Land‐use effects on the composition of organic matter in particle‐size separates of soils:

II. CPMAS and solution ¹³C NMR analysis

authored by

G. Guggenberger, W. Zech, L. Haumaier, B. T. Christensen

Abstract

Soils from A horizons of Eutrochrepts under spruce forest (Sf), mixed deciduous forest (Df), permanent grassland (Gp), and arable rotation (Ar) were fractionated into clay‐ (<2 μm), silt‐(2–20 μm) and sand‐ (20–2000 μm) sized separates. ¹³C NMR spectroscopy was used to compare SOM composition across size separates and between land‐use regimes. CPMAS ¹³C NMR spectroscopy showed that the intensity of signals assigned to carbohydrates (representing most O‐alkyl C) and lignin (phenolic and methoxyl C) declined with decreasing particle size. Concurrently, alkyl C and C‐substitution of aromatic C increased in the order sand, silt, clay. The amount of alkyl C correlated well with microbial resynthesis of carbohydrates. Solution ¹³C NMR spectra suggested that humic acids (HA) extracted from the size separates were richer in carboxyl C and aromatic C than the bulk size separates. Also HA reflected increasing percentage of alkyl C with decreasing particle size. O‐alkyl C were lower in silt HA than in clay HA whereas aromatic C tended to peak in silt HA. These results suggested that sand‐sized separates were enriched in plant residues (primary resources) whereas clay‐sized separates were dominated by products of microbial resynthesis (secondary resources). Silt was rich in selectively preserved and microbially transformed primary resources. ¹³C NMR spectroscopy showed only small differences in SOM composition between land‐use regimes, except that silt and silt HA from Ar were richer in aromatic C than those from the other plots. But enrichment factors (E= content in fraction/content in whole soil) revealed differences in the distribution of C species across the size separates. Relatively high E_aromatic (0.9) and E_o‐alkyl (1.0) for sand from Gp indicated high amounts of plant residues, probably due to intense rhizodeposition and to occlusion of plant debris within aggregates. Low E_aromatic (0.3) and E_o‐alkyl (0.3) for sand from Ar suggested depletion of primary resources, which could be attributed to disintegration of soil aggregates upon cultivation. A pronounced enrichment of alkyl C in Ar clay‐sized separates (E_alkyl= 3.1) suggested large amounts of microbial carbon. Microbial products attached to clay surfaces by a variety of physico‐chemical bondings appeared more stable against mineralization induced by cultivation than plant residues sequestered in aggregates.

External Organisation(s)

University of Bayreuth
Aarhus University

Type

Article

Journal

European journal of soil science

Volume

46

Pages

147-158

No. of pages

12

ISSN

1351-0754

Publication date

03.1995

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Soil Science

Sustainable Development Goals

SDG 15 - Life on Land

Electronic version(s)

https://doi.org/10.1111/j.1365-2389.1995.tb01821.x (Access: Closed)