Wide Range Of Soils Research Articles

Estimating Nitrogen Mineralization from Cover Crop Mixtures Using the Precision Nitrogen Management Model

Core Ideas The precision N management model performed reasonably well for estimating cover crop N mineralization. Model performance was sufficient to justify incorporation into an N decision support tool. Cover crops as a best management practice can be assessed with the calibrated model. Cover crops influence soil N dynamics and N availability to a subsequent crop. Dynamic simulation models, if properly calibrated and tested, can simulate C and N dynamics of a terminated cover crop and estimate crop‐available N over diverse production environments. We calibrated and tested a dynamic simulation model modified to simulate C and N cover crop residue dynamics in maize (Zea mays L.) production systems. Data from a 2‐yr field study of different cover crop residue mixtures, fertilizer rates, and tillage practices were used in model calibration and testing. First order rate constants governing cover crop residue decomposition were calibrated so that statistical measures of model best fit were optimized. Calibration resulted in a good fit between measured and modeled N release from the terminated cover crop mixtures (root mean square error [RMSE] = 14 kg N ha−1; Willmott's index of agreement (IA) = 0.92). The calibrated model performed reasonably well in the testing phase (RMSE = 25; IA = 0.88) with significantly better performance for the no‐till (NT) treatments compared to the incorporated treatment. Accounting for cover crop components (leaf, stem proportions), calibration of the temperature and moisture response functions that modify the calibrated rate constants, and testing over a wider range of soils, management practices and climates are potential areas for model improvement. The revised, calibrated model will be used in a decision support tool for N management in maize production and in studies of N dynamics in maize agro‐ecosystems that include cover crops.

Approaches for evaluating subsurface phosphorus loss potential from soil profiles

Abstract The importance of subsurface phosphorus (P) transport in deterioration of surface water quality is well documented. Practices and treatments have been identified and modeling approaches have been implemented to decrease the subsurface P load to surface waters. Soil phosphorus storage capacity (SPSC) is a site-assessment tool that predicts the potential for P loss from soils. We examined the use of SPSC to assess soil profile P distributions and loss potentials for two contrasting soil types: manure-impacted Spodosols and Ultisols. Our specific objectives were to (i) validate prediction of the Langmuir P bonding strength, K L , the Freundlich adsorption coefficient, K F , or the linear adsorption coefficient, K D , of Spodosols (Ap, E and Bh horizons) and Ultisols (Ap, E and Bt horizons) using SPSC-based equations developed for surface and sub-surface horizons (Ap, E and Bt) of Ultisols and (ii) identify factors affecting retention and release of P. Results showed that SPSC derived from oxalate-extractions data can effectively predict P isotherm parameters used in P transport models. The SPSC also captured differences between soil horizons within a profile pertinent to P loss. Magnesium (Mg) and calcium (Ca) concentrations related to negative SPSC, likely due to the presence of these components in dairy manure. The initial sorbed P in the solid phase (S 0 ) of both soil types was related to soil components such as Mg, Ca and total P when SPSC was positive. The equations developed from A, E and Bt horizons of Ultisol relating isotherm parameters to the PSR and SPSC made successful predictions for all horizons of Ultisols sampled from locations different from those in the current study and Spodosols suggesting that such equations might be applicable across a wide range of soils.

Proximal gamma-ray spectrometry for site-independent in situ prediction of soil texture on ten heterogeneous fields in Germany using support vector machines

