The growth of urban trees is often inhib­it­ed by low soil quality and numer­ous dis­turb­ing factors such as (air) pol­lu­tion or drought stress. There­fore, attempts are gen­er­al­ly made to stim­u­late the growth of urban trees through syn­thet­ic fer­til­iz­ers. In view of neg­a­tive con­se­quences such as green­house gas emis­sions, acid­i­fi­ca­tion, salin­i­sa­tion and losses of soil carbon, more and more cities are looking for alter­na­tive strate­gies. In this study 5 alter­na­tive fer­tilis­er sub­strates were exam­ined for their effect: Biochar (BC), dried sewage sludge (BS), wood chips (WC), compost (COM) and compost tea (ACT).
Dried sewage sludge and biochar had the great­est influ­ence on tree growth and improved soil prop­er­ties. In this study, the growth of urban trees seemed to be limited pri­mar­i­ly by the supply of nitro­gen (N). This was par­tic­u­lar­ly evident in the treat­ment of the soil with dried sewage sludge, where the study showed the largest increase in tree growth and also the largest amount of nitrogen.

Typical urban soils & trees

Three soil types were select­ed for the study to rep­re­sent a gra­di­ent in urban soil quality and pos­si­ble restric­tions on tree growth. On the one hand, the sci­en­tists select­ed an uncom­pact­ed sludge soil as an exem­plary soil of the highest quality. In addi­tion, a com­pact­ed clay soil was used for the exper­i­ments, as an example for the com­pact­ed soil with low organic matter that is typical for cityscapes. As a third variant, the sci­en­tists chose a sandy soil to rep­re­sent a soil with low organic matter, well per­me­able and rel­a­tive­ly dry, which can often be found in street tree sec­tions. The sci­en­tists focused on two tree species: Sugar Maple and Gled­it­sia are common urban trees in the United States. Because of their faster growth reac­tion rates com­pared to old trees, seedlings of these species were used.

Biochar and dried sewage sludge are the better alternatives

Con­ven­tion­al nitro­gen fer­til­iz­ers, compost and wood chips are the most fre­quent­ly used soil improvers in urban areas. However, the results of the study suggest that biochar and dried sewage sludge are prefer­able to these sub­strates. Firstly, the largest increase in tree growth was observed in the treat­ment with dried sewage sludge. The researchers explain this by the fact that dried sewage sludge reduces the pH value of the soil and thus improves the avail­abil­i­ty of nutri­ents (e.g. nitro­gen) and also increas­es micro­bial activ­i­ty. The total tree biomass of both tree species and all three soil types also increased by 44% com­pared to the control for the plant-char­coal vari­ants. In con­trast to the dried sewage sludge vari­ants, the sci­en­tists were unable to detect any changes in the soil pH value of the plant-char­coal vari­ants, but found an increased carbon value. In addi­tion, the micro­bial activ­i­ty only increased in the sandy soils, in all other soils treated with biochar there was no change here.
The researchers were sur­prised that no increase in the total tree biomass was observed for the vari­ants compost, wood chips and compost tea. The desired effect there­fore did not mate­ri­al­ize. The nitrate losses, on the other hand, which can provide an indi­ca­tion of the nutri­ent losses, were the same and minimal for all vari­ants – irrel­e­vant which soil improver was used.
The authors of the study there­fore con­clud­ed that the use of dried sewage sludge or biochar is also attrac­tive for urban tree and soil man­age­ment in view of the many poten­tial envi­ron­men­tal advan­tages. Both sub­strates are pro­duced from waste prod­ucts and recycle mate­ri­als that would oth­er­wise end up in land­fills. An esti­mat­ed 21 million tonnes of wood waste and 12 million tonnes of munic­i­pal green waste are pro­duced annu­al­ly in the USA, of which 54% have so far been simply dis­posed of. Biochar would also have the advan­tage that the carbon con­tained in the biomass would be bound in the long term and stored in the soil and thus would no longer have any climate-dam­ag­ing effects.

Orig­i­nal article: Biochar and Biosolids Increase Tree Growth and Improve Soil Quality for Urban Landscape
Author: Bryant C. Scharen­broch, Elsa N. Meza, Michelle Catania, Kelby Fite
Pub­lished in: Journal of Envi­ron­men­tal Quality 2013, Amer­i­can Society of Agron­o­my. Crop Science Society of America and Soil Science Society of America