n-alkanes reflect the isotopic signal of the local precipitation, aquatic n-alkanes incorporate the isotopic signal of the lake’s water, which can be strongly modulated by evaporative enrichment. So far, the spatial distribution of the terrestrial and aquatic δ2H signal within lakes have not systematically been investigated. Here, we present compound-specific δ2H results of terrestrial (δ2HC31) and aquatic (δ2HC23) n alkanes of surface sediment samples from Lake Khar Nuur, a semi-arid and high-altitude lake in the Mongolian Altai, and additionally investigate the δ2H signal of topsoils from the catchment. Our results show that the majority of the n-alkane δ2H values from the catchment topsoils correspond well with modeled local growing season precipitation (JJAS). However, few samples in the northern catchment show more positive δ2H values possibly due to increased evapo(transpi)ration by southward exposition and shallower soils there. The only small variability of δ2HC31 in the surface sediments is in the range of most topsoils δ2H from the catchment, and thus, well reflects local growing season precipitation. δ2HC23 in surface sediment samples from the central and deepest parts of the lake, i.e., the lake’s sediment accumulation zones, shows distinctly more positive δ2HC23 values due to evaporative lake water enrichment. Consequently, Δaq-terr, which is the isotopic offset between δ2HC23 and δ2HC31, indicates distinct lake water enrichment in the lake’s accumulation zones and is a valuable proxy to investigate past hydrological changes.
... 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). ...
Full-text available
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). ...
Full-text available
Semi-arid Mongolia is a highly sensitive region to climate changes, but the region’s Holocene paleoclimatic evolution and its underlying forcing mechanisms have been the subject of much recent debate. Here we present a continuous 7.4 ka sediment record from the high-altitude Shireet Naiman Nuur (Nuur = lake) in the central Mongolian Khangai Mountains. We extensively dated the sediments and analyzed elemental composition and bulk isotopes for lake sediment characterization. Our results show that ¹⁴ C-dating of bulk organic carbon and terrestrial macrofossils provide a robust and precise chronology for the past 7.4 ± 0.3 cal ka BP at Shireet Naiman Nuur and ¹⁴ C-ages are mostly in stratigraphic order. The ¹⁴ C-based chronology is confirmed by paleomagnetic secular variations, which resemble the predictions of spherical harmonic geomagnetic field models. The very good chronological control makes paleomagnetic secular variation stratigraphy a powerful tool for evaluating and refining regional ¹⁴ C-chronologies when compared to the record presented here. The lake sediment proxies TOC, N, log (Ca/Ti) and log (Si/Ti) reveal increased lake primary productivity and high growing season temperatures from 7.4 ± 0.3 to 4.3 ± 0.2 cal ka BP, which is likely the result of stronger summer insolation and pronounced warming. Reduced summer insolation thereafter results in decreased productivity and low growing season temperatures at Shireet Naiman Nuur from 4.3 ± 0.3 cal ka BP until present day. The globally acknowledged 4.2 ka event also appears as a pronounced cooling event at Shireet Naiman Nuur, and additional abrupt cooling events occurred during minima in total solar irradiance at ∼3.4, 2.8 and 2.4 ka BP. Low lake primary productivity and growing season temperatures are likely the result of longer ice cover periods at the high-altitude (2,429 m a.s.l.) Shireet Naiman Nuur. This leads to shorter mixing periods of the lake water which is supported by more positive δ ¹³ C TOC because of increased incorporation of dissolved HCO 3 ⁻ by aquatic producers during periods of longer ice cover.
... 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. ...