Gamma spectrometric field measurements may provide high resolution information on topsoil texture. Yet, calibrations for the estimation of texture data usually have to be done site-specifically. The lack of site-independent calibrations thus limits the easy and universal use of proximal gamma-ray sensing in soil mapping and precision agriculture. Our objective was to develop a study site-independent prediction model for topsoil texture from gamma-ray spectra. We surveyed ten study sites across Germany with 417 reference samples (291 for calibration, 126 for test set-validation), providing soils from a broad range of parent materials and with widely varying soil texture. First, study site-specific models were calibrated by a linear regression approach. These models provided reliable estimations of sand, silt, and clay for most of the study sites. Second, study site-independent models were calibrated via i) linear regression and ii) support vector machines (SVM), the latter being mathematical methods of data pattern recognition. Based on the non-linear relationship between gamma spectrum and soil texture, which varied widely between the different parent materials the linear models are not appropriate for satisfactory soil texture prediction (averaged R2 of 0.73 for sand, 0.61 for silt, and 0.18 for clay and averaged absolute prediction errors of 9 to 5%, respectively). In contrast, the SVM calibrated prediction models revealed reliable performance also for site-independent calibrations. With the non-linear SVM approach we were able to include all sites in one single prediction model for each texture fraction although the different mineralogical composition of their parent materials led to complex and partly opposing relationships between gamma features and soil texture. Site-independent predictions via SVM were often even better than site-specific linear regression models. The site-independent SVM calibrated predictions yielded an averaged R2 of 0.96 (sand), 0.93 (silt), and 0.78 (clay), and corresponding averaged absolute prediction errors of 2 to 4%, respectively. To summarize, (i) non-linear prediction models are a feasible approach for capable site-independent texture estimations across a wide range of soils and (ii) gamma spectrometry-based texture predictions are a valuable input for applications that require highly resolved texture information at low costs and efforts.

Is the computed speciation of copper in a wide range of Chinese soils reliable?

Free Cu species in soils is a key issue to its bioavailability. However, predictive models for Cu speciation across a wide range of soils were still unavailable. In this study, Cu speciation in 34 contaminated soil samples were investigated via analytical technique and predictive models. The results showed that most of free Cu2+ was underestimated when using default log KCuFA and 65% active fulvic acid as inputs in models of WHAM VI and NICA-Donnan. The best prediction was found when using either adjusted active fulvic acid from 10% to 125% for WHAM VI or from 15% to 65% for NICA-Donnan model with the RMSE < 0.32 and r2 > 0.96. In contrast, NICA-Donnan demonstrated a slightly stronger binding for Cu than WHAM VI due to extra 26% of samples was underestimated. This work presents a comprehensive database of Cu speciation and an effective attempt of free Cu2+ prediction in a wide range of Chinese soils.

Effect of NPK, boron and sulphur on growth and yield of passion fruit

India is bestowed with a wide range of soil and agro-climatic conditions. Therefore, almost all types of fruit can be grown in one or the other parts of the country. China is the largest producer of fruits followed by India which now accounts for about 10 per cent of world production. Passion fruit belongs to the family Passifloracea, distributed throughout the tropical and subtropical regions of the world. There is need to improve and standardize the nutrient doses by which we increase the production of this fruit. Keeping with these views, the experiment was conducted in the experimental farm, Department of Horticulture, BAU, Ranchi. This experiment consisted of ten treatments including control. The treatments were NPK (300: 150:150 g/vine), NPK (250: 125:125 g/vine), boron 1.2 g/vine, sulphur 24 g/vine, the next four treatments were combination of NPK with boron and sulphur, and two controls. Thus, there were ten treatments, replicated thrice in Randomized Block Design. In all the treatments 2 kg of vermicompost and 0.5 kg of lime were applied as basal dose except absolute control. All the treatments exhibited better results over untreated control and absolute control. Highest yield (56.65 q/ha) was obtained by NPK (250: 125:125 g/vine) + boron 1.2 g/vine which was at par with, NPK (300: 150:150 g/vine) + boron 1.2 g/vine (49.15 q/ha) and NPK (250: 125:125 g/vine) + sulphur 24 g/vine (52.48 q/ha). Thus, NPK (250: 125:125 g/ vine ) + boron 1.2 g/vine appeared to be the best treatment in vegetative character, reproductive characters, fruit characters and yield followed by NPK (250:125:125 g/ vine) + sulphur 24 g/vine.

