Top Meter Research Articles

Soil organic carbon in savannas decreases with anthropogenic climate change

Climate models indicate that climate change is likely to affect carbon (C) cycling in drylands, particularly savannas, but the magnitude and direction of change are not fully understood. In this study, we used the Century model to analyze how net primary productivity (NPP), soil respiration and soil C sequestration would respond to an increase in atmospheric CO2 and soil temperature. We also assessed the coupled effects of precipitation and temperature change on C dynamics under future climatic conditions, as well as the decoupled effects of each of the climate variables under three IPCC climate scenarios; historical, Representative Concentration Pathway 2.6 (RCP2.6) and RCP8.5. An increase in soil temperature results in loss of soil organic C (SOC), whereas doubling atmospheric CO2 concentration causes an increase in SOC. The increase in air temperature causes soil respiration to increase, while it causes NPP to decrease. We calculated the total SOC in the Kalahari savannas to be 0.9 Pg C (1Pg=1015g) in the top meter, and the rate of SOC loss due to anthropogenic climate change to be ~1.1TgCyr−1 (1Tg=1012g) and ~2.0TgCyr−1 under RCP2.6 and RCP8.5, respectively until the end of this century. If extrapolated to the global extent of savannas, our results imply net SOC loss of at least ~28.4TgCy−1 and 64.1TgCyr−1 under RCP2.6 and RCP8.5, respectively. The rapid loss of C from dryland soils predicted by Century could accelerate global warming and strengthen positive feedback mechanisms between climate change and processes controlling SOC. Our results vividly support the positive feedback between the SOC and atmospheric C cycles and further indicate that these feedbacks are not adequately accounted for in existing Earth System Models (ESM) that are part of CMIP5. Revisions to these ESM would appear necessary to adequately account for this positive feedback.

Mineralogy, Chemical Characteristics and Upgrading of Beach Ilmenite of the Top Meter of Black Sand Deposits of the Kafr Al‐Sheikh Governorate, Northern Egypt

AbstractEgyptian beach ilmenite occurs in a relatively high content in the naturally highly concentrated superficial black sand deposits at specific beach zones in the northern parts of the Nile Delta at Rosetta. Microscopic study shows that the ilmenite occurs as fresh homogeneous black or heterogeneous multicoloured altered grains and exhibits three types (homogeneous, exsolved and altered) of ilmenite varieties. XRD data of ilmenite indicates their association with minor hematite and quartz, whereas leucoxene shows its association with Nb‐rutile, pseudorutile and hematite. Grain size distribution suggests a very fine sand size of >89% and 80% and a fine sand size of 10.5% and 18.3% for fresh and altered ilmenites, respectively. The density of fresh, altered ilmenite and leucoxene concentrates varies from 2.70, 2.50 to 2.40 ton/m3, suggesting a gradual decrease from high grade fresh to leucoxene and consistent with variation in magnetic susceptibility as a consequence of the leaching of iron. Mass magnetic susceptibility reveals 97.6% of ilmenite and 92% of the altered form are obtained at 0.20 and 0.48 ampere. Fresh ilmenite exhibits variable TiO2 (47.18%) and Fe2O3T (46.10%) with minor MnO, MgO and Cr2O3 (1.22, 1.10 and 0.51%). The altered ilmenite is higher in TiO2 (76.16%) and SiO2 (4.68%) and lower in Fe2O3T (14.45%), MnO, MgO and Cr2O3 (0.39, 0.52 and 0.11%) compared with the fresh form. Three concentrates of ilmenites (G1, G2 and G3) were prepared from crude ore using a Reading cross belt magnetic separator under different conditions, revealing a gradual increase of TiO2, SiO2, Al2O3 and CaO accompanied by a decrease of Fe2O3T, MgO and Cr2O3 with repetition of the separation processes. Several ore dressing techniques were carried out to upgrade the ilmenite concentrate.

