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Abstract: In southern Brazil, the landscape restoration after the exhaustion of open cast coal mines involves the filling of mine cavities with both pedogenic and geological material. The objective of this work was to determine the content and chemical composition of the organic matter of two constructed soils (24 years and 2 years) in a coal mining area in southern Brazil. An undisturbed Acrisol and geological material from three sites were also sampled. Samples were analyzed for carbon and nitrogen contents by dry combustion and for chemical composition of the organic matter by 13C NMR CPMAS and FTIR spectroscopies. Prior to the spectroscopic analyses, the samples were treated with 10% (m/m) hydrofluoric acid solution, which lead to a carbon enrichment in the sample of 2–46 times. The three coal samples were mainly composed of aromatic C (46–63%) and alkyl C (10–28%), and differed largely in the carbon content (18–312gkg−1). The C/N ratio of 27 and the proportions of O-alkyl C (26%) and aromatic C (29%) found in the native soil (18gCkg−1 soil) suggest a coal contamination of the native site. The proportions of O/N-alkyl C (15–17%), alkyl C (22–23%) and aromatic C (39–41%) observed in the organic matter of the A1 horizon of the two constructed soils were intermediate to those of the coal samples and the native soil. In the younger constructed soil the proportion of O/N-alkyl diminished and that of aromatic C increased with depth, whereas in the older constructed soil this trend was less evident. Our results show that, during the 24 years after site reconstruction, the input of vegetation residues diluted the proportion of recalcitrant organic matter. The aromaticity index calculated from the FTIR data (I 1620/I 2920) correlated positively with the aryl C/alkyl C ratio, obtained from the NMR data, evidencing the applicability of the FTIR index for geological and coal contaminated samples. [Copyright &y& Elsevier]

The long-term soil management effects on C and N stocks of soil physical fractions are still poorly understood for South American subtropical soils. This study aimed (i) to evaluate the influence of cereal- and legume-based cropping systems and N fertilisation on C and N stocks of the sand-, silt- and clay-size fractions of a no-tilled subtropical Acrisol in southern Brazil, (ii) to compute the Carbon Management Index (CMI) for those cropping systems using physical fractionation data, and (iii) to investigate the possible existence of finite capacity of those soil physical fractions to store C and N. Soil samples of a long-term experiment were collected from the 0-2.5 and 2.5-7.5 cm layers of three no-till cropping systems [fallow bare soil, oat/maize (O/M) and pigeon pea+maize (P+M)] under two N fertilisation levels (0 and 180 kg N ha-1). However, for fallow bare soil, only the non-fertilised sub-plot was sampled. An adjacent native grassland soil was sampled as a reference. The C and N stocks of the three soil physical fractions were higher in the legume-based cropping system (P+M) than in O/M and bare soil, because of the higher residue input in the former. The P+M cropping system restored the C and N stocks in sand- and silt-size fractions to the same levels found in grassland soil. Higher C and N stocks in all physical fractions were also obtained with N fertilisation. The C and N stocks and the C:N ratio were most affected by cropping systems in the sand- and least in the clay-size fraction. Particulate organic matter was found in the silt-size fraction, showing this fraction is not only constituted by mineral-associated organic mater, as commonly believed. Taking grassland soil as reference (CMI = 100), the CMI ranged from 46, in O/M no N, to 517, in P+M with N, pointing to a better soil management in the latter. The clay-size fraction tended to show a finite capacity to store C and N (48.8 g C kg-1 and 4.9 g N kg-1 of clay), which was not verified in sand- and silt-size fractions. The adoption of no-tillage and legume-based cropping systems with high residue input are adequate soil management strategies to improve soil quality and make the agricultural production systems more sustainable in subtropical regions. [ABSTRACT FROM AUTHOR]

