Additions of ochre to soil increase the retention of organic carbon

Fe oxyhydroxides are known to adsorb organic carbon in soil, protecting it from degradation. In this presentation we will report the results of experiments in which we added waste ochre (Fe oxy-hydroxide) precipitated by waters draining former UK coal mines to soils and observed decreases in carbon lability and thus increased carbon storage.

 

In batch experiments in which ochre was added to soil at rates of 5 wt % and 10 wt % by mass and shaken in 20 mL 0.01 M CaCl₂, organic carbon release into solution was significantly reduced suggesting that the ochre had adsorbed organic carbon. In an initial set of plant growth experiments soil was amended with 5 wt% ochre and wheat plants grown for 6 weeks. The concentration of organic carbon leached from the soil over the course of the experiment and the hot water extractable organic carbon at the end of the experiment were significantly reduced by c. 43 % and 16 % respectively indicating a reduction in carbon lability. However, plant growth was reduced by c. 50 % by the ochre amendments. As Olsen P was reduced by the ochre amendments we ascribe this growth reduction to reduced P availability due to adsorption of P to the ochres. In a follow up set of experiments we added both ochre and KH₂PO₄ to soils. The KH₂PO₄ was added at a rate equivalent to levels of recommended P fertiliser application in the UK. In this second plant experiment, there was no difference in plant biomass between the study control and the ochre-treated soils and carbon lability was still significantly reduced by the ochre treatments. In addition to this the ochre amended soils leached less phosphate over the course of the experiment.

 

These results suggest that Fe oxide amendments to soil may be a practical way of changing soil chemistry in order to increase the amount of carbon retained in soils but that care must be taken to ensure that phosphorus availability is not negatively affected.

 

However, rough calculations indicate that the global supply of Fe-rich waste may be insufficient for Fe-oxide waste amendments to generate significant increases in soil carbon at a global scale. So addition of Fe-oxide wastes may represent a local means of waste-generating industries to offset their carbon footprint.