CO2 released by carbonate sediment production in some coastal areas may offset the benefits of seagrass “Blue Carbon” storage

AbstractSeagrass ecosystems have been identified as long‐term carbon sinks whose conservation could serve as a tool to mitigate carbon emissions. Seagrasses alter landscapes in a way that stimulates carbon biosequestration, but discussions of their role in atmospheric CO2 mitigation disregard the co‐occurring inorganic carbon cycle, whose antagonist effect on CO2 sequestration can buffer and potentially outweigh the effects of Corg production on net carbon exchange with the atmosphere. This study examines the extent of both organic carbon (Corg) and inorganic carbon (Cinorg) stocks as proxies for long‐term production and calcification in the poorly studied seagrass meadows of southeastern (SE) Brazil and compares values to Florida Bay (U.S.A.), a well‐studied system known for both high autotrophy and calcification, representing extremes of CaCO3 soil content. Seagrass soils in SE Brazil contain an average of 67.6 ± 14.7 Mg Corg ha−1 in the top 1 m, compared to an average of 175.0 ± 20.4 Mg Corg ha−1 for their counterparts in Florida Bay. Cinorg as CaCO3 in SE Brazil averaged 141.5 ± 60.0 Mg Cinorg ha−1 in the top meter of soil while the warmer, calcification‐promoting waters of Florida Bay had higher soil Cinorg areal stock, averaging 754.6 ± 26.7 Mg Cinorg ha−1. When the CO2 evasion related to CaCO3 production is considered, seagrass ecosystems with high CaCO3 content may have CO2 sequestered via Corg accumulation negated by CO2 produced by calcification. These findings prompt the reconsideration of carbon inventory methods and encourage regionally‐ and community‐specific assessments of CO2 sequestration abilities of seagrass ecosystems.

Oyster reefs as carbon sources and sinks.

Carbon burial is increasingly valued as a service provided by threatened vegetated coastal habitats. Similarly, shellfish reefs contain significant pools of carbon and are globally endangered, yet considerable uncertainty remains regarding shellfish reefs' role as sources (+) or sinks (-) of atmospheric CO2 While CO2 release is a by-product of carbonate shell production (then burial), shellfish also facilitate atmospheric-CO2 drawdown via filtration and rapid biodeposition of carbon-fixing primary producers. We provide a framework to account for the dual burial of inorganic and organic carbon, and demonstrate that decade-old experimental reefs on intertidal sandflats were net sources of CO2 (7.1 ± 1.2 MgC ha-1 yr-1 (µ ± s.e.)) resulting from predominantly carbonate deposition, whereas shallow subtidal reefs (-1.0 ± 0.4 MgC ha-1 yr-1) and saltmarsh-fringing reefs (-1.3 ± 0.4 MgC ha-1 yr-1) were dominated by organic-carbon-rich sediments and functioned as net carbon sinks (on par with vegetated coastal habitats). These landscape-level differences reflect gradients in shellfish growth, survivorship and shell bioerosion. Notably, down-core carbon concentrations in 100- to 4000-year-old reefs mirrored experimental-reef data, suggesting our results are relevant over centennial to millennial scales, although we note that these natural reefs appeared to function as slight carbon sources (0.5 ± 0.3 MgC ha-1 yr-1). Globally, the historical mining of the top metre of shellfish reefs may have reintroduced more than 400 000 000 Mg of organic carbon into estuaries. Importantly, reef formation and destruction do not have reciprocal, counterbalancing impacts on atmospheric CO2 since excavated organic material may be remineralized while shell may experience continued preservation through reburial. Thus, protection of existing reefs could be considered as one component of climate mitigation programmes focused on the coastal zone.

Separation, upgrading, and mineralogy of placer magnetite in the black sands, northern coast of Egypt

The black sand magnetite is a raw material to produce iron and steel. High content magnetite in the top meter beach sands of the north-west of El-Burullus Lake of Egypt is separated and mineralogically and geochemically investigated. Magnetite content obtained by laboratory techniques varies between 2.92 and 0.11 wt%, with 1.15 wt% average. It represents about one third of the total economic heavy mineral (31.6 wt%). The western and northern halves of the study area are richer in magnetite content (1.33–1.51 wt%) than the eastern and southern halves (0.80–0.98 wt%). On the other hand, the average value of magnetite obtained by ore dressing techniques is 1.13 wt% with a recovery of 98.3 wt%. About 306 samples at a 1-m depth of a grid pattern 200 m × 200 m nearly parallel and perpendicular to the shoreline were covered the area. Microscopic study shows that the magnetite concentrate occurs as irregular, octahedral, or abnormal spherical grains and represented either by fresh homogeneous black or heterogeneous exsolved grains with ilmenite. XRD data indicate that the sample is composed mainly of magnetite grains. Grain size distribution suggests a very fine sand size modal class of more than 93 wt% of the particles. The fine sand size class contains 5.21 wt% magnetite grains. The abnormal spherical grains are mostly more coarser than the associated magnetite concentrate, so they are easily concentrated by sieving process. The contents of iron oxide and titanium dioxide of octahedral magnetite vary from 73.3 to 91.5 and 2.0 to 16.0 wt%, with 85 and 6.0 wt% averages, respectively. On the other hand, these contents of the abnormal spherical one are variable from 81.4 to 95.6 wt%, with 90.3 wt% averages for iron oxide and from 0.3 to 4.0 wt%, with 1.3 wt% average for titanium dioxide. The tonnage of magnetite using laboratory and ore dressing techniques is 182,850 and 179,670 t, respectively. The chemical data of the investigated magnetite suggest their basic volcanic rock origin of the Blue Nile provenance and can be considered as a good indicator for the volcanic source of the Ethiopian plateau.

