Black carbon accrual during 2000 years of paddy-rice and non-paddy cropping in the Yangtze River Delta, China.

Rice straw burning has accompanied paddy management for millennia, introducing black carbon (BC) into soil as the residue of incomplete combustion. This study examined the contribution of BC to soil organic matter and the rate at which it accumulates in paddy soils as a result of prolonged paddy management. Soil depth profiles were sampled along a chronosequence of 0-2000 years of rice-wheat rotation systems and adjacent non-paddy systems (50-700 years) in the Bay of Hangzhou (Zhejiang province, China). The soil BC content and its degree of condensation were assessed using benzene-polycarboxylic acids (BPCAs) as geochemical markers. The results showed that despite regular long term BC input, BC only contributed 7-11% of total soil organic carbon (SOC) in the topsoil horizons. Nevertheless, along with SOC, paddy soils accumulated BC with increasing duration of management until 297 years to reach a steady-state of 13 t BC ha(-1). This was 1.8 times more than in non-paddy soils. The fate of BC in paddy soils (0-1 m) could be modeled revealing an average annual input of 44 kg ha(-1) yr(-1), and a mean residence time of 303 years. The subsoils contributed at least 50% to overall BC stocks, which likely derived from periods prior to land embankment and episodic burial of ancient topsoil, as also indicated by BPCA pattern changes. We conclude that there is a significant but limited accumulation of C in charred forms upon prolonged paddy management. The final contribution of BC to total SOC in paddy soils was similar to that in other aerobic ecosystems of the world.

Fertilizer P-derived uranium continues to accumulate at Rothamsted long-term experiments.

The application of phosphorus (P) fertilizers inevitably contributes to the accumulation of trace elements, such as uranium (U), in agricultural soils. The fertilizer-derived U accumulation was first reported in Rothamsted Research in 1979. In the present study, we expand this early key research by evaluating the fertilizer-derived U accumulation in topsoil (0-23 cm) from 1876 to the 2010s. We found that total U accumulation rates ranged from 2.8 to 6.1 µg U kg-1 yr-1 at the Broadbalk and Park Grass, respectively, being similar to those observed 40 years ago. This highlights that U accumulation is still an ongoing process in Rothamsted. Fortunately, the proportion of fertilizer-derived U did not significantly increase in the ammonium acetate extractable ('proxy' of plant-available) fraction over 130 years. In addition, we compiled an overview of the global rate of mineral P fertilizer-derived U accumulation in agricultural systems using existing literature (36 experimental trials, from 11 countries). The resulting dataset predicts an estimated mean U accumulation of 0.85 µg U kg-1 soil for an annual application of 1 kg P ha-1 in the topsoil of agricultural systems (0.26 µg U kg-1 per kg P ha-1 for arable land and 1.34 µg U kg-1 per kg P ha-1 for grassland). The annual U accumulation per applied kg P ha-1 being 0.08 (Broadbalk) and 0.17 µg U (Park Grass) corresponds to around one-third and one-eighth of the worldwide mean U accumulation for their respective agricultural systems, suggesting 'relatively' low U contents of the applied P fertilizers. Our study underscores that fertilizer-derived U accumulation is a persistent problem on the global scale, even if at different rates, and therewith suggests an evaluation of current regulatory limits and acceptable U input levels from P fertilization.

Groundwater controls on colloidal transport in forest stream waters.

Biogeochemical changes of whole catchments may, at least in part, be deduced from changes in stream water composition. We hypothesized that there are seasonal variations of natural nanoparticles (NNP; 1-100 nm) and fine colloids (<300 nm) in stream water, which differ in origin depending on catchment inflow parameters. To test this hypothesis, we assessed the annual dynamics of the elemental composition of NNP and fine colloids in multiple water compartments, namely in stream water, above and below canopy precipitation, groundwater and lateral subsurface flow from the Conventwald catchment, Germany. In doing so, we monitored meteorological and hydrological parameters, total element loads, and analyzed element concentrations of org C, Al, Si, P, Ca, Mn and Fe by Asymmetric Flow Field Flow Fractionation (AF4). The results showed that colloid element concentrations were < 5 µmol/L. Up to an average of 55% (Fe) of total element concentrations were not truly dissolved but bound to NNP and fine colloids. The colloid patterns showed seasonal variability with highest loads in winter. The presence of groundwater-derived colloidal Ca in stream water showed that groundwater mainly fed the streams throughout the whole year. Overall, the results showed that different water compartments vary in the NNP and fine colloidal composition making them a suitable tool to identify the streams NNP and fine colloid sources. Given the completeness of the dataset with respect to NNP and fine colloids in multiple water compartments of a single forest watershed this study adds to the hitherto underexplored role of NNP and fine colloids in natural forest watersheds.

