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Fluorescence spectroscopy relies on the fluorescence emitted by rigid conjugated systems and thus can be used to assess the soil organic matter (SOM) humification. This technique is generally applied to solution samples of humic substances, and so far no information exists about its applicability to whole untreated soil samples. The laser‐induced fluorescence (LIF) spectroscopy is proposed as a novel technique to assess the organic matter humification in whole soil samples. We sampled the 0‐ to 2.5‐, 2.5‐ to 5‐, 5‐ to 10‐, 10‐ to 15‐, and 15‐ to 20‐cm layers of three Oxisols of long‐term experiments located in two sites of the Brazilian Cerrado. The humification index based on LIF spectroscopy (HLIF) of whole soil samples showed a close correlation with the humification indexes A4/A1, I465/I399, and A465 obtained after fluorescence spectroscopy analysis of the dissolved humic acids. The HLIF in soils under native cerrado or subjected to no‐tillage increased from the top to the deepest layer, which is consistent with the deposition of labile organic matter from plant residues on the soil surface. The soils subjected to conventional tillage, however, showed relatively constant HLIF along the profile, possibly because homogenization imparted by disturbance of the arable layer. Accordingly, for the two top layers, the soils under no‐tillage showed a lower HLIF than for conventionally tilled soils. Laser‐induced fluorescence spectroscopy is a promising technique to assess humification in whole soil samples, particularly in Oxisols, which due to high concentration of Fe3+ are not feasible to electron spin resonance (ESR) and Carbon‐13 nuclear magnetic resonance (13C NMR) spectroscopy, unless previous treatment is employed.

Resumo -A reserva de carbono na matéria orgânica (MO) do solo é uma importante estratégia para atenuar a concentração de dióxido de carbono na atmosfera. O objetivo deste trabalho foi avaliar o efeito do plantio direto (PD), durante seis anos, sob quatro sistemas de cultura de outono na sucessão comercial soja-milho (guanduanão-milheto, crotalária-sorgo, girassol-aveia-preta e nabo forrageiro-milho), no armazenamento de carbono nas frações particulada (>53 µm) e associada aos minerais (<53 µm) da MO de um Latossolo Vermelho argiloso da Região do Cerrado. Em comparação ao preparo convencional (PC) com cultivo de culturas estivais, o PD aumentou os estoques de carbono orgânico total (COT) na camada superficial do solo (0-5 cm). Na camada de 0-20 cm, o solo sob nabo forrageiro-milho manteve um estoque de COT superior em 9% (4,66 Mg ha -1 ) ao do solo em PC (54,30 Mg ha -1 ). Nos demais sistemas de culturas, o estoque de COT nessa camada não diferiu do solo em PC. O estoque de C na MO particulada foi mais sensível ao manejo do solo do que o COT, com um incremento entre 37% e 52% sob PD, em comparação ao PC (0-20 cm). O estoque de C na MO associada aos minerais não foi afetado pelos sistemas de manejo, o que pode estar relacionado ao curto período sob PD e à microagregação altamente estável. O armazenamento de C preferencialmente na fração lábil da MO do solo representa um benefício ambiental cuja manutenção depende fundamentalmente da continuidade do plantio direto e do contínuo aporte de resíduos vegetais.Termos para indexação: solos tropicais, efeito estufa, agricultura conservacionista, manejo do solo. Carbon storage in labile fractions of soil organic matter in a tropical no-tillage OxisolAbstract -Carbon storage in the soil organic matter (SOM) is an important strategy to mitigate carbon dioxide concentration in the atmosphere. The objective of this study was to evaluate the effect of the no-tillage (NT) use for six years, under four autumn (dry-season) cover crops based maize and soybean crop rotations (pigeon peapearl millet, bengan hemp-sorghum, sunflower-black oat, and wild radish-maize), on C storage in the particulate (>53 µm) and mineral-associated (<53 µm) SOM fractions of a clayey Oxisol from Brazilian Cerrado region. In comparison to conventional tillage (CT) under summer cash-crops, NT increased the total organic carbon (TOC) stocks in the surface soil layer (0-5 cm). In the top 20 cm layer, NT soil under wild radish-maize had 9% (4.66 Mg ha -1 ) more TOC than the conventionally tilled soil. On the other hand, C storage in NT soil under other cropping systems was statiscally similar to the CT soil. The C stocks in the particulate SOM fraction increased by 37 to 52% in NT soil (0-20 cm) in comparison to CT. The higher sensitivity to soil management changes made the particulate organic matter a more adequate soil quality index compared to the TOC stocks. The C stock in the mineral-associated SOM fraction was not affected by soil management systems, which can be related to the short-term under NT and or...