Chemical extraction of newly immobilized ¹⁵N and native soil N as influenced by substrate addition rate and soil treatments

A Pakistani silt loam soil was maintained at 30°C with increasing amounts of ¹⁵N-labeled (NH₄)₂SO₄ and glucose (C-to-N ratio of 30 for all additions) and the rate of immobilization and onset of remineralization was followed by exposure at 30°C for up to 460 h. At a stage when all of the applied ¹⁵N was in organic forms (as microbial biomass and products), soil samples were either refluxed with 10 mM CaCl₂, extracted with 10mM NaHCO₃, extracted with 500 mM K₂SO₄ or extracted with 500 mM K₂SO₄ after 24 h chloroform fumigation, freeze-drying and oven-drying. The extracts were analyzed for total N and ¹⁵N. The rate of immobilization and release of applied ¹⁵N was inversely related to the amount of N (and glucose) applied. The extracted ¹⁵N ranged between 14 and 58% of that applied and showed an increase with increase in the amount of. applied ¹⁵N. Similarly, the applied N (and glucose) enhanced the extractability of the native soil N, and the effect increased with an increase in the amount applied, indicating a positive "priming effect". Of the extradants used, 10 mM NaHCO₃ was the most selective for the applied ¹⁵N; maximum native N was extracted by reflux with 10 mM CaCl₂. Extractabilty ratios (% of the applied ¹⁵N/% of the native soil N) ranged from 2 to 9 depending upon extradant and soil treatment (amount of N and glucose applied). Both freeze-drying and oven-drying led to a decrease in the amount of N extracted; a larger amount of the applied ¹⁵N was rendered extractable due to freeze-drying. Chloroform fumigation caused a 2-fold increase in N extracted from both applied and native sources. However, extractability ratios were lower for fumigated than for unfumigated soil, indicating that chloroform fumigation enhanced the extractabil- ity of non-biomass N.