Soil Pools Research Articles

Tree regeneration and soil responses to management alternatives in beetle-infested lodgepole pine forests

Recent mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have caused one of the most widespread and dramatic changes in forest condition in North American forests in more than a century and highlighted challenges facing resource managers. To address uncertainty regarding the consequences of post-harvest woody residue management on soil productivity and tree regeneration following MPB outbreaks in lodgepole pine-dominated forests we compared three treatment prescriptions (bole-only harvest, whole-tree harvest, and whole-tree harvest with scarification) and uncut stands. The study was replicated at twelve sites across a range of operational project areas and stand conditions in northern Colorado. Salvage logging generated a new cohort of lodgepole pine at densities far above the threshold considered adequate to develop into well-stocked stands (1700–2300 t ha−1 in logged compared to 537 t ha−1 in uncut areas). Regeneration density was generally highest in whole-tree harvested areas. Growth of planted and naturally regenerating lodgepole pine recruits was best in the bole-only, residue-retention treatment, where soil moisture and inorganic nitrogen supply was also highest. However, we found no indication that whole-tree harvesting lowered soil moisture, soil nitrogen supply or pools relative to uncut stands. The density of trees regenerating beneath uncut stands indicates that post-outbreak forest structure should recover without management in these forests. The cohort of trees that regenerated following MPB-related overstory mortality, but prior to harvesting, comprise the fastest-growing component of the growing stock and 30% of its density. The broader watershed-scale outcomes of these treatments and their implications for wildfire behavior and other effects remain uncertain. However, the soil and tree patterns we report during the initial post-treatment period inform on-going decisions regarding harvest and residue retention and create a platform to guide future forest management research.

Depth patterns and connections between gross nitrogen cycling and soil exoenzyme activities in three northern hardwood forests

Despite the enormous size of the organic nitrogen (N) pool contained in mineral subsoils, rates of N cycling and soil exoenzyme activities are rarely measured in soils below 10 or 20 cm depth. Furthermore, assumed relationships between N mineralization rates and the activities of various decomposition exoenzymes are poorly characterized. We measured rates of gross and net N mineralization and nitrification as well as the potential activities of hydrolytic and oxidative enzymes at five soil depths (forest floor to 50 cm) in Spodosols at three hardwood forests of varying age (45 and 100 years post-harvest and old growth) at and near the Hubbard Brook Experimental Forest in New Hampshire, USA. As expected, rates of N cycling and potential enzyme activities per unit soil mass correlated strongly with soil carbon (C) concentration, and these parameters declined exponentially with increasing soil depth. After normalization per unit soil organic matter, N cycling rates and specific enzyme activities generally decreased little with depth within the mineral soil. Gross N mineralization rates correlated with specific activities of those enzymes that hydrolyze cellulose (β-glucosidase, cellobiohydrolase) and N-rich glucosamine polymers (N-acetylglucosaminidase), but not those that degrade protein or more complex C compounds. Hence, gross N cycling appear associated with the N released during microbial N recycling, rather than from decomposition of soil organic matter. Across the three stands, the youngest had a larger ratio of N- to-phosphorus-acquiring enzyme activities, indicating a greater N demand in younger than older forests. For all three stands, mineral soil below 10 cm contributed 30–53% of total gross and net N cycling per unit area to 50 cm depth. Overall, even though microbial N cycling and enzyme activities per unit soil mass decreased with depth, microbial processes in subsoils contributed substantially to ecosystem-scale gross N fluxes because of the sustained microbial activity per unit soil organic matter at depth and the large size of the organic matter pool in the mineral soil. These results support the inclusion of often-ignored mineral subsoils and microbial N recycling in both ecosystem N budgets and in model simulations, due to their contribution to soil N fluxes and the importance of microbial N dynamics in forest stands.

