J.M. Verstraten

Soil water content measurements at different scales: accuracy of time domain reflectometry and ground-penetrating radar

Abstract Accurate measurements of soil water content with an appropriate support are important in many research fields. Ground-penetrating radar (GPR) is an interesting measurement technique for mapping soil water content at an intermediate scale in between point and remote sensing measurements. To measure soil water content with GPR, we used the velocity of the ground wave, which is the signal traveling directly from source to receiving antenna through the upper centimeters of the soil. To evaluate GPR performance, we aggregated time domain reflectometry (TDR) and gravimetric soil water content measurements to the support of GPR measurements. The results showed that the calibration equations between GPR measurements and aggregated gravimetrical soil water content were similar to those obtained for TDR measurements, suggesting that available TDR calibrations (e.g. Topps equation) can be used for GPR. Furthermore, we found that the accuracy of GPR to measure soil water content is comparable with the accuracy of TDR, although it depended on the type of data acquisition used for the determination of the ground wave velocity.

Dissolved organic matter, aluminium and iron interactions: precipitation induced by metal/carbon ratio, pH and competition

To better understand the precipitation behaviour of dissolved organic matter induced by interactions with metals, a systematic titration experiment was conducted mimicking the soil solution conditions in an acidic, sandy soil. The variables of interest included the type of metal species (Al, Fe), the redox state [Fe(II), Fe(III)], the pH (3.5, 4.0, 4.5), the metal to organic carbon (M/C) ratio and the competition between Al and Fe. Precipitation of DOM-Al appeared to be strongly correlated with M/C ratio and the pH. For Fe(II) only little precipitation occurred, while the strongest flocculation degree was found after addition of Fe(III). In contrast to Al, hardly any correlation between DOM-Fe precipitation and pH was observed. Both reduction and oxidation of Fe was found and exhibited a strong effect on the precipitated amounts of DOM and Fe. In competition, Al determined the precipitation behaviour at lower M/C ratios (<0.10), while at higher M/C ratios Fe determined the flocculation. Below a M/C ratio of 0.06 Al was the dominant metal in the precipitates, especially at lower pH levels, while the opposite trend was found at M/C ratios above 0.06. Overall, Fe(III) gave the strongest flocculation, although Al influenced the impact of Fe(III) interactions with DOM in relation to pH and M/C ratio.

Occurrence and distribution of ester-bound lipids in Dutch coastal dune soils along a pH gradient

The base-hydrolysable fraction of roots and soil organic matter (SOM) in mineral subsoils from oak forests (coastal dunes, The Netherlands) with a soil pH varying from pH (CaCl2) 6.9 to 3.5 were analysed both quantitatively and qualitatively. Comparison of thermally assisted hydrolysis and methylation (THM) using tetramethylammonium hydroxide (TMAH) with base hydrolysis data confirmed that base hydrolysis is an efficient way of analysing ester-linked compounds in soils. The compounds released from SOM upon base hydrolysis comprised largely suberin-derived lipids, which were clearly related to the oak vegetation and, in particular, to oak roots. Compared with fresh (fine) roots, which are largely responsible for the input of SOM in the subsoils, unsaturated and epoxy compounds decreased in soil, probably due to oxidation. The amounts of the saponifiable fraction increased upon acidification, whilst hardly any changes were observed in their composition. This accumulation, without significant compositional alteration of ester-bound moieties in acidic soils, indicates that these chemically labile compounds can be preserved in soils.

Frequency domain analysis of time domain reflectometry waveforms: 2. A four‐component complex dielectric mixing model for soils

Although time domain reflectometry (TDR) is becoming accepted as an important tool for the measurement of soil water content and bulk soil electrical conductivity, a major part of the method is based on empirical relationships. An improved understanding of dielectric measurements on soils may give more insight into soil properties other than soil water content and bulk soil electrical conductivity. Frequency domain analysis of TDR waveforms enables the measurement of the frequency dependent complex dielectric permittivity of soils. The frequency dependent complex dielectric permittivity of soils can be described with a four-component complex dielectric mixing model based on the volumetric mixing of the refractive indices of the soil components. The four soil components in the model are air, solids, bound water, and free water. Results indicate that the apparent dielectric permittivity obtained from the travel time of the TDR pulse in the soil is the dielectric permittivity at the highest measurement frequency of the cable tester, probe, and soil system. The model based on the volumetric mixing of real permittivities underestimates the measurements in situations with high values of the imaginary part of the dielectric permittivity. Because the model based on the mixing of the complex dielectric permittivities can describe the data, we conclude that the apparent dielectric permittivity is influenced by the imaginary parts in the dielectric, permittivities of the soil components. Combination of the four-component complex dielectric mixing model with the complex dielectric permittivity obtained from the frequency domain analysis of TDR waveforms gives a tool for modeling the bulk soil electrical conductivity by separating the conductivity of the soil water into a bound water conductivity and a free water conductivity.

