Looking a little deeper: A new experiment warming forest soil to 1 m deep

Background/Question/Methods

We are investigating and modeling the microbial, biogeochemical, and mineralogical effects of warming the whole soil profile in a temperate conifer forest.  Globally, most soil organic carbon is found below 30 cm, and deep soil organic carbon tends to have very old radiocarbon-based residence times.  However, little is known about the controls of decomposition of this deeper organic carbon, how vulnerable it is to changes in climate, and what the impacts might be on plant nutrient availability and productivity. We have developed two experiments in California, USA to investigate the effects of warming (+4°C) on whole soil profiles. The first warms coniferous forest soils in situ to 1.3 m. The second warms grassland soils in field lysimeters, and will be used to contrast to the forest in this presentation.  We have added highly ¹³C-enriched root substrates to multiple depths—15, 50, and 90 cm in the forest and 10 and 40 cm in the grassland. This labeled substrate will allow us to trace the transformation of organic inputs (into CO₂and microbial, particulate, and mineral-associated pools) with soil depth. We also hope to be able to detect if and how native SOC decomposition is affected by root inputs.

Results/Conclusions

We will present our experimental design and preliminary results from the first 6 months of warming and its impact on soil microclimate, soil respiration, soil profile CO₂ concentrations, and PRS probe nutrient data. The heaters were turned on in October 2013 and are maintaining an even warming to more than 1 m deep.  In the first months of heating, only some heated plots have higher soil respiration than their control pairs. The project takes advantage of a new fine-resolution, reactive transport model that can be tested and improved with experiment data as well as applied in global land simulations.  This study is one of the first to test whole-soil-profile responses to warming and root carbon inputs, and will enhance our understanding of carbon cycling mechanisms throughout the soil profile to improve predictions of the role of soils in a changing climate.