Whiteley, Geoff (Univ. of Leeds, UK, School of Biology, Miall Building, Leeds,

LS2 9JT, UK; Phone: +44 (0)1133432886; Fax: +44 (0)1133432835; Email: g.m.whiteley@leeds.ac.uk)

Iron Stabilization of Crop Residues as a Novell Land Management Practise for Sequestering Carbon in the Agricultural Sector

G.M. Whiteley *

Current proposals to increase the sequestration of carbon in the agriculture sector are necessarily directed towards financial incentives to shift the balance between well established systems of conventional land management such as afforestation of crop and pasture land or shifts from conventional to conservation tillage. However, it is possible that the availability of subsidies might stimulate innovation in the sector and encourage alternative residue management technologies. Iron mineral impregnation of crop biomass is used in horticulture to slow the rate of decomposition of materials such as wheat straw for mulching. Iron bonding to cell wall polymers creates a physical and chemical barrier to access by cellulose degrading enzymes, with the increased recalcitrance contributing to control of composting in storage, reducing nitrogen immobilisation and an extending the life for surface applied mulches. Efficacy is not a simple function of recalcitrance because the addition of iron can also induce microbial phosphorus deficiency and reduce palatability to earthworms. Residual effects are also evident after soil incorporation as protected straw fragments in the light fraction and as Fe-stabilized humic material. The application of this to the treatment of crop residues in agricultural systems has not been investigated. This paper considers the carbon sequestration potential of a single spray treatment in year 1 consisting of 6 kg of Fe per metric ton of wheat straw applied as a ferrous sulphate solution after harvest. The supply of materials for this spray treatment was assumed to be $8.00 per metric ton of treated straw (assuming a delivered price of $200 per metric ton of ferrous sulphate heptahydrate). Comparisons are made between the increased temporary storage of carbon under conventional tillage and conservation tillage systems over a 15 year projection. For the purposes of this study, three levels of efficacy of the iron impregnation are considered and the annual increases in carbon stored in surface litter, light organic matter fraction and Fe-bound humic carbon are tabulated. Finally, rental payment values are calculated for net sequestration, using a 5% discount rate for payments ranging from $10 to $150 per metric ton of permanent carbon sequestration. Over the 15 year projection, the net increase in stored carbon ranged from 0.33 metric tons per metric ton of treated crop residue in the high scenario with conservation tillage to 0.082 metric tons per metric ton of treated crop residue in the low scenario with conventional tillage. Increased residue cover in the first year after treatment accounted for between 38% and 52% of the total increase in carbon retention under conservation tillage and net storage remains correspondingly higher under conservation tillage than conventional tillage. Rental values never exceeded $2.50 per metric ton of treated crop residue, representing only a fraction of the cost of the treatment. It appears unlikely that incentive payments, however priced, would influence decisions by farmers to adopt this practice in farming systems. However, this is not to say that the process does not have an intrinsic value in areas such at the Pacific North West where available supplies of crop residue are limiting factors to productivity and have an intrinsic value in erosion protection. Large scale take up of the technology on a regional scale might still be shown to have a measurable impact on carbon stocks in the agricultural sector; it is just that this would be insensitive to incentive payment inducements.