Search

Biochar can deliver many agronomic benefits, though effects may be greatest in suboptimal agricultural soils. The objective of this study was to investigate biochar in prime agricultural soils in a Mediterranean climate. In 3‐year irrigated processing tomato (Solanum lycopersicum) field trials, in which three biochars were amended to a sandy and silt loam, biochar did not influence yield. Given the global policy focus on biochar for climate change mitigation and adaptation, however, it is essential to measure more than just agronomic parameters. To this end, a soil health assessment was conducted 2.5 years after biochar amendment. In the fertile silt loam and more nutrient‐limited sandy loam, biochars had a variable but positive effect on pH, electrical conductivity, soil potassium, water stable aggregation, and permanganate oxidizable carbon. There was no effect on soil moisture nor potentially mineralizable nitrogen or carbon. In the silt loam, there was no change in microbial biomass or phospholipid fatty acid (PLFA) composition. In the sandy loam, almond shell biochars were associated with a slight increase in total biomass, an increase in the ratio of fungal to bacteria PLFAs, and reductions in multiple PLFA ratios were typically interpreted as indicators of environmental stress. Results of this study demonstrate that biochar may deliver soil health benefits in agricultural soils in a Mediterranean climate. However, increased soil health does not necessarily engender increased agricultural productivity within a 3‐year period. Furthermore, increases in soil health indicators were inconsistent across soil and biochar types, underscoring the need for site‐ and material‐specific investigation. Climate policies may bolster biochar application to working lands, irrespective of soil texture or fertility.Data on potential agronomic and soil health outcomes are needed, particularly for prime agricultural soils.In 3‐year field trials, biochar did not positively or negatively impact processing tomato yield.Soil health indicators increased more in sandy loam than fertile silt loam, stressing need for site‐specific data.Incentives should cover biochar costs if no agronomic benefits are expected, and negative impacts are unlikely.

Various soil health indicators that measure a chemically defined fraction of nitrogen (N) or a process related to N cycling have been proposed to quantify the potential to supply N to crops, a key soil function. We evaluated five N indicators (total soil N, autoclavable citrate extractable N, water‐extractable organic N, potentially mineralizable N, and N‐acetyl‐β‐D‐glucosaminidase activity) at 124 sites with long‐term experiments across North America evaluating a variety of managements. We found that 59%–81% of the variation in N indicators was among sites, with indicator values decreasing with temperature and increasing with precipitation and clay content. The N indicators increased from 6%–39% in response to decreasing tillage, cover cropping, retaining residue, and applying organic sources of nutrients. Overall, increasing the quantity of organic inputs, whether from increased residue retention, cover cropping, or rotations with higher biomass, resulted in higher values of the N indicators. Although N indicators responded to management in similar ways, the analysis cost and availability of testing laboratories is highly variable. Further, given the strong relationships of the N indicators with carbon (C) indicators, measuring soil organic C along with 24‐h potential C mineralization could be used as a proxy for N supply instead of measuring potentially mineralizable N or any other N indicator directly. Nitrogen indicators of soil health vary predictably across North America with climate and soil texture.The response to soil health practices is similar across all indicators.There are strong correlations among nitrogen indicators, but also between carbon and nitrogen indicators.Soil organic carbon and 24‐h carbon mineralization potential can be used to capture nitrogen cycling.

Farmers, scientists, and other soil health stakeholders require interpretable indicators of soil hydraulic function. Determining which indicators to use has been difficult because of measurement disconformity, spatial and temporal variability, recently established treatments, and the effect of site characteristics on management practice differences. The North American Project to Evaluate Soil Health Measurements includes 124 sites uniformly sampled across a range of soil health management practices in North America in 2019. We compare and recommend indicators of hydraulic function that best characterize soil health. We assessed the relationship of each indicator to a suite of soil inherent properties and climate variables, the response of each indicator to soil health management practices, the effect that soil inherent properties (clay content, sand content, and pH) and climatic variables (10‐yr mean annual precipitation and temperature) had on response to management practices, and the relationship among the responses of the indicators to soil health management practices. Field capacity measured on intact cores (θFC_INTACT) was the best measure of soil hydraulic function, because it responded to management, represents a direct measure of soil hydraulic function, is proximal to stakeholder values, and its response to management was not significantly influenced by inherent and climatic variables. Other suitable indicators are bulk density, soil organic carbon (SOC), and aggregate stability, which are not direct measures of soil hydraulic function but do respond to management and may be practical in situations in which measuring θFC_INTACTis not. This study informs selection of soil health indicators to measure soil hydraulic function. Across sites, management changed aggregate stability, C, and bulk density.Management also changed field capacity for intact cores, but not repacked cores.Increased residue, organic nutrient sources, less tillage, each increased field capacity by 4%.Residue retention, organic nutrients and less tillage reduced bulk density by 2–3%.Field capacity for intact cores was the most responsive and direct indicator.

