P and Phytodiversity

Highest plant diversity has often been found to be correlated with low P availability (Table 6.1). Different shapes of this relationship have been found, e.g., linear or hump-backed shapes. The form of these shapes seems to be independent of the overall amount of P in the soil. However, comparison between different studies is hindered by different methods of P extraction as well as different units. In the

Table 6.1 Literature overview of the relation between species richness and P concentration

Habitat

Relation species richness to P

P concentration

Reference

Old permanent

Negative (hump-

Soil extractable P

Janssens et al.

grassland, Western

backed, optimum

(acetate-EDTA) 0-350

(1998)

and Central Europe

at approx. 30 mg

mg kg-1

kg-1)

Alpine meadows, Italy

Negative (linear)

Soil extractable P (Olsen)

Marini et al.

13-155 mg kg-1

(2007)

Grassland or related

Negative (hump-

Soil extractable P (Olsen)

Critchley

vegetation in

backed, optimum

1-85 mg L-1

et al.

environmentally

at 4-15 mg P L-1)

(2002)

sensitive areas, GB

Arable field

Negative (linear)

Soil extractable P (acid-

Ma (2005)

boundaries, Finland

ammonium-acetate)

2.95-12.21 mg L-1

Park Grass Experiment,

Negative

Fertilization with or

Crawley et al.

England

without 35 kg P ha-1 in

(2005)

combination with other

nutrients since 1856

Low-productive

Negative

Fertilization with

Hejcman

grassland, Germany

or without 80

et al.

kg P annually in

(2007a)

combination with other

nutrients since 1941

Table 6.1 (continued)

Habitat

Relation species richness to P

P concentration

Reference

Semi-natural and urban

Negative

Log soil extractable P

McCrea et al.

meadows, England

(Truog's) 0.15-1.35

(2004)

Mesotrophic grassland,

Negative (not

Soil extractable P

Aerts et al.

Netherlands

significant)

(ammonium lactate)

(2003)

12.4-281 mg kg-1

(means), biomass N/P

ratios: between 4.5 and

15.9 (means)

Open, grassy woodlands,

Negative

Soil extractable P

Dorrough

Australia

(Colwell) 10 - 45 mg

et al.

kg-1 (means)

(2006)

Wet meadows and fens,

Negative correlation

Soil extractable P

Olde

The Netherlands,

with endangered

(ammonium acetic-acid

Venterink

Belgium

species

lactic-acid) 1.3-4.1 g

et al.

m-2 (means)

(2001)

Herbaceous terrestrial

Sites with intermediate

Plant biomass N/P ratios:

Wassen et al.

ecosystems across

N/P ratios most

between 2 and 60

(2005)

Eurasia

species-rich

(hump-backed),

negative

correlation with

endangered species

Degraded broad-leaf

Positive correlation for

Soil extractable P

Fu et al.

forest, China

trees and shrubs,

(ammonium carbonate)

(2004)

negative for forbs

9.85 - 13.33 ppm

(means)

Mediterranean dwarf-

No effect on total

Fertilization with 0, 4.5

Henkin et al.

shrub community,

richness, but

or 9 g P m-2 in 1988,

(2006)

Israel

positive correlation

measurements between

with annual

1989 and 1993

legumes

Fens and wet grasslands,

Plots with high N/P

Plant biomass N/P ratios:

G├╝sewell

Europe and USA

ratio more

between 4 and 36, plant

et al.

species-poor than

P concentrations 0.5-4

(2005)

those with low N/P

mg g-1

ratios

Grassland, New Zealand

No clear relation

Soil P (method not

White et al.

specified) 7-41 ppm

(2004)

Salt marsh, New

Positive relation

Soil extractable P (acetic

Theodose

England

acid, ammonium

and Roths

hydroxide) 15-23 mg

(1999)

kg-1 soil (means)

following, we will discuss the impact of P on phytodiversity compared to the influence of other nutrients.

According to the law of the minimum, P should only influence the growth and competitive strength of plants if it is the limiting factor. The niche dimension hypothesis predicts that a larger number of limiting resources in a habitat leads to a larger number of coexisting species. The prediction was consistent with experimental results and developments in the Park Grass Experiment (Harpole and Tilman 2007). For the often found importance of P limitation for phytodiversity, this could mean that (a) P limitation is often coupled to limitation of other resources. This was for example the case in the studies of a degraded broad-leaved forest in China (Fu et al. 2004), where P limitation was coupled to low potassium concentrations, and of continuously or rotationally grazed pastures in New Zealand (White et al. 2004), where the availability of all nutrients was generally correlated. However, it could also mean that (b) P is the main limiting resource in the examined habitats, so that it controls the dimension of the niche to a large extent. In many habitats, N deposition has reduced the former importance of N limitation. In a meta-analysis of recently published studies, Elser et al. (2007) have shown that N and P limitation are equally important in terrestrial systems, independent of the latitude. An increasing importance of P limitation could increase the correlation between P concentration and the total extent of the niche (Fig. 6.2). Furthermore, habitats with sufficient P but low N availability may promote the growth and N fixing of legumes, leading to higher N concentrations (Almeida et al. 2000; Saber et al. 2005).