INFERRED RESOURCES AND ENVIRONMENTAL IMPACT OF THE TOP METER BLACK SANDS IN THE COASTAL AREA OF MEDITERRANEAN SEA BETWEEN DAMIETTA SEA PORT AND GAMASA CITY, EGYPT.

The study area of the Mediterranean Sea Coast was covered by seawater containing detritus sediments transported by White Nile, Blue Nile and Atbara Rivers composed from amphibolite; granulites grade metamorphic rocks of the central African province and alkali basalts of Ethiopian high lands. The study area exposed to river environment, because of clay and organic matter contents in the raw sands. The apparent specific gravity of the top meter in the studied sediments ranged from 1.36 to 1.71 gm. /cm3 with an average1.533625 gm. /cm3. These sediments contain heavy economic minerals such as magnetite, ilmenite, leucoxene, rutile, monazite, zircon, garnet, cassiterite, xenotime, thorite, thorianite, gold, copper, zinc and lead, and light minerals such as quartz, feldspars, amphiboles, pyroxenes, mica,…. . The heavy minerals have the highest content in the middle area and the lowest content towards west in the study area. The radiometric measurements of the study area are important to determine the human radiation exposure and detect the radiation doses by converting the concentrations of Radium (ppm), equivalent Uranium (ppm), equivalent Thorium (ppm) and Potassium percent into absorbed dose and effective dose rates. The calculated values of absorbed dose and effective dose rates were found below the world average values according to (UNSCEAR, 2000). Therefore, there are no side effects to the human beings or the other habitants found near the study area and it is recommended using the beach sands in this area for building constructions and engineering with taking into consideration increasing the ventilations in these buildings to reduce radon emissions if present.

Modeled CO2 Emissions from Coastal Wetland Transitions to Other Land Uses: Tidal Marshes, Mangrove Forests, and Seagrass Beds

The sediments of coastal wetlands contain large stores of carbon which are vulnerable to oxidation once disturbed, resulting in high levels of CO2 emissions that may be avoided if coastal ecosystems are conserved or restored. We used a simple model to estimate CO2 emissions from mangrove forests, seagrass beds and tidal marshes based on known decomposition rates for organic matter in these ecosystems under either oxic or anoxic conditions combined with assumptions of the proportion of sediment carbon being deposited in either oxic or anoxic environments following a disturbance of the habitat. Our model found that over 40 years after disturbance the cumulative CO2 emitted from tidal marshes, mangrove forests and seagrass beds were approximately 70-80% of the initial carbon stocks in the top meter of the sediment. Comparison of our estimates of CO2 emissions with empirical studies suggests that 1) assuming 50% of organic material moves to an oxic environment after disturbance gives rise to estimates that are similar to CO2 emissions reported for tidal marshes; 2) field measurements of CO2 emissions in disturbed mangrove forests were generally higher than our modeled emissions that assumed 50% of organic matter was deposited in oxic conditions, suggesting higher proportions of organic matter may be exposed to oxic conditions after disturbance in mangrove ecosystems; and 3) the generally low observed rates of CO2 emissions from disturbed seagrasses compared to our estimates, assuming removal of 50% of the organic matter to oxic environments, suggests that lower proportions may be exposed to oxic conditions in seagrass ecosystems. There are significant gaps in our knowledge of the fate of wetland sediment carbon in the marine environment after disturbance. Greater knowledge of the distribution, form, decomposition and emission rates of wetland sediment carbon after disturbance would help to improve models.

