P.A.W. van Hees

Organic acid behavior in soils - misconceptions and knowledge gaps

Organic acids have been hypothesized to perform many functions in soil including root nutrient acquisition, mineral weathering, microbial chemotaxis and metal detoxification. However, their role in most of these processes remains unproven due to a lack of fundamental understanding about the reactions of organic acids in soil. This review highlights some of the knowledge gaps and misconceptions associated with the behavior of organic acids in soil with particular reference to low-molecular-weight organic acids (e.g., citrate, oxalate, malate) and plant nutrient acquisition. One major concern is that current methods for quantifying organic acids in soil may vastly underestimate soil solution concentrations and do not reveal the large spatial heterogeneity that may exist in their concentration (e.g., around roots or microbes). Another concern relates to the interaction of organic acids with the soils solid phase and the lack of understanding about the relative importance of processes such as adsorption versus precipitation, and sorption versus desorption. Another major knowledge gap concerns the utilization of organic acids by the soil microbial community and the forms of organic acids that they are capable of degrading (e.g., metal-complexed organic acids, adsorbed organic acids etc). Without this knowledge it will be impossible to obtain accurate mathematical models of organic acid dynamics in soil and to understand their role and importance in ecosystem processes. Fundamental research on organic acids and their interaction with soil still needs to be done to fully elucidate their role in soil processes.

Low molecular weight organic acids and their Al-complexes in soil solution - composition, distribution and seasonal variation in three podzolized soils.

Low molecular weight organic acids have been determined qualitatively and quantitatively over a growing season in the soil solution of three podzolized soil profiles. Several low molecular weight (LMW) acids such as citric, shikimic, oxalic, fumaric, formic, acetic, malonic, malic, lactic and t-aconitic acids were identified in the range < 1-1100 μM. Citric acid was the acid generally present at the highest concentrations (15-250 μM) in the upper layers while shikimic and oxalic acids could be observed in all horizons at lower concentrations. The levels of LMW organic acids were always greatest in the organic (O) horizon, and declined in the deeper layers. The fraction of the dissolved organic carbon and total acidity made up by LMW acids was generally in the range 0.5-5% and 0.5-15%, respectively. No apparent seasonal variations were observed, and the differences between the sites were little. The fraction of Al in soil solution bound to LMW organic acids was evaluated using ultrafiltration (< 1000D) and size exclusion chromatography. Fe < 1000D was also determined. For the O1 horizon about 40% and 20% of the Al and Fe, respectively, were detected in the LMW fraction (< 1000D). The LMW fraction of the two elements decreased deeper down in the profile, but relative increases were sometimes observed in the B1 horizon. Labile and LMW (< 1000D) Si were determined by FIA and ultrafiltration, respectively. The results show that a major fraction (80-100%) of the Si is labile and of low molecular weight, most likely silicic acid

Advances in understanding the podzolization process resulting from a multidisciplinary study of three coniferous forest soils in the Nordic Countries

Advances in understanding the podzolisation process resulting from a multidisciplinary study at three coniferous forest soils in the Nordic countries

Low molecular weight organic acid adsorption in forest soils: effects on soil solution concentrations and biodegradation rates

Low molecular weight (LMW) organic acids are believed to play a key role in many rhizosphere and pedogenic processes; However, their efficiency is likely to depend on their susceptibility to sorption and biodegradation. The sorption characteristics of three organic acids (citrate, oxalate and acetate) and phosphate were examined over the concentration range 0–1000 μM in three coniferous forest soil profiles. Sorption to the soils solid phase could be adequately described by the Langmuir equation with sorption capacity following the horizon series: B>C>E>O. The strength of anion sorption followed the series: phosphate>oxalate≥citrate≫acetate. Calculations indicated that between 50 and 95% (O and E horizons) and >93% (B horizons) of these LMW organic acids entering the soil will become sorbed to the solid phase. The amount of organic acids predicted to be present on the solid phase at typical soil solution concentrations ranged from <1 to 1100 nmol g−1 yielding adsorbed-to-solution ratios (adsorption coefficients) of between <0.1 and 3100. In the case of citrate, sorption to the solid phase significantly reduced its biodegradation potential by 35–99% depending upon the degree and type of sorption surface. The findings of this work are discussed in the context of the quantitative effects of adsorption on organic acids, their ecological functions and role in soil forming processes.

