Dissolved Organic C Research Articles

Translocating subtropical forest soils to a warmer region alters microbial communities and increases the decomposition of mineral-associated organic carbon

It is not clear how soil organic carbon (SOC) and its related microbial processes respond to climate warming in subtropical forest, which limits our ability to predict the response and feedback of such forests to future warming. Here, we translocated a forest microcosm from a high-elevation site to a low-elevation site (600 m–30 m a.s.l.) in a subtropical forest, to study the responses of SOC fractions, microbial communities and enzyme activities to increases in soil temperature (ca. 1.69 °C). Results showed that translocation to a warmer region significantly decreased the total SOC content by an average of 21.1% after three years of soil warming. Warming non-significantly decreased the particulate organic C (POC) and microbial C (MBC) content by 15.7% and 15.2%, respectively, and increased the light fraction organic C (LFOC) and dissolved organic C (DOC) content by 15.5% and 2.3%, respectively. By contrast, warming significantly decreased the <53 μm fraction organic C (N-POC, −15.3%) and heavy fraction organic C (HFOC, −14.8%) content. Warming significantly decreased the relative abundance of total bacteria (−2.7%), G+ bacteria (−6.1%), G− bacteria (−6.6%) and actinomycetes (−10.8%), but increased the relative abundance of fungi (+22%). The oxidase and mass-specific oxidase activities were significantly increased by 32–70% in the warming soils. The decline in the N-POC was highly correlated to the increases in the relative abundance of fungi, the ratio of fungal to bacterial biomass (F:B), oxidase and mass-specific oxidase activities. Our results suggest that climate warming may increase the potential for fungal decomposition of mineral-associated organic C by increasing oxidase activities, leading to greater C losses in the subtropical forest than previously estimated.

Tail-drain sediments are a potential hotspot for nitrous oxide emissions in furrow-irrigated Vertisols used to grow cotton: A laboratory incubation study.

The impacts of soil properties and urea fertigation on nitrous oxide (N2 O) emissions from uncropped areas of furrow-irrigated Vertisol paddocks are unknown. We sampled soils from the head-ditch end (upslope) and sediments from the tail-drain end (downslope) of 10 Vertisols under irrigated cotton (Gossypium hirsutum L.) production in northeastern Australia. Four replicates of each sample were incubated in open-top polyvinyl chloride (PVC) chambers at 25 ± 2°C for 25 d. Nitrous oxide emissions were measured periodically after simulated irrigations on Days 0 and 15 with either water or, for soils, urea solution. Compared with the soils, sediments were enriched in silt, total organic carbon (TOC), total nitrogen (TN), ammonium N, and dissolved organic C (DOC) but had lower pH and sand content. Sediments emitted more N2 O than soils from the same paddocks after water irrigations. Nitrous oxide fluxes varied by two orders of magnitude between paddocks, with most variation explained by baseline nitrate N, TOC, pH (inversely), and sand content. Urea solution applied to soils at 30kg N ha-1 irrigation-1 increased N2 O emitted, but more so after the second irrigation. In irrigated cotton systems, tail-drain sediments are a potential hotspot for N2 O emissions that has not previously been documented.

Carbon and Nitrogen Dynamics Affected by Drip Irrigation Methods and Fertilization Practices in a Pomegranate Orchard

Knowledge of carbon (C) and nitrogen (N) dynamics under different irrigation practices in pomegranate orchards is novel and essential to develop sustainable production systems. The aim of this research was to determine the effect of high-frequency drip irrigation and different rates of N fertilizer on C and N distribution in the soil and N uptake by pomegranate fruit and leaves. The main treatments were surface drip irrigation (DI) and subsurface drip irrigation (SDI), and the sub-treatments used were three initial N rates (N1, N2, and N3). As trees grew larger, the N application rate increased. From 2013–2015, trees received the following rates of N: 62–113 (N1), 166–263 (N2), or 244–342 kg/ha (N3). Soil and leaf total C (TC) and N (TN), soil dissolved organic C (DOC), soil nitrate (NO3−), and total N uptake by fruit were evaluated between 2012 and 2015. Soil samples were collected to 120 cm depth at 15 cm increments. DI resulted in higher concentrations of TN, TC, NO3−, and DOC in the upper 75 cm depth than SDI. The N3 treatment resulted in higher concentrations of TN, TC, NO3−, and DOC under both DI and SDI. Neither DI nor SDI at the N1 or N2 levels increased TN and NO3− concentrations at 105–120 cm soil depth, indicating reduced leaching risk using high-frequency drip irrigation. Higher N uptake by fruit was observed in SDI than in DI in 2014 and 2015, and in N2 and N3 treatments compared with N1 in 2013 and 2014. The data indicate that the application rate at 166–263 kg/ha (N2) provided sufficient N for a 4–6-year-old pomegranate orchard and that high-frequency SDI is a promising technology for achieving higher N use efficiency and minimizing leaching loss of NO3− and DOC.

