Joshua L. Heitman

Co-Occurring Anammox, Denitrification, and Codenitrification in Agricultural Soils

ABSTRACT Anammox and denitrification mediated by bacteria are known to be the major microbial processes converting fixed N to N2 gas in various ecosystems. Codenitrification and denitrification by fungi are additional pathways producing N2 in soils. However, fungal codenitrification and denitrification have not been well investigated in agricultural soils. To evaluate bacterial and fungal processes contributing to N2 production, molecular and 15N isotope analyses were conducted with soil samples collected at six different agricultural fields in the United States. Denitrifying and anammox bacterial abundances were measured based on quantitative PCR (qPCR) of nitrous oxide reductase (nosZ) and hydrazine oxidase (hzo) genes, respectively, while the internal transcribed spacer (ITS) of Fusarium oxysporum was quantified to estimate the abundance of codenitrifying and denitrifying fungi. 15N tracer incubation experiments with 15NO3 − or 15NH4 + addition were conducted to measure the N2 production rates from anammox, denitrification, and codenitrification. Soil incubation experiments with antibiotic treatments were also used to differentiate between fungal and bacterial N2 production rates in soil samples. Denitrifying bacteria were found to be the most abundant, followed by F. oxysporum based on the qPCR assays. The potential denitrification rates by bacteria and fungi ranged from 4.118 to 42.121 nmol N2-N g−1 day−1, while the combined potential rates of anammox and codenitrification ranged from 2.796 to 147.711 nmol N2-N g−1 day−1. Soil incubation experiments with antibiotics indicated that fungal codenitrification was the primary process contributing to N2 production in the North Carolina soil. This study clearly demonstrates the importance of fungal processes in the agricultural N cycle.

Sensible Heat Observations Reveal Soil-Water Evaporation Dynamics

Soil-water evaporation is important at scales ranging from microbial ecology to large-scale climate. Yet routine measurements are unable to capture rapidly shifting near-surface soil heat and water processes involved in soil-water evaporation. The objective of this study was to determine the depth and location of the evaporation zone within soil. Three-needle heat-pulse sensors were used to monitor soil heat capacity, thermal conductivity, and temperature below a bare soil surface in central Iowa during natural wetting/ drying cycles. Soil heat flux and changes in heat storage were calculated from these data to obtain a balance of sensible heat components. The residual from this balance, attributed to latent heat from water vaporization, provides an estimate of in situ soil-water evaporation. As the soil dried following rainfall, results show divergence in the soil sensible heat flux with depth. Divergence in the heat flux indicates the location of a heat sink associated with soil-water evaporation. Evaporation estimates from the sensible heat balance provide depth and time patterns consistent with observed soil-water depletion patterns. Immediately after rainfall, evaporation occurred near the soil surface. Within 6 days after rainfall, the evaporation zone proceeded 13 mm into the soil profile. Evaporation rates at the 3-mm depth reached peak values 0.25 mm h 1 . Evaporation occurred simultaneously at multiple measured depth increments, but with time lag between peak evaporation rates for depths deeper below the soil surface. Implementation of finescale measurement techniques for the soil sensible heat balance provides a new opportunity to improve understanding of soil-water evaporation.

FIELD CALIBRATION OF THE THETA PROBE FOR DES MOINES LOBE SOILS

Knowledge of soil moisture is needed to understand crop water use, hydrology, and microclimate. A reliable, rapid technique is needed, and recently an impedance soil moisture probe (Theta Probe) has been accepted by the scientific community. The purposes of this study were to calibrate the probe for soils of Central Iowa through field sampling, to determine the number of samples needed for calibration, and to determine the effect of temperature on calibration. Laboratory calibration was conducted on Des Moines lobe soils across a range of water contents and temperatures. Including a temperature term increased the R 2 from 0.85 to 0.87. Field calibration was based on Theta Probe measurements on similar soils combined with gravimetric sampling and soil temperature determination. Although some scatter existed, the field calibration was adequate for Iowa soils (R 2 = 0.77). Inclusion of temperature did not significantly improve the calibration for the field data. To determine the appropriate number of samples needed for the field calibration, regression equations were determined from sample numbers ranging from 2 to 89, and the standard error was determined for each. Based on the standard error analysis, 20 samples was an adequate number, with no further improvement for additional data points.