On the Fundamental Causes of High Environmental Alkalinity (pH ≥ 9): An Assessment of Its Drivers and Global Distribution

AbstractVery alkaline environments exceeding calcite buffering are globally rare but conspicuous in many sedimentary plains of the World. While the deleterious effects of high alkalinity on soils are well understood, less agreement exists on its causes. We revise these causes to understand these exceptional environments and explain the pervasiveness of calcite buffering elsewhere. We argue that the injection of respired CO2 into stagnant hydrological systems subject to evaporative discharge is the key context for high alkalinization. The evolution of evaporites in nature reaches highly alkaline stages only when excess of (bi)carbonate with respect to divalent cations occurs. In most dry landscapes, evaporating groundwater solutions lose this condition as respired inorganic carbon (recharge zone supply) equilibrates with divalent cations from rocks (whole hydro‐trajectory supply). Groundwater in stagnant landscapes avoids this limitation owing to short/shallow trajectories sustaining (bi)carbonate excess until evaporative discharge zones are reached. Flat sedimentary landscapes that are (i) wet enough to develop stagnation and have shallow water tables but (ii) sufficiently dry to expose them to evaporative concentration should host very alkaline soils. This is confirmed with >9,000 soil profiles from the global WISE database, which shows that profiles with pH ≥ 9 in the top meter are 2·7% globally but 18% in areas with low slope (<0·05%, 25‐km radius, SRTM digital elevation model (SRTM DEM)) and semiarid–subhumid climate (annual precipitation to potential evapotranspiration ratio = 0·2–1, CRU database). Understanding how climate and vegetation change as well as irrigation practices influence hydrological stagnation and evaporative concentration may provide the key to manage very alkaline environments. Copyright © 2017 John Wiley & Sons, Ltd.

Phosphorus Sequestration in Sediments Along the Salinity Gradients of Chesapeake Bay Subestuaries

We used a sequential extraction technique to compare the forms and amounts of particulate phosphorus (PP) deposited in the top meter of sediment spanning salinities from 0 to 10 in three Chesapeake Bay subestuaries: the Potomac, Choptank, and Bush Rivers. Fe-bound P (i.e., citrate-bicarbonate-dithionite (CDB)-extractable P) was the most dynamic fraction of PP, dominating oligohaline (salinity <3) sediments, but declining to near zero with depth in the most saline sediments of all three subestuaries. In contrast, we previously found Fe-P dominating the sediment PP at salinities ranging 0–11 in the Patuxent subestuary. Particulate organic P was relatively constant with depth and salinity and became the dominant form of PP in the most saline sediments. Fe-P was not replaced with diagenetic authigenic carbonate fluorapatite. In all the subestuaries, Fe-P generally persisted in the oligohaline sediments to 1 m depth, where sediment ages ranged from 60 to 200 years, based on 210Pb dating. At one site, PP burial reflected changing P loads from a nearby wastewater treatment plant. About 67% of the PP discharged from the Potomac watershed at the head of tide was buried in the sediments in the upper reaches of that subestuary. The form and amount of PP buried depended on the amount of CDB-extractable Fe in the sediments and on the distribution of sediment deposition along the salinity gradient. Even though CDB targets iron oxides, the predominant form of Fe in the sediments was ferrous iron suggesting that the predominant form of PP in oligohaline sediments may be ferrous phosphate.

Deep soil water dynamics in an undisturbed primary forest in central Amazonia: Differences between normal years and the 2005 drought

Understanding how Amazonian rainforests deal with extended droughts is critical in the face of changing climate. This research analyze the physical properties and the soil water dynamics of a deep soil profile in an area of primary forest in Central Amazonia to elucidate these processes under drought and non-drought conditions. Physical soil properties derived from soil cores exhibited a distinctive layer between 480 and 880 cm deep, characterized by higher microporosity and low plant water availability. In situ soil moisture measurements collected during the period from January 2003 through February 2006 and for depths ranging from 10 cm to 1430 cm suggest that, in the study site, the top 480 cm of the soil profile satisfied most of the transpirational demands in normal climatological years. However, during exceptionally dry periods, such as the 2005 drought, root uptake occur below 480 cm. As concluded by previous studies, most of the uptake is concentrated in the first meter of the soil profile: more than 40% of the total demand for transpiration is supplied by the top meter of soil. Since deep root uptake occurred at greater depths than normal during the 2005 drought, our results suggest that this is a fundamental mechanism to cope with prolonged droughts.