Currently accepted pedotransfer functions show negligible effect of management‐induced changes to soil organic carbon (SOC) on plant available water holding capacity (θAWHC), while some studies show the ability to substantially increase θAWHCthrough management. The Soil Health Institute's North America Project to Evaluate Soil Health Measurements measured water content at field capacity using intact soil cores across 124 long‐term research sites that contained increases in SOC as a result of management treatments such as reduced tillage and cover cropping. Pedotransfer functions were created for volumetric water content at field capacity (θFC) and permanent wilting point (θPWP). New pedotransfer functions had predictions of θAWHCthat were similarly accurate compared with Saxton and Rawls when tested on samples from the National Soil Characterization database. Further, the new pedotransfer functions showed substantial effects of soil calcareousness and SOC on θAWHC. For an increase in SOC of 10 g kg–1(1%) in noncalcareous soils, an average increase in θAWHCof 3.0 mm 100 mm–1soil (0.03 m3m–3) on average across all soil texture classes was found. This SOC related increase in θAWHCis about double previous estimates. Calcareous soils had an increase in θAWHCof 1.2 mm 100 mm–1soil associated with a 10 g kg–1increase in SOC, across all soil texture classes. New equations can aid in quantifying benefits of soil management practices that increase SOC and can be used to model the effect of changes in management on drought resilience. New pedotransfer functions show organic C increases plant available water.Noncalcareous soils show greater effects of organic C on plant available water.Increase in plant available water from organic C is more than double previous estimates.These pedotransfer functions can easily be used in hydrologic models.A gap is bridged for modeling the effect of increased soil organic C on plant available water.

Copper (Cu) products, including copper oxide nanoparticles (nCuO), are critically important agricultural fungicides and algaecides. Foliar application onto crops and subsequent aerosol drift of these Cu products, especially nCuO, on to soil may alter nutrient cycling and microbial communities in both managed and unmanaged environments. We measured the influence of land use on soil microbial biomass and respiration in response to the addition of nCuO to an alluvial soil. Different land uses included grassland, forest and both organic and conventional managed row crops. Soil samples were amended with 1000 mg Cu per kg soil as CuCl2, 16 nm CuO (16nCuO), 42 nm CuO (42nCuO), and larger than nanoparticle sized bulk CuO (bCuO). Copper availability immediately increased in all soils following Cu addition in the order of CuCl2> 16nCuO > 42nCuO > bCuO. After 70 days Cu availability was diminished across land uses and lowest in soils treated with bCuO. Using X-ray absorption near edge structure (XANES) spectroscopy, we determined that the relatively high availability of Cu after treatment with nanoparticle sized CuO was due to the dissolution of CuO particles and subsequent adsorption by soil materials. Respiration, an indicator of microbial activity, was suppressed by Cu additions, especially CuCl2. Copper effects on soil microbial biomass were sensitive to land use. In agricultural soils, microbial biomass was unaltered by Cu form, regardless of concentration, whereas in unmanaged soils, it decreased following exposure to CuCl2and 42nCuO. Our results suggest that land use history has little impact on Cu chemical fate in soils, but strongly modulates microbial response to Cu exposure. These results are especially important for organic agricultural systems where copper fungicides are widely used but may suppress microbial mineralization of nutrients from soil organic matter.

Supplemental Digital Content is available in the text

Elevated carbon dioxide (eCO2) concentrations in the atmosphere, along with decreased precipitation regimes expected in the next decades, can alter soil microbe functioning and hence soil organic matter (SOM) dynamics causing important impacts on the global C cycle. We investigated the effects of eCO2and reduced soil moisture, further referred to as reduced precipitation treatment (RP), on SOM fractions and soil microbial distribution across different soil zones (i.e., rhizosphere vs. bulk soil) and physically‐separated SOM fractions (coarse particulate organic matter (cPOM; >250 mm), microaggregate (53–250 mm), and silt‐and‐clay fraction (<53 mm)) at the soybean free air concentration enrichment (SoyFACE) experiment in Illinois. To quantify the abundance of microorganisms, we used real‐time quantitative polymerase chain reaction (PCR) for the total bacterial (16S rRNA) and denitrifier (nosZ) genes. We did not detect any significant effects of eCO2on bacterial abundance, soil C, and N concentrations either in the whole soil or in the SOM fractions. These findings corroborate the previously reported absence of soil C responses to eCO2. The mass of microaggregates was highest in RP treatments in both rhizosphere and bulk soil. Furthermore, the abundance of 16S rRNA and nosZgenes increased in the microaggregates under RP compared to ambient conditions of soil moisture. Hence, RP increased the formation of microhabitats in which the microorganisms are partly protected from the adverse effects of reduced soil moisture.

