Kyungsoo Yoo

Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere

Streams, rivers, lakes, and other inland waters are important agents in the coupling of biogeochemical cycles between continents, atmosphere, and oceans. The depiction of these roles in global-scale assessments of carbon (C) and other bioactive elements remains limited, yet recent findings suggest that C discharged to the oceans is only a fraction of that entering rivers from terrestrial ecosystems via soil respiration, leaching, chemical weathering, and physical erosion. Most of this C influx is returned to the atmosphere from inland waters as carbon dioxide (CO2) or buried in sedimentary deposits within impoundments, lakes, floodplains, and other wetlands. Carbon and mineral cycles are coupled by both erosion–deposition processes and chemical weathering, with the latter producing dissolved inorganic C and carbonate buffering capacity that strongly modulate downstream pH, biological production of calcium-carbonate shells, and CO2 outgassing in rivers, estuaries, and coastal zones. Human activities substantially affect all of these processes.

Process-based model linking pocket gopher (Thomomys bottae) activity to sediment transport and soil thickness

Burrowing organisms assist in shaping earth surfaces and are simultaneously affected by the environment they inhabit; however, a conceptual framework is not yet available to describe this feedback. We introduce a model that connects the population density of soil-burrowing animals to sediment transport via energy. The model, combined with avail- able data from California hillslopes where soil erosion is driven by pocket gophers ( Thom- omys bottae), suggests that a gopher annually expends ;9 kJ of energy, or ;1% of re- ported burrowing energy expenditure, in generating sediment transport. The model is used to evaluate the case that gophers prefer to populate thicker soils. The results suggest that this behavior may drastically dampen the spatial and temporal variations of soil thickness and gopher populations, implying that burrowing organisms may create landscapes dis- tinct from those affected by abiotic processes.

Twelve testable hypotheses on the geobiology of weathering

Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earths surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical Zone. (8) Rising atmospheric P(CO2) will increase rates and extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

Using hilltop curvature to derive the spatial distribution of erosion rates

[1] Erosion rates dictate the morphology of landscapes, and therefore quantifying them is a critical part of many geomorphic studies. Methods to directly measure erosion rates are expensive and time consuming, whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents. If hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates. In this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high-resolution (1 m) digital elevation data. Published and new cosmogenic radionuclide analyses in the Feather River basin, California, suggest that erosion rates vary by over an order of magnitude (10 to 250 mm kyr � 1 ). Hilltop curvature increases with erosion rates, allowing calibration of the hillslope sediment transport coefficient, which controls the relationship between gradient and sediment flux. Having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature. Additionally, we show that hilltop curvature continues to increase with rising erosion rates after gradient-limited hillslopes have emerged. Hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient, providing soil production on hilltops can keep pace with erosion. Finally, hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels.

An explorative study of mercury export from a thawing palsa mire

Thawing of permafrost and a subsequent accelerated loss of mercury from the soil constitute a possible threat to the quality of high-latitude surface waters. In this paper we estimate the export of ...

Invasive Earthworms Deplete Key Soil Inorganic Nutrients (Ca, Mg, K, and P) in a Northern Hardwood Forest

Hardwood forests of the Great Lakes Region have evolved without earthworms since the Last Glacial Maximum, but are now being invaded by exotic earthworms introduced through agriculture, fishing, and logging. These exotic earthworms are known to increase soil mixing, affect soil carbon storage, and dramatically alter soil morphology. Here we show, using an active earthworm invasion chronosequence in a hardwood forest in northern Minnesota, that such disturbances by exotic earthworms profoundly affect inorganic nutrient cycles in soils. Soil nutrient elemental concentrations (Ca, Mg, K, and P) were normalized to biogeochemically inert Zr to quantify their losses and gains. This geochemical normalization revealed that elements were highly enriched in the A horizon of pre-invasion soils, suggesting tight biological recycling of the nutrients. In the early stage of invasion, epi-endogeic earthworm species appeared to have been responsible for further enriching the elements in the A horizon possibly by incorporating leaf organic matter (OM). The arrival of geophagous soil mixing endogeic earthworms, however, was associated with near complete losses of these enrichments, which was related to the loss of OM in soils. Our study highlights that elemental concentrations may not be sufficient to quantify biogeochemical effects of earthworms. The geochemical normalization approach, which has been widely used to study soil formation, may help when determining how invasive soil organisms affect soil elemental cycles. More generally, this approach has potential for much wider use in studies of belowground nutrient dynamics. The results support the existing ecological literature demonstrating that invasive earthworms may ultimately reduce productivity in formerly glaciated forests under climate change.

