Timothy B. Parkin

Effect of sampling frequency on estimates of cumulative nitrous oxide emissions.

It is generally recognized that soil N(2)O emissions can exhibit pronounced day-to-day variations; however, measurements of soil N(2)O flux with soil chambers typically are done only at discrete points in time. This study evaluated the impact of sampling frequency on the precision of cumulative N(2)O flux estimates calculated from field measurements. Automated chambers were deployed in a corn/soybean field and used to measure soil N(2)O fluxes every 6 h from 25 Feb. 2006 through 11 Oct. 2006. The chambers were located in two positions relative to the fertilizer bands-directly over a band or between fertilizer bands. Sampling frequency effects on cumulative N(2)O-N flux estimation were assessed using a jackknife technique where populations of N(2)O fluxes were constructed from the average daily fluxes measured in each chamber. These test populations were generated by selecting measured flux values at regular time intervals ranging from 1 to 21 d. It was observed that as sampling interval increased from 7 to 21 d, variances associated with cumulative flux estimates increased. At relatively frequent sampling intensities (i.e., once every 3 d) N(2)O-N flux estimates were within +/-10% of the expected value at both sampling positions. As the time interval between sampling was increased, the deviation in estimated cumulative N(2)O flux increased, such that sampling once every 21 d yielded estimates within +60% and -40% of the actual cumulative N(2)O flux. The variance of potential fluxes associated with the between-band positions was less than the over-band position, indicating that the underlying temporal variability impacts the efficacy of a given sampling protocol.

Relationship of soil ergosterol concentration and fungal biomass

We studied the relationship between amounts of fungi and ergosterol in soil to further evaluate the use of ergosterol as an indicator of soil fungal biomass. Soils from seven sites in central Iowa were analyzed for soil ergosterol content, total fungal hyphal length (living and non-living, using calcofluor as stain), and living fungal hyphal length (using fluorescein diacetate as stain) to: 1) determine how soil ergosterol concentration correlates with total fungal hyphal length and living fungal hyphal length; and 2) determine an approximate value for ergosterol concentration in living fungal biomass present in soil. Correlation significance tests and analysis of variance indicated highly significant positive correlation between soil ergosterol content and both measures of hyphal length, but regression analysis demonstrated only a moderate degree of linear correlation between these variables (coefficients of linear correlation, r = 0.638 to 0.874). Calculated values for ergosterol concentration in living fungal biomass present in the soils examined ranged from 5 to 31 mg ergosterol g−1 living fungal biomass. Data indicate that the wide range in specific ergosterol content of living fungal biomass in soils is related to the total amount of fungal hyphae (living and non-living) in a soil. We propose a method to estimate living fungal biomass from soil ergosterol content which compensates for the variability in fungal ergosterol concentrations by accounting for this relationship. A preliminary evaluation of this approach using independent data from the literature provides support for this method, in that a high correlation (r2 = 0.999) between predicted and measured living fungal biomass was observed.

Calculating the detection limits of chamber-based soil greenhouse gas flux measurements.

Renewed interest in quantifying greenhouse gas emissions from soil has led to an increase in the application of chamber-based flux measurement techniques. Despite the apparent conceptual simplicity of chamber-based methods, nuances in chamber design, deployment, and data analyses can have marked effects on the quality of the flux data derived. In many cases, fluxes are calculated from chamber headspace vs. time series consisting of three or four data points. Several mathematical techniques have been used to calculate a soil gas flux from time course data. This paper explores the influences of sampling and analytical variability associated with trace gas concentration quantification on the flux estimated by linear and nonlinear models. We used Monte Carlo simulation to calculate the minimum detectable fluxes (α = 0.05) of linear regression (LR), the Hutchinson/Mosier (H/M) method, the quadratic method (Quad), the revised H/M (HMR) model, and restricted versions of the Quad and H/M methods over a range of analytical precisions and chamber deployment times (DT) for data sets consisting of three or four time points. We found that LR had the smallest detection limit thresholds and was the least sensitive to analytical precision and chamber deployment time. The HMR model had the highest detection limits and was most sensitive to analytical precision and chamber deployment time. Equations were developed that enable the calculation of flux detection limits of any gas species if analytical precision, chamber deployment time, and ambient concentration of the gas species are known.

