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Institute of Soil and Water Conservation made new progress in global soil respiration research

Update time:2022-06-20

2020-04-01, Juying Jiao’s research team at the Institute of Soil and Water Conservation, in collaboration with Pacific Northwest National Laboratory and Stanford University, published a paper entitled “Historically inconsistent productivity and respiration fluxes in the global terrestrial carbon cycle” in Nature Communications. Professor Jinshi Jian was the first author and corresponding author of the paper, and the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University was the first affiliation. This research was supported by the second Tibetan Plateau Scienti？c Expedition and Research Program (STEP) (2019QZKK0603), the Strategic Priority Research 482 Program of the Chinese Academy of Sciences (XDA20040202).

Terrestrial GPP is the photosynthetic gain of C by plants; soil respiration, the soil-to-atmosphere CO2 flux, the sum of root respiration and heterotrophic respiration as measured at the soil surface, and represents carbon fixed by plants at some point in the past. While GPP and R_S may diverge significantly at local scales and for short time periods, they should however be coupled to a degree consistent with our understanding of the C cycle. Plant autotrophic respiration (including leaf and stem respiration, R_shoot, and root respiration, R_root) consumes part of GPP, and the remainder is termed net primary productivity (NPP). Parts of NPP are consumed by heterotrophs (R_H) and herbivores (C_herb), burned in fires (C_fire), exported as dissolved organic carbon (DOC), or returned to the atmosphere by plants’ biogenic volatile organic compound emissions (BVOC). The remainder comprises long-term carbon storage–the terrestrial carbon sink (C_sink). Theoretically, if we know how GPP is partitioned at each of these steps, we can produce an estimate of the RS implied by a GPP value (here termed Rs_GPP) at site or global scales; a similar process can be used to derive GPP from R_S. For the first time, this study explored the consistency of global gross primary productivity (GPP) and soil respiration (R_S) estimates in the global terrestrial carbon (C) cycle.

When we estimate GPP based on R_S measurements and some assumptions about R_S:GPP ratios, we found the resulted global GPP values (bootstrap mean 149⁺²⁹_-23 Pg C yr^-1) are significantly higher than most GPP estimates reported in the literature (113⁺¹⁸_-18Pg C yr^-1). Similarly, historical GPP estimates imply a soil respiration flux (Rs_GPP, bootstrap mean of 68⁺¹⁰_-8Pg C yr^-1) statistically inconsistent with most published R_S values (87⁺⁹_-8 Pg C yr^-1) (Fig. 1), although recent, higher, GPP estimates are narrowing this gap. This discrepancy has implications for our understanding of carbon turnover times and the terrestrial sensitivity to climate change. Future efforts should reconcile the discrepancies associated with calculations for GPP and Rs to improve estimates of the global carbon budget.

We used R_S data from a recently updated global daily R_S database (DGRsD) to parameterize Random Forest (RF) models for each month, and estimated global monthly R_S at a spatial resolution of 0.1°. Such daily data can provide more robust estimates than do annual numbers used until now to estimate global-scale R_S. The resulting global annual R_S was 93 Pg C yr^-1, with a corresponding GPP_Rs of 157 Pg C yr^-1 (Fig. 2), close to the meanRs_lit (87⁺⁹_-8Pg C yr^-1) and GPP_Rs (149⁺²⁹_-23 Pg C yr^-1). This also suggests that higher GPP is a possible explanation for any discrepancy between GPP_lit and Rs_lit, but it should be noted that DGRsD is not independent from the global annual soil respiration database, and therefore more evidence is needed to ensure there are no systematic biases in Rs_lit.

Terrestrial plants sequester carbon through photosynthesis, and that carbon is eventually returned to the atmosphere through respiration by plants and soil microbes. Here we show a large, unexpected gap in estimations of these two carbon fluxes, the results thus reinforce the importance of cross-comparison of data from different sources for understanding the terrestrial carbon cycle.

Fig.1 Distribution and comparison of annual global soil respiration (R_S) and gross primary productivity (GPP). a Distributions of global gross primary productivity (GPP_lit and GPP_Rs); b Joint distribution of annual global soil respiration (R_S) and gross primary productivity (GPP); c Distribution of global soil respiration (R_slit and Rs_GPP) estimates.

Fig.2 Spatial distribution of soil respiration (R_S) sites and predicted global R_S. (a) Spatial distribution of sites used in the Random Forest model; (b) Global spatial distribution of R_S predicted by the Random Forest model, with spatial resolution of 0.1°latitude × 0.1°longitude.