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

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.

[1]  G. Guggenberger,et al.  Factors controlling humification and mineralization of soil organic matter in the tropics , 1997 .

[2]  W. Amelung,et al.  Losses and biogeochemical cycling of soil organic nitrogen with prolonged arable cropping in the South African Highveld — evidence from d- and l-amino acids , 2004 .

[3]  R. Mayer,et al.  Long-term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. V. Rate of loss of total nitrogen from the soil profile and changes in carbon : nitrogen ratios , 1986 .

[4]  Eric A. Davidson,et al.  Changes in soil carbon inventories following cultivation of previously untilled soils , 1993 .

[5]  M. Schnitzer.  Organic Matter Characterization , 1982 .

[6]  J. Mcdonagh,et al.  Soil organic matter decline and compositional change associated with cereal cropping in southern Tanzania , 2001 .

[7]  I. Kögel‐Knabner,et al.  The effect of 10% HF treatment on the resolution of CPMAS 13C NMR spectra and on the quality of organic matter in Ferralsols , 2003 .

[8]  P. Srivastava,et al.  Characteristics of humic substances in cultivated and natural forest soils of Sikkim , 1998 .

[9]  L. Janik,et al.  Changes in chemical nature of soil organic carbon in vertisols under wheat in south-eastern Queensland , 2001 .

[10]  J. Oades,et al.  Paramagnetic effects on solid state carbon-13 nuclear magnetic resonance spectra of soil organic matter. , 2002, Journal of Environmental Quality.

[11]  Wulf Amelung,et al.  Losses of carbon and nitrogen with prolonged arable cropping from sandy soils of the South African Highveld , 2001 .

[12]  John Doe,et al.  Soil Map of the World , 1962 .

[13]  P. Nye,et al.  Changes in the soil after clearing tropical forest , 1964, Plant and Soil.

[14]  K. Giller,et al.  Long‐term changes in organic matter of woodland soils cleared for arable cropping in Zimbabwe , 2005 .

[15]  J. Lehmann,et al.  Land use effects on soil organic matter properties of chromic luvisols in semi-arid northern Tanzania: carbon, nitrogen, lignin and carbohydrates. , 2000 .

[16]  A. Möller,et al.  Forms of organic C and P extracted from tropical soils as assessed by liquid-state 13C- and 31P-NMR spectroscopy , 2000 .

[17]  Biqing Liang,et al.  Carbon K‐Edge NEXAFS and FTIR‐ATR Spectroscopic Investigation of Organic Carbon Speciation in Soils , 2005 .

[18]  Chris Jacobsen,et al.  Origin and mobility of fulvic acids in the Gorleben aquifer system: implications from isotopic data and carbon/sulfur XANES , 2005 .

[19]  K. Paustian,et al.  Bacterial and Fungal Cell‐Wall Residues in Conventional and No‐Tillage Agroecosystems , 1999 .

[20]  R. Gifford,et al.  Soil carbon stocks and land use change: a meta analysis , 2002 .

[21]  H. Ade,et al.  NEXAFS Spectroscopy and Microscopy of Natural and Synthetic Polymers , 2000 .

[22]  Denecke,et al.  Soft X-Ray Spectromicroscopy Investigation of the Interaction of Aquatic Humic Acid and Clay Colloids. , 2000, Journal of colloid and interface science.

[23]  R. Bol,et al.  Molecular dynamics of organic matter in a cultivated soil. , 2002 .

[24]  D. S. Jenkinson,et al.  THE TURNOVER OF SOIL ORGANIC MATTER IN SOME OF THE ROTHAMSTED CLASSICAL EXPERIMENTS , 1977 .

[25]  H. Duschner,et al.  Characterization of humic and fulvic acids from Gorleben groundwater , 1990 .

[26]  W. Gates,et al.  Soil Organic Matter Decomposition and Turnover in a Tropical Ultisol: Evidence from δ13C, δ15N and Geochemistry , 2002, Radiocarbon.

[27]  Sunghwan Kim,et al.  MODERN ANALYTICAL STUDIES OF HUMIC SUBSTANCES , 2001 .

[28]  F. Scheffer,et al.  Lehrbuch der Bodenkunde , 1971, Anzeiger für Schädlingskunde und Pflanzenschutz.

[29]  W. Amelung,et al.  Climatic effects on soil organic phosphorus in the North American Great Plains identified by phosphorus-31 nuclear magnetic resonance , 1998 .

[30]  Hari Eswaran,et al.  Organic Carbon in Soils of the World , 1993 .

[31]  V. Bagnato,et al.  Characterization of humic acids from a Brazilian Oxisol under different tillage systems by EPR, 13C NMR, FTIR and fluorescence spectroscopy , 2004 .

