Retenção de carbono orgânico e nitrogênio em solos do Sul de Minas Gerais: efeito da altitude

Abstract

Studies on the variability of soil organic carbon (SOC) and nitrogen (total N) stocks are critical because of the ecosystem services played by soil organic matter and the importance of soils in the carbon cycle and greenhouse gases balance. However, the controlling factors of SOC and N retention are not often approached in the literature, especially in tropical forest soils. The aim of this work was to quantify the contents and stocks of SOC and total N, and to determine the influence of altitude, soil physical and chemical properties on SOC and N retention. Thus, different forest soils were sampled on two toposequences in southern Minas Gerais, Brazil: a gneiss rock toposequence (GNE) at altitudes of 1,480, 1,710 and 2,050 m a.s.l., and another metapelitical rock toposequence (Metap) at altitudes of 800, 975 and 1,240 m a.s.l. The toposequences presented the same parent material, vegetation, climate, drainage and an altitudinal gradient at least of 150 m between each sampling site. We determined in 3 replicates SOC and N concentrations, soil bulk density, aggregate stability in water, particle size distribution at each altitude up to 1 m depth, aside with soil profile description and chemical analysis, and the micromorphology of the surface layer. The SOC and total N contents increased linearly with increasing altitude, and decreased exponentially with increasing depth. The effect of altitude on SOC and N concentrations occurred to 1 m depth in the GNE toposequence, with a cooler climate and coarser soils. However, in the soils of the Metap toposequence, with slightly warmer climate finer textures, the effect of altitude on the SOC and total N occurred only to a 40 cm depth. Soil bulk density was strongly influenced by SOC contents. Overall, all soils showed high Al saturation, low pH and CEC, in accord with the gneiss and metapelitic parent materials, indicating a intermediate weathering stage. The SOC and N stocks up to a 1 m depth increased linearly with increasing altitude. The soils of the GNE toposequence presented SOC stocks (0-1 m) between 228, 7 Mg ha -1 (1,480 m) and 606 Mg ha -1 (2,050 m), and total N ranging from 13,3 Mg ha - 1 to 36 Mg ha -1 . The SOC stocks (0-100 cm) of the Metap toposequence ranged from 154.8 Mg ha -1 (800 m) to 237.1 Mg ha -1 (1,240 m), and total N stocks between 12.1 Mg ha -1 (800 m) to 18.6 Mg ha -1 (1,240m). Soil micromorphology illustrated the textural and mineralogical differences between soils of distinct lithologies, and the outstanding presence of SOM in the 0-10 cm layer, especially at the higher altitudes of each toposequence. There was also an effect of soil texture (silt + clay) on SOC and N retention, although weaker than the main effect of microclimate, resulting from the altitudinal variation in the studied areas.