Measurements of diffusive and convective gas transport parameters can be used to describe soil functional architecture and reveal key factors for soil structure development. Undisturbed 100-cm3soil samples were sampled at the Long-term Research on Agricultural Systems experiment located at the University of California, Davis. The 18 plots used in this study represented fairly wide ranges in organic carbon (0. 0072–0.0153 kg kg−1) and clay (0.30–0.44 kg kg−1). Soil-air permeability, ka, and soil-gas diffusivity, DPD0, were determined at field-moist conditions (fm) and, subsequently, after saturation and drainage to −100 cm of matric potential (pF2). Gas diffusivity in intact samples at fm conditions exhibited a general, linear relationship with air-filled porosity ([Latin Small Letter Open E]), independent of soil texture and treatment. Comparing intact and repacked samples drained to pF2, repacked soil displayed markedly lower DPD0values at similar air-filled porosity, illustrating soil structure effects on DPD0. The Currie tortuosity-connectivity parameter, X= Log(DPD0)Log([Latin Small Letter Open E]), decreased with increasing bulk density in the intact samples at both moisture conditions, suggesting less tortuous and well-connected pathways for gas diffusion at higher bulk density. Pore organization, PO = ka[Latin Small Letter Open E], showed a treatment effect with typically higher values for the organic plots, implying that an improved possibility for formation of organomineral soil aggregates resulted in better-connected macropore networks. Fitting a linear model to DPD0versus [Latin Small Letter Open E] measurements revealed different slopes at the two moisture conditions, suggesting short-term nonsingularity (hysteretic) effects after rewetting and drainage. Defining the ratio of slopes at the fm and pF2 moisture conditions as a nonsingularity index (), the nonsingularity in DPD0increased with lower bulk density and higher organic carbon content.

Growing plants in extraterrestrial environments, for example on a space station or in a future lunar or Martian outpost, is a challenge that has attracted increasing interest over the last few decades. Most of the essential plant needs for optimal growth (air, water, and nutrient supply, and mechanical support) are closely linked with the basic physical properties of the growth media. Diffusion is the main process whereby oxygen and nutrients are supplied to plant roots, and gas and solute diffusivity are the key parameters controlling the diffusive movement of oxygen and nutrients in the root zone. As one among several essential aspects of optimal porous media design for plant growth, this study presents a diffusion‐based characterization of four commercial, aggregated growth media. To account for the observed large percolation threshold for gas diffusivity in the selected media, an inactive pore and density corrected (IPDC) model was developed and excellently described measured gas diffusivity in both inter‐ and intraaggregate pore regions. A strong relation (r2= 0.98) between percolation threshold for gas diffusivity and mean particle (aggregate) diameter was found and suggested to be used in future design models. Also, critical windows of diffusivity (CWD) was defined identifying the air content range where gas diffusivity (hence, oxygen supply) and solute diffusivity or the analogous electrical conductivity (hence, nutrient supply) are above pre‐defined, critical minimum values. Assuming different critical values for gas diffusivity under terrestrial and Martian conditions, the four growth media were compared and it was found that one medium did not fulfill the pre‐set criteria. Overall, the analyses suggested that particle (aggregate) sizes below 0.25 and above 5 mm should likely be avoided when designing safe plant growth media for space. The CWD concept was also applied to a natural volcanic ash soil (Nishi‐Tokyo, Japan), and the natural soil was found competitive or better than the tested commercial growth media. This could bear large perspectives for Martian outpost missions, since NASA has found that Martian dust/soil mostly resembles volcanic ash soil among terrestrial materials.

The article presents a study on the importance of soil organic carbon (OC) and clay for developments of soil functional structure. The study involved soil samples from the Long-term Research on Agricultural Systems (LTRAS) at the University of California in Davis. The results showed that the Currie tortuosity-connectivity parameter decreased with increasing bulk density in the intact samples, suggesting less tortuous and well-connected pathways for gas diffusion at higher bulk density.

It is important to understand the impact of texture and organic carbon (OC) on soil structure development. Only few studies investigated this for silt-dominated soils. In this study, soil physical properties were determined on samples from a controlled experiment (Static Fertilization Experiment, Bad Lauchstädt, Germany) on a loess soil that started more than 100 years ago with six different combinations of organic and mineral fertilizers. The parameters measured include soil texture, water retention curve, air-connected porosity, gas diffusion coefficient, air permeability, and saturated hydraulic conductivity. The management resulted in a distinct gradient in OC. A bulk density gradient developed from differences in amount of clay not complexed with OC. This gradient in bulk density mainly affected content of pores larger than 3 m. The air-connected porosity measured by a pycnometer was highly similar to the total air-filled porosity calculated from gravimetric water content. For all six treatments, diffusivities and permeabilities were quite similar; both suggested that air-filled pore space was inactive for gas transport for air saturation below 0.1, but became highly connected around 0.2 to 0.25. Furthermore, diffusion data from intact cores compared well with data from repacked samples measured at low air-filled porosities and another high-silt soil (Yolo silt loam, USA) measured at higher air-filled porosities. A two-parameter fitting model was used to analyze gas diffusion coefficient data; the model pore-connectivity factor was fairly constant, whereas the water blockage factor was markedly different. Water and air parameters both implied that change in bulk density was the major driver for diffusive and convective parameters in the experiment.

The article discusses a loess soil long-term experiment on the relation of soil physical parameters with density gradients. It mentions that soil physical properties were determined from a controlled experiment samples on a loess soil with six different combinations of organic and mineral fertilizers. It states that the major driver for diffusive and convective parameters was the change in bulk density.

