SOIL CHEMICAL CHANGES AND RESEMBLANCES IN A CHRONOSEQUENCE RAINFOREST-SUGARCANE-PASTURELAND IN THE ATLANTIC FOREST BIOME

ABSTRACT This study evaluated soil chemical and isotopic changes in soils of a chronosequence rainforest-sugarcane-pasture in the Atlantic Forest biome, Brazil. Soil samples were collected (0-20 cm) in areas of native Brazilian Atlantic rainforest, sugarcane plantation and pastures of Brachiaria decumbens. The soil analyses performed were: pH (water 1:2.5), P (Mehlich-I), (Al+3, H+Al, K+, Ca+2, Mg+2 and Na+), soil organic matter (SOM), N, organic carbon and δ13C and δ15N stable isotopes. The conversion of rainforest to sugarcane and pastures resulted in a reduction of the soil natural acidity. Forest areas had greater Al+3 and H+Al concentrations than cultivated areas. The conversion from forest to agricultural soil reduced Al+3 (44%) and H+Al (11%), approximately. Soils from pasture had a greater percentage of base saturation (37.3%) than forest soils (25.4%). Cation exchange capacity was strongly influenced by concentrations of K+, Ca+2 and Mg+2, but not by Na+. Carbon stable isotope (δ13C) was more depleted in forest areas (-28.14‰), followed by sugarcane (-21.33‰), and pastures (-19.54‰). The greatest δ15N values were found in sugarcane areas. The short chronosequence studied, had a strong influence of the conversion of the forest on the decrease of the natural acidity and modifications of the isotopic profile. The enrichment of soil δ13C was attributed to the changes from predominant C3 vegetation to C4 grasses.

摘要 本研究针对巴西大西洋森林生物群区内一条由原生林-甘蔗种植园-牧场组成的时间序列（chronosequence）样地的土壤，开展了土壤化学性质与同位素特征变化的评估。研究采集了巴西原生大西洋雨林、甘蔗种植园以及俯仰臂形草（Brachiaria decumbens）牧场三类样地的0~20 cm土层土壤样品。本次测定的土壤指标包括：水土比1:2.5的pH值、梅利奇-I法（Mehlich-I）提取的有效磷（P）、铝离子（Al³+）、氢铝复合体（H+Al）、钾离子（K+）、钙离子（Ca²+）、镁离子（Mg²+）与钠离子（Na+）含量、土壤有机质（SOM）、全氮（N）、有机碳含量，以及碳稳定同位素（δ¹³C）与氮稳定同位素（δ¹⁵N）。将原生林转换为甘蔗种植园或牧场后，土壤自然酸度显著降低。原生林样地的Al³+与H+Al含量均高于耕作样地。由原生林转换为农用土壤后，Al³+含量约降低44%，H+Al含量约降低11%。牧场土壤的盐基饱和度（37.3%）高于原生林土壤（25.4%）。阳离子交换量主要受K+、Ca²+及Mg²+含量的影响，而与Na+含量无关。碳稳定同位素（δ¹³C）在原生林样地中最为贫化（-28.14‰），其次为甘蔗种植园（-21.33‰），牧场样地相对富集（-19.54‰）。甘蔗种植园样地的δ¹⁵N值最高。本次研究的短时间序列样地表明，原生林向农用土地的转换对土壤自然酸度降低与同位素特征改变具有显著影响。土壤δ¹³C的富集现象主要归因于优势植被由C3植物转变为C4草本植物。