C.C. Cerri

Organic carbon and 13C contents in soils and soil size-fractions, and their changes due to deforestation and pasture installation in eastern Amazonia

Abstract In soils of the eastern Amazonian forest, modifications in soil organic matter (SOM) contents as a consequence of deforestation and pasture installation were investigated. Profile distribution of total organic carbon (C) and nitrogen (N), and of 13C isotope abundance (expressed in δ13C%o units) were compared. The two soils, one under native forest and the other one after ten years under pasture of Pennisetum purpureum had similar C/N values, which slightly decreased with increasing depth, from 13.6 to 11.9–12.7 within the first 40 cm. In the pasture soil, the C content was slightly lower than in the forest soil, and reached 29 t ha−1 compared with 31 t ha−1, and 15 t ha−1 compared with 16 t ha−1, in the 0–20 and 20–40 cm layers, respectively. The δ13C values reflected the origin of SOM, from either the forest (which had a C3 photosynthetic cycle), or the pasture (which had a C4 cycle). They were higher by at least 6.5 and 1.5 units, in the respective 0–10 and 10–20 cm layers of the pasture soil, than in the corresponding layers of the forest soil. These δ13C values were used to estimate the proportions of C derived from the forest (Cdf) and from the pasture (Cdp). The calculations clearly indicated a strong input of Cdp, which reached 46–49% and 21–24% of total C in the respective layers. Particle size fractionation showed that SOM changes were predominant in the upper soil layer (0–10 cm), and vanished with increasing depth: in the pasture soil, the coarse organic residues accumulated, whereas the amount of C in the finest fractions decreased, which suggested a slowing down in humification processes. In the forest soil, the δ13C values were lower in the coarsest than in the finest fractions, the latter being less affected than the coarsest ones by the vegetation change; in the 0–10 cm layer, these values increased by about 7–10 units in the sand-size fraction, and only 4–5 units in the clay-size fraction. Thus, the replacement of Cdf by Cdp was greater in the sand-size fraction (55–65% of total C), than in the clay-size fraction (34–45% of total C). Based on 14C dating, the oldest pool (i.e. that of mean age 5000 years) of forest SOM was calculated at each depth. In the soil surface, it was still smaller than the more labile fraction of Cdf, and represented about 17% and 26% of total C in the 0–10 cm and 0–20 cm layers, respectively.

Organic matter dynamics and aggregation in soils under rain forest and pastures of increasing age in the eastern Amazon Basin

Abstract In the eastern Amazon basin, four neighbouring clayey Oxisols with similar particle-size distributions were selected, one under rain forest and three under pasture for 7, 12 and 17 years, respectively. These soils were sampled at depth intervals of 0.1 m down to 1 m. Although no clear effect of pasture establishment on aggregate stability was assessed, significant negative effects of cattle trampling on porosity and water retention and of vegetation change on clay water-dispersion were observed in the organic-rich horizons (0–0.40 m layers). Indirect evidence of a great change of the nature of organic materials was also suggested, particularly with (i) an increase in both negative surface charges and clay dispersibility, attributed to an increase in organic functional groups, and (ii) a decrease in clay fabric porosity, attributed to an increase in the hydrophilic-hydrophobic balance on organic surfaces (short-range adhesion forces during drying processes). Studies of soil organic matter (SOM) changes along this forest-to-pasture sequence were based on total C and 13C measurements, which allowed to calculate the distribution of C derived from forest (Cdff) and from pasture (Cdfp) throughout the profiles. The distribution of C and 13C in the whole soil, in water-stable (WSA), in not stable (NWSA) aggregates, and in particle fractions, was compared. Young organic residues derived from pasture were trapped in WSA, from where they were released by dispersion. After 17 years, the decrease in forest-derived SOM and the input of about 25% of pasture-derived SOM were suggested to be more effective on clay dispersability than on aggregate stability.