Two agricultural soil microbial communities were analyzed using phospholipid fatty acid (PLFA) analysis to identify coarse differences between community compositions after a 3-year nitrogen (N) rate study on cotton followed by a rotational cereal crop. The soils studied were a Wasco sandy loam and Panoche clay loam. The study compared the influence of soil type, soil N levels, and how rotation crops affected soil microbial biomass. Principal component analysis of the PLFA signatures indicated that microbial communities were significantly different between the two soil types after 3 years of similar management and cotton residue inputs. The Panoche soil showed a greater total microbial biomass than the Wasco soil. Phospholipid fatty acid biomarker results showed that the Panoche soil was characterized by a fungal population, and the Wasco soil showed evidence of a bacterial composition. The observed microbial response to different soil N levels was negligible, but crop yields were highly responsive to N treatments. Cereal rotations after 3 years of cotton produced a substantial increase in microbial biomass, but did not alter the community composition of either soil. Results suggested that microbial communities found in the two soils responded to changes in residue inputs from subsequent crop rotations by increasing total microbial biomass, but community composition was not influenced by changes in crop type or N additions. Based on the results of this study, soil texture had a more decisive role in soil microbial composition and consequent microbial activity than N fertilization or different rotational crops after 3 years of similar management.

The article examines two agricultural soil microbial communities with the use of phospholipid fatty acid (PLFA) analysis to determine differences between compositions after a nitrogen rate study on cotton followed by a rotational cereal crop. Two agricultural soils being studied include a Wasco sandy loam and Panoche cereal crop. The study aims to compare the impact of soil type, nitrogen fertility, and rotations on soil microbial biomass.

Abstract: Plant invasions pose a serious threat to native ecosystem structure and function. However, little is known about the potential role that rhizosphere soil microbial communities play in facilitating or resisting the spread of invasive species into native plant communities. The objective of this study was to compare the microbial communities of invasive and native plant rhizospheres in serpentine soils. We compared rhizosphere microbial communities, of two invasive species, Centaurea solstitialis (yellow starthistle) and Aegilops triuncialis (barb goatgrass), with those of five native species that may be competitively affected by these invasive species in the field (Lotus wrangelianus, Hemizonia congesta, Holocarpha virgata, Plantago erecta, and Lasthenia californica). Phospholipid fatty acid analysis (PLFA) was used to compare the rhizosphere microbial communities of invasive and native plants. Correspondence analyses (CA) of PLFA data indicated that despite yearly variation, both starthistle and goatgrass appear to change microbial communities in areas they invade, and that invaded and native microbial communities significantly differ. Additionally, rhizosphere microbial communities in newly invaded areas are more similar to the original native soil communities than are microbial communities in areas that have been invaded for several years. Compared to native plant rhizospheres, starthistle and goatgrass rhizospheres have higher levels of PLFA biomarkers for sulfate reducing bacteria, and goatgrass rhizospheres have higher fatty acid diversity and higher levels of biomarkers for sulfur-oxidizing bacteria, and arbuscular mycorrhizal fungi. Changes in soil microbial community composition induced by plant invasion may affect native plant fitness and/or ecosystem function.

Abstract: The types and amounts of carbon (C) and nitrogen (N) inputs, as well as irrigation management are likely to influence gaseous emissions and microbial ecology of agricultural soil. Carbon dioxide (CO2) and nitrous oxide (N2O) efflux, with and without acetylene inhibition, inorganic N, and microbial biomass C were measured after irrigation or simulated rainfall in two agricultural fields under tomatoes (Lycopersicon esculentum). The two fields, located in the California Central Valley, had either a history of high organic matter (OM) inputs (“organic” management) or one of low OM and inorganic fertilizer inputs (“conventional” management). In microcosms, where short-term microbial responses to wetting and drying were studied, the highest CO2 efflux took place at about 60% water-filled pore space (WFPS). At this moisture level, phospholipid fatty acids (PLFA) indicative of microbial nutrient availability were elevated and a PLFA stress indicator was depressed, suggesting peak microbial activity. The highest N2O efflux in the organically managed soil (0.94 mg N2O-N m−2 h−1) occurred after manure and legume cover crop incorporation, and in the conventionally managed soil (2.12 mg N2O-N m−2 h−1) after inorganic N fertilizer inputs. Elevated N2O emissions occurred at a WFPS >60% and lasted <2 days after wetting, probably because the top layer (0–150 mm) of this silt loam soil dried quickly. Therefore, in these cropping systems, irrigation management might control the duration of elevated N2O efflux, even when C and inorganic N availability are high, whereas inorganic N concentrations should be kept low during times when soil moisture cannot be controlled.

