Divergences of soil carbon turnover and regulation in alpine steppes and meadows on the Tibetan Plateau

Soil carbon isotopes (&#948;13C) provide reliable insights at the long-term scale for the study of soil carbon turnover and topsoil &#948;13C could well reflect organic matter input from the current vegetation. Qinghai-Tibet Plateau (QTP) is called &#8220;the third pole of the earth&#8221; because of its high elevation, and it is one of the most sensitive and critical regions to global climate change worldwide. Previous studies focused on variability of soil &#948;13C at in-site scale. However, a knowledge gap still exists in the spatial pattern of topsoil &#948;13C in QTP. In this study, we first established a database of topsoil &#948;13C with 396 observations from published literature and applied a Random Forest (RF) algorithm (a machine learning approach) to predict the spatial pattern of topsoil &#948;13C using environmental variables. Results showed that topsoil &#948;13C significantly varied across different ecosystem types (p < 0.05).&#160; Topsoil &#948;13C was -26.3 &#177; 1.60 &#8240; for forest, 24.3 &#177; 2.00 &#8240; for shrubland, -23.9 &#177; 1.84 &#8240; for grassland, -18.9 &#177; 2.37 &#8240; for desert, respectively. RF could well predict the spatial variability of topsoil &#948;13C with a model efficiency (pseudo R2) of 0.65 and root mean square error of 1.42. The gridded product of topsoil &#948;13C and topsoil &#946; (indicating the decomposition rate of soil organic carbon, calculated by &#948;13C divided by logarithmically converted SOC) with a spatial resolution of 1000 m were developed. Strong spatial variability of topsoil &#948;13C was observed, which increased gradually from the southeast to the northwest in QTP. Furthermore, a large variation was found in &#946;, ranging from -7.87 to -81.8, with a decreasing trend from southeast to northwest, indicating that carbon turnover rate was faster in northwest QTP compared to that of southeast. This study was the first attempt to develop a fine resolution product of topsoil &#948;13C for QTP using a machine learning approach, which could provide an independent benchmark for biogeochemical models to study soil carbon turnover and terrestrial carbon-climate feedbacks under ongoing climate change.

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A dual isotope approach to isolate soil carbon pools of different turnover times

Biogeosciences ◽

10.5194/bg-10-8067-2013 ◽

2013 ◽

Vol 10 (12) ◽

pp. 8067-8081 ◽

Cited By ~ 32

Author(s):

M. S. Torn ◽

M. Kleber ◽

E. S. Zavaleta ◽

B. Zhu ◽

C. B. Field ◽

...

Keyword(s):

Environmental Change ◽

Soil Carbon ◽

Carbon Storage ◽

Co2 Efflux ◽

Carbon Turnover ◽

Soil Carbon Storage ◽

Density Fraction ◽

Dual Isotope ◽

Slow Cycling ◽

Soil Carbon Turnover

Abstract. Soils are globally significant sources and sinks of atmospheric CO2. Increasing the resolution of soil carbon turnover estimates is important for predicting the response of soil carbon cycling to environmental change. We show that soil carbon turnover times can be more finely resolved using a dual isotope label like the one provided by elevated CO2 experiments that use fossil CO2. We modeled each soil physical fraction as two pools with different turnover times using the atmospheric 14C bomb spike in combination with the label in 14C and 13C provided by an elevated CO2 experiment in a California annual grassland. In sandstone and serpentine soils, the light fraction carbon was 21–54% fast cycling with 2–9 yr turnover, and 36–79% slow cycling with turnover slower than 100 yr. This validates model treatment of the light fraction as active and intermediate cycling carbon. The dense, mineral-associated fraction also had a very dynamic component, consisting of ∼7% fast-cycling carbon and ∼93% very slow cycling carbon. Similarly, half the microbial biomass carbon in the sandstone soil was more than 5 yr old, and 40% of the carbon respired by microbes had been fixed more than 5 yr ago. Resolving each density fraction into two pools revealed that only a small component of total soil carbon is responsible for most CO2 efflux from these soils. In the sandstone soil, 11% of soil carbon contributes more than 90% of the annual CO2 efflux. The fact that soil physical fractions, designed to isolate organic material of roughly homogeneous physico-chemical state, contain material of dramatically different turnover times is consistent with recent observations of rapid isotope incorporation into seemingly stable fractions and with emerging evidence for hot spots or micro-site variation of decomposition within the soil matrix. Predictions of soil carbon storage using a turnover time estimated with the assumption of a single pool per density fraction would greatly overestimate the near-term response to changes in productivity or decomposition rates. Therefore, these results suggest a slower initial change in soil carbon storage due to environmental change than has been assumed by simpler (one-pool) mass balance calculations.

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Climate exerts stronger control on topsoil carbon persistence than plant input in alpine grasslands

10.22541/au.162610069.98503366/v1 ◽

2021 ◽

Author(s):

Donghai Wu ◽

Xiangtao Xu ◽

Haicheng Zhang

Keyword(s):

Soil Carbon ◽

Direct Effect ◽

Carbon Turnover ◽

Alpine Grasslands ◽

Soil Carbon Turnover

Chen et al. (2021) concluded that plant input governs topsoil carbon persistence in alpine grasslands. We demonstrated that the excluded direct effect of precipitation on topsoil Δ14C in their analysis was in fact significant and strong. Our results provide an alternative viewpoint on the drivers of soil carbon turnover.

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