Towards a global-scale soil climate mitigation strategy.

Sustainable soil carbon sequestration practices need to be rapidly scaled up and implemented to contribute to climate change mitigation. We highlight that the major potential for carbon sequestration is in cropland soils, especially those with large yield gaps and/or large historic soil organic carbon losses. The implementation of soil carbon sequestration measures requires a diverse set of options, each adapted to local soil conditions and management opportunities, and accounting for site-specific trade-offs. We propose the establishment of a soil information system containing localised information on soil group, degradation status, crop yield gap, and the associated carbon-sequestration potentials, as well as the provision of incentives and policies to translate management options into region- and soil-specific practices.

Pesticides and antibiotics in permanent rice, alternating rice-shrimp and permanent shrimp systems of the coastal Mekong Delta, Vietnam.

BACKGROUND: Salinity intrusion into coastal regions is an increasing threat to agricultural production of salt sensitive crops like paddy rice. In the coastal Mekong Delta, farmers respond by shifting to more salinity tolerant agricultural production systems such as alternating rice-shrimp and permanent shrimp. While shrimps are sensitive to pesticide residues used on rice, the use of antibiotics in shrimp farming can cause contamination in rice crops. These patterns of cross-contamination are not well documented empirically in the rapidly changing agricultural landscape. OBJECTIVE AND METHODS: Our objective was to understand changing pollution patterns induced by shifts in agricultural land use system. We addressed this by i) documenting pesticide and antibiotic use in three different agriculture land use systems (permanent rice, alternating rice-shrimp and permanent shrimp), and by ii) determining residues of pesticides and antibiotics in top soil layers of these three land use systems. Samples were taken in Sóc Trang and Ben Tre province in the Mekong Delta, Vietnam. Chemical analyses comprised 12 of the most commonly used pesticides in rice paddies and six common antibiotics used in shrimp production. RESULTS: Results showed that residues of pesticides were present in all agricultural land use systems, including shrimp aquaculture. Active ingredients were mostly fungicides with a maximum concentration of 67 µg kg-1 found for isoprothiolane in permanent rice systems, followed by alternating rice-shrimp and permanent shrimp systems. Furthermore, antibiotics were present ubiquitously, with fluoroquinolones accumulating to larger amounts than sulfonamides and diaminopyrimidines. All concentrations were below critical lethal threshold values. CONCLUSION: Overall, farmers were most conscious of agrochemical use in alternating rice-shrimp systems to prevent harm to shrimps, which was reflected in overall lower concentrations of agrochemicals when compared to rice systems. Thus, alternating rice-shrimp systems present a low risk option in terms of food safety, which may bring additional benefits to this so far rather low-input system in brackish water transition zone.

Analysis of aged sulfadiazine residues in soils using microwave extraction and liquid chromatography tandem mass spectrometry.

An efficient extraction of sulfadiazine residues from soils is difficult, as sulfadiazine is known to form quickly sequestering residues. The objective of this study was to optimize an exhaustive extraction for aged residues of sulfadiazine and its two major metabolites, N-acetylsulfadiazine and 4-hydroxysulfadiazine, from soil. For this purpose two representative used agricultural soils (Luvisol, Cambisol) were blended with manure derived from [(14)C]sulfadiazine-treated pigs and incubated at 10 degrees C in the laboratory. After different extraction tests with various solvent mixtures (two- to four-component mixtures with water, methanol, acetonitrile, acetone, and/or ethyl acetate), different pH values (pH 4 and 9), and extraction temperatures (up to 200 degrees C), soil extracts were measured by liquid scintillation counting and liquid chromatography coupled to tandem mass spectrometry. With respect to sulfadiazine yields, stability of soil extracts, and the amount of coextracted matrix, a microwave extraction of soil (15 min, 150 degrees C) using acetonitrile/water 1:4 (v/v) is the method of choice for the exhaustive extraction of aged sulfadiazine residues from soils.

