Frações da matéria orgânica do solo sob povoamentos de eucalipto no Brasil e simulação de sua dinâmica com modelos processuais / Soil organic matter fractions under eucalypt plantations in Brazil and simulation of their dynamics with mecanistic models

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

Soil C sequestration is one of the most important alternatives to minimize CO2 emissions to the atmosphere. Eucalypt afforestation of poorly managed pastures and agriculture lands in Brazil is among the most attractive strategies to mitigate greenhouse gas emissions to the atmosphere. Despite the utilization of adequate management practices to obtain high wood productivity, information on their impact on soil organic matter (SOM) stocks in areas afforested with eucalypt in Brazil is scarce. Thus, the aims of this study were: i) to evaluate the impact of short-rotation eucalypt cultivation on C and N stocks of SOM fractions, up to 1 m deep, in soils with contrasting textures in comparison to those under native forest, pasture and sugar cane in the São Paulo State, Brazil; ii) to evaluate SOM dynamics utilizing the Century model to simulate SOC stocks in two eucalypt chronosequences, and for different soils orders. It was also simulated the impact of eucalypt debarking on site during the harvest on SOC stocks in the Minas Gerais State, Brazil and, iii) to calibrate the FullCAM model to simulate the SOM dynamics in short-rotation eucalypt plantations, pasture and native forest located in four eucalypt growing states of Brazil (São Paulo SP, Espírito Santo ES, Minas Gerais MG, and Bahia BA). To accomplish the first aim, we selected commercial eucalypt plantations located nearby native forest (Atlantic forest or Cerrado), pasture and sugar cane in two soil groups: clayey ( 66% of clay) and sandy ( 9% of clay). So, for these soil it was determined the total organic C (TOC)and total N (TN) stocks, and C in the humic substances (HS), in the light fraction (LF), and in the microbial biomass (MB) in the 0-10, 10-20, 20-40, 40-60 and 60-100 cm layers. The results showed that the clayey soils, in general, have higher C and N stocks in SOM fractions than the sandy soils. The eucalypt soil has a larger TOC stock (146.6 t ha-1), and stocks of C in the humic substances (HS) (139.1 t ha-1), and in the LF (6.9 t ha-1) than those under pasture pasture (TOC = 123.4 t ha-1, HS = 109.9 t ha-1, LF = 3.4 t ha-1) and sugar cane soil (TOC = 127.1 t ha-1, HS = 119.9 t ha-1, LF = 3.7 t ha-1), up to 100 cm deep, in the clayey soils group. There were no differences in the TN stocks among eucalypt, pasture and sugar cane soils in both clayey and sandy soil groups. All soil uses led to lower N stocks in humic fractions as compared to the native forest soil up to the 60 cm depth. The eucalypt soil had higher N stock in the LF (0.16 t ha-1) than the pasture (0.08 t ha-1) and sugar cane (0.10 t ha-1) soil, up to 60 cm deep. In the sandy soils group, the eucalypt, the pasture and the sugar cane soils had lower C stocks in HS, fulvic acid fraction (FAF), and humic acid fraction (HAF) than the native forest soil up to 100 cm deep. A similar pattern was observed for the C and N stocks in the LF throughout the soil profile. To undertake our second aim, two chronosequences were constructed by selecting areas under shortrotation eucalypt cultivation for 4.0, 13.0, 22.0, 32.0 and 34.0 years in Belo Oriente (BO) and 8.0, 19.0 and 33.0 in Virginópolis (VG). The results indicated that the C stocks simulated by the Century model decreased after 37 years of poor pasture management in areas originaly covered by native forest in the BO and VG regions. The substitution of poorly managed pastures by short-rotation eucalypt in the early 70`s led to an average increase of 0.28 and 0.42 t ha-1 year-1 of C in BO and VG, respectively. The measured soil C stocks under eucalypt cultivated in distinct soil orders in independent regions with distinct edapho-climatic conditions closely resemble the values estimated by the Century model (root mean square error - RMSE = 20.9; model efficiency EF = 0.29) despite the opposite result obtained with the statistical procedure to test the identity of analytical methods (Leite and Oliveira, 2000). Using the calibrated model it was found that the maintenance of eucalypt bark on site after harvesting resulted in an increase in C sequestration by the soil. The third aim was accomplished by comparing the observed TOC stocks, and C stocks in the HS, the LF and the MB with the C stocks in these fractions simulated by the FullCAM. The results showed that in ES and BA, the simulated TOC stocks decreased 0.37 and 0.30 t ha-1 year-1, respectively, after the short-rotation eucalyptus establishment in formerly well managed pastures. A similar pattern was observed in SP where the simulated TOC stocks, and stocks of C in HS and in LF decreased after replacement of native forest by eucalypt. On the other hand, after 33 years of eucalypt cultivation the TOC stocks increased 5.6 % in relation to Cerrado in the Jequitinhonha Valley MG. The FullCAM model satisfactorily described the TOC stocks (EF=0.74) and HS (EF= 0.65). So, the FullCAM model constitutes an appropriate tool to simulate the changes in soil C after eucalypt afforestation.

ASSUNTO(S)

ciencia do solo carbon sequestration modelagem sequestro de carbono modelling century fullcam century fullcam

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