Changes in Organic Matter in an Oxisol from the Central Amazonian Forest During Eight Years as Pasture, Determined by 13C Isotopic Composition

The consequences of burning and deforestation on soil organic matter (SOM) content were studied on two managed areas of Oxisol (Yellow Latosol) located near Manaus, in Central Amazonia. The variations of organic carbon (OC) content were studied on the 2 mm sieved fraction of surface soils collected under natural forest, before and after slash and burning, and under pasture of Brachiaria humidicola established for one, two and eight years. Under natural forest, OC content reached maximal values of 28 T.ha−1 in the 0–3 cm layer, 62 T.ha−1 in the 3–20 cm layer, and decreased rapidly with depth. Burning removed about 4 T.ha−1 of OC, chiefly from the 0–3 cm soil layer, but this loss was recovered after one year of pasture. Decreases of about 8 T.ha−1 after one year and 28 T.ha−1 after two years were observed in the 3–20 cm layers, probably because the humification of grass root residues did not balance the decomposition of pre-existing SOM. Contrarily, after eight years of pasture, the initial OC content of surface soil had been almost entirely restored, including the 3–20 cm layer. Changes in the isotopic ratios 13C/12C of SOM were used to determine the respective contribution of OC derived from forest (Cf) and pasture (Cp) in each situation. After one year, the proportion of Cp was already 5% in the whole 0–20 cm layer. After two years, it was of 20%, whereas about 40% of Cf had disappeared. After eight years, the proportion of Cp was close to 40%, whereas almost no further decrease of Cf had been observed between two and eight years of permanent pasture.

Sorption of atrazine, terbutryn and 2,4-D herbicides in two Brazilian Oxisols

The adsorption by Oxisols of 3 herbicides: atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine), terbutryn (2-(ter-butylamino)-4-(ethylamino)-6-(methyltio)-s-triazine), and 2,4-D (2,4-dichlorophenoxy acetic acid) was studied in two sites (MPR and MSP sites) with plots under natural vegetations and plots cropped for up to 56 years after clearing. The main mineralogical difference between the soils from the two sites was the higher content of gibbsite and hematite of the MPR than of the MSP site. In both locations, the soil organic matter (SOM) content decreased after clearing and cropping. The adsorption of atrazine and terbutryn (models of neutral and weakly basic herbicides, respectively) decreased with years of cultivation because of their significant correlation with decreasing SOM content. While the atrazine K∝ values were relatively constant (average K∝ = 54 ± 22 l·kg−1, the terbutryn K∝ varied greatly with different soils (average K∝ = 308 ± 184 l·kg−1). The terbutryn adsorption depended on soil pH: it was maximum between pH = 4.5 and 5.0, and between 5.0 and 5.5 for soils from the MSP and MPR sites respectively. The adsorption of 2,4-D (model of acidic herbicides) was always higher in soils from the MPR than from the MSP plots. In soils from the MPR site, 2,4-D adsorption decreased with years of cultivation. An opposite tendancy was observed with soils from the MSP site. The adsorption of 2,4-D increased strongly with decreasing soil pH. Soils from the MPR plots were more sensitive to pH modifications than those from the MSP plots. In order to compare adsorption of polar herbicides on soils with different pH, we proposed to calculate the Kd and K∝ coefficients for a controlled pH.

Carbon limitations to nitrous oxide emissions in a humid tropical forest of the Brazilian Amazon

The availability of labile organic C for microbial metabolic processes could be an important factor regulating N2O emissions from tropical soils. We explored the effects of labile C on the emissions of N2O from a forest soil in the State of Rondônia in the southwestern quadrant of the Brazilian Amazon. We measured emissions of N2O from a forest soil after amendments with solutions containing glucose, water only or NO3–. Addition of glucose to the forest soil resulted in very large increases in N2O emissions whereas the water only and NO3– additions did not. These results suggest a strong C limitation on N2O production in this forest soil in the southwestern Amazon.