The early years of soil genesis during mine spoil reclamation are critical for vegetative establishment and may help predict reclamation success. Mine spoils in the Halle‐Leipzig region of Germany were analyzed for microbial changes following a hay mulch‐seeding treatment without topsoil or fertilizer application. Microbial biomass carbon (Cmic) and dehydrogenase activity (DHA) of spoils were measured each year in the first 3 yr after treatment. In the third year, bacterial community DNA fingerprints were compared with those from a reference soil. Microbial indicators were measured at three depths in the upper 10 cm of spoils at three sites with contrasting parent materials: glacial till (sandy loam), limnic tertiary sediments (high‐lignite sandy clay loam), and quaternary sand and gravel (loamy sand). Before reclamation, Cmicmeans and standard deviations of surface spoils (0–1 cm) were 9 ± 6, 39 ± 11, and 38 ± 16 mg kg−1for the loamy sand, high‐lignite sandy clay loam, and sandy loam spoils, respectively. Within one year, mean Cmicat the surface increased to 148 ± 70, 229 ± 64, and 497 ± 167 mg kg−1, respectively, and was significantly higher at 0 to 1 cm than at lower depths. Highest DHA and DNA yields were obtained in the 0‐ to 1‐cm depth of the sandy loam spoils. Microbial biomass C values exhibited significant correlations with DHA, DNA yield, and extractable C for all three mine spoils. Soil microbial indices were more responsive than plant measurements to differences in parent materials.

SORPTIONmechanisms and kinetics govern contaminant fate and bioavailability in soil to a great extent. In this study, temporal development of naphthalene sorption was measured on four soils and showed continuously increasing sorption with time. Two mechanistically based analytical models were fitted to the sorption kinetics data (i) a Two-Rate Sorption Kinetics model (TRSK) and (ii) an Intraparticle Diffusion model (ID) with an initial sorption capacity, Xi. Both models described measured data well, with the TRSK model providing the best fit (lowest root mean square error). Although the two models are mathematically different, the parameters describing the slow sorption rate in the two models (in ID the effective diffusion coefficient, Da/l2, and in TRSK the slow rate coefficient, rs) exhibited comparable values. Both Da/l2and rsshowed a minor concentration dependency. Sorption isotherms measured at different time scales were described well by the Freundlich equation and showed that the Freundlich coefficient, K′F, increased with time. The Freundlich exponent, n′a, decreased at short sorption times (t < 4 days) but was reasonably constant at longer sorption times (t > 4 days). An empirical, two-term K′F(t) model, used together with a constant value of the Freundlich exponent, n′a(defined as n′ameasured at t > 4 days), gave good predictions of the measured sorption at different time scales and represented a simple alternative to the ID and TRSK models. Calculating the naphthalene retardation factor as a function of time, R(t), using the two-term K′F(t) model implied that sorption non-linearity is highly important when evaluating both short- and longer-term naphthalene mobility in soil and will often overshadow the effects of sorption time-dependency.

Diffusion of hydrophobic organic compounds (HOC) is a key process controlling transport of contaminants in soils. However, the separate effects of sorption and diffusion on net (effective) HOC diffusion are not fully understood. In this study, effective diffusion of naphthalene in five unsaturated soils was evaluated by: (i) naphthalene adsorption‐desorption experiments (batch method), (ii) naphthalene effective diffusion experiments (half‐cell method), and (iii) trace‐gas diffusivity experiments (chamber method). There was no soil type effect on gas diffusivity in repacked unsaturated soil, but a pronounced soil type effect on naphthalene sorption behavior. Varying degree of adsorption nonlinearity (Freundlich n′a) and apparent adsorption‐desorption nonsingularity (ω) and ω increased with decreasing n′awere observed. In the half‐cell experiments, gas diffusion was the governing naphthalene transport mechanism. Three effective diffusion coefficients were calculated from the half‐cell experiments, based on concentration profile from either the whole (Deff) cell, the source (desorption) half‐cell (Deff,D), or the recipient (adsorption) half‐cell (Deff,A). Generally, the observed Deffdecreased with naphthalene‐soil contact time, because of aging effects. The Deff, Deff,A, and Deff,Dvalues (half‐cell method) could only to some extend be estimated from Freundlich isotherm parameters (batch method). A suggested index of effective diffusion nonsingularity, H= Deff,D/Deff,A, showed that Hwas correlated with ω and inversely correlated with n′aThus, the sorption nonlinearity (n′a) was found to provide good indications of degree of nonsingularity in both HOC adsorption‐desorption and effective diffusion. The combination of batch and half‐cell experiments generally gave useful insight towards understanding and predicting the influence of sorption nonlinearity and nonequilibrium on HOC diffusion.

Perchlorate (ClO−4) contamination of ground water and surface water is a widespread problem, particularly in the western United States. This study examined the effect of biodegradation on perchlorate fate and transport in soils. Solute transport experiments were conducted on two surface soils. Pulses of solution containing perchlorate and Br−were applied to saturated soil columns at steady state water flow. Perchlorate behaved like a nonreactive tracer in Columbia loam (coarse‐loamy, mixed, superactive, nonacid, thermic Oxyaquic Xerofluvent) but was degraded in Yolo loam (fine‐silty, mixed, superactive, nonacid, thermic Mollic Xerofluvent). Batch experiments demonstrated that perchlorate removal from solution in Yolo loam was caused by biodegradation. Other batch experiments with Yolo loam surface and subsurface soils, Columbia loam surface soil, and dredge tailings demonstrated that perchlorate biodegradation required anaerobic conditions, an adequate carbon source, and an active perchlorate‐degrading microbial population. The sequential reduction of perchlorate and NO−3by an indigenous soil microbial community in Yolo loam batch systems was also studied. Nitrate reduction occurred much sooner than perchlorate reduction in soils that had not been previously exposed to perchlorate, but NO−3and perchlorate were simultaneously reduced in soils previously exposed to perchlorate. The results of this study have implications for in situ remediation schemes and for agricultural soils that have been contaminated by perchlorate‐tainted irrigation water.

