Soil Nitrogen Storage and Fractions as Influenced by Tillage, Cropping System, and Nitrogen Source.

            Management practices are needed to increase soil N storage and reduce N fertilization rate and N leaching by replacing inorganic N fertilizer by organic manure, such as poultry litter. We evaluated the 10-yr effects of tillage [No-till (NT), mulch till (MT), and conventional till (CT)], cropping systems [rye/cotton-rye/cotton-corn (R/C-R/C-CN) and cotton-cotton-corn (C-C-CN)], and N sources and rates [0 and 100 kg N ha^-1 from NH₄NO₃ and 100 and 200 kg N ha^-1 from poultry litter] on soil N fractions and N balance in northern Alabama. Nitrogen fractions were soil total N (STN), particulate organic N (PON), microbial biomass N (MBN), potential N mineralization (PNM), NH₄-N, and NO₃-N. Crop residue N returned to the soil varied with treatments and years and total residue N from 1997 to 2005 was greater in R/C-R/C-CN than in C-C-CN and greater with NH₄NO₃ than with poultry litter. Concentrations of soil N fractions at 10- to 20-cm depth were not influenced by treatments. The STN and PON contents at 0- to 20-cm were greater in NT with poultry litter than in NT and CT with NH₄NO₃ and in MT with poultry litter. As a result, poultry litter stored N at 38 kg N ha^-1 yr^-1 compared with -4 kg N ha^-1 yr^-1 with NH₄NO₃. Similarly, poultry litter increased MBN, PNM, and NO₃-N contents compared with NH₄NO₃. Cropping increased STN, MBN, and PNM contents compared with fallow in NT. Estimated N loss after 10 yr was greater in NT with C-C-CN and 100 kg N ha^-1 from NH₄NO₃ than in other treatments, except in NT with R/C-R/C-CN and 100 kg N ha^-1 from NH₄NO₃ and poultry litter. Long-term poultry litter application increased soil N storage and N fractions and reduced N loss probably by N leaching compared with inorganic N fertilization.