Use of a Pre-Drilled Hole for Implementing Thermal Needle Probe Method for Soils and Rocks

The thermal needle probe method, which is widely used for measuring the thermal conductivity λ of soils, deploys a long and thin metallic probe that houses a line heater and a temperature sensor. However, inserting such probes into consolidated or densely compacted soils or rocks is difficult, frequently causing buckling of the probe and severe disturbance to the surrounding ground, leading to unreliable measurements. We found that the use of a pre-drilled hole filled with thermally conductive grease for installing a thermal needle probe was feasible to overcome such challenges, and still yielded reliable measurements of thermal conductivity. The proposed method, i.e., the pre-drilling thermal needle probe method, was verified by finite element calculations and laboratory experiments by varying various parameters, such as the pre-drilled hole diameter, probe diameter, and thermal conductivity of thermal grease. It was observed that increases in the pre-drilled hole diameter and probe diameter and a decrease in the thermal conductivity of the thermal grease caused delays in temperature increase owing to the slowed heat transfer. Nevertheless, all the estimated λ values agreed well with the reference λ values with acceptable errors. Thus, the proposed method yields reliable measurements and can be applied for a wide range of soils from compacted soils to hard rocks.

Factors and Predictions for Cadmium Transfer from Soils into Tomato Plants

ABSTRACTSoil-plant transfer models are needed to predict levels of cadmium (Cd) in vegetables when evaluating the food chain risks of Cd contamination in agricultural soils. In the present study, the transfer of Cd from a wide range of soils into tomato plants was investigated in order to identify the major factors and to develop predictive models. It was found that soil pH was the main factor controlling Cd uptake and lower pH was more favorable for Cd bioaccumulation. Furthermore, the hydrogen ion activity, other than pH, was correlated linearly with the ratio of plant Cd to soil Cd. Combining soil Cd, and the interaction between soil Cd and hydrogen ion activity greatly improved the model to predict the transfer of Cd from soil to plant. Compared with the previous log transformation model, the predicted model improved the regression coefficient from 0.799 to 0.973 and has important significance in practice.

Salinity and High Temperature Tolerance in Mungbean [Vigna radiata (L.) Wilczek] from a Physiological Perspective.

Biotic and abiotic constraints seriously affect the productivity of agriculture worldwide. The broadly recognized benefits of legumes in cropping systems—biological nitrogen fixation, improving soil fertility and broadening cereal-based agro-ecologies, are desirable now more than ever. Legume production is affected by hostile environments, especially soil salinity and high temperatures (HTs). Among legumes, mungbean has acceptable intrinsic tolerance mechanisms, but many agro-physiological characteristics of the Vigna species remain to be explored. Mungbean has a distinct advantage of being short-duration and can grow in wide range of soils and environments (as mono or relay legume). This review focuses on salinity and HT stresses on mungbean grown as a fallow crop (mungbean-rice-wheat to replace fallow-rice-wheat) and/or a relay crop in cereal cropping systems. Salinity tolerance comprises multifaceted responses at the molecular, physiological and plant canopy levels. In HTs, adaptation of physiological and biochemical processes gradually may lead to improvement of heat tolerance in plants. At the field level, managing or manipulating cultural practices can mitigate adverse effects of salinity and HT. Greater understanding of physiological and biochemical mechanisms regulating these two stresses will contribute to an evolving profile of the genes, proteins, and metabolites responsible for mungbean survival. We focus on abiotic stresses in legumes in general and mungbean in particular, and highlight gaps that need to be bridged through future mungbean research. Recent findings largely from physiological and biochemical fronts are examined, along with a few agronomic and farm-based management strategies to mitigate stress under field conditions.

Progress in upscaling Miscanthus biomass production for the European bio‐economy with seed‐based hybrids

AbstractField trials in Europe with Miscanthus over the past 25 years have demonstrated that interspecies hybrids such as M. × giganteus (M × g) combine both high yield potentials and low inputs in a wide range of soils and climates. Miscanthus hybrids are expected to play a major role in the provision of perennial lignocellulosic biomass across much of Europe as part of a lower carbon economy. However, even with favourable policies in some European countries, uptake has been slow. M × g, as a sterile clone, can only be propagated vegetatively, which leads to high establishment costs and low multiplication rates. Consequently, a decade ago, a strategic decision to develop rapidly multiplied seeded hybrids was taken. To make progress on this goal, we have (1) harnessed the genetic diversity in Miscanthus by crossing and progeny testing thousands of parental combinations to select several candidate seed‐based hybrids adapted to European environments, (2) established field scale seed production methods with annual multiplication factors &gt;1500×, (3) developed the agronomy for establishing large stands from seed sown plug plants to reduce establishment times by a year compared to M × g, (4) trialled a range of harvest techniques to improve compositional quality and logistics on a large scale, (5) performed spatial analyses of yield potential and land availability to identify regional opportunities across Europe and doubled the area within the bio‐climatic envelope, (6) considered on‐farm economic, practical and environmental benefits that can be attractive to growers. The technical barriers to adoption have now been overcome sufficiently such that Miscanthus is ready to use as a low‐carbon feedstock in the European bio‐economy.

