Constraining the carbonate system in soils via testing the internal consistency of pH, pCO(2) and alkalinity measurements

Inorganic carbon exists in various dissolved, gaseous and solid phase forms in natural waters and soils. It is important to accurately measure and model these forms to understand system responses to global climate change. The carbonate system can, in theory, be fully constrained and modelled by measuring at least two of out of the following four parameters: partial pressure (pCO2), total alkalinity (TA), pH and dissolved inorganic carbon (DIC) but this has not been demonstrated in soils. In this study, this "internal consistency" of the soil carbonate system was examined by predicting pH of soil extracts from laboratory measurement of TA through alkalinity titration for solutions in which pCO2 was fixed through equilibrating the soil solution with air with a known pCO2. This predicted pH (pHCO2) was compared with pH measured on the same soil extracts using spectrophotometric and glass electrode methods (pHspec and pHelec). Discrepancy between measured and calculated pH was within 0.00-0.1 pH unit for most samples. However, more deviation was observed for those sample with low alkalinity (≤ 0.5 meq L-1). This is likely attributable to an effect of dissolved organic matter, which can contribute alkalinity not considered in the thermodynamic carbonate model calculations; further research is required to resolve this problem. The effects of increasing soil pCO2 was modelled to illustrate how internally consistent models can be used to predict risks of pH declines and carbonate mineral dissolution in some soils.Sima Bargrizan, Ronald J. Smernik and Luke M. Mosle

Frequency versus quantity: phenotypic response of two wheat varieties to water and nitrogen variability

Due to climate change, water availability will become increasingly variable, affecting nitrogen (N) availability. Therefore, we hypothesised watering frequency would have a greater impact on plant growth than quantity, affecting N availability, uptake and carbon allocation. We used a gravimetric platform, which measures the unit of volume per unit of time, to control soil moisture and precisely compare the impact of quantity and frequency of water under variable N levels. Two wheat genotypes (Kukri and Gladius) were used in a factorial glasshouse pot experiment, each with three N application rates (25, 75 and 150 mg N kg−1 soil) and five soil moisture regimes (changing water frequency or quantity). Previously documented drought tolerance, but high N use efficiency, of Gladius as compared to Kukri provides for potentially different responses to N and soil moisture content. Water use, biomass and soil N were measured. Both cultivars showed potential to adapt to variable watering, producing higher specific root lengths under low N coupled with reduced water and reduced watering frequency (48 h watering intervals), or wet/dry cycling. This affected mineral N uptake, with less soil N remaining under constant watering × high moisture, or 48 h watering intervals × high moisture. Soil N availability affected carbon allocation, demonstrated by both cultivars producing longer, deeper roots under low N. Reduced watering frequency decreased biomass more than reduced quantity for both cultivars. Less frequent watering had a more negative effect on plant growth compared to decreasing the quantity of water. Water variability resulted in differences in C allocation, with changes to root thickness even when root biomass remained the same across N treatments. The preferences identified in wheat for water consistency highlights an undeveloped opportunity for identifying root and shoot traits that may improve plant adaptability to moderate to extreme resource limitation, whilst potentially encouraging less water and nitrogen use

Fire-derived organic matter retains ammonia through covalent bond formation

Fire-derived organic matter, often referred to as pyrogenic organic matter (PyOM), is present in the Earth's soil, sediment, atmosphere, and water. We investigated interactions of PyOM with ammonia (NH₃) gas, which makes up much of the Earth's reactive nitrogen (N) pool. Here we show that PyOM's NH₃ retention capacity under ambient conditions can exceed 180 mg N g⁻¹ PyOM-carbon, resulting in a material with a higher N content than any unprocessed plant material and most animal manures. As PyOM is weathered, NH₃ retention increases sixfold, with more than half of the N retained through chemisorption rather than physisorption. Near-edge X-ray absorption fine structure and nuclear magnetic resonance spectroscopy reveal that a variety of covalent bonds form between NH₃-N and PyOM, more than 10% of which contained heterocyclic structures. We estimate that through these mechanisms soil PyOM stocks could retain more than 600-fold annual NH₃ emissions from agriculture, exerting an important control on global N cycling.Rachel Hestrin, Dorisel Torres-Rojas, James J. Dynes, James M. Hook, Tom Z. Regier, Adam W. Gillespie, Ronald J. Smernik, Johannes Lehman

