Dissolved Organic C Concentrations Research Articles

Landscape regulation of microbial use of terrestrial carbon in boreal streams

Microbes decomposing leaf litter in aquatic ecosystems are exposed to two major sources of carbon (C), namely, particulate organic C (POC) and dissolved organic C (DOC). The use of DOC relative to POC during litter decomposition likely depends on the availability of DOC, which in turn is regulated by the characteristics of the surrounding landscape, although this extrinsic indirect control of DOC use remains largely unexplored. We have investigated how variations in stream physical and chemical characteristics, distribution of major landscape elements (i.e., forest, mires, and lakes), and riparian vegetation community composition (i.e., relative cover of deciduous vs. coniferous tree species) influence DOC use by leaf-associated microbes (LAM). Specifically, in a boreal stream network of ten first- to third-order streams, we related in-stream characteristics, landscape elements, and riparian vegetation community composition to DOC/POC respiration ratios (i.e., the amount of CO2 produced by LAM respiration of DOC + POC divided by the amount of leaf C mass lost through decomposition). The results showed that DOC/POC respiration ratios were > 1 in most of the study sites, indicating that LAM use a substantial amount of DOC during leaf litter decomposition. This microbial reliance on DOC varied with in-stream DOC and nutrient concentrations, proportional mire and forest cover, and riparian vegetation community composition. In particular, high mire and coniferous cover increased DOC use by LAM. As such, landscape configuration and how it is modified by land use and climate change must be considered in order to understand microbial turnover of terrestrial C in boreal streams.

The impacts of nitrogen addition on the active carbon pools in forest soils along an urban–rural gradient

The increased nitrogen (N) deposition accompanying urbanization alters the dynamics of soil carbon (C) in urban and peri-urban forests. However, our understanding of the magnitude and mechanism of this impact is still minimal. In this study, experimental plots were established in urban, suburban, and rural forests to investigate the effects of three levels of N addition: no addition (0), low addition (50 kg N·ha−1·yr−1), and high addition (100 kg N·ha−1·yr−1). The responses of the active components in the soil C pool, including dissolved organic C (DOC) and microbial biomass C (MBC), were monitored after simulated N deposition treatments in forests along an urban–rural gradient. The results showed that the concentrations of DOC and MBC in forest soils along the urban–rural gradient exhibited significant seasonal and site variation. Soil DOC and MBC concentrations in the rural forest were significantly affected by N addition, increasing by 13.39 % and 45.90 % under the low N treatment, respectively, and MBC concentration increased by 28.40 % under the high N treatment. On the contrary, soil DOC and MBC concentrations in urban and suburban forests did not change significantly. Among studied influencing factors, the total dissolved N provided the crucial nutrient sources for microbial growth and activity and thus became the primary factor affecting the active components in the soil C pool. The insensitivity of urban and suburban forests' active soil C pools to added N indicates that the N fertilization effect brought by urban expansion may not have a lasting impact on soil C pools of rural forests.

Addressing the C/N imbalance in the treatment of irrigated agricultural water by using a hybrid constructed wetland at field-scale

To mitigate excess of nitrate-N (NO3−-N) derived from agricultural activity, constructed wetlands (CWs) are created to simulate natural removal mechanisms. Irrigated agricultural drainage water is commonly characterized by an organic carbon/nitrogen (C/N) imbalance, thus, C limitation constrains heterotrophic denitrification, the main biotic process implicated in NO3−-N removal in wetlands. We studied a pilot plant with three series (169 m2) of hybrid CWs over the first two years of functioning to examine: i) the effect of adding different C-rich substrates (natural soil vs. biochar) to gravel on NO3−-N removal in a subsurface flow (Phase I), ii) the role of a second phase with a horizontal surface flow (Phase II) as a source of dissolved organic C (DOC), and its effect in a consecutive horizontal subsurface flow (Phase III) on NO3−-N removal, and iii) the contribution of each phase to global NO3−-N removal. Our results showed that the addition of a C-rich substrate to gravel had a positive effect on NO3−-N removal in Phase I, with mean efficiencies of 40% and 17% for soil and biochar addition, respectively, compared to only gravel (0.75%). In Phase II, the algae growth turned into a DOC concentration increase, but it did not enhance NO3−-N removal in Phase III. In series with C-rich substrate addition, the largest contribution to NO3−-N removal was found in Phase I. However, in series with only gravel, Phase II was the most effective on NO3−-N removal. Contribution of Phase III to NO3−-N removal was almost negligible.

