The effect of nitrogen management in agricultural production on water and air quality: evaluation on a regional scale

The purpose of this study was to use a nitrogen (N) budget calculation appr oach to estimate the effect of agricultural production in the Lower Fraser Valley, British Columbia, Canada, on N loss to the environment under 1991 c rop and animal management practices, the change in N loss to the environmen t between 1981 and 1991, and the extent to which these losses could be redu ced through improved management practices, Ammonia losses to the atmosphere from animal production were substantial, in excess of 7250 (t N) year(-1), but could be reduced by up to 30% through improved manure management and a nimal diets. Root-zone N losses to surface and groundwater were estimated a s the N surplus (the excess of N additions from inorganic fertilizer, manur e, and atmospheric deposition over N removals by the crop and denitrificati on) for the root zone of land in agricultural production. The N surplus for the study area as a whole was 68 (kg N) ha(-1) land in agricultural produc tion, whereas an N surplus of < 50 (kg N) ha(-1) would be expected under op timal N management. Calculation of N surplus values on the basis of individ ual districts as compared to the entire study area predicted a greater effe ct of agricultural production on water quality because of a non-optimal dis tribution of nutrients. High N surplus values within individual districts, up to 238 (kg N) ha(-1), were attributed to animal intensification, particu larly poultry and swine production which does not require a local land base for feed production. Root-zone N losses increased between 1981 and 1991, p rimarily because of the concentration of animal production in specific geog raphical areas and on a diminished agricultural land base. Improved fertili zer and manure management was predicted to reduce the N surplus of the stud y area from 68 to 5 (kg N) ha(-1'); however, less than one-half of this red uction could be practically realized at this time. Improved animal diets we re predicted to reduce the N surplus from 68 to 45 (kg N) ha(-1), with most of the reduction occurring in areas with high root-zone N losses. The resu lts demonstrate the value of using an N balance approach to evaluate the en vironmental effect of agricultural production on air and water quality, and to assess how this effect could be reduced through improved management pra ctices. (C) 1999 Elsevier Science B.V. All rights reserved.