Long‐term impacts of anthropogenic perturbations on dynamics and speciation of organic carbon in tropical forest and subtropical grassland ecosystems

Abstract

Anthropogenic perturbations have profoundly modified the Earth's biogeochemical cycles, the most prominent of these changes being manifested by global carbon (C) cycling. We investigated long-term effects of human-induced land-use and land-cover changes from native tropical forest (Kenya) and subtropical grassland (South Africa) ecosystems to agriculture on the dynamics and structural composition of soil organic C (SOC) using elemental analysis and integrated 13 C nuclear magnetic resonance (NMR), near-edge X-ray absorption fine structure (NEXAFS) and synchrotron-based Fourier transform infrared-attenuated total reflectance (Sr-FTIR-ATR) spectroscopy. Anthropogenic interventions led to the depletion of 76%, 86% and 67% of the total SOC; and 77%, 85% and 66% of the N concentrations from the surface soils of Nandi, Kakamega and the South African sites, respectively, over a period of up to 100 years. Significant proportions of the total SOC (46-73%) and N (37-73%) losses occurred during the first 4 years of conversion indicating that these forest-and grassland-derived soils contain large amounts of labile soil organic matter (SOM), potentially vulnerable to degradation upon human-induced land-use and land-cover changes. Anthropogenic perturbations altered not only the C sink capacity of these soils, but also the functional group composition and dynamics of SOC with time, rendering structural composition of the resultant organic matter in the agricultural soils to be considerably different from the SOM under natural forest and grassland ecosystems. These molecular level compositional changes were manifested: (i) by the continued degradation of O-alkyl and acetal-C structures found in carbohydrate and holocellulose biomolecules, some labile aliphatic-C functionalities, (ii) by side-chain oxidation of phenylpropane units of lignin and (iii) by the continued aromatization and aliphatization of the humic fractions possibly through selective accumulation of recalcitrant H and C substituted aryl-C and aliphatic-C components such as (poly)-methylene units, respectively. These changes appeared as early as the fourth year after transition, and their intensity increased with duration of cultivation until a new quasi-equilibrium of SOC was approached at about 20 years after conversion. However, subtle but persistent changes in molecular structures of the resultant SOM continued long after (up to 100 years) a steady state for SOC was approached. These molecular level changes in the inherent structural composition of SOC may exert considerable influence on biogeochemical cycling of C and bioavailability of essential nutrients present in association with SOM, and may significantly affect the sustainability of agriculture as well as potentials of the soils to sequester C in these tropical and subtropical highland agroecosystems.

Synchrotron‐Based Near‐Edge X‐Ray Spectroscopy of Natural Organic Matter in Soils and Sediments

“…They showed peaks at 285.2 eV indicating aromatic carbon, at 286.7 eV indicating phenolic carbon, and at 288.6 eV indicating carboxyl C. These peak positions were also found in the river NOM of the IHSS standard (Figure 17.7 in Section 17.3.1) and have been reported for a wide variety of NOM (Rothe et al, 2000;Scheinost et al, 2001;Schäfer et al, 2003). Also, a significant decline in total soil organic matter from more than 10% to about 1% did not change the peak positions in a chronosequence study in Western Kenya (Solomon et al, 2007a). The positions of these three dominant peaks appear to be constant across a wide climatic, edaphic, and degradational gradient.…”

“…Clay size separates (<2 µm) had more reduced organic sulfur (12-28% of its total organic sulfur contents) than did silt size fractions (2-20 µm) (7-15%) (Solomon et al, 2003) and had more aromatic carbon (11-16% of total organic carbon) than did silt size separates (6-8%) (Solomon et al, 2007a) regardless of vegetation cover. Density separates obtained from a Kenyan Hapludox showed little difference in peak heights at the carbon K-edge between free light, intra-aggregate light, and organo-mineral fractions (NaI at 1.8 g cm −3 ; Lehmann et al, unpublished data 2007).…”

“…The available data from grassland soils may indicate a slightly lower proportion of phenolic carbon compounds (Figure 17.14). With progressive degradation, Solomon et al (2007a) found a clear relative decrease in phenolic carbon and increase in aromatic carbon as well as the most oxidized carbon forms (compare agricultural and forest soil from Kenya in Figure 17.14). A greater oxidation through longer agricultural activity was also observed for organic sulfur forms in grassland soils from South Africa (Solomon et al, 2005b).…”

“…NEXAFS spectroscopy, microspectroscopy and the development of NEXAFS spectromicroscopy (explained in Section 2.3) have been one of the most promising tools for the study of soils and sediments since its development in the 1980s (Kirz et al, 1995;Ade and Urquhart, 2002), but was only more widely used for investigations of NOM since the mid-1990s. The scope of studies has included contaminant and metal speciation, uptake, and distributions (Schulze et al, 1995), model humic and natural substances (Vairavamurthy et al, 1997;Plaschke et al, 2004Plaschke et al, 2005Rothe et al, 2004), microorganisms (Thieme et al, 1994;Lawrence et al, 2003;Liang et al, 2006), soils (Schmidt et al, 2003;Kinyangi et al, 2006;Schumacher et al, 2005;Lehmann et al, 2007;Wan et al, 2007) and soil extracts (Solomon et al, 2005a(Solomon et al, 2007a, dissolved organic matter (DOC) (Schumacher et al, 2006), sediments Benzerara et al, 2006;Haberstroh et al, 2006, Brandes et al, 2007 or aquatic colloids (Niemeyer et al, 1994;Thieme et al, 1998;Rothe et al, 2000;Brandes et al, 2004), and DNA (Ade et al, 1992). NEXAFS has also been successfully used for the environmental study of coal (Cody et al, 1998), plant fossils (Boyce et al, 2002) or soot (Braun et al, 2007), which are not the subject of this contribution.…”

Long term change in chemical properties of preindustrial charcoal particles aged in forest and agricultural temperate soil

“…A1), as conversion of land from forest to cropland decreases soil C stock (e.g. Nye and Greenland, 1964;Guo and Gifford, 2002;Goidts and van Wesemael, 2007;Solomon et al, 2007). Soils were sampled in November 2014.…”