The effects of long‐term drainage and short‐term restoration on dissolved organic C concentrations and optical properties in the Baijianghe Peatland

AbstractPeatlands are important reservoirs of terrestrial carbon (C), whose fate can have implications for the global C cycle and climate change. Many peatlands around the world have been drained for agriculture and forestry and restored later on hoping to reduce C emissions and improve water quality. However, the actual effects of these management practices remain unclear. To address this, we investigated the concentration and quality of dissolved organic C (DOC) and water chemistry (nutrient and Fe concentrations) in the surface water (creeks and ditches) of the natural, drained, and restored areas along an elevation gradient of the Baijianghe (BJH) peatland in northeast China from 2017 to 2020. We found significantly higher DOC concentrations in the natural than the drained area, with a heavier, more fulvic, and less aromatic DOC composition upslope, but the difference in DOC quality became smaller downslope. Restoration, on the other hand, did not bring DOC concentrations or quality back to the natural conditions within 2 years. These variations in DOC concentration and quality were closely associated with dissolved Fe concentration, highlighting the role of the Fe redox cycle, as Fe(II) can catalyse phenol oxidation activities and promote DOC production. Vegetation also affected DOC characteristics, as the drained and restored areas were dominated by shrubs with woodier tissues, which probably released less in amount but more recalcitrant DOC. Our study features rare side‐by‐side comparisons between pristine, drained, and restored areas of the same peatland, and sheds light on factors affecting peatland DOC under different management practices.

Impact of Forest Harvesting Intensity and Water Table on Biodegradability of Dissolved Organic Carbon in Boreal Peat in an Incubation Experiment

Boreal peatlands are vast carbon (C) stores but also major sources of dissolved organic C (DOC) and nutrients to surface waters. Drainage and forest harvesting accelerates DOC leaching. Continuous cover forestry (CCF) is considered to cause fewer adverse environmental effects. Yet, the effects of CCF on DOC processes are unrecognised. We study DOC production and quality in unharvested, CCF, and clear-cut drained peatland forests and in a non-forested alluvial sedge fen. Parallel replicate peat columns with ground vegetation are collected from the uppermost 50 cm at each site, and the water table (WT) is set to −20 or −40 cm depths on the columns. During the eight-month ex situ incubation experiment, the soil water samples are extracted monthly or bi-monthly. The samples are incubated at 15 °C for multiple 72 h incubation cycles to study pore water quality and biodegradation of DOC. The CO2 production occurs during the first three days. The DOC concentrations and the CO2 release per volume of water are significantly lower in the sedge fen than in the drained peatland forests. The WT has a negligible effect on DOC concentrations and no effect on DOC quality, but the higher WT has generally higher CO2 production per DOC than the lower WT. The results suggest that peat in the drained peatlands is not vulnerable to changes per se but that forest management alters biotic and abiotic factors that control the production, transport, and biodegradation of DOC.