Biochar has been shown as a potential mean to enhance carbon sequestration in the soil. In Brazil, approximately 15% of the produced charcoal is discarded as charcoal fines, which are chemically similar to biochar. Therefore, we aimed to test charcoal fines as a strategy to increase soil carbon sequestration. Charcoal fines of hardwood Mimosa scabrella were incorporated into a Cambisol down to 10 cm (T1 = 0 and T4 = 40 Mg ha−1) in Southern Brazil. Soil samples were collected (0–30 cm) 20 months after charcoal amendment. Soil organic matter (SOM) acid extract, humic acid, fulvic acid, and humin fractions were separated. Solid-state 13C nuclear magnetic resonance (NMR) spectra from charcoal and SOM in T1 and T4 were obtained before and after 165 days of incubation under controlled conditions. Charcoal increased soil carbon as fulvic (10–20 cm) and humic acids (10–30 cm) and, especially, as humin (0–5 cm), which probably occurred due to the hydrophobic character of the charcoal. The 13C NMR spectra and mean residence times (MRT) measured from incubation essays indicated that the charred material decomposed relatively fast and MRT of T1 and T4 samples were similar. It follows that the charcoal fines underwent similar decomposition as SOM, despite the high charcoal dose applied to the soil and the high aryl C contribution (78%) to the total 13C intensity of the charcoal NMR spectra. [ABSTRACT FROM AUTHOR]

• Py-GC/MS confirmed stronger Eucalyptus litter recalcitrance compared to Acacia litter. • Eucalyptus modified more C and N stocks and less the chemistry of light SOM pools. • n-alkane molecular proxies in SOM pools sensitively responded to the exotic forests. • Acacia caused stronger chemical shifts in light SOM pools, mainly up to 10 cm depth. • Seven-year cycle of forests did not affect C and N stocks of heavy pool and bulk SOM. In Southern Brazil, exotic species as Acacia (A) and Eucalyptus (E) are often planted over native pasturelands and may change bulk soil organic matter (SOM) composition as verified in our previous study with Cambisols (0–5 cm). Here we aimed to follow the impact of seven-year A and E plantation on the composition of the free light- (FLF), occluded light- (OLF) and heavy fraction (HF) of SOM along the soil profile. We hypothesized that A and E may have shifted the molecular composition and carbon (C) stocks (C s) of SOM fractions, at least at 0–5 cm; with stronger shifts caused by A due to greater E litter recalcitrance. Litter and soil samples (0–20 cm) were collected at A and E and neighboring native pasturelands without A (WA) and without E (WE). Litter, FLF, OLF and HF samples were subjected to C, nitrogen (N), pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) and lipid biomarkers analysis. In E soil, the C s of FLF at 0–5 cm (0.5 Mg ha−1) and OLF at 5–10 cm (1.7 Mg ha−1) were 194 and 70 % greater than in WE, whereas in A soil the C s of OLF at 0–5 cm (0.2 Mg ha−1) was 44 % lower than in WA. Nevertheless, A changed more remarkably the composition of SOM fractions, confirming our hypothesis partially, likely due to greater A litter biodegradability (polysaccharides abundance) compared to E. The contribution of A litter to FLF (0–10 cm) was evidenced by abundance of long chain and the predominance of odd-over-even n-alkanes (particularly >C29), and to OLF (0–20 cm) by the greatest abundance of n-alkanes at C31, resembling A litter. Loss of C and N of OLF in A compared to WA (0–5 cm) was compensated by fresh A litter additions to FLF and OLF and microbial-derived compounds association to soil minerals, equaling soil C s in A and WA. The lower soil N stock in A compared to WA likely resulted from depletion of occluded microbial-derived N-compounds, supposedly reflecting the breakdown of soil aggregates at forest plantation. The increase of Cs in FLF and OLF of E compared to WE soil was associated with increased abundance of aromatics and n-alkane/alkenes and decrease of fatty acids. Similar patterns of n-alkanes observed for OLF of E and WE soil confirmed the incipient contribution of E litter to OLF. Conversion of these pastures to A and E modifies SOM composition and protection, requiring policies in view of the highly invasive potential and possible negative implications of A and E to native pasture regeneration. [ABSTRACT FROM AUTHOR]