Does Long-Term Storage of Air-Dried Soils Effect the Results of Chemical Analyses Commonly Performed on Forest Soils?.

Repeated sampling to monitor changes in forest soils is a relatively new practice that involves the use of soil archiving to avoid bias that can be introduced by inconsistencies in chemical analyses that can occur over time.  Soil samples from previous collections can be reanalyzed with new collections to ensure analysis comparability.  However, this procedure depends on the stability of the archived soil over the period it has been stored, usually in an air-dried form.  Limited studies of the stability of air-dried soils over time suggest that a number of measurements commonly done on forest soils are reasonably stable, but there are also results that suggest caution with regard to assumptions of stability.   Further work is needed to (1) clearly define which chemical analyses are appropriate to perform on air-dried samples that have been subjected to long-term storage, and (2) determine how the length of storage effects analysis stability. 

As a project of the Northeastern Soil Monitoring Cooperative (NESMC), data from six laboratories have been compiled on repeated analyses of reference soil samples extending over periods of up to 18 years.  In each case, the reference samples were created from large quantities of natural forest soil that were air-dried, sieved and homogenized to enable repeated analyses for quality control purposes.  Because these samples were run for periods of several years to nearly two decades, they also provide useful information for evaluating possible storage effects.  Trends in measurements that change significantly with time can be caused by storage effects, but could also reflect drifting results from subtle methodological influences or step changes from methodological changes.  Analysis of data combined from multiple laboratories provides an effective method for resolving these issues.  Results of this analysis demonstrate the value of archived soil in reducing uncertainty in results of soil monitoring.