Enhanced Lateral Exchange of Carbon and Nitrogen in a Coastal Wetland With Invasive Spartina alterniflora

AbstractLateral movements of materials and energy in coastal wetlands, due mainly to tidal activities, have been recognized as key processes in understanding the biogeochemical cycles of ecosystems. However, our understanding of the roles of lateral movement in shaping ecosystem functions remains limited. Here we quantified the effects of lateral sediment transport on total carbon (C, inorganic + organic) and nitrogen (N) pools in plants and soils in two dominant wetland types: invasive Spartina alterniflora (Spartina) marshes and native Phragmites australis (Phragmites) marshes in coastal Shanghai of the Yangtze Estuary. We found that the accreted sediments across the water‐marsh gradients caused by lateral movement resulted in contrasting C and N contents between the two communities. The sediment load and C and N pools in the plants and soils of the Spartina marshes were significantly higher than those in the adjacent Phragmites marshes. The shifts in species composition and community structure not only altered the C and N balance but also enhanced the ecosystem net primary productivity. Our findings highlight the importance of lateral transport in altering ecosystem structure after Spartina invasion. The ecosystem C and N pools were significantly higher in the invaded ecosystems than in the native community. Our study also reveals that the plant density and structures can alter tidal hydrodynamics and the lateral transportations of sediments, which in turn influence ecosystem C and N cycle. The C accumulation processes of the native and invaded marshes were further complicated by the contrasting productivities of the ecosystems.

Nitrogen addition change soil N pools with litter removal or not in subtropical forest

ABSTRACT There are many nitrogen (N) pools in soil, so their availability and different status can give information about bulk soil response to N deposition. However, the different size of N pools in forest soils and the relationship between them have not been well studied under N deposition when considering the role of litter. Here soil in an N-deposition experiment carried out for 5 years in a broad-leaved forest was used as an object to study the response of N pools to N deposition by stepwise extraction using water or solutions containing 0.5 M K2SO4, 2.5 M H2SO4 (LPI), or 13 M H2SO4 (LPII), and calculation of recalcitrant (RC) N pool. Under N control (CT), soil with the presence of litter had a higher N of 23.8–106.8% in the first four pools, but lower of 80.6% in recalcitrant N pool compared with soil with the absence of litter. In the absence of litter, N addition increased soil N in labile pool but decreased N in the RC pool compared to CT and these impacts were greater at high added N (HN) than low-added N (LN) rates. However, in the presence of litter, LN increased the amount of N in the K2SO4- extracted pool and HN reduced that in the water extracted pool. Additionally, LN and HN increased TN in the RC pool and HN increased the total soluble N (TSN) in the LPI and LPII pool. N changes in the water extraction pool were attributed to inorganic N, whereas they were NH4 + and soluble organic N (SON) in the K2SO4-extracted, LPI, and LPII pools. In the presence of litter, HN increased the SON concentration in the K2SO4, LPI, and LPII extractions; thus, SON may be a potentially important N form for N availability. These results suggested that N additions improve the accumulation of N in RC pool with the presence of litter. The different effects of N additions on soil N pool or N form in each pool depend on litter present or not.

Threshold for labile phosphate in a sandy acid sulfate soil

In acid soils, plant availability of fertiliser P is affected by phosphate binding to Fe or Al oxide minerals. However, little is known about the P pools in acid sulfate soils after phosphate fertilisation, or the capacity of the oxyhydroxy sulfate mineral jarosite (KFe3(OH)6(SO4)2) found in these soils, to bind or release added phosphate. Sulfuric acid sulfate soil (initial pH 3.5, adjusted to 5.5) was amended with phosphate at 0, 96, 385, 578 and 770 mg P kg−1 and incubated under submerged conditions. Soil phosphate pools (labile phosphate, moderately labile phosphate, non-labile phosphate and residual phosphate) and amorphous Fe/Al were measured after two and four weeks. Phosphate sorption and its release were determined in incubated soils and jarosite, separately. In soil, maximum phosphate sorption was about 350 mg phosphate kg−1. Labile phosphate was higher than the control (without phosphate addition) at phosphate rates above 350 mg P kg−1 and was more than 50% of added phosphate. The length of submergence had little effect on phosphate pools. Below the threshold of 350 mg phosphate kg−1, absorbed phosphate was strongly bound to the soil whereas 50% of sorbed phosphate was released above the threshold. The high phosphate binding capacity of jarosite (maximal 360 mg phosphate kg−1) confirmed the importance of Fe oxyhydroxy sulfate minerals in P binding in acid soils. However, at maximal phosphate binding, up to 20% of bound phosphate could be released, suggesting that phosphate bound to jarosite may become plant available. The results of this study will help inform fertiliser requirements for crop growth or remediation of acid sulfate soils using plants.