Full-text available
As the primary producer in terrestrial ecosystems, plants are the ultimate source of organic carbon supplied into soils and fueling microbial decomposition. Hence, plants play a key role in the formation and accumulation of soil organic matter. However, due to the complexity of plant‐derived organic matter and soil processes, the fate, persistence and storage of carbon in the form of plant‐derived molecules in the soil remain poorly understood. Plant‐derived organic matter is composed of a wide array of organic molecules from simple organic acids and carbohydrates to polymeric molecules such as cellulose, lignin and proteins, etc. These molecules have distinct chemical structures, physiochemical properties, interactions with soil minerals and propensities to associate with soil aggregates. Such characteristics, coupled with soil environmental conditions and microbial capacities, play a decisive role on the stability and biodegradability of plant‐derived molecules in the soil. In this chapter, I summarize the properties and behavior of major plant macromolecules as inputs into soils and describe various molecular‐level methods to trace the transformation and turnover of plant molecules in soils, with a specific emphasis on biomarker and isotopic analyses that are specific to plant‐derived molecules. Major soil processes that affect the persistence of different plant‐derived compounds are also reviewed, with several aspects summarized for prioritizing future research in order to improve our understanding on the fate of plant‐derived organic matter and its contribution to soil carbon storage.
... Urbanisation is known to have caused historical declines in soil fertility and organic matter content (based on a study of Mayan cities by Douglas et al. 2018). Although these declines were not caused by climate change, they represent processes which decrease the resilience of urbanised ecosystems to climate change effects. ...
Urban soils are a global resource which presents many challenges and opportunities for human populations in cities. This chapter addresses several of these opportunities and threats, and highlights where scientific knowledge is uncertain or incomplete with implications for the direction of further research. Climate change and global warming is discussed in terms of soil resilience, carbon cycling and storage, pollutant fluxes, and changes in water inputs. We review studies finding that biodiversity change in the context of urban soils may be negative or positive and suggest that some urban environments can promote conservation of flora and fauna. Urban agriculture is a significant opportunity for beneficial use of urban soils, and the benefits and constraints of growing food and trees in cities are examined. Water Sensitive Urban Design is unevenly implemented globally and offers multiple benefits for sustainable development which are not fully realised. Urban soil contamination is presented as an ongoing issue, with discussions of both legacy contamination and emerging contaminants. We highlight the potential for urban soil remediation to be performed more sustainably by widespread adoption of life cycle assessment and emphasise the need to promote environmental justice in the context of urban soils worldwide. Finally, we draw attention to the opportunities to include indigenous, traditional, and local soil knowledge in parallel with scientific and technical understanding of urban soils.
... Common Era (CE) 250 to 900 (14). Classic Maya cities of the Mesoamerican lowlands were densely populated, and urban centers relied on intensive agricultural practices, which were associated with environmental impacts such as deforestation (15) and soil erosion (16)(17)(18). In the past few decades, evidence of profound climate changes during the period of Maya occupation has emerged, and ancient droughts in the Maya region were shown to have been temporally correlated with times of sociopolitical disintegration (19)(20)(21)(22)(23)(24). ...
Full-text available
Human-induced deforestation and soil erosion were environmental stressors for the ancient Maya of Mesoamerica. Furthermore, intense, periodic droughts during the Terminal Classic Period, ca. Common Era 830 to 950, have been documented from lake sediment cores and speleothems. Today, lakes worldwide that are surrounded by dense human settlement and intense riparian land use often develop algae/cyanobacteria blooms that can compromise water quality by depleting oxygen and producing toxins. Such environmental impacts have rarely been explored in the context of ancient Maya settlement. We measured nutrients, biomarkers for cyanobacteria, and the cyanotoxin microcystin in a sediment core from Lake Amatitlán, highland Guatemala, which spans the last ∼2,100 y. The lake is currently hypereutrophic and characterized by high cyanotoxin concentrations from persistent blooms of the cyanobacterium Microcystis aeruginosa . Our paleolimnological data show that harmful cyanobacteria blooms and cyanotoxin production occurred during periods of ancient Maya occupation. Highest prehistoric concentrations of cyanotoxins in the sediment coincided with alterations of the water system in the Maya city of Kaminaljuyú, and changes in nutrient stoichiometry and maximum cyanobacteria abundance were coeval with times of greatest ancient human populations in the watershed. These prehistoric episodes of cyanobacteria proliferation and cyanotoxin production rivaled modern conditions in the lake, with respect to both bloom magnitude and toxicity. This suggests that pre-Columbian Maya occupation of the Lake Amatitlán watershed negatively impacted water potability. Prehistoric cultural eutrophication indicates that human-driven nutrient enrichment of water bodies is not an exclusively modern phenomenon and may well have been a stressor for the ancient Maya.
