W. Daniel Reynolds

Time domain reflectometry: a seminal technique for measuring mass and energy in soil

Abstract Soil water exerts a strong influence on the transfer and storage of solutes, heat, air, and even water itself, within the soil profile. Soil water also dominates the mass and energy balance of the soil–atmosphere interface. Over the last decade or so, the development and continuing refinement of the time-domain reflectometry (TDR) technique for in situ, nondestructive measurement of water, ionic solutes and air has revolutionized the study and management of the transfer and storage of mass and energy within the soil profile. TDR-measured water content has been applied successfully to water balance studies ranging from the km scales of small watersheds to the mm scale of the root–soil interface. TDR-measured ionic solute status, which applies to the same sample volume as the water content measurement, has been used successfully on soil column, field plot and whole field scales for in situ determination of solute transport parameters, such as pore water velocity and dispersivity. TDR-measurement of air-filled porosity in space and time has given new insights into the mechanisms controlling aeration and gaseous exchange in the crop root zone. The combined water content – solute mass measurement capability of TDR has made this technique a very powerful tool for characterizing solute leaching characteristics, as well as for evaluating solute transport theories and solute transport models. The portability of TDR instrumentation coupled with the simplicity and flexibility of TDR soil probes has allowed the separation of water and solute content measurement error from soil variability, resulting in the capability for determining the mechanisms behind the spatial and temporal variability in field-based soil water content distributions and solute leaching patterns. The usefulness and power of the TDR technique for characterizing mass and energy in soil is increasing rapidly through continuing improvements in operating range, probe design, multiplexing and automated data collection.

AN ANALYSIS OF THE PERCOLATION TEST BASED ON THREE-DIMENSIONAL SATURATED-UNSATURATED FLOW FROM A CYLINDRICAL TEST HOLE

The percolation test is commonly used to determine site suitability and filter field design for on-site wastewater treatment facilities. As now practiced, however, the test is neither standardized nor scientifically sound. The test hole radius, the depth of water in the hole, and the measurement procedure vary widely within and between jurisdictions; and more importantly, the present interpretation of the percolation test does not take into account the capillarity component of flow in unsaturated soils. This paper presents a new analysis of the percolation test, based on three-dimensional, saturated-unsaturated flow theory. It accounts for capillarity, hole radius, and water depth, and it explains much of the anomalous behavior previously observed in percolation rates. The new analysis identifies the field-saturated hydraulic conductivity and the matric flux potential, rather than the percolation rate, as the main soil hydraulic properties relevant to determining site suitability and filter field design. Procedures are suggested for determining the field-saturated hydraulic conductivity and matric flux potential, and for dealing with spatial variability.

Influence of current and previous crops on soil basal and potential denitrification rates

Soil and crop management including crop rotation influences available organic carbon and soil nitrate levels, which may in turn affect denitrification losses from soils. The objective of this paper was to determine how the current and previous crop affect denitrification by comparing the basal denitrification rate (BDR), denitrification rate with added nitrate (DAN), and potential denitrification rate (PDR) (amended with glucose and nitrate) of a clay loam soil under monoculture corn (C), soybean (S), and winter wheat (WW) with or without underseeded red clover (RC) and under each phase of a 2-year crop rotation (C-S) and two 3-year crop rotations (C-S-WW, C-S-WW+RC). The BDRs were greater in the 3-year C-S-WW rotation treatments than in the 2-year C-S rotation and monoculture C, S, and WW treatments. The WW+RC phase of the C-S-WW+RC treatment was found to have a greater BDR and DAN than the corn phase of the rotation. Available organic carbon was found to limit denitrification in the BDR and DAN incubations as evidenced by the 2- to 21-fold increase in denitrification when glucose was added in the PDR assay. Further, the significant relationship between soil respiration and BDR suggests that available carbon was a limiting factor. This study found that both the current crop and previous crops in a rotation affected soil denitrification rates substantially; and that denitrification rate was increased when a mixture of crop residues were added to soil (i.e., from growing crops in rotation) relative to when only a single residue was added (i.e., monoculture cropping).

