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The interaction of soil organic matter (SOM) and minerals is a critical mechanism for retaining SOM in soil and protecting soil fertility and long-term agricultural sustainability. The chemical speciation of carbon (C) and nitrogen (N) in mineral-associated SOM can be sensitive to both anthropogenic management practices and landscape positions, but these two aspects are rarely examined in tandem. Here we examined the effects of long-term (>100 years) agricultural management and erosion on mineral-associated SOM along grassland and agricultural hillslope transect. The mineral-associated SOM was obtained using particle size and density fractionation approaches. Chemical speciation of C and N in mineral-associated SOM was characterized using micro X-ray absorption near-edge fine structure (XANES) spectroscopy. The extent of SOM coverage and contribution of iron oxyhydroxides (Fe oxides) to the total specific mineral surface area (SSA) were determined using the BET–N2 adsorption method of soil samples under three conditions: untreated, SOM removal, and Fe oxides removal. The amount of SSA covered by SOM (SSASOM-covered) was lower by 61% and 37% in cultivated eroding and depositional topsoils, respectively, compared with the corresponding grassland. Depositional soils had higher SSASOM-covered than eroding positions. In the cultivated hillslopes, aromatic and phenolic C species were more abundant in depositional soils than in the eroding topsoils, indicating that deposition and burial of eroded or in-situ plant-derived phenolic C protected them from further transformation. Our results, therefore, highlight the importance of anthropogenic activities in the interaction of SOM and minerals, including C speciation changes, which may exert a considerable influence on SOM retention in soils.

Interactions between organic carbon (OC) and minerals represent a critical mechanism for stabilizing organic matter in soils. Because both mineral weathering and plant productivity are negatively affected by soil erosion, mineral-associated organic carbon (MOC) chemistry is also expected to vary with erosion intensity. Here we show that MOC chemistry, determined by carbon X-ray absorption near-edge fine structure spectroscopy (XANES), exhibits little difference across a large (10-fold) gradient in erosion-derived soil turnover times. Mineral-associated OC chemistry further fails to explain the variation in radiocarbon-based MOC turnover times. Our results suggest that soil OC longevity is largely independent of organic matter chemistry in steep mountainous landscapes where soil development is constrained by erosion.

Steep soil-mantled hillslopes are thought to be important sources of sediments and organic carbon (OC) to rivers. Minerals in these sediments may protect OC from decomposition, yet the significance of such interactions in steep upland soils remains poorly constrained particularly in relation to erosion rates. We examined a tributary basin draining to the Middle Folk Feather River in California, where knickpoint migration has created a series of hillslopes with erosion rates varying over an order of magnitude (35 to 250 mm kyr−1). This setting provides a unique opportunity to study soil OC pools and their erosional exports as a function of changes in erosion rates. Soil OC inventories were 37% lower at rapidly eroding sites relative to slowly eroding sites. This difference was driven by coarse rock contents as rapidly eroding soils had more rock fragments, limiting their capacities to store OC. Although clay contents in soils were negatively correlated with erosion rates, the total mineral specific surface area remained relatively invariant. Based on secondary phyllosilicate minerals present in the studied soils and our field observations of saprocks, we suggest that this discrepancy may have originated from different clay mineralogy (types and abundance) associated with different degrees of deep subsurface chemical weathering. Across the erosion gradient, the radiocarbon age of mineral associated organic matter (MOC) in saprock varied by a factor 2 (from 1045 to 2110 14C years), while soil turnover times estimated from soil thickness and erosion rates varied from 17 to 5.4 kyr. At the site eroding at the fastest rate, the soil turnover time approaches the 14C age of MOC, suggesting erosion can potentially limit the timescale over which MOC is replaced. We found that organic matter generally covered

We test the hypothesis that erosion driven soil movement on hillslopes results in an increase in new organomineral associations and overall organic matter storage in colluvial deposits within a forested hillslope. We measured mineral specific surface area (SSA), organic carbon (OC), meteoric radioisotopes (210Pb, 137Cs, 10Be), soil physical properties, C/N, δ15N, δ13C, and ∆14C in bulk soil and density fractions in a hillslope transect of soil pits. The quantity of OC per unit of mineral surface area (OC/SA) and OC inventories increased by a factor of 2–3 in depositional sites as result of soil mixing due to erosional movement as confirmed by 210Pb, 137Cs, and 10Be profiles and inventories. Soil mixing systematically decreased C/N and enriched stable isotopes of δ13C and δ15N, revealing that formation of organomineral associations instead of microbial processing was responsible for depth trends in organic matter composition. Our findings indicate that the processes that associate organic matter and minerals are fundamentally linked with organic matter composition, and OC/SA, C/N, δ13C, and δ15N provide proxies for organic matter stabilization by soil minerals.

