387-6 Leaching Methods to Determine Soil Inorganic N Underestimate Net Mineralization.



Wednesday, October 19, 2011: 3:05 PM
Henry Gonzalez Convention Center, Room 213B, Concourse Level

Rafael Otto, ESALQ/USP, Piracicaba, Brazil, Kai Su, Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu, China, Richard L. Mulvaney, Dep. of Natural Resources and Environ. Sci., Univ. of Illinois, Urbana, IL and Paulo Cesar O. Trivelin, Lab. Stable Isotopes, CENA/USP, Piracicaba, Brazil
The profitability of N fertilizer usage can be increased by accounting for soil N mineralization that usually serves as the main source of N for crop uptake. One approach is to estimate net mineralization through aerobic incubation, utilizing a technique developed by Stanford and Smith in 1972 that involves sequential leaching of soil samples with 0.01 M CaCl2. This technique has been widely used throughout the world, despite the absence of evidence that the prescribed leaching approach ensures quantitative recovery of inorganic soil N. A study was therefore conducted to evaluate the accuracy of leaching for inorganic N analysis, relative to conventional extractions using 2 M KCl. Ten air-dried soils differing widely in physicochemical properties were used, five each from Illinois and Brazil, with and without addition of NH4+-N (100 mg kg-1) and NO3-N (100 mg kg-1). In the leaching method, 15 g of soil was mixed with 15 g of acid-washed silica sand and transferred onto Whatman no. 42 filter paper in a polypropylene Buchner funnel, and the mixture was then leached under vacuum with 100 mL of 0.01 M CaCl2. In the extraction method, 5 g of soil was shaken for 1 h with 50 mL of 2 M KCl in a 100-mL polyethylene bottle, and the soil suspension was filtered under vacuum through Whatman no. 42 filter paper in a Buchner funnel. Leachates and extracts were analyzed by accelerated diffusion methods to determine exchangeable NH4+-N and (NO3 + NO2)-N. Inorganic N concentrations were usually lower by leaching than by extraction and the difference was often significant, particularly for exchangeable NH4+-N. The disparities increased with cation-exchange capacity (CEC), indicating incomplete displacement of exchangeable cations by 0.01 M CaCl2. This flaw can lead to serious underestimation of net N mineralization, in cases where there is limited autotrophic conversion of NH4+-N to NO3-N.
See more from this Division: S08 Nutrient Management & Soil & Plant Analysis
See more from this Session: Strategies for Improved Nitrogen Use, Management and Fertilization