[32]  Ingrid Kögel-Knabner,et al.  The macromolecular organic composition of plant and microbial residues as inputs to soil organic matter , 2002 .

[33]  N. Batjes,et al.  Total carbon and nitrogen in the soils of the world , 1996 .

[34]  C. E. Evans,et al.  Nitrogen and carbon changes in great plains soils as influenced by cropping and soil treatments , 1957 .

[35]  W. Schlesinger.  Biogeochemistry: An Analysis of Global Change , 1991 .

[36]  G. Sposito,et al.  Molecular structure in soil humic substances: the new view. , 2005, Environmental science & technology.

[37]  J. Skjemstad,et al.  The effects of cultivation on the composition of organic matter and structural stability of soils , 1995 .

[38]  Rattan Lal,et al.  Global Potential of Soil Carbon Sequestration to Mitigate the Greenhouse Effect , 2003 .

[39]  F. J. Stevenson.  HUmus Chemistry Genesis, Composition, Reactions , 1982 .

[40]  I. Kögel‐Knabner.  Analytical approaches for characterizing soil organic matter , 2000 .

[41]  D. Anderson,et al.  Organic carbon and sulphur compounds in wetland soils: insights on structure and transformation processes using K-edge XANES and NMR spectroscopy , 2003 .

[42]  R. Spaccini,et al.  Influence of land use on the characteristics of humic substances in some tropical soils of Nigeria , 2005 .

[43]  M. Lemenih,et al.  Soil carbon stocks and turnovers in various vegetation types and arable lands along an elevation gradient in southern Ethiopia , 2004 .

[44]  W. Amelung,et al.  Influence of prolonged arable cropping on lignin compounds in sandy soils of the South African Highveld , 2002 .

[45]  N. Hertkorn,et al.  Functional group analysis of natural organic colloids and clay association kinetics using C(1s) spectromicroscopy , 2003 .

[46]  R. Dalal,et al.  Long-term trends in fertility of soils under continuous cultivation and cereal cropping in southern queensland. ii* Total organic carbon and its rate of loss from the soil profile , 1986 .

[47]  J. Reeder,et al.  CHANGES IN SOIL PROPERTIES IN A CENTRAL PLAINS RANGELAND SOIL AFTER 3, 20, AND 60 YEARS OF CULTIVATION1 , 1990 .

[48]  C. Palm,et al.  Carbon and nitrogen dynamics in a phosphorus-deficient soil amended with organic residues and fertilizers in western Kenya , 2005, Biology and Fertility of Soils.

[49]  W. Amelung,et al.  Land-use effects on amino sugars in particle size fractions of an Argiudoll , 1999 .

[50]  Chris Jacobsen,et al.  Near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy for mapping nano‐scale distribution of organic carbon forms in soil: Application to black carbon particles , 2005 .

[51]  J. Agbenin,et al.  The microbial biomass properties of a savanna soil under improved grass and legume pastures in northern Nigeria , 2005 .

[52]  J. Baldock,et al.  Assessing the extent of decomposition of natural organic materials using solid-state 13C NMR spectroscopy , 1997 .

[53]  J. Skjemstad,et al.  Structural units in humic acids from south-eastern Queensland soils as determined by 13nmr spectroscopy , 1983 .

[54]  R. Houghton.  The annual net flux of carbon to the atmosphere from changes in land use 1850–1990* , 1999 .

[55]  J. Lehmann,et al.  Soil Organic Matter Composition in the Subhumid Ethiopian Highlands as Influenced by Deforestation and Agricultural Management , 2002 .

[56]  Robert J. Naiman,et al.  Soil texture and nitrogen mineralization potential across a riparian toposequence in a semi-arid savanna , 2006 .

[57]  G. Cody,et al.  Determination of chemical-structural changes in vitrinite accompanying luminescence alteration using C-NEXAFS analysis , 1998 .

[58]  I. Kögel‐Knabner,et al.  Composition of organic matter in a subtropical Acrisol as influenced by land use, cropping and N fertilization, assessed by CPMAS 13C NMR spectroscopy , 2005 .

[59]  D. Lewis,et al.  Soil organic carbon content and composition of 130‐year crop, pasture and forest land‐use managements , 2004 .

[60]  J. Skjemstad,et al.  Spectroscopic and chemical differences in organic matter of two vertisols subjected to long periods of cultivation , 1987 .

[61]  Johannes Lehmann,et al.  Sulphur speciation and biogeochemical cycling in long-term arable cropping of subtropical soils: evidence from wet-chemical reduction and S K-edge XANES spectroscopy , 2005 .

[62]  A. Michelsen,et al.  Carbon stocks, soil respiration and microbial biomass in fire-prone tropical grassland, woodland and forest ecosystems , 2004 .