Enzyme activities have been measured in soil particles to better understand the relationships between biotic and abiotic constituents with enzyme activity of soil, by using a preservative fractionation method by Stemmer et al. (1998), which is quite soft to allow complete recovery of soil enzyme activities (Kandeler et al. 1999a,b,d; Gerzabek et al. 2002; Poll et al. 2003). In the case ofxylanase the sum of the enzyme activities of all isolated fractions considerably surpassed the activity of the bulk soil likely due to aggregate breakdown releasing new enzymatically active surfaces (Stemmer et al. 1998). The same work conducted by using low-energy sonication showed an enrichment of invertase activity in the silt- or clay-sized fractions in relation to the organic C content. Urease activity was mainly located in the smaller fractions, whilst xylanase activity was enriched in sand fractions. Different rates of organic and inorganic amendments increased urease activity in all particle-size fractions, whilst the increase of xylanase activity was variable depending on the fraction when particle-size fractions were prepared by a long-term managed soil (organic and inorganic fertilisation, crop rotation; Kandeler et al. 1999b). The highest enzyme activity-to-C ratios were observed in the smaller fractions and in the coarse particles.
The silt-sized particles were the principal medium-term sink for organic C, applied as organic manures and/or plant residues to an Eutric Cambisol treated for 42 years (Gerzabek et al. 2002). The sand-sized fractions accumulated organic C by increasing organic C levels of bulk soil, thus possibly acting as indicators of organic C content of the investigated soils, whereas the clay-sized fraction contained more stable organic C and was less affected by treatments than the other particle-size fractions. Invertase activity was found largely in silt- and clay-sized particles, whereas xylanase was found in fine sand particles. An increasing bacterial diversity and abundance with decreasing particle size was also evidenced by 16S rRNA-based analysis (Gerzabek et al. 2002). Both fungal and bacterial communities were differently distributed among coarse sand, silt and clay fractions of a Luvic Phaeozem soil subjected to farmyard manure application over 120 years (Poll et al. 2003).
Xylanase activity was mainly located in the coarse sand fraction of the conventional tillage treatment and alkaline phosphatase activity in the silt and clay fraction independently of tillage treatment (Kandeler et al. 1999d). Protease activity was higher in the coarse and in the clay fractions and invertase activity in the silt fraction of the reduced and minimum tillage treatments. Cultivation reduced the activities of arylsulfatase and acid phosphatase in all aggregate size fractions (Gupta and Germida 1988). Microaggregates ofboth native and cultivated soils contained lower enzyme activities than their respective macroaggregates.
The highest numbers of microorganisms and the highest ^3-glucosidase and ^-acetylglucosaminidase activities were found in organic particles of a long-term (100 years) wastewater irrigated soil (Filip et al. 2000), whereas protease activity was concentrated in the smaller organic and silt+clay fractions despite the decrease in microbial counts in these two fractions. The overall effects remained detectable even 20 years after the wastewater irrigation was terminated.
In soil polluted by heavy metals, urease activity was mainly located in the clay fraction, alkaline and arylsulfatase activities in the silt-sized and clay particles, and xylanase activity was almost equally distributed among the particle-size fractions, with a slightly lower activity in the clay fractions. In agreement with earlier studies, the predominance of enzyme activities in different size fractions appeared independent of soil type, soil management and soil pollution (Kandeler et al. 2000).
Both invertase and xylanase activities of silt and clay fractions were hardly affected by plant litter amendment, whereas the enzyme activity of the coarser fractions containing most of the particulate organic matter was directly affected by litter quantity and quality (Stemmer et al. 1999). These studies demonstrated that several soil microhabitats, other than rhizosphere (Sect. 12.2), have fairly distinct physical, chemical and microbiological properties that operate at different spatial scales and support the temporal and spatial heterogeneity of microbial communities (Beare et al. 1995). Fresh organic residues added to soil offer new microbial sites. Thus at the litter-soil interface (detritusphere) a stimulation of the mi-crobial growth was accompanied by an increase in protease, invertase and xylanase activities which exponentially decreased at a distance of a few millimetres from the litter surface probably corresponding to the depletion of dissolved organic C (Kandeler et al. 1999c).
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