Significant Diel Variation of Soil Respiration Suggests Aboveground and Belowground Controls in a Tropical Moist Forest in Puerto Rico

Abstract

Soil respiration in tropical forests represents a major source of carbon dioxide (CO2) to the atmosphere. The magnitude of this large flux is projected to change in response to climate change, with global implications due to the disproportionate role of tropical forests in the carbon cycle. Evaluating diel patterns of soil respiration concomitantly with biophysical drivers is a valuable approach for elucidating the mechanisms controlling soil respiration. We measured hourly soil respiration rates in a tropical moist forest in Puerto Rico over a 3‐year period using automated chambers, as well as soil temperature/moisture, air temperature, relative humidity, and photosynthetically active radiation. Hourly soil respiration exhibited as much as threefold variation on diel time scales (monthly diel amplitude ranged from 1 to 7 μmol CO2 m−2 s−1), and both the magnitude and shape of diel patterns changed significantly from month to month. Soil respiration peaked in the morning and late afternoon with a midday decline that was evident during the warmest summer months. The relationship between soil respiration and soil temperature differed during daytime versus the night, with a nonlinear relationship in the daytime but a significant positive linear relationship at night. These findings suggest factors other than/in addition to temperature could be controlling soil respiration during the day; however, soil respiration did not correlate with any of the other measured biophysical variables. Overall, our results highlight the potential role of aboveground processes as drivers of soil respiration at diel time scales, especially in closed canopy tropical forests with low diel variation in soil temperature/moisture.

Day–night discrepancy in soil respiration varies with seasons in a temperate forest

“…In contrast to our results, a slightly lower nighttime Q 10 was observed in a temperate plantation forest (Yan et al, 2019), while our results are in line with the lower daytime Q 10 in another subalpine forest (Hu et al, 2016) and a boreal region in north‐eastern Alberta (Chang et al, 2016). The reason for higher Q 10 at night in these study areas lies in the fact of hysteresis between Rs and STP on the diel scale caused by the different control of ambient factors on Rs variation, especially the hysteresis effect of allocation of photosynthesis assimilates on below‐ground C supply (Chang et al, 2016; Gutiérrez del Arroyo & Wood, 2020; Lei et al, 2022). In addition, we speculate that the higher but insignificant daytime Q 10 in other study areas (Yan et al, 2019) could be due to the changing rate of soil moisture versus soil temperature between daytime and nighttime (Nottingham et al, 2020; Wang, Liu, et al, 2014).…”

“…Different from other studies utilizing a fixed timeslot to classify day and night throughout the year, for example, 7 am to 6 pm as daytime and 7 pm to 6 am as nighttime (Gómez-Gener et al, 2021;Gutiérrez del Arroyo & Wood, 2020;Hu et al, 2016), we used photosynthetically active radiation (PAR; 50Y; Vaisala, Helsinki, acquired above the canopy near the top of the tower) to classify the daytime (PAR >0 μmol m −2 s −1 ) and nighttime (PAR = 0) periods (Yang et al, 2009), which could minimize uncertainty and bias in estimating Rs day and Rs night .…”

Large seasonal variation of soil respiration in a secondary tropical moist forest in Puerto Rico

“…We explore patterns of soil respiration at diel timescales in detail in a separate publication (Gutiérrez del Arroyo & Wood, 2020).…”

“…While the goal of this study was to evaluate seasonal variation in soil respiration, these data also allow for analyses at diel timescales. We explore patterns of soil respiration at diel timescales in detail in a separate publication (Gutiérrez del Arroyo & Wood, 2020).…”

“…In this context, we collected hourly R s measurements over a 3‐year period to (a) quantify the magnitude of R s and (b) identify the role of climatic, substrate, and nutrient controls on the seasonality of R s in a tropical moist forest in Puerto Rico. Controls on soil respiration at diel timescales are explored in a separate publication (Gutiérrez del Arroyo & Wood, 2020).…”