Comparison of procedures for estimating sample numbers

Abstract

Published standard methods for estimating sample numbers entail hidden tradeoffs. This note will describe the nature of these tradeoffs and their consequences. A modified version of the standard method is presented. The modification recognizes that the desired answer is implicit in the t‐statistic. The modified version uses a succession of estimates until achieving convergence. Examples are presented showing that either under‐ or overestimation is possible when using the original methods compared with the modified method. The variation in estimates reflects the variation in the tolerance probability. We recommend when to select specific methods.     

A simplified method for estimating soil compaction

We describe a simplified model that allows users to explore some of the main aspects of soil compaction. It is intended for use by non-experts, such as students, and is written as an easy-to-use spreadsheet. It estimates soil bulk density under the centre-line of a wheel track from readily available tyre details. The model uses an analytical method to estimate the propagation of stress in the soil. It contains compactibility data for contrasting soils and it accounts for both rebound and recompression realistically. We present examples that show the potential of the model in selecting tyres and wheel systems to minimise compaction.     

A quick method of soil moisture determination

Abstract

The method described here is useful for the quick determination of soil moisture, especially when many determinations are to be made at a time. The only pieces of apparatus required are some wide‐mouthed conical flasks, a few pieces of glass rod and a balance. The principle involves recording the weight of the flask filled with water and soil sample of known weight (A). This flask weight (H), and the predetermined soil particle density (D_p) and weight of the water‐filled flask (G) are then used to calculate moisture percentage (MP) in the soil sample from the formula:

The method was compared with conventional oven‐drying technique for a wide range of soil textures, moisture contents and other contrasting soil properties. The results obtained showed that the accuracy, precision and simplicity of the method are good, particularly for rapid practical uses.     

A proposal for re-evaluating the most probable number procedure for estimating numbers of Bradyrhizobium spp.

The reliability of the most probable number (MPN) method for estimating bradyrhizobial numbers was evaluated by comparison with the plate count procedure. MPN estimates increased with time of nodulation scoring after seedling inoculation through 6 weeks of incubation. Ratios of MPN to plate counts increased as the bradyrhizobial cell suspension concentration increased. The MPN method could not detect Bradyrhizobium japonicum numbers at concentrations of 10³ colony forming units (CFU) ml^-1 and below. A proposal for re-evaluating MPN estimates is discussed.     

A colorimetric method for estimating vesicular-arbuscular mycorrhizal infection in roots

A colorimetric method measuring the conversion of fungal chitin to glucosamine has been used to estimate the intensity of vesicular-arbuscular mycorrhizal infection in roots. The technique has been used successfully with four plant genera and with four different endophytes.     

Approximate method for estimating soil charcoal contents

Abstract

Dark‐colored soil organic matter could be oxidized with 6% H₂O₂ to make graphitized charcoal more visible. A pretreatment with 4 N HC1 removed the more soluble compounds that might react with H₂O₂ and a post treatment with 48% HF volatilized Si to concentrate the charcoal in the remaining residue. The pre‐weighed H₂O‐washed residue was ignited in a muffle and the charcoal estimated from the weight loss.     

A quick method for assessing the importance of open spaces in towns for urban nature conservation

The present study explains a quick method for the assessment of the importance of open spaces in towns as habitat for plants and animals; the assessment criteria include the period of development, area, rarity and variety of habitat conditions.     

A simple method for estimating nitrate removal from floodwaters in river marginal wetlands

Abstract. Soil cores from river marginal wetlands in the UK, France and Ireland, sampled and contained within PVC piping, were flooded with nitrate-N enriched water. Half of the cores were sterilized prior to flooding to destroy the denitrifying bacteria. The change in nitrate-N concentration in the floodwater was measured over time. It is argued that the observed nitrate depletion rates (from 0.4 to 2.3 kg/ha per day) may be identified With microbially-mediated denitrification. The results show the method to be a simple and direct procedure for the assessment of spatial variation in nitrate-sink capacity. The depth of the denitrifying layer at the soil-water interface was confirmed to be of the order of a few mm.     

