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A long‐term field experiment was conducted to determine the effect of the additions of urban waste compost on the physical and chemical properties and enzymatic activities in a calcareous soil (Fluventic Xerochrept). Total porosity (pores >50 µm measured on thin soil sections from undisturbed samples by image analysis) was greater in the plots treated with compost than the control plots due to a larger amount of elongated pores. In the amended plots total and humified organic C, Pb, Cu, and Zn showed a significant increase compared with nonamended plots. Enzymatic activities (L‐asparaginase, arylsulphatase, dehydrogenase, phosphodiesterase, and alkaline phosphomonoesterase) were significantly enhanced by the compost addition thus indicating no inhibiting influence of the heavy metals present. The increased levels of the arylsulphatase, dehydrogenase, phosphodiesterase, and phosphomonoesterase activities were significantly correlated with total porosity: the first three with pores ranging from 50 to 1000 µm, mainly with pores 50 to 200 µm in size, and phosphomonoesterase only with pores whose size was <500 µm. L‐asparaginase activity was not correlated with porosity. Only arylsulphatase, dehydrogenase, and phosphodiesterase were negatively correlated with bulk density.

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Modifications induced in soil porosity and in stability of soil aggregates were studied for 2 years on an Italian sandy loam soil. Aerobic and anaerobic sludges and their composted mixtures with the organic fraction of urban refuse were used and compared with manure. Addition rates were equivalent to 50 and 150 metric tons/ha of manure on the organic carbon basis. A control plot was also present. Porosity and pore size distribution were measured on thin sections prepared from undisturbed soil samples by using electro‐optical image‐analysis equipment. The stability of soil aggregates was determined by a wet‐sieving method.All organic materials increased the total porosity significantly at all sampling times. Differences between the two application rates were generally not significant. The improvement of total porosity caused by sludges and composts was comparable to that of manure. Modifications of pore size distribution were also observed. Stability of soil aggregates increased slightly in treated samples. The best stabilizing effect was shown by the anaerobic sludge.

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