Solute dynamics and the Ontario nitrogen index: I. Chloride leaching1

Abstract: The nitrogen (N) index for humid temperate southern Ontario, Canada (Ontario N index) incorporates previous and current crop type, fertilizer and (or) manure management, and hydrologic soil group (HSG) to estimate risk for contamination of tile drainage water and groundwater by nitrate leached below the primary crop root zone (top 60 cm of soil). The Ontario N index has received limited ground-truthing, and the leaching component was assessed using chloride tracer (ClTR) on five soils (one sandy loam, two loams, and two clay loams) representing four HSG-based risk levels (HSG-A, high risk; HSG-B, medium risk; HSG-C, low risk; HSG-D, very low risk). A square-wave pulse of ClTR was applied to the soil surfaces in fall 2007 as KCl, and movement and loss of ClTR was tracked over 1-1.2 years using monthly soil core samples collected from the top 60-80 cm. For all five soils, 60-96% of ClTR was leached out of the primary crop root zone (below 60 cm depth) during the noncropping period (October 2007 to March 2008 inclusive), and >80% was leached out of the root zone within 1 year. The percentage of ClTR that leached did not correlate with precipitation or HSG designation, but produced significant (P < 0.05) power function regressions with minimum and harmonic mean saturated soil hydraulic conductivity (Ksat) measured in the top 50-60 cm. ClTR leaching rate appeared to be controlled primarily by Ksat in a manner consistent with infiltration and solute transport theory. It was consequently proposed that solute leaching loss versus Ksat relationships may improve N index risk estimates for both southern Ontario and other humid temperate regions.

Enrichment of antibiotic resistance genes in soil receiving composts derived from swine manure, yard wastes, or food wastes, and evidence for multi-year persistence of swine Clostridium spp.

The impact of amendment with swine manure compost (SMC), yard waste compost (YWC), or food waste compost (FWC) on the abundance of antibiotic resistance genes in soil was evaluated. Following a commercial-scale application of the composts in a field experiment, soils were sampled periodically for a decade, and archived air-dried. Soil DNA was extracted and gene targets quantified by qPCR. Compared with untreated control soil, all 3 amendment types increased the abundance of gene targets for up to 4 years postapplication. The abundance of several gene targets was much higher in soil amended with SMC than in soil receiving either YWC or FWC. The gene target ermB remained higher in the SMC treatment for a decade postapplication. Clostridia were significantly more abundant in the SMC-amended soil throughout the decade following application. Eight percent of Clostridium spp. isolates from the SMC treatment carried ermB. Overall, addition of organic amendments to soils has the potential to increase the abundance of antibiotic resistance genes. Amendments of fecal origin, such as SMC, will in addition entrain bacteria carrying antibiotic resistance genes. Environmentally recalcitrant clostridia, and the antibiotic resistance genes that they carry, will persist for many years under field conditions following the application of SMC.

Earthworm population dynamics as a consequence of long-term and recently imposed tillage in a clay loam soil

Abstract: Earthworm abundances were tracked from 1997 to 2012 in established tillages (since 1983) and recently imposed tillages (since 1997) from a Brookston clay loam soil (Orthic Humic Gleysol) at Woodslee, ON. The tillages included the following systems: long-term fall moldboard plowing (CT83) and its 1997 conversion to no tillage (NT97-CT83), long-term no tillage (NT83) and its conversion to moldboard plowing (CT97-NT83), long-term ridge tillage (RT83) and its conversion to moldboard plowing (CT97-RT83), and long-term bluegrass sod (BG83) and its conversion to moldboard plowing (CT97-BG83). Lumbricus terrestris and Aporrectodea turgida were the most abundant of six species identified. The NT83 system had the greatest earthworm numbers except for 2012 when RT83 had equal abundance because of increased Ap. turgida juveniles. Populations in NT97-CT83 increased significantly from 1997 to 2012 because of reduced mechanical disturbance and greater surface residues. During 1997, 1999, and 2003, mean abundance in CT97-BG83 was not different from that of BG83, which likely occurred because buried sod continued to provide ample food. The CT97-RT83 system showed a decline in earthworm populations relative to RT83. The CT97-NT83 treatment had the most significant earthworm decline, reflecting a substantial increase in soil disturbance. Characterizing tillage system effects on earthworm dynamics (e.g., diversity, occurrence, adult, and juvenile abundance) will provide essential data for landscape models.