Responses Of Litter Decomposition And Nutrient Release To Different Environmental Conditions

Decomposition of aquatic macrophytes is an essential ecological process influencing nutrient cycling and energy flow in aquatic ecosystems.Especially in eutrophic shallow lakes,incomplete decomposition of detritus leads to sediment accumulation so as to accelerate lake aging.Decomposition of aquatic organic matter is influenced by various factors such as water temperature,pH,litter quality,complexity of litters(i.e.interactions between different litter types)and nutrient availability.It has been well-documented that higher nutrient availability in the environment can pose effects directly and indirectly on decomposition rates and nutrient release.Previous studies mainly examined effects of nutrient availability in water columns on aquatic macrophyte decomposition.Little is known about the responses of aquatic macrophyte decomposition to lake sediment nutrients,although most dead materials of aquatic plant sink on the surface layer of lake sediment.Lake sediments with different nutrient characteristics have contrasting properties,which is reflected by their susceptibility to microbial decomposition.Thus,it is important to investigate the effect of sediments on aquatic macrophytes decomposition rate and nutrient release.Litter quality,defined as chemical characteristics of litter,is considered as one of the important determinants for its decomposition.In general,high litter quality(i.e.high initial nitrogen(N)and phosphorus(P)contents as well as low C:N,C:P and N:P ratios),facilitates fast decomposition.In addition,natural litters are most likely a mixture of litterfall of diverse species.Many pervious studies indicate that the decomposition rates of litter mixtures of diverse species may differ significantly from the expected rates estimated from additive decomposition rates of single species,probably due to the chemical interactions between litter substances.However,these studies are often conducted only at the level of plant,without considering interaction between litter quality and sediment nutrient,although there are the complex interaction between aquatic macrophytes and the sediment due to the synergistic effects.In order to assess the effects of sediment nutrient and synergistic effects between sediment and litter quality on macrophytes decomposition,we here presented data from an experimental study with three sediment treatments which represent different nutrient levels.Concretely,a litter bag experiment was conducted to investigate the effects of sediment nutrient and litter quality on the decomposition rates and nutrient release of four different life forms macrophytes(Phragmites australis,Hydrocharis dubia,Nymphoides peltatum and Ceratophyllum demersum)and species mixture.We study the decomposition progress by repeatedly measuring the remaining biomass,C,TN,and TP content in litter.Our results showed that litter quality significantly influenced macrophytes decomposition and N release,and initial C/N and C/P ratio were good indicators of the decomposition rate.Sediment nutrient conditions had little effect on the decomposition rate yet strong effect on nitrogen(N)and phosphorus(P)release,although the interaction between litter quality and sediment nutrient significantly affect the decomposition rate.Repeated-measures ANOVA analysis indicated that litter quality posed a stronger effect on the decomposition rate and N and P release than sediment nutrient conditions.Results suggest that interactions between the sediment nutrient status and litter quality may be a driver controlling macrophyte decomposition in shallow lakes.In addition,shallow lakes and ponds are likely to be particularly susceptible to climate change,because of their large surface-to-volume ratios and absence of stratification in summer,leading to a more direct response to the prevailing weather conditions than in deeper systems.The influence of extreme climatic events,such as heat waves,has become an issue of considerable concern in both ecological research and the global change agenda.Most studies have,however,been focused on the effects of an increasing mean temperature.Although these studies are informative,they have predominantly considered temperature effects as either increases to a fixed mean or fixed increment studies,and usually fail to incorporate either current or predicted future patterns of variability.Our future climate is likely to include extreme events which may have more pronounced impact on ecological systems than changes in mean temperatures alone.Therefore,investigating degradation mechanisms of aquatic plants under extreme temperature conditions is significant.In order to assess the effects of heat waves on macrophyte decomposition,a litter bag experiment was conducted with three temperature treatments(ambient,4? above ambient and heat wave,i.e.,4 ? oscillation around 4? increment).Similar to the sediment treatment experiment,the temperature treatment experiment study the litter decomposition progress by repeatedly measuring the remaining biomass,C,TN,and TP content in litter.The results showed that temperature treatment significantly influence macrophyte decomposition;the litter decomposition progress(change of remaining biomass and nutrient release)in heat waves treatment was similar with warming treat,but differ from normal temperature;litter quality significantly influenced macrophytes decomposition and nutrient release;the influence of litter quality on decomposition rate and nutrient release was stronger than temperature treatment;there is significant correlation between the initial C/N ratio of different plants and plant degradation rate;The interactions of different species mixture were different under different temperature treatments;the response of floating-leaved plant and submerged plant to temperature change was more sensitive than that of emerged plant.