Mother Nature doesn’t care if you’re having fun.Larry Niven Mother Nature doesn’t care if you’re having fun. Larry Niven The left-hand side of Equation (1.1) (repeated below for convenience) is environmental impacts (I). While there are natural processes that have (some) negative impacts on the environment, such as volcanic eruptions and wildfires, this book is primarily concerned with human impacts on the environment. Greenhouse-gas emissions, such as CO2, are our prime example. Other examples of human impact include deforestation, desertification, aquifer depletion, plant and animal extinctions, ground water and soil contamination, ozone depletion, smog creation, habitat destruction, and many other “tions.”
The chronology of geological records in lacustrine, peatland and marine sediments for the late glacial depends mainly on ¹⁴C dating technology, which provides the basic database for global paleoclimate research. However, ¹⁴C reservoir correction always challenges the accuracy of the ¹⁴C chronology of terrestrial and marine sediments and the uncertainty of the ¹⁴C chronology associated with the carbon reservoir effect becomes critical for high resolution paleoclimate studies. Here, based on the hypothesis of synchronization of precipitation isotopes, we verify and identify a series of in-phase points of precipitation isotopes (IPPIs) between sediment leaf-wax hydrogen and stalagmite-calcite oxygen isotopes. Because stalagmite oxygen isotope records are accurately dated by U–Th dating technology, the ages of stalagmite IPPIs could be used to improve ¹⁴C reservoir correction of lacustrine IPPIs. We found that reservoir-corrected ¹⁴C ages of lacustrine, peat, and marine IPPIs are scattered with an average uncertainty of 1 ka when compared with the corresponding IPPIs on U–Th age scales. We suggest that the reservoir age correction at different time intervals could be further adjusted by using the U–Th age-backed IPPIs, which largely reduces the uncertainty of the ¹⁴C chronology and thus provides more accurate paleoclimate records.
Full-text available
Soils play an essential role in the global cycling of carbon and understanding the stabilisation mechanisms behind the preservation of soil organic carbon (SOC) pools is of globally recognised significance. Until recently, research into SOC stabilisation has predominantly focused on acidic soil environments and the interactions between SOC and aluminium (Al) or iron (Fe). The interactions between SOC and calcium (Ca) have typically received less attention, with fewer studies conducted in alkaline soils. Although it has widely been established that exchangeable Ca (CaExch) positively correlates with SOC concentration and its resistance to oxidation, the exact mechanisms behind this relationship remain largely unidentified. This synthesis paper critically assesses available evidence on the potential role of Ca in the stabilisation of SOC and identifies research topics that warrant further investigation. Contrary to the common view of the chemistry of base cations in soils, chemical modelling indicates that Ca²⁺ can readily exchange its hydration shell and create inner sphere complexes with organic functional groups. This review therefore argues that both inner- and outer-sphere bridging by Ca²⁺ can play an active role in the stabilisation of SOC. Calcium carbonate (CaCO3) can influence occluded SOC stability through its role in the stabilisation of aggregates; however, it could also play an unaccounted role in the direct sorption and inclusion of SOC. Finally, this review highlights the importance of pH as a potential predictor of SOC stabilisation mechanisms mediated by Al- or Fe- to Ca, and their respective effects on SOC dynamics.
