Sistemas agrosilvipastoris no semiárido brasileiro: efeito na estrutura das comunidades microbianas e formação da matéria orgânica do solo

Abstract

Environmental degradation and climate change are the main threats to food and agricultural security, largely caused by anthropic action, mainly in the Brazilian semiarid region. Soil degradation results in a decrease in soil carbon (C) stocks, with consecutive losses of its functions. Land management to increase C sequestration and soil organic matter content is therefore a key strategy for agricultural sustainability and environmental quality. Thus, this work has two general objectives. First, simulate the dynamics of C in traditional management systems and in agroforestry systems using the Century model during a 100-year scenario. Second, evaluate the contribution of different compositions of plant species from the Caatinga to the formation of soil organic matter. For the first objective, data from a long-term experiment implemented in the Brazilian semiarid region were used to simulate the dynamics of C in traditional management systems (TRAD) and agroforestry (AFs), with the natural vegetation (NV) "Caatinga" as a reference. The Century model was used for data simulation. TRAD scenarios were simulated with different fallow periods (0, 7, 15, 30, 50 and 100 years) between crops. Also, two types of AFs (agrosilvopastoral-AGP and silvopastoral-SILV) were simulated under two contrasting conditions: permanent and rotation. The adoption of TRAD without a fallow period (0 years) resulted in a reduction of about 50% of C, approximately 20 Mg ha-1, after the first 10 years. However, the SOC stocks recovered when the fallow was adopted and reached the initial stocks after 50 years of fallow, considering the SOC VN simulated at equilibrium. The permanent (p) and rotating (r) AFs management systems quickly recovered the original SOC stocks, resulting in higher stocks than NV at equilibrium. The Century model performed well in simulating SOC stocks in AFs and can be considered a useful tool to assess temporal SOC dynamics in these complex agroecosystems. For the second objective, a microcosm experiment was conducted under controlled conditions, using native species of the herbaceous (HERB) and arboreal (ARB) strata and an exotic grass (GRASS). The combinations of these species in different proportions were also evaluated: (i) 55% herbaceous and arboreal + 45% grass (MIX1) and (ii) 75% herbaceous and arboreal + 25% grass (MIX2). The residues were incubated in a soil with intermediate isotopic abundance (δ 13C -20 ‰), in which the established treatments considered shoots (s) and roots (r) separately. The experiment lasted 120 days, periodically quantifying C-CO2 emissions. At the end of the incubation, soil samples were collected for phospholipid fatty acid analyzes (PLFA), to assess the microbial community structure and pH and macronutrient analyses. Another soil sample was collected for the physicalfractionation of SOM into particulate organic matter (POM) and mineral-associated organic matter (MAOM). Then, the determination of the natural abundance of δ 13C, the contributions of C derived from the residues to the SOM fractions was carried out. Thermochemolysis was also performed in the presence of tetramethylammonium hydroxide (TMAH) in the POM and MAOM fractions, for a biochemical characterization of SOM. In general, the lowest C-CO2 emissions from shoots residues were observed in GRASSs, while the highest occurred in HERBs and ARBs. In roots, the highest C-CO2 emissions were observed in HERBr. The largest contributions of C by the residues were observed in the POM fraction, both by shoots and by the roots component. The results showed that the shoots residues of ARBs provided greater biomass of total PLFA in the soil. There was a greater abundance of fungi with the incorporation of shoots residues and greater abundance of bacteria with the addition of residues from the roots. Regarding the biochemical composition of SOM, there was a greater abundance of aliphatic and nitrogen compounds in the MAOM fraction. In POM there was greater abundance of lignin and cutin derivates. The results demonstrate the importance and role of agroforestry systems, especially in the semiarid region, since the preservation of native species through the system contributes to the sequestration of C in the soil, preservation of biodiversity, formation of soil organic matter and conservation of resources natural areas of the region.