Blueberry rootstock: selection, evaluation, and field performance of grafted blueberry plants

There is currently no rootstock available for blueberry production in nurseries. Vaccinium arboreum, a native species in the United States posesses several desirable traits for use as a blueberry rootstock. Such traits include a sturdy single trunk, and a root system which can thrive in a wide range of soils that differ in soil pH and organic matter. Seeds of V. arboreum from Florida (FL), Oklahoma (OK), and eastern Texas (TX) were germinated through various treatments in the greenhouse. The best germination rate (43%) was obtained from 90 days of cold stratification of seeds from dried V. arboreum fruits. Seedlings were then planted at the North Willamette Research and Extension Center (NWREC) in Aurora, OR. The plot had over 500, 200, and 50 V. arboreum selected seedlings from TX, OK, and FL seed sources, respectively. Foliar nutrient levels varied among the three seedling sources, with plants from TX and OK showing higher N, P, K, B, and Mn levels than plants from the FL source. Plants from TX and OK were more vigorous than those from FL. In another plot at the NWREC, three blueberry cultivars ('Draper', 'Liberty', and 'Aurora') were grafted onto V. arboreum seedlings. Grafted blueberry plants were compared to their own rooted plants under two soil treatments. Results demonstrated that fruit firmness, size, total soluble solids, and titratable acidity were not affected by grafting and soil treatments. Variations in foliar nutrient levels N, P, K, Ca, Mg and Mn observed in the grafted plants may be due to grafting because no cultivar and rootstock interactions were found. The potential for using V. arboreum as blueberry rootstock is promising.

A Simple and Farmer-Friendly Decision Support System for Enhancing Water Use Efficiency in Agriculture:Tool Development, Testing and Validation

In the semi-arid tropics (SAT) farmers practice calendar- based irrigation scheduling, which generally results in over irrigation and poor water use efficiency. The lack of a simple decision tool to decide timing and quantity of water to be applied is a bottleneck. An Excel-based decision support system termed Water Impact Calculator (WIC) is developed using data collected at the ICRISAT, which were validated at three pilot sites on farmers' fields in Rajasthan, Gujarat and Telangana. Field studies were conducted under two land-form treatments (broad bed and furrow (BBF) and flat fields); and irrigation water was applied following two different methods (drip and flood). The data collected at micro-watershed at the ICRISAT and three other sites showed that WIC could be used under wide range of soil and rainfall conditions. WIC simulated soil moisture was comparable with the observed moisture data, which forms the basis of irrigation scheduling. The WIC-based water balance at these experimental sites showed that number and amount of irrigation could be reduced by 30-40% using WIC-based irrigation scheduling without compromising the crop yield. The WIC could be a potential tool for water resources planning and efficient management at the field and watershed scale in the SAT.

Inhibition of nitrification to mitigate nitrate leaching and nitrous oxide emissions in grazed grassland: a review

