Increased Sequestration of Organic Carbon in Soil by Hydrophobic Protection

 The impact of soil organic carbon dynamics on the global carbon cycle is still largely uncertain despite studies of agricultural activities and control emissions of greenhouse gases to the earth's atmosphere. Improved knowledge of organic matter dynamics should lead to reduction in CO₂ emissions. We used stable carbon isotope analysis to detect small changes in organic carbon storage and turnover upon soil treatments with a ¹³C-labeled aliphatic alcohol previously partitioned into soluble humic substances of varying hydrophobicity. We found that labeled organic carbon is increasingly protected from mineralization with increased hydrophobic character of humic matter. The stabilization of organic carbon by hydrophobic protection significantly reduced decomposition during incubation time in soil. Hydrophobic protection can become an useful tool to limit decomposition of fresh organic matter in soil and thus reduce CO₂ emission from agricultural soils on a global scale. Received: 1 March 1999 / Accepted in revised form: 16 June 1999     

Investigations of nitrogen fluxes and pools on a limestone site in the Alps

In the North Tyrolean Limestone Alps a site was investigated over a four-year period (1998–2001) in order to assess the nitrogen saturation status, the nitrogen budget (quantification of the net uptake of nitrogen by the canopy and of the nitrogen mineralization, nitrogen uptake from roots and N₂O emission rates, proof of the origin of nitrate in the soil water with stable isotope analyses), and the effects of the actual nitrogen input on ground water quality. The main goals were to quantify the nitrogen input rate, the nitrogen pools in above-ground and below-ground compartments, nitrogen turnover processes in the soil as well as the output into the groundwater and into the atmosphere. The findings are based on continuous and discontinuous field measurements as well as on model results.While nitrogen input exceeded the Critical Loads of the WHO (1995), nitrogen deficiency and nutrient imbalances were verified by needle analyses. The atmospheric input of inorganic nitrogen was higher than the nitrogen output in 50 cm soil depth. A tracer experiment with¹⁵N helped to prove that not more than half of the applied nitrate could be discharged. This allows the conclusion that nitrogen is stored in the system and that the site cannot yet be said to be saturated with nitrogen. The same result was also obtained by modelling. In addition, it was proved that the nitrogen discharge did not stem from deposition but from processes within the system.     

Nitrate dynamics in an alpine forest site (Mühleggerköpfl) O and N stable isotope analysis in natural water samples

Stable isotope analysis of¹⁵N/¹⁴N and¹⁸O/¹⁶O - nitrate was used to investigate the nitrate dynamics and potential groundwater pollution in an Alpine forest stand in Tyrol/Austria. The dynamics of δ¹⁵−N_nitrate values were followed in a forest ecosystem. The stable isotopic values of the throughfall are comparable with other studies. The completely decoupled dynamics of the δ¹⁵−N_nitrate of the precipitation and the surface water was observed. High variations in δ¹⁵-N - nitrate values in rainfall indicate that nitrate of different sources is deposited at that site. A significant correlation between the δ¹⁵N_nitrate values of the surface water and soil water was obtained, while no significant correlation between the δ¹⁵N_nitrate values of any precipitation sample with the surface water could be found. This suggests that the main source of nitrate in soil water originates from microbiological activity such as nitrification reactions and less from nitrate input by deposition. The results of δ¹⁸O_nitrate measurements strongly supported the microbiological origin of nitrate in the surface and soil water. In an additional lysimeter experiment,¹⁵N - labelled nitrate was applied to study nitrate transport in soil. After 130 days and the collection of 300 L leachate, a total of 52% of the applied nitrate was detected in seepage water.

     

Effects of elevated ozone concentration on the degradation of dichlorprop in soil.

An aerobic degradation study was conducted to estimate possible effects of elevated ozone concentration in air on the behaviour of dichlorprop. An average ozone concentration of 80 nL L-1 was chosen, which often occurs close to congested areas during late spring and summer. A control soil and an ozone exposed soil were kept under same conditions such as temperature, air flow and soil humidity. The use of 14C-labelled dichlorprop allowed to examine the fate of dichlorprop and follow the degradation products in soil. Exhaustive extraction of both soils yielded several fractions containing dichlorprop residues. Half lives of dichlorprop of both treatments were 5 days. After 32 days most of the residues in soil remained in the non extractable fraction. The elevated ozone concentration showed no significant effects on the degradation behaviour of dichlorprop and its metabolites but significant differences were found for the behaviour of the nonextractable residues and of the release of carbon dioxide, which were higher for control soil in comparison to the ozone variant. These findings suggest that even moderately elevated ozone concentration in air can effect mineralisation and fixation processes of dichlorprop.     