Reservoir theory for studying the geochemical evolution of soils

[1] Linking mineral weathering rates measured in the laboratory to those measured at the landscape scale is problematic. In laboratory studies, collections of minerals are exposed to the same weathering environment over a fixed amount of time. In natural soils, minerals enter, are mixed within, and leave the soil via erosion and dissolution/leaching over the course of soil formation. The key to correctly comparing mineral weathering studies from laboratory experiments and field soils is to consistently define time. To do so, we have used reservoir theory. Residence time of a mineral, as defined by reservoir theory, describes the time length between the moment that a mineral enters (via soil production) and leaves (via erosion and dissolution/leaching) the soil. Age of a mineral in a soil describes how long the mineral has been present in the soil. Turnover time describes the time needed to deplete a species of minerals in the soil by sediment efflux from the soil. These measures of time are found to be sensitive to not only sediment flux, which controls the mineral fluxes in and out of a soil, but also internal soil mixing that controls the probability that a mineral survives erosion. When these measures of time are combined with published data suggesting that a mineral’s dissolution reaction rate decreases during the course of weathering, we find that internal soil mixing, by partially controlling the age distribution of minerals within a soil, might significantly alter the soil’s mass loss rate via chemical weathering.

Rates of soil mixing and associated carbon fluxes in a forest versus tilled agricultural field: Implications for modeling the soil carbon cycle

In natural ecosystems, bioturbation is an essential component of soil formation, whereas tillage drives soil mixing in agricultural soils. Yet soil mixing is commonly neglected in modeling soil org ...

Soil carbon accumulation in the dry tundra: Important role played by precipitation

A positive relationship between the mean annual precipitation (MAP) and soil organic carbon (SOC) is found in most surveys covering the subarctic and boreal region. In this paper we assess mechanis ...

Impact of exotic earthworms on organic carbon sorption on mineral surfaces and soil carbon inventories in a northern hardwood forest.

Exotic earthworms are invading forests in North America where native earthworms have been absent since the last glaciation. These earthworms bioturbate soils and may enhance physical interactions between minerals and organic matter (OM), thus affecting mineral sorption of carbon (C) which may affect C cycling. We quantitatively show how OM-mineral sorption and soil C inventories respond to exotic earthworms along an earthworm invasion chronosequence in a sugar maple forest in northern Minnesota. We hypothesized that mineral surface area in A horizons would increase as burrowing earthworms incorporated clay minerals from the B horizons and that enhanced contacts between OM and minerals would increase the OM sorption on mineral surfaces and mineral-associated C inventories in A horizons. Contrary to our hypotheses, mineral surface areas within A horizons were lowered because earthworm burrows only extended into the silt-rich loess that separated the A and clay-rich B horizons. Furthermore, where endogeic earthworms were present, a smaller fraction of mineral surface area was covered with OM. OM sorption on minerals in the A horizons shifted from a limitation of mineral surface availability to a limitation of OM availability within a decade after the arrival of endogeic earthworms. C-mineral sorption depends on earthworm consumption of OM as well as the composition and vertical distribution of minerals. This finding may thus explain the contradictory results reported in earlier investigations. Our results highlight the rapid and drastic effects of exotic earthworms on key ecosystem processes in deciduous forests in post-glacial settings.