SEASONAL TRENDS IN SOIL-NITROGEN FROM INJECTED OR SURFACE-INCORPORATED SEWAGE-SLUDGE APPLIED TO CORN

Use of sewage sludge on cropland may contribute to nitrate leaching or runoff from fields if mineralization of organic nitrogen (N) is not i n synchrony with crop N uptake. Differences in sludge application meth od may influence sludge N use efficiency. A two-year field experiment was conducted to determine how method of applying lime-stabilized dige sted sewage sludge in the spring affected seasonal patterns of soil mi neral N. Sludge rate was such as to supply 157 kg ha(-1) plant availab le N as determined by State of Maryland regulations. Corn (Zea mays) y ield in year 2 and corn earleaf N at tasseling in both years were sign ificantly lower for surface-incorporated sludge than for injected slud ge, although equivalent N was applied to both. Vertical distribution o f soil ammonium-N and nitrate-N within the surface 30 cm of soil in ye ar 2 suggested that, compared to injected sludge, surface-incorporated sludge lost more nitrate by leaching before the period of rapid plant N uptake. Release of nitrate was delayed by at least one month with i njected as compared to surface-incorporated sludge. In both years, soi l pH and electrical conductivity measured about one month after sludge application were sufficiently high to inhibit nitrification in the in jected sludge band. Residual mineral soil N remaining in the upper 30 cm of soil after plant N uptake ceased in fall was greatest with injec ted sludge in both years. Delayed nitrification in the injected band s hould be considered in planning method and timing of sludge applicatio ns to cropland.