For a given soil bulk density increases as soil organic matter decreases.  When an area is  deforested soil organic matter tends to decline as decomposition is more rapid than when well vegetated with forests.  Also, the addition of soil organic matter is lower when the site is not vegetated. So, for any given soil, bulk density would be greater under deforested than forested conditions.  

For a given soil bulk density increases as soil organic matter decreases.  When an area is  deforested soil organic matter tends to decline as decomposition is more rapid than when well vegetated with forests.  Also, the addition of soil organic matter is lower when the site is not vegetated. So, for any given soil, bulk density would be greater under deforested than forested conditions.  

Es correcto lo que dice Marianne K. Burke, no obstante, debería tenerse en cuenta algunos aspectos adicionales. Cuando se produce la deforestación, generalmente, se realiza con maquinaria muy pesada. Estas máquinas, en su segunda pasada por un mismo sitio densifica los horizontes subsuperficiales del suelo, situación que se vuelve irreversible en los trópicos por gran cantidad de años, independientemente que luego se desarrolle la vegetación o bien se deje sin ella. 

Por otro lado, si bien es correcto que mayor cantidad de materia orgánica disminuye la densificación del suelo, es probable que el año siguiente a la deforestación la MO del suelo sea mayor. Porqué? Porque el material vegetal no aprovechable como madera (ramas y hojas) queda en el sitio y, al ingresar mayor energía lumínica por falta del dosel arbóreo, se aceleran los procesos de descomposición, cuyo porcentaje de MO en los años sucesivos, será MAYOR. No obstante, si la tarea se realizó con maquinaria, encontraras en el sitio MAYORES valores de densidad aparente.

Dear Dr. Kinyanjui,

Soil bulk density is influenced by factors such as organic matter content, management practices, texture, soil fauna, etc. As Mrs. Burke said, an increase in the organic matter content will cause a decrease in the soil bulk density. The density of the organic compounds is smaller than the minerals of the soil and the aggregation power of these compounds becomes the soil more porous. However, it is important to remember that the soil bulk density will usually increase with the depth in soils under forest. Soils under forest present larger total porosity than when cultivated in the surface layers, and, consequently, small soil bulk density. This occur mainly due to the influence of the organic matter as pointed out. The increase of the soil bulk density with depth are related to the decrease of the organic matter amount and the pressure caused by the superior layers. Changes in the soil texture among soil horizons can also explain differences in the soil bulk density. However, forested soils can also present higher soil bulk densities than areas without forest due to some compaction event such as animal or machinery traffic and root development. You should also consider the influence of the methods used to evaluate the soil bulk density between the areas under study. Sampling errors can affect the quality of the data measured.