Effects of nitrogen addition on DOM-induced soil priming effects in a subtropical plantation forest and a natural forest

Dissolved organic matter (DOM) plays a key role in soil organic matter (SOM) decomposition via the priming effect (PE). The DOM-induced soil PE is closely related to nutrient availability, especially nitrogen (N). Regardless of the widespread of chronic N addition, how elevated N deposition affects DOM-induced PEs remains poorly understood. To fill this knowledge gap, we studied the effects of N addition, 13C-labeled leaf-DOM (herein DOM) addition, and leaf-DOM plus N addition (DOM+N) on soil PEs in soils of a subtropical Chinese-fir (Cunninghamia lanceolata) plantation and a natural Castanopsis carlesii forest (hereafter referred to as Chinese-fir soil and Castanopsis soil, respectively). Soil properties (e.g., soil organic C, total N, available phosphorus, and ratio of C and N), dissolved organic C (DOC), soil microbial biomass C (MBC), phospholipid fatty acid (PLFA), and enzyme activities were also investigated, because these parameters predominantly affect the intensity and direction of soil priming. The addition of DOM induced positive PEs in the Castanopsis soil but negative PEs in the Chinese-fir soil. In addition, DOM addition increased MBC and fungal abundance and the activities of phenol oxidase (PhOx) and peroxidase (Perox) in the Castanopsis soil but not in the Chinese-fir soil. Compared with DOM-only addition, DOM+N addition significantly enhanced PEs in the Chinese-fir soil but not in the Castanopsis soil. Furthermore, compared with DOM-only addition, DOM+N addition significantly increased MBC, abundance of fungi and AMF, fungi to bacteria ratio (F:B), and activities of four enzymes [β-glucosidase (βG), N-acetyl glucosaminidase (NAG), PhOx, and Perox] in the Chinese-fir soil but not in the Castanopsis soil. The DOM+N addition also had a significant effect on composition of main microbial groups in the Chinese-fir soil but not in the Castanopsis soil. These results suggest the enhanced PE following DOM+N in the Chinese-fir soil was likely mediated by enhanced enzyme production associated with increased fungal abundance. Our study highlights that the effects of increases of DOM on soil C cycling is largely affected by N availability and mediated by the effects on the abundance of soil microbial groups and enzyme activities. Our result also demonstrated a case in which effects of DOM and N addition on soil C cycling differ between a Castanopsis forest and a Chinese-fir plantation forest, with Chinese-fir soil being more sensitive to N addition. This is an important finding that needs to be taken into consideration in estimating the soil C pools.

Biochar suppresses N2O emissions and alters microbial communities in an acidic tea soil.

Biochar has been considered as a promising soil amendment for improving fertility and mitigating N2O emission from the arable land. However, biochar's effectiveness in acidic tea soil and underlying mechanisms are largely unknown. We conducted a short-term microcosm experiment using two biochars (1% w/w, LB, generated from legume and NLB, non-legume biomass, respectively) to investigate the effects of biochar amendments on soil chemical properties, N2O emission, and microbial community in an acidic soil. Soil and headspace gas samples were taken on 1, 10, and 30 day's incubation. Biochar amendment increased soil pH and DOC, however, significantly reduced soil inorganic N. Both biochars at ~ 1% addition had little effect on microbial CO2 respiration but suppressed soil N2O emission by ~ 40% during the incubation. The divergence in N2O efflux rates between soils with and without biochar addition aligned to some degree with changes in soil pH, inorganic N, and dissolved organic C (DOC). We also found that biochar addition significantly modified the fungal community structure, in particular the relative abundance of members of Ascomycota, but not the bacterial community. Furthermore, the copy number of nosZ, the gene encoding N2O reductase, was significantly greater in biochar-amended soils than the soil alone. Our findings contribute to better understanding of the impact of biochar on the soil chemical properties, soil N2O emission, and microbial community and the consequences of soil biochar amendment for improving the health of acidic tea soil.

