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DIVISION S-7—FOREST & RANGE SOILS

Total Soil Nitrogen in the Coarse Fraction and at Depth

Univ. of Washington, College of Forest Resources, Box 352100, Seattle, WA 98195

^* Corresponding author (zabow{at}u.washington.edu).

Historically, studies estimating total soil N are based almost exclusively on the 2-mm soil particles (fine soil fraction), and generally have not included that portion of soil >2-mm (coarse soil fraction) or soil >1-m depth. This study examined the underestimation of the total soil N pool by following traditional or standard soil sampling and analysis. Seventeen varied soil series plus one soil classified to the level of Great Group were sampled from Alaska, Oregon, Puerto Rico, and Washington. Total soil N in each profile was quantified for the soil fine and coarse fractions, as well as for soil depths >1 m. In soils with a coarse fraction, the average percentage of total soil N contained in the coarse soil fraction ranged from 0.3 to 37%, increasing with an increase in coarse soil mass (the highest percentage of N for an individual profile was 67% in a Cryorthent with 92% coarse fragments). For soils with depths >1 m, the percentage of soil N below 1 m relative to the whole profile ranged from 7 to 35%. An average of 0.03% N concentration in the coarse soil fraction was found for all major genetic mineral horizons. Results of this study indicated that up to one-half of the total soil N of a profile would have been missed by the combined exclusion of the coarse fraction and soil below 1-m depth from analysis. The coarse soil fraction may reflect storage and/or potential sources of N not considered in traditional soil N budgets.

Abbreviations: CSM, coarse soil mass • CSN_tot, coarse soil total nitrogen • D_b, bulk density • Fr, percentage of coarse or fine fraction by weight • FSM, fine soil mass • FSN_tot, fine soil total nitrogen • H, horizon thickness • N_con, total nitrogen concentration • N_tot, total nitrogen

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