29 research outputs found
Decadal-scale litter manipulation alters the biochemical and physical character of tropical forest soil carbon
© 2018 Elsevier Ltd Climate change and rising atmospheric carbon dioxide (CO2) concentrations are likely to alter tropical forest net primary productivity (NPP), potentially affecting soil C storage. We examined biochemical and physical changes in soil C fractions in a humid tropical forest where experimental litter manipulation changed total soil C stocks. We hypothesized that: (1.) low-density soil organic C (SOC) fractions are more responsive to altered litter inputs than mineral-associated SOC, because they cycle relatively rapidly. (2.) Any accumulation of mineral-associated SOC with litter addition is relatively stable (i.e. low leaching potential). (3.) Certain biomolecules, such as waxes (alkyl) and proteins (N-alkyl), form more stable mineral-associations than other biomolecules in strongly weathered soils. A decade of litter addition and removal affected bulk soil C content in the upper 5 cm by +32% and â31%, respectively. Most notably, C concentration in the mineral-associated SOC fraction was greater in litter addition plots relative to controls by 18% and 28% in the dry and wet seasons, respectively, accounting for the majority of greater bulk soil C stock. Radiocarbon and leaching analyses demonstrated that the greater mineral-associated SOC in litter addition plots consisted of new and relatively stable C, with only 3% of mineral-associated SOC leachable in salt solution. Solid-state13C NMR spectroscopy indicated that waxes (alkyl C) and microbial biomass compounds (O-alkyl and N-alkyl C) in mineral-associated SOC are relatively stable, whereas plant-derived compounds (aromatic and phenolic C) are lost from mineral associations on decadal timescales. We conclude that changes in tropical forest NPP will alter the quantity, biochemistry, and stability of C stored in strongly weathered tropical soils
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Combined effects of rhizodeposit C and crop residues on SOM priming, residue mineralization and N supply in soil
Fluxes of rhizodeposit carbon (C) to soil stimulate microbial activity affecting soil organic matter (SOM)decomposition and, in turn, nutrientfluxes in soil. In agricultural soils, residues from previous crops alsohave major impacts on SOM and nutrient cycling, and their turnover by microbes is likely to be indirectlyimpacted by rhizodeposition. However, the combined effects of rhizodeposit C and inputs of C from deadplant materials in soil on native SOM decomposition are unclear. In this study, we assessed (i) the in-dividual and combined effects of barley rhizodeposition and ryegrass root residue inputs (as a model forresidue input from previous crop) on SOM mineralization, (ii) the intraspecies variation within barley inimpacting residue mineralization, and (iii) whether genotypes that stimulate high mineralization rates ofplant residues in soil also directly benefit through increased nutrient uptake from these residues. Wecontinuously applied13C depleted CO2to selected barley recombinant chromosome substitution lines(RCSLs) to trace theflow of barley root-derived C in surface soil CO2efflux, soil microbial biomass and soilparticle-size fractions. In addition,13C and15N enriched ryegrass root residues were mixed into soil totrace the mineralization of residue-derived C and the residue-derived nitrogen (N) uptake by plants. Ourresults show (i) genotype-specific variation in impacting total soil CO2efflux and its component sources:SOM-derived C, barley root-derived C and/or ryegrass residue-derived C, (ii) residue effects on total C andSOM-derived C respired as CO2, (iii) genotype-residue combined effects on SOM primed C, that were verysimilar to the sum of primed C caused by planting or residue addition alone (except for the last samplingdate), and (iv) that plant uptake of residue released N between genotypes was linked to genotype im-pacts on residue mineralization. These results suggest that impacts of plant rhizodeposition and residueinputs had additive effects on SOM priming. Furthermore, these results demonstrate, for thefirst time,genotype differences in impacting the mineralization of recent plant-derived organic materials in soil,and reveal that this process directly contributes to plant nutrition
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The development of soil organic matter in restored biodiverse Jarrah forests of South-Western Australia as determined by ASE and GCMS
Background, aim and scope
Soil organic matter (SOM) is known to increase with time as landscapes recover after a major disturbance; however, little is known about the evolution of the chemistry of SOM in reconstructed ecosystems. In this study, we assessed the development of SOM chemistry in a chronosequence (space for time substitution) of restored Jarrah forest sites in Western Australia.
Materials and methods
Replicated samples were taken at the surface of the mineral soil as well as deeper in the profile at sites of 1, 3, 6, 9, 12, and 17 years of age. A molecular approach was developed to distinguish and quantify numerous individual compounds in SOM. This used accelerated solvent extraction in conjunction with gas chromatography mass spectrometry. A novel multivariate statistical approach was used to assess changes in accelerated solvent extraction (ASE)-gas chromatography-mass spectrometry (GCMS) spectra. This enabled us to track SOM developmental trajectories with restoration time.
Results
Results showed total carbon concentrations approached that of native forests soils by 17 years of restoration. Using the relate protocol in PRIMER, we demonstrated an overall linear relationship with site age at both depths, indicating that changes in SOM chemistry were occurring.
Conclusions
The surface soils were seen to approach native molecular compositions while the deeper soil retained a more stable chemical signature, suggesting litter from the developing diverse plant community has altered SOM near the surface. Our new approach for assessing SOM development, combining ASE-GCMS with illuminating multivariate statistical analysis, holds great promise to more fully develop ASE for the characterisation of SOM