The effect of arsenic on soil intracellular and potential extracellular β-glucosidase differentiated by chloroform fumigation

Arsenic (As) contamination of soil is a global issue of serious ecological and human health concern. For better use of soil enzymes as biological indicators of As pollution, the response of soil β-glucosidase in different pools of soil (total, intracellular and potential extracellular) to As(V) stress was investigated. Chloroform fumigation method was employed to distinguish the intracellular and potential extracellular β-glucosidase in three soils. The intracellular and potential extracellular β-glucosidase accounted about 79% and 21% of the total β-glucosidase activity in the tested soils. Moreover, it was found that the response of these three enzyme pools to As(V) pollution was different. Under the stress of 400 mg kg−1 As(V), the β-glucosidase activities decreased by 69%, 79%, and 28% for the total, intracellular and potential extracellular pools, respectively. The calculated median ecological dose (ED50) showed the highest value for potential extracellular β-glucosidase (19.55–27.63 mg kg−1 for total, 18.49–27.42 mg kg−1 for intracellular, and 32.27–52.69 mg kg−1 for potential extracellular β-glucosidase). As(V) exhibited an uncompetitive inhibition for total and intracellular β-glucosidase and non-competitive inhibition for potential extracellular enzyme. The inhibition constant (Kiu) is biggest for potential extracellular β-glucosidase among the three enzyme pools (0.61–0.79 mmol L−1 for total, 0.34–0.36 mmol L−1 for intracellular, and 4.01–23.90 mmol L−1 for potential extracellular β-glucosidase). Thus, compared to potential extracellular β-glucosidase, the total and intracellular β-glucosidases are more suitable for their use as sensitive indicators of As(V) pollution.

Soil Organic Carbon Pools under <i>Terminalia chebula</i> Retz. based Agroforestry Systemin Himalayan Foothills, Indiax

Knowledge of carbon (C) pools in soils is helpful in devising practices for efficient carbon management in intensive cropping systems. Carbon fractions of soil organic carbon are used asan indicator for land-use induced change in soil quality. The present study evaluated carbon pools under Terminalia chebula(chebulic myrobalan) based agroforestry system supplied with different nutrient sources, viz. farmyard manure, poultry manure, vermicompost, wheat straw and inorganic fertilizer (NPK @ 100:80:60). Carbon fractions, viz. very labile (C 1 frac ), labile (C 2 frac ), less labile (C 3 frac ) and non-labile (C 4 frac ), were analysed at 0–15 and 15–30 cm soil depth. The higher value of C 1 frac (13.8%), C 2 frac (4.8%), C 3 frac (8.3%) and C 4 frac(11.1%) were recorded under agroforestry as compared to open system. Among the nutrient sources, all the carbon fractions were higher under 100% integrated nutrient sources as compared to controlled treatment. Microbial biomass carbon (MBC) was recorded higher (298.31 μg g –1 ) under agroforestry system compared to the open system (290.63 μg g –1 ) at 0–15 cm. Among the different nutrient sources, higher MBC (458.66 μg g –1 ) at 0–15 cm and lower (340.59 μg g –1 ) at 15–30 cm soil depth was recorded in 100% integrated treatment.Thus, agroforestry-based land-use types and integrated nutrient management are more efficient for soil health and carbon management in Himalayan foothills.

Factors Governing Total and Permanganate Oxidizable C Pools in Agricultural Soils from Southern Italy

The present investigation was aimed to quantify the three principal components of the soil carbon (C) stock, namely inorganic, organic and permanganate oxidizable, in 0–5 cm and 5–30 cm soil layers, of the main Mediterranean agricultural land coverages: olive grove, olive forest, citrus grove, vineyard, arable irrigated, arable rainfed and natural soil covered by Mediterranean scrub and garrigue. We assessed the contribution of soil properties and climatic variables on soil TOC and POxC by Pearson’s correlations, multiple linear regressions, principal component analyses and variance partitioning. NAT and both olive cropping systems showed the highest TOC concentration while the other land covers showed values ranging between 8.0 and 26.6 g kg−1. Soil POxC represented between 0.5% and 2.2% of the total soil C. A large number of interrelated factors govern soil C accumulation in Mediterranean agroecosystems. Rainfall and temperature had a selective effect on soil C distribution between the sampling areas, while among soil properties, texture exerted a prominent effect, resulting in positive effects from clay and negative effects from sand. Soil POxC concentration showed a similar trend to soil TOC in all land uses although on each of them represent a different TOC portion.