A method for estimating coefficients of soil organic matter dynamics based on long-term experiments

The one-compartment C model C_t=C₀e^−k₂t+k₁A/k₂(1−e^−k₂t) is being long used to simulate soil organic C (SOC) stocks. C_t is the SOC stock at the time t; C₀, the initial SOC stock; k₂, the annual rate of SOC loss (mainly mineralization and erosion); k₁, the annual rate to which the added C is incorporated into SOC; and A, the annual C addition. The component C₀e^−k₂t expresses the decay of C₀ and, for a time t, corresponds to the remains of C₀ (C_0 remains). The component k₁A/k₂(1−e^−k₂t) refers, at time t, to the stock of SOC derived from C crops (C_crop). We herein propose a simple method to estimate k₁ and k₂ coefficients for tillage systems conducted in long-term experiments under several cropping systems with a wide range of annual C additions (A) and SOC stocks. We estimated k₁ and k₂ for conventional tillage (CT) and no-till (NT), which has been conducted under three cropping systems (oat/maize −O/M, vetch/maize −V/M and oat + vetch/maize + cowpea −OV/MC) and two N-urea rates (0 kg N ha⁻¹ −0 N and 180 kg N ha⁻¹ −180 N) in a long-term experiment established in a subtropical Acrisol with C₀ = 32.55 Mg C ha⁻¹ in the 0–17.5 cm layer. A linear equation (C_t = a + bA) between the SOC stocks measured at the 13th year (0–17.5 cm) and the mean annual C additions was fitted for CT and NT. This equation is equivalent to the equation of the model C_t=C₀e^−k₂t+k₁A/k₂(1−e^−k₂t), so that a=C₀e^−k₂t and bA=k₁A/k₂(1−e^−k₂t). Such equivalences thus allow the calculation of k₁ and k₂. NT soil had a lower rate of C loss (k₂ = 0.019 year⁻¹) than CT soil (k₂ = 0.040 year⁻¹), while k₁ was not affected by tillage (0.148 year⁻¹ under CT and 0.146 year⁻¹ under NT). Despite that only three treatments had lack of fit (LOFIT) value lower than the critical 5% F value, all treatments showed root mean square error (RMSE) lower than RMSE 95% indicating that simulated values fall within 95% confidence interval of the measurements. The estimated SOC stocks at steady state (C_e) in the 0–17.5 cm layer ranged from 15.65 Mg ha⁻¹ in CT O/M 0 N to 60.17 Mg ha⁻¹ in NT OV/MC 180 N. The SOC half-life (t_1/2 = ln 2/k₂) was 36 years in NT and 17 years in CT, reflecting the slower C turnover in NT. The effects of NT on the SOC stocks relates to the maintenance of the initial C stocks (higher C_0 remais), while increments in C_crop are imparted mainly by crop additions.     

Methods for estimating types of soil organic carbon and their application to surveys of UK urban areas

The occurrence of substantial quantities of black carbon (BC) in urban soil due to local dispersal following incomplete combustion of fossil fuel complicates the determination of labile soil organic carbon (SOC). Estimates of SOC content were made from loss on ignition (LOI) analyses undertaken on samples (0–15 cm depth) from comprehensive soil geochemical surveys of three UK urban areas. We randomly selected 10 samples from each decile of the LOI distribution for each of the surveys of Coventry (n = 808), Stoke‐on‐Trent (n = 737) and Glasgow (n = 1382) to investigate the proportions of labile SOC and BC. We determined their total organic carbon (TOC) and BC contents, and by difference the labile SOC content, and investigated the linear relationship of the latter with SOC estimates based on LOI analyses. There was no evidence for a difference in the slope of the regression for the three urban areas. We then used a linear regression of labile SOC based on LOI analyses (r² = 0.81) to predict labile SOC for all survey samples from the three urban areas. We attribute the significantly higher median BC concentrations in Glasgow (1.77%, compared with 0.46 and 0.59% in Coventry and Stoke‐on‐Trent) to greater dispersal of coal ash across the former. An analysis of the 30 samples showed that LOI at 450 °C accounts for a consistent proportion of BC in each sample (r² = 0.97). Differences between TOC (combustion at 1050 °C after removal of inorganic carbon) and an LOI estimate of SOC may be a cost‐effective method for estimation of BC. Previous approaches to estimation of urban SOC contents based on half the mean SOC content of the equivalent associations under pasture, underestimate the empirical mean value.     