Effects of long-term organic material applications on soil carbon and nitrogen fractions in paddy fields

Soil organic carbon (C) and nitrogen (N) pools are the most important soil components, and their characteristics directly or indirectly determine the stability of the soil structure and the soil fertility. Using a rice fields experiment in the coastal areas of Southeast China, how a long-term (10 years) continuous application of organic materials consisting of green manure (Chinese milk vetch), pig manure, and rice straw affected the soil C and N fractions were studied. The results showed that 10-year continuous applications of organic materials to the paddy fields significantly increased the C fraction content in terms of parameters such as the soil organic C (SOC), microbial biomass C (MBC), and active organic C (AOC). It also increased the soil N fraction content, including the soil total N (TN), dissolved total N (DTN), microbial biomass N (MBN), and active N. The rice straw return had the most significant effect in relation to increasing the SOC and dissolved organic C (DOC), with 23.9% and 42.3% increases, respectively, compared to applying chemical fertilizer alone. Organic pig manure had the most significant effect on the soil MBN and DTN, with 49.3% and 50.7% increases, respectively, compared to the application of chemical fertilizer alone. MBC and MBN are the primary components of soil AOC and active N, respectively. The application of organic materials increased the ratio of soil MBC and AOC to SOC, as well as the ratio of MBN, DTN, and active N to the soil TN. Our findings indicated that long-term applications of organic materials could improve and enhance paddy soil fertility and promote increased rice yields.

Mitigation of N2O emissions from urine treated acidic soils by liming

Nitrous oxide (N2O) is a devastating greenhouse gas mainly released from soils to the atmosphere. Pasture soils, particularly acidic in nature, are large contributors of atmospheric N2O through deposition of urine-N. Devising strategies for reducing N2O emissions in acidic soils are the utmost need of the time. Therefore, the present study was carried out to investigate the possible efficacy of dolomite application to reduce N2O emissions from urine treated acidic soil. Application of urine to soil enlarged the production of NH4+-N, NO3−-N, microbial biomass C (MBC) and dissolved organic C (DOC), resulting in higher N2O emissions as compared to the control (soil only). The highest N2O emission rate (1.35 μg N2O-N kg−1 h−1) and cumulative flux (408 μg N2O-N kg−1) occurred in urine only treated soil. Dolomite addition, especially higher application dose, greatly reduced N2O emissions through improved soil pH. The results suggest that increasing pH of acidic soils is a good applicable approach for reducing N2O emissions from urine-treated soils.

PH and exchangeable aluminum are major regulators of microbial energy flow and carbon use efficiency in soil microbial communities

The microbial partitioning of organic carbon (C) into either anabolic (i.e. growth) or catabolic (i.e. respiration) metabolic pathways represents a key process regulating the amount of added C that is retained in soil. The factors regulating C use efficiency (CUE) in agricultural soils, however, remain poorly understood. The aim of this study was to investigate substrate CUE from a wide range of soils (n = 970) and geographical area (200,000 km2) to determine which soil properties most influenced C retention within the microbial community. Using a 14C-labeling approach, we showed that the average CUE across all soils was 0.65 ± 0.003, but that the variation in CUE was relatively high within the sample population (CV 14.9%). Of the major properties measured in our soils, we found that pH and exchangeable aluminum (Al) were highly correlated with CUE. We identified a critical pH transition point at which CUE declined (pH 5.5). This coincided exactly with the point at which Al3+ started to become soluble. In contrast, other soil factors [e.g. total C and nitrogen (N), dissolved organic C (DOC), clay content, available calcium, phosphorus (P) and sulfur (S), total base cations] showed little or no relationship with CUE. We also found no evidence to suggest that nutrient stoichiometry (C:N, C:P and C:S ratios) influenced CUE in these soils. Based on current evidence, we postulate that the decline in microbial CUE at low pH and high Al reflects a greater channeling of C into energy intensive metabolic pathways involved in overcoming H+/Al3+ stress (e.g. cell repair and detoxification). The response may also be associated with shifts in microbial community structure, which are known to be tightly associated with soil pH. We conclude that maintaining agricultural soils above pH 5.5 maximizes microbial energy efficiency.