Does sea-dyke construction affect the spatial distribution of pesticides in agricultural soils? - A case study from the Red River Delta, Vietnam.

The Red River Delta is a major agricultural production area of Vietnam with year-round use of pesticides for paddy rice cultivation and other production systems. The delta is protected from flooding, storm surges and saline water intrusion by a sophisticated river and sea-dyke system. Little is known about the effects of such a dyke system on pesticide pollution in the enclosed landscape. Our aim was to address this gap by i) determining pesticide prevalence in soils and sediments within a dyked agricultural area, and by ii) assessing whether and to which degree this dyke system might affect the spatial distribution of pesticides. After sampling paddy rice fields (topsoil) and irrigation ditches (sediment) perpendicular to the dyke in Giao Thuy district, we analysed 12 of the most commonly used pesticides in this area. In soils, we detected most frequently isoprothiolane (100% detection frequency), chlorpyrifos (85%) and propiconazole (41%) while in sediments isoprothiolane (71%) and propiconazole (71%) were most frequently found. Maximum concentrations reached 42.6 µg isoprotiolane kg-1 in soil, and 35.1 µg azoxystrobin kg-1 in sediment. Our results supported the assumption that the dyke system influenced residue distribution of selected pesticides. More polar substances increasingly accumulated in fields closer to the sea-dyke (R2 = 0.92 for chlorpyrifos and 0.51 for isoprothiolane). We can thus support initiatives from local authorities to use the distance to dykes as a mean for deliniating zones of different environmental pollution; yet, the degree at which dykes influence pesticide accumulation appear to be compound specific.

Water flow paths are hotspots for the dissemination of antibiotic resistance in soil.

Antibiotic resistance genes in soil pose a potential risk for human health. They can enter the soil by irrigation with untreated or insufficiently treated waste water. We hypothesized that water flow paths trigger the formation of antibiotic resistance, since they transport antibiotics, multi-resistant bacteria and free resistance genes through the soil. To test this, we irrigated soil cores once or twice with waste water only, or with waste water added with sulfamethoxazole (SMX) and ciprofloxacin (CIP). The treatments also contained a dye to stain the water flow paths and allowed to sample these separately from unstained bulk soil. The fate of SMX and CIP was assessed by sorption experiments, leachate analyses and the quantification of total and extractable SMX and CIP in soil. The abundance of resistance genes to SMX (sul1 and sul2) and to CIP (qnrB and qnrS) was quantified by qPCR. The sorption of CIP was larger than the dye and SMX. Ciprofloxacin accumulated exclusively in the water flow paths but the resistance genes qnrB and qnrS were not detectable. The SMX concentration in the water flow paths doubled the concentration of the bulk soil, as did the abundance of sul genes, particularly sul1 gene. These results suggest that flow paths do function as hotspots for the accumulation of antibiotics and trigger the formation of resistance genes in soil. Their dissemination also depends on the mobility of the antibiotic, which was much larger for SMX than for CIP.

Pesticide fate in tropical wetlands of Brazil: an aquatic microcosm study under semi-field conditions.

A contamination of off-site aquatic environments with pesticides has been observed in the tropics, yet only sparse information exists about pesticide fate in such ecosystems. The objective of our semi-field study was to elucidate the fate of alachlor, atrazine, chlorpyrifos, endosulfan, metolachlor, profenofos, simazine, and trifluralin in the aqueous environment of the Pantanal wetland (MT, Brazil). To this aim, water and water/sediment microcosms of two sizes (0.78 and 202 l) were installed in the outskirts of this freshwater lagoon environment and pesticide dissipation was monitored for up to 50 d after application. The physical-chemical water conditions that developed in the microcosms were reproducible among field replicates for both system sizes. Pesticide dissipation was substantially enhanced for most pesticides in small microcosms relative to the large ones (reduced DT(50) by a factor of up to 5.3). The presence of sediment in microcosms led to increased persistence of chlorpyrifos, endosulfan, and trifluralin in the test systems, while for polar pesticides (alachlor, atrazine, metolachlor, profenofos, and simazine) a lesser persistence was observed. Atrazine, simazine, metolachlor, and alachlor were identified as the most persistent pesticides in large water microcosms (DT(50) > or = 47 d); in large water/sediment systems endosulfan beta, atrazine, metolachlor, and simazine showed the slowest dissipation (DT(50) > or = 44 d). A medium-term accumulation in the sediment of tropical ecosystems can be expected for chlorpyrifos and endosulfan isomers (11-35% of applied amount still extractable at 50 d after application). We conclude that the persistence of the studied pesticides in aquatic ecosystems of the tropics is not substantially lower than during summer in temperate regions.

