UN report warns of major threats to NZ's productive soils and freshwater contamination risk

An international report giving a health check to the world's soils has recognised New Zealand's highly productive soils, but warns it is not all green shoots.

The new Status of the World's Soil Resources report by the United Nation Food and Agriculture Organization delved into the threats to soil health globally, and associated food production and environmental risks by region.

The microscope on New Zealand revealed signficant soil threats including erosion, excessive nutrients risking water systems, soil compaction and urban encroachment.

It found some negative environmental effects to soils and waterways today were the legacy of decades of land clearance like forest conversion in steep North Island hill country, agricultural intensification and nutrient inputs.

"Continuing agricultural intensification, along with lag and legacy effects from historic high fertilizer additions to soil since the beginning of the twenty-first century are likely to continue affecting water quality into the near future," the report said.

"Despite best management practices and national policy direction, including a cap on the use of synthetic nitrogen fertilizer, many New Zealand rivers are still showing excess nitrogen and phosphorus concentrations, indicating a risk of environmental harm."

The report said with New Zealand considered a "very high" user of pesticides, averaging 8.8 kilograms per hectare in 2020, nutrient mismanagement posed environmental risks.

It found most soil nitrogen levels were within normal limits, but phosphorus relatively high on particularly farmland, and the two chemicals were presenting in excessive levels in many rivers.

"Soils in New Zealand are experiencing an array of complex and interconnected challenges that may become exacerbated by storm events associated with climate change," it said.

"Some impacts (such as severe erosion) are irreversible, and it is important to take an intergenerational perspective when considering, for example, the increasing vulnerability of the food system that soil supports."

It warned about the fragmentation of highly productive land in Aotearoa and the subsequent loss of commercial food production was escalating, in areas like Auckland.

"Although government and industry guidelines exist to promote [sustainable soil management], no legislation or policy in New Zealand currently addresses soil health directly," it said.

"However, freshwater management policy now has some guidance on practices that negatively affect soil compaction, such as damage to soil structure in wet conditions through pugging."

NZ soils stressed, under pressure - soil scientist

Report contributor and peer reviewer, soil scientist Dr Fiona Curran-Cournane is also the principal science advisor for the new mega-Ministry for Cities, Environment, Regions and Transport.

Curran-Cournane said decades of soil quality testing revealed longstanding soil issues in Aotearoa that were still present today.

"What we're seeing is that these soils are stressed and under pressure and they have been for some time, with things like land use intensification," she said.

"These things could be exacerbated under an increasingly erratic climate as well."

She said soil tests were established to provide early warnings of chemical imbalances.

"Some of the key soil quality indicators that come up again as being problematic include things like soil compaction, particularly for some of our pastoral land use activities, so on dairy farms across monitored sites, as well as some of our horticultural land uses," she said.

"The other indicator would be elevated phosphorus levels within our soil, so more than the plants can take up. So anything in excess to what the soil can absorb risks being lost when it starts to rain in surface runoff.

"The third issue would also be the loss of soil carbon levels, particularly under some of our cropping systems, so outdoor vegetable production and some of our arable lands."

Climate change expected to increase erosion risk

Erosion remained a major soil threat in New Zealand, the report said, with an estimated 182 million tonnes of New Zealand soil lost into waterways each year, and worsening impacts expected due to climate change.

"Human-accelerated soil erosion remains a problem in New Zealand," the report read.

"Hillslope mass movement (landslides, earthflow and gully erosion) is expected to increase in soft rock hill country by the middle of the twenty-first century, primarily through more extreme and frequent storms, leading to regional increases in coastal sediment loads ranging from 1 to 233 percent."

It said Cyclone Gabrielle caused an estimated 300 million tonnes of landslides, costing the country $1.5 billion, which prompted national environmental standard amendments for plantation forestry.

But it noted sediment management regulatory settings may not be adequate in the future, citing a 2023 study by Andrew Neverman of Manaaki Whenua Landcare Research.

"Current policies on freshwater management address sediment loads but may not be adequate with continued climate change impacts."

Curran-Cournane said a lot of soft rock hill country in the North Island was particularly vulnerable to erosion, and soil conservation and planting may be able to address further loss in future weather events.

Retaining soil carbon

The report said the challenge ahead was preventing the loss of soil carbon, key for growing plants, due to historic intensive grazing, irrigation and cropping.

It cited earlier Government research that showed 80 percent of soil tests on monitored sites nationwide between 2014 and 2018 failed on at least one soil-quality indicatory target, such as pH acidity, carbon or nitrogen.

The Ministry for the Environment and StatsNZ study also found soil carbon was below the target range in just over a quarter of the cropping sites tested.

It noted nutrient mismanagement as an issue, citing that half of the dairy, cropping and horticulture sites tested had nutrient levels greater than what the plants needed, risking run-off.

Of that research, nearly half the sites showed signs of soil compaction, including 65 percent of dairy farms assessed. Dense and compacted soils, usually from livestock or heavy machinery use, could impact the ability for plants to soak up nutrients and prevent leaching nutrients into the environment.

Curran-Cournane said there was definitely an opportunity for more extension and knowledge transfer on the ground to address the soil issues.

"I think we definitely need to be doing more when it comes to the extension and the applied science and more boots on the ground to try and help close that gap between what we know and seeing more improved soil outcomes."

The report also identified intergenerational and holistic perspectives to soil were inherent and core to Māori values and principles.