Decomposition of an N-fixing invasive plant compared with a native species: consequences for ecosystem.
Invasion by the Australian Acacia longifolia has significantly altered the characteristics of both vegetation and soil of Portuguese coastal dunes, including the accumulation of a thick litter layer. Although many aspects of impacts by this species in invaded dunes are known, decomposition of the litter is not well studied, but this can be crucial to disentangle both impacts and recovery. In order to better understand the impacts at soil level and the recovery of the system after removal of the invader, decomposition of A. longifolia litter was studied and compared with decomposition of the native Cistus salviifolius, for >3 years. Litterbags were deployed in a coastal sand dune and samples retrieved after 2, 5, 11, 17, 24, 36 and 41 months. At the final sampling, ca. 41% and 48% of the initial litter mass of A. longifolia and C. salviifolius, respectively, remained, showing that in these dunes both species decompose slowly. During the first 17 months, A. longifolia decomposed faster than C. salviifolius, but thereafter the rate slowed down and C. salviifolius was decomposed at a faster rate. In general, throughout decomposition, N and lignin content, lignin/cellulose ratio, cellulase and chitinase activity, and the number of microfungi morphotypes were significantly higher in A. longifolia than in C. salviifolius litter. On the other hand, C/N ratio, cellulose content and the number of microfungal isolates were generally higher in C. salviifolius litter. Mostly for A. longifolia, lignin content and lignin/N ratio increased as the remaining litter mass decreased. Despite high N content and low lignin/N and C/N ratios, almost 43% of the A. longifolia litter is recalcitrant/very slowly decomposed, accumulating on soil surface for long periods. The slow decomposition rate of A. longifolia litter together with the production of much more litter than the native vegetation (ca. 3 times more than C. salviifolius) is reflected in the accumulation of a thick N-rich litter layer on soil surface of invaded areas. This promotes a continuous input of C and nutrients to the soil, and leaves a hidden legacy long after the removal of the invader that, amongst other impacts, prevents the regeneration of native species.