Dynamics of carbon sources supporting burial in seagrass sediments under increasing anthropogenic pressure

AbstractSeagrass meadows are strong coastal carbon sinks of autochthonous and allochthonous carbon. The aim of this study was to assess the effect of coastal anthropogenic pressure on the variability of carbon sources in seagrass carbon sinks during the last 150 yr. We did so by examining the composition of the sediment organic carbon (Corg) stocks by measuring the δ13Corg signature and C : N ratio in 210Pb dated sediments of 11 Posidonia oceanica seagrass meadows around the Balearic Islands (Spain, Western Mediterranean) under different levels of human pressure. On average, the top meter sediment carbon deposits were mainly (59% ± 12%) composed by P. oceanica derived carbon whereas seston contribution was generally lower (41% ± 8%). The contribution of P. oceanica to the total sediment carbon stock was the highest (∼ 80%) in the most pristine sites whereas the sestonic contribution was the highest (∼ 40–80%) in the meadows located in areas under moderate to very high human pressure. Furthermore, an increase in the contribution of sestonic carbon and a decrease in that of seagrass derived carbon toward present was observed in most of the meadows examined, coincident with the onset of the tourism industry development and coastal urbanization in the region. Our results demonstrate a general increase of total carbon accumulation rate in P. oceanica sediments during the last century, mainly driven by the increase in sestonic Corg carbon burial, which may have important implications in the long‐term carbon sink capacity of the seagrass meadows in the region examined.

The whole-soil carbon flux in response to warming

Soil organic carbon harbors three times as much carbon as Earth's atmosphere, and its decomposition is a potentially large climate change feedback and major source of uncertainty in climate projections. The response of whole-soil profiles to warming has not been tested in situ. In a deep warming experiment in mineral soil, we found that CO2 production from all soil depths increased with 4°C warming; annual soil respiration increased by 34 to 37%. All depths responded to warming with similar temperature sensitivities, driven by decomposition of decadal-aged carbon. Whole-soil warming reveals a larger soil respiration response than many in situ experiments (most of which only warm the surface soil) and models.

A method of estimating in situ salmon louse nauplii production at fish farms

Abundance and depth distribution of planktonic sea lice were investigated in rela- tion to hydrodynamics and diurnal solar insolation at a salmon farm in Sundalagið, Faroe Islands. Plankton surveys were conducted by surface tows with a plankton net around the farm and by using a plankton pump at 1, 4 and 6 m depth in a fish cage. The entire sample content was investigated under a stereomicroscope and sea lice were identified. Sea lice of the species Lepeophtheirus salmonis and Caligus elongatus were present at the farm. Nauplii dominated the planktonic stages (>95%) while copepodids were absent from most samples. The highest observed copepodid density was 0.3 ind. m �3 , which is within the range found in open water. No diurnal vertical distribution pattern was observed for salmon lice nauplii in the net cages, which were most abundant in the top meter of the water column, and density decreased with depth. At 1 m depth, nauplii density was inversely proportional to the current speed at the same depth. From this relation, and the abundance of adult female sea lice on the farmed fish, the in situ nau- plii production was calculated to be within 26�68 nauplii female �1 d �1 . The lower end of this range is similar to production rates suggested by laboratory studies at similar temperatures (7.8°C).

Bioturbation by the Fungus-Gardening Ant, Trachymyrmex septentrionalis.

Soil invertebrates such as ants are thought to be important manipulators of soils in temperate and tropical ecosystems. The fungus gardening ant, Trachymyrmex septentrionalis, is an important agent of biomantling, that is, of depositing soil excavated from below onto the surface, and has been suggested as an agent of bioturbation (moving soil below ground) as well. The amount of bioturbation by this ant was quantified by planting queenright colonies in sand columns consisting of 5 layers of different colored sand. The amount of each color of sand deposited on the surface was determined from April to November 2015. In November, colonies were excavated and the color and amount of sand deposited below ground (mostly as backfill in chambers) was determined. Extrapolated to one ha, T. septentrionalis deposited 800 kg of sand per annum on the surface, and an additional 200 kg (17% of the total excavated) below ground. On average, this mixes 1.3% of the sand from other layers within the top meter of soil per millennium, but this mixing is unlikely to be homogeneous, and probably occurs as "hotspots" in both horizontal and vertical space. Such mixing is discussed as a challenge to sediment dating by optically stimulated luminescence (OSL).