Effect of Land Use on Organic Carbon Storage Potential of Soils with Contrasting Native Organic Matter Content

This study aimed to determine the impact of land use on organic carbon (OC) pools of soils with contrasting native organic matter (OM) content. Surface (0–15 cm) soils of four land uses (cropland, orchard, grassland, and fallow) were collected from four agroecological zones (AEZs) of Bangladesh with different OM content (AEZ-7: very low, −3: low, −9: medium, and −5: high). Bulk soils were physically fractionated into particulate and mineral associated OM (POM and MOM: >53 and <53 µm, respectively). Both bulk and fractionated soils were analyzed for OC and nitrogen (N). Among the land uses, undisturbed soils (grassland and fallow land) had significantly higher total OC (0.44–1.79%) than disturbed soils (orchard and cropland) (0.39–1.67%) in all AEZs. The distribution of OC and N in POM and MOM fractions was significantly different among land uses and also varied with native OM content. In all AEZs, cropland soils showed the lowest POM-C content (0.40–1.41%), whereas the orchard soils showed the highest values (0.71–1.91%). The MOM-C was highest (0.81–1.91%) in fallow land and lowest (0.53–1.51%) in orchard, and cropland had a moderate amount (0.70–1.61%). In croplands, distribution of a considerable amount of OC in the MOM pool was noticeable. These findings reveal that total OC in soils can be decreased with cultivation but does not inevitably indicate the loss of OC storage in the stable pool. Carbon storage potential of soils with both high- and low-native OM contents can be increased via proper land use and managements.

Organic carbon fractional distribution and saturation in tropical soils of West African savannas with contrasting mineral composition

Savannas are dry tropical biomes characterized by high above- and belowground biomass production, but at the same time there is extensive C loss by fire regimes. Soil-mediated responses are crucial in biogeochemical cycling and the role of soil minerals may be important in controlling organic carbon (OC) storage and stabilization. The objective of this study was to determine OC fractional distribution and saturation capacity in three soil groups (Vertisols, Lixisols, Luvisols) that are characterized by specific and different 1:1 and 2:1 clay mineral dominance. For each soil group, top- and subsoils (0–10 cm, 40–60 cm) were sampled in protected Soudanian savannas across Burkina Faso and fractionated into free and aggregate-occluded particulate organic matter (fPOM, oPOM) and mineral-associated organic matter (mOM). The results showed that oPOM and mOM were significantly affected by soil groups and topsoil/subsoil layers. We consider this to be due to interactions with fine particles (clay, fine and medium silt), the presence of exchangeable Ca and Mg and extractable Al (D) and Fe (D) as revealed by significant positive correlations. We found that oPOM (13–29% of total OC) and mOM < 20 µm (54–76%) were the most important OC pools in these tropical savanna soils. Vertisols exhibited the highest OC contents and held three times more OC in the fine mineral fraction mOM < 20 µm than Lixisols and Luvisols. Luvisols were affected by mineral illuviation processes which lowered their OC contents. Overall, topsoils contained three times higher OC amounts of mOM < 20 µm than subsoils. Indications were found that all investigated topsoils are below OC saturation, likely due to prescribed fire and soil erosion effects, while the OC saturation deficit was important in Vertisols (−49 to −65%) and Lixisols (−41 to −45%). We conclude that tropical savanna soils have a high protective capacity for OC in top- and subsoils but require adequate management in order to increase OC storage.