Pesticide pollution of multiple drinking water sources in the Mekong Delta, Vietnam: evidence from two provinces.

Pollution of drinking water sources with agrochemicals is often a major threat to human and ecosystem health in some river deltas, where agricultural production must meet the requirements of national food security or export aspirations. This study was performed to survey the use of different drinking water sources and their pollution with pesticides in order to inform on potential exposure sources to pesticides in rural areas of the Mekong River delta, Vietnam. The field work comprised both household surveys and monitoring of 15 frequently used pesticide active ingredients in different water sources used for drinking (surface water, groundwater, water at public pumping stations, surface water chemically treated at household level, harvested rainwater, and bottled water). Our research also considered the surrounding land use systems as well as the cropping seasons. Improper pesticide storage and waste disposal as well as inadequate personal protection during pesticide handling and application were widespread amongst the interviewed households, with little overall risk awareness for human and environmental health. The results show that despite the local differences in the amount and frequency of pesticides applied, pesticide pollution was ubiquitous. Isoprothiolane (max. concentration 8.49 µg L(-1)), fenobucarb (max. 2.32 µg L(-1)), and fipronil (max. 0.41 µg L(-1)) were detected in almost all analyzed water samples (98 % of all surface samples contained isoprothiolane, for instance). Other pesticides quantified comprised butachlor, pretilachlor, propiconazole, hexaconazole, difenoconazole, cypermethrin, fenoxapro-p-ethyl, tebuconazole, trifloxystrobin, azoxystrobin, quinalphos, and thiamethoxam. Among the studied water sources, concentrations were highest in canal waters. Pesticide concentrations varied with cropping season but did not diminish through the year. Even in harvested rainwater or purchased bottled water, up to 12 different pesticides were detected at concentrations exceeding the European Commission's parametric guideline values for individual or total pesticides in drinking water (0.1 and 0.5 µg L(-1); respectively). The highest total pesticide concentration quantified in bottled water samples was 1.38 µg L(-1). Overall, we failed to identify a clean water source in the Mekong Delta with respect to pesticide pollution. It is therefore urgent to understand further and address drinking water-related health risk issues in the region.

Leaching and degradation of corn and soybean pesticides in an Oxisol of the Brazilian Cerrados.

Pesticide pollution of ground and surface water is of growing concern in tropical countries. The objective of this pilot study was to evaluate the leaching potential of eight pesticides in a Brazilian Oxisol. In a field experiment near Cuiabá, Mato Grosso, atrazine, chlorpyrifos, lambda-cyhalothrin, endosulfane alpha, metolachlor, monocrotofos, simazine, and trifluraline were applied onto a Typic Haplustox. Dissipation in the topsoil, mobility within the soil profile and leaching of pesticides were studied for a period of 28 days after application. The dissipation half-life of pesticides in the topsoil ranged from 0.9 to 14 d for trifluraline and metolachlor, respectively. Dissipation curves were described by exponential functions for polar pesticides (atrazine, metolachlor, monocrotofos, simazine) and bi-exponential ones for apolar substances (chlorpyrifos, lambda-cyhalothrin, endosulfane alpha, trifluraline). Atrazine, simazine and metolachlor were moderately leached beyond 15 cm soil depth, whereas all other compounds remained within the top 15 cm of the soil. In lysimeter percolates (at 35 cm soil depth), 0.8-2.0% of the applied amounts of atrazine, simazine, and metolachlor were measured within 28 days after application. Of the other compounds less than 0.03% of the applied amounts was detected in the soil water percolates. The relative contamination potentials of pesticides, according to the lysimeter study, were ranked as follows: metolachlor > atrazine = simazine >> monocrotofos > endsulfane alpha > chlorpyrifos > trifluraline > lambda-cyhalothrin. This order of the pesticides was also achieved by ranking them according to their effective sorption coefficient Ke, which is the ratio of Koc to field-dissipation half-life.

