Paulo Cesar Conceição

Variáveis relacionadas à estabilidade de complexos organo-minerais em solos tropicais e subtropicais brasileiros

A estabilidade de complexos organo-minerais e uma caracteristica importante quanto a quimica e fisica de solos tropicais e subtropicais. O objetivo deste estudo foi identificar variaveis relacionadas a estabilidade de complexos organo-minerais, avaliada pela energia de ultra-som necessaria para a dispersao total do solo em particulas primarias, em seis solos das regioes Sul e Centro-Oeste do Brasil com textura e mineralogia distintas. A energia de ultra-som necessaria para dispersao total dos solos variou de 239 a 2.389J mL-1, sendo diretamente relacionada aos teores de carbono orgânico (R2=0,799, P<0,05). A mineralogia da fracao argila teve um papel determinante na estabilidade dos complexos organo-minerais, a qual foi relacionada aos teores de oxidos de ferro de baixa cristalinidade (R2=0,586, P<0,10), mas nao teve relacao com os teores totais de oxido de ferro pedogenicos. A analise qualitativa da fracao argila, realizada por difratometria de raios-X, demonstrou que a estabilidade dos complexos organo-minerais variou positivamente em funcao das proporcoes de gibbsita e de goethita na fracao argila, o que reforca o papel desses minerais na protecao fisica e estabilidade coloidal da materia orgânica em solos tropicais e subtropicais.

Fracionamento físico da matéria orgânica e índice de manejo de carbono de um Argissolo submetido a sistemas conservacionistas de manejo

The physical fractionation of soil organic matter (SOM) is an alternative in assessing the amount of labile fraction that is used to calculate the carbon management index (CMI). The objective this research was to assess the efficiency of particle-size physical fractionation, (53-µm mesh), and density physical fractionation, with sodium iodide 1.8Mg m-3 (NaI) or sodium polytungstate 2.0Mg m-3 (PTS) solutions, at recovering SOM labile fractions (particulate and light, respectively) and at estimating the CMI. Soil samples of the 0-20cm layer of a sandy clay loam Acrisol under conventional tillage (CT) and no-tillage (NT) combined with two cropping systems (oat/maize and oat+vetch/maize+cowpea) were analyzed for stocks of total organic carbon (TOC), particulate organic carbon fraction (POC) and light fraction carbon (LF-C). In density physical fractionation, with the use of NaI was recovered less LF-C than PTS. The recovery of POC by particle-size fractionation was intermediate between the recovery of NaI-LF and PTS-LF. The CMI calculated after particle-size fractionation showed high correlation with CMI after density fractionation with NaI or PTS, although results were rather underestimated with NaI. Conservation management systems without soil disturbance and with higher crop residue addition enhanced the CMI, because of increments of both lability index and carbon pool index. The CMI based on SOM physical densimetrical fraction shows evidence of being and efficient index to discriminate management systems that can be used to assess soil management practices.

O uso de amostras peneiradas a dois milímetros subestima a proteção física da matéria orgânica no solo

Density fractionation is an important tool to determine the physical protection of organic matter within soil aggregates, but results may be affected by the mesh size used to sieve soil samples. The hypothesis of this study is that in comparison to 9.5 mm sieve, sieving through 2 mm mesh results in excessive soil disaggregation by releasing organic matter originally protected within aggregates and consequently leads to underestimations of the carbon content in the occluded light fraction (OLF-C) and of the magnitude of physical protection of organic matter within soil aggregates. Soil samples (0-5 cm) of five Brazilian soils under long-term experiments (9-18 yrs) were collected in the regions Center West (4 soils) and South (1 soil), under conventional and conservation soil management systems. Based on 2 mm (r = 0.90, p < 0.02) and aggregate stability index (r = 0.94, p < 0.01). In three of the evaluated soils, the use of < 2.0 mm soil samples led to the erroneous conclusion that conservation soil management systems are not efficient to stabilize organic matter within soil aggregates. Therefore, a mesh size of 9.5 mm is indicated to evaluate the physical protection of organic matter in aggregates, or alternatively, the sieves used to prepare soil samples for aggregate stability analysis, i.e., usually greater than 8 mm.