Recovery of fertilizer nitrogen from continuous corn soils under contrasting tillage management

Tillage systems influence soil properties and may influence the availability of applied and mineralized soil N. This laboratory study (20°C) compared N cycling in two soils, a Wooster (fine, loamy Typic Fragiudalf) and a Hoytville (fine, illitic Mollic Epiaqualf) under continuous corn (Zea mays) production since at least 1963 with no-tillage (NT), minimum (CT) and plow tillage (PT) management. Fertilizer was added at the rate of 100 mg ¹⁵N kg^–1–1 soil as 99.9% ¹⁵N as NH₄Cl or Ca(NO₃)₂ and the soils were incubated in leaching columns for 1 week at 34 kPa before being leached periodically with 0.05 M CaCl₂ for 26 weeks. As expected, the majority of the ¹⁵NO₃^– additions were removed from both soils with the first leaching. The majority of applied ¹⁵NH₄⁺ additions were recovered as ¹⁵NO₃^– by week 5, with the NT soils demonstrating faster nitrification rates compared with soils under other tillage practices. For the remaining 22 weeks, only low levels of ¹⁵NO₃^– were leached from the soils regardless of tillage management. In the coarser textured Wooster soils (150 g clay kg^–1), mineralization of native soil N in the fertilized soils was related to the total N content (r² 0.99) and amino acid N (r² 0.99), but N mineralization in the finer textured Hoytville (400 g clay kg^–1) was constant across tillage treatments and not significantly related to soil total N or amino acid N content. The release of native soil N was enhanced by NH₄⁺ or NO₃^– addition compared to the values released by the unfertilized control and exceeded possible pool substitution. The results question the use of incubation N mineralization tests conducted with unfertilized soils as a means for predicting soil N availability for crop N needs.