Effects Of Forest Gap On Foliar Litter Humification In An Alpine Forest

Plant litter is a major source of soil organic matter,and the slowly humified rather than rapidly decomposed components in decomposing litter are more important for long-term formation and stabilization of soil organic matter.In alpine forest,low temperature limits litter decomposition,and soil development is slow,thus litter humification is important for forming soil organic matter and maintaining soil fertility.However,the mechanisms involved in litter humification are poorly understood.Traditional paradigm suggests that humus forms at very late periods(humus near stages)during litter decomposition.In recent years,increasing evidence have suggested that recalcitrant compounds are not selectively preserved in litter(plant-derived)and that labile components are precursors of humic substances(microbial-derived),but more experimental concerns should be addressed to the latter hypothesis.The limited isotope labeling studies traced the rapid transformation of labile carbon(C)from plant litter to soil,but how does humic substances accumulate in the decomposing litter substrate remains unclear.In alpine forest,the subluxation between rapid release of labile components and constrained microbial activity during cold winter may modify the microbial-derived hypothesis.Moreover,the temporal shift in snow cover during snow formation,snow coverage and snowmelt in winter as well as the spatial shift driven by forest gap may impact the release of labile components from decomposing litter,and further modulate the microbial-derived litter humification.In this study,we used a litterbag method to measure mass loss,labile C release,humic substances accumulation,spectroscopic properties and functional C in six dominant foliar litters that were incubated in gap center,canopy gap edge,extended gap edge and under closed canopy during snow formation,snow coverage and snowmelt stages and during growing seasons from 2012 to 2016 in an alpine forest on the eastern Tibetan Plateau.We assessed the formation and stabilization of humic substances in decomposing litter,the controlling factors for litter humification and the potential effects of forest gap on the formation and stabilization of humic substances during early litter humification in this alpine forest.We found that:(1)After four years of decomposition,mass remaining in the six dominant foliar litters ranged from 28% to 52%.44-62% of litter mass was lost during winter,suggesting that winter litter decomposition is a significant ecological process in this alpine forest.Forest gap increased litter mass loss during the first two years of decomposition,but had non-significant effect in the subsequent two years,suggesting that the stimulative effect of forest gap on litter decomposition is decreasing as decomposition proceeds.Moreover,forest gap increased the percent changes of litter mass that was decomposed in winter,suggesting that winter snow cover in forest gap stimulates litter decomposition.(2)Initial labile C content ranged from 16% to 25% in the six fresh foliar litters and decreased to 1-5% after four years of decomposition.Labile C released rapidly during snow formation stage in the first year of decomposition,but increased markedly during the subsequent snow coverage and snowmelt stages,suggesting that low temperature limited microbial activity,which decreased the uptake of labile C,resulting in subluxation between rapid release of labile components and constrained microbial activity during early litter decomposition in this alpine forest.After four years of decomposition,only 2-12% of labile C was remained in the six foliar litters,and forest gap increased labile C release from decomposing litter.(3)Initial content of humic substances was 8-13% in the six foliar litters and consistently increased during litter decomposition,reachingd at 18-21% after four years of decomposition,suggesting that a rapid accumulation of humic substances occurrs during early litter decomposition.However,the net accumulation of humic substances markedly decreased,and this decrease was significantly correlated with rapid release of labile C.Initial humification degree was 16-25% for the six fresh foliar litters and increased to 38-58% after four years of decomposition.Furthermore,humification rate was high during the four years of decomposition,suggesting that accumulation of humic substances remains high level during winter in this alpine forest.Forest gap increased the content and net accumulation of humic substances as well as humification degree during the first three years,but decreased them during the fourth year,suggesting that forest gap increases litter humification during early periods but decreases litter humification during later periods.This difference was because forest gap increased the rapid release of labile C during early periods and thus stimulated the accumulation of humic substances,resulting in decrease in microbial substrate use efficiency,which decreased litter humification during late periods.(4)Initial contents of humic acid and fulvic acid,the two major components of humic substances,were 2-3% and 6-10% respectively in the six fresh foliar litters.Humic acid content increased in early periods and decreased in later periods,but fulvic acid content decreased in early periods and increased in later periods.After four years of decomposition,humic acid and fulvic acid contents increased to 5-11% and 10-14% respectively.Humic acid to fulvic acid ratio was lower than 1 in the first year of decomposition,but greater than 1 in the subsequent three years,suggesting that early litter humification was primarily depended on the accumulation of humic acid.Fulvic acid formed earlier,but was unstable,suggesting that early litter humification was a dynamical accumulation and degradation equilibrium rather than a consistent accumulation process.(5)Humic substances were type Rp during the first two years,but can be classified as type A,which was more developed with high stability,after four years of decomposition.When the litter has been decomposed for two years,the humic substances was stabilized as organo-mineral interactions by associating with mineral elements,such as manganese,calcium and potassium,suggesting that stable humic substances are formed during early litter humification.(6)Initial contents of alkyl C,O-alkyl C,aromatic C and carbonyl C in fir needle litter were 31%,52%,12% and 5%,and remained at 48%,45%,92% and 89% respectively after two years of decomposition.Initial contents of these C in birch leaf litter were 22%,49%,19% and 7%,and remained at 47%,46%,50% and 66% after two years of decomposition.Methoxyl C,carbohydrate C and di-O-alkyl C remaining in fir needle litter were 56%,41% and 51%,and remaining aryl C and phenolic C were 102% and 84% respectively after two years of decomposition.These C remaining in birch leaf litter were 72%,43%,50%,47% and 53% respectively.Alkyl C to O-alkyl C ratio increased in fir needle litter during decomposition,but did not change in birch leaf litter.Forest gap increased O-alkyl C loss,but had non-significant effect on alkyl C,aromatic C and carbonyl C,suggesting that forest gap only increased the release of labile components but not for recalcitrant compounds.(7)Overall,labile C increased the accumulation of humic substances,but organic soluble substances and acid unhydrolyzable residues decreased the accumulation of humic substances over the four year period.At time scale,labile C and water soluble substances had positive effects on the accumulation of humic substances in early periods,but this effect was decreased in later periods.In contrast,acid soluble substances and acid unhydrolyzable residues had negative effects on the accumulation of humic substances in early periods,and this effect was positive in later periods.These results suggest that litter humification was controlled by labile components during early stages but by recalcitrant components during late stages.Furthermore,the effect of functional C on the accumulation of humic substances decreased as increase in litter recalcitrance in the order of O-alkyl C < alkyl C < aromatic C < carbonyl C,also suggesting that labile components stimulate early litter humification.In conclusion,stable humic substances are rapidly accumulated in decomposing litter during early decay periods,challenging the traditional paradigm that humic substances accumulates at a very late period(humus near stage)of litter decomposition.This result suggests that considerable C and nutrients are not only decomposed from plant litter and thus uptook rapidly by soil microorganisms and plant roots,but also be retained as humic substances and thus sequestrated in soils.This result also suggests that litter decomposition is accompanied by rapid humification,challenging the hierarchical view that plant litter is firstly decomposed and then humified.Early litter humification is primarily controlled by the rapid release of labile components,supporting the microbial-derived hypothesis from a perspective of accumulation of humic substances during litter decomposition.Our results provide a new idea for improving current theory about formation of soil organic matter.Gap formation during forest regeneration stimulates litter decomposition,particularly for rapid release of labile components,and thus accelerates nutrient cycling in this alpine forest.Meanwhile,forest gap stimulates the accumulation of humic substances in early periods but diminishes the accumulation of humic substances in late periods,and further changes soil C sequestration.This effect of forest gap greatly maintains soil fertility and sustainable management in this alpine forest.