Characteristics and environmental significance of organic carbon in sediments from Taihu Lake, China

The rapid economic development in the Yangtze River Delta has affected the carbon (C) pool in lake sediments, which has become a serious environmental management issue. To explore the characteristics of composition, spatial distribution and burial of organic C (OC) in lacustrine sediments, a typical shallow lake was targeted to compare the heavy fraction OC (HFOC), light fraction OC (LFOC), and dissolved OC (DOC) in the surface sediments of different lake areas using the specific gravity separation approach. The effects of environmental factors on the form of OC present were identified by a redundancy analysis. The results showed a regional difference of DOC, LFOC, and HFOC concentrations in Taihu Lake, and HFOC was dominant in the OC buried in lake sediments. The deposition rate of OC in Taihu Lake sediments varied in a ranged of 31.91–114.47 g C m−2 a-1. The LFOC levels in the sediments were lower than those in other lakes. Both sedimentary LFOC and HFOC were significantly correlated with the nitrogen (N) content, indicating that the C sink was enhanced by an increase in N in the sediments. The grass-type lake zone stimulated C accumulation in Taihu Lake sediments.

Organic matter in aqueous soil extracts: Prediction of compositional attributes from bulk soil mid-IR spectra using partial least square regressions

Water-extractable organic matter (WEOM) is labile and a key component of soil organic matter. Thus, the prediction of soil WEOM concentration and composition-related characteristics is of great interest. The main objective of this study was to model and predict dissolved organic C (DOC) concentration and WEOM composition-related attributes of aqueous soil extracts using mid-infrared (IR) spectra of bulk soils coupled with partial least square (PLS) regression. Absorbance of UV light at 254 nm (Abs254), considered proportional to the concentration of aromatic substances in soil extracts, and light emission intensities proportional to concentrations of some components controlling WEOM fluorescence, were used as composition-related attributes of the soil extracts. The DOC-normalized Abs254 and emission intensities were used as composition-related attributes of WEOM. Application of PLS regressions for predicting spectroscopy-based composition-related attributes of aqueous soil extracts, using bulk soil IR spectra, is novel. Mid-IR spectra were determined for 216 soil samples collected from different (i) Israeli climate regions (Mediterranean and Semi-arid), (ii) types of land use (field crops, orchard and non-cultivated land), (iii) two depths (0–10 and 30–60 cm), (iv) sampling seasons (Fall and Spring). Prediction of DOC concentrations in the soil extracts was of limited and variable success evaluated by the coefficient of determination and slope of the linear regression of predicted vs measured values, with some soil subsets yielding no satisfactory predictions. However, Abs254 and emission intensities of fluorescent humic-like components were predicted more successfully than DOC concentrations, suggesting that WEOM aromatic and fluorescent components are better presented in soil IR spectra compared to WEOM aliphatic substances. Yet, prediction of the specific UV absorbance (SUVA), using bulk soil IR spectra, was less successful than prediction of Abs254. Prediction of DOC-normalized emission intensities of fluorescent components (analogous to SUVA in fluorescence spectroscopy) was not successful. The differences between the success of predicting extract properties, i.e., Abs254 and fluorescence emission intensities, and the prediction of DOC-normalized derivatives, suggest that concentrations of aromatic (fluorescent) components in soil extracts are better predicted using PLS regression analysis of bulk soil mid-IR spectra than the WEOM composition.

Nitrogen addition altered the microbial functional potentials of carbon and nitrogen transformation in alpine steppe soils on the Tibetan Plateau

