Management Of The Cerrado Soils Of Brazil

Author : Y. L. Zinn
Publisher :
ISBN 13 :
Total Pages : 366 pages
Book Rating : 4.:/5 (79 download)

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Download or read book Textural, Mineralogical and Structural Controls on Soil Organic Carbon Retention in Brazilian Cerrados written by Y. L. Zinn and published by . This book was released on 2005 with total page 366 pages. Available in PDF, EPUB and Kindle. Book excerpt: The retention of soil organic carbon (SOC) is commonly considered the result of climate, vegetation, internal drainage and management interactions. However, a sparse but considerable set of evidence in the literature suggests that soil texture, mineralogy and aggregation also affect SOC retention. The objective this research was to qualitatively and quantitatively assess the controls that these three soil properties exert on SOC levels in the Brazilian Savanna (Cerrado) region, in tropical humid South America. Thus, three soils under similar climate and slope, but of contrasting texture and under native vegetation and Eucalyptus plantation, were sampled in triplicate to 1 m depth, and characterized by physical, chemical, mineralogical, Yoder wet sieving, and microscopic analyses. That C and N concentration were determined in bulk soils, particle size separates (clay, silt, sand) and water-stable aggregates (WSA). A basic assumption was made that the SOC particle size is inherently associated to its retention mechanism: colloidal and soluble forms are retained by sorption to clays; particulate organic matter (POM,>20 um) are retained outside (free-POM) or inside aggregates (occluded-POM), and silt-sized SOC has intermediate properties. The rationale is that these mechanisms are necessarily determined by soil texture, mineralogy and structure, which then control SOC retention. The three soils were highly weathered and composed mostly of quartz and kaolinite, with minor and variable contents of Fe-Al oxides, hydroxyl-interlayered vermiculite and illite. These soils were classified as clayey Haplustox, loamy Haplustox and sandy soils (one Quartzipsamment and two sand Haplustoxes). In all soils, Eucalyptus plantation increased the C/N ratio and decreased aggregation and aggregate-occluded POM in the top 10 cm layer in relation to native vegetation, but bulk SOC concentrations and stocks were not affected. In the sandy soils only, the relative content of SOC in the sand fraction was enriched, whereas it decreased in the clay fraction. Soil texture and depth strongly affected bulk SOC concentrations, which could be credibly estimated as a function of clay+silt contents and depth, by means of a movel mathematical model (using data from samples under the two land uses). The specific surface area (SSA) of soil 2mm under native Cerrado was modeled as a function of clay, silt and SOC contents but not depth. This suggests that SOC levels increase with higher SSA associated with higher clay contents. In a single soil profile, however, SSA decreases near the soil surface because of slightly lower clay contents but also because higher SOC levels promote clay flocculation and aggregation. The SOC concentration in particle size separates was inversely related to the proportion of that size fraction in soil (SOC dilution effect), which precluded its use in modeling SOC size partition. However, the calculation of relative amounts of total SOC in each fraction allowed for prediction of clay-sized SOC (as percent of total SOC), based on clay contents and depth. A quantitative assessment of clay mineralogy showed that, for the bulk soil, SOC concentrations were better correlated with contents of crystalline and amorphous Feoxides in surface layers and amorphous Al oxides in the subsoil, with higher coefficients of determination than those of SOC vs. clay+silt. These trends were even stronger when the clay-sized SOC pool was correlated to the same mineral phases. Aggregation, as indicated by the mean weight diameter (MWD) and percent of WSA2mm, was strongly correlated with clay+silt contents, but bulk SOC was poorly correlated with MWD and WSA>2mm except for the 0-5 cm depth. The fraction of POM occluded inside aggregates was strongly affected by soil texture, varying from ca. 25% in the sandy soils to ca. 50% in the clayey Haplustox. The activity of soil fauna resulted in three types of zoogenic aggregates (fecal pellets, cocoons and aggrotubules), the latter more common and SOC-enriched in soils of loamy and coarser texture. Because texture directly affects the contents of Fe and Al oxides and the protection of POM within aggregates, SOC retention in comparable aerobic Cerrado soils in controlled, in decreasing order of importance, by: 1)texture, 2) mineralogy, and 3) structure (including pedogenic and faunal peds).