Fate of pesticides in tropical soils of Brazil under field conditions.

The potential of pesticides for nonpoint ground water pollution depends on their dissipation and leaching behavior in soils. We investigated the fate of 10 pesticides in two tropical soils of contrasting texture in the Brazilian Cerrado region near Cuiabá during an 80-d period, employing topsoil dissipation studies, soil core analyses, and lysimeter experiments. Dissipation of pesticides was rapid, with field half-lives ranging from 0.8 to 20 d in Ustox and 0.6 to 11.8 d in Psamments soils. Soil core analyses showed progressive leaching of polar pesticides in Psamments, whereas in Ustox pesticides were rapidly transported to 40 cm soil depth regardless of their sorption properties, suggesting that leaching was caused by preferential flow. In lysimeter experiments (35 cm soil depth), cumulative leaching was generally low, with < or = 0.02% and < or = 0.19% of the applied amounts leached in Ustox and Psamments, respectively. In both soils, all pesticides but the pyrethroids were detected in percolate at 35 cm soil depth within the first 6 d after application. Cumulative efflux and mean concentrations of pesticides in percolate were dosely correlated with their Groundwater Ubiquity Score (GUS). The presence of alachlor (2-chloro-2', 6'-diethyl-N-methoxymethylacetanilide), atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine), metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], simazine [2-chloro-4,6-bis(ethylamino)-1,3,5-triazine], and trifluralin (2,6-dinitro-N,N-dipropyl-4-trifluoromethylaniline) throughout the soil profile and in percolate of wick lysimeters at 95 cm soil depth indicated that a nonpoint pollution of ground water resources in tropical Brazil cannot be ruled out for these substances.

Perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) in surface waters, sediments, soils and wastewater - A review on concentrations and distribution coefficients.

The sorption of perfluorinated compounds (PFCs) to soils and sediments determines their fate and distribution in the environment, but there is little consensus regarding distribution coefficients that should be used for assessing the environmental fate of these compounds. Here we reviewed sorption coefficients for PFCs derived from laboratory experiments and compared these values with the gross distribution between the concentrations of PFCs in surface waters and sediments or between wastewater and sewage sludge. Sorption experiments with perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) suggest that their sorption can be described reasonably well as a partitioning-like process with an average log K(oc) of approximately 2.8 for PFOA and 3.0 for PFOS. However, median concentrations in sediments (PFOA, 0.27 ng g(-1); PFOS, 0.54 ng g(-1)) or sewage sludge (PFOA, 37 ng g(-1); PFOS, 69 ng g(-1)) in relation to median concentrations in surface water (PFOA, 3ngl(-1); PFOS, 3ngl(-1)) or wastewater treatment effluent (PFOA, 24 ng l(-1); PFOS, 11 ng l(-1)), suggest that effective log K(oc) distribution coefficients for the field situation may be close to 3.7 for PFOA and 4.2 for PFOS. Applying lab-based log K(oc) distribution coefficients can therefore result in a serious overestimation of PFC concentrations in water and in turn to an underestimation of the residence time of PFOA and PFOS in contaminated soils. Irrespective of the dissipation kinetics, the majority of PFOA and PFOS from contaminated soils will be transported to groundwater and surface water bodies.

Sequestration of manure-applied sulfadiazine residues in soils.

