Carbon balance and soil carbon input in a poplar short rotation coppice plantation as affected by nitrogen and wood ash application

The increasing importance of short rotation coppice (SRC) plantations for bioenergy production makes it crucial to evaluate their carbon (C) sink potential and to understand which sustainable practices could increase productivity and C sequestration. This study examined the combined effect of woody ash and nitrogen (N) fertilization on the productivity, net C balance (NECB) and soil C sink capacity of a poplar SRC plantation established on a former arable land in northern Italy. Above- (ANPP) and below-ground (BNPP) primary productivity, including both root litter and rhizodeposition (Croot litter), were estimated by a combination of inventory methods, soil respiration measurements and C budgeting approaches. NECB was estimated as changes in both plant and soil C pools, and the portion of soil organic carbon (SOC) derived from poplar leaf and root litter was estimated using a combination of in-growth cores and isotopic measurements. Nitrogen application at 100 kg ha−1 increased ANPP in terms of C accumulation only in the first year, while NECB was not affected by treatments and was 9.96 Mg C ha−1 year−1 on average. Irrespective of the treatment, the plantation accumulated on average 0.7 Mg C ha−1 year−1 of SOC and the poplar-derived soil C input was 3.0 Mg ha−1 year−1. Positive NECB shows that the studied SRC resulted in an important C sink even in the first years after the establishment. However, long-term studies are needed to better assess the effect of N and ash application on C and nutrient cycles over more than one rotation period.