Physical and chemical interactions between soil organic matter (OM) and minerals is one of the primary mechanisms for stabilizing OM in terrestrial ecosystems. Focusing on OM association with mineral surfaces, this study sought to examine mineral-associated OM from the perspectives of both mineral surface characteristics and organic matter chemistry. The research was conducted at paired-sites under North American Mid-Atlantic Coastal forest and crop production with shared environmental factors. Using carbon (C) and nitrogen (N) 1s micro- X-ray absorption near-edge fine structure (XANES) spectroscopy, we investigated the amounts and types of mineral-associated OM. Mineral specific surface area (SSA) of bulk soil was determined for three conditions: untreated, post OM removal and post iron (Fe) (oxyhydr)oxides removal. Amounts of mineral-associated OM were smaller in the agricultural soil, where greater SSA sourced from clay-sized phyllosilicates and Fe (oxyhydr)oxide minerals did not result in greater OM coverage of the mineral surface area. Although agricultural surface soil showed less abundance of phenolic C, speciation of mineral-associated OM did not differ between comparable horizons. Our results suggest that despite the plow-derived mixing of soil, which increased SSA and secondary minerals available to interact physically and chemically with OM in the plowed layer, the formation of mineral-associated OM in agricultural soil is ultimately limited by available OM.

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.

The interactions of organic matter and minerals contribute to the capacity of soils to store C. Such interactions may be controlled by the processes that determine the availability of organic matter and minerals, and their physical contacts. One of these processes is bioturbation, and earthworms are the best known organisms that physically mix soils. Earthworms are not native species to areas previously glaciated, and the introduction of earthworms to these regions has been associated with often dramatic changes in soil structure and geochemical cycles. The authors are studying C mineral interaction along an approximately 200 m long earthworm invasion transect in a hardwood forest in northern Minnesota. This transect extends from the soils where earthworms are absent to soils that have been invaded by earthworms for nearly 30–40 years. Pre-invaded soils have an approximately 5 cm thick litter layer, thin (5 cm) A horizon, silt rich E horizon, and clay-rich Bt horizons. The A and E horizons formed from aeolian deposits, while the clay-rich Bt horizons probably developed from underlying glacial till. With the advent of earthworm invasion, the litter layer disappears and the A horizons thicken at the expense of the O and E horizons. In addition, organic C contents in the A horizons significantly increase with the arrival of earthworms. Simultaneously, measured mineral specific surface areas suggest that minerals’ capacities to complex the organic matter appear to be greater in soils with active earthworm populations. Based on the data from two end member soils along the transect, mineral specific surface areas in the A and E horizons are larger in the earthworm invaded soil than in the pre-invasion soil. Additionally, within < 5 a of earthworm invasions, A horizon materials are turned from single grain to a strong medium granular structure. While A horizon organic matter content and organic C-mineral complexation increase after earthworm invasion, they are also more vigorously mixed. This growing data set, when ultimately combined with ongoing measurements of (1) the population dynamics of earthworms along the invasion transect, (2) C-mineral association (via surface adsorption and physical collusion in mineral aggregates) and (3) dissolved organic C will show how and how much soil capacity to store C is affected by burrowing organisms, which are often the keystone species of given ecosystems.

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.

Contaminants as Tracers for Studying Dynamics of Soil Formation: Mining an Ocean of Opportunities.

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 ...

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 ...

[1] We compiled new and published data on the natural abundance N isotope composition (δ15N values) of soil and plant organic matter from around the world. Across a broad range of climate and ecosystem types, we found that soil and plant δ15N values systematically decreased with increasing mean annual precipitation (MAP) and decreasing mean annual temperature (MAT). Because most undisturbed soils are near N steady state, the observations suggest that an increasing fraction of ecosystem N losses are 15N-depleted forms (NO3, N2O, etc.) with decreasing MAP and increasing MAT. Wetter and colder ecosystems appear to be more efficient in conserving and recycling mineral N. Globally, plant δ15N values are more negative than soils, but the difference (δ15Nplant-δ15Nsoil) increases with decreasing MAT (and secondarily increasing MAP), suggesting a systematic change in the source of plant-available N (organic/NH4+ versus NO3−) with climate. Nitrogen isotopes reflect time integrated measures of the controls on N storage that are critical for predictions of how these ecosystems will respond to human-mediated disturbances of the global N cycle.