This study investigated the effects of the quantity of methylene chloride, used as a carrier solvent for phenanthrene when added to soil, on phenanthrene mineralization kinetics, soil phospholipid fatty acid profiles (PLFA), and phenanthrene distribution. Methylene chloride dosages of 25 μL/g soil or more resulted in an enrichment of saturated PLFAs, suggesting soil microorganisms had adjusted their cell membranes in response to the solvent. A greater fraction of phenanthrene mineralized when spiked in 5 μL/g than in 25 μL/g methylene chloride suggesting that the methylene chloride became toxic to phenanthrene‐degrading organisms in soil. Phenanthrene was more equally distributed among 0.1 g soil subsamples if spiked in 25 than 5 or 1 μL methylene chloride per gram soil. Thus the amount of methylene chloride used to spike phenanthrene in soil strongly impacted the mineralization kinetics, phenanthrene distribution, and microbial community in soil. Because a variety of spiking methods are used in biodegradation research, scientists should consider the quantity of solvents used when comparing results among different studies.

The high‐value, large‐scale crop production systems in the San Joaquin Valley (SJV) of California typically entail intensive tillage and large fertilizer and water inputs but few C additions to the soil. Such practices often contribute to a decline in soil quality. Our objective for this participatory study was to examine the effects of supplemental C management practices (SCMPs) on various soil quality indicators. To increase farmer participation, we conducted the study on farms using a variety of SCMPs, including cover crops, compost and manure amendments, and several different crop rotations common to the region. The SCMPs significantly changed a number of soil properties, including soil organic matter (SOM); total Kjeldahl N; microbial biomass C and N; exchangeable K; Olsen P; and extractable Fe, Mn, and Zn. A comparison including previously established, adjacent organic, conventional, and transitional fields in addition to the treatment fields at one farm revealed significant differences in 16 of 18 soil quality indicators. A soil quality index computed for this farm scored the established organic system significantly higher than the conventional system. Our results suggest that significant changes in several soil quality indicators occur with a variety of SCMPs. This is especially noteworthy considering the intensive tillage, irrigation, and hot, semiarid environment of the SJV, California, where increases in SOM and related soil properties are generally not expected in a 3‐yr study.

The Sustainable Agriculture Farming Systems project (SAFS) was established in 1988 to study the transition from conventional to lowinput and organic crop production practices. The project includes four-year crop rotations under conventional (conv-4), low-input, and organic management and a conventionally managed, two-year rotation (conv-2). Positive effects on soil quality resulting from low-input and organic management include increased soil organic matter, a reduction in soil-borne diseases, increased pools of P and K, higher microbial biomass and activity, an increase in mobile humic acids and increased water infiltration rates and soil water-holding capacity. Pesticide use in the low-input cropping system is about 25% of that used in the conventional systems. The most profitable farming system continues to be the conv-2 system due to the greater frequency of tomato in that rotation. Among the four-year rotations, the organic system, in which the produce commands premium prices, is the most profitable, although least profitable if premium prices are not applied. Information generated from SAFS research has been disseminated via videotape, workshops, annual field days, field tours, educational materials, peer-reviewed articles and an Internet homepage. Future challenges for the SAFS project include develo pment of reduced-tillage and cover crop management strategies to optimize N availability following cash crops, weed management in organic and low-input systems, improvement of water-use efficiency in alternative systems and sequestration of C in the soil.

The Sustainable Agriculture Farming Systems project (SAFS) was established in 1988 to study the transition from conventional to low-input and organic crop production practices. The project includes four-year crop rotations under conventional (conv-4), low-input, and organic management and a conventionally managed, two-year rotation (conv-2). Positive effects on soil quality resulting from low-input and organic management include increased soil organic matter, a reduction in soil-borne diseases, increased pools of P and K, higher microbial biomass and activity, an increase in mobile humic acids and increased water infiltration rates and soil water-holding capacity. Pesticide use in the low-input cropping system is about 25% of that used in the conventional systems. The most profitable farming system continues to be the conv-2 system due to the greater frequency of tomato in that rotation. Among the four-year rotations, the organic system, in which the produce commands premium prices, is the most profitable, although least profitable if premium prices are not applied. Information generated from SAFS research has been disseminated via videotape, workshops, annual field days, field tours, educational materials, peer-reviewed articles and an Internet homepage. Future challenges for the SAFS project include development of reduced-tillage and cover crop management strategies to optimize N availability following cash crops, weed management in organic and low-input systems, improvement of water-use efficiency in alternative systems and sequestration of C in the soil.