Full-text available
The stability and potential vulnerability of soil organic matter (SOM) to global change remains incompletely understood due to the complex processes involved in its formation and turnover. Here we combine compound-specific radiocarbon analysis with fraction-specific and bulk-level radiocarbon measurements in order to further elucidate controls on SOM dynamics in a temperate and sub-alpine forested ecosystem. Radiocarbon contents of individual organic compounds isolated from the same soil interval generally exhibit greater variation than those among corresponding operationally-defined fractions. Notably, markedly older ages of long-chain plant leaf wax lipids (n-alkanoic acids) imply that they reflect a highly stable carbon pool. Furthermore, marked 14C variations among shorter- and longer-chain n-alkanoic acid homologues suggest that they track different SOM pools. Extremes in SOM dynamics thus manifest themselves within a single compound class. This exploratory study highlights the potential of compound-specific radiocarbon analysis for understanding SOM dynamics in ecosystems potentially vulnerable to global change.
Full-text available
Understanding the effect of land use on soil carbon, nitrogen, and microbial activity associated with aggregates is critical for thorough comprehension of the C and N dynamics of karst landscapes/ecosystems. We monitored soil organic carbon (SOC), total nitrogen (TN), microbial biomass carbon (MBC), and Cmic: Corg ratio in large macro- (>2 mm), small macro- (0.25–2 mm), and micro- (0.053–0.25 mm) aggregates to determine the changes in soil properties under different land uses in the karst area of Southwest China. Five common land-use types—enclosure land (natural system, control), prescribed-burning land, fuel-wood shrubland, pasture and maize fields—were selected. Results showed that pasture and maize fields remarkably decreased the SOC and TN concentrations in aggregates. Conversion of natural system to other land uses decreased MBC (except for prescribed-burning) and increased Cmic: Corg ratios in aggregates. The extent of the response to land uses of SOC and TN concentrations was similar whereas that of MBC and Cmic: Corg ratios differed across the three aggregate sizes. Further, the SOC concentrations were significantly higher in macro-aggregates than micro-aggregates; the MBC and Cmic: Corg ratios were highest in small macro-aggregates. Therefore, small macro-aggregates might have more active C dynamics.
We systematically investigated the concentration and distributions of saturated fatty acids (FAs) in 66 submerged plants (including 48 Potamogeton, 7 Myriophyllum and 11 Ruppia), 59 algae (including 26 Chara, 20 Cladophora and 13 Spirogyra) and 32 terrigenous plants from 18 lakes on the northeastern Tibetan Plateau. The results indicate that C14-C32 FAs in algae, submerged plants and terrigenous plants were dominated mainly by C16 and C20-C32. FAs in algae and submerged plants were predominantly C24, but also contained a relatively high abundance of C26. The submerged plants had high C26-C32 concentration, with average values of 216 μg/g for Potamogeton, 52 μg/g for Myriophyllum and 134 μg/g for Ruppia, close to those of terrigenous plants (avg. 161 μg/g). Algae exhibited low C26-C32 FA concentration, with mean values of 7 μg/g for Chara, 8 μg/g for Cladophora and 18 μg/g for Spirogyra. The C26-C32 FAs in algae and submerged plants accounted for a large proportion of C20-C32, yielding average ratios (C26-C32 vs. C20-C32) of 33, 35, 31, 15, 19 and 23% for Potamogeton, Myriophyllum, Ruppia, Chara, Cladophora and Spirogyra, respectively. Therefore, the contribution of submerged plant C26-C32 FAs to lake sediments should be considered due to their high concentration, whereas the influence of algae was minor. Meanwhile, by comparing the values of average chain length (ACL), alga/terrigenous ratios (ATRs) and submerged/terrestrial ratios (STRs), we found that ACL14-32 and ATRs could be used to distinguish algal FAs from those of other plants and that STRs couldn be used to differentiate FA sources from submerged and terrestrial plants. Thus, we suggest that using specific chain lengths to determine source inputs is not adequate when applying sedimentary FAs for paleoclimatic reconstruction, while ACL 14-32, ATRs and STRs may aid in determining FA sources in sedimentary records.
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.
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.
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.
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.
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.