Spectrophotometric measurement of the pH of soil extracts using a multiple indicator dye mixture

This paper describes the development of a spectrophotometric method with an expanded pH range of 3–9 that uses a mixed indicator solution (equimolar bromophenol blue, bromocresol purple, m‐cresol purple and thymol blue). The method uses measurements of absorbance of the dye mixture at two wavelengths (434 and 585 nm), chosen to represent the average acid and base peak maxima of the individual dyes within the mixture. The ratio of absorbance at these two wavelengths was used to calculate pH based on measured dye properties (pKa, molar absorptivity) and fundamental equations derived from Beer's law. The mixed dye spectrophotometric pH measurement was tested using certified pH buffers (pH (NBS/NIST) 4.00, 6.86, 9.18) and was found to be accurate to within ± 0–0.06 pH units. Measurements made with the mixed dye showed good correlation against conventional soil pH measurement using a glass electrode (r = 0.99), and also an alkalinity titration (r = 0.99) through the pH range 3–9. The average standard deviation was 0.07 for spectrophotometric soil pH measurement (n = 30) using the dye mixture. The mixed dye technique expands the working range of spectrophotometric pH measurement methods in soils and other applications.S. Bargrizan, R.J. Smernik, L.M. Mosle

Arbuscular mycorrhizas increased tomato biomass and nutrition but did not affect local soil P availability or 16S bacterial community in the field

While interest in arbuscular mycorrhizal (AM) fungal effects on soil phosphorus (P) have recently increased, field ex- periments on this topic are lacking. While microcosm studies provided valuable insights, the lack of field studies rep- resents a knowledge gap. Here, we present a field study in which we grew a mycorrhiza-defective tomato (Solanum lycopersicum L.) genotype (named rmc) and its mycorrhizal wild-type progenitor (named 76R) with and without addi- tional fertiliser, using a drip-irrigation system to examine the impacts of the AM symbiosis on soil P availability and plant growth and nutrition. AM effects on fruit biomass and nutrients, soil nutrient availability, soil moisture and the soil bacterial community were examined. At the time of harvest, the AM tomato plants without fertiliser had the same early season fruit biomass and fruit nutrient concentrations as plants that received fertiliser. The presence of roots reduced the concentration of available soil P, ammonium and soil moisture in the top 10 cm soil layer. Arbuscular mycorrhizas did not significantly affect soil P availability, soil moisture, or 16S bacterial community composition. These findings suggest an indirect role for AM fungi in tomato production but not necessarily a direct role in determin- ing soil physicochemical traits, during the one season that this experiment was conducted. While longer-term field studies may be required in the future, the present study provides new insights into impacts of AM fungi on P availability and uptake in a field soil system.Cuc T.K. Tran, Stephanie J. Watts-Williams, Ronald J. Smernik, Timothy R. Cavagnar

Root and arbuscular mycorrhizal effects on soil nutrient loss are modulated by soil texture

Available online 9 June 2021Despite their importance, there is a lack of knowledge on the impact of forming arbuscular mycorrhizas (AM) on soil phosphorus (P) leaching in soils with different textures. Therefore, the objective of this study was to investigate the impacts of mycorrhizal and non-mycorrhizal roots on P leaching in two non-sterilised soils of contrasting texture. A mycorrhiza-defective tomato (Solanum lycopersicum L.) genotype (named rmc), and its wild-type progenitor that is able to form AM (named 76R), were used to investigate the effects of AM on soil P loss via leaching. Concentrations of reactive and un-reactive P in the leachate and soil were measured and related to plant growth, plant P uptake, soil water relations and leachate dissolved organic carbon (DOC) concentration. Soil texture affected mycorrhizal colonization, plant growth and plant P concentration, and influenced the concentration and chemical composition of P and the concentration of DOC leached. The chemical composition of P leached and P remaining in soil varied with soil texture, the presence or absence of roots, and their arbuscular mycorrhizal status. Mycorrhizal plants reduced P lost via leaching in the sandy soil substrate, where DOC leached was also high. The roots, regardless of mycorrhizal colonization, appeared to have the greatest impact on increasing P and DOC leached. Taken together, this study provides new insights into the role of AM on soil P loss via leaching in soils of contrasting texture.Cuc T.K. Tran, Stephanie J. Watts-Williams, Ronald J. Smernik, Timothy R. Cavagnar

