Abstract

The Acacia caven (Mol) is one of the most widespread tree species of the Mediterranean-type ecosystem of South America, forming the "Espinal" ecosystem characterized by a complex and heterogeneous structure like savannas. In South-Central Chile, these Espinal ecosystems include 2 million hectares of agricultural and agroforestry soil. We analyzed the effect of A. caven (land coverage) on C-distribution and chemical composition of soil organic carbon (SOC) fractions. Two ecosystems with a gradient of A. caven vegetation were studied: a well-preserved Espinal (WPE; with 51-80% land coverage) and Degraded Espinal (DE; with less than 25% of land coverage). The soil under the trees was physically fractionated by wet sieving to obtain light (> 212 μm; LF), intermediate (53-212 μm; IF) and heavy (< 53 μm; HF) fractions of SOC. The SOC fractions were analyzed by using Fourier-transform infrared spectroscopy (FTIR), natural abundance of 13C relative to 12C (δ13C) and Nuclear Magnetic Resonance Spectroscopy of 13C (13C NMR) techniques. The results indicate that the A. caven coverage in the Espinal ecosystem affected the distribution (dry mass of each fraction) and C content of SOC fractions, decreasing both parameters in the more degraded ecosystem (DE), especially in the most labile fractions (LF and IF). Increases in δ13C value in the profiles of soils, due microbial isotopic discrimination were observed for IF and HF fractions, but not for LF fractions. The SOC fractions showed important chemical structural differences, while the LF fraction was the most reactive fraction (with more carboxylic groups), followed by IF and HF fractions, in consecutive order. The chemical composition of SOC fractions was affected by the A. caven coverage. In ecosystems with less land coverage increase of aromaticity and decrease of aliphaticity in LF and IF fractions was noted. © 2007 Elsevier B.V. All rights reserved.