Abstract
Studies have proven that the agroforestry systems in the semi-arid region of the State of Ceará, Brazil, induce an increase in soil organic C levels. Notwithstanding, there is no information if this increase also results in qualitative changes in different pools of soil organic matter. The objective of this study was to verify the possible chemical and structural alterations in fulvic and humic acids of a Luvisol in areas adopting agroforestry, traditional intensive cultivation and native forest in a long-term experiment conducted in the semi-arid region of Ceará State, Brazil. The study was conducted in an experimental area of the National Goat Research Center (Embrapa) in Sobral, CE. The following treatments were evaluated: agrosilvopasture (AGP), silvopasture (SILV), intensive cultivation under fallow (ICF), and areas with native forest (NF). Soil fulvic and humic acids fractions were extracted from the 0-6 and 6-12 cm layers and characterized by elemental composition, thermogravimetry and infrared spectroscopy analyses. The elemental composition analysis of humic acids confirmed the data found for fulvic acids, showing reduction in the C, H and N levels, followed by an increase in O contents in the AGP and ICF treatments over SILV and NF. In all treatments, except to SILV in the 0-6 cm layer, the percentage of mass loss was highest (300-600 °C) for humic acids in the thermally most stable region. Despite the similarity between infrared spectra, soil fulvic acids in the SILV treatment extracted from 6-12 cm depth decrease the absorption bands at 1708 and 1408 cm-1 followed by an increase in the absorption band at 1608 cm-1 attributed to aromatic C=C groups. This behavior suggests an increase in the aromatic character of the structure. The AGP and ICF treatments, which increase the soil tilling, favored the maintenance of humic substances with a more aromatic character in the soil than SILV and NF. The less aromatic humic substances in the SILV treatment resulted in an increase of exchange sites of soil organic matter, indicating improved nutrient cycling and maintenance of productivity in the system.
Highlights
International concerns about climate changes attributed to land use have triggered discussions on proposals of environmentally less harmful agricultural production systems
In the AGP treatment, soil organic C (SOC) content was reduced by 41 % when compared to the reference native forest (NF)-1
The C-to-N atomic ratio of the humic fractions Table 3) shows that chemically more stable organic forms are predominant in the upper soil layer compared to those extracted from the deeper layer. These results show that soil organic matter (SOM) transformation processes occurred more intensely in the top soil, given the influence of climate, biological and biochemical factors, especially when intensified by anthropic activities in cultivated areas
Summary
International concerns about climate changes attributed to land use have triggered discussions on proposals of environmentally less harmful agricultural production systems. Among the possibilities of environmental-friendly agroecosystems, agroforestry (AGF) is considered a viable option for a better soil use, favoring environmental functions (Jose, 2009) and increasing C sequestration (Montagnini & Nair, 2004; Takimoto et al, 2009). In the semi-arid Northeastern region of Brazil, AGF may be considered a promising alternative to restore the soil quality of areas degraded by intense soil cultivation (Araújo Filho & Carvalho, 2001). These agroecosystems can improve nutrient cycling and enhance soil structure (Maia at al., 2006), reducing water erosion (Aguiar et al, 2010) and increasing soil organic C stocks (Maia et al, 2007). In spite of this increase, little is known about qualitative changes in SOM as a result of the adoption of AGF, mainly in the systems developed in the semi-arid region of Brazil
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