Effects of plant roots and arbuscular mycorrhizas on soil phosphorus leaching

Available online 10 March 2020While the impact of arbuscular mycorrhizal fungi (AMF) on phosphorus (P) uptake is well understood, the mechanism(s) of how these fungi affect P leaching from soil is still unclear. Here we present results of a study in which we grew a mycorrhiza-defective tomato (Solanum lycopersicum L.) genotype (named rmc) and its mycorrhizal wild-type progenitor (named 76R) in microcosms containing non-sterile soil, to examine the influence of roots and AMF on P leaching. More P was leached from the planted microcosms as compared to the plant-free controls. Further, although there was more plant biomass and greater P uptake in the mycorrhizal plant treatments, these treatments were associated with the most leaching of total P, reactive P, and dissolved organic carbon (DOC). There was a strong correlation between the total P and DOC leached, suggesting that root and fungal exudates may have affected P leaching. These findings provide new insights into the impact of roots and AMF on nutrient leaching in soils.Cuc T.K. Tran, Stephanie J.Watts-Williams, Ronald J. Smernik, Timothy R. Cavagnar

Does solid-state 15N NMR spectroscopy detect all soil organic nitrogen?

The original publication can be found at www.springerlink.comVirtually all of the N detected by 15N cross polarization (CP) NMR spectra of four HF-treated soil clay fractions is amide N. However, the intensity of this 15N CP NMR signal (per unit N) is 27–57% lower than detected for a wheat protein, gliadin. There are two possible explanations – either the amide N in the soil clay fractions produces proportionately less NMR signal than does the amide N in gliadin, or part of the N in the soil clay fractions produces little or no NMR signal. The cross polarization dynamics of the gliadin amide resonance and amide resonances detected for the soil clay fractions are very similar and thus should produce similar amounts of signal, ruling out the first possibility. Therefore up to half or even more of the organic N in these soil clay fractions must be in a form that is insensitive to NMR detection. For a model compound (caffeine), non-protonated heterocyclic N produced less than 20% of the signal of an equivalent amount of amide N in gliadin. Results from several 13C NMR techniques provide further evidence that much of the undetected N in the soil clay fractions may be heterocyclic.Ronald J. Smernik and Jeffrey A. Baldoc

Soil phosphorus pools with addition of fertiliser phosphorus in a long-term grazing experiment

Published Online 8 November 2019Grasslands are a globally important use of land for food and fibre production, which often require the addition of phosphorus (P) fertiliser to maximise plant production. However, a large proportion of the added P can accumulate in pools of poorly available inorganic and organic P in the surface soil layer under grasslands. The aim of this study was to identify the chemical nature of the organic P in soils from a long-term fertiliser by grazing permanent pasture experiment that have received varying additions of phosphatic fertiliser (cumulative P input of 27, 169, 311, 513, 745 and 1035 kg P ha⁻¹) over a period of 37 years. The design of the experiment uniquely provides insight into the response of soil organic P to the addition of fertiliser P on the decadal scale. On average, 46% of the added fertiliser P was recovered as total P in the 0–100 mm soil layer after 37 years of phosphate addition. The content of both inorganic and organic forms of soil P increased with the addition of fertiliser P. The accumulation of organic P increased linearly up to a cumulative P input of 745 kg P ha⁻¹ and plateaued thereafter. The majority of organic P in all treatments was detected as a broad signal in the phosphomonoester region of solution ³¹P nuclear magnetic resonance (NMR) spectra; this also accounted for 79% of the accumulated organic P in fertilised soil. Our results indicate that accumulation of P in the organic portion as complex forms eventually reaches a new equilibrium where no net accumulation would be expected with further addition of phosphate.Timothy I. McLaren, Ronald J. Smernik, Michael J. McLaughlin, Therese M. McBeath, Malcolm R. McCaskill, Fiona A. Robertson, Richard J. Simpso