Pronounced effect of farmyard manure application on P availability to rice for paddy soils with low total C and low pH in the central highlands of Madagascar

ABSTRACT Farmyard manure (FYM) – mixtures of animal droppings, crop residues, and fodder that are piled nearby homesteads – is a major nutrient source for smallholder farmers in Sub-Saharan Africa. However, its application effect has not been fully understood on typical P-deficient soils in tropics and in particular under anaerobic conditions. This study assessed the effect of FYM on irrigated rice in relation to soil properties – oxalate-extractable P (POx), pH, and total C (TotC) – that are important indicators of soil P deficiency in the region. The first pot experiment was conducted with a factorial combination of FYM (0 and 20 g kg–1) and mineral P (0 and 100 mg kg–1) applications using six paddy soils differing in the aforementioned soil properties. The second pot experiment was conducted in a factorial combination of FYM and mineral P using the isotope dilution technique. In both experiments, the effect of FYM application on biomass and P uptake of rice per P applied was nearly equivalent to that of mineral P and was greater in soils with lower TotC and lower pH with negligible effect of POx. The isotope tracing suggested that the FYM application might increase rice P uptake by solubilizing non-labile P pools in soils while mineral P was directly used by rice from labile P pools. The results indicated that the FYM should be most effective in soils with low TotC and low pH, and its application could enable the use of insoluble P pools in soils and enhance P uptake of rice under P-deficient and anaerobic conditions.

Carbon and Nitrogen Stocks in Three Types of Larix gmelinii Forests in Daxing’an Mountains, Northeast China

Studying carbon and nitrogen stocks in different types of larch forest ecosystems is of great significance for assessing the carbon sink capacity and nitrogen level in larch forests. To evaluate the effects of the differences of forest type on the carbon and nitrogen stock capacity of the larch forest ecosystem, we selected three typical types of larch forest ecosystems in the northern part of Daxing’an Mountains, which were the Rhododendron simsii-Larix gmelinii forest (RL), Ledum palustre-Larix gmelinii forest (LL) and Sphagnum-Bryum-Ledum palustre-Larix gmelinii forest (SLL), to determine the carbon and nitrogen stocks in the vegetation (trees and understories), litter and soil. Results showed that there were significant differences in carbon and nitrogen stocks among the three types of larch forest ecosystems, showing a sequence of SLL (288.01 Mg·ha−1 and 25.19 Mg·ha−1) &gt; LL (176.52 Mg·ha−1 and 14.85 Mg·ha−1) &gt; RL (153.93 Mg·ha−1 and 10.00 Mg·ha−1) (P &lt; 0.05). The largest proportions of carbon and nitrogen stocks were found in soils, accounting for 83.20%, 72.89% and 64.61% of carbon stocks and 98.61%, 97.58% and 96.00% of nitrogen stocks in the SLL, LL and RL, respectively. Also, it was found that significant differences among the three types of larch forest ecosystems in terms of soil carbon and nitrogen stocks (SLL &gt; LL &gt; RL) (P &lt; 0.05) were the primary reasons for the differences in the ecosystem carbon and nitrogen stocks. More than 79% of soil carbon and 51% of soil nitrogen at a depth of 0–100 cm were stored in the upper 50 cm of the soil pool. In the vegetation layer, due to the similar tree biomass carbon and nitrogen stocks, there were no significant differences in carbon and nitrogen stocks among the three types of larch forest ecosystems. The litter carbon stock in the SLL was significantly higher than that in the LL and RL (P &lt; 0.05), but no significant differences in nitrogen stock were found among them (P &gt; 0.05). These findings suggest that different forest types with the same tree layer and different understory vegetation can greatly affect the carbon and nitrogen stock capacity of the forest ecosystem. This indicates that understory vegetation may have significant effects on the carbon and nitrogen stocks in soil and litter, which highlights the need to consider the effects of understory in future research into the carbon and nitrogen stock capacity of forest ecosystems.

