WATERSHED LIMING EFFECTS ON THE FOREST FLOOR N CYCLE

The forest floor was expected to play a major role in determining the total ecosystem response to watershed liming because of its high conce ntration of nutrients and its high level of activity. Net N mineraliza tion and net nitrification were estimated in a field survey using the buried-bag approach. In a laboratory incubation experiment, forest flo or humus was mixed with 6 doses of lime to determine the sensitivity o f N mineralization and nitrification to lime dose. Forest floor microc osms with and without live tree roots were used to calculate a N budge t for the system. The pH of the forest floor increased from 3.6 to 4.9 in the Oe and to 4.0 in the Oa two years after liming. The extractabl e ammonium pool in both the field survey and microcosm study was subst antially smaller after liming and was probably a result of the 36% to 55% lower net N mineralization rate in limed plots than in reference p lots. The laboratory incubation results agreed with the field survey r esults and further demonstrated that at higher lime doses (pH 5 to 6), N mineralization increased above controls. Net nitrification in limed humus in both the buried bags and laboratory incubation was as much a s three times higher than controls, which could explain why nitrate le aching in limed microcosms was greater than in control microcosms. How ever, nitrate leaching from microcosms with live roots was not affecte d by liming, suggesting that roots in the forest floor may prevent exc ess nitrate leaching. Reductions in N mineralization had no effect on N leaching or N uptake, but reduced the extractable ammonium pool.