Soil pH influences differential accumulation Phosphorus compounds in cultivated lands

Turner and Blackwell (European Journal of Soil Science; 2013) studied the pH effect on the organic phosphorus speciation using the unique acid gradient strip on a soil fertilization / exhaustion long-term experiment (Hoosfield) at Rothamsted Research, Harpenden, UK. This experimental site offered a singular opportunity to study a wide pH gradient in a single soil type that has been under uniform land use for 150 years. The soil acidification caused by progressive leaching of bases was associated with a reduction on soil carbon and a relative increase in organic phosphorus. Among the main findings, the authors reported that inositol phosphates, DNA and phosphonates were preferentially accumulated under acidic conditions. Possible causes to the observed behaviour were: a) metal chelation of inorganic phosphorus compounds; b) chemical interactions between organic phosphorus compounds and clay surfaces; c) pH dependency of soil phosphatase enzymes; d) Phosphatases inactivation by sorption; e) differential abundance of organisms that synthesize phosphatases. The acid pHs strongly limited barley growth and therefore the P extraction and cycling; therefore the observed results are not just a function of soil chemical factors, but were most likely were also a function of the complex interactions and mass balances associated with the different plant growth and survival across the studied acid strip.
This study brings new insights about the effect of long term soil acidification on the abundance of different forms of soil phosphorus. The issues involved are transversal to soil P fertilizer application and management strategies in agricultural soils.

A visitor from down under

We had the privileged of welcoming Dr Tim McLaren, Research Fellow in soil nutrient dynamics at the University of Adelaide, Australia on Tuesday. Tim presented some recent data on the fate of fertilser P using P-33 labelled single superphosphate in clover pasture systems. The team were able to identify the amount of fertiliser P that was recovered in clover shoots and roots, what remained in the granule, and in soil fractions as either inorganic or organic P, under different fertiliser management treatments (placement, timing and initial soil P fertility) (results to be published). He also discussed some of his data on the speciation of organic P in soils under pasture using solution P-31 NMR spectroscopy. It was a great meeting, full of discussion, followed by great food and drink in the pub later!

Some thoughts on DGT from the Community…

At a recent meeting (December 2014), we discussed the dependence of the relationship between soil P availability and plant response on the soil P test that is used.

Our rationale:

Identifying appropriate methods for quantifying P, in addressing different research questions, remains a challenge. Comparing between soil P analysis methods, in addition to soil properties and experimental variables, limits the progress we can make towards understanding how different conditions affect P dynamics. Whilst a globally-applicable test to quantify each P fraction is unattainable, questioning our rationale for the measures we select is essential. Furthermore, it is useful to consider what an alternative measure would mean in terms of the results we acquire and how we interpret them.

We discussed the following paper:

Six L, Smolders E, Merckx R. 2013. The performance of DGT versus conventional soil phosphorus tests in tropical soils—maize and rice responses to P application. Plant and Soil, 366, 49–66.

We focused the conclusion:

For predicting yield response to applied P, an intensity measure (DGT) was most effective for maize, whereas conventional quantity measures (e.g. Olsen P) were most effective for rice.

This suggested that rice does not depend on diffusion of P in the soil (as measured by DGT). Compared to maize, rice has a greater ability to acquire P, via: secretion of organic acids to solubilise non-labile P, symbiotic associations with mycorrhizal fungi, and a more efficient root system for P uptake. Thus availability of soil P depends on the crop species.

In response, we considered the question:

  • How to identify the most representative soil P tests for specific soil-plant systems, without laborious preliminary experimentation or requiring numerous (costly) analyses?

Our main points were:

  • The standard test for plant-available P for UK soils is Olsen P. How did this arise? From a comprehensive review of the available tests and selection of the most replicable and appropriate measure? Assessed for what purpose and soil-plant system?

Key outcomes:

  • Six et al. (2013) raised three criteria for soil P tests. If these are satisfied by numerous different tests, does it matter which we select? This paper and our discussion suggested it does, in which case, what other criteria are we applying to our judgement?
  • How to rectify the perhaps contradictory aims for greater harmonisation of methods (at laboratory group/ university/ regional/ global scales) and for soil-plant system specific measures?

European Sustainable Phosphorus Platform

On the 2nd December 2014, Kirsty Ross on behalf of Lancaster University attended the European Sustainable Phosphorus Platform Constitutive General Assembly in Brussels, Belgium. Here, the Platform was established as a legal entity by the agreement and signing of the statutes.

The European Sustainable Phosphorus Platform (ESPP) has been functioning since March 2013 as a means for stakeholders (rather than individuals) in both industry and research to positively contribute and facilitate EU policy, first and foremost regarding phosphorus (P). In addition, ESPP aims to raise the awareness of P not only in the political arena, but also in the scientific and public domain as the sustainable management of P is vital for industry, agriculture, food, water and the environment. Discussions at the meeting included policy development, ESPP actions and outreach, status of P/P-rock on the EU Critical Raw Materials list.

One of the most notable and discussed topics was the recent publication (May, 2014) of the updated EU Critical Raw Materials list, on which ‘Phosphate Rock’ is identified among 19 other raw materials that are considered to be critical by the European Commission. It was proposed that ‘Phosphate Rock’ should be replaced by simply ‘Phosphorus’ (the element in any form), as it was felt that ‘Phosphate Rock’ was unclear and misleading in terms of its recyclability (rock perhaps cannot be recycled by phosphate by-products can be and are). However, the listing of P on such a list can and will only drive developments in both political and research agendas to enhance its sustainable use.

Lancaster aims to continue contributing to the SCOPE newsletter, and all community members are invited to review relevant, new, exciting research in the phosphorus arena!