Transformations of non-symbiotically fixed 15N2 were studied under laboratory conditions in two soils from Pakistan (Hafizabad silt loam, Khurrarianwala silt loam) and one from Illinois, U.S.A. (Drummer silty clay loam). Up to 50μg N was fixed g-1 soil after a 5-day exposure in an 15N2-enriched atmosphere under waterlogged conditions with glucose as the C substrate. Efficiency of N2 fixation was of the order of 11.6-12.7 mg N2 g-1 substrate C used and was greatest for the Drummer soil, which had the highest content of organic matter. Percentage recovery of organically-fixed 15N in available mineral forms (exchangeable NH4+ and NO3-) by incubation for up to 112 days following exposure to 15N2 was rather low (< 10% in the Drummer and Hafizabad soils and 28% in the Khurrarianwala soil). From 12 to 46% of the 15N was solubilized by alkaline sodium pyrophosphate, the lower value (12%) being obtained with the Drummer soil. In comparison to the native soil N, a much higher proportion of the fixed N (88-90% vs 62-78% for the native N) was released to soluble forms (i.e. amino acids, amino sugars, ammonium and hydrolyzable unknown forms) by acid hydrolysis, and a higher proportion of the hydrolyzable 15N was accounted for as amino acid-N (40-55% vs 30-40% for the soil N). Overall, the findings suggest that the newly fixed N in soil is incorporated into stable humus fractions rather rapidly and will be available to crop plants over an extended period.
ASJC Scopus subject areas
- Soil Science