Climate change conditions impacting critical element for plant growth, fertiliser need

07 April 2025

Rising carbon dioxide levels in the atmosphere are altering phosphorus (P) available for plant growth, potentially leading to costly and unnecessary fertiliser application by farmers.

New research published in the Global Change Biology journal by scientists from AgResearch and Lincoln University has found that the test used in New Zealand to estimate the amount of available P in the soil may be underestimating the true accessibility of this crucial element for pasture plant growth.

The research, which drew on data from a long-running AgResearch experiment* in which carbon dioxide is artificially elevated on an area of grazed pasture, is the latest to throw up surprising new results about what happens under rising carbon dioxide levels tied to climate change.

“For decades, New Zealand farmers have relied on the Olsen P test to determine the P available in soils and to guide decisions about fertiliser use,” says lead author Zac Beechey-Gradwell.

“The data from the AgResearch experiment shows a substantial and sustained reduction in available P in the topsoil extracted by the Olsen P measure under elevated carbon dioxide, prior to annual P fertiliser application. In addition, the effectiveness of P fertiliser in raising Olsen P was significantly reduced.”

Scientist Zac Beechey-Gradwell, who was one of the authors of the new research

“Scientists have known for a while that elevated carbon dioxide reduces the amount of P extracted by the Olsen P test, but we didn’t fully understand why or what the consequences of this might be."

"Our latest research confirms that more of the fertiliser P that is applied under elevated carbon dioxide is rapidly converted into organic forms in the topsoil, in a process called ‘biological immobilisation’. This essentially makes it invisible to standard soil P tests.”

“If a soil P test indicates phosphorus deficiency, farmers may respond by applying more fertiliser than is necessary, driving up costs without improving pasture performance. Phosphorus is an essential macro- nutrient for plant growth. Without it, production will decline.”
“At a minimum, this research suggests we are going to need to recalibrate soil P test targets under elevated carbon dioxide to ensure that P fertiliser recommendations remain cost-effective and environmentally sustainable in the years ahead.”

AgResearch principal scientist Alec Mackay says the phosphorus findings have landed against a backdrop of expected gains in pasture productivity not materialising under climate change.

“We need further research to see if similar trends are occurring in different soil types, and what the impact is on clovers and nitrogen fixation.”

Research co-author, Professor Leo Condron of Lincoln University, says: “Phosphorus fertiliser manufacture in New Zealand is dependent on imported phosphate rock and the cost has increased and become more volatile since 2007-08, and it is therefore important to ensure that fertiliser P is being used efficiently”.

Read the full research here(external link).

*New Zealand’s globally unique Free Air Carbon Dioxide Enrichment (FACE) experiment has allowed scientists to study plant and soil responses to elevated carbon dioxide in a sheep grazed pasture for almost 24 years (1997-2021). Read more here(external link).