Little is known about the short‐term effects (hours to days) of tillage on labile pools of C and N, or microbial activity and community composition. We examined the effects of rototillage on microbial biomass C (MBC) and N (MBN), respiration (i.e., soil CO2production in 1‐h incubations), CO2efflux from the soil surface, inorganic N, nitrification potential, denitrification rate, and phospholipid fatty acids (PLFA). A fallow silt loam soil was rototilled in the field and soil cores were immediately obtained from tilled and adjacent control soils. The soil cores were then incubated at constant temperature and sampled throughout a 2‐wk period. Tilled soil had higher CO2efflux than the control soil. This increase occurred immediately after tillage and lasted for 4 d. Respiration was similar in both soils until the fourth day after tillage, and then declined in the tilled soil. Tilled soil showed increases in MBN, nitrate, and denitrification rates, suggesting that tillage makes available previously protected organic N. The overall reduction in respiration together with the lack of response in MBC, however, suggests that tillage did not make available significant amounts of readily decomposable C. These combined C and N dynamics suggest that low C/N ratio compounds may have been mineralized following tillage. Denitrification rates increased in the tilled soil even though the bulk of the soil had reduced respiration and bulk density. Tillage caused temporary changes in PLFA profiles, suggesting changes in soil microbial community structure. Phospholipid fatty acid 18:1 ω7t, which marks the presence of eubacteria, decreased in the tilled soil. In contrast, 19:0 cy, a marker for anaerobic eubacteria, increased in the tilled soil. Our results show that tillage causes short‐term changes in nutrient dynamics that may potentially result in N losses through denitrification and nitrate leaching, as well as C losses through degassing of dissolved CO2These changes are accompanied by concomitant shifts in microbial community structure, suggesting a possible relationship between microbial composition and ecosystem function.

We measured growth of a phenanthrene-degrading bacterium, Arthrobacter, strain RP17, in Forbes soil, amended with 500 µg g−1phenanthrene using a quantitative competitive polymerase chain reaction method. The inoculum, which was not indigenous to Forbes soil, grew from 5.55×105colony forming units (cfu) g−1to 1.97×107cfu g−1within 100 h after the cells were added to the soil. Maximum population density was reached before the highest degradation rate was observed 150 h after the cells were added to soil. Population density remained stable even after 56% of the phenanthrene had mineralized. This study is one of the few documented examples of growth by a non-indigenous bacterium in a non-sterile soil amended with a pollutant.

We measured growth of a phenanthrene‐degrading bacterium, Arthrobacter, strain RP17, in Forbes soil, amended with 500 μg g−1phenanthrene using a quantitative competitive polymerase chain reaction method. The inoculum, which was not indigenous to Forbes soil, grew from 5.55×105colony forming units (cfu) g−1to 1.97×107cfu g−1within 100 h after the cells were added to the soil. Maximum population density was reached before the highest degradation rate was observed 150 h after the cells were added to soil. Population density remained stable even after 56% of the phenanthrene had mineralized. This study is one of the few documented examples of growth by a non‐indigenous bacterium in a non‐sterile soil amended with a pollutant.

Trichloroethylene (TCE) biodegradation in soil under aerobic conditions requires the presence of another compound, such as toluene, to support growth of microbial populations and enzyme induction. The biodegradation kinetics of TCE and toluene were examined by conducting three groups of experiments in soil: toluene only, toluene combined with low TCE concentrations, and toluene with TCE concentrations similar to or higher than toluene. The biodegradation of TCE and toluene and their interrelationships were modeled using a combination of several biodegradation functions. In the model, the pollutants were described as existing in the solid, liquid, and gas phases of soil, with biodegradation occurring only in the liquid phase. The distribution of the chemicals between the solid and liquid phase was described by a linear sorption isotherm, whereas liquid‐vapor partitioning was described by Henry's law. Results from 12 experiments with toluene only could be described by a single set of kinetic parameters. The same set of parameters could describe toluene degradation in 10 experiments where low TCE concentrations were present. From these 10 experiments a set of parameters describing TCE cometabolism induced by toluene also was obtained. The complete set of parameters was used to describe the biodegradation of both compounds in 15 additional experiments, where significant TCE toxicity and inhibition effects were expected. Toluene parameters were similar to values reported for pure culture systems. Parameters describing the interaction of TCE with toluene and biomass were different from reported values for pure cultures, suggesting that the presence of soil may have affected the cometabolic ability of the indigenous soil microbial populations.

The rates of biodegradation in soil often do not conform to the predictions of kinetic models, such as the first‐order and Michaelis‐Menten models, developed to describe metabolic processes occurring in solutions in which microorganisms and their substrates are well mixed. To test whether the kinetics of biodegradation in the presence of aggregates could be described by explicitly accounting for chemical diffusion, studies were conducted in well‐defined experimental systems. The kinetics of biodegradation of low concentrations of 14C‐labeled phenol and glutamate by Pseudomonassp. Strain K in buffer containing spherical aggregates of kaolinite that exclude bacteria were significantly different from the kinetics measured in the absence of aggregates. Both the biodegradation rate and the percentage of the initial compound degraded were lower in the presence of aggregates than in their absence. Using measurements of biodegradation, diffusion rates, and physical properties of the experimental system as input parameters, the diffusion‐sorption‐biodegradation (DSB) model simulated the biodegradation of phenol and glutamate originating inside aggregates. The model also simulated the initial period of biodegradation of glutamate in an experimental system containing gel‐exclusion chromatography beads. Clay aggregates reduced the concentration of available p‐nitrophenol sufficiently to lower the apparent rate constant for its biodegradation. Microscopic mass‐transfer processes, such as diffusion, may be important to consider in quantitative descriptions of the biodegradation of organic chemicals in soil.

