Aggregate Density Fractionation Research Articles

Chemical Composition of Organic Carbon in Aggregate Density Fractions Under Cacao Agroforestry Systems in South Bahia, Brazil

Chemical Composition of Organic Carbon in Aggregate Density Fractions Under Cacao Agroforestry Systems in South Bahia, Brazil

Effects of substitution of chemical fertilizer by Chinese milk vetch on distribution and composition of aggregates-associated organic carbon fractions in paddy soils

PurposeGreen manure plays a key role in reducing chemical fertilizer applications and increasing soil organic carbon (SOC) stock. The effects of chemical fertilizer substitution by Chinese milk vetch (MV) on the distribution and composition of organic carbon fractions in macroaggregates and microaggregates and SOC stability mechanism were investigated in paddy soils in southern China.MethodsA 10-year (2008–2018) field experiment was conducted, including no fertilizer (CK), 100% NPK fertilizer (F100), MV with different percentages of chemical fertilizer (MV + F100, MV + F80, MV + F60 and MV + F40). The soil was separated into distinct organic carbon fractions using aggregate density fractionation and SOC chemical structure was analyzed by fourier-transform infrared and nuclear magnetic resonance.ResultsChemical fertilizer substitution by MV increased SOC contents in the bulk soil by 9.1% (MV + F80), 5.8% (MV + F60), 17.9% (MV + F40) compared to F100. Organic carbon fraction mainly existed in mineral associated organic carbon (mSOC), accounting for 70.3–83.7% and 69.4–84.0% of the relative mass of macroaggregates and microaggregates, respectively. Compared to F100, aromatic C increased by 11.6% and 29.1% under MV + F60 within mSOC in macroaggregates and microaggregates. Within the mSOC in macroaggregates, compared to CK, MV + F80 and MV + F60 promoted the proportion of alkyl C by 9.6% and 6.7%, and decreased the content of O-alkyl C by 14.1% and 11.0%, respectively, which correspondingly increased alkyl C/O-alkyl C ratio.ConclusionsSubstitution of chemical fertilizer by MV (especially MV + F80 and MV + F60) improved the stability of SOC via increasing recalcitrant structure in mSOC, and was conducive to the sequestration of SOC in paddy soils.

N, P and straw return influence the accrual of organic carbon fractions and microbial traits in a Mollisol

Soil organic carbon (SOC) is the key to soil fertility and quality. However, SOC has severely declined in the past several decades due in part to inappropriate nitrogen (N) and phosphorus (P) input levels. Thus far, the influences of N and P input levels on the accrual of different SOC fractions and relevant microbial traits at aggregate scales remain uncertain. Here, a 3-year field experiment was used to reveal the effects of N and P input levels on SOC fractions in a Mollisol. Treatments included straw return (SR) and combinations of high nitrogen (N) and high phosphorus (P) inputs (N + P + SR), high N and low P inputs (N + P1/2 + SR), and low N and high P inputs (N1/2 + P + SR). Macroaggregates (>0.25 mm), microaggregates (0.25–0.053 mm), and clay and silt (<0.053 mm) were separated, and then particulate organic carbon (POC) and mineral-associated organic carbon (MOC) were determined after aggregate density fractionation. C-cycling genes (cbhI, GH48 and cbbL genes) were quantified by quantitative PCR, and aggregate bacterial community compositions were analyzed by amplicon sequencing of the 16S rRNA genes. The results showed that the free POC (fPOC) content increased after SR, and SR with N input (irrespective of N level) increased bacterial C-cycling gene abundance (GH48 and cbbL genes) and MOC content; however, only combinations with high N inputs increased the fungal cbhI gene abundance and occluded POC (oPOC) content, while P input levels had little impact on SOC fractions. Structural equation modelling revealed that pH was a determinant of GH48 and cbbL gene abundances, which were positively associated with MOC, while N:P was positively related to cbhI gene abundance and oPOC. Network analysis revealed that SOC fractions increased with Nitrosomonadaceae, Xanthobacteraceae, Gaiellaceae and Subgroup_6 and decreased with Gemmataceae and Betaproteobacteriales, suggesting a functional role in SOC sequestration. Our study suggests that moderate high N input and reduced P input can promote the accrual of SOC fractions in a Mollisol with straw return.

Density fractions versus size separates: does physical fractionation isolate functional soil compartments?

Abstract. Physical fractionation is a widely used methodology to study soil organic matter (SOM) dynamics, but concerns have been raised that the available fractionation methods do not well describe functional SOM pools. In this study we explore whether physical fractionation techniques isolate soil compartments in a meaningful and functionally relevant way for the investigation of litter-derived nitrogen dynamics at the decadal timescale. We do so by performing aggregate density fractionation (ADF) and particle size-density fractionation (PSDF) on mineral soil samples from two European beech forests a decade after application of 15N labelled litter. Both density and size-based fractionation methods suggested that litter-derived nitrogen became increasingly associated with the mineral phase as decomposition progressed, within aggregates and onto mineral surfaces. However, scientists investigating specific aspects of litter-derived nitrogen dynamics are pointed towards ADF when adsorption and aggregation processes are of interest, whereas PSDF is the superior tool to research the fate of particulate organic matter (POM). Some methodological caveats were observed mainly for the PSDF procedure, the most important one being that fine fractions isolated after sonication can not be linked to any defined decomposition pathway or protective mechanism. This also implies that historical assumptions about the "adsorbed" state of carbon associated with fine fractions need to be re-evaluated. Finally, this work demonstrates that establishing a comprehensive picture of whole soil OM dynamics requires a combination of both methodologies and we offer a suggestion for an efficient combination of the density and size-based approaches.

Origin of the enriched labile fraction (ELF) in Russian Chernozems with different site history

It was assumed that protected soil organic matter (SOM) inside small macroaggregates is indicated by the enriched labile fraction (ELF) (2.07–2.22 g cm −3 of aggregated fine silt). The ELF should be of fungal origin and sensitively reflect management effects on SOM. To test these hypotheses, we have isolated ELF from mollic epipedons (0–10 cm) of Chernozems under native steppe, meadow, forest, and long-term arable land. The ELF was characterized by C, N, amino-sugar and 13C-CP/MAS-NMR measurements. The results showed that the glucosamine-to-muramic acid ratio was higher in ELF than in other aggregate-density fractions. This indicated that microbial residues in the ELF, stronger than in other fractions, are of fungal origin. The remaining SOM composition as identified by 13C-CP/MAS-NMR spectroscopy was not different from that of the other density fractions. The ELF was therefore enriched in fungal debris but it did not solely consist of microbial N. Nevertheless, ELF seemed to reflect land-use effects on SOM. The C and N loss from ELF due to arable land use or afforestation resulted in a relative preservation of its bacterial-derived muramic acid relative to glucosamine, however, due to the lack of independent replicates such effects could not be statistically evaluated.