Climate change is arguably the biggest environmental challenge facing humanity today. Livestock production systems are a major source of greenhouse gases that contribute to climate change. Nitrous oxide (N2O) is a potent greenhouse gas with a long-term global warming potential 298 times that of carbon dioxide (CO2). Nitrate (NO3 −) leaching from soil causes water contamination, and this is a major environmental issue worldwide. Agriculture is identified as the dominant source for NO3 − in surface and ground waters. In grazed grassland systems where animals graze outdoor pastures, most of the N2O and NO3 − are from nitrogen (N) returned to the soil in the excreta of the grazing animal, particularly the urine. This paper reviews published literature on the use of nitrification inhibitors (NI) to treat grazed pasture soils to mitigate NO3 − leaching and N2O emissions. This paper provides a review on: ammonia oxidisers, including ammonia oxidising bacteria (AOB) and ammonia oxidising archaea (AOA), that are responsible for ammonia oxidation in the urine patch areas of grazed pastures; the effectiveness of NIs, such as dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP), in inhibiting the growth and activity of ammonia oxidisers; the efficacy of DCD and DMPP in reducing NO3 − leaching and N2O emissions in grazed pastures; additional benefits of using NI in grazed pasture, including increased pasture production, decreased cation leaching and decreased NO3 − concentrations in plants; and major factors that may affect the efficacy of NIs. Research from a number of laboratory and field studies have conclusively demonstrated that treating grazed pasture soils with a NI, such as DCD, is an effective means of reducing NO3 − leaching and N2O emissions from grazed livestock production systems. Results show that N2O emissions from animal urine-N can be reduced by an average of 57 % and NO3 − leaching from animal urine patches can be reduced by 30 to 50 %. The NI technology has been shown to be effective under a wide range of soil and environmental conditions. The NI technology also provides other benefits, including increased pasture production, reduced cation (Ca2+, Mg2+ and K+) leaching and reduced NO3 − concentration in pasture plants which would reduce the risk of NO3 − poisoning for the animal. The use of NIs such as DCD to treat grazed pasture soil is a scientifically sound and practically viable technology that can effectively mitigate NO3 − leaching and N2O emissions in grazed livestock production systems.

Biological Flora of the British Isles:Fraxinus excelsior

SummaryThis account presents information on all aspects of the biology ofFraxinus excelsiorL. (Ash) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of theBiological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history, and conservation.Fraxinus excelsioris a large forest tree, native throughout the main islands of Britain and much of mainland Europe. Seedlings are shade tolerant, but adults are not so it tends to be an intermediate successional species, invading gaps in mixed stands rather than forming extensive pure stands. Ash grows on a wide range of soils but is commonest on nutrient‐rich soils with a high base status andpH &gt; 4.2, and is at its best on dry calcareous screes and fertile alluvial soils.Fraxinus excelsioris trioecious or subdioecious with male, hermaphrodite and female flowers and trees. Seed production is prolific with periodic higher producing mast years. Seeds are primarily wind‐dispersed, but they can float and be moved considerable distances along waterways. Germination is delayed by dormancy until usually the second spring after being shed.Ash is tolerant of drought, but intolerant of spring frosts and so is predicted to fare well under current climate change scenarios, and indeed has recently been expanding in range in Europe. However, ash health and survival is currently seriously compromised by ash dieback caused by the fungusHymenoscyphus fraxineus(Chalara fraxinea) that has the potential to kill all but a very few resistant trees. Moreover, the emerald ash borer beetleAgrilus planipennis, a serious pest of ash species in N. America, has reached Europe (though not yet the British Isles) and poses an equally if not more serious long‐term threat to ash.

Lime Requirement Using Mehlich-III Extraction and Infrared-Inferred Cation Exchange Capacity