The microbial-mediated functional potentials of soil can be reflected by the abundance of corresponding microbial functional genes (MFGs). Few studies have simultaneously examined the responses of multiple key MFGs involving soil carbon (C) and nitrogen (N) transformation to N deposition, particularly under multiple application levels. In this study, we treated the alpine soil from the Tibetan Plateau with N fertilization at six addition rates. Absolute quantitative analysis was used to detect the abundance of MFGs related to soil C process: carbon dioxide (CO 2 ) fixation( cbbL ), methane (CH 4 ) oxidation ( pmoA ) and production ( mcrA ), and soil N process: ammonia oxidation (AOA- amoA : archaea amoA , AOB- amoA : bacterial amoA ), hydroxylamine oxidation ( hao ), nitrate, nitrite, nitric oxide and nitrous oxide (N 2 O) reduction ( narG / napA , nirS / nirK , norB , and nosZ ). The abundance of MFGs involved in both the soil C and N transformation processes were reduced as N rates increased (except AOB- amoA gene). Specifically, the abundance of the mcrA , AOA- amoA , nirK , norB , and nosZ genes were decreased non-linearly (U-shape) with the increasing N rates ( P < 0.05). The soil pH and dissolved organic C (DOC) concentration were positively correlated with the abundance of those MFGs, suggesting that the decreases in abundance of MFGs were more likely to be driven by changes of soil pH and DOC concentration. In contrast, the abundance of AOB- amoA gene increased linearly with the increasing N rates, which was strongly and positively correlated with soil nitrate-N (NO 3 – -N) concentration ( P < 0.001, r = 0.82). In addition, soil greenhouse gases emissions linked with the abundance of corresponding MFGs. Specifically, the soil CO 2 and N 2 O emissions were strongly and positively correlated with the AOB- amoA gene abundance ( P < 0.001, r = 0.78, r = 0.84), and the soil CH 4 emissions were significantly correlated with the pmoA ( P < 0.01, r = 0.60) and AOA- amoA ( P < 0.001, r = 0.72) gene abundance. The above results showed that the AOB- amoA gene had a positive response to increasing N rates, which was different from other genes. Indicating that elevated N deposition may improve the ammonia-oxidation process in alpine steppe soil mediated by AOB communities, and AOB communities may play an important role in donating the soil-atmosphere C and N exchange via CO 2 and N 2 O emissions. • The abundance of both C- and N-related MFGs were affected by N input rates. • N addition oppositely influenced on AOA- amoA and AOB- amoA gene. • The MFGs abundances were primarily regulated by soil DOC and pH under N addition. • Soil CO 2 and N 2 O emissions were strongly and positively correlated to AOB- amoA gene abundance.

Decomposition of carbon adsorbed on iron (III)-treated clays and their effect on the stability of soil organic carbon and external carbon inputs

The interaction of organic carbon (OC) with clay and metals stabilizes soil carbon (C), but the influence of specific clay-metal-OC assemblages (flocs) needs further evaluation. This study aimed to investigate the stability of flocs in soil as affected by external C inputs. Flocs representing OC-mineral soil fractions were synthesized using dissolved organic C (DOC) combined with kaolinite (1:1 layer structure) or montmorillonite (2:1 layer structure) clays in the absence or presence of two levels of Fe (III) (named low or high Fe). Flocs were mixed with soil (classified as Luvisol) and incubated with or without 13C labelled plant residue (i.e., ryegrass) for 30 days. The CO2 emissions and DOC concentrations as well as their 13C signatures from all treatments were examined. Total C mineralization from flocs was approximately 70% lower than non-flocced DOC. The flocs made with montmorillonite had 16–43% lower C mineralization rate than those made with kaolinite with no Fe or low Fe. However, when flocs were made with high Fe, clay mineralogy did not significantly affect total C mineralization. A positive priming effect (PE) of flocs on native soil OC was observed in all treatments, with a stronger PE found in lower Fe treatments. The high-Fe clay flocs inhibited ryegrass decomposition, while the flocs made without clay had no impact on it. Interestingly, flocs significantly decreased the PE of ryegrass on native soil OC decomposition. These results indicate that the adsorption of DOC onto clay minerals in the presence of Fe (III) stabilizes it against decomposition processes and its stability increases as Fe in flocs increases. Flocs also protect soil OC from the PE of external degradable plant C input. This study showed that Fe level and clay mineralogy play an important role in controlling soil C stability.