Farm-scale carbon and nitrogen fluxes in pastoral dairy production systems using different nitrogen fertilizer regimes

The nitrogen (N) fertilizer application rate (kg ha−1 year−1) in pastoral dairy systems affects the flow of N through the soil, plant and animal pools of the system. With better understanding of the magnitude of these pools and their fluxes, dairy systems could be managed to improve N use efficiency, therefore reducing losses to the environment. A study with three levels of N fertilizer, 0 (N0), 150 (N150) and 300 (N300) kg N ha−1 year−1, was conducted in the Canterbury region of New Zealand from 1 June 2017 till 31 May 2018. Farm measurements, e.g. pasture and milk production, were used to calibrate three different farm-scale models, DairyNZ’s Whole Farm Model, DairyMod, and Overseer®. The models were used to extrapolate periodic farm measurements to predictions of carbon (C) and N pools and fluxes on an annual basis. Pasture and milk production per hectare increased from N0 to N300 by 70 and 58%, respectively. There was a concomitant increase in farm-gate N surplus (input–output) of 43%, resulting in predicted increases in N leaching and greenhouse gas emissions of 72 and 67%, respectively. By increasing N fertilizer from 0 to 300 kg N ha−1 year−1, 53% more feed N flowed through the dairy herd with surplus N deposited as urinary N increasing by 49%. Plant uptake and soil immobilization increased by 58 and 343%, respectively, but not enough to avoid substantial increases in leaching and emission losses. Carbon flux through the soil system increased through increased litter and faecal deposition, but with very little C sequestration because of accelerated microbial respiration rates.

Assessment of Organic Carbon Stocks in Forest Soils on a Regional Scale

Results of the comparison of organic carbon stocks in the automorphic and semihydromorphic forest soils are discussed. The database includes information on 289 soil profiles in the forest zone of European Russia. On the average, the total organic carbon pool (including forest litter) in semihydromorphic soils is three times higher than that in automorphic soils. The difference decreases with an increase in the thickness of the considered soil layer from 3.2 times for the layer of 0–30 cm to 2.6 times for the layer of 0–100 cm. The greatest difference in carbon stocks is noted for the organic horizons; in semihydromorphic soils, the carbon storage in them averages 73 ± 8.2 t C/ha; in automorphic soils, it is seven times smaller. The neglect of the contribution of semihydromorphic soils results in underestimation of the total soil organic carbon stock of the region. The degree of underestimation depends on the soil cover pattern of particular areas and on the thickness of the considered soil layer. Calculations made for Karelia show that the underestimation of organic carbon stocks in various landscapes ranges from 10 to 40%. For the total area of this republic, it is estimated at 22, 19, and 13% for the soil layers of 0–30, 0–50, and 0–100 cm, respectively. A comparative analysis of known pedotransfer functions for calculating soil bulk density shows that the best results (RMSE = 0.15; R2 = 0.36) for mineral horizons of forest soils in European Russia are provided by the pedotransfer function suggested by O.V. Chestnykh and D.G. Zamolodchikov.

Methodology and validation of a new tandem mass spectrometer method for the quantification of inorganic and organic 18O-phosphate species.

Phosphorus (P) fertilizers are crucial to achieve peak productivity in agricultural systems. However, the fate of P fertilizers via microorganism incorporation and the exchange processes between soil pools is not well understood. 18Oxygen-labelled phosphate (18O- Pi) can be tracked as it cycles through soil systems. Our study describes biological and geochemical P dynamics using a tandem mass spectrometry (MS/MS) method for the absolute quantification of 18O- Pi. Soil microcosms underwent three treatments: (i) 18O- Pi, (ii) unlabelled phosphate (16O- Pi) or (iii) Milli-Q control, dissolved in a bio-stimulatory solution. During a 6-week series the microcosms were sampled to measure P by Hedley sequential fractionation and DNA extraction samples digested to 3′-deoxynucleoside 5′-monophosphates (dNMP). A MS/MS attached to a HPLC analyzed each P-species through collision-induced dissociation. The resin-extractable and bicarbonate 18O- Pi and 16O- Pi fractions displayed similar precipitation and adsorption-desorption trends. Biotic activity measured in the NaOH and dNMP fractions rapidly delabelled 18O- Pi; however, the MS/MS measured some 18O that remained between the P backbone and deoxyribose sugars. After 6 weeks, the 18O- Pi had not reached the HCl soil pool, highlighting the long-term nature of P movement. Our methodology improves on previous isotopic tracking methods as endogenous P does not dilute the system, unlike 32P techniques, and measured total P is not a ratio, dissimilar from natural abundance techniques. Measuring 18O- Pi using MS/MS provides information to enhance land sustainability and stewardship practices regardless of soil type by understanding both the inorganic movement of P fertilizers and the dynamic P pool in microbial DNA.

