Continuous measurements of soil respiration with and without roots in a ponderosa pine plantation in the Sierra Nevada Mountains

Abstract

Continuous measurements of soil respiration and its components help us understand diurnal and seasonal variations in soil respiration and its mechanism. We continuously measured CO2 concentration at various depths in the soil and calculated surface CO2 efflux based on CO2 gradients and diffusivity in a young ponderosa pine plantation in the Sierra Nevada Mountains of California. We determined soil respiration both in a control plot that included roots and in a trenched plot that had no roots. The difference between these plots was used to partition soil respiration into root respiration and heterotrophic respiration.We found that both CO2 concentration in the soil and surface CO2 efflux in the control plot were higher than in the trenched plot. The diurnal range of soil respiration in the trenched plot was larger than in the control. We observed dramatic pulses of soil respiration in response to rain events in summer and fall during the dry season. We modeled the seasonal variation in soil respiration without the pulses using soil temperature and moisture as driving variables and simulated soil respiration pulses using an exponential decay function in response to the volume of rain. Daily mean soil respiration peaked at 5.0 mmol m_2 s_1 in the control and at 2.7 mmol m_2 s_1 in the trenched plot in June before the rain pulses. Soil respiration increased from 4.9 to 8.2 and to 12.1 mmol m_2 s_1 after the first and second rain events in the control, and increased from 2.2 to 4.1 and to 6.6 mmol m_2 s_1 in the trenched plot. After incorporating the pulse effect, the model simulated measured data well. Annual soil respiration in 2003 was estimated as 1184 g C m_2 y_1. The average ratio of root over total respiration was 0.56 during the growing season and 0.16 during the nongrowing season with an annual average of 0.44.