The use of nitrogen 15 in nitrogen immobilization studies in the soil 1 effect of the method of fertilizer placement and straw application on the biological immobilization of nitrogen 15 labeled ammonium sulfate

Msumali, G.P.; Racz, G.J.

East African Agricultural and Forestry Journal 43(3): 223-231


ISSN/ISBN: 2313-450X
Accession: 033969509

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N was either mixed or banded in the soil. Straw was applied at 1% soil (wt/wt basis). Other treatments included a nitrification inhibitor (N-serve) to show the effect of maintaining N in the NH4-form on immobilization. The soil was incubated at field capacity moisture for 12 wk and samples taken at 0, 2, 4, 8 and 12 wk of incubation for analysis of NO3--N, NH4+-N, NO2--N (the sum of which gave total mineral N), total (Kjeldahl) N and percent N15 excess. There was very little or no fixation of NH4+-N by expanding clay minerals. Straw applied at 1% (wt/wt soil) reduced percent recovery of mineral N and therefore increased immobilization. Less N was immobilized when the fertilizer was banded than when it was mixed with the soil; but straw application doubled immobilization. Using mineral N data (NH4+ + NO3- + NO2-) there was no apparent advantage of banding fertilizer with respect to N immobilization. This was inconsistent with N15 data, and the discrepancy could not be explained. Percent recovery of mineral N was constantly high throughout the incubation period. Biological interchange of N probably took place between the freshly added tracer N and mineralized N from the native soil organic matter. N15 data also showed that a nitrification inhibitor (N-serve) favored immobilization of N. Biological immobilization alone apparently could account for 40% of N fertilizer ineffectiveness when the fertilizer is mixed with the soil in the presence of an organic residue with a wide C:N ratio, but this could be reduced to 1/2 if the fertilizer is localized in bands.