Abstract Fourier transform infrared spectroscopy (FTIR) assists in investigating functional groups of soil organic matter (SOM). However, the use of this tool is impaired given the low organic carbon levels and high content of oxides in tropical soils, resulting in low quality spectra and in turn requiring the use of hydrofluoric acid (HF). The objective of this study was to verify the efficiency of using HF in removing the mineral fraction and to increase the C concentration to enable visualizing the bands related to SOM in infrared spectra in soil samples under forest system. The HF treatment was efficient in removing mineral components and proportionally increasing C. The FTIR with HF enabled identifying differences between coarse and fine fractions. The spectra of the HF samples showed that the coarse fractions presented bands related to aromatic material and the fine fractions presented more labile components, with the absence of more recalcitrant components.

RESUMEN En los pastos es común utilizar el fuego para controlar las malas hierbas justificado por el incremento de fertilidad en el suelo que pueden generar las cenizas. Sin embargo, los beneficios de este proceso son de corta duración, además de aumentar la exposición del suelo. La permanencia de las malezas puede contribuir a la cobertura vegetal en áreas de pastos deficientes. Este estudio tuvo como objetivo evaluar el efecto de la quema prescripta y corte de malezas en la cobertura vegetal de los pastos de U. brizantha y en la fertilidad de la superficie del suelo en pastos ubicados en la Amazonía brasileña. El estudio se realizó en Itupiranga, estado de Pará, Amazonía brasileña. Cinco pasturas de U. brizantha cv. Marandu y un área de bosque fueron evaluados. El suelo era un “Latosol Rojo-Amarillo" (Ferralsols). La cobertura vegetal se estimó utilizando un cuadrante. Se tomaron cuatro muestras de suelo compuestas en cada área, recolectadas de 0-0,05 m de profundidad. Se determinó el porcentaje de cobertura vegetal de U. brizantha, malezas y suelo expuesto y la fertilidad en muestras de suelo (pH, Al3+, Ca2+, Mg2+, K+, Na+, P y H+Al). Se identificaron tres grupos entre las parcelas de pastos evaluadas. El grupo I tuvo el porcentaje promedio más alto de suelo expuesto (33,5%) y se asoció con los valores de H+Al y P. El grupo II tuvo el promedio más alto de cobertura de U. brizantha (90%) y se asoció con Ca2+ + Mg2+ y K+. El grupo III tuvo el promedio más alto de cobertura de hierbas (53,4%) y se asoció con Al3+. El mayor porcentaje de U. brizantha en el grupo II se vio favorecido por la quema posterior. Este grupo presentó mejores niveles de K+, Mg2+ y suma de bases. El manejo adoptando el menor uso del fuego en los pastos, con un mayor tiempo de regeneración de forrajes, combinado con el control mecánico de malezas, puede ser la mejor alternativa para mantener la cobertura del suelo, trayendo beneficios a la calidad química de la capa superficial del suelo.

ABSTRACT In pastures it is common to use fire to control weeds, justified by the increase in soil fertility that ashes can generate. However, the benefits of this process is short-lived, and also increase the soil exposure. The permanence of weeds can contribute to the vegetation cover in areas of deficient pastures. This study aimed to evaluate the effect of prescribed burning and mowing on vegetation cover of Urochloa brizantha (U. brizantha) pastures and soil surface fertility in pastures located in the Brazilian Amazon. The study was conducted in Itupiranga, Pará state, Brazilian Amazon. Five pastures of U. brizantha cv. Marandu and a forest area were evaluated. The soil was a Ferralsol. Vegetation cover was estimated using a quadrat. Four composite soil samples were taken in each area, collected from 0-0.05 m depth. The percentage of vegetal cover of U. brizantha, weeds and bare soil was measured and fertility in soil samples (pH, Al3+, Ca2+, Mg2+, K+, Na+, P, and H+Al) were determined. Three groups were identified in the evaluated pasture plots. Group I had the highest average percentage of bare soil (33.5%) and was associated with H+Al and P values. Group II had the highest average of U. brizantha cover (90%) and was associated with Ca2++Mg2+ and K+. Group III had the highest average of weed cover (53.4%) and was associated with Al3+. The higher percentage of U. brizantha in Group II was favored by the burning later. This group presented better levels of K+, Mg2+ and sum of bases. The management adopting the least use of fire on the pastures, with longer time for the regeneration of forage, combined with the mechanical control of weeds, can be the best alternative for maintaining the soil cover and bringing benefits for quality surface layer of the soil.

Resumo O objetivo deste trabalho foi selecionar os indicadores de qualidade de solo mais relacionados à degradação de pastagens, e estabelecer os limites de diferentes níveis de degradação a partir desses indicadores. Cinco pastagens sobre Latossolo Vermelho-Amarelo foram selecionadas, em terço médio da encosta, com diferentes níveis visuais de degradação, tendo-se considerado as variáveis: perda de vigor, presença de espécies espontâneas, solo exposto e erosão. Avaliou-se a cobertura do solo, e coletaram-se amostras nas profundidades de 0,00-0,05, 0,05-0,10 e 0,10-0,20 m, para as análises físicas, químicas e de matéria orgânica. Diferentemente da classificação visual, a análise de agrupamento das variáveis de cobertura indicou apenas dois níveis de degradação: de baixo a moderado e de alto a muito alto, com percentual de classificações corretas acima de 87% na análise discriminante, em que o solo exposto foi a variável mais assertiva para discriminar os níveis de degradação. A matéria orgânica leve e o componente Ca+Mg foram os indicadores de qualidade mais relacionados aos índices de degradação de pastagens à profundidade de 0,00-0,05 m; enquanto o teor de potássio foi o indicador mais relacionado aos índices às profundidades de 0,05-0,10 e 0,10-0,20 m.

Abstract The objective of this work was to select the soil quality indicators more related to pasture degradation and to establish the limits of the different degradation levels using these indicators. Five pasture areas on an Oxisol were selected in the medium third of the slope, with different levels of visual degradation, considering the following variables: loss of vigor, presence of weeds, exposed bare soil, and erosion. Soil cover was evaluated, and soil samples were collected at the 0.00-0.05, 0.05-0.10, and 0.10-0.20-m soil depths, for physical, chemical, and organic matter analyses. Differently from the visual classification, the cluster analysis of the cover variables only indicated two degradation levels: low to moderate and high to very high, with classification percentage above 87% in the discriminant analysis, in which bare soil was the most assertive variable for discriminating the degradation levels. Light organic matter and the Ca+Mg component were the indicators most related to pasture degradation indices at 0.00-0.05 m; while potassium content was the indicator most related to those indices at the 0.05-0.10 and 0.10-0.20-m soil depths.