Identification of barley genetic regions influencing plant\u2013microbe interactions and carbon cycling in soil

PurposeRhizodeposition shapes soil microbial communities that perform important processes such as soil C mineralization, but we have limited understanding of the plant genetic regions influencing soil microbes. Here, barley chromosome regions affecting soil microbial biomass-C (MBC), dissolved organic-C (DOC) and root biomass were characterised.MethodsA quantitative trait loci analysis approach was applied to identify barley chromosome regions affecting soil MBC, soil DOC and root biomass. This was done using barley Recombinant Chromosome Substitution Lines (RCSLs) developed with a wild accession (Caesarea 26-24) as a donor parent and an elite cultivar (Harrington) as recipient parent.ResultsSignificant differences in root-derived MBC and DOC and root biomass among these RCSLs were observed. Analysis of variance using single nucleotide polymorphisms genotype classes revealed 16 chromosome regions influencing root-derived MBC and DOC. Of these chromosome regions, five on chromosomes 2H, 3H and 7H were highly significant and two on chromosome 3H influenced both root-derived MBC and DOC. Potential candidate genes influencing root-derived MBC and DOC concentrations in soil were identified.ConclusionThe present findings provide new insights into the barley genetic influence on soil microbial communities. Further work to verify these barley chromosome regions and candidate genes could promote marker assisted selection and breeding of barley varieties that are able to more effectively shape soil microbes and soil processes via rhizodeposition, supporting sustainable crop production systems.

Methane and nitrous oxide have separated production zones and distinct emission pathways in freshwater aquaculture ponds

Aquaculture systems receive intensive carbon (C) and nitrogen (N) loadings, and are therefore recognized as major anthropogenic sources of methane (CH4) and nitrous oxide (N2O) emissions. However, the extensively managed aquaculture ponds were identified as a hotspot of CH4 emission but just a weak N2O source. Here, we investigate annual CH4 and N2O fluxes from three earthen ponds used for crab culture, of different sizes, in southeast China. Our purposes are to identify the spatiotemporal variations of CH4 and N2O emissions and their components among ponds and to evaluate the zone for CH4 and N2O production. Static chamber-measured CH4 flux ranged from 0.03 to 64.7 mg CH4 m‒2 h‒1 (average: 9.02‒14.3 mg CH4 m‒2 h‒1), and temperature, followed by dissolved organic C (DOC) concentration, and redox potential, were the primary drivers of seasonal CH4 flux patterns. Annual mean diffusive CH4 flux was 1.80‒2.34 mg CH4 m‒2 h‒1, and that by ebullition was up to 7.20‒12.0 mg CH4 m‒2 h‒1 (79.1‒83.5% of the total CH4 flux). Annual CH4 emission was positively correlated with sediment DOC concentration but negatively (P < 0.05) correlated with water depth across ponds, with the highest CH4 emission occurred in a pond with low water depth and high DOC concentration. The calculated diffusive N2O flux by the gas transfer velocity was 0.32‒0.60 times greater than the measured N2O emission, suggesting that N2O in water column can not only evade as water-air fluxes but diffuse downwards and to be consumed in anaerobic sediments. This also indicates that N2O was primarily produced in water column. The highly reduced condition and depletion of NO3‒-N in sediments, can limit N2O production from both nitrification and denitrification but favor N2O consumption, leading the ponds to become a weak source of N2O annually and even a sink of N2O in summer. Our results highlight that the current global CH4 budget for inland waters is probably underestimated due to a lack of data and underestimation of the contribution of ebullitive CH4 flux in small lentic waters. The downwards N2O diffusion from the water column into sediment also indicates that the extensively-used model approach based on gas transfer velocity potentially overestimates N2O fluxes, especially in small eutrophic aquatic ecosystems.