Permafrost Hydrology Drives the Assimilation of Old Carbon by Stream Food Webs in the Arctic

Permafrost thaw in the Arctic is mobilizing old carbon (C) from soils to aquatic ecosystems and the atmosphere. Little is known, however, about the assimilation of old C by aquatic food webs in Arctic watersheds. Here, we used C isotopes (δ13C, Δ14C) to quantify C assimilation by biota across 12 streams in arctic Alaska. Streams spanned watersheds with varying permafrost hydrology, from ice-poor bedrock to ice-rich loess (that is, yedoma). We measured isotopic content of (1) C sources including dissolved organic C (DOC), dissolved inorganic C (DIC), and soil C, and (2) stream biota, including benthic biofilm and macroinvertebrates, and resident fish species (Arctic Grayling (Thymallus arcticus) and Dolly Varden (Salvelinus malma)). Findings document the assimilation of old C by stream biota, with depleted Δ14C values observed at multiple trophic levels, including benthic biofilm (14C ages = 5255 to 265 years before present (y BP)), macroinvertebrates (4490 y BP to modern), and fish (3195 y BP to modern). Mixing model results indicate that DOC and DIC contribute to benthic biofilm composition, with relative contributions differing across streams draining ice-poor and ice-rich terrain. DOC originates primarily from old terrestrial C sources, including deep peat horizons (39–47%; 530 y BP) and near-surface permafrost (12–19%; 5490 y BP). DOC also accounts for approximately half of fish isotopic composition. Analyses suggest that as the contribution of old C to fish increases, fish growth and nutritional status decline. We anticipate increases in old DOC delivery to streams under projected warming, which may further alter food web function in Arctic watersheds.

Warming enhances the stimulatory effect of algal exudates on dissolved organic carbon decomposition

Abstract The current paradigm in peatland ecology is that the organic matter inputs from plant photosynthesis (e.g. moss litter) exceed that of decomposition, tipping the metabolic balance in favour of carbon (C) storage. Here, we investigated an alternative hypothesis, whereby exudates released by microalgae can actually accelerate C losses from the surface waters of northern peatlands by stimulating dissolved organic C (DOC) decomposition in a warmer environment expected with climate change. To test this hypothesis, we evaluated the biodegradability of fen DOC in a factorial design with and without algal DOC in both ambient (15°C) and elevated (20°C) water temperatures during a laboratory bioassay. When DOC sources were evaluated separately, decomposition rates were higher in treatments with algal DOC only than with fen DOC only, indicating that the quality of the organic matter influenced degradability. A mixture of substrates (½ algal DOC + ½ fen DOC) exceeded the expected level of biodegradation (i.e. the average of the individual substrate responses) by as much as 10%, and the magnitude of this effect increased to more than 15% with warming. Specific ultraviolet absorbance at 254 nm (SUVA254), a proxy for aromatic content, was also significantly higher (i.e. more humic) in the mixture treatment than expected from SUVA254 values in single substrate treatments. Accelerated decomposition in the presence of algal DOC was coupled with an increase in bacterial biomass, demonstrating that enhanced metabolism was associated with a more abundant microbial community. These results present an alternative energy pathway for heterotrophic consumers to breakdown organic matter in northern peatlands. Since decomposition in northern peatlands is often limited by the availability of labile organic matter, this mechanism could become increasingly important as a pathway for decomposition in the surface waters of northern peatlands where algae are expected to be more abundant in conditions associated with ongoing climate change.

