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Summary Band 20

Schlüter, W. (1997): Cations and anions contents in the soil solution and in the high level ground water under organically and conventionally cultivated farm land of the river flood plain Sieg as well as the simulation of the nitrogen-dynamic and nitrate-leaching.

 

Over a period of two years the dynamics of cations and anions were investigated in weakly to moderately acidic Fluvi-Eutric Cambisols of a river flood plain of the Rhine tributary Sieg (Nordrhein-Westfalen, Germany). The soils were cultivated according to organic and conventional farming systems. Under a surface area of 9 m2 in different depth (every 30 cm) of different thick soils (up to 180 cm) soil solutions were sampled by ceramic suction cups and analyzed for the contents of selected elements.

The investigation of the small scale variability in different depths of two soils showed, that the installation of 6 suction cups per depth step was sufficient to get representative values of the Na+-, Mg2+-, Ca2+-, NO3--, SO42-- and Cl--concentrations, whereas the values of the K+- and DOC-concentrations are partly uncertain.

For the most of the analyzed soil solution contents the investigations showed typical time series which reflect the seasonal and agricultural conditions very well. The leaching of the cations Na+, K+, Mg2+ and Ca2+ as well as the leaching of the anions SO42- and Cl- were quantified with the help of their concentrations in the soil solutions underneath the rooting zone. The interpretation of the NO3--dynamic was possible by model calculations using the DELPHI model, which first had been calibrated by tracer experiments with Cl-, Br-, NO3- and SO42-.

During the tracer experiments in addition to the sampling of soil solution by suction cups at definite time intervalls soil material was taken from different depth to produce soil saturation extracts.

The concentrations of the non-adsorbed anions Cl-, Br- and NO3- were very similar in the suction cup solutions and in the saturation extracts, whereas the concentrations of the weakly adsorbed SO42- partially showed considerable differences. Obviously both methods extract in acid soils different fractions of the dissolved sulfate.

By comparing marked sulfate peaks with those of chloride, bromide or nitrate, which occur at the same time in different soil depth, a Kd value for sulfate of 0,16 l/kg can be calculated for the Ap horizon and of 0,02 l/kg for the subsurface horizons of one Fluvi-Eutric Cambisol.

Values of the dispersivity of the different horizons were estimated by varying the values of the dispersivity reported in the literature until the best fit of the simulated anion depth distribution to the measured values was found. The estimated dispersivities varied between 1 cm in the Ap horizon and 10 cm in the subsurface horizons.

In the course of simulating the NO3--dynamic by means of the calibrated model N-mineralization and N-leaching were quantified. For different crop residues reaction coefficients for the mineralization of the easily decomposed organic material were calculated. They were extremly high for bean residues (19.0 x 1012 x exp (-9800/T)) and mustard-vetch residues (12.0 x 1012 x exp (-9800/T)). The lowest reaction coefficient of 3.0 x 1012 x exp (-9800/T) was calculated for the decomposition process during winter time.

For the two farming systems a detailed N-balance was calculated. It is shown that under conventional farming over a time period of two years 199 kg N/ha were leached. Within the same period 53 kg N/ha were leached under organic farming with the cultivation of intercrop, which reduced the leaching of nitrate. The partly considerable losses of NO3- in the investigated soils results in increasing NO3--concentrations in the high level ground water under conventional and organic farming. Temporary the treshold value for NO3--concentration of the drinking-water was exceeded. 

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