Frequent burning causes large losses of carbon from deep soil layers in a temperate savanna

Abstract Fire activity is changing dramatically across the globe, with uncertain effects on ecosystem processes, especially below‐ground. Fire‐driven losses of soil carbon (C) are often assumed to occur primarily in the upper soil layers because the repeated combustion of above‐ground biomass limits organic matter inputs into surface soil. However, C losses from deeper soil may occur if frequent burning reduces root biomass inputs of C into deep soil layers or stimulates losses of C via leaching and priming. To assess the effects of fire on soil C, we sampled 12 plots in a 51‐year‐long fire frequency manipulation experiment in a temperate oak savanna, where variation in prescribed burning frequency has created a gradient in vegetation structure from closed‐canopy forest in unburned plots to open‐canopy savanna in frequently burned plots. Soil C stocks were nonlinearly related to fire frequency, with soil C peaking in savanna plots burned at an intermediate fire frequency and declining in the most frequently burned plots. Losses from deep soil pools were significant, with the absolute difference between intermediately burned plots versus most frequently burned plots more than doubling when the full 1 m sample was considered rather than the top 0–20 cm alone (losses of 98.5 Mg C/ha [−76%] and 42.3 Mg C/ha [−68%] in the full 1 m and 0–20 cm layers respectively). Compared to unburned forested plots, the most frequently burned plots had 65.8 Mg C/ha (−58%) less C in the full 1 m sample. Root biomass below the top 20 cm also declined by 39% with more frequent burning. Concurrent fire‐driven losses of nitrogen and gains in calcium and phosphorus suggest that burning may increase nitrogen limitation and play a key role in the calcium and phosphorus cycles in temperate savannas. Synthesis. Our results illustrate that fire‐driven losses in soil C and root biomass in deep soil layers may be critical factors regulating the net effect of shifting fire regimes on ecosystem C in forest‐savanna transitions. Projected changes in soil C with shifting fire frequencies in savannas may be 50% too low if they only consider changes in the topsoil.

The influence of tree and soil management on soil organic carbon stock and pools in dehesa systems

This study evaluated the effect on SOC concentration, stock and fractions in a dehesa divided into two areas of similar soil type but different soil management. The first area was a pastured dehesa (P) with young Holm oaks, planted in 1995 (70 trees ha−1, 12 m × 12 m) and, since 2000, grazed by sheep (3 sheep ha−1) with an average period of grazing of six months a year. Prior to this it was managed in the same way as the second adjacent area. The second area was a cropped dehesa (C) with widely spaced mature Holm oak (14 trees in a 12-ha dehesa), on which a mixture of vetch and oats was cultivated every three years and tilled with a chisel plough. After 22 years both dehesas showed similar SOC stock distribution amongst areas with different soil management, with approximately 40 t ha−1 in the top 100 cm of the soil. The P dehesa only showed higher SOC stock than the C dehesa on the surface 0–2 cm (5.86 ± 0.56 t ha-1vs 3.24 ± 0.37 t ha−1). The influence of the trees, increasing SOC concentration and content when compared to the area outside the canopy projection, was only detected under the mature trees in the C dehesa. In the area outside the tree canopy, both systems showed a similar distribution of soil organic carbon among their different fractions, with the unprotected fraction being the dominant one, followed by the physically and chemically protected fractions. In the C dehesa, the mature trees’ presence significantly modified the distribution of soil organic carbon in their surroundings, increasing the relevance of the unprotected fraction. The distribution of soil organic carbon in the unprotected and physically and chemically protected fractions were strongly correlated to the overall organic carbon concentration in the soil, indicating the rapid response of these three fractions to management, with the biochemically protected fraction showing no correlation, suggesting a high resilience to the changes in carbon budget.

