Patterns of Nutrient Loss from Unpolluted, Old‐Growth Temperate Forests: Evaluation of Biogeochemical Theory

Abstract

Atmospheric pollution and other human activities have altered natural element cycles over large regions of the world. Much current understanding of nutrient dynamics in temperate region ecosystems has been derived from such biogeochemically altered areas. While it is increasingly difficult to find regions of temperate forest that can be classified as free from human influences, studies of unpolluted and undisturbed areas can (1) provide important °baseline° information about natural patterns of element cycling, against which disturbed cycles can be compared; (2) provide insights into the biogeochemical conditions that acted as evolutionary and selective constraints on biotic communities before the advent of regional—scale human impacts; and (3) provide an opportunity to evaluate the general nature of biogeochemical theories that have been developed in areas subject to strong human influences. We here report on patterns of hydrologic nutrient loss from old—growth temperate forest ecosystems in southern Chile based on chemical analyses of small streams draining 31 watersheds. These forests have not been subject to air pollution inputs and have remained floristically stable throughout the Holocene. For major elements our results support predictions from current biogeochemical theory (the °nutrient retention hypothesis°) that net biotic retention of elements should be minimal in old—growth ecosystems. Despite the dilute nature of watershed streams, the overall patterns of loss of most elements could be explained almost exclusively by atmospheric inputs of marine aerosols. In contrast, patterns of nitrogen loss were more complex than predicted by current conceptual models. Hydrolic N losses occurred nearly exclusively (95% of total N) as dissolved organic forms of nitrogen (DON), rather than as the inorganic forms NO3 — (0.2% of total N) and NH4+ (4.8% of total N). The strong dominance of organic over inorganic losses of N indicates that losses of N from old—growth forest ecosystems are not exclusively subject to traditional mechanisms of direct biotic control (i.e., mineralization supply or biotic uptake), but are also subject to indirect biotic control associated with the long—term accumulation, humification, and leaching of soil organic N during ecosystem succession. The view of unpolluted old—growth forests as °leaky° vs. °non—leaky° with respect to N depends on whether ecosystem budgets or models consider dissolved organic, as well as inorganic, forms of N. High— and mid—elevation forests in our study area showed the lowest afflux concentrations of N as NO3 — reported from any Old—growth temperate forest ecosystem (0.l0 vs. 0.30 mg/L, respectively). Comparisons against other old—growth forests, subject to varying levels of N deposition, indicated that levels of NO3— and the relative abundance of NO3— vs. NH4+ in Northern Hemisphere forests may be strongly influenced by atmospheric N inputs. Our analyses suggest that patterns of N cycling, as well as evolutionary selective pressures on plant and microbial species, may have changed as a function of regional scale increases in atmospheric N deposition to European and North American temperate forests. We hypothesize that, because they inherently are more poor than aggrading forests at retaining added nutrients, old—growth forest ecosystems are particularly sensitive indicators of N deposition.