Study of the fate of fertilizer nitrogen in soil

Abstract

1. Fertilizer NO₃-N was applied to fallow and planted soil. In a field experiment the rates of application in factorial combinations were 30 and 60 kg N0₃-N/ha in the autumn (December) and 60 and 90 kg N0₃-N/ha in the spring (May). Samples of soil cores down to 60 cm., and plant material (winter wheat) were taken periodically throughout the December to September growing season.Greenhouse experiments were carried out using topsoil and subsoil separately. In the first greenhouse experiment* 1|5 cm. columns of topsoil and of subsoil were used under plant (rye grass) and fallow with and without 60 kg N0₃-N/ha. Duration was 7 weeks. Three watering regimes were applied, i.e. 60, 90, and 120 percent of the water holding capacity. The excess water from the highest water regime was collected. The second experiment was carried out on topsoil only. Soil was ignited to destroy organic matter and was compared with the unignited soil under fallow and planted (rye grass) receiving N0₃-N at rates of 23 and 50 ppm.A laboratory experiment was carried out investigating the effect of two aggregate sizes (1-3 and 3_6 mm. in diameter) on leaching losses of added N0₃-N.2. Soil samples were analysed for NO₃-N and NH₄-N in 4 horizons from the field experiment (3 from the Greenhouse Experiment I) each 15 cm deep, and from each of the Greenhouse Experiment II and the laboratory experiment. Leachates were analysed for the same two mineral fractions of nitrogen. Plant samples were analysed for total N.3. The field experiment showed that the number of soil cores is a limiting factor and better results could be obtained only by taking more replicates.4. Recovery of fertilizer N at the end of the season ranged from 67-77% in the field experiment (9 months) and from 60-95% in the greenhouse experiments (nearly 2 months) under conditions of no leaching. However, when leaching was excessive, recovery of fertilizer N was as low as 2%.5- Leaching losses of fertilizer N could be as high as 100%. Movement of fertilizer N occurred within 3 weeks of application, translocating fertilizer N from the surface to 45-60 cm deep in soil. Precipitations ranging from 38 to 1025 mm resulted in downward translocation of fertilizer N. The size of soil aggregates showed their important role in holding fertilizer against loss by leaching. Planted soils lost about 30% by leaching; fallow ones lost more than 75% within 3 weeks under 482 mm precipitation.6. The topsoil possessed greater capacity for mineralization and immobilization than the subsoil. This was proved by experiments comparing behaviour of fertilizer N in topsoil vs. subsoil and in ignited vs. unignited topsoil.7- The proportion of fertilizer N immobilized was greater in the fallow (about 90%) than in the planted. Indications of remineralization were shown in both field and greenhouse experiments.8. Denitrification and reduction of NO₃-N was shown in both topsoil and subsoil and in the ignited topsoil. Fertilizer NO₃-N moving from topsoil down to the subsoil - under field conditions - seems to have been reduced to elemental N rapidly in the subsoil.9. There are indications that application of fertilizer enhances mineralization immediately (up to 3 weeks) after application; but within 3-ip months it seems that fertilizer enhances immobilization. Exudation of fertilizer N by plants may have had an important effect on its status in soil.