The impact of fertilizer management on the oxidation status of terrestrial organic matter

The oxidative ratio (the ratio of moles of O-2 produced per mole CO2 sequestered - OR) of the organic matter in the terrestrial biosphere governs the ability of the terrestrial biosphere to uptake CO2. The value of OR is known to vary between environments, but it would also be expected to vary with management. This study measured the OR of plant and soil samples from the long-term grassland plots on the Park Grass experiment at Rothamsted (SE England). The selected plots included those with different fertilizer inputs, including farmyard manure or inorganic fertilizers and an unfertilized control, each with and without lime. The measurements show that: (i) Use of inorganic fertilizer caused the OR of soil organic matter to increase. (ii) Farmyard manure (FYM) caused OR of the soil to increase but that of the vegetation decreased. (iii) Liming had the effect of decreasing OR and counteracting effects of fertilizer. (iv) The OR of the ecosystem increased with FYM application but decreased with inorganic fertilizer application. The global pattern in the use of organic amendments and inorganic fertilizers suggests that the likely impact of the predicted increase in global inorganic fertilizer use will result in a net decrease in the OR of the organic matter of the terrestrial biosphere, and an increase in its ability to act as a carbon sink. Corresponding increases in global FYM use and its impact upon global OR are unlikely to be large enough to counteract this effect.