Technical Comments

Response to Comment on “The whole-soil carbon flux in response to warming”

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Science  23 Feb 2018:
Vol. 359, Issue 6378, eaao0457
DOI: 10.1126/science.aao0457

Abstract

Temperature records and model predictions demonstrate that deep soils warm at the same rate as surface soils, contrary to Xiao et al.’s assertions. In response to Xiao et al.’s critique of our Q10 analysis, we present the results with all data points included, which show Q10 values of >2 throughout the soil profile, indicating that all soil depths responded to warming.

In their comment, Xiao et al. (1) raise several questions about our whole-soil warming experiment and results (2), which we address in this response. We agree that this is an important topic and appreciate the opportunity to clarify points that perhaps were not sufficiently clear in our original report.

Xiao et al. question the relevance of heating the whole profile because they assert that deep soil will not warm as much as surface soil due to low thermal diffusivity. Although the exact rates of warming in any location will depend on a host of factors, both direct observations and soil thermal modeling find that nearly synchronous warming of the subsurface is a realistic climate change scenario. Analyses of temperature records for 38 stations across North America showed no difference in average warming trends at 10 cm and 100 cm depth between 1967 and 2002 [0.31° and 0.31°C decade−1, respectively (3)]. Analyses of soil temperature predictions from IPCC models (Coupled Model Intercomparison Project; CMIP5) show that both surface soils (0 to 2 cm) and deep soils (80 to 140 cm) will warm at roughly the same rate throughout this century, closely following air warming trends under scenario RCP 8.5 (Fig. 1), except in permafrost regions. The thermal diffusivity of soils does not impose meaningful lags to warming at 1 m depth over climatic time scales.

Fig. 1 Surface and deep soil temperatures are predicted to rise in synchrony.

Within the next century, global air temperatures over land are projected to increase 4°C according to Community Earth System Model Version 1–Biogeochemistry (CESM1-BGC) scenario RCP 8.5. Globally, soil temperature at the surface and at depth will lag slightly behind air temperature in the later part of the century, as a result of permafrost/snow feedbacks at high latitudes, but will also warm by approximately 4°C by 2100. Temperature change is reported as the difference between average 2081–2100 global temperatures for scenario RCP 8.5 and average 1986–2005 global temperatures simulated under a historical scenario.

In addition, the lack of deep soil warming in most warming experiments is not evidence that deep soils will exhibit reduced warming relative to the surface under future climates. The attenuation of warming with depth measured in the top-down warming (i.e., warming applied at or near the surface only) experiments cited by Xiao et al. (4, 5) was caused not by the low thermal diffusivity of the soil but as a result of lateral heat transfer. In top-down warming experiments, the area being warmed is adjacent to areas of ambient temperature to which heat is lost. Thus, many top-down warming experiments have soil heating profiles that attenuate much more steeply with depth than would be predicted in climate change scenarios in which the entire surface is warmed. Our warming design corrects for this experimental artifact that occurs when surface warming is implemented over a limited soil volume. Our study, wherein the entire profile to 1 m was warmed by +4°C while allowing for natural differences in diurnal and seasonal temperature fluctuations among depths, is a more realistic scenario of future soil warming. Warming by a similar amount at all depths not only approximates future climate change scenarios, it also facilitated quantification of the temperature response of the whole profile.

Moreover, Xiao et al. critiqued our soil profile Q10 analysis on the basis of an apparent misunderstanding of data treatment and a lack of clarity on our part regarding mechanisms. In setting up our analysis, we tried many ways of calculating Q10, including curve fitting, before deciding on a comparison between the heated and control plots of each plot pair. This method avoided confounding seasonal effects that can arise when warmer and cooler temperatures from the same site are used to fit a curve. We dropped unrealistically high Q10 values (>30) from our analysis because these values were likely caused by differences in substrate availability and microbial communities among paired samples and were not a response to the warming manipulation. Unlike laboratory incubation experiments that calculate Q10, we could not measure the temperature response of the same soil sample. In most laboratory incubations, either the soil is homogenized, split, and subjected to different temperatures in parallel (6) or the same soil sample is subjected to different temperatures in series (7). In such experiments, the effects of natural spatial heterogeneity in substrate availability and microbial communities are reduced. Furthermore, in contrast to the description provided by Xiao et al., we did not exclude Q10 values greater than 6.4 and less than 30, and we took into account the nonindependence of repeated measures.

Rather than removing data points as Xiao et al. did, we present the Q10 analysis with all data (Fig. 2). Q10 values calculated using all data are still >2 throughout the soil profile, with more extreme variability at the shallowest and deepest depths. Furthermore, although the Q10 results of Xiao et al. differ in magnitude from ours, their analysis shows a pattern similar to the pattern we originally published (2), with a tendency toward stronger Q10 responses in the shallower soil at depths of 0 to 15 cm and 15 to 30 cm.

Fig. 2 The soil profile of mean Q10 (±SE) with values of >30 retained.

All depths have Q10 values of >2, showing that all soil depths are responding to warming.

Putting aside the different ways to calculate Q10, our conclusions are also supported by the CO2 production data. All depths responded to warming with an increase in CO2 production. As stated in the original article, the warming response was greater (on an absolute basis) toward the surface, but it was an unexpected finding that the deeper soils responded at all. Although deeper soils (>30 cm) only contributed 10% of the total warming response, neglecting their contributions, as has been standard in most experiments, has major implications when scaling up soil carbon feedbacks to climate change from the site level to the global scale.

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