The State of New Zealand’s Soil Carbon
The government’s plan to plant a billion trees is a crucial plank in its climate change strategy. It’s also no secret that our huge dairy industry is one of our biggest greenhouse gas emitters. But France’s “4 per 1000” climate initiative has highlighted a different issue at the heart of New Zealand agriculture: the state of our soil-carbon stocks.
Professor Louis Schipper at the University of Waikato has studied New Zealand’s soil-carbon levels extensively. He reckons our soils are actually in pretty good shape. The challenge, he says, is to maintain the soil carbon we’ve got – and to build a better understanding of the consequences of recent management practices and landuse conversions, such as the move from dry stock to dairy.
The science, Schipper points out, is only partially complete. It’s clear both pasture and forestry are efficient ways to sequester carbon (with pines, there’s less in the soil but that’s offset by the amount stored in the plant itself ). Where things get trickier is around how pastures are managed, and the external inputs used to support growth.
Several studies have shown that the use of phosphorus fertiliser, while increasing pasture growth, has no measurable effect on soil-carbon levels. But there haven’t yet been any long-term trials of the effects of nitrogen-based fertilisers (crucial to intensive dairying and at the heart of the freshwater pollution debate) on soil-carbon levels, which Schipper says is a big, important gap in our knowledge. New Zealand research does, however, already show that irrigation can decrease soil-carbon stocks.
What everyone in the field agrees on is that maintaining or increasing soil-carbon levels could play a vital role in offsetting CO² emissions, while improving soil quality. And the best way to do this – whether with pasture, pine forest, natives or vegetables – is to keep soil covered with growing plants as much as possible.