Ciencia del Suelo (Dec 2008)
La materia orgánica particulada: comparación de métodos para su determinación y su valor como indicador de calidad de suelos del Chubut Particulate organic-matter: a comparison of methods for its determination and its role as an indicator of soil quality for some soils in the province of Chubut
Abstract
La materia orgánica particulada (MOP) es un componente dinámico del suelo, que responde selectiva y rápidamente a los cambios de manejo y distintos tipos de disturbios. El objetivo de este estudio fue comparar dos técnicas de determinación de MOP en distintos suelos y evaluar el potencial de esta variable como indicadora de calidad de los mismos. Se estudiaron tres sitios ecológicos de la provincia del Chubut con distinto manejo o afectados por distintos disturbios: Media Luna Clausura (ML C), Media Luna Pastoreo (ML P), Puerto Madryn bioperturbado (PM B), Puerto Madryn no bioperturbado (PM NB), Punta Ninfas no erosionado (PN NE) y Punta Ninfas erosionado (PN E). En muestras de suelos extraídas de 0 a 5 cm (PM y PN) y de 0 a 10 cm (ML) de profundidad se determinó la materia orgánica total (MOT) y la MOP por vía húmeda (MOPh) (Método Walkley y Black) y por ignición en mufla (MOPi) a 430 °C. En los Molisoles de ML y en los Aridisoles de PM la MOPi fue mayor que la MOPh, mientras que para los Aridisoles de PN no se registraron diferencias significativas entre los dos métodos. Para los suelos con carbonatos (PM) o con alofanos (ML), el método por ignición sobreestimó los contenidos de MOP. La MOT presentó una alta correlación con la MOPh y con la MOPi para todos los suelos en conjunto (r² = 0,89 y r² = 0,73, respectivamente). El suelo que presentó una mayor relación MOP/MOT fue PN NE. En contraste, el suelo PN E, mostró los valores más bajos para esta relación, lo que indicaría, por un lado, que la poca MO que contienen estos suelos está predominantemente asociada a la fracciónParticulate organic-matter (MOP) is a dynamic soil component that quickly and selectively responds to management changes and to different disturbance regimes. We compared two techniques to determine the MOP content in different soils and assessed the potential of this variable as an indicator of soil quality. We studied the soils of three ecological sites located in the province of Chubut under different management schemes or affected by different disturbance regimes: Media Luna exclosure (ML C), Media Luna-grazing (ML P), Puerto Madryn bioturbed (PM B), Puerto Madryn nonbioturbed (PM NB), Punta Ninfas uneroded (PN NE) and Punta Ninfas eroded (PN E). We collected soil samples from the 0 to 5 cm (PM and PN) or 0 to 10 cm (ML) depths to determine total soil organic-matter (MOT) and MOP using wet combustion MOPh (Walkley and Black method) and weight loss-on-ignition in a high temperature oven set at 430° C (MOPi). In the Mollisols and the Aridisols from Puerto Madryn, MOPi was higher than MOPh; in the Aridisols from Punta Ninfas, however, the differences between MOPi and MOPh were not significant. In soils with high calcium carbonate (PM B and PM NB) or allophane (ML C and ML P) contents, the loss-on-ignition method overestimated MOP contents. The MOT contents were highly correlated to both MOPh and MOPi for all soils (r²= 0.89 and r² = 0.73, respectively). Both MOT and MOP were sensitive to land management and soil erosion. The soil that displayed the greatest MOP/ MOT ratio was the PN NE. On the contrary, the PN E site showed the lowest values for this ratio, which would indicate that the low MOT content of this soil is predominantly associated to the < 0.053 mm fraction, and that the erosion process decreased the MOP reserves more than the MOT. For the Puerto Madryn soils, the MOPi / MOT ratio was greater than 1. This seems to be mainly due to the overestimation of the loss-on-ignition method in soils with abundant carbonates. Both MOT and MOP, determined by the two methods, can be used to detect differences in management or the state of soil degradation. However, MOP seemed to be more affected by soil erosion and sheep grazing compared to MOT. These results would allow us to consider MOP as a good indicator of changes in soil quality by either soil erosion or sheep grazing.
