Carbon Sequestration Capacity of Biomass Crops - Perennial Versus Annual.

Bioenergy crops can have a major impact in agricultural on-site and off-site environmental impacts. It is expected that feedstock for biofuel will be originated from a mixture of annual and perennial crops occupying different agroecological niches or having a complementary role in the rotation. The carbon allocation and distribution with depth of carbon and other nutrient in the roots of annual and perennial crops are however dramatically different, which can alter significantly the carbon balance and distribution in soil profiles. In addition, the growing cycle controls the soil water balance which in turn affects carbon turnover. Assessing the effect of annual and perennial systems through long-term experiments has obvious limitations. We used simulation models (EPIC and C-Farm) to estimate the effect of the carbon partitioning to roots and its distribution with depth on the soil carbon balance of annual crops like corn, biomass sorghum, and perennials like switchgrass and miscanthus. Allocation of biomass to roots was altered using simple partitioning rules, and the distribution with depth determined using normalized functions of depth. The range of situations tested encompasses what is expected for annual and perennial crops. The simulations were performed along a climate gradient artificially varying the soil texture to represent different soils. It is shown that retention and mixing of aboveground biomass becomes relevant in annual systems with minor allocation of carbon to roots. Uncertainties remain regarding the controls of organic carbon, nitrogen, and phosphorus cycling in the subsoil (depth > 0.2 m), with major carbon losses in the subsoil when the soils have a high initial soil carbon content.