The fate of 15N-labelled fertilizer N was studied in a field microplot experiment (MPE) lasting from 1983 to 1989 (one 7-course grain-grass rotation). The Ap horizon of the dern-podzolic loamy sand soil for the MTE was removed from the Ap horizon of the plots of the long-term experiment (LTE) started in 1948.
The LTE had received the following treatments: (1) no fertilizer; (2) mineral fertilizers only (240, 105, and 199 kg/ha of N, P, and K over each 7-year crop rotation period); (3) the same amount of mineral fertilizers plus 40 t/ha of half-rotted manure per the crop rotation period. These long-term treatments served as a prior fertilization backgrounds for the MPE and are designated by the symbols F0, NPK and NPK+M, respectively.
Long-term application of mineral fertilizer (NPK) did not decrease the uptake of the newly-added 60 kg/ha of fertilizer N by the spring cereal crops, compared to F0 background (table). Where both mineral fertilizer and organic manure (NPK+M) had been applied there was a 10-35 % increase in the uptake of fertilizer N.
The immobilization of fertilizer N was the highest for NPK+M background and the lowest for NPK one. All 15N remained into the soil was N assimilated by microorganisms because the nonexchangeable fixation of NH4+ by clay was insufficient. The high amount of the immobilized fertilizer N (up to 55 % of total labelled) we obtained for our MPE is typical for Siberian seasonally frozen soils. It is because the period of high microbial activity after soil thawing out and heating coincide the time of fertilizer application at the beginning of June.
The leaching of fertilizer N did not take place in frost-free period because of evaporation dominates over precipitation and of intensive immobilization, and in winter - because of soil freezing at the beginning of November. So, the losses of N fertilizer in the MPE happened only in gaseous form. The long-term application of mineral fertilizer increased the gaseous losses of newly added N on 53-79 % compared to F0 background. Minimal losses of fertilizer N was obtained for NPK+M background. It is also positive for the environment. Significant effect of such low dose of organic fertilizer as 5.7 t ha-1 year-1 on the fertilizer N transformations might cause extensive decomposing manure so far as the soil stay in frozen condition during 6 months.
Table. The fate of fertilizer N in soil (0-100 cm) - plant system
in microplot experiment, g/m2
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Plant uptake Immobilization Gaseous losses
Crop ------------------------------------------------------------
F0 NPK NPK+M F0 NPK NPK+M F0 NPK NPK+M
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1 1.81a 1.76a 2.37b 3.57b 3.13a 3.81b 0.62b 1.11c 0.00a
2 2.00a 2.05a 2.28b 3.22b 2.73a 3.51c 0.78b 1.22c 0.21a
3 2.07a 2.12a 2.33b 3.08b 2.58a 3.25b 0.85b 1.30b 0.42a
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1 - barley, 1984; 2 - wheat, 1987; 3 - oats, 1989.
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