NITROGEN TRANSFORMATIONS ASSOCIATED WITH EARTHWORM CASTS

Abstract Earthworms are intimately involved in the cycling of C and N in soil. Earthworm casts are enriched in mineral N; however, there have been few studies of the dynamics of microbial N transformations associated with earthworm casts. We evaluated the N-transformations in earthworm casts as affected by organic residues used as a food source by earthworms. Denitrification rate, nitrification potential and mineral N content of the casts of two earthworm species ( Octolasian tyrtaeum Savigny and Aporrectodea tuberculata Eisen) were assessed in laboratory trials. Trials were made in plastic chambers (600 g soil) with three organic-C treatments: 20 g fresh hairy vetch ( Vicia villosa Roth), 5.5 g air-dried hairy vetch or 5.5 g air-dried horse ( Equus caballus ) manure. Earthworm casts were enriched in mineral N, relative to surrounding soil, and that the amount of N accumulated in earthworm casts was a reflection of the N content of the organic matter used as a food source by the earthworms. Casts had elevated denitrification rates, compared to soil, however, rates were low relative to the elevated NO 3 − concentrations in the casts (80–100 μg NO 3 − -Ng −1 dry wt). Observed denitrification rates appeared to be related to the quality of organic matter available to the earthworms, but were not significantly affected by species of worm.

Greenhouse Gas Emissions from Two Soils Receiving Nitrogen Fertilizer and Swine Manure Slurry

The interactive effects of soil texture and type of N fertility (i.e., manure vs. commercial N fertilizer) on N(2)O and CH(4) emissions have not been well established. This study was conducted to assess the impact of soil type and N fertility on greenhouse gas fluxes (N(2)O, CH(4), and CO(2)) from the soil surface. The soils used were a sandy loam (789 g kg(-1) sand and 138 g kg(-1) clay) and a clay soil (216 g kg(-1) sand, and 415 g kg(-1) clay). Chamber experiments were conducted using plastic buckets as the experimental units. The treatments applied to each soil type were: (i) control (no added N), (ii) urea-ammonium nitrate (UAN), and (iii) liquid swine manure slurry. Greenhouse gas fluxes were measured over 8 weeks. Within the UAN and swine manure treatments both N(2)O and CH(4) emissions were greater in the sandy loam than in the clay soil. In the sandy loam soil N(2)O emissions were significantly different among all N treatments, but in the clay soil only the manure treatment had significantly higher N(2)O emissions. It is thought that the major differences between the two soils controlling both N(2)O and CH(4) emissions were cation exchange capacity (CEC) and percent water-filled pore space (%WFPS). We speculate that the higher CEC in the clay soil reduced N availability through increased adsorption of NH(4)(+) compared to the sandy loam soil. In addition the higher average %WFPS in the sandy loam may have favored higher denitrification and CH(4) production than in the clay soil.

Denitrification in Wood Chip Bioreactors at Different Water Flows

Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO(3)-N) and concentrations frequently exceed 10 mg L(-1). A laboratory column study was conducted to investigate the ability of a wood chip bioreactor to promote denitrification under mean water flow rates of 2.9, 6.6, 8.7 and 13.6 cm d(-1) which are representative of flows entering subsurface drainage tiles. Columns were packed with wood chips and inoculated with a small amount of oxidized till and incubated at 10 degrees C. Silicone sampling cells at the effluent ports were used for N(2)O sampling. (15)Nitrate was added to dosing water at 50 mg L(-1) and effluent was collected and analyzed for NO(3)-N, NH(4)-N, and dissolved organic carbon. Mean NO(3)-N concentrations in the effluent were 0.0, 18.5, 24.2, and 35.3 mg L(-1) for the flow rates 2.9, 6.6, 8.7, and 13.6 cm d(-1), respectively, which correspond to 100, 64, 52, and 30% efficiency of removal. The NO(3)-N removal rates per gram of wood increased with increasing flow rates. Denitrification was found to be the dominant NO(3)-N removal mechanism as immobilization of (15)NO(3)-N was negligible compared with the quantity of (15)NO(3)-N removed. Nitrous oxide production from the columns ranged from 0.003 to 0.028% of the N denitrified, indicating that complete denitrification generally occurred. Based on these observations, wood chip bioreactors may be successful at removing significant quantities of NO(3)-N, and reducing NO(3)-N concentration from water moving to subsurface drainage at flow rates observed in central Iowa subsoil.