A quick and sensitive method for the quantification of peroxidase activity of organic surface soil from forests

The purpose of the present study was to test the non-mutagenic compound 3,3′,5,5′-tetramethylbenzidine (TMB) as a model substrate for peroxidase in forest topsoil, as an alternative to the conventional substrate l-3,4-dihydroxyphenylalanine (l-DOPA). TMB was highly sensitive; linear absorbance changes of 0.6 were achieved within 20 min for 1000-fold diluted soil. Brief heating (denaturation) of the soil suspension gave a 34-fold reduction of TMB oxidation, indicating that the reaction measured by TMB was indeed an enzymatic reaction. TMB oxidation showed a narrow peak at pH 4.4. A proportional decrease in peroxidase activity, when the soil suspension was diluted, demonstrated that TMB estimates of peroxidase activity are directly comparable when corrected for differences in sample size. Oxidation of TMB was slow in the absence of H₂O₂ suggesting that TMB is a poor substrate for phenol oxidases. TMB oxidation was tested in nine different forest topsoils. The peroxidase activity, when normalised to the amount of soil organic matter, ranged from 1.4±0.1 Δabs₄₅₀ h^?1 mg^?1 to 34.9±4.3 Δabs₄₅₀ h^?1 mg^?1. In comparison, l-DOPA oxidation by soil peroxidases and commercial peroxidases gave inconsistent results, suggesting that one should be cautious when using l-DOPA as a soil peroxidase substrate. The high sensitivity of TMB, compared to l-DOPA, and the low interference from phenol oxidase and humic substances suggest that TMB is a better substrate than l-DOPA for estimation of peroxidase activity of forest topsoil.     

Monitoring distributions using call surveys: estimating site occupancy, detection probabilities and inferring absence

Monitoring programs serve to track changes in the distribution and abundance of species. A major problem with most monitoring programs is that species detection is imperfect and some populations are inevitably missed. Therefore, in most monitoring programs the true distribution of a species will be underestimated. Here, we report a field test of the reliability and performance of a commonly used method to monitor the distribution of amphibians (anuran call surveys). We surveyed the distribution of four anuran species in western Switzerland, and estimated detection probabilities to account for imperfect species detection and used these estimates to adjust our estimate of site occupancy (i.e., distribution). Next, we assessed how detection probabilities were affected by weather and how site occupancy was affected by site specific covariates. For one species (Hyla arborea), call surveys proved efficient in determining the regional distribution with only few site visits because detection probabilities were relatively high. The call surveys apparently missed many populations of another common species (Bufo calamita) because detection probabilities were lower. Two other species (Bombina variegata and Alytes obstetricans) were uncommon and strong inference from the analysis is not possible. Thus, multispecies surveys may be inefficient for rare species. Estimates of detection probabilities were used to calculate how many site visits are necessary to infer the absence of a species with some predetermined statistical certainty. The implications of “false absences” are important in ecology as they are known to bias usual habitat suitability models and overestimate extinction/colonization events in metapopulations. Large-scale monitoring programs would benefit from the application of an estimation-based approach to monitoring the distribution of species.     

A simple structure-based calculator for estimating vapor pressure

The development of an estimator for vapor pressure based upon organic functional groups is described. This vapor pressure calculator permits prediction of vapor pressure for a wide range of structural classes. The statistical quality of the derived coefficients is presented as well as the quality of the prediction of the training set of compounds. The calculator is then used to predict the vapor pressure of recently introduced agrochemicals to illustrate its performance. The significance of this calculator is that the agrochemical scientist can readily estimate the effects on vapor pressure of altering specific structural features of a molecule.     

Simple method for estimating 14C in formic acid solutions using liquid scintillation counting

Abstract

The feasibility of using a simple radiochemical method to determine the extent of formylation resulting from interaction of formic acid solutions with soil constituents was investigated. The interaction between formic acid‐^l4C solvents and starch was used as a model reaction. Radioassay of 1 cm³ formic acid solution by liquid scintillation counting in 10 cm³ of the scintillator‐solubilizer Insta‐gel was found to be suitable. The method is simple to use and sample preparation is easy.     

An improved nitrogen difference method for estimating biological nitrogen fixation in legume-based intercropping systems

