Interpreting a Carbon Budget for a Rehabilitated Mine Site
A mining company revegetates a 200-hectare waste-rock landform. Monitoring over five years gives these annual figures: gross primary production (GPP) 4.2 t C/ha/yr, autotrophic respiration 2.9 t C/ha/yr, heterotrophic respiration from soil 1.6 t C/ha/yr, and carbon export in erosion and runoff 0.4 t C/ha/yr. A nearby reference woodland has GPP of 7.8 t C/ha/yr and stores about 90 t C/ha in biomass and soil. The company claims the site is a net carbon sink of 1.3 t C/ha/yr and therefore offsets part of its emissions.
1. Calculate the net ecosystem carbon balance (NECB) and evaluate the company's claim.
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Calculation. Net primary production (NPP) = GPP − autotrophic respiration = 4.2 − 2.9 = 1.3 t C/ha/yr. This is the figure the company quotes, but it is not the ecosystem balance. Net ecosystem production (NEP) = NPP − heterotrophic respiration = 1.3 − 1.6 = −0.3 t C/ha/yr. The net ecosystem carbon balance (NECB) also subtracts losses to erosion and runoff: −0.3 − 0.4 = −0.7 t C/ha/yr.
Evaluation. The claim is wrong. The company has used NPP, which ignores the carbon respired by soil organisms and the carbon physically exported from the site. On these figures the landform is a net carbon source of 0.7 t C/ha/yr, not a sink of 1.3. The error is a category mistake: NPP measures plant growth, not ecosystem storage. Over 200 ha the discrepancy is 1.3 − (−0.7) = 2.0 t C/ha/yr, or 400 t C/yr, which is material to any offset claim. The site also stores far less carbon than the reference woodland (90 t C/ha), consistent with a young, disturbed ecosystem still accumulating carbon but not yet net-positive.
2. The company argues that as vegetation matures, heterotrophic respiration will fall and the site will become a sink. Assess this argument.
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The argument is questionable on the evidence given. Heterotrophic respiration typically rises as a rehabilitated site accumulates litter and soil organic matter, because there is more substrate for decomposers. A young site often has low heterotrophic respiration simply because there is little organic matter to decompose, not because decomposition is inefficient. As biomass and litter build up, heterotrophic respiration tends to increase toward the level supported by NPP, and a mature ecosystem approaches a near-zero NECB rather than a large positive one. The reference woodland, with much higher GPP (7.8 vs 4.2 t C/ha/yr), illustrates the scale of production needed to support substantial storage. The site could become a modest sink if GPP rises faster than heterotrophic respiration, but that requires improved growth, not a fall in decomposition. The claim should be tested by continued monitoring of all four fluxes rather than asserted, and erosion loss (0.4 t C/ha/yr) should be reduced, since it is a direct and avoidable carbon export.