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In Situ Transformations of Nitrogen-15 Labeled Materials in Sonoran Desert Soils¹

ABSTRACT

Influences of ammonium, nitrate, organic N sources and organic C amendments on microbial respiration, N transformations and gaseous losses were measured under natural environmental conditions in two field experiments. Microbial respiration was increased by addition of wheat straw and barley grain. Addition of N as (¹⁵NH₄)₂SO₄ and K¹⁵NO₃ alone did not increase CO₂ evolution significantly.

Significant immobilization of added inorganic-¹⁵N occurred with few exceptions in all treatments except nitrate alone. Organic-¹⁵N in soils amended with wheat straw increased with increasing N rates with both inorganic N sources. Inorganic-¹⁵N was practically void in both soils at the end of 12 months. Mineralization of ¹⁵N-labeled barley grain occurred but at a very low rate and no significant accumulations of inorganic-¹⁵N were observed.

Gaseous losses of N at the end of 12 months varied from 73 to 94% of the inorganic-N added alone, with the greatest loss occurring from K¹⁵NO₃. The incorporation of wheat straw stimulated immobilization of added N and reduced gaseous losses to 30 to 86% of the total N applied and the smallest loss occurred with (¹⁵NH₄)₂SO₄. The average loss of mineralized organic-¹⁵N derived from barley grain for the two soils was 19% of the total N added. Most of the gaseous losses occurred within the first 3 months, the period of the measurable rainfall.

Analyses of subdivided soil sections (0–5 and 5–10 cm) showed that 95 to 100% of ¹⁵N remaining in the soil was in the upper 5-cm layer indicating that leaching was not appreciable in any treatment.

¹ Contribution from Department of Soils, Water and Engineering, Arizona Agr. Exp. Sta., Tucson 85721. Published with approval of the Director as journal article No. 2702. This work was supported in part by a grant from US IBP Desert Bionie Program of the National Science Foundation.

² Associate Professor and Professor, Department of Soils, Water and Engineering, the Univ. of Arizona, respectively. Address of senior author is now Dept. of Agron., Oklahoma State Univ., Stillwater, Oklahoma 74074.

Received for publication December 9, 1977. Accepted for publication August 24, 1978.