Lateral Carbon Exports From Drained Peatlands: An Understudied Carbon Pathway in the Sacramento‐San Joaquin Delta, California

AbstractDegradation of peatlands via drainage is increasing globally and destabilizing peat carbon (C) stores. The effects of drainage on the timing and magnitude of lateral C losses from degraded peatlands remains understudied. We measured spatial and temporal variability in lateral C exports from three drained peat islands in the Sacramento‐San Joaquin Delta in California across the 2017 and 2018 water years using measurements of dissolved inorganic C (DIC), dissolved organic C (DOC), and suspended particulate organic C (POC) concentration combined with discharge. These measurements were supplemented with stable isotope data (δ13C‐DIC, δ13C‐POC, δ15N‐PON, and δ2H‐H2O values) to provide insight into hydrological and biogeochemical controls on lateral C exports from drained peatlands. Drainage DOC and DIC concentrations were seasonally variable with the highest values in the winter rainy season, when discharge was also elevated. Seasonal differences in the mobilization of dissolved C appeared to result from changing water sources and water table levels. Peat island drainage C contributions to surrounding waterways were also greatest during the winter. Although temporal variability in C cycling processes and trends were generally similar across islands, baseline drainage DIC, DOC, and POC concentrations were spatially variable, likely a result of sub‐island‐scale differences in soil organic matter content and hydrology. This spatial variability complicates system‐wide assessments of C budgets. Net lateral C exports were water year dependent and comparable to previously published vertical C emission rates for this system. This work highlights the importance of including lateral C exports from drained peatlands in local and regional C budgets.

Impact of surface soil manuring on particulate carbon fractions in relevant to nutrient stoichiometry in a Mollisol profile

Organic matter application significantly influences soil carbon storage and sequestration in cropland. The investigation of C sequestration through soil organic carbon (SOC) fractions in the soil profile and its relation to nutrient stoichiometry are essential for evaluating the effect of farming practice on soil fertility. This study aimed to examine the effect of cattle manure application on nutrient stoichiometry, which would be associated with soil C stock in SOC fractions, such as microbial biomass C (MBC), dissolved organic C (DOC) and water-soluble organic C (WSOC), coarse particulate organic carbon (cPOC), fine particulate organic carbon (fPOC) and the mineral-associated organic carbon (MOC). A four-year manure application experiment was conducted in a Mollisol, in which fertilization treatments comprised (1) NoF: the non-fertilizer control, (2) F: synthetic fertilizer, i.e. 20.3, 21.2 and 12.2 kg ha−1 of nitrogen (N), phosphorus (P) and potassium (K) in soybean, and 69.0, 28.1 and 6.1 kg ha−1 of N, P and K for corn on an annual basis, respectively, (3) F + M: synthetic fertilizer plus 15 Mg ha-1 of cattle manure and (4) F + 2M: synthetic fertilizer plus 30 Mg ha−1 of cattle manure in a soybean-corn rotation. SOC concentration in top 10 cm of soil was 18 % and 19 % higher under F + M than that under NoF and F, respectively. Whereas, F+2 M significantly increased SOC concentration by 32 %, 32 %, 14 %, and 11 % in 0−10, 10−20, 20−30, and 30−40 cm of soil depth compared to that in respective soil depth under NoF. Compared to NoF and F, manure application (F + M and F+2 M) significantly increased MBC in the top 30 cm of soil depth and DOC concentration in the top 20 cm of soil depth, but significantly decreased WSOC concentration in the top 20 cm of soil depth. F+2 M significantly increased cPOC concentration in the top 30 cm of soil depth compared to NoF and F. F + M significantly increased fPOC concentration in the top 50 cm of soil depth, compared to NoF, and F. F+2 M further increased the fPOC concentration in the top 30 cm of soil depth, compared to F + M. There was no difference among fertilization treatments in MOC concentration in any soil depth. There were significant exponential correlations of C concentration in POC fractions with soil N, P and N/P ratio. These results indicate that the combination of manure and synthetic fertilizer increased C accumulation in the POC fraction especially in the top 30 cm of soil depth. The change of nutrient stoichiometry was responsible to the accumulation of soil organic matter, highlighting that N, rather than P mainly regulated C accumulation in POC. Increasing input of N fertilizer combined with manure application would benefit the formation of soil organic matter in agricultural Mollisols.

