Aggregate and soil organic carbon dynamics in South Chilean Andisols

Abstract

Abstract. Extreme sensitivity of soil organic carbon (SOC) to climate and land use change warrants further research in different terrestrial ecosystems. The aim of this study was to investigate the link between aggregate and SOC dynamics in a chronosequence of three different land uses of a south Chilean Andisol: a second growth Nothofagus obliqua forest (SGFOR), a grassland (GRASS) and a Pinus radiata plantation (PINUS). Total carbon content of the 0-10 cm soil layer was higher for GRASS (6.7 kg C m −2 ) than for PINUS (4.3 kg C m −2 ), while TC content of SG-FOR (5.8 kg C m −2 ) was not significantly different from either one. High extractable oxalate and pyrophosphate Al concentrations (varying from 20.3-24.4 g kg −1 , and 3.9-11.1 g kg −1 , respectively) were found in all sites. In this study, SOC and aggregate dynamics were studied using size and density fractionation experiments of the SOC, δ 13 C and total carbon analysis of the different SOC fractions, and C mineralization experiments. The results showed that electrostatic sorption between and among amorphous Al components and clay minerals is mainly responsible for the formation of metal-humus-clay complexes and the stabilization of soil aggregates. The process of ligand exchange between SOC and Al would be of minor importance resulting in the absence of aggregate hierarchy in this soil type. Whole soil C mineralization rate constants were highest for SGFOR and PINUS, followed by GRASS (respectively 0.495, 0.266 and 0.196 g CO 2 -C m −2 d −1 for the top soil layer). In contrast, incubation experiments of isolated macro organic matter fractions gave opposite results, showing that the recalcitrance of the SOC decreased in another order: PINUS>SGFOR>GRASS. We deduced that electrostatic sorption processes and physical protection of SOC in soil aggregates were the main processes determining SOC stabilization. As a result, high aggregate carbon concentraCorrespondence to: D. Huygens (dries.huygens@ugent.be) tions, varying from 148 till 48 g kg −1 , were encountered for all land use sites. Al availability and electrostatic charges are dependent on pH, resulting in an important influence of soil pH on aggregate stability. Recalcitrance of the SOC did not appear to largely affect SOC stabilization. Statistical correlations between extractable amorphous Al contents, aggregate stability and C mineralization rate constants were encountered, supporting this hypothesis. Land use changes affected SOC dynamics and aggregate stability by modifying soil pH (and thus electrostatic charges and available Al content), root SOC input and management practices (such as ploughing and accompanying drying of the soil).

Stabilization of recent soil carbon in the humid tropics following land use changes: evidence from aggregate fractionation and stable isotope analyses

“…The absence of an aggregate hierarchy suggests that in volcanic ash soils aggregates are not predominantly stabilized by particulate organic matter. Formation and stabilization of aggregates is more likely to occur by electrostatic attractions among amorphous Al components and clay minerals in addition to metalhumus complexes acting as binding agents through ligand exchange between functional groups of soil C and Al (Nanzyo et al 1993;Huygens et al 2005). Comparable results have been reported in Chile, where no aggregate hierarchy was found in a volcanic ash soil.…”

“…Huygens et al (2005) found no differences in the aggregate distribution of natural forest, grassland, and a pine plantation. The absence of an aggregate hierarchy suggests that in volcanic ash soils aggregates are not predominantly stabilized by particulate organic matter.…”

Nature, properties and function of aluminum–humus complexes in volcanic soils

“…Research has shown that plantderived SOM is strongly degraded and that it is the microbial SOM fraction that contributes substantially to SOM pools in Andosols (Buurman et al, 2007). Stabilization of SOM in Andosols has been attributed to i) formation of the SOM in organo-mineral and/or organo-metallic (Al/Fe-humus) complexes (Inoue and Higashi, 1988;Nanzyo et al, 1993;Neculman et al, 2013;Percival et al, 2000;Rumpel et al, 2012;Torn et al, 1997), ii) low activity of soil microorganisms due to low soil pH, Al toxicity, low base cation content, and/or P deficiency (Tokashiki and Wada, 1975;Tonneijck, 2009), iii) physical protection of the SOM in stable microaggregates characteristic of variable charge soils (Huygens et al, 2005;Baldock and Broos, 2011), iv) sorption and deactivation of exoenzymes involved in the extracellular depolymerization component of SOM decomposition (Saggar et al, 1994;Miltner and Zech, 1998), v) burial of organic-rich surface horizons by repeated additions of airfall tephra deposition, and vi) the presence of microbiallyrecalcitrant charcoal (especially in melanic epipedons) (Miyazaki et al, 2009(Miyazaki et al, , 2010Nishimura et al, 2006). The stabilization of SOM is reflected in the 14 C age of humic acids extracted from A horizons of Andosols which is reported to range from modern to 30,000 YBP, with the majority in the range 1000-5000 YBP (Inoue and Higashi, 1988).…”

Distinctive organic matter pools among particle-size fractions detected by solid-state13C-NMR, δ13C and δ15N analyses only after strong dispersion in an allophanic Andisol

“…Recent studies reported an apparent lack of aggregate hierarchy in South American Andisols based on the absence of sizedependent changes in C concentration using the dispersion treatments commonly used for non-volcanic soils (Hoyos and Comerford 2005;Huygens et al 2005;Paul et al 2008;Candan and Broquen 2009). By applying 5-10-fold higher sonication energy following sodium saturation pretreatment, on the other hand, we were able to disrupt most of the macro-and microaggregates and found strong evidence of aggregate hierarchy present below the 53-μm scale for an allophanic Andisol (Asano and Wagai 2014).…”