Enhancing ecosystem services in cities using wood wastes

The most common green spaces in our cities are mown pasture on road verges, parks and private lawns.  We think adding biowastes to these ubiquitous green spaces could improve the ‘ecosystem’ services they deliver, for example retaining storm water (helping reduce flooding) and supporting pollinators (bees) and beneficial predator insects. Woody organic wastes and composted greenwastes are used in large volumes overseas to enhance degraded urban soils. Spreading and/or incorporating minimum depths and qualities of organic soil conditioners is now mandated in parts of USA and Canada through rules and policies [1] and was mandated in Long Bay, North Shore City [2]. Poor quality urban pasture can have high ongoing financial costs (for mowing) and environmental costs where compacted, or intensively irrigated, fertilised and/or sprayed.

In 2015 a corner of old pasture at the Auckland Botanic Gardens (ABG) was divided into 49, 2 by 2 m plots. Two rates of coarse wood chip from macrocarpa trees, were spread over 14 of the plots. 

Macrocarpa mulch ResizedImageWzYwMCw0NTBd

 

Five other treatments included a control (untreated) and additions of sawdust, sugar and nitrogen.  A high rate (8 litres/m2) was defined as the maximum ground staff considered was practical without smothering the turf or impeding mowing; we also used a half rate of 4 litres/m2 9. We wanted to maintain public use of the area and so didn’t want to disturb the surface by rotary hoeing. Rotary hoeing is the usual way organic amendments are added, and happens at the time of turf renewal or renovation. We hypothesised that an annual addition of woody biowaste to the surface would:

  • lower soil nitrogen levels and increase competitiveness of a variety of herbs, including ones used by beneficial insects (clovers, plantains and wild carrots)
  • reduce turf growth, hence reduce the mowing frequency (saving $)
  • increase infiltration rates as the wood chip broke down, reducing surface runoff and flooding, and improving drought resilience

Unfortunately, mowing plots with the high rate of coarse wood chip damaged mower blades, so in the second year we substituted a much finer biowaste (compost). An advantage of the compost was that rain could wash it down to the soil surface - this minimised visual impact, improving aesthetics compared with the coarse mulch.

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