Effects of forest conversion on carbon-degrading enzyme activities in subtropical China

The mineralization of soil organic carbon (SOC) is primarily mediated by carbon (C) degrading enzyme. In the current study, we determined how the activities of four soil C-degrading enzymes, the hydrolases β-glucosidase (BG) and cellobiohydrolase (CBH) and the oxidases polyphenol oxidase (PPO) and peroxidase (POD), responded to forest conversion of natural broadleaf forests (BF) to secondary forests (SF) and plantation forests (PF) in subtropical China. We also quantified SOC, dissolved organic C (DOC), permanganate oxidase organic C (PXC), recalcitrant C (RC), microbial biomass C (MBC), mineral-associated C (MOC), soil particle-sizes distribution, pH, and moisture content, and C: nitrogen (N) ratio. Results showed that, the activities of all four C-degrading enzymes (BG, CBH, PPO and POD) decreased by 23.1, 9.5, 6.9 and 1.8%, respectively by forest conversion of BF to SF and 30.5, 15.3, 28.1 and 27.8%, respectively by conversion of BF to PF and were higher in the topsoil than in the subsoil. Relative to SF and PF, BF had higher hydrolase activities, which were related to its higher concentrations of MBC, DOC, and PXC, and to its lower C:N ratio. The BF also had higher oxidase activities, which were related to its higher concentrations of MBC, RC, and MOC, and to its lower C:N ratio. PF had higher specific enzyme activities (i.e., enzyme activities per unit of SOC) than BF and SF, indicating faster C turnover rates in PF. In addition to being affected by the concentrations of SOC and SOC components, forest conversion-induced changes in soil enzyme activities were affected by clay content and soil moisture content. These results revealed the different underlying mechanisms between soil hydrolases and oxidases in their responses to forest conversion.

Responses of soil labile organic carbon fractions and stocks to different vegetation restoration strategies in degraded karst ecosystems of southwest China

The effective restoration of degraded areas, resulting in the sequestration of significant amounts of soil organic carbon (SOC) and labile soil organic carbon fractions (LOCF), is essential for mitigating global climatic changes. It is therefore important to improve our understanding of the responses and magnitudes of SOC and LOCF during restoration processes in degraded land like karst ecosystems. In this study, we selected four different restoration strategies: (1) not subjected to any management practices, mainly bare land (BL); (2) Pinus yunnanensis plantation (PY), native and characterized by slow growth and high drought resistance; (3) Eucalyptus maideni plantation (EM), non-native and characterized by rapid growth and high adaptability; (4) natural regeneration of secondary forest (SF). The SOC concentrations, SOC stocks, sensitive indicators of LOCF (microbial biomass C (MBC), permanganate oxidizable C (KMnO4-C), dissolved organic C (DOC), water soluble organic C (WSOC)), non-labile C (NLC), and the carbon pool management index (CPMI) were investigated. The different reforestation strategies had significant effects (p < 0.05) on SOC stocks; compared with BL, the SOC stocks of SF, EM, and PY increased by 57.7, 38.0, and 26.9%, respectively. Both the LOCF and the NLC stocks were significantly increased by the different restoration strategies relative to the BL, while natural vegetation restoration was more beneficial for C sequestration than afforestation. Compared with BL, in SF, EM, and PY, the CPMI increased by 70.1, 35.8, and 32.6%, respectively. These findings suggest that although afforestation can significantly increase C sequestration, natural regeneration may be a more effective approach in terms of SOC sequestration. Soil C accumulation would be limited by nitrogen and phosphorus during the vegetation restoration in the karst areas. In degraded karst ecosystems, the CPMI could be used as a representative index in evaluating the impacts of vegetation restoration on SOC concentration and soil quality.

Impacts of slash burning on soil carbon pools vary with slope position in a pine plantation in subtropical China

Slash burning is commonly used to clear harvest residues and improve forest health in tropical and subtropical forest plantations. However, whether the effects of slash burning on soil carbon pools (C) varied with slope position remains unknown. In this study, soils samples were collected at three depths (0–5, 5–10 and 10–20 cm) in three slope positions (upper, middle, and lower) one day before the slash burning and one week afterwards in a Pinus massoniana plantation in southeastern China. We measured soil physiochemical properties (bulk density, pH and moisture) and six soil C pools (total C, dissolved organic C (DOC), microbial biomass C (MBC), coarse particulate organic C (CPOC), fine particulate organic C (FPOC), and mineral-associated organic C (MAOC)). The results showed that slash burning increased soil pH (p < 0.001) and total C content (p < 0.001), but decreased soil DOC content (p < 0.001). Soil BD and moisture were not altered by the slash burning (p > 0.05). Slope position significantly affected soil moisture, DOC, MBC and MAOC. Soil moisture at 0–5 cm depth was significantly higher in the lower than upper slope position. The effects of slash burning on soil MBC varied with slope position and soil depth. Slash burning significantly decreased soil MBC at the 5–10 cm depth in the middle and lower slope position, but not in the upper slope position. Slash burning significantly decreased soil MBC at the 0–5 cm depth, while had no influence on soil MBC at the 10–20 cm depth irrespective of slope position. Averaged across slope positions for each soil depth, slash burning resulted in 48.0%, 41.3% and 25.7% decrease in soil MBC content at the 0–5, 5–10 and 10–20 cm depth, respectively. Slash burning had no effect on the distribution of the mass, but altered the C content in soil physical fractions. Slash burning increased FPOC (p < 0.01), but had no influence on CPOC and MAOC. The effects of slash burning on soil FPOC did not vary with slope position (p > 0.05). Our findings suggested that slope position should be considered as an important factor influencing the impact of slash burning on soil microbial mediated C pools, and future research is needed to investigate the post-fire recovery of vegetation and soil C dynamics.