The nitrogen difference method (NDM) for quantifying N₂ fixation, based on the same amount of soil N exploited by N₂-fixing and non-N₂-fixing plant, may not be suitable to plants with different root traits. We tested the reliability of NDM in legume-based intercropping systems by two field experiments in Northwest China. In experiment 1, faba bean (Vicia faba), pea (Pisum sativum), and soybean (Glycine max) grew solely or intercropped with maize (Zea mays) with two N application rates (0, 225 kg ha^?1). The biomass of faba bean, pea, and maize was significantly increased, whereas that of soybean was decreased when intercropped than solely grown. Aggressivity analyses demonstrate greater N competition ability of faba bean and pea, but not soybean, than maize. An improved NDM (INDM) could mitigate these effects: $ {N_{{\text{fix - int}}}} = \left[ {{N_{{\text{leg - int}}}} + \frac{{1 - x}}{x}{N_{{\text{ref - int}}}} - \frac{{{N_{{\text{ref - sole}}}}}}{x}} \right] + \left[ {{\text{soil}}{N_{{\text{leg - int}}}} + \frac{{1 - x}}{x}{\text{soil}}{N_{{\text{ref - int}}}} - \frac{{{\text{soil}}{N_{{\text{ref - sole}}}}}}{x}} \right] $ , where x and 1???x are planting area of legume and non-legume in the intercropping system. Compared to traditional NDM (TNDM, $ {N_{{\text{fix - int}}}} = \left[ {{N_{{\text{leg - int}}}} - {N_{{\text{ref - sole}}}}} \right] + \left[ {{\text{soil}}{N_{{\text{leg - int}}}} - {\text{soil}}{N_{{\text{ref - sole}}}}} \right] $ ), %N _dfa (N derived from air) by INDM was decreased by 54.3% and 39.8% for faba bean, 44.7% and 5.0% for pea, but increased by 113.5% and 191.0% for soybean at the two N application rates, indicating different %N _dfa quantifications between the two methods. In experiment 2, %N _dfa of sole or intercropped faba bean was quantified by TNDM, INDM, and ¹⁵N natural abundance method (NA). The %N _dfa only by INDM correlated significantly with that from NA. Both interspecific root interactions and N loss affect %N _dfa estimation. Our results suggested that INDM could be more suitable than TNDM for quantifying %N _dfa of a N₂-fixing plant in intercropping systems.     

A rapid procedure for estimating nitrogen mineralization in manured soil

 A routine soil testing procedure for soil N mineralization is needed that is rapid and precise. Not accounting for N mineralization can result in the over-application of N, especially in soils with a history of manure application. Our objectives were to compare results from a recently proposed rapid laboratory procedure with: (1) long-term N mineralization under standard laboratory conditions, and (2) actual forage N uptake from soil receiving dairy cattle (Bos taurus) manure in a 2-year field study. The rapid procedure is based on the quantity of CO₂-C evolved during 24 h under optimum laboratory conditions following the rewetting of dried soil. Dairy cattle manure was surface applied beginning in 1992 at annual rates of 0, 112, 224, or 448 kg N ha^–1 to field plots on a Windthorst fine sandy loam soil (fine, mixed, thermic Udic Paleustalf) near Stephenville, Texas (32°N, 98°W). Results of the one-day CO₂ procedure were highly correlated with soil N mineralized from samples collected in March of 1995 (P=0.004) and 1996 (P<0.001) and with forage N uptake (P<0.001) both years of the study. Residual inorganic N in the same soil samples was poorly correlated with soil N mineralization and forage N uptake. Received: 23 February 2000     

A quick and efficient screen for resistance to iron toxicity in lowland rice

Iron (Fe) toxicity is a major stress to rice in many lowland environments worldwide. Due to excessive uptake of Fe²⁺ by the roots and its acropetal translocation into the leaves, toxic oxygen radicals may form and damage cell structural components, thus impairing physiological processes. The typical visual symptom is the “bronzing” of the rice leaves, leading to substantial yield losses, particularly when toxicity occurs during early vegetative growth stages. The problem is best addressed through genotype improvement, i.e., tolerant cultivars. However, the time of occurrence and the severity of symptoms and yield responses vary widely among soil types, years, seasons, and genotypes. Cultivars resistant in one system may fail when transferred to another. Better targeting of varietal improvement requires selection tools improving our understanding of the resistance mechanisms and strategies of rice in the presence of excess iron. A phytotron study was conducted to develop a screen for seedling resistance to Fe toxicity based on individual plants subjected to varying levels of Fe (0–3000 mg L^–1 Fe supplied as Fe(II)SO₄), stress duration (1–5 d of exposure), vapor‐pressure deficit (VPD; 1.1 and 1.8 kPa), and seedling age (14 and 28 d). Genotypes were evaluated based on leaf‐bronzing score and tissue Fe concentrations. A clear segregation of the genotypic tolerance spectrum was obtained when scoring 28 d old seedlings after 3 d of exposure to 2000 mg L^–1 Fe in a high‐VPD environment. In most cases, leaf‐bronzing scores were highly correlated with tissue Fe concentration (visual differentiation in includer and excluder types). The combination of these two parameters also identified genotypes tolerating high levels of Fe in the tissue while showing only few leaf symptoms (tolerant includers). The screen allows selecting genotypes with low leaf‐bronzing score as resistant to Fe toxicity, and additional analyses of the tissue Fe concentration of those can identify the general adaptation strategy to be utilized in breeding programs.     

