Organic Matter and Sustainability - Sustainable Management of Soil Organic Matter - page 27

order to test our hypothesis that higher organic fertilizer application rates
will affect the composition of hot water-extractable SOM, we included a
long-term field experiment on a sandy site which had received 1.6 times
more organic fertilizer than the one at Müncheberg. Stevenson (1982)
speculated that the effect of SOM on soil properties may be smaller in a
loamy than in a sandy soil. Soil properties, such as structure, water storage
capacity or cation exchange capacity (CEC), may therefore be affected
more by fertilization via changes in SOM in a sandy soil than in a loam.
Therefore, an additional objective of this study was to investigate whether
differences in SOM composition due to fertilization could be found in soils
with higher clay contents.

Materials and Methods

Long-term field experiments

A detailed description of the field experiments can be found in Körschens
et al . (1990). Some site and soil characteristics are listed in Table 1.2.1.
The fertilizer treatments were as follows: no organic manure and no
nitrogen fertilizer (control), mineral nitrogen (N), cattle manure with no
mineral nitrogen (FYM) and cattle manure with mineral nitrogen
(FYM + N). Table 1.2.2 gives the mean annual input rates of organic
matter (excluding stubble and roots) and nitrogen fertilizer for the different
sites. Soil samples were taken at 0-25 cm depth using an auger. The soil
samples from each plot were air dried, and sieved to pass 2 mm.

Chemical analyses and spectroscopy

Organic carbon content (C org ) was analysed by elemental analysis (LECO,
CNS 2000) according to DIN ISO 10694 (1994). The cation-exchange
capacity was determined according to DIN 19684 part 8 (1977). SOM was
extracted in two ways:
1. Hot water : 10 g of the soil were mixed with 0.1 dm 3 of distilled water
and heated under reflux for 1 h according to Körschens et al . (1990). The
solid residue was separated by centrifugation followed by a membrane
filtration (0.45 m, Nytran from Schleicher&Schüll®). Ions were removed

from solution by dialysis, then the solution was freeze dried.
2. Sodium pyrophosphate solution : 10 g of soil were mixed with 0.1 dm 3
of 0.1 M Na 4 P 2 O 7 solution and shaken for 6 h at room temperature
(Hayes, 1985). The solid residue was separated by centrifugation. The
remaining solution was adjusted with 1 M HCl to pH = 2, to precipitate
the SOM. After 12 h, the precipitation was completed and the mixture was

µ