Efficient tools for the assessment and prediction of impacts on soil organic matter (SOM)
are necessary for the development of sustainable management practices aimed at maintaining
and enhancing soil biodiversity. The usefulness of direct SOM measurements in these matters
is constrained by the impossibility of establishing experimentally verifiable changes within a
relatively short time. Soil microbial biomass (SMB) is among the most promising tools for
timely detection and prediction of changes in SOM induced by soil management. The purpose
of this work was to make use of the advantages of long-term field experiments to identify
microbial parameters that could be used as an effective tool for SOM-related assessments and
predictions. The long-term field experiments of the study included 2 soil types (the moderately
and the poorly humified Typical chernozems), 6 traditional (10-field) and 5 ecological (7-field)
crop rotations with and without alfalfa, mineral fertilizers and/or farmyard manure;
continuous black fallow and 5 continuous crops with and without mineral fertilizers with
farmyard manure. Soil samples were collected in the springtime from a depth of 0-20 cm;
passed through a 2 mm sieve and plant material, stones and visible organisms removed
manually; adjusted to 40 percent water holding capacity and pre-incubated for 10 days at 25oC
in the dark, in aerated plastic bags with periodic adjustment of moisture. SOM was determined
by dichromate oxidation followed by back titration of the excess dichromate. SMB was
determined by substrate-induced respiration and/or fumigation-extraction. Basal respiration
was determined by using IRGA or gas chromatography. Metabolic quotient was calculated as
basal respiration expressed per unit of SMB carbon, and microbial quotient was expressed as
the percentage of SMB carbon to the total soil organic carbon.
Significant correlations between SOM and soil microbial parameters (SMB, and/or basal
respiration, and/or metabolic quotient) were observed in the moderately and poorly humified
Typical chernozems of the studied long-term field experiments. These correlations
demonstrated that with time-specific relationships between SOM and SMB related parameters
are reached in the chernozem soils of Moldova, relationships that remain the same across a wide
range of experimental variants with different levels of SOM and SMB, and are conditioned by
peculiarities of soil management. These correlations as well as the difference in the turnover
rates between SMB and SOM permit a practical possibility of using SMB as a tool for SOM-related
assessments and predictions. Once a new soil management practice is introduced and SMB is
allowed sufficient time (comparable to the SMB turnover rates) to approach the new
equilibrium determined by peculiarities of this practice, the quantitative prediction of the
future SOM level becomes predictable from the new microbial parameters and the established
correlational relationship (assuming that given enough time SOM will tend to fit the same
correlational relationship that was observable in the long-term field experiments). These
predictions may be beneficial in such important fields as protection and enhancement of soil
quality and biodiversity, carbon sequestration, development/assessment of sustainable soil
management practices, and others. The implementation of the possibility will provide farmers
with better opportunities for investing in soil quality/biodiversity and will contribute to solving
problems related to climate change and others.