Application of respiration and FDA hydrolysis measurements for estimating microbial activity during composting processes

Olive-tree leaves (OL) were mixed with olive press cake (OPC) and extracted olive press cake (EPC) at 1:1 dw/dw ratios to prepare two composting mixtures (OL+OPC and OL+EPC). Both CO₂–C evolution and fluorescein diacetate (FDA) hydrolysis, determined as estimates of the microbial activity during composting, were related to temperature fluctuations in the compost piles, showing greater values at the temperature peaks, compared to the end, of each thermophilic phase. This, however, was only shown after handling and incubating samples at the temperatures of the compost mixtures at the sampling times and not at a low standard temperature. Incubating samples from thermophilic phases at low standard temperatures resulted in underestimation of the microbial activity occurring during composting. The effect of incubation temperature was less dramatic for FDA hydrolysis compared to CO₂–C evolution measurements, probably reflecting the reduced dependence of enzymes involved in FDA hydrolysis on the respective temperatures. However, FDA hydrolysis was a less sensitive indicator of microbial activity, probably due to extracellular cleavage of fluorescein by persistent esterases, at lowered microbial activity phases. Total microbial biomass, estimated by the fumigation–extraction method, was not consistently related to temperature fluctuations during composting and showed a clear increase at the end of composting, probably resulting from a large slow-growing mycelial community colonising the end products. Since high temperatures did not induce significant non-microbial CO₂–C release and FDA degradation, we propose the performance of microbial activity measurements during thermophilic composting phases at the actual temperatures evolving in the composts.     

A gamma-shaped detection function for line-transect surveys with mark-recapture and covariate data

We have developed a procedure for estimating animal population size from aerial survey data collected simultaneously by two observers on the same sighting platform. We used a line transect sample design where transects follow elevation contours in mountainous terrain. Because our 10 data sets from aerial line transect surveys, conducted over a terrestrial environment, consistently show unimodal detection shapes, we chose a gamma-shaped detection function that is unimodal and admits covariates. We fit models separately to data from each observer, and then used all of the data to estimate the probabilities at the apex of the detection curves. We used a Horvitz-Thompson estimator to estimate the population size. We illustrate our procedure on a recently collected brown bear data set.     

A comparison of operational remote sensing-based models for estimating crop evapotranspiration

The integration of remotely sensed data into models of evapotranspiration (ET) facilitates the estimation of water consumption across agricultural regions. To estimate regional ET, two basic types of remote sensing approaches have been successfully applied. The first approach computes a surface energy balance using the radiometric surface temperature for estimating the sensible heat flux (H), and obtaining ET as a residual of the energy balance. This paper compares the performance of three different surface energy balance algorithms: an empirical one-source energy balance model; a one-source model calibrated using inverse modeling of ET extremes (namely ET = 0 and ET at potential) which are assumed to exist within the satellite scene; and a two-source (soil + vegetation) energy balance model. The second approach uses vegetation indices derived from canopy reflectance data to estimate basal crop coefficients that can be used to convert reference ET to actual crop ET. This approach requires local meteorological and soil data to maintain a water balance in the root zone of the crop. Output from these models was compared to sensible and latent heat fluxes measured during the soil moisture–atmosphere coupling experiment (SMACEX) conducted over rain-fed corn and soybean crops in central Iowa. The root mean square differences (RMSD) of the estimation of instantaneous latent and heat fluxes were less than 50 W m⁻² for the three energy balance models. The two-source energy balance model gave the lowest RMSD (30 W m⁻²) and highest r² values in comparison with measured fluxes. In addition, three schemes were applied for upscaling instantaneous flux estimates from the energy balance models (at the time of satellite overpass) to daily integrated ET, including conservation of evaporative fraction and fraction of reference ET. For all energy balance models, an adjusted evaporative fraction approach produced the lowest RMSDs in daily ET of 0.4–0.6 mm d⁻¹. The reflectance-based crop coefficient model yielded RMSD values of 0.4 mm d⁻¹, but tended to significantly overestimate ET from corn during a prolonged drydown period. Crop stress can be directly detected using radiometric surface temperature, but ET modeling approaches-based solely on vegetation indices will not be sensitive to stress until there is actual reduction in biomass or changes in canopy geometry.