Soil Microbial Biomass Size and Nitrogen Availability Regulate the Incorporation of Residue Carbon into Dissolved Organic Pool and Microbial Biomass

Microbial biomass (MB) plays a critical role in residue decomposition and soil organic matter (SOM) turnover. We investigated the effects of the initial size of soil MB pool and nitrogen (N) availability on the incorporation of ryegrass residue carbon (C) into microbial biomass C (MBC) and dissolved organic C (DOC). Soils from a row crop system (CS) and an adjacent grass sod (GS) were preincubated with (i.e., increased MB) and without (i.e., unchanged MB) glucose to produce soils with two levels of initial MB size before residue additions. Ammonium sulfate was added to test the effect of N availability. Residue addition increased DOC production in both soils, regardless of initial MB size and N availability, contributing 9 to ∼22% to the total pool. Residue quality was observed to affect the incorporation of residue C into DOC, which depended on soil type and initial MB size. However, the assimilation of residue C into MBC was not affected by the quality of ryegrass residue. Compared with the control (which did not have residue and N additions), a significant increase in SOM‐derived MBC by residue addition was observed in CS without glucose preincubation but not in the glucose preincubated CS and in GS. This indicated that the primed MBC by residue addition depended on initial MB size and soil type. The assimilation of residue C into MB was marginally inhibited by the preincubation with glucose in GS but was promoted in CS. With the addition of extra N, higher ryegrass‐derived MBC was observed in CS with glucose preincubation compared with the corresponding treatments without preincubation, which was not found in GS. These results suggested that residue‐derived MBC was not only regulated by initial MB size and N availability but also was affected by soil management history. However, N addition reduced ryegrass‐derived DOC production in GS without glucose preincubation. Apparently, both soil MB size and N availability largely affected the assimilation and incorporation of residue C in soil labile pools (i.e., DOC and MBC), and the extent of this relationship varied between two agricultural soils.Core IdeasRyegrass residue addition primed the production of SOM‐derived DOC and MBC.Residue quality affected ryegrass‐derived DOC in crop soils.N addition reduced ryegrass‐derived DOC in grass soils.N addition increased ryegrass‐derived MBC in crop soils.Initial MB and N availability regulated the incorporation of residue C into DOC and MBC.

American chestnut soil carbon and nitrogen dynamics: Implications for ecosystem response following restoration

American chestnut (Castenea dentata), once dominant throughout the eastern deciduous forest of North America, was extirpated from its native range by chestnut blight fungus. Through development of blight-resistant trees, the reintroduction of chestnut is likely, though little is known about the biogeochemistry of forests influenced by chestnut. We performed a one-year laboratory incubation experiment with soil and litter from 10-year-old monoculture plantings of pure American chestnut, black cherry, and northern red oak, in addition to a field-based 13C isotopic analysis of soil C. Parameters included litter decomposition, C respiration, N leaching, soil oxidizable C, extracellular enzyme activity related to nutrient acquisition, and litter chemistry. Results indicate that chestnut litter decayed more rapidly than that of oak or cherry (19.0%, 10.8%, 14.1% mass loss in chestnut, oak, and cherry litter, respectively). Chestnut had lower N leaching rates than soils beneath oak or cherry (7.8, 11.5, and 12.0 mg N kg−1 in chestnut, cherry, and oak soils, respectively), greater dissolved organic C (DOC) in leachate than soils influenced by oak (32.2 and 26.4 mg kg−1 in chestnut and oak soil, respectively). No differences in soil respiration or total soil C by species were detected. We conclude that surface soils influenced by chestnut have large inputs of C through rapid litter decomposition and low inorganic N availability, indicating potential for accumulation of C in surface soil over the long-term.