It is not the total but the (bio)accessible concentration of veterinary medicines that determines their toxicity in the environment. We elucidate the changes in (bio)accessibility of manure-applied sulfadiazine (SDZ) with increasing contact time in soil. Fattening pigs were medicated with 14C-labeled SDZ, and the contaminated manure (fresh and aged) was amended to 2 soil types (Cambisol, Luvisol) and incubated for 218 days at 10 degrees C in the dark. Antibiotic residues of different bioaccessibility were approached by sequential extractions with 0.01 M CaCl2 (CaCl2 fraction), methanol (MeOH fraction), and finally acetonitrile/water (residual fraction, microwave extraction at 150 degrees C). In each fraction, total radioactivity, SDZ, and its major metabolites were quantified. The results showed that both SDZ and,to a lesser extent 4-hydroxysulfadiazine (4-OH-SDZ) were rapidly reformed from N-acetylsulfadiazine (N-ac-SDZ) during the first 2-4 weeks after fresh manure application, i.e., the N-acetylated metabolite does not sequester in soil to a significant extent Yet, the water and methanol extractable SDZ and 4-OH-SDZ also dissipated rapidly (DT50 = 6.0-32 days) for the fresh manure treatment with similar rate constants for both soil types. In the residual fractions, however, the concentrations of both compounds increased with time. We conclude that the residual fraction comprises the sequestered pool of SDZ and its hydroxylated metabolite. There they are entrapped and may persist in soil for several years. Including the residual fraction into fate studies thus yields dissipation half-lives of SDZ which exceed those previously reported for sulfonamides by a factor of about 100.

Field aging of insecticides after repeated application to a northern Thailand ultisol.

Field aging immobilizes pollutants and reduces their toxicity, but it also boosts their accumulation and holds the risk of future release. We investigated the aging of six insecticides (water solubilities: 0.33 mg L (-1)-completely miscible) applied five times (10-day intervals) to a tropical fruit orchard under natural weather conditions. After sequential extractions of soil samples with 0.01 M CaCl 2, methanol (MeOH), and acetone/ethylacetate/water (AEW), a conventional ( K OC(app) = [ c(MeOH) + c(AEW)]/ c(CaCl 2), normalized to soil organic carbon) and a newly introduced distribution ratio (MAR = MeOH/AEW ratio; c(MeOH)/ c(AEW)) were calculated. Field half-lives of the insecticides correlated with K OC(app) but not with MAR, which might reflect that dissipation was significantly affected by abiotic processes. The extent of aging was related to hydrophobicity of the compounds and most pronounced for endosulfan (3-fold increase in K OC(app) within 84 days). For dimethoate, this increase was even steeper (5- to 10-fold within 10 days), which was, however, mostly caused by dissipation from labile pools rather than by aging. The K OC(app) of chlorpyrifos remained constant, but a significant decrease in MAR ( r = -0.78) revealed that sorption strength increased nevertheless. Results for malathion were ambiguous. Within the time frame of our study, neither K OC(app) nor MAR gave evidence for the aging of mevinphos. The different dynamics of K OC(app) and MAR for the six insecticides studied indicate that different aging mechanisms or rates, or both control the fate of the individual insecticides, which can potentially be revealed by sequential exctraction procedures.

In vitro quantification of hydrolysis-induced racemization of amino acid enantiomers in environmental samples using deuterium labeling and electron-impact ionization mass spectrometry.

D-amino acids indicate aging, bacterial origin, and pathogenic properties of peptides in the environment, but the reliable assessment of D-enantiomers must account for a yet unknown formation during hydrolyses. Here, we introduce a method for the in vitro determination of the hydrolysis-induced racemization (HIR) of amino acids in environmental samples. It involves hydrolyses with hydro- and deuteriochloric acid (6 M, 12 h, 105 degrees C), desalting, and selective detection of chiral mass fragments of amino acid-N-pentafluoropropionyl derivatives. D-Amino acids formed in 2HCl incorporated deuterium into their C(alpha) position. This resulted in a relative signal loss of the nondeuterated fragment compared with the 1HCl hydrolysate. Mathematically evaluating the relative target signal intensities of both hydrolysates allowed the quantification of the proportion of D-amino acids formed during sample processing. Side-chain incorporations of deuterium were no limitations for this method as they could be estimated from that of the respective L-enantiomers. In soil and litter samples, between 0 (D-glutamic acid) and 85% (D-alloisoleucine) of the detected D-amino acids were formed upon hydrolysis (standard error, 5-11%). For a given amino acid, the HIR varied by a factor of 2-10 between samples, thereby confirming that HIR must be individually assessed for samples from different environments.

Natural 13C abundance: a tool to trace the incorporation of dung-derived carbon into soil particle-size fractions.

