RESUMO
We used fine root and litter mass from a tropical mountain cloud forest to assess their relative contribution to nutrient content and to examine mineralization processes during a laboratory incubation experiment. Our results showed that average fine root mass density ranged from 2.86kgm-3 to 11.59kgm-3, while litter mass density ranged from 72.5kgm-3 to 177.3kgm-3. On average, fine root mass density represented 4.7% of the mass density of the O horizon. Fine root mass density followed an exponentially declining trend with soil depth. On average, 83% of fine root mass density within the soil profile was concentrated in the O horizon. Mean element pools in litter decreased from 44.08mgcm-3 to 0.49µgcm-3 in the following sequence: C>N>Fe>S>Ca>P>K>Mg>Na>Mn>Zn>Cu. For fine roots, a different mean element pool sequence (C>N>Ca>K>Fe>S>Mg>Na>P>Mn>Zn>Cu) in decreasing abundance (from 2.88mgcm-3 to 0.13µgcm-3) was observed with respect to litter. Regarding C, litter mineralized faster than fine roots, with a mean k value of 0.25d-1 for litter and 0.13d-1 for fine roots. Principal component analysis (PCA) combined with stepwise regression analysis revealed that the main mass density predictors were N, S, Zn, and Mn for litter (p<0.0001, R2=0.92), and S and C/N ratio for fine roots (p<0.0001, R2=0.82). These results demonstrate the potential of chemical composition to influence the mineralization of fine root and litter mass and therefore the nutrient availability and C sequestration.
RESUMO
This study addressed the effects of land use and slope position on soil inorganic nitrogen and was conducted in small watersheds. The study covered three land use types: tropical cloud forest, grassland, and coffee crop. To conduct this research, typical slope small watersheds were chosen in each land use type. Slopes were divided into three positions: shoulder, backslope, and footslope. At the center of each slope position, soil sampling was carried out. Soil inorganic nitrogen was measured monthly during a period of 14 months (July 2005-August 2006) with 11 observations. Significant differences in soil NH(4) (+)-N and NO(3) (-)-N content were detected for both land use and sampling date effects, as well as for interactions. A significant slope position-by-sampling date interaction was found only in coffee crop for NO(3) (-)-N content. In tropical cloud forest and grassland, high soil NH(4) (+)-N and low NO(3) (-)-N content were recorded, while soil NO(3) (-)-N content was high in coffee crop. Low NO(3) (-)-N contents could mean a substantial microbial assimilation of NO(3) (-)-N, constituting an important mechanism for nitrogen retention. Across the entire land use set, the relationship between soil temperature and soil inorganic N concentration was described by an exponential decay function (N = 33 + 2459exp(-0.23T), R (2) = 0.44, P < 0.0001). This study also showed that together, soil temperature and gravimetric soil water content explained more variation in soil inorganic N concentration than gravimetric soil water content alone.
