Distribution of nitrogen in soils of the southern Mississippi river alluvial plain

Abstract

Nitrogen (N) dynamics in the agriculturally important alluvial soils of the southern Mississippi Delta are not well understood, and little information is available regarding the amounts of various forms of N present in these soils. Profiles of nine alluvial soils were selected to represent the principal agricultural acreage in the southern Mississippi Delta. Soils were sampled by horizon to a depth of 150 cm and the distribution of various N fractions were characterized. Forty‐one additional chemical, physical and mineralogical properties were measured, and regression techniques were used to determine if these soil properties were related to N distribution in the highly heterogeneous soils typical of this region. These profiles contained 11.6 to 26.5 Mt N/ha (average 18.8 Mt N/ha). The surface 15 cm contained an average of 4.8 Mt N/ha and accounted for about 26% of the profile N. Most of N in the surface 15 cm was recovered as organic N (78.4–87.4%), and the balance recovered primarily as nonexchangeable ("clay‐fixed") NH₄+. In subsurface horizons, nonexchangeable NH₄ ⁺ represented a substantially larger fraction of total N (average 35.6 %). The amounts of exchangeable NH₄ ⁺ and NO₃‐ were very low in most samples and accounted for only 0.2–0.7% of surface N and 0.3–2.5% (average 0.7%) of the total N accumulated within horizons. The proportion of total N recovered as organic N was most closely related to organic carbon (C) content and the amounts of 2: 1 type of clay minerals present in the horizon. Even though subsurface horizons contained an appreciable portion of their N as inorganic nonexchangeable NH₄ ⁺, organic C content was the best single indicator of total N content (r²= 0.931) within the 52 horizons studied.