Calculation of Theoretical and Empirical Nutrient N Critical Loads in the Mixed Conifer Ecosystems of Southern California

Joan Breiner,Benjamin S Gimeno,Mark Fenn

doi:10.1100/tsw.2007.65

Joan Breiner, Benjamin S Gimeno + Show 1 more

Open Access

https://doi.org/10.1100/tsw.2007.65

Copy DOI

Abstract

Edaphic, foliar, and hydrologic forest nutrient status indicators from 15 mixed conifer forest stands in the Sierra Nevada, San Gabriel Mountains, and San Bernardino National Forest were used to estimate empirical or theoretical critical loads (CL) for nitrogen (N) as a nutrient. Soil acidification response to N deposition was also evaluated. Robust empirical relationships were found relating N deposition to plant N uptake (N in foliage), N fertility (litter C/N ratio), and soil acidification. However, no consistent empirical CL were obtained when the thresholds for parameters indicative of N excess from other types of ecosystems were used. Similarly, the highest theoretical CL for nutrient N calculated using the simple mass balance steady state model (estimates ranging from 1.4–8.8 kg N/ha/year) was approximately two times lower than the empirical observations. Further research is needed to derive the thresholds for indicators associated with the impairment of these mixed conifer forests exposed to chronic N deposition within a Mediterranean climate. Further development or parameterization of models for the calculation of theoretical critical loads suitable for these ecosystems will also be an important aspect of future critical loads research.

Highlights

The anthropogenic use of fossil fuels and agricultural fertilizers has increased the formation of reactive N species (Nr) by 90% during the last 150 years with continued growth in the emission rate expected over the 50 years[1]
The well-documented N deposition gradient occurring in the mixed conifer forests of the Sierra Nevada, San Gabriel Mountains, and San Bernardino National Forest[12,17] provides a useful dataset to compare the theoretical N critical levels (CL) calculated with the Simple Mass Balance (SMB) model and the empirical CL derived from the analysis of actual measurements of environmental parameters that have been proposed as indicators of ecosystem N status[18]
Evidence of disturbance of the N cycle of the Mediterranean mixed conifer forest by increased N deposition was found when comparing the values recorded for the different parameters along the N deposition gradient (Table 2)

Summary

Introduction

The anthropogenic use of fossil fuels and agricultural fertilizers has increased the formation of reactive N species (Nr) by 90% during the last 150 years with continued growth in the emission rate expected over the 50 years[1]. Considerable disagreement has been found between the theoretical CL resulting from SMB calculations and those derived either empirically[7] or through ecosystem-process modeling[8].

Results

Conclusion