Randall J. Miles

Temporal and spatial distributions of ammonia-oxidizing archaea and bacteria and their ratio as an indicator of oligotrophic conditions in natural wetlands

Ammonia-oxidizing organisms play an important role in wetland water purification and nitrogen cycling. We determined soil nitrification rates and investigated the seasonal and spatial distributions of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in three freshwater wetlands by using specific primers targeting the amoA genes of AOA and AOB and real-time quantitative polymerase chain reaction (qPCR). The nitrifying potentials of wetland soils ranged from 1.4 to 4.0 μg g(-1) day(-1). The specific rates of ammonia oxidation activity by AOA and AOB at the Bee Hollow wetlands were 1.9 fmol NH(3) cell(-1) day(-1) and 36.8 fmol NH(3) cell(-1) day(-1), respectively. Soil nitrification potential was positively correlated with both archaeal and bacterial amoA abundance. However, the gene copies of AOA amoA were higher than those of AOB amoA by at least an order of magnitude in wetland soils and water in both summer and winter over a three year study period. AOB were more sensitive to low temperature than AOA. The amoA gene copy ratios of AOA to AOB in top soils (0-10 cm) ranged from 19 ± 4 to 100 ± 11 among the wetland sites. In contrast, the ratio of the wetland boundary soil was 10 ± 2, which was significantly lower than that of the wetland soils (P < 0.001). The NH(4)(+)-N concentrations in wetland water were lower than 2 mg/L throughout the study. The results suggest that ammonium concentration is a major factor influencing AOA and AOB population in wetlands, although other factors such as temperature, dissolved oxygen, and soil organic matter are involved. AOA are more persistent and more abundant than AOB in the nutrient-depleted oligotrophic wetlands. Therefore, ratio of AOA amoA gene copies to AOB amoA gene copies may serve as a new biological indicator for wetland condition assessment and wetland restoration applications.

Long-Term Agroecosystem Research in the Central Mississippi River Basin: Introduction, Establishment, and Overview

Many challenges currently facing agriculture require long-term data on landscape-scale hydrologic responses to weather, such as from the Goodwater Creek Experimental Watershed (GCEW), located in northeastern Missouri, USA. This watershed is prone to surface runoff despite shallow slopes, as a result of a significant smectitic clay layer 30 to 50 cm deep that restricts downward flow of water and gives rise to a periodic perched water table. This paper is the first in a series that documents the database developed from GCEW. The objectives of this paper are to (i) establish the context of long-term data and the federal infrastructure that provides it, (ii) describe the GCEW/ Central Mississippi River Basin (CMRB) establishment and the geophysical and anthropogenic context, (iii) summarize in brief the collected research results published using data from within GCEW, (iv) describe the series of papers this work introduces, and (v) identify knowledge gaps and research needs. The rationale for the collection derives from converging trends in data from long-term research, integration of multiple disciplines, and increasing public awareness of increasingly larger problems. The outcome of those trends includes being selected as the CMRB site in the USDA-ARS Long-Term Agro-Ecosystem Research (LTAR) network. Research needs include quantifying watershed scale fluxes of N, P, K, sediment, and energy, accounting for fluxes involving forest, livestock, and anthropogenic sources, scaling from near-term point-scale results to increasingly long and broad scales, and considering whole-system interactions. This special section informs the scientific community about this database and provides support for its future use in research to solve natural resource problems important to US agricultural, environmental, and science policy.

Combining Proximal and Penetrating Soil Electrical Conductivity Sensors for High-Resolution Digital Soil Mapping

Proximal ground conductivity sensors produce high spatial resolution maps that integrate the bulk electrical conductivity (ECa) of the soil profile. For meaningful interpretation, variability in conductivity maps must either be inverted to profile conductivity or be directly calibrated to profile properties. Penetrating apparent electrical conductivity (ECa–P) sensors produce high depth resolution data at relatively fewer spatial locations. The objectives of this research were to (i) investigate the profile source of ECa in claypan soils via a detailed examination of ECa–P profiles; (ii) examine the potential for feature detection with ECa–P in claypan soils; and (iii) determine if ECa sensors can be calibrated to ECa–P features. Two study areas were chosen representing the claypan soils of north-east Missouri, USA. Profile conductivity was measured at high depth resolution on soil cores using a miniaturised Wenner conductivity probe and in the field using a conductivity penetrometer. Proximal ground conductivity was mapped with one direct contact sensor and two non-contact sensors, providing five distinct coil/electrode geometries. Increasing ECa–P was observed below the claypan, correlated with decreasing clay and water content and increasing bulk density. Depth to the claypan was successfully calibrated to derivative peaks on ECa–P profiles (R 2 = 0.72, p < 0.001). Relationships between ECa and ECa–P features were poor (R 2∼ 0.21) to good (R 2∼ 0.87) on a field-specific basis. Results show that ECa–P can be used for calibration of ECa to the depth to claypan.

LIMING OF TWO ACIDIC SOILS IMPROVED GRASS TETANY RATIO OF STOCKPILED TALL FESCUE WITHOUT INCREASING PLANT AVAILABLE PHOSPHORUS

Previous research demonstrated that phosphorus (P) fertilization of tall fescue (Festuca arundinacea Schreb.) pastures on acidic soils increased leaf P, magnesium (Mg), and calcium (Ca) concentrations and reduced the risk of grass tetany. It has been suggested that liming could have a similar effect by increasing soil pH and remobilizing sorbed P. To investigate this option, calcitic or dolomitic limestone was applied to stockpiled tall fescue plots on two acidic soils, a fragipan and a claypan. As expected, increasing rates of limestone were correlated with higher soil pHs. Contrary to predictions, limestone applications had very little effect on soil or plant P. However, both types of limestone at both locations altered leaf macronutrient concentrations in a manner that improved the grass tetany equivalent ratio: increased Ca and/or Mg and decreased K concentrations, thus lowered the likelihood of grass tetany in grazing beef cattle.

125 Years of Soil and Crop Management on Sanborn Field: Effects on Soil Physical Properties Related to Soil Erodibility

ABSTRACT Long-term management systems cause changes to soil physical properties that may affect soil erosion and erodibility. A study was conducted to evaluate the effects of 125 years of continuous crop management on Sanborn Field for selected soil physical properties. Intact soil cores were collected from continuous corn (Zea mays L.), continuous wheat (Triticum aestivum L.), continuous timothy (Phleum pratense L.), and a rotation of corn–wheat–red clover (Trifolium pratense L.). Soil samples were collected from the surface horizon throughout 1 year (April, July, and November 2014 sampling dates). Aggregate stability, soil splash detachment, bulk density, and soil strength were measured. Significant differences in aggregate stability (P < 0.01), splash detachment (P < 0.01), soil shear strength (P < 0.05), and bulk density (P < 0.05) were found among the treatments. Continuous timothy had three to four times better aggregate stability and 50% to 75% less splash detachment compared with continuous wheat and corn, respectively. Lowest aggregate stability, lowest soil strength, highest bulk density, and highest soil splash detachment were found under continuous corn. Highest aggregate stability was found during July. Annual crops with tillage have a negative effect on soil quality and erodibility. Comparing results after 125 years with data collected after 105 years illustrates that properties have not changed dramatically during the past 20 years. Assessing the effects of long-term soil management on soil quality and erodibility is critical for society to determine the amount of soil erosion associated with selected soil management and to develop appropriate conservation practices to minimize this challenge and promote long-term sustainability.