SPECIES, ROTATION, AND LIFE‐FORM DIVERSITY EFFECTS ON SOIL CARBON IN EXPERIMENTAL TROPICAL ECOSYSTEMS

Extensive areas of species-rich forests in the tropics have been replaced by tree monocultures over the last two decades, and the impact on biogeochemical cycles is unclear. We characterized effects on soil carbon dynamics of species identity and rotation frequency in experimental plantations containing three native, non-N-fixing tree species, Hyeronima alchoreoides, Cedrela odorata, and Cordia alliodora, grown in monocultures and in polycultures with two monocot species, Euterpe oleracea and Heliconia imbricata. Over all treatments, change in total soil organic carbon (TSOC, 0–15 cm) after 10 years ranged from a loss of 24% (0.9 mg/ha in 1-yr rotation of Cedrela) to an increase of 14% (0.6 mg/ha under Hyeronima polycultures). Species differed in their effects on quantities of TSOC (P = 0.038), but differences were more pronounced in light particulate organic matter (LPOM; P = 0.001), a biologically active, sand-size soil fraction that constituted 6% of TSOC. Effects of rotation frequency were strong; in ...

Evaluation of potential inhibitors of methanogenesis and methane oxidation in a landfill cover soil

Abstract Biological methane (CH 4 ) production is an anaerobic process, while CH 4 consumption occurs predominantly under aerobic conditions; however, both processes can occur simultaneously in soil. Thus, field measurements of CH 4 flux reflect the net result of both consumption and production reactions. Specific inhibitors of either CH 4 consumption or production processes offer the opportunity for assessing the rates of these two processes independently. The objective of this work was to identify potential gaseous inhibitors of either CH 4 oxidation or methanogenesis, and to evaluate the effect of inhibitor concentration on these two processes. For these studies, sieved cover soil from a municipal landfill was treated with a variety of compounds including acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), ethane (C 2 H 6 ), methyl chloride (CH 3 Cl) and methyl fluoride (CH 3 F). Each experiment consisted of six different treatments which included sterile soil, untreated soil and soil with test compound concentrations of 0.00%, 0.01%, 0.1% and 1%. Incubations were conducted under both aerobic and anaerobic conditions. Several compounds completely inhibited CH 4 oxidation while not significantly influencing CH 4 production; including C 2 H 2 at 0.001%, C 2 H 4 at 0.1%, and CH 3 F at 0.1%. One compound (CH 3 Cl) was unique; in that a concentration of 0.1% inhibited methanogenesis by 88.9% but CH 4 oxidation was not significantly affected. We recommend the use of C 2 H 2 or C 2 H 4 for inhibition of CH 4 oxidation, and CH 3 Cl for inhibition of methanogenesis.

Fungal presence in paired cultivated and uncultivated soils in central Iowa, USA

Abstract Amounts of fungal biomass in adjacent cultivated and uncultivated soils in central Iowa were estimated and compared by quantifying soil ergosterol concentrations and lengths of fungal hyphae present. Both indices of fungal biomass, with one exception, indicated that there was at least twice as much fungal biomass in uncultivated soil as in cultivated soil. Levels of microbial biomass carbon in uncultivated soils were also determined to be at least twice that in cultivated soils. Data collected in this study indicate that fungi may be more significantly affected by agricultural soil management practices than other components of the soil microbial community. For two of the soils examined, calculated estimates denote that fungal biomass carbon represented approximately 20% of the total microbial biomass carbon in cultivated soil and about 33% of the microbial biomass carbon in uncultivated soil. Results of this study indicate that conventional agricultural practices result in a significant reduction of fungal biomass production in soil. Implications of differences in fungal biomass between the soils are discussed.

Comparison of DAYCENT-Simulated and Measured Nitrous Oxide Emissions from a Corn Field

Accurate assessment of N(2)O emission from soil requires continuous year-round and spatially extensive monitoring or the use of simulation that accurately and precisely predict N(2)O fluxes based on climatic, soil, and agricultural system input data. DAYCENT is an ecosystem model that simulates, among other processes, N(2)O emissions from soils. The purpose of the study was to compare N(2)O fluxes predicted by the DAYCENT model to measured N(2)O fluxes from an experimental corn field in central Iowa. Soil water content temperature and inorganic N, simulated by DAYCENT were compared to measured values of these variables. Field N(2)O emissions were measured using four replicated automated chambers at 6-h intervals, from day of year (DOY) 42 through DOY 254 of 2006. We observed that DAYCENT generally accurately predicted soil temperature, with the exception of winter when predicted temperatures tended to be lower than measured values. Volumetric water contents predicted by DAYCENT were generally lower than measured values during most of the experimental period. Daily N(2)O emissions simulated by DAYCENT were significantly correlated to field measured fluxes; however, time series analyses indicate that the simulated fluxes were out of phase with the measured fluxes. Cumulative N(2)O emission calculated from the simulations (3.29 kg N(2)O-N ha(-1)) was in range of the measured cumulative N(2)O emission (4.26 +/- 1.09 kg N(2)O-N ha(-1)).