During the decay of 13C enriched dung patches, the; delta 13C signal of surface soil (1-5 cm) increased with a temporary maximum after 42 d. To understand the underlying processes, we investigated the incorporation of dung-derived C into soil particle-size fractions. Dung, collected from beef steers fed on maize (delta 13C = -15.36/1000) or ryegrass (delta 13C = -25.67/1000), was applied in circular patches to a C3 pasture at North Wyke, UK. Triplicates were sampled from surface soil (1-5 cm) at 14, 28, 42, and 70 d after application, pooled, separated into fine (< 0.2 micron) and coarse clay (0.2-2 microns), silt plus fine sand (2-250 microns), and coarse sand (250-2000 microns), and analyzed for total C, N, and delta 13C. As particle-size diameter decreased, the C/N ratios decreased and delta 13C values increased at all plots due to increasing microbial alteration of soil organic matter. After dung application, ca. 60% of dung-derived C in soil was recovered in the 0.2-250 microns fractions during the whole experiment. The proportion of dung-derived C in the fine clay peaked 42 d after dung application, coinciding with the delta 13C maximum in the bulk soil and the maximum leaching rate measured in lysimeters at this time in another study at the same sites. The percentage of dung-derived C as particulate C in the coarse sand fraction increased until the end of the experiment. We conclude that incorporation of C into soil from decomposing dung patches involved both temporary sorption of leached dung C to < 0.2 micron fractions and continuous accumulation of particulate C (> 250 microns).

Quantification of priming and CO2 respiration sources following slurry-C incorporation into two grassland soils with different C content.

The fate of incorporated slurry-C was examined in a laboratory experiment using two UK grassland soils, i.e. a Pelostagnogley (5.1 %C) and a Brown Earth (2.3 %C). C3 and C4 slurries were incorporated into these two wet-sieved (C3) soils (from 4-10 cm depth). Gas samples were collected 0.2, 1, 2, 3, 4, 6, 9, 20, 30 and 40 days after slurry application and analyzed for CO2 concentration and delta13C content. Slurry incorporation into the soil strongly increased soil CO2 respiration compared with the unamended soil. Total (40 day) cumulative CO2 flux was higher for the Pelostagnogley than the Brown Earth. The 13C natural abundance tracer technique enabled quantification of the sources of respired CO2 and priming effects (days 0-9). Proportionally more slurry-derived C was respired from the Pelostagnogley (46%) than the Brown Earth (36%). The incorporated slurry-C was lost twice as fast as the native soil C in both soils. Slurry incorporation induced a priming effect, i.e. additional release of soil-derived C, most pronounced in the Pelostagnogley (highest C content). The majority of respired soil-derived C (>70%) was primed C. The study indicated that potential reductions in ammonia volatilisation following slurry injection to grasslands might be negated by enhanced loss of primed soil C (i.e. pollution swapping).

The Influence of Dung Amendments on Dissolved Organic Matter in Grassland Soil Leachates - Preliminary Results from a Lysimeter Study.

Understanding the carbon (C) cycle in grassland pasture systems requires more information about the fate of decomposing dung material within the soil. In this soil lysimeter study we successfully applied the natural 13C abundance labelling technique to trace dung-C within a temperate grassland soil. Dung was collected from beef steers fed on either maize (a C4 plant) or perennial ryegrass (a C3 plant) silages, and applied to a freely draining (C3) grassland soil. Leachates were collected from soil lysimeters (0-2.5) and (0-10 cm soil depth) to determine the organic carbon and 13C content of < 0.7 µm filtered solution. Leachates were taken from (i) control, no dung added, (ii) C3 dung and (iii) C4 dung amended soil. Results showed that, (i) the addition of dung resulted in a tenfold increase in C lost from the lysimeters in drainage waters, (ii) up to 50% of the C present in the leachates was 'native' soil C and (iii) the application of dung produced a 'priming' effect. Further work is required to verify; (i) whether increased leaching of native C following dung application is a 'true priming' phenomenon, or merely the result of 'displacement' or 'pool substitution' of soil C, and (ii) the precise conditions and mechanisms under which organic amendments induce a true 'priming' effect in grassland and other agricultural soils.