Phosphorus, Plant Biodiversity and Climate Change

Abstract

Phosphorus (P) is a major plant nutrient. Its increasing use as a fertilizer has helped to raise crop and fodder production. However, the global reserves and resources of P are finite, demanding an efficient use of P. Under natural conditions, it is often in limited supply. Plants have developed adaptations to small soil P concentrations. Increased P levels can have unwanted side effects like eutrophication and algal blooms. Besides, P concentrations in the soil have often been found to be negatively correlated with plant diversity. For sustainable agriculture, it is essential to understand (1) adaptations of plants to small P concentrations in soils to maintain production with decreasing P reserves, (2) influences of P on phytodiversity to minimize unwanted effects, and (3) future developments of P and phytodiversity in relation to climate change to adjust agricultural practices.

P is essential for the energy and sugar metabolism of plants. As it moves in soils by diffusion only, the geometry of the root system is essential for its uptake. Plants have developed different adaptations for P uptake: e.g., localized or overall increases in the number of roots, the development of cluster roots that increase the root surface area by up to 140 times, exudation of different phosphatases and organic acids in reaction to specific forms of P, or symbiosis with mycorrhiza that may be responsible for up to 75% of the P acquired by plants. Gradual differences in these adaptations decrease interspecific competition and facilitate coexistence. Low P concentrations increase plant diversity by favoring stress-tolerant rather than ruderal species or by restricting the growth of competitive grasses more than that of forbs. According to the niche dimension hypothesis, more limiting resources lead to more coexisting species. Worldwide, P limitation is as relevant for plant production as Nitrogen (N) limitation. Thus, P could regulate the size of ecological niches by being the main growth-limiting factor or by being coupled to other limiting resources.

Global climate change influences soil P availability. Increasing temperatures tend to increase P mineralization of litter. Furthermore, temperature increases by 5°C have been found to double the colonization of roots by mycorrhiza. Nitrogen mineralization was enhanced by on average 48% by temperature increases of between 0.3°C and 6.0°C. Larger amounts of N stimulate phosphatase exudation and plant P uptake. This could result in increased soil P availability, which is further enhanced by increased P mobilization due to human activities. Such a development would reduce phytodiversity and promote the growth of ruderal, fast-growing species. In the long run, this could cause mining of soil P, which would then again increase plant diversity. However, diversity needs a long time to recover from P additions. Therefore, in sustainable agriculture, increases in soil P relative to other factors limiting plant growth have to be prevented to guarantee large phytodiversity.