Equilibrium models of aluminium and iron complexation with different organic acids in soil solution

Abstract The percentage of Al and Fe bound to identified low molecular weight (LMW) organic acids and phosphate in soil solution was calculated using a chemical equilibrium model. The highest fractions were obtained for the O1 horizon solutions with median values between 38–49% for Al and 18–29% for Fe. Generally the percentage declined in the deeper horizons Acceptable agreement with experimental values using ultrafiltration ( 85% of the Al and >95% of the Fe were organically bound. The opposite was seen for Ca and Mg for which >85% was modelled to occur as inorganic ions. The modelling results support the theory that LMW organic acids contribute to the translocation of Al and Fe in the podzolization process.

Distribution and mobilization of Al, Fe and Si in three podzolic soil profiles in relation to the humus layer.

Distribution and mobilization of Al, Fe and Si in three podzolic soil profiles in relation to the humus layer

Effects of Acidification and its Mitigation with Lime and Wood Ash on Forest Soil Processes: A Review

Anthropogenic acid deposition causes forest soil acidification and perturbation of the soil forming processes. The impact of soil acidification on tree growth is discussed in view of the role of mycorrhizal fungi in weathering and nutrient uptake. A review has been carried out of experiments involving treatments of forest soil by lime and wood ash, where soil properties and soil solution composition have been investigated. Results from these experiments in Europe and North America are summarized. In general, the content of C in the mor layer decreased as a result of treatment due to higher microbial activity and soil respiration as well as increased leakage of DOC. In addition, the content of N in the mor layer, in general, decreased after treatment and there are occasional peaks of high NO3concentrations in soil solution. In nearly all reviewed investigations the pH of the deep mineral soil solution decreased and Al, SO4and NO3concentrations increased after treatment. These effects are probably due to the high ionic strength and increased microbial activity as a consequence of the treatments. In the soil, pH, CEC and base saturation increased in the upper horizons, but decreases in the upper mineral soil are also reported. In general, there was no increase in tree growth as a result of these treatments. The positive effects of the treatments on soil processes and tree growth are therefore questionable. In view of these conclusions, an investigation was carried out on the soil and soil solution chemistry and the role of mycorrhizal fungi in a spruce stand treated with two doses of lime and another treated with lime/ash in southern Sweden. The results of this investigation is reported in this volume.

Dissolution of microcline and labradorite in a forest O horizon extract : the effect of naturally occurring organic acids.

Dissolution of microcline and labradorite in a forest O horizon extract : the effect of naturally occurring organic acids.

Factors influencing aluminium concentrations in soil solution from podzols.

Abstract Mechanisms controlling the concentrations of free Al 3+ and total Al in the soil solution from three podzolic soils were evaluated. Saturation indices (SI) were calculated for five mineral phases, but no single phase determined the Al 3+ activity. E horizon samples were found to be undersaturated with respect to all phases. A more detailed equilibrium study indicated that formation of proto-imogolite sols was commonly possible in B horizon solutions with pH>5. The poor relationship between pH and pAl 3+ in the E and B1 horizons did not support control by exchange reactions with solid organic matter, although partial least square (PLS) regressions indicated importance of soil-exchangeable Al in the B and C horizons. It was concluded that in the E and most of the B1 horizon solutions, Al 3+ activity is controlled by equilibrium with soluble organic acids which was supported by the PLS regressions.

The acid-base properties of high and low molecular weight organic acids in soil solutions of podzolic soils.

The acid properties of the organic acids and identified low molecular weight (LMW) organic acids in soil solutions of podzolic soils were determined using proton affinity spectra, based on EMF titration data, and Gran titrations. The proton affinity spectra showed apparent pKa values of 2.6, 4.1, 5.4 and 6.7 for the dissolved organic material. The LMW organic acids had similar pKa values. The average specific buffer capacity as determined by Gran titrations was 8.8±0.5 μmol H+/mg DOC. The specific buffer capacity for the identified low molecular acids was 40±2 μmol H+/mg DOC.