Spatial and diurnal below canopy evaporation in a desert vineyard: Measurements and modeling

Evaporation from the soil surface (E) can be a significant source of water loss in arid areas. In sparsely vegetated systems, E is expected to be a function of soil, climate, irrigation regime, precipitation patterns, and plant canopy development and will therefore change dynamically at both daily and seasonal time scales. The objectives of this research were to quantify E in an isolated, drip-irrigated vineyard in an arid environment and to simulate below canopy E using the HYDRUS (2-D/3-D) model. Specific focus was on variations of E both temporally and spatially across the inter-row. Continuous above canopy measurements, made in a commercial vineyard, included evapotranspiration, solar radiation, air temperature and humidity, and wind speed and direction. Short-term intensive measurements below the canopy included actual and potential E and solar radiation along transects between adjacent vine-rows. Potential and actual E below the canopy were highly variable, both diurnally and with distance from the vine-row, as a result of shading and distinct wetted areas typical to drip irrigation. While the magnitude of actual E was mostly determined by soil water content, diurnal patterns depended strongly on position relative to the vine-row due to variable shading patterns. HYDRUS (2-D/3-D) successfully simulated the magnitude, diurnal patterns, and spatial distribution of E, including expected deviations as a result of variability in soil saturated hydraulic conductivity.

Turbidimetric Determination of Anionic Polyacrylamide in Low Carbon Soil Extracts

Concerns over runoff water quality from agricultural lands and construction sites have led to the development of improved erosion control practices, including application of polyacrylamide (PAM). We developed a quick and reliable method for quantifying PAM in soil extracts at low carbon content by using a turbidimetric reagent, Hyamine 1622. Three high-molecular weight anionic PAMs differing in charge density (7, 20, and 50 mol%) and five water matrices, deionized (DI) water and extracts from four different soils, were used to construct PAM calibration curves by reacting PAM solutions with hyamine and measuring turbidity development from the PAM-hyamine complex. The PAM calibration curve with DI water showed a strong linear relationship ( = 0.99), and the sensitivity (slope) of calibration curves increased with increasing PAM charge density with a detection limit of 0.4 to 0.9 mg L. Identical tests with soil extracts showed the sensitivity of the hyamine method was dependent on the properties of the soil extract, primarily organic carbon concentration. Although the method was effective in mineral soils, the highest charge density PAM yielded a more reliable linear relationship ( > 0.97) and lowest detection limit (0.3 to 1.2 mg L), compared with those of the lower charge density PAMs (0.7 to 23 mg L). Our results suggest that the hyamine test could be an efficient method for quantifying PAM in environmental soil water samples as long as the organic carbon in the sample is low, such as in subsurface soil material often exposed at construction sites.

Granular and Dissolved Polyacrylamide Effects on Erosion and Runoff under Simulated Rainfall.

