By Sally Brown, University of Washington
Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members.
Ecological impacts of long-term application of biosolids to a radiata pine plantation
Long-term biosolids application alters the composition of soil microbial groups and nutrient status in a pine plantation https://nwbiosolids.org/resource/long-term-biosolids-application-alters-composition-soil-microbial-groups-and-nutrient
Economic analysis of growth response from a pine plantation forest applied with biosolids
Economic analysis of a pine plantation receiving repeated applications of biosolids
Long-term response of forest plantation productivity and soils to a single application of municipal biosolids
This library was supposed to be a travelogue of my recent trip to New Zealand. I was an invited speaker at their version of Biofest - The New Zealand Land Treatment Collective. Much like Biofest, the conference was a relative small group who were happy to see each other and very welcoming to newcomers. Unlike Biofest there was almost no mention of biosolids. Most of the talks were about land treatment of wastewaters including wastewater from dairies and small communities. One step up from septic was about as big as it got. It turns out that a majority of the solids produced in NZ end up in the landfill- not quite in harmony with their reputation as the home of the Shire. But it is really pretty and we did take a hike that took us past the mountain that was used as Mt Doom in the movies. And they do have some forest applications along with a long-term forest research site. So this month we will focus on forest application of biosolids, on Rabbit Island in NZ with a detour to Canada at the very end.
The tree of choice for commercial plantations in NZ is Radiata pine. They are massive and really pretty. The site on Rabbit Island was established in 1997 when the trees were 6 years old. The soil there is a low fertility sand. This is not the location that was used to film Fanghorn Forest. Biosolids were applied at 300 and 600 kg total N per hectare every three years. There was also a control treatment. Each treatment was also replicated +/- thinning. The first two papers focus on the ecosystem impacts of this practice. The first paper includes data on changes in metals in the soil after 15+ years. Zinc and copper, both essential plant nutrients, were higher in the biosolids amended in at least one of the measured horizons. So was Cd, with a whopping increase from 0.07 ppm in the control to 0.26 ppm in the high biosolids rate. The fact that that increase was statistically significant says that the authors had really tight data. There were also large increases in total C, with smaller but significant increases in total N and P in the biosolids amended soils. The microbial population also increased with the biosolids as did the collembolans (springtails or bigger than microbes but still tiny creatures that live in soil). No changes in soil physical properties. So some improvement, certainly no degradation but the big deal is that the trees grew bigger. Stem volume for both the biosolids rates were similar and higher than the control.
The second paper visits the same site with a slightly different focus a few years later. Here authors have done a more detailed study of soil microbes with measures both in winter (Rabbit Island is on the northern tip of the S Island, so winter is a relative term). Here the data seem to be a bit of a mess with larger numbers in the summer over winter and similar numbers for many variables in the control and high biosolids, with smaller number in the standard biosolids rate. They do say that mycorrhizal fungi are lower in the biosolids amended soils, likely because of improved phosphorus fertility. Again - pretty much all fine.
The next two papers discuss impacts on that other kind of green stuff- the financial implications of biosolids use. The first paper is from 2004. The authors say that the growth response seen in the biosolids amended treatments resulted in increased value of the then 11 year- old stand of $217 and $411 NZ dollars per hectare. And that was before NZ got really expensive. Fast forward a few years to the 4th paper and the authors see a 27% increase in stem volume for the regular application and a 36% increase in the high rate in comparison to the control. They see some negative impacts on wood quality with increased number of larger branches and reduced stiffness and density but the increased growth outweighs the negatives. They say that the net value of the logs was increased by 14 and 24% for the standard and high rates of biosolids application.
Take home points: Biosolids makes growing trees greener both by improving soil carbon and nutrients and by making the whole enterprise more profitable.
The last paper in the library takes us to Canada. Results from NZ are one thing, but just to show that these aren’t the work of Gandalf the Grey, it is good to do a comparison. Here trees from 4 different sites near Quebec with multiple tree species were revisited 16-19 years after biosolids were applied. One application was all that it took. Increased tree growth, increased soil carbon and phosphorus concentrations, all echoes of what was seen in NZ. Seems that Ents in NZ and in Canada would all approve of biosolids. As would the microbes and the plantation owners.