Competitive interactions between two meadow grasses under nitrogen and phosphorus limitation

Abstract

Summary1. Different grass species dominate grasslands fertilized with nitrogen (N) or phosphorus (P), possibly due to the impact of N : P stoichiometry on competitive interactions. How species compete for nutrients, and whether the mechanisms are similar for N and P, is still not fully understood. 2. We investigated whether the outcome of competition between Alopecurus pratensis and Agrostis capillaris depends on N : P stoichiometry, and on the ability to acquire supplied N or P. Monocultures and mixtures of the two species were grown in pots at nine combinations of N and P supply (N : P ratios 1AE7, 15 and 135, all at three supply levels). After 3 months, we determined plant biomass, morphological traits and nutrient concentrations of plant tissues. 3. N : P supply ratios had similar effects on the growth of the two species but contrasting effects on their competitive strength, as assessed by comparing growth in competition to growth in monoculture: Alopecurus was the stronger competitor under N limitation (N : P ratio 1AE7), whereas Agrostis was an equal or stronger competitor under P limitation (N : P ratio 135). This result resembled patterns of species distribution observed in the field. 4. The strong competitive response of Alopecurus at the low N : P supply ratio was associated with a high investment in root biomass and root length, and a high nitrogen productivity. The competitive response of Agrostis at the high N : P supply ratio was associated with low root mortality and high root phosphatase activity. 5. Our results obtained under N limitation support the pre-emption theory in which plants with the highest root length are able to acquire more N from the soil than their competitors, and therefore can suppress their growth. Under P limitation, however, plant investment in root length could not explain competitor suppression. Here, other factors important in competition for P, such as mycorrhizal hyphal length, root longevity, or exudation rates of P releasing compounds, merit investigation.

Legumes have a higher root phosphatase activity than other forbs, particularly under low inorganic P and N supply

“…The plants were germinated from seeds 2-3 weeks prior to the experiment. In this experiment, I aimed to compare the maximal PME activities of these species, and therefore grew the plants with very little P, an excess of N, and non-limiting doses of other macro-and micronutrients (K, Ca, Mg, S, Fe, Bo, Mn, Zn, Mo and Cu; for forms and doses see (Olde Venterink and Güsewell 2010)). Thus, the pots received 0.2 mg P (total in 8 weeks applied as NaH 2 PO 4 ) and 128 mg N (applied as NaNO 3 ), yielding a N:P supply mass ratio of 640.…”

“…Generally, root phosphatase activity increases as either soil P availability decreases or N supply increases (Li et al 1997;Hayes et al 1999;Treseder and Vitousek 2001;Phoenix et al 2004;Fujita et al 2010;Olde Venterink and Güsewell 2010). These effects can both be seen as responses to increased P limitation, since N and P are the most common growth-limiting nutrients in terrestrial ecosystems (Elser et al 2007).…”

Does phosphorus limitation promote species-rich plant communities?

“…In the case of competition for nutrients, the superior competitor will acquire a resource from the rhizosphere more rapidly than its competing neighbours, and the total root length of the competing plants is the decisive trait to enable this (Craine et al 2005). Indeed, Olde Venterink and Güsewell (2010) recently demonstrated that this pre-emption competition mechanism was supported for two competing grass species under N limitation, but not under P limitation. Under P-limitation, traits other than root length, such as root phosphatase activity and root mortality appeared to be more important for the outcome of competition for these two grasses.…”

Root system variability in common legumes in Central Europe

“…The root systems of individual plants can be highly modified by root competition as the competition for nutrients provokes higher energy investment into roots in order to increase their density and length (Olde Venterink & Güsewell 2010). The presence of nutrient-rich patches in the soil profile may also modify the root system because roots are able to proliferate and grow preferentially into nutrient-rich patches (Robinson 1994;Forde & Lorenzo 2001;Hejcman et al 2011).…”