Gusewell (2004) has pointed out that interspecific competition in P-limited habitats might be less than that in N-limited ones, so that species' coexistence could be favored under P limitation. For example, there are several forms of P in soils, so that different species can exploit distinct P pools (van Ray and van Diest 1979; Haynes 1992; Nuruzzaman et al. 2006). We have already seen that plant species

Fig. 6.2 (a) Niche dimension of a habitat that is co-limited by nitrogen (N), phosphorus (P), and potassium (K), and (b) niche dimension of the same habitat after N addition. The different shapes indicate soil areas that are limited by N, P, or K. If one nutrient is added, e.g., N by N deposition, the total soil area with limiting amounts of one or more nutrients for plant growth becomes smaller. The habitat becomes more uniform and may offer less niches for plants to coexist

Fig. 6.2 (a) Niche dimension of a habitat that is co-limited by nitrogen (N), phosphorus (P), and potassium (K), and (b) niche dimension of the same habitat after N addition. The different shapes indicate soil areas that are limited by N, P, or K. If one nutrient is added, e.g., N by N deposition, the total soil area with limiting amounts of one or more nutrients for plant growth becomes smaller. The habitat becomes more uniform and may offer less niches for plants to coexist have various adaptations to low-P environments. As they differ in their efficiency of P uptake, species with different adaptations will colonize distinct niches. In Western Australia, species with cluster roots are generally found on spots with lowest P concentrations and those with mycorrhiza on intermediately P-rich soils (Lambers et al. 2006). Klironomos et al. (2000) proposed a generalized niche model similar to the one by Tilman et al. (1997), with two resources limiting species abundance, but including mycorrhizal fungi. It became clear that mycorrhiza may be able to expand the range covered by plant species, since they can access more soil resources. That will affect the outcome of competition between plants as well as plant community diversity (van der Heijden et al. 1998a, b; Klironomos et al. 2000).

Coexistence of species may also be enabled by facilitated nutrient uptake of one species due to the presence of another species. Thus, in silvopastoral systems with combined over and understorey species, P availability for one species can be affected by the other (Scott and Condron 2003). Gillespie and Pope (1989) found that P uptake by black walnut (Juglans nigra) was larger when grown with alfalfa (Medicago sativa) than with other walnuts, black locust (Robinia pseudoacacia) or orchard grass (Dactylis glomerata). They attributed this to acidification of the root zone during N2 fixation by alfalfa, which solubilized P. Different rooting depths can also help to reduce competition between species (Jackson et al. 2000). Although roots of the same species generally avoid contact, intertwining of different species has been observed, especially of legumes and nonlegumes (Gardner and Boundy 1983). Furthermore, transfer of N and P between coexisting plants has been observed (H0gh-Jensen and Schjoerring 2000). For P, it has been shown to occur via connecting mycorrhizal hyphae (Whittingham and Read 1982).

Different life history strategies of plants also interact with their reaction to and influence on P availability. The plant strategy types according to Grime (2001) have been suggested to differ in their requirements of P and N/P ratios (Gusewell 2004). Stress-tolerant (S) and competitive (C)/stress-tolerant species have low P and high N/P requirements, while ruderal (R) and mixed strategists (CSR) have high P and low N:P requirements. This is consistent with results by Hill et al. (2005) in extensive grazing systems. It also fits the finding that during succession, ecosystems are first N and later P limited (Verhoeven et al. 1996), as they would first be colonized by ruderals, which are later replaced by competitive and stress-tolerant species. Against this background, lower diversity in P-rich systems could be explained by the quick growth of R strategists, leading to competitive exclusion of other species. However, Hill et al. (2005) also pointed out that the relation between plant strategy types and nutrients did not hold in intensive grazing systems, where tolerance or avoidance of grazing became the most crucial plant traits for survival.

Halsted and Lynch (1996) examined the reaction of C3 and C4 species to P limitation. They could not find different reactions between C3 and C4 species, but concluded that monocots can better cope with P stress than dicots due to contrasting allocation of P and biomass. Combined with the finding that graminoids are favored by N applications more than dicots (Falkengren-Grerup 1998) and usually have a larger N/P ratio (Gusewell 2004), graminoids may grow better in P-limited conditions rich in N (Fig. 6.3). Systems both high in N and P are dominated by c o

N concentration

Fig. 6.3 Conceptual drawing of the distribution of different groups of herbaceous plants in relation to major plant nutrients. Cycles represent the distribution of grasses, herbs, and legumes. Species with mycorrhiza are able to exploit sites low in both nitrogen (N) and phosphorus (P). Highly productive species, such as ruderal plants, need conditions abundant in N and P.

N concentration

Fig. 6.3 Conceptual drawing of the distribution of different groups of herbaceous plants in relation to major plant nutrients. Cycles represent the distribution of grasses, herbs, and legumes. Species with mycorrhiza are able to exploit sites low in both nitrogen (N) and phosphorus (P). Highly productive species, such as ruderal plants, need conditions abundant in N and P.

fast-growing species that are very productive, usually grasses. Species-rich systems are often dominated by forbs (Willems and van Nieuwstadt 1996; Theodose and Roths 1999). As these prefer habitats with lower N/P ratios (Gusewell 2004), this could be another explanation for the often found relation between species richness and P availability.

To sum up, there are several possible explanations for interactions between P and phytodiversity:

1. P determines the size of the niche because

(a) P is the main limiting resource in the system

(b) P limitation is coupled to other limiting resources

(c) P-rich systems favor growth of legumes, leading to an increase in N, another nutrient often determining the niche size

2. P-limited habitats have lower interspecific competition than N-limited ones due to:

(a) A range of available P forms that may be exploited by different species

(b) Favoring of stress-tolerant rather than ruderal species in low-P environments

(c) Larger restriction of the growth of grasses than of forbs in habitats with low

Thus, P may not have a larger impact on phytodiversity than other nutrients per se, but can gain it due to its availability relative to other resources. This explains also why the relation between P and phytodiversity may be overruled easily by other factors, e.g., management factors like heavy grazing (Hill et al. 2005; Dorrough et al. 2006) or environmental factors like soil salinity (Theodose and Roths 1999).

N/P ratios

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