Biogenic amorphous silica as main driver for plant available water in soils

More frequent and longer drought periods are predicted threatening agricultural yield. The capacity of soils to hold water is a highly important factor controlling drought stress intensity for plants. Biogenic amorphous silica (bASi) pools in soils are in the range of 0–6% and are suggested to help plants to resist drought. In agricultural soils, bASi pools declined to values of ~1% or lower) due to yearly crop harvest, decreasing water holding capacity of the soils. Here, we assessed the contribution of bASi to water holding capacity (WHC) of soil. Consequently, ASi was mixed at different rates (0, 1, 5 or 15%) with different soils. Afterwards, the retention curve of the soils was determined via Hyprop method. Here we show that bASi increases the soil water holding capacity substantially, by forming silica gels with a water content at saturation higher than 700%. An increase of bASi by 1% or 5% (weight) increased the water content at any water potential and plant available water increased by up to > 40% or > 60%, respectively. Our results suggest that soil management should be modified to increase bASi content, enhancing available water in soils and potentially decreasing drought stress for plants in terrestrial ecosystems.

Nitrogen pools in soil covered by biological soil crusts of different successional stages in a temperate desert in Central Asia

Biological soil crusts (BSCs) are a primary source of nitrogen (N) in deserts through N-fixation. N-fixation rates may be determined by the species present within BSCs, and environmental factors like temperature and moisture, which vary seasonally. These same factors may also govern dynamics of within-soil N transformations. Few studies have explored the dynamics of different N forms in response to seasonal microclimate variations in BSCs, especially across both the growing season and the snow-covered season (winter). In this study, monthly changes in multiple soil N pools across a full year and beneath three soil cover types – bare soil, cyanobacteria BSC, and lichen BSC – were assessed across a full year in the Gurbantunggut Desert, Central Asia. We focused on multiple organic and inorganic N pools including: total N, alkali-hydrolyzable N, DON, inorganic N (NH4+-N, NO3−-N), free amino acids, and microbial biomass N. We found that different N forms had divergent trends during the year, peaked in different months, and showed non-synchronous responses related to seasonal temperature and moisture patterns. Most N forms were most abundant in lichen BSC > cyanobacteria BSC > bare soil, although significant differences were only observed in a few months. In the growing season, available N forms such as soil DON and inorganic N were related to both water and temperature. In winter, available N was strongly related to temperature variability prior to the sampling dates. The high NH4+-N/NO3−-N in the early growing season suggest that N fixation or N mineralization were highest at this time. Our results indicate non-synchronous availability of different N forms, and elevated N availability beneath lichen BSCs, which may maintain the diverse N uptake requirements for different plant species.

A tale of three cities: Mercury in urban deciduous foliage and soils across land-uses in Poughkeepsie NY, Hartford CT, and Springfield MA USA.

Mercury is a global pollutant that harms human and wildlife health through chronic exposure. The role of urban forests in Hg biogeochemistry has been understudied in cities without historical mining or current coal combustion. This study aimed to quantify total Hg concentrations and pools in urban forests to determine whether adjacent land-use impacts Hg accumulation. Three cities in the northeastern United States were studied: Hartford, Connecticut; Poughkeepsie, New York; and Springfield, Massachusetts. We identified ~20 urban forests sites in a ~10km by ~10km grid for each city and sampled foliage and soil at each site. Foliage from Populus exhibited significantly lower Hg concentrations (15.6±2.1ngg-1) than mean foliar Hg concentrations (23.7±0.6ngg-1) but most deciduous genera had comparable concentrations. Average forest floor Hg concentrations (195±21ngg-1) and Hg pools (1.9±0.5mgm-2) were similar to previous, non-urban studies in the region. Average A horizon (182±19ngg-1) and B horizon (125±14ngg-1) Hg concentrations were double those of regional forest soils. Mineral soil Hg pools for the top 30cm (49±6mgm-2) averaged two to ten times higher than rural, montane forests in the region. Soil pH, LOI, and %clay were poorly correlated with mineral soil Hg concentrations. Instead, highest foliar and soil Hg concentrations and pools were in urban forests adjacent to high and medium intensity developed areas in Springfield and Hartford. To differentiate the impact of land-uses not captured by the National Land Cover Database (NLCD) system, we implemented new land-use categories. Industrial areas had highest foliar and soil Hg concentrations and pools of any land use. Our results show increasing land-use increases Hg accumulation in urban forests.