Changes in photosynthesis and soil moisture drive the seasonal soil respiration-temperature hysteresis relationship

Authors: ZHANG, Q. - Indiana University; PHILLIPS, R.P. - Indiana University; MANZONI, S. - Stockholm University; Scott, Russell - Russ; QISHI, A.C. - Us Forest Service (FS); FINZI, A. - Boston University; DALY, E. - Monash University; VARGAS, R. - University Of Delaware; NOVICK, K.A. - Indiana University

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2018
Publication Date: 5/10/2018
Citation: Zhang, Q., Phillips, R., Manzoni, S., Scott, R.L., Qishi, A., Finzi, A., Daly, E., Vargas, R., Novick, K. 2018. Changes in photosynthesis and soil moisture drive the seasonal soil respiration-temperature hysteresis relationship. Agricultural and Forest Meteorology. 259:184-195. https://doi.org/10.1016/j.agrformet.2018.05.005.
DOI: https://doi.org/10.1016/j.agrformet.2018.05.005

Interpretive Summary: Soil respiration, the term used to describe the efflux of CO2 from the soil surface,owing to plant and soil respiration, is the largest land carbon source to the atmosphere. In nearly all models, soil respiration is represented as a function of soil temperature. However, the relationship between soil respiration and soil temperature is highly variable, and there is often a pronounced hysteresis or lag in seasonal soil respiration-temperature relationships. We developed a simple numerical model to demonstrate how photosynthesis, soil moisture, and soil temperature, alone and in combination, affect the hysteresis relationship. Then, we compared measurements of soil respiration, soil temperature, soil moisture and photosynthesis from multiple mesic and semiarid ecosystems to quantify the frequency of hysteresis and identify its potential controls. Using the numerical model, we show that the hysteresis can result from the seasonal cycles of photosynthesis, which supplies carbon to fuel root respiration, and soil moisture, which limits microbial respiration when too low or too high. Using the field data, we found evidence of seasonal hysteresis in most of the years across 8 sites. We found that across all sites, much of the respiration-temperature lag was explained by the decoupling of photosynthesis and temperature, highlighting the importance of recently assimilated carbon to soil respiration. Collectively, our results show that incorporating photosynthesis and soil moisture in the standard soil respiration-temperature equation improves model predictions of soil respiration at local scales.

Technical Abstract: In nearly all large-scale models, CO2 efflux from soil (i.e., soil respiration) is represented as a function of soil temperature. However, the relationship between soil respiration and soil temperature is highly variable at the local scale, and there is often a pronounced hysteresis in the soil respiration-temperature relationship at the seasonal timescale. This phenomenon indicates the importance of biophysical factors beyond just temperature in controlling soil respiration. To identify the potential mechanisms of the seasonal soil respiration-temperature hysteresis, we developed a simple numerical model to demonstrate how photosynthesis, soil moisture, and soil temperature, alone and in combination, affect the hysteresis relationship. Then, we compared measurements of soil respiration, soil temperature, soil moisture and photosynthesis from multiple mesic and semi-arid ecosystems to quantify the frequency of hysteresis and identify its potential controls. Using the numerical model, we show that the hysteresis can result from the seasonal cycles of photosynthesis, which supplies carbon to rhizosphere respiration, and soil moisture, which limits heterotrophic respiration when too low or too high. Using the field measured time series data, we found evidence of seasonal hysteresis in 9 out of the 15 site-years across 8 sites. Specifically, clockwise hysteresis occurred when photosynthesis precedes seasonal temperature and counterclockwise hysteresis occurred when photosynthesis lags soil temperature. We found that across all sites, much of the respiration-temperature lag was explained by the decoupling of photosynthesis and temperature, highlighting the importance of recently assimilated carbon to soil respiration. Collectively, our results show that incorporating photosynthesis and soil moisture in the standard exponential soil respiration-temperature equation (Q10) improves model predictions of soil respiration at local scales.