THE EFFECTS OF LONG-TERM APPLICATIONS OF INORGANIC NITROGEN-FERTILIZER ON SOIL-NITROGEN IN THE BROADBALK WHEAT EXPERIMENT

The Broadbalk Wheat Experiment at Rothamsted (UK) includes plots given the same annual applications of inorganic nitrogen (N) fertilizer eac h year since 1852 (48, 96 and 144 kg N/ha, termed N-1, N-2 and N-3 res pectively). These very long-term N treatments have increased total soi l N content, relative to the plot never receiving fertilizer N (N-0), due to the greater return of organic N to the soil in roots, root exud ates, stubble, etc (the straw is not incorporated). The application of 144 kg N/ha for 135 years has increased total soil N content by 21%, or 570 kg/ha (0-23 cm). Other plots given smaller applications of N fo r the same time show smaller increases; these differences were establi shed within 30 years. Increases in total soil N content have been dete cted after 20 years in the plot given 192 kg N/ha since 1968 (N-4). Th ere was a proportionally greater increase in N mineralization. Crop up take of mineralized N was typically 12-30 kg N/ha greater from the N-3 and N-4 treatments than the uptake of c. 30 kg N/ha from the N-0 trea tment. Results from laboratory incubations show the importance of rece ntly added residues (roots, stubble, etc) on N mineralization. In shor t-term (2-3 week) incubations, with soil sampled at harvest, N mineral ization was up to 60% greater from the N-3 treatment than from N-0. In long-term incubations, or in soil without recently added residues, di fferences between long-term fertilizer treatments were much less marke d. Inputs of organic N to the soil from weeds (principally Equisetum a rvense L.) to the N-0-N-2 plots over the last few years may have parti ally obscured any underlying differences in mineralization. The long-t erm fertilizer treatments appeared to have had no effect on soil micro bial biomass N or carbon (C) content, but have increased the specific mineralization rate of the biomass (defined as N mineralized per unit of biomass). Greater N mineralization will also increase losses of N f rom the system, via leaching and gaseous emissions. In December 1988 t he N-3 and N-4 plots contained respectively 14 and 23 kg/ha more inorg anic N in the profile (0-100 cm) than the N-0 plot, due to greater N m ineralization. These small differences are important as it only requir es 23 kg N/ha to be leached from Broadbalk to increase the nitrate con centration of percolating water above the 1980 EC Drinking Water Quali ty Directive limit of 11.3 mg N/I. The use of fertilizer N has increas ed N mineralization due to the build-up of soil organic N. In addition , much of the organic N in Broadbalk topsoil is now derived from ferti lizer N. A computer model of N mineralization on Broadbalk estimated t hat after applying 144 kg N/ha for 140 years, up to half of the N mine ralized each year was originally derived from fertilizer N. In the sho rt-term, the amount of fertilizer N applied usually has little direct effect on losses of N over winter. In most years little fertilizer-der ived N remains in Broadbalk soil in inorganic form at harvest from app lications of up to 192 kg N/ha. However, in two very dry years (1989 a nd 1990) large inorganic N residues remained at harvest where 144 and 192 kg N/ha had been applied, even though the crop continued to respon d to fertilizer N, up to at least 240 kg N/ha.