Precipitation Events, Soil Type, and Vineyard Management Practices Influence Soil Carbon Dynamics in a Mediterranean Climate (Lodi, California)

Core Ideas Vineyard management practices create spatial heterogeneity in CO2 efflux and SOM content. Soil C dynamics are influenced by water availability rather than temperature in comparatively warm Mediterranean climates. Soil tillage, organic amendments, cover crops, irrigation and precipitation stimulate CO2 efflux. To characterize the effect of precipitation events, management practices, and soil type on vineyard carbon (C) dynamics, we monitored CO2 emissions and labile C pools from nine vineyards in Lodi Wine Grape District, California, from April 2011 to December 2012. These commercial vineyards are replicates of three soil series (Redding, San Joaquin, and Tokay), representing a spectrum of soil texture and degree of soil development. We hypothesized that soil characteristics would influence the magnitude of CO2 efflux occurring in response to precipitation and management events in a Mediterranean climate. During each field visit—bimonthly (April–October) and monthly (November–March)—we measured CO2, soil temperature, and gravimetric water content (GWC) from vine and intervine (alleys) rows. Monthly, we collected soil samples for dissolved organic C (DOC), which tended to be greater in the alleys of San Joaquin and Redding than Tokay in summer but decreased after the onset of precipitation. In mid‐May and mid‐October 2012, CO2 efflux was higher in Tokay than in San Joaquin or Redding. Carbon dioxide efflux across all soils increased as a result of seasonal management practices (i.e., tillage and mowing of cover crops). Management practices distinguished soil DOC between vine rows and alleys from June to October 2012. Soil type or clay content influenced CO2 efflux across these vineyards, as did GWC and soil temperature. This 20‐mo study indicated that CO2 efflux responded to soil disturbance from management practices, precipitation, and irrigation.

Factors influencing student success in a senior-level horse management course

The Baltic Sea is one of the most eutrophied water bodies in northern Europe and more than 50% of its total anthropogenic waterborne phosphorus (P) and nitrogen (N) loads derive from agricultural sources. Sweden is the second largest contributor of waterborne N and the third largest contributor of waterborne P to the Baltic Sea. Horse farms now occupy almost 10% of Swedish agricultural land, but are not well investigated with regard to their environmental impact. In this study, potential P, N and carbon (C) leaching losses were measured from two representative horse paddock topsoils (0–20 cm; a clay and a loamy sand) following simulated rainfall events in the laboratory. Results showed that the leachate concentrations and net release of P, N and dissolved organic C (DOC) from paddock topsoils were highest in feeding and excretion areas and considerably higher from the loamy sand than the clay paddock topsoil. Leaching losses of dissolved reactive P (DRP) were significantly (p < 0.05) correlated with concentrations of water-soluble P and ammonium acetate lactate-extractable P (P-AL) in the soil, while leaching losses of dissolved organic P and total organic N were significantly correlated with DOC concentration in leachate. Leaching loads of P and N from paddock topsoils greatly exceeded average figures for Swedish agricultural topsoils. It was concluded that: i) horse paddocks pose a potential threat to water quality via leaching of excess P and N, ii) feeding and excretion areas are potential hotspots for highly enhanced leaching losses, and iii) paddocks established on sandy soils are particularly susceptible to high N leaching losses.