Polyacrylamide (PAM) has been demonstrated to reduce erosion under many conditions, but less is known about the effects of its application method on erosion and concentrations in the runoff water. A rainfall simulation study was conducted to evaluate the performance of an excelsior erosion control blanket (cover) and two PAM application methods. The treatments were (i) no cover + no PAM (control), (ii) cover + no PAM, (iii) cover + granular PAM (GPAM), and (iv) cover + dissolved PAM (DPAM) applied to soil packed in wooden runoff boxes. The GPAM or DPAM (500 mg L) was surface-applied at a rate of 30 kg ha 1 d before rainfall simulation. Rainfall was applied at 83 mm h for 50 min and then repeated for another 20 min after a 30-min rest period. Runoff samples were analyzed for volume, turbidity in nephelometric turbidity units (NTU), total suspended solids (TSS), sediment particle size distribution, and PAM concentration. The cover alone reduced turbidity and TSS in runoff by >60% compared with the control (2315 NTU, 2777 mg TSS L). The PAM further reduced turbidity and TSS by >30% regardless of the application method. The median particle diameter of eroded sediments for PAM treatments was seven to nine times that of the control (12.4 μm). Loss of applied PAM in the runoff water (not sediment) was 19% for the GPAM treatment but only 2% for the DPAM treatment. Both GPAM and DPAM were effective at improving groundcover performance, but DPAM resulted in much less PAM loss.

Effects of tillage and compost amendment on infiltration in compacted soils

Soils are compacted during land development through soil excavation and heavy equipment traffic. Compacted soils have limited infiltration and are susceptible to erosion. Infiltration can be enhanced by various approaches including tillage and compost addition. The objective of this study was to determine the efficacy of tillage and adding compost to reduce stormwater runoff and sediment loss by improving infiltration in simulated postconstruction soils. Tillage treatments were tested at two sites in the Piedmont region of North Carolina (Piedmont 1 and 2). Prior to applying tillage and amendment, soils at both sites were graded to remove the surface horizon and compacted with a vibratory roller. At Piedmont 1, the treatments were compacted with no tillage, shallow (15 cm [5.9 in] depth) tillage (ST), and deep (30 cm [11.8 in] depth) tillage (DT). At Piedmont 2 the treatments were compacted, DT, and DT with incorporated compost (DT+Com). The grass seed mixtures recommended by the North Carolina Department of Transportation for the location (Piedmont) and time of planting were applied at each site. Runoff volumes (RV) and total suspended solids were measured after each of the first 12 and 13 storm events at Piedmont 1 and 2, respectively. Infiltration rate (IR) and bulk density (BD) were determined five and seven months after establishment at Piedmont 1 and 2, respectively. At both sites, RV and total amount of soil loss were reduced with tillage by 60% to 82% during the monitoring period. Neither deeper tillage nor incorporating compost significantly affected these results. Grass establishment was significantly better with tillage. The IRs measured at the end of the monitoring period were around 1 cm h−1 (0.4 in hr−1) in the compacted treatment but ranged from 19 to 33 cm h−1 (7.5 to 13 in hr−1) in the tilled treatments, again with no effects of tillage depth or compost. The results suggest that tillage to a depth of at least 15 cm (6 in) can be highly effective for improving soil conditions and reducing runoff and erosion from soils compacted as the result of construction activities.

A multi-year study of tillage and amendment effects on compacted soils

Constructing roads and buildings often involves removal of topsoil, grading, and traffic from heavy machinery. The result is exposed, compacted subsoil with low infiltration rate (IR), which hinders post-construction vegetation establishment and generates significant runoff, similar to impervious surfaces. Our goal was to assess tillage and adding amendments for improving density and maintaining perviousness of subsoils compacted during construction. The effects of tillage with and without amendments on (1) soil compaction, (2) IR, and (3) vegetative growth at five sites in North Carolina, USA were evaluated over a period of up to 32 months. The sites, representing a range of soil conditions, were located at three geographic regions; one in the Sandhills (located in Coastal Plain), one in the mountains, and three in the Piedmont. Amendments varied by site and included: (1) compost, (2) cross-linked polyacrylamide (xPAM), and (3) gypsum. Bulk density (BD) and soil penetration resistance (PR) tests were used to characterize soil physical condition. The IR was measured using a Cornell Sprinkle Infiltrometer. Vegetative growth was evaluated by measuring shoot mass and vegetative cover at all sites and root density at the Piedmont sites. Tillage decreased BD and PR compared to the compacted soil at four out of five sites for observations ranging from 24 to 32 months. Compost was applied to four sites prior to tillage and reduced BD in two of them compared to tillage alone. The IR in the tilled plots was maintained at about 3-10 times that of the compacted soil among the five sites over the monitoring periods. Adding amendments did not increase IR relative to tillage alone except at one Piedmont site, where compost and xPAM increased IR at 12 months and compost at 24 months after site establishment. Vegetative responses to tillage and amendments were inconsistent across sites. Results suggest that tillage is a viable option to reduce bulk density and increase infiltration for areas with compacted soils where vegetation is to be established, and that the effect is maintained for at least several years.

