Coupling of aboveground and belowground responses to warming in a grass-dominated temperate old field

Background/Question/Methods

In northern temperate ecosystems, increased grass productivity resulting from warming and an extended growing season can be offset by increased frost damage, which results from reduced snow cover and increased temperature variability.  Belowground processes can also respond directly to warming or frost, but the question remains as to how much their responses to environmental change are indirect, and instead driven by aboveground responses.  We explored aboveground and belowground responses to warming for seven years in a grass-dominated temperate old field using overhead infrared heaters.  To further explore the effects of freezing on aboveground and belowground processes, we used a snow removal experiment in the field and froze plant-soil mesocosms in a controlled environment chamber. For aboveground responses, we measured aboveground biomass and cover at the plant species level.  For belowground responses, we measured soil respiration using steady-state flow-through chambers, litter decomposition using litter bags, and net nitrogen mineralization and nitrogen retention using buried bags and ¹⁵N tracer, respectively.  We also examined soil microbial biomass using chloroform fumigation and epifluorescence staining, and potential soil extracellular enzyme activity using microplate assays.

Results/Conclusions

Aboveground plant biomass did not respond to warming in all years, but it increased in response to warming when earlier plant growth was initiated in the spring, and it decreased in response to warming in a year with a severe late spring frost.  Soil respiration increased directly in response to warming, but there was no evidence of an indirect effect.  Warming increased litter decomposition for some species in the first year, but there were no longer-term effects.  Nitrogen mineralization responded to winter warming only, and warming increased the retention of added 15N.  Soil microbial biomass and extracellular enzyme activity were generally unresponsive to warming, although there were increases in microbial C and phosphatase activity.  The snow removal and mesocosm freezing experiments revealed that soil freezing can both increase soil N losses over winter and reduce the interception of growing season N deposition.  While the immediate soil N and C freezing responses were mostly transient, legacy effects of soil freezing on soil N and C pools and microbial biomass materialized, possibly as a result of changes in plant biomass.  However, only the most severe freezing treatments exhibited legacy effects on plant biomass and soil processes over multiple growing seasons.