Interactions between soil nutrient and organic matter content, tree density, and grass cover in Brazilian Cerrado

Background/Question/Methods: Brazilian savanna (cerrado) contains several vegetation and soil types that are spatially variable and has been subject to extensive deforestation over the last 40 years.  It was hypothesized that soil carbon storage and fertility would be highly correlated with aspects of vegetation composition, including tree density and grass cover.  Data were collected in arboreal cerrado stands (cerradão and sensu stricto) and grass-dominated cerrado stands (campo limpo and campo sujo) located near Cuiaba, Mato Grosso, Brazil.  Ten centimeter deep soil samples were analyzed for soil organic matter (SOM), pH, cation exchange capacity (CEC), total nitrogen (N), and total phosphorus (P), tree density was measured with the point-center method, and grass cover was estimated using a line-intercept method across 100 m long transects within each stand.   

Results/Conclusions: Preliminary results indicate that SOM was significantly correlated with tree density (p<0.05) but negatively correlated with grass cover, indicating arboreal forms of cerrado have significantly higher soil C storage in the upper 10 cm soil layer than campo forms of cerrado.  Moreover, the interaction between SOM, N, and CEC was significant (p<0.05), and SOM content explained almost all of the spatial variation in CEC and soil N content.  These data suggest that tree abundance has a large effect on SOM content and surface soil fertility, which has implications for the maintenance of soil C storage and fertility under land use practices that result in tree mortality and cause degradation of cerrado.