Characterization of organic matter in topsoils under rain forest and pasture in the eastern Brazilian Amazon basin

Abstract In topsoils under forest and 7-, 12- and 17-year-old pastures, organic matter was characterized by analysing C and N distribution in particle-size fractions, the C decomposition rates of soil and particle-size fractions and by employing density-fractionation of macro-organic matter (>150 μm). The C and N associated with clay fractions increased with increasing age of pasture. The weight (%) of macro-organic matter and its heavy fractions (>1.37 g cm–3) also increased with increasing age of pasture. However, in a long-term incubation (100 days), these changes seemed to involve an increase in the C decomposition rate in the topsoil of the oldest pasture. Using the C decomposition rates of particle-size fractions, it appeared that silt and clay contributed differently to C decomposition in the whole soil. C associated with silt contributed to the C decomposition rate during the first 40 days of incubation, while C associated with clay contributed to C decomposition in the long-term incubation (after 40 days), especially when the clay fraction appeared to reach saturation point with respect to its ability to bind organic compounds and thus protect the soil from C loss.

Factors influencing carbon decomposition of topsoils from the Brazilian Amazon Basin

Abstract The influence of texture and local climate, especially precipitation, on C decomposition was evaluated in topsoils under forest and pastures at different sites located in the western, central, and eastern Brazilian Amazon Basin. Independent of the geographical location and vegetation cover, C decomposition was lower in topsoils of fine texture with a sand/clay ratio of less than 1, compared to topsoils of coarse a texture with a sand/clay ratio ranging from 2 to 8. In the former, the low sand/clay ratio appears to reduce C decomposition. In the latter, besides the effect of the high sand/clay ratio, the local climate, especially the annual precipitation, seems to play an important role in controlling C decomposition.

Interrelationships Among Soil Total C and N, Microbial Biomass, Trace Gas Fluxes, and Internal N-Cycling in Soils Under Pasture of the Amazon Region

ABSTRACT The aim of this study was to investigate the interrelationships and seasonal weather variations on soil total C and N concentrations, microbial biomass C pool, trace gas fluxes, and rates of internal N-cycling from soils under degraded pasture at Nova Vida ranch, Ron-d

Carbon decomposition of the topsoils and soil fractions under forest and pasture in the western Brazilian Amazon basin, Rondônia.

oCnia, Brazilian Amazon. We collected samples from soils and gas fluxes at the 5 soil types in April 2000 (end of the wet season) and in October 2001 (end of the dry season). Samples were analyzed in both seasons for soil microbial biomass C, soil total C and N, inorganic N, net N mineralization and net nitrification rates and some other soil properties. Overall, the effects of the season events upon soil properties were highly variable. The studied soils with high rates of CO2 evolution but low rates of net N mineralization had high rates of N immobilization. The high immobilization rates suggest that the high CO2 evolution rates were associated with a growing microbial biomass. Moreover, in the wet season, the lower NO3 −-N pools and rates of net mineralization and net nitrification we measured are indicative of lower pasture N2O fluxes. In general, net nitrification rates exceeded net N mineralization rates as the relatively NH4 +-N pools. The interrelationships were investigated in order to understand the major biogeochemical cycles that influence soil fertility under pasture, which is important for devising management technologies that enhance the sustainability of these areas and thus slow further deforestation.

Effect of sewage sludge on microbial biomass, basal respiration, metabolic quotient and soil enzymatic activity

Abstract The topsoils of two sites, comprising natural forest and 4- and 20-year-old pastures, respectively, were selected in Rondônia to evaluate the changes of soil organic matter due to pasture establishment. These changes were evaluated by measuring the proportions of the C and N associated with clay and silt fractions, and by the C decomposition (CD) rate of the whole topsoils and their size fractions. The topsoils studied had large proportions of C and N associated with fine fractions, especially with clay fractions. The CD rate of the silt fractions was higher than that of the clay fractions under the two forest topsoils and under the 20-year-old pasture. The CD rate of the silt fractions under forest vegetation at each site was significantly higher than that of the silt fractions under pasture vegetation at the same site. The CD of clay fractions followed the same trend as the silt fractions, showing an improvement in the stability of C associated with clay and silt fractions under pasture vegetation.