Attenuation of wind-induced pressure perturbations in alpine snow

ABSTRACTWindpumping has been identified as a process that could potentially enhance sublimation of surface snow at high forcing frequency and spawn air movement deeper in firn at lower frequencies. We performed an experiment to examine the relationship between high-frequency wind and pressure measurements within the top meter of an alpine snowpack and compared experimental results with two theoretical predictions. We find that both theoretical predictions underestimate high-frequency perturbation pressure attenuation with depth in the near-surface snowpack and the discrepancy between theory and measurement increases with perturbation pressure frequency. The impact of this result for near-surface snow is that potential enhanced sublimation will occur over a shallower snow depth than these two theories predict. Correspondingly, interstitial air mixing at depth in firn will be driven by lower frequencies than these two theories predict. While direct measurement of these energy-rich lower frequencies is beyond the scope of this paper, stationary pressure measurements validate the presence of a pressure field that could drive near-surface circulation.

Predictions of depth-to-ice on asteroids based on an asynchronous model of temperature, impact stirring, and ice loss

Water ice near the surface of main belt asteroids is gradually lost to space. A mantle of low thermal conductivity causes large surface temperature amplitudes, and thus increased cooling by thermal re-radiation, lowering temperatures well below the fast-rotator limit. A computational barrier for modeling this ice loss is the multi-scale character of the problem: accurate temperatures require many time steps within a solar day, but ice retreats slowly over billions of years. This barrier is overcome with asynchronous coupling: Models of temperature, ice loss, and impact stirring each use their own time steps and are coupled with one another. The model is applied to 1 Ceres and 7968 Elst-Pizarro. On Ceres, ice can be expected in the top half meter poleward of 60° latitude on both hemispheres, even if excursions of the axis tilt took place, and even in the presence of impact gardening. At the poles, ice can be expected within a centimeter of the surface. The retreating ice crust leads to emission of water from the surface, mainly at the equator; the gradually retreating ice supplies a water exosphere less dense than has been observed by the Herschel telescope. For Main Belt Comet Elst-Pizarro, depths to ice depend on the properties of the surface mantle. For a dust mantle estimated depths are on the order of a decimeter; for a rocky surface the depth at the pole is on the order of one meter. Hence, it could have been activated by a small impact that exposed buried ice.

Rain and channel flow supplements to subsurface water beneath hyper-arid ephemeral stream channels

Summary In hyper-arid regions, ephemeral stream channels are important sources of subsurface recharge and water supply for riparian vegetation, but few studies have documented the subsurface water content dynamics of these systems. This study examines ephemeral channels in the hyper-arid western Sonoran Desert, USA to determine how frequently water recharges the alluvial fill and identify variables that affect the depth and persistence of recharge. Precipitation, stream stage, and subsurface water content measurements were collected over a three-year study at six channels with varying contributing areas and thicknesses of alluvial fill. All channels contain coarse alluvium composed primarily of sands and gravels, and some locations also have localized layers of fine sediment at 2–3 m depth. Rain alone contributed 300–400 mm of water input to these channels over three years, but water content responses were only detected for 36% of the rain events at 10 cm depth, indicating that much of the rain water was either quickly evaporated or taken up by plants. Pulses of water from rain events were detected only in the top meter of alluvium. The sites each experienced ⩽5 brief flow events, which caused transient saturation that usually lasted only a few hours longer than flow. These events were the only apparent source of water to depths >1 m, and water from flow events quickly percolated past the deepest measurement depths (0.5–3 m). Sustained saturation in the shallow subsurface only developed where there was a near-surface layer of finer consolidated sediments that impeded deep percolation.

