A lysimeter investigation of nitrogen gains and losses under various systems of covercropping and fertilization, and a discussion of error sources

Abstract

The Results in Brief The net gain or loss of soil nitrogen was studied in lysimeters at Riverside for ten years under four series of treatments: (1) fall applications of cereal straw (5,000 pounds per acre); (2) a mustard winter covercrop; (3) purple-vetch winter covercrop; (4) sweet-clover winter covercrop. In each series, one lysimeter was given no fertilizer, one 100 pounds, and one 200 pounds of nitrogen per acre per year as nitrate. A summer-harvested crop (first barley, later Sudan grass) was grown in all lysimeters. Total nitrogen and organic matter in the soil at the outset and at five-year intervals, amounts of nitrogen removed by cropping and leaching, and amounts added in seed, rain, irrigation water, and fertilizer were recorded. Unaccounted-for net losses of nitrogen, thought to be due to gaseous volatilization, amounted to more than 60 pounds per acre per year with a mustard covercrop when 200 pounds of nitrogen per acre was added in fertilizer. Similar losses occurred with cereal-straw applications and 200 pounds of nitrogen. Net gains of nitrogen when legume covercrops were grown without fertilization amounted to about 150 pounds per acre per year. These net gains were progressively reduced when 100 and 200 pounds of nitrogen per acre per year was added in fertilizer. Net gains of nitrogen due to nonsymbiotic fixation amounted to 48 pounds per acre per year when dry cereal straw was applied without fertilization; they amounted to 40 pounds per acre per year when a mustard covercrop was grown without fertilization. Average loss of nitrogen through leaching varied from 4 to 88 pounds per acre per year. It was highest when no winter covercrop was grown and with heavy nitrogen fertilization, considerably higher with legume covercrops than with mustard. Most of the losses occurred in years of heavy rainfall or when rain fell just after an irrigation, since irrigation was designed to just meet the water requirements of the crops grown. The nitrogen requirements of Sudan grass—about 125 pounds of nitrogen per acre per year for maximum yields—were fully met through the nitrogen fixed by legume covercrops: yields were not increased by nitrogen fertilizer when legume covercrops were grown, although the fertilizer increased the growth of the legumes. The nitrogen added in cereal straw (22 pounds per acre per year), in rain and irrigation water (7 pounds), and by nonsymbiotic fixation (48 pounds) was not enough to meet the requirements of the Sudan grass. These results indicate that high-level nitrogen fertilization is wasteful. If much more nitrogen is applied than the crops need, substantial amounts will be lost by gaseous volatilization and by leaching, fixation will be reduced, and luxury consumption of nitrogen by crops may occur without benefit to yield or quality. Therefore, greatest nitrogen economy will be achieved when nitrogen is maintained at the lowest possible point consistent with satisfactory crop performance.