Monitoring Soil Processes after Biochar and Compost Amendment of a Technosol.

City parks and gardens are often established on substrates of excavated materials, containing rubble and garbage. In order to support plants, the porosity and the nutrient contents of these materials need to be improved. This can be achieved by adding compost or biochar, two amendments made out of organic wastes, which are produced in large quantities in cities.

Within the frame of a postgraduate teaching course on soil monitoring, we set up a field experiment in a small garden established on a plot that was filled up with construction debris. Half of the garden was shaded by a large tree, while the other half was mostly exposed to the sun. We applied three treatments in three-fold replication to 1 m² plots, namely compost or biochar in doses of 4 kg m^-2 mixed in 0-10 cm depth, and a control plot without amendments. We established 9 plots in the section under shade, and 9 plots  in the section exposed to the sun. During 3 months we monitored the soil temperature, water content, water tension and volumetric water content and CO₂ emissions. We also measured the height of 2 kitchen herbs (Chenopodium ambrosoides and Rosmarinus officinalis), four plants of each herb per plot, just after planting and 8 weeks later and calculated their growth increment in this period.

In the shade, compost and biochar additions improved the water content at small tensions when compared to the control. But in the sun plots no effect of the amendments was detectable. Compost emitted the largest quantities of CO₂. Also Chenopodium ambrosoides had larger growth increments in the compost treatment, while Rosmarinus officinalis had a better survival.