Core Ideas Exchangeable acidity estimated from bases and CEC was related to lime requirement. CEC of Québec soils was inferred accurately from infrared spectroscopy. Model using CEC inferred within ±2.3 cmolc kg−1 was more accurate than the SMP buffer. Lime requirement (LR) to reach and maintain target soil pH levels could be determined using environmentally friendly methods to avoid buffer solutions containing toxic chemicals. Infrared (IR) spectroscopy can predict soil properties related to soil pH buffering capacity, potentially supporting LR models. The aim of this study was to develop IR-based LR models across a wide range of soils in Québec, Canada. The estimated exchangeable acidity (EEA) index was computed from the cation exchange capacity determined by the NH4OAc (pH 7.0) method (CEC-NH4OAc) minus the sum of Mehlich-III extractable non-acidic cations. The performance was high for IR-inferred CEC-NH4OAc predicted from mid-infrared (MIR) and near-infrared (NIR) spectra or computed from quantified organic C and NIR- and MIR-predicted clay content (0.82 ≤ R2 ≤ 0.91 in validation). The performance in cross-validation of EEA using quantified CEC-NH4OAc (0.81 ≤ R2 ≤ 0.89) was higher than Shoemaker–McLean–Pratt (SMP) buffer (0.15 ≤ R2 ≤ 0.77) for target pHH2O values between 5.5 and 6.5, and similar for a target pHH2O of 7.0. To support LR determination of cropping systems such as potato (Solanum tuberosum L.) that requires low pH (<6.5) or to maintain adequate pH levels at low LRs (<1.5 g CaCO3 kg−1), the EEA index using CEC-NH4OAc inferred within ±2.3 cmolc kg−1 was found to be more accurate than the SMP buffer. Direct prediction of the EEA-derived LR using NIR and MIR spectroscopy showed low performance (validation R2 ≤ 0.48) compared with EEA computed from IR-inferred CEC-NH4OAc. The IR spectroscopy can improve routine LR determination at low cost.

Depth distribution of mineralizable nitrogen pools in contrasting soils in a semi-arid climate

A better understanding of the depth distribution of soil mineralizable nitrogen (N) pools is important to improve prediction of net soil N mineralization. However, our understanding of the depth distribution of these N pools under the semi-arid conditions of western Canada is limited. This study examined the depth distribution of soil mineralizable N pools (kS, the rate constant of a nondepleting zero-order stable N pool, and NL, the size of a depleting first-order labile N pool) of six sites in western Canada chosen to vary with respect to soil zone, soil texture, and cropping system. The depth distribution of mineralizable N pools varied substantially among sites, indicating that this distribution needs to be considered in making predictions of net soil N mineralization. A single regression equation including soil total nitrogen (STN), Pool I (a labile mineralizable N pool determined through a 14-day aerobic incubation), and soil pH explained 67% of the variation in kS across sites and soil depths. In addition, 95% of the variation in NL was explained by a regression model with Pool I. Thus, although the depth distribution of soil mineralizable N pools can vary substantially among sites, the mineralizable N parameters can be adequately predicted across sites and soil depths from simple soil properties. Comparison with a study using surface soils under humid conditions in New Brunswick suggests that the relationship between NL and Pool I is applicable across a wide range of soils, climatic zones, and cropping systems, whereas the regression model to predict kS varied with climatic zone, perhaps reflecting different pedogenic processes stabilizing the organic matter in these climatic zones.

Semichemical fibres of Leucaena collinsii reinforced polypropylene: Macromechanical and micromechanical analysis

Abstract Leucaena collinsii (LCN) is a fast growing legume. Due to the ability of legumes to fix nitrogen in the ground and the wide range of soils where they can grow up, Leucaena genus has been proposed for recovering deserted soils. Furthermore, the incorporation of fibres into the bosom of a polymeric matrix increases the tensile strength. This study examines the options of a treated fibre of L. collinsii to be added as reinforcement to a polypropylene matrix. The influence on the tensile properties of different percentages of coupling agent and reinforcement were studied. A local maximum for the tensile strength was noticed when increasing the amount of coupling agent up to 6% for 30% w/w LCN contents. Afterwards, a tensile strength characterisation for 20–50% w/w LCN contents with the same amount of coupling agent was performed. The fibres from the composite reinforced with a 30% of LCN were recovered by extraction. The theoretical tensile strengths for composites from 20 to 50% w/w LCN content were modelled using the fibre distribution of the 30% composite material and compared with the experimental results. A good correlation between experimental and theoretical values was confirmed.

