Production of dissolved organic carbon and low-molecular weight organic acids in soil solution driven by recent tree photosynthate
ABSTRACT Dissolved organic carbon (DOC) is an important component in the terrestrial carbon cycle. Yet, the relative importance of
different inputs of DOC to the soil solution remains uncertain. Here, we used a large-scale forest girdling experiment to
examine how the supply of recent photosynthate to tree roots and their mycorrhizal fungi affects DOC, in particular low-molecular
weight organic acids (LMWOA). We also studied effects of tree girdling on non-structural carbohydrates in microorganism, and
examined the effects of freezing of soil and the presence of roots in the soil samples on soil solution DOC and LMWOA in this
The concentration of DOC was reduced by 40%, while citrate was reduced by up to 90% in the soil solution by the girdling treatment.
Other LMWOA such as oxalate, succinate, formate and propionate were unaffected by the girdling. We also found that girdling
reduced the concentrations of trehalose (by 50%), a typical fungal sugar, and of monosaccharides (by 40%) in microorganisms
in root-free soil. The effect of freezing on DOC concentrations was marked in samples from control plots, but insignificant
in samples from girdled plots. Release of DOC from cell lysis after freezing was attributed equally to roots and to microorganisms.
Our observations suggest a direct link from tree photosynthesis through roots and their mycorrhizal fungi to soil solution
chemistry. This direct link should impact solute transport and speciation, mineral weathering and C dynamics in the soil compartment.
Importantly, our finding of a substantial photosynthate driven production of DOC challenges the paradigm that DOC is mainly
the result of decomposition of organic matter.
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ABSTRACT: Land use changes such as savannah afforestation with eucalypts impact the soil carbon (C) balance, therefore affecting soil CO2 efflux (F s ), a major flux in the global C cycle. We tested the hypothesis that F s increases with stand age after afforestation, due to an increasing input of fresh organic matter to the forest floor. In a Eucalyptus plantation established on coastal savannahs in Congo, bimonthly measurements of F s were carried out for 1 year on three adjacent stands aged 0.9, 4.4 and 13.7 years and presenting similar growth patterns. Litterfall and litter accumulation on the forest floor were quantified over a chronosequence. Equations were derived to estimate the contribution of litter decomposition to F s throughout the rotation. Litterfall increased with stand age after savannah afforestation. F s , that was strongly correlated on a seasonal basis with soil water content (SWC) in all stands, decreased between ages 0.9 year and 4.4 years due to savannah residue depletion, and increased between ages 4.4 years and 13.7 years, mainly because of an increasing amount of decomposing eucalypt litter. The aboveground litter layer therefore appeared as a major source of CO2, whose contribution to F s in old stands was estimated to be about four times higher than that of the eucalypt-derived soil organic C pool. The high litter contribution to F s in older stands might explain why 13.7 years-old stand F s was limited by moisture all year round whereas SWC did not limit F s for large parts of the year in the youngest stands.Biogeochemistry 01/2012; 111(1-3). · 3.53 Impact Factor
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ABSTRACT: Elevated CO2 generally increases soil C pools. However, greater available C concentrations can potentially stimulate soil organic matter (SOM) decomposition. The effects of climate warming on C storage can also be positive or negative. There is a high degree of uncertainty on the combined effects of climate warming and atmospheric CO2 increase on SOM dynamics and its potential feedbacks to climate change. Semi-arid systems are predicted to show strong ecosystem responses to both factors. Global change factors can have contrasting effects for different SOM pools, thus, to understand the mechanisms underlying the combined effects of multiple factors on soil C storage, effects on individual C pools and their kinetics should be evaluated. We assessed SOM dynamics by conducting long-term laboratory incubations of soils from PHACE (Prairie Heating and CO2 Enrichment experiment), an elevated CO2 and warming field experiment in semi-arid, native northern mixed grass prairie, Wyoming, USA. We measured total C mineralization and estimated the size of the labile pool and the decomposition rates of the labile and resistant SOM pools. To examine the role of plant inputs on SOM dynamics we measured aboveground biomass, root biomass, and soil dissolved organic C (DOC). Greater aboveground productivity under elevated CO2 translated into enlarged pools of readily available C (measured as total mineralized C, labile C pool and DOC). The effects of warming on the labile C only occurred in the first year of warming suggesting a transient effect of the microbial response to increased temperature. Experimental climate change affected the intrinsic decomposability of both the labile and resistant C pools. Positive relationships of the rate of decomposition of the resistant C with aboveground and belowground biomass and dissolved organic C suggested that plant inputs mediated the response by enhancing the degradability of the resistant C. Our results contribute to a growing body of literature suggesting that priming is a ubiquitous phenomenon that should be included in C cycle models.Biogeosciences Discussions 03/2010; 7(2):1575-1602.
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ABSTRACT: Soil low-molecular-weight (LMW) organic acids play important roles in the soil-forming process and the cycling of nutrients in Karst regions. In this study, we quantified the contents of LMW organic acids (including lactate, acetate, formate, malate, and oxalate) in soil solution over the Karst region of Guizhou Province, China using ion chromatography. The concentration of total LMW organic acids in topsoil solution ranged from 0.358 to 1.823 μmol·g−1, with an average of 0.912 μmol·g−1. The mean concentrations of lactate, acetate, formate, malate, and oxalate were 0.212±0.089, 0.302±0.228, 0.301±0.214, 0.014±0.018 and 0.086±0.118 μmol·g−1, respectively. There were also significant difference in the contents of these acids among four phases of rocky desertification, and their concentrations decreased with the aggravation of rocky desertification. The concentrations of the LMW organic acids were significantly positive correlated each other. Significant positive correlations were also observed among individual LMW organic acids in soil solution, and between them and soil available P, available K, exchangeable Ca, respectively. Furthermore, the concentrations of LMW organic acids were significantly positively correlated with inorganic anions (chlorides, nitrates, and sulfates) in Karst topsoil solution. Therefore, the concentrations of soil LMW organic acids might be one of driving force in the Karst rock desertification process in Guizhou Province.Frontiers of Environmental Science & Engineering. 6(2).