Organic chemicals subject to biodegradation by microorganisms in soil are often limited in their availability by sorption and diffusion to unavailable sites. To quantitatively describe biodegradation under conditions of substrate limitation, a process‐based deterministic model called the diffusion‐sorption‐biodegradation (DSB) model was developed to describe the biodegradation of an organic chemical in the presence of spherical aggregates. Diffusion is described by Fick's second law, sorption by a linear isotherm, and biodegradation by one of various rate equations including first‐order kinetics. Sensitivity analyses were performed to determine the dependence of a chemical's half‐life on soil physical and chemical properties. Half‐lives in the presence of aggregates were substantially greater than equivalent half‐lives in the absence of aggregates when the sorption partition coefficient was ≥3.2 dm3kg‐1and the aggregate radius was ≥0.32 cm. The two‐compartment model provided a better fit than did a simple first‐order model to simulations of the biodegradation of a sorbing chemical in the presence of large (0.25‐cm radii) aggregates, whereas there was no difference between the two models for aggregates of much smaller radii. When simple first‐order kinetics was assumed without accounting for diffusion and sorption, long‐term extrapolation of chemical persistence in the presence of aggregates overestimated the degradation of a chemical with a sorption partition coefficient of 10 dm3kg‐1. Considering that many organic pollutants sorb strongly and numerous soils have some degree of aggregate structure and organic matter, taking into account the physical and chemical processes affecting a chemical's concentration may greatly improve kinetics models of biodegradation in soil.

The presence of significant denitrification activity in an aerobic toluene-treating biofilter was demonstrated under batch and flow-through conditions. N2O concentrations of 9.2 ppmv were produced by denitrifying bacteria in the presence of 15% acetylene, in a flow-through system with a bulk gas phase O2 concentration of >17%. The carbon source for denitrification was not toluene but a byproduct or metabolite of toluene catabolism. Denitrification conditions were successfully used for the reduction of 60 ppmv nitric oxide to 15 ppmv at a flow rate of 3 L min⁻¹ (EBRT of 3 min) in a fully aerated, 17% v/v O2 (superficially aerobic) biofilter. Higher NO removal efficiency (97%) was obtained by increasing the toluene supply to the biofilter. © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 58:408-415, 1998.

Soil chemical properties during the transition from conventional to organic and low‐input farming practices were studied over 8 yr in California's Sacramento Valley to document changes in soil fertility status and nutrient storage. Four farming systems differing in crop rotation and external inputs were established on land previously managed conventionally. Fertility in the organic system depended on animal manure applications and winter cover crops; the two conventional systems received synthetic fertilizer inputs; the low‐input system used cover crops and animal manure during the first 3 yr and cover crops and synthetic fertilizer for the remaining 5 yr. At 4 and 8 yr after establishment, most changes in soil chemical properties were consistent with predictions based on nutrient budgets. Inputs of C, P, K, Ca, and Mg were higher in the organic and low‐input systems as a result of manure applications and cover crop incorporations. After 4 yr, soils in the organic and low‐input systems had higher soil organic C, soluble P, exchangeable K, and pH. Ceasing manure applications in the low‐input system in Year 4 resulted in declining levels of organic C, soluble P, and exchangeable K. Crop rotation (the presence or absence of corn) also had a significant effect on organic C levels. Differences in total N appeared to be related in part to inputs, but perhaps also to differing efficiency of the farming systems at storing excess N inputs: the low‐input system appeared to be most efficient, and the conventional systems were least efficient. Electrical conductivity (EC), soluble Ca, and soluble Mg levels were tightly linked but not consistently different among treatments. Relatively stable EC levels in the organic system indicate that animal manures did not increase salinity. Overall, our findings indicate that organic and lowinput farming in the Sacramento Valley result in small but important increases in soil organic C and larger pools of stored nutrients, which are critical for long‐term fertility maintenance.

Locomotor activity in hamsters, Mesocricetus auratus. in 24-h LD cycles with light from 06.00 to 18.00 h, and simultaneously in imposed 26-h weak magnetic cycles (ranges varying from 0.260 to 0.008 gauss), significantly reflected both periods. A 24-h circadian pattern (ca. 200% of mean) was modulated by a concurrent 26-h circadian pattern (range ca. 14% of mean).

The article offers information on impact of mineral fertilizer on soil microbial biomass. It mentions that long term use of mineral fertilizer increases microbial biomass by 15 percent and soil organic carbon by 13 percent. It highlights that meta-analysis method was used to check the effect of nitrogen fertilization.