Physiological responses to extreme hydrological events in the Pantanal wetland: heterogeneity of a plant community containing super‐dominant species

AbstractAimsWe tested two mechanisms of adaptation to extreme hydrological stresses (flooding and drought) of species making up a tropical wetland plant community by measuring leaf gas exchange and water potential. We hypothesized that anoxic conditions that occur during flooding will decrease leaf gas exchange when compared to the dry season, and that ‘super‐dominant’ species will have a distinctive physiological advantage when compared to other plants within the community.LocationNorthern Pantanal wetland, Private Natural Heritage Reserve of the Brazilian Social Service of Commerce (RPPN‐SESC Pantanal), Mato Grosso, Brazil.MethodsTwo periods representing typical extreme hydrological conditions in the Pantanal wetland were selected based on historical soil and meteorological measurements: (1) a drought period when plants experience stress due to soil moisture deficits during a dry season that persists for several months (May to Sept), and (2) a flooding period when oxidation‐reduction potential is negative for 30 d or more (Mar or Apr), indicating anoxic stress. Measurements of gas exchange and leaf water potential were made on seven species in drought and flood stress conditions. The seven species represent the majority of the plant community.ResultsAs a whole, the plant community showed significantly lower potential net photosynthesis (PN) during flooding when soil oxidation‐reduction potential reached close to −900 mV when compared to the dry season, but the magnitude of the decline in PN was species specific. Not all super‐dominant species showed higher PN compared to non‐dominant species, but they did demonstrate higher stomatal conductance and transpiration leading to lower water use efficiency. The combination of higher PN despite low soil water content suggests that the plant community had access to deep water resources. This access was also confirmed by the midday leaf water potential, which was similar for the flood and dry seasons.ConclusionsResults suggest that the plant community may have high physiological performance under a wide range of soil oxidation‐reduction potentials. Higher PN rates of super‐dominant species indicate a physiological advantage of these species in the different hydrological conditions.

Validation of Long-Term Pavement Performance Prediction Models for Resilient Modulus of Unbound Granular Materials

Reliable characterization of unbound granular material and subgrade resilient modulus requires repeated load triaxial testing, advanced dynamic loading equipment, and technical experience not typically available in many geotechnical laboratories. For Level 2 design inputs, the Mechanistic–Empirical Pavement Design Guide recommends correlation models to estimate the resilient modulus of unbound granular material and subgrade from basic material properties (e.g., gradation, unconfined compressive strength, and California bearing ratio). These correlation models are developed from data on a wide range of soil and material types; this wide range increases the associated error with resilient modulus prediction. This paper compares laboratory measured resilient modulus for two types of locally available unbound granular materials to the predicted values; models developed under the Long-Term Pavement Performance (LTPP) program were used. Resilient modulus tests of six gradations of two types of material—100% crushed limestone and gravel—were conducted at two levels of moisture content. Results showed that the LTPP models significantly underestimated resilient modulus for the limestone material and showed high residuals associated with evaluating resilient modulus of both gravel and limestone materials. The coefficient of variation of the root mean square error was 50.3% for gravel and 55.6% for limestone. The high residuals associated with evaluating resilient modulus with the LTPP models justify the development of local prediction models to improve the reliability of Level 2 design inputs for unbound granular materials.

