Soil quality. Determination of soil microbial biomass

Soil microbial biomass (SMB) measurements are often used in soil biological analysis; however, their interpretation can be problematic. In this review, both the limitations and benefits of indirect (both CHCl3 fumigation incubation and fumigation extraction, and substrate-induced respiration) SMB measurements are outlined, along with their value and interpretation as attributes or indicators to assess some soil quality (SQ) functions (e.g., enhance plant growth, maintain aggregation, regulate energy) for mainly humid, temperate soils, with specific emphasis on research conducted in eastern Canada and New Zealand. Indirect SMB methods are subject to limitations analogous to "soil test" procedures (e.g., soil sampling and handling, water content, storage prior to treatment), and also the difficulties with establishing an acceptable "control" and fraction (i.e., k value) of SMB mineralized or extracted. In many cases, such limitations present a need for some degree of standardization (e.g., pre-conditions of 7- to 10-d incubation at 25°C and −0.001 MPa water potential) prior to SMB measurement. However, for SQ assessment, where "comparative" rather than "absolute" values of SMB are often of interest, use of commonly derived k values seem appropriate for surface soils.Soil ecological factors govern SMB and often underlie much of the spatial and temporal variation in SMB. Plant species composition, mainly through net primary productivity and litter quality, can affect SMB measurements along with trophic cascades in soil, where interactions among soil organisms can influence microbial activity. Benefits of SMB measurements relate mainly to the assessment of both soil C turnover and management induced changes in organic matter. The combination of SMB and δ13C to elucidate the transformations and fate of organic C in cropping and soil management systems has also shown that both temporal and spatial redistribution of C inputs, and soil type (i.e., particle size distribution) are dominant factors in turnover and nutrient flow through the SMB.For SQ assessment, SMB is not a useful indicator for the function of soil as a "medium for plant growth" in regard to plant productivity for intensively farmed temperate soils. For the function of soil to "maintain aggregation", where SMB is one agent only of a multi-faceted process, the relationship between SMB and soil aggregation is not always present and tends to be site-specific. In regard to the "regulate energy" soil function, SMB is related to some degree with decomposition and mineralization processes. The main role of SMB for SQ assessment is to serve within a minimum data set of other indicators (e.g., macroorganic C) to monitor soil organic C storage and change. Key words: Soil microbial biomass, humid climate, soil quality

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Long-term effects of tillage intensity on the distribution of microbial biomass and activity in four arable field-sites across Europe

10.5194/egusphere-egu2020-9043 ◽

2020 ◽

Author(s):

Ilka Schmoock ◽

Deborah Linsler ◽

Mignon Sandor ◽

Rainer Georg Joergensen ◽

Martin Potthoff

Keyword(s):

Microbial Biomass ◽

Soil Quality ◽

Soil Profile ◽

Soil Microbial Biomass ◽

Reduced Tillage ◽

No Tillage ◽

Tillage Systems ◽

Minimum Tillage ◽

Soil Microbial

Over the last decades, reduced tillage became more and more important as a suitable soil management practice. Moreover, reduced tillage is expected to promote a healthy and active soil life as a feature of sustainable agricultural. The determination of soil microbial biomass and microbial indices are suitable indicators for estimating soil quality. This study follows a regional approach and focusses at four different countries with varying environmental conditions at long-term experimental field-sites (LTE&#180;s) across Europe. Soil microbial biomass carbon (SMB-C), the metabolic quotient (qCO2) and the ratio of SMB-C to soil organic carbon (SOC) were measured as microbial properties.Our contribution to the ongoing discussion of the effectiveness of non-conventional tillage systems is (i) the comparison between conventional ploughing (CT) and minimum tillage (MT), (ii) the comparison of inversion vs. not inversion tillage at the same working depth, (iii) the comparison of ploughing vs. no-tillage (NT), (iv) the comparison between reduced tillage systems with each other (MT vs. NT).We found a significant difference of SMB-C for CT and MT between 0 and 10 cm in Germany and Sweden, but no difference between tillage treatments for the sampled soil profile (0-30 cm). We highlight that tillage changed the vertical distribution of SMB-C, showing similar values among soil depths under CT and a depth gradient with decreasing values for MT.The comparison of inversion vs. not inversion tillage at the same working depth in Romania showed no differences between CT and MT at all. This suggests that humus-rich soils seem to be more resistant to tillage-related disturbances. The working depth might have a greater impact for both, inversion and non-inversion tillage than the type of the tillage system itself.For the comparison of CT and NT, we used the field-sites in Spain and Sweden. In Spain, NT was clearly of advantage for microbial biomass and activity, compared to CT. This was true for the whole sampled soil profile (0-30&#160;cm) whereas in Sweden differences could only be detected between SMB-C levels in two soil depths. Our results indicate that the effect of tillage seems to be smaller in cold-temperate areas.Comparing MT and NT in Sweden, we found no difference in SMB-C between these two forms of conservation tillage, neither in the first centimeters, nor in the whole sampled profile. Consequently, minimum tillage seems to be an alternative in cold and moist regions if no-tillage is not possible to apply without reducing soil quality or crop yields.We conclude that even if minimum and no-tillage are generally beneficial for microorganisms, there is a big variance between the different forms of reduced tillage systems. Thus, statements cannot be made across different soils and machine types, but have to be made on a regional scale.&#160;

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