The idea that nutrients are good is deeply embedded in society – driving the profitable vitamin-popping industry and the sale of fertilisers. But nutrients added to the environment can do great harm, including by promoting weed invasions.
Like most people, I haven’t thought much about the destination of nutrients I excrete or discard but I’ve had to confront it recently while searching for an onsite waste treatment system for a property with very infertile soils surrounded by weeds that would welcome more nutrients. Unfortunately, the ‘nutrients are good’ mentality applies to waste water systems.
Most so-called ‘environmentally friendly’ waste treatment systems for unsewered properties promote nutrient-rich effluent as a benefit. ‘A healthy ecosystem thrives on clean water, soil nutrients and minimal pollutants and contamination,’ says a company promoting a worm farm system that ‘provides liquid fertiliser for your soil’. Advertising for an aerobic system trumpets it as ‘environmentally friendly producing nutrient rich water’.
One in 20 Australian homes is not connected to a municipal sewage system and dispose of effluent on their property. This is an increasing trend as houses proliferate in the urban fringe and rural areas beyond the reach of reticulated sewage systems. It will undoubtedly exacerbate weed problems – many studies in different vegetation types show that increasing soil nutrients facilitates weed invasion.
The nutrient problem
Sydney’s bushland reserves on Hawkesbury sandstone soils are a botanical delight with more than 1500 native plant species. The sandstone soils are naturally very low in phosphorus, with concentrations on ridge tops and slopes ranging from 30 to 100 mg/kg, much less than the Australian average of about 300 mg/kg, which is much less than averages elsewhere, such as 650 mg/kg in the UK and 500 mg/kg in the US.[1]
The future of the sandstone reserves depends on maintaining nutrient poverty. Sites receiving stormwater runoff have become very weedy – where soil phosphorus levels exceed 350 mg/kg, weed cover exceeds 80%.[1] Other sources of nutrients are dumped garden rubbish, septic tank effluent and sewage overflows.
Michelle Leishman and Vivien Thomson have found in glasshouse and field experiments that nutrients added to Hawkesbury sandstone soils diminish native plant survival and benefit weeds.[2,3] The survival of most native plants tested (seedlings and 6 months old plants) declined as phosphorus levels increased, and under high phosphorus levels (typical of areas downslope of roads, 260 mg/kg) most plants died. Conversely, higher nutrient levels increased the survival and growth of exotic invaders compared to other plants.
Nutrient enrichment is thought to generally facilitate weed invasion by increasing the amount of unused resources and reducing competition from resident species for those resources.[4] The availability of resources (water, nutrients, light) can increase for many reasons – reduced use by resident vegetation (due to damage or destruction, eg. from heavy grazing or disease) or additional resources (higher rainfall, fertilisation or more light from canopy removal). This means, say Mark Davis and colleagues, who have called it the ‘fluctuating resource theory’, that the invasibility of vegetation communities changes over time as the amount of unused resources fluctuates, and that weed invasions are likely to be episodic events.[5] It means that maintaining or restoring ecological health in many communities requires limiting nutrient additions and maintaining native species diversity to help ensure that available resources are comprehensively used.
Restoring natural nutrient levels
[O]vercoming mechanisms that favour weeds above native species (weed persistence), and conversely re-establishing mechanisms that favour natives above weeds (ecological resistance), remain a foremost restoration challenge.
Suzanne Prober and Georg Whiel[6]
It’s easy to add nutrients but not so easy to remove them. Carbon supplementation (added sugar) has successfully reduced soil nitrate concentrations (by stimulating growth of soil micro-organisms) and reduced weed abundance, but its effect is only temporary.[7] However, Suzanne Prober and Ian Lunt found that short-term sugar treatment or spring burning of a weed-infested temperate grassland (endangered grassy white box woodlands) allowed kangaroo grass to re-establish, which then reduced soil nitrates to natural levels. They concluded that kangaroo grass is ‘a keystone species that regulates nitrate cycling, thereby imparting ecological resistance to invasion by nitrophilic annuals’.[8]
As well as striving to limit nutrient additions in the first place, it should be a high priority to investigate the potential of other native plants to sequester soil nutrients to restore low-nutrient resistance to weed invasion.
References
[1] Leishman M, Hughes M, Gore D. 2004. Soil phosphorus enhancement below stormwater outlets in urban bushland: spatial and temporal changes and the relationship with invasive plants. Australian Journal of Soil Research 42: 197-202.
[2] Thomson V, Leishman M. 2004. Survival of native plants of Hawkesbury Sandstone communities with additional nutrients: effect of plant age and habitat. Australian Journal of Botany 52: 141-147.
[3] Leishman M, Thomson V. 2005. Experimental evidence for the effects of additional water, nutrients and physical disturbance on invasive plants in low fertility Hawkesbury Sandstone soils, Sydney, Australia. Journal of Ecology 93: 38–49.
[4] Davis M, Grime J, Thompson K. 2000. Fluctuating resources in plant communities: a general theory of invasibility. Journal of Ecology 88:528–534.
[5] Davis et al. (2000) identify the following testable predictions arising out of their theory that invasibility is related to fluctuating resource availability:
- Environments subject to pronounced fluctuations, either by periodic enrichment or by release from the resident organisms, will be more susceptible to invasions than environments with more stable resource supply rates.
- Environments will be more susceptible to invasion immediately following an abrupt increase in the rate of supply or decline in the rate of uptake of a limiting resource.
- Invasibility will increase following disturbances, disease and pest outbreaks that increase resource availability
- Invasibility will increase when there is a long interval between an increase in the supply of resources and the eventual capture or recapture of the resources by the resident vegetation.
[6] Prober S, Whiel G. 2011. Resource heterogeneity and persistence of exotic annuals in long-ungrazed Mediterranean-climate woodlands. Biological Invasions 13:2009–2022.
[7] Prober S, Thiele K, Lunt I, Koen T. 2005. Restoring ecological function in temperate grassy woodlands—manipulating soil nutrients, annual exotics and native perennial grasses through carbon supplements and spring burns. Journal of Applied Ecology 42:1073–1085.
[8] Prober S, Lunt I. 2009. Restoration of Themeda australis swards suppresses soil nitrate and enhances ecological resistance to invasion by exotic annuals. Biological Invasions 11:171–181.
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