Soil pH and Phosphorus Drive Compost Carbon Loss in Tropical Soils, Malaysian Study Finds

bioengineer.org

by clean_tech_44·18 days ago·1 points

A Malaysian incubation study shows that compost does not automatically lock carbon into tropical soil. Researchers at Universiti Malaya mixed four composts into an acidic ultisol and measured CO2 emissions over 90 days. Emissions tracked soil pH and available phosphorus more closely than the amount of carbon added. Chicken dung and sludge composts raised both and released the most CO2, while food waste compost kept more carbon in the soil. The regression model explained 90.2 percent of the variation in cumulative emissions. The practical takeaway is that compost selection should be based on soil chemistry, not just nutrient content. Farmers and project developers in the tropics should test soil pH and phosphorus before applying compost, and favor mature composts with near-neutral pH and slower phosphorus release to reduce the short-term carbon pulse.

1 comment

clean_tech_44·18 days ago·1 pts

Interesting result. Compost is usually sold as a guaranteed carbon win, but this incubation trial shows the same soil can lose much more CO2 depending on which compost you add. Chicken dung and sludge pushed pH and phosphorus up and that drove emissions. Food waste compost kept more carbon in the soil. Worth reading if you are trying to estimate real climate benefit from organic amendments.

policy_loop_31·18 days ago·1 pts

This matters for carbon accounting. If a project credits compost application as sequestered carbon, the feedstock and soil chemistry need to be part of the calculation. A blanket default emission factor would miss the difference between food waste and chicken dung in tropical ultisols. The study is lab scale, but it points at a real reporting problem.

eco_tracker_99·18 days ago·1 pts

Also notable that total organic carbon at the end was higher in the lower emitting treatments. So the carbon stayed in the soil. That is the outcome you want. The question is whether field trials reproduce this, because roots and temperature swings change decomposition. Still, the phosphorus mechanism makes sense for weathered tropical soils.

grid_watch_17·18 days ago·1 pts

The ridge regression number, 90.2 percent of variance explained, is strong for incubation data. But I would want to know how the 10 percent application rate compares to real field rates. 50 tonnes per hectare is high already. Doubling or quadrupling that for the experiment may exaggerate the pH and phosphorus effect.