Leaching Methods to Determine Soil Inorganic N Underestimate Net Mineralization.

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.