Hydroseeding for degraded/devastated landscapes

Hydroseeding – A Global Road Technology perspective. Post-2019 the bushfire recovery efforts in Australia and New Zealand created unique opportunities for innovative erosion control techniques such as hydroseeding to the vast amounts of areas that succumbed to the bushfires. Marked as the largest fire event ever recorded in eastern Australia it generated hazardous dust and soil exposure of areas that had been ravaged by the bushfires. In New Zealand, the bushfires spread extensively across 2,300 hectares within a 25 km radius of the Nelson-Tasman areas which included a large area of sensitive terrain. Regardless of the extent of damage done by the bushfires the most important priority became finding cost effective and time sensitive solutions to render revegetation of slopes and uneven terrains whilst providing cover to minimize erosion, boost the soil water retention capacity and accelerate seed germination. The efforts to implement hydroseeding in the bushfire recovery also depended on the quality of the soil hence the best results would have been obtained in larger areas of high quality soil therefore there was need for soil analysis for effective hydroseeding. The article seeks to understand the effects of fire on soils, followed by evaluating what hydroseeding entails, where it is utilized and the benefits and drawbacks of using hydroseeding from a Global Road Technology perspective. 

The long and short terms effects of fire

The effects of fire on soil properties can either be direct or indirect, where the direct effects are usually short, and heat related with exception of smoldering fires and burning logs and piles owing to the poor heat conductivity of soil the fire is limited to the first few centimeters of soil. The actual effects depend on the soil type, texture, pre-fire conditions such as moisture content, type and structure of vegetation in terms of density and connectivity, the ecosystem, fire intensity, severity and recurrence, meteorological conditions during the fire and topography in terms of the slope and aspect. The indirect effects of fire are normally related to the ash-bed effects, degree of vegetation recuperation, post-fire weather patterns, topography and post-fire management. The severity of the fires also plays a role in the after effects on the soil properties. Low severity fires can yield beneficial impacts on soil properties depending on the temperature reached which when not high the loss of nutrients by volatilization and with smoke are minimized. Advantages include production of ash rich in carbon, there is an increase in soil organic matter, pH, electrical conductivity and extractable cations such as calcium, magnesium, sodium and some form of nitrogen such as ammonia which is important for vegetation recuperation. The downside of low severity fires is the production of hydrophobic ash that can temporarily increase water repellency resulting in leaching of hydrophobic compounds. 

On the contrary, high-severity fires such as the bushfires experienced in Australia and New Zealand combust large amount of fuel and have extremely negative effects on the soil. Most importantly the large reduction in soil cover. High soil surface temperatures reduce the quantity of organic materials, destroy soil aggregates, increases soil redness and the proportion of sand, increase soil hydrophobicity which increases water repellency, volatilize carbon, nitrogen, and result in loss of nutrients such as potassium, sodium, phosphorus, calcium, magnesium, aluminium and manganese at even higher temperatures. A marked increase in soil pH is noted as a result of organic acid denaturation and the electrical conductivity as a consequence of the mineralization of organic matter. Soil extractable anions that are highly soluble in burned material are leached with the increase in pH supporting solubility of cations such as calcium, magnesium, sodium and potassium whist reducing others such as copper and zinc. Phosphorus solubility is limited which when in solution is easily precipitated with calcium, aluminium or iron. Majority of the soil microbes suffer from thermal shock which changes the composition, size and activity of microbial biomass.

After the burn

Vulnerability of soil to degradation is prevalent on steep slopes, more especially after high severity fires because steep slopes spread flames easily if the fire line heads upslope owing to convection of heat from the fire which preheats the fuel, reducing moisture content before combustion. The effects are mostly felt on dry south facing slopes that are the most susceptible to soil erosion. High fire severities affect seed abundance in the soil as high temperatures can extend for prolonged periods of contact with the soil surfaces and high post fire soil pH from ash leachates decrease the rate of seed germination capacity. Seed germination rate reduces with increasing temperatures and the resultant bare soil areas increase the vulnerability of seed to erosion and prolonged nutrient loss. Post fire seeding in the form of hydroseeding is applied to the burned areas affected by high severity fires and utilized to accelerate the development of vegetation and reduce the nutrient losses. 

The hydroseeding technique involves sparring a homogenous slurry of seeds, fertilizer, binder, mulch and other components such as moisture retention agents and plant growth promoters over large and inaccessible areas. The drawbacks of using hydroseeding are in that the seeding treatments can cause deterioration in plant community diversity through delaying native plant emergence and introduction of non-native species, which compete with regenerating native plants for available resources therefore constraining vegetative dynamics in the long-term. Hydroseeding without hydromulching is also susceptible to erosion by both wind and water and is only really suited to flat terrain or low risk climates. Another proven methodology is to protect seeded areas with polymer erosion control technologies such as GRT: Enviro-Binder – a technique used with great success in the revegetation of Nelson bushfire rehabilitation project.

In Conclusion

Hydroseeding is an alternative technique to the traditional processes of broadcasting or sowing dry seed. It finds its use as an erosion control technique for revegetation, restoration of degraded areas, improve soil properties, nutrient levels and water conditions, increasing plant cover and biomass and is deal for over seeding too. Its cost effectiveness has been approximated to be 66% less than sod and 80% more effect than dry seeding based on 240 m2 local area. Species performance at the germination stage determines the success of hydroseeding with evidence showing that species are able to germinate earlier and at high germination rates they manage to colonize road slopes more successfully than species with low germination rates especially during periods of low water availability. Although this is the case, other schools of thought suggest that the success of native seed mixtures focuses more on plant establishment and less of germination. Overall, Global Road Technology offers both GRT: Enviro-Binder mentioned above and also GRT Nature Plus a spray over liquid polymer which provides phosphorus and nitrogen to increase germination and strike rate for the short and long term hence can be effectively utilized as a key solution for all revegetation needs. 

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REFERENCES 

Clemente, A.S., Moedas, A.R., Oliveira, G., Martins-Loucao, M.A., and Correia, O. 2016. Effect of hydroseeding components on the germination of Mediterranean native plant species. Journal of Arid Environments. 125. 68-72. 

Pereira, P., Francos, M., Brevik, E.C., Ubeda, X., and Bogunovic, I. 2018. Post-fire soil management. Current Opinion in Environmental Science & Health. 1-18. 

Texas A&M Foreset Service. 2012. Wildfire Recovery : Soil Erosion Control Practice Guide. 1-16. 

Vourlitis, G.L., Griganavicius, J., Gordon, N., Bloomer, K., Grant, T., and Hentz, C. 2017. Hydroseeding increases ecosystem nitrogen retention but inhibits natural vegetation regeneration after two years of chaparral post fire recovery. Ecological Engineering. 102. 46-54.