23 November 2011

Dirty needs non dirt heap


As I mow my relatively small lawn I am always faced with what I should do with the lawn clippings. I could put them in a green waste bin or put them back into my garden soils. I usually go for the latter - it's easier and I feel like it's a better option for my garden. Nutrients go back into the soil, carbon is contained in my property, I don't get a sore back from carrying the catcher to the bin. The same decision making probably happens all over the world in people's yards. Given the sheer area of urban soils around the world this is probably an instance of where lots of small decisions add up to a major impact on the world around us. Just how useful is putting greenwaste back into the soil?
Nick Dickinson moved to New Zealand last year to take up a position as the Head of the Department of Ecology at Lincoln University. In between earthquake aftershocks he has found time to publish on this issue of urban soils with Luke Beesley. In a study published in Soil Biology and Biochemistry, Nick looks at what happens to dissolved organic carbon and heavy metals under different soil treatments.

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Nick set up experimental containers of soil and added greenwaste to some, biochar (a coal-like product) and noncomposted woody material in others. He also looked at the affect of adding earthworms as they churn the soil. The experiments were left for two months before water was collected from each and analysed for dissolved organic carbon and heavy metals. Addition of the various materials all contributed to increased mobility of dissolved organic carbon and heavy metals (rather than staying 'locked' into the soil they were moving around in water). Greenwaste seemed to increase this mobility compared to the biochar and woody materials. Earthworms also contributed to this mobility.
What does this mean for urban soils? While these additions increase the health of the soil, often urban soils are high in heavy metals. Ideally we do not want these metals to move out of the soils where they could contaminate ground water. Urban soils are also a useful place to lock up carbon and, again, we don't want it to move out of the system. So Nick and Luke conclude that non-composted additions are probably better to add to soils. However, they are quick to point out that doing these experiments at larger scales (like a backyard) are important before we read too much into the results. So I'll go on adding the lawn clippings to my garden soil in the meantime while knowing that acting locally really may be affecting globally!

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