Atmospheric Wet Deposition of Organic Carbon and Dissolved Nitrogen in City, Countryside and Nature Reserve of Subtropical China

AbstractThe atmospheric wet deposition was higher in subtropical China because of rapid economic growth in past decades and high humidity weather. One‐year measurement of wet deposition of organic carbon (C) and dissolved nitrogen (N) was carried out at three sites (Changsha city, Huitong agriculture county, and Dashanchong nature reserve) in Hunan Province. Taking the nature reserve as the control, we make a comparative analysis of the characteristics in atmospheric wet deposition between city and countryside. The concentrations and wet depositions of total and each component of C and N were the lowest in the nature reserve, indicating that human activities significantly increase the atmospheric wet deposition. The concentrations and the wet deposition fluxes of particulate organic C, ‐N, and dissolved organic N (DON) were the highest in the city, whereas the corresponding values of dissolved organic C (DOC) and ‐N were the highest in the countryside. Except for DON, the concentrations of DOC, ‐N, and ‐N changed significantly with rainfall characteristics (rainfall amount, intensity, and weather conditions). Our study supplements the wet deposition data as a baseline for exploring future changes in subtropical China. Meanwhile, the lowest wet depositions in nature reserve imply that the reduction of anthropogenic emission is essential to minimizing the adverse effects of the increased atmospheric deposition.

Microbial-derived carbon components are critical for enhancing soil organic carbon in no-tillage croplands: A global perspective

Soil organic carbon (SOC) is an important component of the global carbon (C) cycle, while tillage has been shown to significantly impact SOC stocks, its impact on the different SOC fractions is not well understood at global level, making it difficult to predict the behavior of SOC over time. Using data from 95 studies on a global level, we investigated the changes of various SOC fractions in 0-400 mm [dissolved organic C (DOC), particulate organic C (POC), easily oxidizable organic C (EOC), microbial biomass C (MBC), and mineral-associated organic C (MOC)] with NT application. This study also quantified the roles of different environmental and agronomic factors (e.g., climatic conditions, soil texture and nutrient conditions, study duration, and cropping intensity) in the changes of these SOC fractions. Compared to conventional tillage (CT), NT and reduced tillage (RT; tillage systems that are less intensive, with fewer trips in the field than CT) increased SOC concentrations, which were 7.4% higher in NT than in RT (percentage change based on weighted response ratio). Compared with CT, NT significantly increased DOC (17.6%), POC (11.7%), EOC (14.8%), MBC (33.1%), and MOC concentration (16.0%). Furthermore, POC, MBC, and MOC concentrations were 8.2, 34.1, and 4.4% higher in NT than in RT, respectively. Importantly, the increased SOC concentrations under NT were significantly correlated with MBC (R2 = 0.62) and POC (R2 = 0.55). Soil depth and mean annual temperature were the dominant factors affecting the response ratios of SOC fractions (DOC, EOC, MBC, MOC, and POC) under NT, followed by the duration of NT application. Our results suggest that SOC fractions, especially those driving soil biological activities (rather than total SOC), should be considered when evaluating the effects of conservation tillage practices on SOC sequestration. We highlight the development of site-specific strategies to effectively manage C stocks and to enhance local parameterization while developing biogeochemical and Earth system models.

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.

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.

Non-additive responses of soil C and N to rice straw and hairy vetch (Vicia villosa Roth L.) mixtures in a paddy soil

