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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Assessing Indices for Predicting Potential Nitrogen Mineralization in Soils under Different Management Systems

^a USDA-ARS, J. Phil Campbell, Sr., Natural Resource Conservation Center, Watkinsville, GA 30677
^b Empresa Brasileira de Pesquisa Agropecuária (Embrapa Trigo), Passo Fundo, Rio Grande do Sul, Brazil 99001-970
^c Friedman School of Nutrition Sci. and Policy, Tufts Univ., Boston, MA 02111
^d Univ. of Georgia, Dep. of Crop and Soil Sciences, Athens, GA 30602
^e USDA-ARS, Coastal Plains Soil, Water, and Plant Res.Ctr., Florence, SC 29501
^f USDA-ARS, National Soil Dynamics Lab., Auburn, AL 36832
^g USDA-ARS, Cropping Systems and Water Quality Res. Unit, Columbia, MO 65211
^h USDA-ARS, National Sedimentation Lab., Oxford, MS 38655
ⁱ USDA-ARS, Grassland Soil and Water Res. Lab., Temple, TX 76502
^j USDA-ARS, Southern Plains Conserv.Production Rese. Lab., Bushland, TX 79012
^k USDA-ARS, Southeast Watershed Res. Lab., Tifton, GA 31793
^l Univ. of Tennessee, Biosystems Engineering and Soil Science Dep., Jackson, TN 38301

^* Corresponding author (harry.schomberg{at}ars.usda.gov).

A reliable laboratory index of N availability would be useful for making N recommendations, but no single approach has received broad acceptance across a wide range of soils. We compared several indices over a range of soil conditions to test the possibility of combining indices for predicting potentially mineralizable N (N₀). Soils (0–5 and 5–15 cm) from nine tillage studies across the southern USA were used in the evaluations. Long-term incubation data were fit to a first-order exponential equation to determine N₀, k (mineralization rate), and N₀* (N₀ estimated with a fixed k equal to 0.054 wk^–1). Out of 13 indices, five [total C (TC), total N (TN), N mineralized by hot KCl (Hot_N), anaerobic N (Ana_N), and N mineralized in 24 d (Nmin_24)] were strongly correlated to N₀ (r > 0.85) and had linear regressions with r² > 0.60. None of the indices were good predictors of k. Correlations between indices and N₀* improved compared with N₀, ranging from r = 0.90 to 0.95. Total N and flush of CO₂ determined after 3 d (Fl_CO2) produced the best multiple regression for predicting N₀ (R² = 0.85) while the best combination for predicting N₀* (R² = 0.94) included TN, Fl_CO2_, Cold_N, and NaOH_N. Combining indices appears promising for predicting potentially mineralizable N, and because TN and Fl_CO2 are rapid and simple, this approach could be easily adopted by soil testing laboratories.

Abbreviations: Ana_N, anaerobic N mineralization • TC, total carbon • Ca_hypcl, calcium hypochlorite • Cold_N, KCl extractable NO₃–N • CT, conventional tillage • Fl_CO2, flush of CO₂ during 3 d • Hot_N, hot KCl extractable NH₄–N • Hyd_N, hydrolyzable N • k, mineralization rate constant • TN, total nitrogen • NaOH_N, sodium hydroxide distillable N • N₀, potentially mineralizable N • N₀*, value of N₀ determined using a fixed value for k • Nmin_24, N mineralization during 24 d • NP, not plowed (prairie soil) • NT, no-Tillage • NT+SS, no-tillage with non-inversion subsurface deep tillage • PB_N, phosphate-borate distillable N • POMC, particulate organic matter C • POMN, particulate organic matter N • SM, stubble mulch tillage (sweeps to undercut weeds) • ST, strip tillage (in-row subsoil for disruption of subsurface pan and coulters for preparation of narrow strip of tilled soil)