A new view of nitrogen saturation based on experimental fertilization in an oak forest

Background/Question/Methods The process of forest nitrogen saturation is often considered to follow the conceptual model of Aber et al. (1998), who proposed that N additions would lead to increases in foliar N, which would be followed by sequential increases in N mineralization, nitrification, and finally nitrate leaching.  Our results from long-term N fertilization of a forest in southeastern New York State are inconsistent with this model. 

Results/Conclusions Six plots in this mixed oak/hickory forest were treated with NH₄NO₃ (4 applications/year) at a rate of 100 kg N/ha/y from 1996-1999 and 50 kg N/ha/y from 2000-present.  Six paired plots were left as untreated controls.   Unlike other fertilization experiments in oak forests, nitrate leaching began soon after the initiation of fertilization and reached levels of near 50 kg N/ha/y in 1999-2000.  Nitrate leaching from the control plots has remained very low, < 1 kg N/ha/y. Foliar N concentration increased by about 15% when the fertilization began, but the concentration soon leveled off.  N mineralization has showed no consistent change, and potential nitrification did not increase until several years after nitrate leaching had begun.  The excess nitrate leaching has led to soil acidification and mortality of some trees, although the surviving trees are growing faster than they were before the fertilization.    The rapid increase in N leaching, the delayed increase in nitrification, and the lack of response in N mineralization all stand in contrast to expectations from the Aber et al. model.  This paper describes a new view of nitrogen saturation that takes into account contingent effects and multiple pathways of added N within the ecosystem.