Soil Fertility and Invasive Species

Soil Fertility and Invasive Species

Exotic plants can alter ecosystems by altering the soil community, soil structure, and soil fertility. 

After invasion, exotic plants can alter the soil community composition and the relationship between soil and plant community indirectly by altering plant-derived soil inputs. The timing, quality, and quantity of litter production affects nutrient inputs to the soil.  At one extreme, an invasive plant could add so much litter that the potential for more frequent and more intense fires can occur and effect the above-ground community, as well as the soil community structure, function, and soil fertility.  Alternatively, invasive plants can alter the soil community directly by releasing secondary compounds from their roots into the soil (Wolfe 2005).  Allelopathic exudates are highly inhibitory to plants and/or soil microbes in invaded communities, but ineffective against natural neighbors that had adapted over time.  While there are many thousands of different LMW (low molecular weight) products, there are only a relatively few for which a function has been identified.  These jobs include soil nutrient acquisition, defense against herbivory, root communication, and antimicrobial protection (Calloway 2004).

Diffuse knapweed (Centaurea diffusa) is a weed that was imported from Eurasia and has spread widely in the semiarid grasslands of western North America.  It is a short-lived perennial that produces a rosette in the first year, and in the second year, flowers, sets seeds, and dies.  The dead plant can act like a tumbleweed and spread the seeds.  Its spread has resulted in loss of native plants for grazing animals (Invasive Plants, Seastedt 2005). 

 Centaurea diffusa  rosette                          mature plant                                    flower

                                                                Centaurea maculosa 

Diffuse knapweed is known to produce a root exudate, 8-hydroxyquinoline, that alters soil fertility by chelating phosphorus and suppressing the ability of North American native plants to acquire phosphorus.  Curiously, plants that had evolved with knapweed in Asia were less affected.  A related species that has successfully invaded North America is spotted knapweed (Centaurea maculosa). 

It produces a root exudate, (+/-)-catechin, a racemic mixture of which one enantiomer suppresses plant pathogens, and the other has phytotoxic activity. The latter activity is caused by triggering intracellular reactive oxygen, eventually leading to cell death.  In addition, this plant is able to alter the soil biota, arbuscular mycorrhizal fungi, so that it can parasitize carbon (up to 15% of plant carbon) from neighboring native bunchgrass (Invasive Plants, Weidenhamer 2005).

               

Garlic mustard (Alliaria petiolata) was introduced to the U.S. in 1868 from Eurasia and is now widely distributed.  It is an obligate biennial that produces first year rosettes that over-winter under snow, bolt early in the spring, set seed, and die by mid-summer.  It is unusual as an invasive herbaceous plant because it is shade tolerant, non-mycorrhizal, and can invade forest understory, even when there is no disturbance.  Like knapweed, it produces a number of secondary compounds.  Among these are glucosinolates, sulfur and nitrogen containing compounds that degrade into volatile cyanide molecules.  These compounds then inhibit herbivory, plant growth, and fungal growth (Rodgers 2008). 

                                                                       Alliaria petiolata

Experimental findings suggest that garlic mustard suppresses growth of arbuscular and ectomycorrhizal fungi.  While garlic mustard is non-mycorrhizal, most plant species do form symbiotic relationships with these fungi so an advantage is gained over competitors by suppressing this mutualism.  Garlic mustard does not appear to affect soil bacteria and the non-mycorrhizal fungi, and was found to increase litter decomposition, increase soil pH, and increase N, P, and base cation availability.  Phosphatase activity is low, so increased phosphate is felt to be due to the garlic root exudates that increased pH and decreased sorption of inorganic phosphorus by aluminum and iron.  Finally, positive feedback is seen by the fact that garlic mustard grows better in soils it has previously occupied, compared to soils occupied by native plants.  Changes in nutrient availability caused by garlic mustard are detrimental to native plants but beneficial for its own growth (Rodgers 2008).

           

Non-indigenous plants have developed weapons in the lands where they evolved that are less effective there because other species have adapted to those characteristics.  In new lands, these weapons affecting soil characteristics and mutualisms are used to their advantage to suppress competition, expand the invasive species abundance and territorial size, and alter the ecosystems.

References

1. Calloway, Ragan M., Ridenour, Wendy M. (2004). Novel Weapons: Invasive Success and the Evolution of Increased Competitive Ability. Frontiers in Ecology and the Environment, 2:436-443.

2. Rodgers, Vikki L., et al. (2008). The invasive species Alliaria petiolata (garlic mustard) increases soil nutrient availability in northern hardwood-conifer forests. Oecologia, 157:459-471.

3. Seastedt, Timothy R. et al. (2005). Understanding invasions: the rise and fall of diffuse knapweed (Centaurea diffusa) in North America. Invasive Plants: Ecological and Agricultural Aspects (Birkhauser Verlag, Boston) pp 129-139.

4. Weidenhamer, Jeffrey D., Romeo, John T. (2005). Allelopathy as a mechanism for resisting invasion: the case of Polygonella myriophylla. Invasive Plants: Ecological and Agricultural Aspects (Birkhauser Verlag, Boston) pp 167-177.

5. Wolfe, Benjamin E., Klironomos, John N., (2005). Breaking New Ground: Soil Communities and Exotic Plant Invasion. BioScience. 55:477-487.