LEACHING OF N-15-LABELED FERTILIZER NITRATE IN UNDISTURBED SOIL COLUMNS AFTER SIMULATED HEAVY RAINFALL

A laboratory experiment was conducted at +5 degrees C with undisturbed columns (0.19 m i.d., 0.60 m long) of fine sand, heavy clay, and peat to determine the effect of soil and timing of the rainfall event on t he amount of fertilizer nitrogen (N) that might be leached after ferti lizer application. The N-15-labeled calcium nitrate [Ca(NO3)(2)] equal to 120 kg N ha(-1) was applied 13 or 1 days before adding a total of 70 mm of water. Drainage water was collected through an inlet attached to the bottom of each column until drainage had ceased nine days late r. From the sand columns, 27 and 19% of the fertilizer nitrate-nitroge n (NO3-N) applied was leached out when the columns were irrigated 1 an d 13 days after fertilizer application, respectively. From the three w ater-permeable clay columns containing earth worm burrows, 23 and 53% of the fertilizer NO, applied was leached out when irrigation took pla ce 1 and 13 days later, respectively; the other five clay columns were impermeable. Less than 0.1% of fertilizer NO3 was leached out from th e peat columns with either incubation treatment. After the experiment, the content of fertilizer N was highest at a depth of 0.12-0.24 m in the peat and clay columns, and at a depth 0.24-0.33 m in the sand colu mns. In the sand and peat columns, less fertilizer N moved down in the column when soil and fertilizer were incubated before irrigation. It is concluded that in a rainy and cool spring, substantial amounts of f ertilizer N can be leached beyond the reach of plant roots and out of the soil profile. The time passing between fertilizer application and rain may reduce the risk of leaching of applied N at low temperatures by diffusion into smaller soil pores rather than by biological immobil ization. With the intensity of irrigation applied here, fertilizer N m oved down in the sand and clay columns as bypass flow and hydrodynamic dispersion, whereas in peat columns it tended to move more as a front .