We studied the effects of mixing rice straw and hairy vetch plant residues in a subtropical paddy soil, on subsequent carbon (C) and nitrogen (N) dynamics. Using a theoretical framework, we designed two groups of experiments (involving equal amounts of residual C or N addition, referred to as either C or N treatments). Each experiment included mixed residues of rice straw and hairy vetch at different mixing ratios. Soils together with residues were incubated at 25 °C under waterlogged conditions for 100 days. Greenhouse gas (GHG) emissions and soil C and N fractions were measured continuously. Both C and N treatments affected soil C and N dynamics, and these dynamics were quantitatively dependent on residue C/N ratios. The effect of residue mixtures on C and N dynamics could not be predicted from single residues, since there were non-additive effects of residue mixtures. Synergistic effects were generally more frequent than antagonistic effects. Residue mixtures tended to enhance CO2 and CH4 emissions in both C and N treatments but decreased N2O emissions in the N treatment. In the N treatment, dissolved organic C (DOC), dissolved organic N (DON), and microbial biomass C (MBC) concentrations increased. DOC and DON concentrations decreased in the C treatment. Residue mixtures enhanced the global warming potentials (GWP) of greenhouse gases (GHG) emitted from soil by non-additive synergistic effects. The C/N ratio of residue mixtures affected the non-additive responses of soil C and N dynamics, for example mixtures with a C/N ratio of 25 had higher CO2 emissions and DOC concentrations than those with a C/N ratio of 35 as a consequence of non-additive effects, however, CH4 emissions and MBC concentrations were higher in mixtures with a C/N ratio of 35 than in mixtures with a C/N ration of 25. These results indicated that non-additive effects can impact soil C and N dynamics and that residue C/N ratios play an important role in influencing non-additive effects. Applying a single residue to paddy soils may be better than residue mixtures from a GWP mitigation perspective.

Mechanisms of rice straw biochar effects on phosphorus sorption characteristics of acid upland red soils

An important pathway for biochar to alter the availability of soil phosphorus (P) is to change P sorption characteristics of the soil. The aim of this study was to understand the mechanisms of biochar effects on P sorption in acid upland red soils in the presence of different concentrations of exogenous P. Rice straw biochar (RSB) was prepared and applied at rates of 0, 1%, 3%, and 5% (w/w) to three red soils (MZ1, MZ2, and QY1) differing in initial pH (pH = 4.31, 4.82, and 5.68, respectively). The P sorption characteristics of these red soils were described using the Langmuir and Temkin equations and their relationships with soil basic physicochemical properties were analyzed. Furthermore, a representative red soil (MZ2) was selected to analyze the zeta potential of soil colloids and the chemical properties of sorption equilibrium solution, in order to understand their relationships with P sorption characteristics. Results showed that within a certain range of P concentration in the equilibrium solution, the amount of P sorbed by the three red soils decreased and the corresponding amount of P desorbed increased with increasing amendment rate of RSB. RSB showed the greatest effect on P desorption characteristics of MZ2 soil in the presence of higher exogenous P concentration. With increasing RSB amendment rate, the maximum P sorption of MZ1 soil decreased, while those of MZ2 and QY1 soils increased after an initial decrease. Phosphate sorption equilibrium constant and maximum P buffer capacity of each soil first increased and then decreased. However, a single physicochemical property could not interpret complex changes in multi-factors that jointly determine the P sorption characteristics of red soils. In the case of MZ2 soil, RSB amendment shifted the zeta potential of soil colloids to the negative direction; this decreased the positive charge and increased the negative charge on the soil surface, thus reducing P sorption in the MZ2 soil. In the presence of the same concentration of exogenous P, RSB amendment altered the pH, dissolved organic C (DOC), humification index (HIX), and maximum fluorescence intensity (Fmax) in the sorption equilibrium solution. In most cases, the amount of P sorbed by the MZ2 soil was negatively correlated with the pH value, DOC concentration, HIX value, and Fmax value of humic-like dissolved organic matter (DOM), and positively correlated with the Fmax value of protein-like DOM (P < 0.05 or P < 0.01). The relative fractional distribution of the contents for humic-like and protein-like DOM might determine the difference in the P sorption characteristics of MZ2 soil. In conclusion, different amendment rates of RSB affected the release of phosphate from soil surfaces into the solution by altering basic physicochemical and electrochemical properties of red soils and chemical properties of sorption equilibrium solution.