Understanding rates of change in storage of carbon and nutrients in forest soils is essential to global carbon budgets as well as to local management decisions. However, change in soils can be slow, non-linear, and may vary with the depth of soil examined. Furthermore, variability in forest soils can make these changes difficult to detect. The purpose of this study is to quantify uncertainty in detection of soil change following reforestation, and to inform soil monitoring designs, including intensity and schedule of sampling.
We analyzed soils collected over almost 50 years from the Calhoun Critical Zone Observatory in South Carolina, USA. Over these decades, this long-term soil-ecosystem experiment has undergone secondary forest succession from an eroded agricultural field to a loblolly pine plantation to a mature pine stand with emerging hardwood understory. The soil changes that accompany this ecosystem development have been monitored at 7-10 year intervals since the forest was planted in 1957, and dramatic changes have been observed in soil carbon, acidity, and nutrients. In this study, we conducted power analysis using a dataset of soil carbon (C) and nitrogen (N) concentrations at four soil depths in eight forest plots over eight resampling dates from 1962 to 2008.
Results/Conclusions
The power to detect soil change was high in surface soils (0-7.5 cm) where concentrations of C and N were high, and in subsoils (35-60 cm) where concentrations were uniformly low, depending on forest development phase. N concentrations were more variable than C concentrations except in subsoil; as a result, smaller changes were detectable for C than N through most of the soil profile in this study system.
During the establishment and thinning phases of forest development, power to detect surface soil change was higher for N than for C, and power to detect C and N changes in subsoil was higher than in surface soil. During the transition phase of forest development, power to detect C and N change in surface soils was higher than during establishment and thinning, higher for surface soil N than C, and power to detect any changes in subsoil C and N contents was low.
Power analysis is a useful tool for reporting uncertainty in long-term change and for guiding the design of future monitoring studies. Our study emphasizes the importance of considering ecosystem development stages in optimizing statistical power of monitoring studies.