Mobility of Fe(II), Fe(III) and Al in acidic forest soils mediated by dissolved organic matter: influence of solution pH and metal/organic carbon ratios

Abstract The mobility of Al and Fe in acidic sandy forest soils is greatly influenced by interactions with dissolved organic matter (DOM). We determined the distribution of Al, Fe(II) and Fe(III) over dissolved ‘free’ metal, dissolved metal–DOM complexes and metal–DOM precipitates as influenced by solution pH, redox potential and metal/organic carbon (M/C) ratios. For this, we used water extracts of the Oh horizon of a Fimic Anthrosol brought at pH=3.5, 4.0 and 4.5, and added increasing amounts of Fe(II), Fe(III) and Al. For all three metals, soluble metal–DOM complexes were dominant at low M/C ratios (

Humus form development and hillslope runoff, and the effects of fire and management, under Mediterranean forest in NE-Spain

Study of a series of plots on acid to intermediate rocks under well preserved mediterranean type forests in NE Spain showed that soils have well developed mor or moder type humus forms. Mor type humus forms were observed in soils with a shallow lithic contact or an abrupt textural change, and consist of an ectorganic layer (L, Fq, H) abruptly overlying a generally water repellent Ae or E horizon. Moder type humus forms, with a gradual transition between the ectorganic layer (L, Fa and the Ah horizon, were found in soils with more favourable rooting and soil moisture conditions, indicating that these conditions have a strong control over humus form development. Quantities of organic matter (ectorganic layer + Ah/Ae) ranged from about 5 kg/m2 in mor to about 10 kg/m2 in moder humus forms, the difference being due to the presence of a well developed Ah horizon in the latter. Rainfall simulator experiments showed that slopes with mor type humus forms are likely to produce hillslope runoff during summer rain storms in spite of the relatively high storage capacity of the ectorganic layer. This is due to the hydrophobicity of the mineral top soil (if dry), which hampers infiltration. Furthermore, during the wet season the soils commonly have a perched water table, inducing saturated overlandflow. Slopes with moder type humus forms, on the contrary, are very unlikely to produce any hillslope runoff, because of the high storage and infiltration capacity of the soils. On slopes with mor type humus forms, traditional management (cutting of the undertsory) leads to erosion of the ectorganic layer and thus to a reduction of its storage capacity. Forest fires have similar effects, since they lead to the (partial) destruction of the ectorganic layer and destroy the existing vegetation. As evidenced by the rainfall simulation experiments, in both cases hillslope runoff will strongly increase, inducing erosion of the mineral soil, in particular after fire. On slopes with moder type profiles the effects of fire and traditional management will be far more limited, because of the high storage and infiltration capacity of these soils, and hillslope runoff is not likely to increase, if occurring at all.

Nitrate production in nitrogen-saturated acid forest soils: Vertical distribution and characteristics

Abstract In order to study the vertical distribution, nature and pH-dependence of nitrate production in nitrogen-saturated acid forest soils, an incubation experiment was carried out using soil cores from five Dutch forests. Soil cores were left intact during incubation and divided into L, F, H horizons and top 5 cm of the mineral soil before analysis. One of the five forest soils revealed no significant NO 3 production during incubation, while in the other four sites different patterns with respect to vertical distribution of NO 3 production were found. The two coniferous sites snowed a dominance of NO 3 production in the ectorganic horizon (L + F + H), while in the two deciduous forests the production of NO 3 in the ectorganic horizon and in the top 5 cm of the mineral soil contributed about equally to total nitrification. At one of the deciduous sites, nitrification in the 5 cm top layer of the mineral soil was higher than the separate NO 3 productions in the L, F and H layer. Experiments with acetylene as a selective inhibitor indicated that the measured NO 3 production in the F layer of all four nitrifying forest sites was mainly carried out by chemolithotrophic bacteria. In the dominant nitrifying compartments, nitrification was mainly found to be acid-tolerant, which underlines the importance of this type of nitrification in N-saturated forest soils.

