South Africa - Inkaba.org

Fate of soil organic matter in South African crop and pasture
land
C.C. du Preez 1 , W. Amelung 2
1. Department of Soil, Crop and Climate Sciences, University of the Free State, P O Box 339,
Bloemfontein 9300, Republic of South Africa, dpreezcc@ufs.ac.za
2. Department of Soil Science and Soil Ecology, Rheinische Friedrich-Wilhelms-Universität Bonn,
Nussallee 13, Bonn, Germany, wulf.amelung@uni-bonn.de
ABSTRACT
Organic matter influences properties of mineral soils disproportionately to the quantities present. It is thus not
surprising that organic matter is regarded as the most important indicator of soil quality and health. Soil with good
quality and health usually promotes sustainable land use in agriculture. A proper knowledge on the fate of organic
matter in agricultural soils is therefore essential in the design of management practices enhancing sustainable land
use. Several studies were conducted over the past 10 years to establish the fate of organic matter in South African
crop (0-98 years old) and pasture (0-31 years old) land. These studies were done in three agro-ecosystems
dominated by plinthic soils (clay ~ 14%), namely Harrismith (MAP ~ 625 mm and MAT ~ 13.8°C), Kroonstad
(MAP ~ 563 mm and MAT ~ 16.6°C), and Tweespruit (MAP ~ 516 mm and MAT ~ 15.9°C). Representative soil
samples were collected for analyses. The concentrations of organic carbon and nitrogen were determined in bulk
soil, particle size separates, and water-stable aggregate classes. In addition, lignin derived phenols and amino
sugars as markers for plant and microbial residues were measured. Long-term cultivation of native grassland
reduced organic matter in bulk soil by 60%, reaching equilibrium after 30 years. Losses of organic matter occurred
from all particle sizes, with increasing rates as particle size increased. Aggregates decreased in size into finer
classes. However, organic matter contents decreased in all aggregate classes, but fastest in the 2.8 to 8.0 mm class.
Increased ratios of phenolic acids to aldehydes suggested that side chains were increasingly oxidized. Amino sugar
analyses suggested that fungal residues were better preserved than bacterial residues, while microbial residues
declined by 60%. It appears that these losses and changes in organic matter cannot easily be rectified by the
conversion of crop to pasture land.
KEYWORDS: aggregate size, amino sugar, microbial residue, phenolic aldehyde
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