SOIL NITROGEN CYCLE IN HIGH NITROGEN DEPOSITION FOREST: CHANGES UNDER NITROGEN SATURATION AND LIMING

Abstract

This study focuses on the microbial N cycle in the acid soil of a beech forest that falls in the upper range of the N saturation continuum. Our objectives were: (1) to quantify microbial N cycling under long-term N-saturated and limed conditions and (2) to determine the factors controlling the differences in microbial N cycling. Our study site has a long history of high N deposition: ?25 kg N-ha- Iyr-' since measurements began in 1971. This was further enhanced by 11 yr (1983-1993) of fertilization (140 kg ammonium sulfate- N-ha-I-yr-') to create an N saturation plot. Another plot was limed with 30 Mg/ha dolomitic limestone in 1982. In 1999-2000, gross rates of microbial N cycling were measured using '5N pool dilution techniques. Despite the chronic high N deposition, the control plot showed a tightly coupled microbial N cycle; NH4 and NO3- immobilization rates were comparable to gross N mineralization and nitrification rates, respectively. These were supported by low levels of NH4+, NO3-, and dissolved organic N (DON) in percolate. Liming increased gross N mineralization and nitrification rates but did not cause similar increases in microbial biomass or NH4 +, and NO~- immobilization rates. In addition, NO3- immobilization rates were somewhat less than gross nitrification rates; relatively high levels of NO3- and DON in percolate were also observed. The N-saturated plot suggested an uncoupled microbial N cycle; NH4+ immobilization rates were lower than gross N mineralization rates, and NO3- immobilization rates were somewhat less than gross nitrification rates. These were corrob- orated by high levels of NH4+, NO3-, and DON in percolate. The reduced NH4+ and N03- immobilization rates in the N-saturated plot could be attributed to the measured decreases in microbial biomass, and the low microbial biomass was likely due to decreases in the supply of labile C. Our study demonstrates that while hydrological N input/output budgets can indicate whether or not a forest ecosystem is in a state of N saturation, the microbial N cycle can provide quantitative information on key processes that govern N losses.