Organic matter formation in sandy subsurface horizons of Dutch coastal dunes in relation to soil acidification

Abstract Subsurface horizons contain considerable amounts of soil organic matter (SOM), which has generally a relatively recalcitrant nature and may be an important key in the examination of the role of soils in the sequestration of carbon. Nonetheless, this part of SOM is hardly studied. This paper focuses on the effects of soil acidification on the formation of SOM in sandy subsurface horizons under Corsican pine ( Pinus nigra var. maritime ) and common oak ( Quercus robur L.) forests in coastal dunes (The Netherlands) as characterized by pyrolysis and thermally assisted hydrolysis and methylation. In the pine forests of 50–70 y old SOM appeared to be only slightly affected by soil pH, whereas SOM from oak forests (100–200 years) showed pronounced changes upon soil acidification. With decreasing soil pH in the oak forests, lignin was more degraded (decrease in syringyl/guaiacyl ratio, reduction of the relative concentration) and the contribution of suberin-derived aliphatic moieties increased. The latter compounds may therefore play an important role in the formation of SOM in the studied subsoils.

Influence of pH and metal/carbon ratios on soluble organic complexation of Fe(II), Fe(III) and Al(III) in soil solutions determined by diffusive gradients in thin films

Abstract Soluble complexation of Al(III), Fe(II) and Fe(III) with dissolved organic matter (DOM) greatly influences the bioavailability and mobility of the metals as well as the DOM itself in acidic forest soils. A scarcity of analytical tools to distinguish between ‘free’ Al(III), Fe(II) and Fe(III) and soluble organic complexes in acidic soil solutions has limited research in this area. We further tested diffusive gradients in thin films (DGT) for this purpose and used it to assess the influence of pH, redox potential and metal/organic carbon (M/C) ratios on soluble organic complexation of Al(III), Fe(II) and Fe(III). We used water extracts of an organic soil (H) horizon from a Fimic Anthrosol at pH 3.5, 4.0 and 4.5 to which we sequentially added the three metal species. DGT worked well for all but Fe(II) at pH 3.5 where a correction for proton competition was needed. For all three metal species, the ‘free’ fraction increased with increasing M/C ratios. The order of soluble complexation strength was Fe(III)>Al(III)>Fe(II). At low M/C ratios the ‘free’ fraction was highest at the lowest pH value due to more deprotonated functional groups, at high M/C ratios increased (organic) precipitation at higher pH reversed this effect. Both reduction of Fe(III) and oxidation of Fe(II) were found. Selective precipitation of Fe(III) complexes led to predominantly soluble Fe(II)–DOM complexes at higher M/C ratios. Therefore, in studies of the mobility and bioavailability of Fe and Al in acidic forest soils, both M/C ratios and Fe(II)/Fe(III) speciation in solution must be determined.

Natural 15N abundance in two nitrogen saturated forest ecosystems.

Abstract Natural 15N abundance values were measured in needles, twigs, wood, soil, bulk precipitation, throughfall and soil water in a Douglas fir (Pseudotsuga menziesii (Mirb.) and a Scots pine (Pinus sylvestris L.) stand receiving high loads of nitrogen in throughfall (>50 kg N ha−1 year−1). In the Douglas fir stand δ15N values of the vegetation ranged between −5.7 and −4.2‰ with little variation between different compartments. The vegetation of the Scots pine stand was less depleted in 15N and varied from −3.3 to −1.2‰δ15N. At both sites δ15N values increased with soil depth, from −5.7‰ and −1.2‰ in the organic layer to +4.1‰ and +4.7‰ at 70 cm soil depth in the Douglas fir and Scots pine stand, respectively. The δ15N values of inorganic nitrogen in bulk precipitation showed a seasonal variation with a mean in NH4+-N of −0.6‰ at the Douglas fir stand and +10.8‰ at the Scots pine stand. In soil water below the organic layer NH4+-N was enriched and NO3−-N depleted in 15N, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean δ15N values of NH4+ and NO3− were very similar in the drainage water at 90 cm soil depth at both sites (−7.1 to −3.8‰). A dynamic N cycling model was used to test the sensitivity of the natural abundance values for the amount of N deposition, the 15N ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Simulated δ15N values for the N saturated ecosystems appeared particularly sensitive to the 15N ratio of atmospheric N inputs and discrimination factors during nitrification and mineralization. The N-saturated coniferous forest ecosystems studied were not characterized by elevated natural 15N abundance values. The results indicated that the natural 15N abundance values can only be used as indicators for the stage of nitrogen saturation of an ecosystem if the δ15N values of the deposited N and isotope fractionation factors are taken into consideration. Combining dynamic isotope models and natural 15N abundance values seems a promising technique for interpreting natural 15N abundance values found in these forest ecosystems.