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Inorganic Nitrogen in Soil Solution Collected with Tensionic Samplers

Joint FAO/IAEA Division, Soil Fertility, Irrigation and Crop Production Section, P.O. Box 100, A1400 Vienna, Austria

J. C. Fardeau

CEA, Cadarache Research Center, DEVM, 13108 St-Paul-lez-Durance, France

*Corresponding author (moutonne{at}ripo.iaea.or.at).

ABSTRACT

Traditionally, porous ceramic cups are used for monitoring ion concentration in soil solutions with time and depth. To calculate the actual N losses through leaching, the cups must be associated with tensiometers, which provide the hydraulic head gradient. This study was conducted to assess how tensionics can determine mineral-N concentration in soil solution, compared with traditional porous ceramic cups. In field conditions under maize (Zea mays L.), five sets of four tensionics buried at depths between 40 and 130 cm were used throughout the 1992 growing season to assess the concentrations of NO₃-N, NO₂-N, and NH₄-N in the soil solution. Samples were obtained every 10 d for 60 d using two successive methods. The first method was the "tensionic mode" functioning by ionic diffusion through the porous ceramic cup and requiring an 8- to 10-d time step. The second method was the more standard method using soil solution samplers kept 4 h under vacuum to extract the soil solution. For both methods, the same devices were used. Regression analysis on >100 samples showed that NO₃-N recovery was similar for both methods (slope = 0.91; r² = 0.93). With the tensionic method, NO₂-N and NH₄-N concentrations were, respectively, 5.4 and 7.9 times greater than those obtained by the standard method. These results are explained on the basis of the different ionic concentrations contained in the macro- and micropores of the soil. Samples obtained throughout the season (12 series) with devices working in "tensionic mode" were analyzed for inorganic-N content. In 1992, a rather rainy year, NO₃-N leaching occurred with a surprising downward flux of NO₂-N, which at certain times during the year might pose a threat to groundwater quality.

Received for publication October 3, 1995.

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