Effect of forced convection on soil water content measurement with the dual-probe heat-pulse method

The dual-probe heat pulse (DPHP) method is useful for measuring soil volumetric water content (θ) near heterogeneities such as the soil surface, but it does not consider convective heat transfer that may result from soil water movement (forced convection). In this study, we examined the effect of forced convection on estimates of soil water content using three different DPHP sensor orientations. Heat transfer theory that explicitly accounts for forced convection was used to test this effect. For Orientation I, the parallel heater and temperature probes were in a plane normal to the direction of steady water flow. The temperature probe was directly downstream from the heater probe for Orientations II and the temperature probe was upstream from the heater probe for Orientation III. A simple model based on instantaneous heating of the sensor gave excellent approximations of error in θ for Orientations II and III. Estimates of absolute error in 6 (Δθ) for Orientation I required a model based on pulsed heating of the sensor. Forced convection causes θ to be underestimated for Orientation II and overestimated for Orientation I and III. The magnitude of these errors increased logarithmically with increasing water flux density, but the error for Orientation I was substantially smaller than that for Orientations II and III. We conclude that the effect of forced convection may be large enough to render the DPHP method useless for Orientations II and III. It does not, however, appear to limit the practical utility of DPHP sensors when placed in Orientation I.

Measurement of soil‐surface heat flux with a multi‐needle heat‐pulse probe

Summary Soil-surface heat flux (G0), an important component of the surface energy balance, is often determined by summing soil heat flux (Gz) at a depth (z) below the surface and the rate of change in soil heat storage (ΔS) in the layer above z. The soil heat flux Gz is commonly measured with passive heat flux plates, but self-calibrating plates or additional corrections are required to obtain accurate data. In some cases, ΔS is neglected because of the difficulty of monitoring the dynamics of volumetric heat capacity (C), which might lead to erroneous estimates of G0. To overcome these limitations, we introduce the heat-pulse method for measuring G0 with a multi-needle heat-pulse probe (HPP). Soil temperature (T) distribution, thermal conductivity (λ) and C of the 0–52-mm layer were measured hourly on five consecutive days with an 11-needle HPP, and Gz at 50-mm depth (G50) and ΔS of the 0–50-mm layer (ΔS0–50) were determined by the gradient and calorimetric methods, respectively. Independent measurements of G50 with a self-calibrating heat flux plate and ΔS0–50 calculated with the de Vries model C were used to evaluate the HPP data. With reliable G50 and ΔS0–50 measurements, the HPP-based G0 data agreed well with those estimated from the independent method (with a mean absolute difference of 4.5 W m−2). Supporting measurements showed that determining Gz at the 50-mm depth minimized the likelihood of errors from evaporation below the measurement depth. The multi-needle HPP provides a reliable way to determine G0 in situ. Additional analysis demonstrated that by reducing the number of needles from 11 to 5, the datalogging requirement was reduced by half and G0 was still determined with acceptable accuracy. Highlights Heat flux at the soil surface (G0) was monitored by the heat-pulse technique. A multi-needle heat-pulse probe (HPP) was used to measure subsurface soil heat flux (Gz) and heat storage concurrently. Appropriate measurement depths of Gz were determined to minimize the effects of subsurface latent heat sink on G0. A simplified calculation reduced the datalogging requirement of the multi-needle HPP.