Miscanthus and hemp as alternative bedding material for horses

The Baltic Sea is one of the most eutrophied water bodies in northern Europe and more than 50% of its total anthropogenic waterborne phosphorus (P) and nitrogen (N) loads derive from agricultural sources. Sweden is the second largest contributor of waterborne N and the third largest contributor of waterborne P to the Baltic Sea. Horse farms now occupy almost 10% of Swedish agricultural land, but are not well investigated with regard to their environmental impact. In this study, potential P, N and carbon (C) leaching losses were measured from two representative horse paddock topsoils (0–20 cm; a clay and a loamy sand) following simulated rainfall events in the laboratory. Results showed that the leachate concentrations and net release of P, N and dissolved organic C (DOC) from paddock topsoils were highest in feeding and excretion areas and considerably higher from the loamy sand than the clay paddock topsoil. Leaching losses of dissolved reactive P (DRP) were significantly (p < 0.05) correlated with concentrations of water-soluble P and ammonium acetate lactate-extractable P (P-AL) in the soil, while leaching losses of dissolved organic P and total organic N were significantly correlated with DOC concentration in leachate. Leaching loads of P and N from paddock topsoils greatly exceeded average figures for Swedish agricultural topsoils. It was concluded that: i) horse paddocks pose a potential threat to water quality via leaching of excess P and N, ii) feeding and excretion areas are potential hotspots for highly enhanced leaching losses, and iii) paddocks established on sandy soils are particularly susceptible to high N leaching losses.

Characteristics of annual greenhouse gas flux and NO release from alpine meadow and forest on the eastern Tibetan Plateau

The importance of the world’s largest plateau (i.e., the Tibetan Plateau) as a source/sink for atmospheric greenhouse gases (GHGs) still remains uncertain as field observations covering entire years and contrasting ecosystems are scarce. Here we report on year-round measurements of methane (CH4), nitrous oxide (N2O) and nitric oxide (NO) fluxes from two representative ecosystems (cattle-grazed meadow and natural forest, >3300 m a.s.l) on the eastern Tibetan Plateau. In our study, the alpine soils generally functioned as sinks for atmospheric CH4. The pronounced seasonality of CH4 uptake, with higher uptake in the growing season and lower uptake in the non-growing season, was mainly regulated by the combined changes in soil moisture and temperature. Soil CH4 uptake was lower at the meadow site due to high soil moisture and grazing-induced soil compaction reducing gas diffusivity and hampering CH4 uptake. For the alpine meadow pulses of N-oxide fluxes were observed during the freeze-thaw period, while freeze-thaw related pulses did not occur at the forest site, where the seasonality of N-oxide fluxes was primarily regulated by changes in soil temperature (Q10 = 3.0). Annual cumulative fluxes for the meadow and forest ecosystems ranged between −1.15 and −2.24 kg CH4-C ha−1 yr−1, 0.13-0.65 kg N2O-N ha−1 yr−1 and 0.022-0.047 kg NO-N ha−1 yr−1. Non-growing season fluxes contributed 34–45% and 21–51% to the annual budgets of CH4 and N-oxides, respectively. Multiple regression analyses showed that soil N-oxide fluxes were significantly positively correlated to concentrations of soil dissolved organic C (DOC) and inorganic N, with DOC and inorganic N concentrations being higher at the forest site. Our study also shows that alpine forest is a stronger source for non-CO2 GHGs relative to alpine meadow (higher by 195 kg CO2-eq ha−1 yr−1). This difference in GHG fluxes significantly offsets the net climate benefits due to increased C-sequestration associated with afforestation/reforestation.

The Case for Digging Deeper: Soil Organic Carbon Storage, Dynamics, and Controls in Our Changing World

Most of our terrestrial carbon (C) storage occurs in soils as organic C derived from living organisms. Therefore, the fate of soil organic C (SOC) in response to changes in climate, land use, and management is of great concern. Here we provide a unified conceptual model for SOC cycling by gathering the available information on SOC sources, dissolved organic C (DOC) dynamics, and soil biogeochemical processes. The evidence suggests that belowground C inputs (from roots and microorganisms) are the dominant source of both SOC and DOC in most ecosystems. Considering our emerging understanding of SOC protection mechanisms and long-term storage, we highlight the present need to sample (often ignored) deeper soil layers. Contrary to long-held biases, deep SOC—which contains most of the global amount and is often hundreds to thousands of years old—is susceptible to decomposition on decadal timescales when the environmental conditions under which it accumulated change. Finally, we discuss the vulnerability of SOC in different soil types and ecosystems globally, as well as identify the need for methodological standardization of SOC quality and quantity analyses. Further study of SOC protection mechanisms and the deep soil biogeochemical environment will provide valuable information about controls on SOC cycling, which in turn may help prioritize C sequestration initiatives and provide key insights into climate-carbon feedbacks.