Understanding rates of change in storage of carbon and nutrients in soils is essential to global carbon budgets as well as local management decisions. However, change in soils can be slow, and variability in forest soils can make these changes difficult to detect. The purpose of this study is to illustrate an approach to quantifying uncertainty in the detection of soil change, which can inform important decisions regarding soil monitoring designs, such as the needed intensity and time frame of sampling. We analyzed soils collected over almost 50 years from the Calhoun Critical Zone Observatory in South Carolina, USA. Dramatic changes have been observed in soil carbon, nitrogen, acidity, and macro- and micronutrients since the old agricultural field was planted to loblolly pine in 1957. In this study, we conducted power analysis using a dataset of soil carbon and nitrogen concentrations at four soil depths in eight forest plots and eight resampling dates from 1962 to 2008. Surprisingly, the power to detect soil change was high in both surface soils (0-7.5 cm), where concentrations of carbon and nutrients were high, and at great depth (60 cm), where concentrations were uniformly low. At intermediate depths, greater sampling intensity would be required to detect a similar rate of change over time. Nitrogen concentrations were more variable than carbon concentrations except at the 60-cm depth; as a result, smaller changes are detectable for C than N through most of the soil profile, in this study system. Power analysis is a useful tool for reporting uncertainty in long-term change. In cases where changes are not statistically significant, power analysis can tell us whether the changes are small or whether spatial or temporal variability is high, and this can help improve investments in environmental monitoring.
Mobley, Megan, Kevin Nelson, Daniel Richter, Ruth Yanai (2018): Detecting change over time with depth in forest soils: an example from the Calhoun Critical Zone Observatory, USA. North American Forest Soils Conference – International Symposium on Forest Soils, Quebec City, Quebec, Canada, June 10-16, 2018.
This Paper/Book acknowledges NSF CZO grant support.