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Microbial indices of P availability across a forest productivity gradient in South Africa
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

Soil microorganisms have the capability to solubilize different fractions of phosphorus (P) and can probably access P fractions unavailable to plants in the short term. However, there are few studies available that attempt to estimate the potentially available microbial P. We compare a wet-chemical extraction procedure for P with a microbial bioassay based on respiration kinetics after amending carbon (C), nitrogen (N), and sub-saturation levels of P in laboratory incubations across a plantation forest productivity gradient in South Africa. We found that the estimated microbial available P was at least 10-fold higher than P fractions conventionally defined as easily plant available based on wet-chemical methods. The microbial P was strongly positively related to sorbed P (i.e. NaOH extractable P) P (r2=0.63, p<0.001) and indicates that this P fraction contributes to the microbial P utilization within a relatively short time frame (<300 hrs) when C and N are not limiting. This was further emphasized by the change in respiration kinetics when the amendment of C and N were combined with phosphate. Addition of phosphate-P increased the growth and maximum respiration (max resp) rates and decreased the amount of time needed to reach max resp in comparison to amendments with only C and N. Our study indicates that sorbed P), a dominant P fraction in highly weathered soils, is most likely accessible to microorganisms.

Keyword [en]
Phosphorus availability, weathered soil, soil respiration, microbial bioassay, Hedley fractionation, plantation forest
URN: urn:nbn:se:umu:diva-33735OAI: diva2:317685
Available from: 2010-05-04 Created: 2010-05-04 Last updated: 2010-05-06
In thesis
1. Phosphorus availability and microbial respiration across biomes:  from plantation forest to tundra
Open this publication in new window or tab >>Phosphorus availability and microbial respiration across biomes:  from plantation forest to tundra
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Phosphorus is the main limiting nutrient for plant growth in large areas of the world and the availability of phosphorus to plants and microbes can be strongly affected by soil properties. Even though the phosphorus cycle has been studied extensively, much remains unknown about the key processes governing phosphorus availability in different environments.

In this thesis the complex dynamics of soil phosphorus and its availability were studied by relating various phosphorus fractions and soil characteristics to microbial respiration kinetics. The soils used represent a range of aluminium, iron, carbon and total phosphorus content, and were located in four different biomes: subtropical forest, warm temperate forest, boreal forest and tundra.

The results showed that NaOH extractable phosphorus, a fraction previously considered to be available to plants only over long time scales, can be accessed by microbes in days or weeks. Microbial phosphorus availability was not related to aluminium or iron content in any of the studied systems, not even in highly weathered soils with high aluminium and iron content. This is in contrast with other studies of soils with high sorption capacity and shows the variability of factors that govern phosphorus availability in different environments.

In the boreal forest chronosequence, no difference could be seen with age in total phosphorus content or concentrations of occluded phosphorus forms. However, there were lower concentrations of labile phosphorus forms in older systems, which were correlated with a decrease in microbial respiration. This was most likely related to organic matter quality in the system, and not to geochemical factors.

Phosphorus availability was linked to differences in topography (water regime) and vegetation in the tundra ecosystems. The results suggest that the availability of phosphorus, both for microbes and plants, was lower on the meadow vegetation sites compared to the two types of heath vegetation.

Many factors are important for phosphorus availability in soils, but these results suggest that microbes can access less available phosphorus if not restricted by carbon, and this may be important in regard to forest management practices as well as effects of environmental change.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, Institutionen för ekologi, miljö och geovetenskap, 2010. 32 + 4 papers p.
phosphorus availability, microbial bioassay, soil respiration, microbial growth rate, Hedley fractionation, soil sorption, weathered soils, boreal forest, subarctic and tundra
urn:nbn:se:umu:diva-33732 (URN)978-91-7264-989-1 (ISBN)
Public defence
2010-05-27, Institutet för Rymdfysik, Aulan, Rymdcampus 1, Kiruna, 09:00 (English)
Available from: 2010-05-06 Created: 2010-05-04 Last updated: 2010-05-06Bibliographically approved

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