DOI: 10.6073/pasta/abf59a218a5868baf5b2c073a7dd1d7f
title: Prospect Hill Soil Warming Experiment at Harvard Forest since 1991

description: The soil warming experiment was installed on the Prospect Hill tract in 1991 to allow us to investigate the effects of a 5 deg C temperature increase on soil processes fundamental to the global cycling of carbon and nitrogen. The experiment is located in an even-aged mixed hardwood forest. Six replicates of three treatments, Heated (resistance heating cables buried at 10cm and maintained at a 5 deg C differential from the control plots), Disturbance Control (cables installed but not powered) and Control treatments make up the randomized block design. The temperature differential is maintained with monitoring at five minute increments by an automated thermistor network in the plots, wired to a multiplexer and a datalogger in the control shed. In the plots, measurements of trace gasses (CO2, N2O and CH4), nitrogen mineralization, soil moisture and soil water chemistry have allowed us to quantify changes in the soil system.
 Ten years of elevated soil temperatures at the Harvard Forest soil warming experiment suggest that there are limits to a positive feedback to the global warming cycle. After many early years of increased CO2 fluxes from the warmed plots, years nine and ten have revealed no significant differences in releases of CO2 between the heated and control plots. Nitrogen mineralization has shown a large response to warming as well, with twice the rate of N mineralized in years 1-4, followed by a gradual decrease in rates to about the 40% level in 1998. Resumption of mineralization measurements in 2001 reveals a continued decrease in mineralization rates.
 Field results from the soil warming experiment indicate that only a small fraction of the soil carbon in this mid-latitude forest ecosystem will be lost to the atmosphere in response to warming. We find that a 5 deg C warming of the soil for a decade results in a loss of about 11% of the carbon stored in the top 60 cm of soil, with most of this loss occurring in the first four to five years. By the end of the decade, warming no longer stimulates soil carbon loss. In addition, we find that warming accelerates the soil nitrogen cycle, particularly in the early years of warming. This acceleration has the potential to stimulate plant carbon storage.
 Calculations based on nitrogen uptake in vegetation at the Chronic N Experiment (Magill, Aber et al. 2000) allow us to estimate how much additional carbon might be stored in plants as a result increased N availability at soil warming. Our calculations show that the potential increase in carbon stored is large enough to more than compensate for the soil carbon loss, so that warming may actually stimulate carbon storage at the ecosystem level in this mid-latitude forest.
 We are currently expanding our research from the original soil warming experiment to investigate this important feedback at the Harvard Forest. The existing 6x6m plots on the Prospect Hill tract are not large enough to allow for the study of treatment effects on trees, so we are installing a larger warming experiment. The new experiment will have two 30x30m plots, a heated and a control plot, in the Slab City Tract of the Harvard Forest. Many aspects of this experiment will be the same as the original. Using resistance heating cables and thermistors, we will maintain a 5 deg C differential between the heated and control plots, year-round. In addition to the trace gas, nitrogen mineralization and soil water parameters measured to date, we will also be measuring woody increment and other vegetation metrics to allow us to investigate the plant response to perturbations in the carbon and nitrogen cycles.