Influence of dissolved humic substances on the leaching of MCPA in a soil column experiment

The influence of dissolved humic substances on the transport of (4-chloro-2-methylphenoxy) acetic acid (MCPA) in a sandy soil with a low organic carbon content was studied in a column experiment. Soil columns were eluted with aqueous solutions containing different fractions of humic substances. More than 70% of the applied compound was found in the leachate in all sandy soil experiments, but distinct differences were obtained depending on the composition of the eluent. The addition of both humic and fulvic acids to the eluent affected the leaching behaviour of MCPA. While the presence of humic acids increased and accelerated the movement of MCPA in the investigated sandy soil, fulvic acids caused the opposite effect: increased retention was observed relative to the control. We concluded that a possible carrier transport or retention strongly depends on the composition of the dissolved organic matter. Thus, changes in the composition of dissolved organic matter may affect MCPA movement into deeper soil layers.     

Sorption of heavy metals on organic and inorganic soil constituents

Sorption of heavy metals to organic matter and mineral soil constituents can hardly be separated experimentally. Here we studied the retention capacity of organic matter and minerals from soils in a long-term field experiment in which the organic carbon content had been altered, but the mineral phase had remained constant over time. The sorption of Cu, Cd and Zn showed a non-additive contribution of soil organic matter and minerals to the sorption capacity of soil. Sorption on organic matter exceeded mineral sorption from 6 to 13 times. This is the first time that sorption to soil organic matter is quantified in bulk soils.     

An alternative method to measure carbonate in soils by FT-IR spectroscopy

Measurements of calcium carbonate contents in soils were performed with FT-IR (Fourier transform infrared) spectroscopy and with the gas volumetric Scheibler method. To the authors’ knowledge it is the first time that carbonate was quantified in soil samples by FT-IR spectroscopy. The carbonate contents of the test soils ranged from 11.3 to 13.1%. Both methods gave similar results, however, results obtained from FT-IR spectra depend on the spectral band used for the carbonate determination. In our investigation we used the bands at 875 and 2506 cm⁻¹. In case of the band at 2506 cm⁻¹ the difference between FT-IR and Scheibler method was a factor of 1.56, in case of 875 cm⁻¹ the respective factor was 1.16. It can be concluded that FT-IR with both bands has a potential to be used in practice as substitution of the Scheibler method. The advantages of the FT-IR method are better reproducibility and the simultaneous characterization of soil organic matter in bulk samples. The disadvantage is its higher cost.     

Sorption of naphthalene derivatives to soils from a long-term field experiment

The influence of long-term farming practices on the soil's behaviour to adsorb hydrophobic organic compounds (HOCs) over long times was investigated. Adsorption of five naphthalene derivatives (naphthalene, 1-naphthol, 1-naphthylamine, 1-hydroxy-2-naphthoic acid, 1,4-naphthoquinone) was examined on soils with varying amounts and origins of soil organic matter obtained after amendment with different organic materials over more than 40 years. Soil organic matter, pore sizes and aggregate stability were significantly altered influencing the adsorption behaviour of the soils. Samples of soil amended with peat having an organic carbon content of 3.4% sorbed naphthalene derivatives stronger than the soil treated with sewage sludge (2.6% C(org)). All other treatments, calcium nitrate, plots without nitrogen fertilizers, grassland, animal manure, green manure and the fallowed soil sorbed less and no significant difference was found between them although the organic carbon content ranged from 1.0% to 2.6%. Thus, a decrease of the carbon content of a soil does not necessarily imply a reduction of sorption capacities for hydrophobic compounds such as naphthalene derivatives. Furthermore, the importance of protonation of HOCs for the adsorption on soil surfaces was shown. Different polarities of electronic structures of HOCs distinctly influence their adsorption behaviour.