Mixtures of bioenergy species yield as well as monocultures

For those hoping to sow some diversity back into the Corn Belt with dedicated bioenergy crops, mixtures of native prairie species seem like an obvious choice. Farmers, though, need to manage biofuels crops for maximum yields and returns. So for them, simple monocultures of switchgrass fertilized with nitrogen may be best. A study in the September–October 2015 issue of Agronomy Journal (http://bit.ly/1NusPO2) now suggests the options aren't quite so black or white. In a seven-year experiment spanning nine locations in Minnesota and North Dakota, an eight-species mixture, or polyculture, of perennial grasses and legumes produced as much biomass as switchgrass monocultures when plots weren't fertilized. And yields of another polyculture—an assemblage of four grasses—rivaled those of switchgrass when nitrogen was added. Although the bioenergy industry has taken a hit recently from plummeting oil and gas prices, a few commercial cellulosic ethanol plants are now in operation–notably the POET-DSM plant in Emmetsburg, IA. In the meantime, many Midwest farmers are interested in restoring native prairie vegetation for non-commercial reasons, says Jacob Jungers, the study's lead author. But they also can't neglect the bottom line. “We've spoken to producers who want to manage a polyculture because it provides wildlife habitat and other ecosystem services. But they also want to make sure it's an economically feasible system that will produce some level of biomass to make a return,” explains the University of Minnesota postdoctoral research fellow. “So we hope the results from this study can help inform that decision-making process.” Although it remains controversial, the hypothesis that systems with higher plant diversity are more productive is well supported by ecological research. In a seminal 2006 paper, for example, University of Minnesota ecologist David Tilman reported that on an abandoned farm with unproductive soils, diverse mixtures of prairie grasses and forbs produced 238% more bioenergy, on average, than monocultures of switchgrass or other species. However, Tilman's study was “a classical plant community ecology experiment,” not an agricultural one, says Jungers, who worked on the project as an undergrad—leaving many unconvinced that a real bioenergy production system would yield similar results. In 2006, Jungers’ advisers Don Wyse and Craig Sheaffer decided to put this to the test. In true agronomic fashion, their experiment compared yields of mixtures and monocultures across a wide range of soil, temperature, and other growing conditions. They tested the effects of adding nitrogen; selected biofuels species—such as Canada wild rye and big blue stem—that are relatively cheap and easy to establish; and harvested plant biomass at the end of each season just like producers would. They also collected data for seven years. “Natural succession takes place in any polyculture, whether it's managed or not, and we didn't know how management would steer the succession,” Jungers explains. A particular question was how adding nitrogen might alter the mixtures, since previous research has documented the loss of species with fertilization. Most of the time, switchgrass monocultures were the star performers, the team found. What they didn't expect is that an eight-species mixture of grasses and legumes would generate as much biomass as switchgrass in unfertilized plots: 5.1 Mg/ha–1 on average. With nitrogen fertilizer, switchgrass yields jumped to 6.8 Mg/ha–1. But even then, a fertilized, four-species grass mixture kept pace, producing 6.4 Mg/ha–1, on average. In contrast to previous work, nitrogen fertilizer also didn't reduce diversity over time—even in polycultures with as many as 12 and 24 species. The reason, Jungers believes, is that biomass was harvested each season. Additions of fertilizer tend to favor species that can take up nitrogen quickly and efficiently, allowing them to outcompete and eventually exclude other plants. “However, when you harvest, you're giving another set of species an advantage,” he says—specifically those that flourish in the extra light that biomass removal lets in. Put another way, harvesting “resets” the system each year, helping ensure no one species can take over. All in all, the findings suggest that while managing agricultural lands for greater diversity and ecological functions will never be easy, it's more feasible than people once thought. “Natural phenomena do still play out”—and can be leveraged—in plant communities subjected to harvesting, fertilizing, and other management practices, Jungers says. Still, even he admits to being surprised—and relieved, he adds with a laugh, since he wants to make a career in agroecology. “All the work that's been done in the past, the basic ecology, is holding up, even though we're manipulating these systems.” Photos courtesy of Jacob Jungers.

An Isotope Labeling Technique to Investigate Atom Exchange during Phosphate Sorption and Desorption

Iron and manganese oxides are common minerals in a wide range of soils, and they largely control the fate of P. It is often implicitly assumed that phosphate does not exchange O atoms with minerals and water during adsorption, desorption, and precipitation reactions, but this has not yet been fully verified. In this study, we applied an ¹⁸O stable isotope labeling technique to individually label O atoms in Fe and Mn oxides, water, and phosphate, and tested the exchange of O among them during phosphate sorption and desorption reactions. Physico-chemical properties of ¹⁸O-labeled goethite, hematite, hausmannite, and birnessite minerals were characterized using X–ray diffraction, Brunauer–Emmett–Teller (BET) surface area and pore size distribution, and scanning electron microscopy. All sorption and desorption results showed fast initial kinetics, with hausmannite having the highest sorption capacity, followed by a relatively slower sorption–desorption reaction before reaching equilibrium. Oxygen isotope ratios in Fe and Mn oxides, phosphate, and water showed no exchange of O between phosphate and mineral or water during either sorption or desorption reactions. These results attest to the integrity of the P–O bond during these reactions and thus enable the phosphate O isotope composition to be used as a reliable tool for tracing phosphate sources in soils and other environments.