Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 357

We used chloroform fumigation extraction to measure microbial
biomass C (Vance et al ., 1987). CO 2 evolution from soils was measured
according to Jäggi (1976) with pre-incubated samples. Solid-state 13 C
nuclear magnetic resonance spectra ( 13 C-NMR) were measured on a
Bruker ASX-400 spectrometer (9.4 Tesla, 100.6 MHz) with cross-
polarization and magic angle spinning (1.0 ms, 4000 Hz). Samples were
pre-treated with 10% hydrofluoric acid for 24 h in order to remove para-
magnetic substances and increase C concentration (Schmidt et al. , 1997).
Light fraction organic matter was extracted from soils by sieving and
density fractionation in Ludox™ (

< 1.13) (Hassink, 1995). A substrate
utilization test was performed according to Garland and Mills (1991) using
GN microplates with 95 different C sources (BIOLOG Inc., Hayward,
California). A functional diversity index was calculated from the intensities
of substrate utilization and the number of substrates used (Fließbach and
Mäder, 1997).

ρ

Results and Discussion

Crop yields were lower in the organic as compared with the conventional
farming systems by 20% for winter wheat, and up to 40% for potatoes. Soil
organic matter and pH were markedly higher in the BIODYN system
compared with the conventional and the unmanured systems (Table 5.2.1).
We suggest these findings to be an effect mainly of the different fertilization
regimes that fertilize the plant either directly or via soil mineralization
processes.
We expected to find changes in soil organic matter composition due
to the long-term application of manure with different degree of maturity.
13 C-NMR spectra, however, did not show any significant differences
between the treatments (Fig. 5.2.1) due to the variation of field replicates.
Uniformity of organic matter from different farming systems has already
been stated by Randall et al . (1995). NMR from whole samples, therefore,
does not seem to be sufficiently sensitive to detect subtle differences in soil
organic matter composition due to farming systems.
Nevertheless, differences in soil organic matter quality were found with
respect to the amount of light fraction particulate organic matter (POM,
ρ

< 1.13) (Fließbach and Mäder, 2000). The light fraction POM was
found to reflect the amount of crop residues that remained in the field, but
on the other hand the light fraction and microbial biomass showed an
inverse relationship. With a high microbial biomass, the amount of the light
fraction was low, indicating that microbial biomass is a regulator of organic
residues in soil, whilst the light fraction may indicate the degree to
which soil organisms are able to decompose residues. We assume, there-
fore, that the quotient of microbial biomass (C mic ) to light fraction POM