Land-use change from poplar to switchgrass and giant reed increases soil organic carbon

Switchgrass and giant reed can provide a dual contribution in reducing greenhouse-gas emissions through displacing fossil fuels and derivatives and increasing soil organic carbon. However, if it is generally true that displacing fossil fuels with biomass brings favorable effects, there is not as much evidence that perennial grasses increase soil organic carbon, as it mainly depends on the land-use change. The present study investigated, for the first time, the effects on soil organic carbon of the land-use change from poplar to switchgrass and giant reed. We addressed the soil organic carbon variation over 10 years of switchgrass and giant reed succeeding a 30-year poplar. Soil samplings were performed after 3 and 10 years from establishment down to 0.6 m depth. The results show that although the ability of poplar to store large quantities of soil C is widely demonstrated, the two perennial crops allowed to further increase soil organic carbon stocks; particularly, giant reed increased soil organic carbon at a double rate than switchgrass (0.19 and 0.09 g kg−1 year−1). The variation in soil organic carbon highly affected total greenhouse gas savings as estimated by a life-cycle assessment: 11–35 and 20–42% of total savings from switchgrass and giant reed, respectively, derived from increasing soil C stocks. These results highlight the importance of understanding long-term environmental- and crop-specific land-use-change effects in life-cycle assessments instead of applying coefficients to generic crop categories (e.g., perennial tree/crop) and crop sequences, as it normally happens.