Predicted change in soil carbon following afforestation or reforestation, and analysis of controlling factors by linking a C accounting model (CAMFor) to models of forest growth (3PG), litter decomposition (GENDEC) and soil C turnover (RothC)

A complete carbon (C) accounting model for forest systems, GRC3, links a C tracking model (CAMFor) with independently verified models of forest growth (3PG), litter decomposition (GENDEC) and soil C turnover (RothC). GRC3 was tested in seven regional case studies of eucalypt or Pinus radiata plantations in Australia to predict rates of change in soil C after afforestation and to determine controlling factors. The model was calibrated as far as possible to above-ground growth of plantations, litterfall, accumulation of litter and in some cases root biomass, and was then run to determine expected change in soil C. Between 0 and 10 years soil C was predicted to decrease by an average of 1.7% per year (0.79 t C ha-1 per year) and between 10 and 40 years it was predicted to increase by 0.82% per year (0.46 t C ha-1 per year). The mean rate of change after 40 years was 0.09% per year (0.06 t C ha-1 per year). These values and pattern of change were consistent with a recent review of the global literature of change in soil C after afforestation [For. Ecol. Manage. (2002a)]. Modelling analyses suggests the main reasons for this pattern are: (i) initially, there are limited inputs of C to soil as plantation net primary production (NPP) is small and goes to building biomass. Residues from the previous crop decompose leading to net loss of C unless a groundcover (intercrop or weeds) is maintained in the inter-rows, (ii) much of the plantation NPP is allocated to long-lived woody components (stems, branches, and coarse roots), which are temporarily or permanently (by harvesting) removed from the soil C cycle, and (iii) as the stand develops, inputs from the more lignified, resistant material increases. The amount of input may be less under plantation than pasture but the quality of residues is the over-riding factor and in the long-term soil C accumulates. Actual trends in soil C may vary according to site and management conditions, but the main controlling factors will be different between pasture and plantation in the amount and allocation of NPP, and the quantity and quality of residue inputs to soil. Sensitivity (Monte Carlo) analyses showed that model parameters and processes for which it will be important to have good estimates include the amount of NPP and its allocation to various plant components, rate constants for decomposition of litter and root residues, the proportion of C lost to respiration during decomposition of litter and soil C, and rate constants for humification (the proportion of decomposing above-ground litter that is transferred to soil). Changes in soil C were small compared with other forest pools and fluxes - after 40 years of afforestation less than 3% of the cumulative NPP was predicted to accumulate in soil. It is debatable whether it will be feasible or cost-effective to directly measure change in soil C over short-time frames (such as 5 years) for the purpose of claiming C credits under an emissions trading scheme. Modelling provides a useful alternative and at the very least can be used to identify sites and time frames where investment in soil C measurement may be warranted. Crow