Fine Root and Soil Organic Carbon Depth Distributions are Inversely Related Across Fertility and Rainfall Gradients in L
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Fine Root and Soil Organic Carbon Depth Distributions are Inversely Related Across Fertility and Rainfall Gradients in Lowland Tropical Forests Daniela F. Cusack1,2*
and Benjamin L. Turner2
1
Department of Ecosystem Science and Sustainability, Colorado State University, Campus Delivery 1476, Fort Collins, Colorado 80523, USA; 2Smithsonian Tropical Research Institute, Apartado, 0843-03092 Balboa, Ancon, Republic of Panama
ABSTRACT Humid tropical forests contain some of the largest soil organic carbon (SOC) stocks on Earth. Much of this SOC is in subsoil, yet variation in the distribution of SOC through the soil profile remains poorly characterized across tropical forests. We used a correlative approach to quantify relationships among depth distributions of SOC, fine root biomass, nutrients and texture to 1 m depths across 43 lowland tropical forests in Panama. The sites span rainfall and soil fertility gradients, and these are largely uncorrelated for these sites. We used fitted b parameters to characterize depth distributions, where b is a numerical index based on an asymptotic relationship, such that larger b values indicate greater concentrations of root biomass or SOC at depth in the profile. Root b values ranged from 0.82 to 0.95 and were best predicted by soil pH and extractable potassium (K) stocks. For
Received 16 May 2020; accepted 12 September 2020
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s10021-020-00569-6) contains supplementary material, which is available to authorized users. Daniela F. Cusack conceived of and designed the study, performed research, analyzed data, and wrote the paper; Benjamin L. Turner helped design the study, performed research, and helped write the paper. *Corresponding author; e-mail: [email protected]
example, the three most acidic (pH < 4) and Kpoor (< 20 g K m-2) soils contained 76 ± 5% of fine root biomass from 0 to 10 cm depth, while the three least acidic (pH > 6.0) and most K-rich (> 50 g K m-2) soils contained only 41 ± 9% of fine root biomass at this depth. Root b and SOC b values were inversely related, such that a large fine root biomass in surface soils corresponded to large SOC stocks in subsoils (50–100 cm). SOC b values were best predicted by soil pH and base cation stocks, with the three most base-poor soils containing 34 ± 8% of SOC from 50 to 100 cm depth, and the three most base-rich soils containing just 9 ± 2% of SOC at this depth. Nutrient depth distributions were not related to Root b or SOC b values. These data show that large surface root biomass stocks are associated with large subsoil C stocks in strongly weathered tropical soils. Further studies are required to evaluate why this occurs, and whether changes in surface root biomass, as may occur with global change, could in turn influence SOC storage in tropical forest subsoils. Key words: Panama; Base cations; Potassium; Soil pH; Dissolved organic carbon (DOC); Root depth distribution.
D. F. Cusack and B. L. Turner
HIGHLIGHTS Root
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