Response of Litter Decomposition to Simulated N Deposition in Disturbed, Rehabilitated and Mature Forests in Subtropical China

Abstract

The response of decomposition of litter for the dominant tree species in disturbed (pine), rehabilitated (pine and broadleaf mixed) and mature (monsoon evergreen broadleaf) forests in subtropical China to simulated N deposition was studied to address the following hypothesis: (1) litter decomposition is faster in mature forest (high soil N availability) than in rehabilitated/disturbed forests (low soil N availability); (2) litter decomposition is stimulated by N addition in rehabilitated and disturbed forests due to their low soil N availability; (3) N addition has little effect on litter decomposition in mature forest due to its high soil N availability. The litterbag method (a total of 2880 litterbags) and N treatments: Control-no N addition, Low-N: )5 g N m )2 y )1 , Medium-N: )10 g N m )2 y )1 , and High-N: )15 g N m )2 y )1 , were employed to evaluate decomposition . Results indicated that mature forest, which has likely been N saturated due to both long-term high N deposition in the region and the age of the ecosystem, had the highest litter decomposition rate, and exhibited no significant positive and even some negative response to nitrogen additions. However, both disturbed and rehabilitated forests, which are still N limited due to previous land use history, exhibited slower litter decomposition rates with significant positive effects from nitrogen additions. These results suggest that litter decomposition and its responses to N addition in subtropical forests of China vary depending on the nitrogen status of the ecosystem.

Effects of experimental nitrogen additions on plant diversity in tropical forests of contrasting disturbance regimes in southern China

“…The rehabilitated forest originated as a pine plantation in 1930's that was naturally invaded and colonized by regional broadleaf species from animal and wind dispersal. However, it has yet to recover fully to its pre-disturbance condition over the past several decades (Mo et al, 2003(Mo et al, , 2004(Mo et al, , 2006, creating and maintaining N-limiting conditions (Mo et al, 2006), i.e., N demand by recovering woody vegetation continues to exceed supply of available N. Under N limitation, increasing N commonly decreases biodiversity by enhancing competitive abilities of fast-growing nitrophilous plants with high maximum growth rates, at the expense of slower growing neighbors of smaller stature (Aerts and Chapin, 2000;Gilliam, 2006;Hautier et al, 2009;Bobbink et al, 2010). Because there were no significant increases in understory plant growth in the rehabilitated forest, it is unlikely that competitive exclusion among understory plants contributed to the observed decline of understory cover.…”

“…In addition, it has been reported that D. dichotoma, which dominated understory layer in this study, is a highly shade-intolerant species and commonly declines in growth with decreases in light intensity (Takeuchi, 1988). Unlike the rehabilitated forest, the disturbed forest has been constantly subjected to anthropogenic pressure (primarily harvesting of understory vegetation and litter) since time of planting in 1930's and continuing until the late 1990's Mo et al, 1995Mo et al, , 2003Mo et al, , 2006. This type of land use may mitigate effects of N additions, resulting in no significant changes in understory community, and do so for two reasons.…”

“…This type of land use may mitigate effects of N additions, resulting in no significant changes in understory community, and do so for two reasons. First, understory/litter harvesting greatly erodes and degrades the disturbed forest (Wang et al, 1982;Mo et al, 1995Mo et al, , 2003, leading to a decrease in soil N stock and maintaining N limitation (Mo et al, 2006), hindering successful development of the understory community (Mo et al, , 2003. Second, it may simultaneously minimize competitive interaction among understory plants and maintain overstory dominance by P. massoniana by eliminating seedlings of potential dominant/co-dominant species.…”

“…Nitrogen addition experiments were initiated in July 2003 (Mo et al, 2006). Three N addition treatments (each in three replicates) were established in both rehabilitated and disturbed forests: Control (without N added), N50 (50 kg N ha À1 yr À1 ), and N100 (100 kg N ha À1 yr À1 ).…”

“…Previous work has demonstrated that both rehabilitated and disturbed forests in tropical China are N limited (Mo et al, 2006(Mo et al, , 2007. Little is known, however, about responses of plant diversity of secondary forests to N deposition, which is currently increasing in this region.…”