Estoque e qualidade da matéria orgânica do solo em áreas cultivadas com café sob sistemas agroflorestal e a pleno sol / Soil organic matter stock and quality in agroforestry and full sun coffee systems

AUTOR(ES)
DATA DE PUBLICAÇÃO

2009

RESUMO

Soil organic matter (SOM) restoring is considered a main goal in the enhancement of soil quality. The evaluation of changes in the SOM as a result of soil management practices is fundamental to identify strategies to increase agricultural production, avoiding soil degradation and decreasing the emission of greenhouse gases. In Brazil, no-tillage has been broadly spread as a management alternative to enhance soil C sequestration in different ecosystems. However, little attention has been paid to other options of land use in restoring SOM. Agroforestry-coffee systems have been considered a suitable option of soil management to solve part of the agricultural problems in the Atlantic Costal Rain Forest domain, since it can contribute to reduce soil erosion, enhance nutrient cycling and increase SOM levels. In 1995, a group of farmers and researchers, supported by the Centre for Alternative Technologies of the Zona da Mata (CTAZM) in partnership with the Universidade Federal de Viçosa, especially with the Department of Soils, started a participatory agroforestry-coffee system experimentation. The potential of agroforestry systems to increase soil C, N and P stocks and the effect of this type of land use on the SOM quality are not completely understood in specific agro-ecosystems in the Zona da Mata. We hypothesized that (i) due to the constant input of organic residues derived from tree-components, agroforestry-coffee favors the increase of C, N and P stocks in the soil and in different SOM pools in comparison to a full sun coffee system; and (ii) that the diversity and quality of the organic residues in the agroforestry systems affect the chemical and structural characteristics of the SOM, reflecting the different forms of its role in the nutrient cycling. Our general objectives were: (i) to measure the stocks of C, N and P in the soil and in different pools of the SOM in areas under agroforestry-coffee and full sun coffee systems, and (ii) to evaluate the effect of the diversity and quality of the organic residues on the chemical and structural characteristics of humic substances in both systems. The work was carried out in three smallholding coffee production areas under agroforestry-coffee (AGF) and full sun coffee (FSC) systems situated at the municipalities of Divino and Araponga in the Atlantic Coastal Forest biome in the state of Minas Gerais, Brazil. In each selected site, areas under natural forest fragments (NF) were sampled and used to represent the native or steady state condition of soil. Soil samples were collected from 0-5, 5-10, 10-20 and 20-40 cm depth intervals. Chapter 1 presents a general introduction of the thesis, emphasizing the main study problems. In chapter 2, we evaluated the total organic C, N and P stocks in the soil and humic substances. Soil organic C stocks in the coffee-cultivated lands varied from 1.36 to 3.92 kg m-2 among all evaluated sites. In general, there were no significant differences in the soil C, N and P stocks between AGF and FSC systems in all evaluated sites. Other soil physical characteristics are likely related to this behavior. In comparison to a reference area, AGF systems presented lower C stock reductions than FSC systems. The adoption of AGF management resulted in the increase in the carbon management index in all sites, suggesting the rehabilitation or enhancement of SOM levels. The potential of agroforestry management to increase soil C storage depended on system-characteristics in each particular environmental condition. The dynamics of C, N and P in the soil has been strongly influenced by the quality of organic residues in the agroforestry systems. The study of C and N dynamics in aggregatesized fractions is presented in Chapter 3. The C concentration in the 2502000 μm fraction represented the most part of soil organic carbon. In Divino, free particulate organic matter amounts in AGF system were 2.8 and 2.0 times greater than in FSC in the 0-5 and 5-10 cm layers, respectively. In Araponga(I), these proportions were 1.3- and 1.8-fold greater in AGF, compared to the FSC system. In Divino and Araponga(I), agroforestry systems enhanced soil aggregation by increasing the amount of more stable macroaggregates as well aspromoting an increased protection of C and N in the microaggregates within macroaggregates. Therefore, the long-term stability of C and N in these sites probably depends upon the continuation of agroforestry-coffee cultivation. In Chapter 4, we characterized the soil inorganic and organic P in different pools in agroforestry-coffee and fullsun coffee systems by adopting the sequential fractionation technique. The distribution of inorganic and organic P (Pi and Po) pools varied among the different studied sites, suggesting that P cycling depends on the inherent characteristic of each agro-ecosystem. The dynamics of P fractions in agroforestry systems seems depend on the organic P pool. In Divino and Araponga(I), agroforestry increased the proportion of mineralizable Po (NaHCO3-Po + HClconc.-Po), which can favor biological processes of the soil P cycling. In Araponga (II), the agroforestry system increased moderately the labile Po pool (NaOH-Po), suggesting that P cycling occurs in a medium-term scale, since such pool has been considered as a source of P when most labile P fractions are depleted. The chemical and structural characterization of soil humic acids (HA) and fulvic acids (FA) was the focus of the study in Chapter 5. Soil HA and FA extracted from 0-5 and 5-10 cm layers were characterized comparatively by using chemical and spectroscopic techniques including: elemental analysis (C, H, N and O contents), analysis of functional groups, thermogravimetry (TG), Fourier transform infra-red (FTIR) and solid-state 13C nuclear magnetic resonance (13C CP-MAS/NMR). At soil surface, the HA from AGF presented higher H/C and lower O/C ratio than the FSC system in Divino, suggesting that AGF management favored the genesis of HAs with less oxidative character. The patterns of elemental composition were supported by the findings of the TG analysis. The FTIR spectra of the HAs and FAs were quite similar when comparing the AGF and FSC systems. In all sites 13C NMR assignment of HAs was dominated by the O-alkyl C resonance in both soil layers followed by a resonance line assigned to alkyl C. Solid-state 13C NMR spectroscopy confirmed the data from the previous analysis in which HA from the AGF system in Divino and Araponga(I) are less humified than in the FSC system, whereas in Araponga(II) this clear evidence was not verified. Conclusively, the chemical changes on the structure of HA and FA, as influenced by agroforestry management depend on inherent characteristics of the environment in which it is developed. Thus, the effect of this type of land use on the structural characteristics of humic substances in comparison to a monoculture system should not be broadly generalized. Finally, Chapter 6 describes a summary of the thesis and final remarks, pointing out that some important future research is necessary for the agroforestry in the Zona da Mata.

ASSUNTO(S)

matéria orgânica do solo zona da mata farms ciencia do solo zona da mata soil organic matter agricultura familiar

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