Variations in the Isotopic Composition of Dissolved Inorganic Carbon in the Unsaturated Zone of a Semi-Arid Region

A study of water and carbon isotopes was conducted in a bare plot in the unsaturated zone of the Yatir Forest in the northern Negev of Israel. Sediment cores were collected in three different seasons. Measurements include profiles of mineralogy, moisture and its δ 18 O and tritium content, dissolved inorganic carbon (DIC) and its δ 13 C (‰) and Δ 14 C (‰) content, and δ 13 C (‰) and Δ 14 C (‰) in the solid sediment. The profiles of moisture and δ 18 O in the cores show clearly the effect of evaporation. The tritium profile indicates infiltration of water (0.11 m yr –1 ). The source of carbon in the DIC is CO 2 released by biotic activity through roots of trees and of seasonal plants, which show seasonal variations, and by decay of organic debris. The δ 13 C (‰) profiles show clearly the chemical transition from dissolved CO 2 (δ 13 C = –22‰) to bicarbonate (δ 13 C = –14‰). At greater depth (–11.3‰), the δ 13 C becomes similar to the δ 13 C in the aquifer below (–12.5‰). The effect of secondary processes is evident in the profile of Δ 14 C in the DIC. It shows a clear decrease with depth due to exchange with the sediment at a rate of 10‰ yr –1 . Precipitation of carbon from the DIC on the sediment is 1.1 mg C L sed –1 yr –1 , negligible compared to the 28 g C in 1 L sed . In the solid sediment, there is a gradient in Δ 14 C carb at the top meter. The net precipitation of 14 C from the DIC on the sediment (0.25 to 1.1‰ yr –1 ), corrected for decay, cannot be observed in the deeper sediment. The presence of 14 C in the top 1 m of the sediment is explained by two possible processes: accumulation of 14 C-tagged dust (~0.05 mm yr –1 ) and/or long-term cumulative precipitation from the DIC. DOI: 10.2458/azu_rc.57.18356

Tropical soil nutrient distributions determined by biotic and hillslope processes

Understanding the drivers of nutrient distributions across heterogeneous tropical landscapes, and the implication of those distributions for ecosystem processes, remains a challenge in biogeochemistry. Guided by a high-resolution digital elevation model, we determined soil chemical characteristics, including rock-derived nutrient availability and total element concentrations, along 16 hillslopes with two different morphologies on a terra firme landscape in the lowland Peruvian Amazon. We assessed the role of geomorphic processes in rejuvenating soils, and the role of biotic processes in retaining nutrients in surface soils along toposequences. Our results show that the chemical weathering state of subsurface soils decreases down convex slopes, suggesting a role for erosion-driven rejuvenation. Total phosphorus (P) and calcium (Ca) are less depleted in surface soils than subsurface soils, in contrast to the vertical patterns observed for total magnesium (Mg) and potassium (K). Exchangeable Ca and Mg are highly concentrated in surface soils relative to the total available in the top meter of soil, which remains true as the total amount of these nutrients available in the top meter of soil increases along convex hillslopes. The combined results suggest that Ca is especially tightly cycled by this tropical forest ecosystem, and that steep, eroding hillslopes play a major role in the distribution and rejuvenation of rock-derived nutrients in regions far from marine aerosol and dust inputs. Geomorphically-mediated distributions of these nutrients, in turn, may underpin spatial variation in tropical forest carbon dynamics.

On the rebound: soil organic carbon stocks can bounce back to near forest levels when agroforests replace agriculture in southern India

Abstract. Tropical agroforestry has an enormous potential to sequester carbon while simultaneously producing agricultural yields and tree products. The amount of soil organic carbon (SOC) sequestered is influenced by the type of the agroforestry system established, the soil and climatic conditions, and management. In this regional-scale study, we utilized a chronosequence approach to investigate how SOC stocks changed when the original forests are converted to agriculture, and then subsequently to four different agroforestry systems (AFSs): home garden, coffee, coconut and mango. In total we established 224 plots in 56 plot clusters across 4 climate zones in southern India. Each plot cluster consisted of four plots: a natural forest reference, an agriculture reference and two of the same AFS types of two ages (30–60 years and &gt; 60 years). The conversion of forest to agriculture resulted in a large loss the original SOC stock (50–61 %) in the top meter of soil depending on the climate zone. The establishment of home garden and coffee AFSs on agriculture land caused SOC stocks to rebound to near forest levels, while in mango and coconut AFSs the SOC stock increased only slightly above the agriculture SOC stock. The most important variable regulating SOC stocks and its changes was tree basal area, possibly indicative of organic matter inputs. Furthermore, climatic variables such as temperature and precipitation, and soil variables such as clay fraction and soil pH were likewise all important regulators of SOC and SOC stock changes. Lastly, we found a strong correlation between tree species diversity in home garden and coffee AFSs and SOC stocks, highlighting possibilities to increase carbon stocks by proper tree species assemblies.