The Decomposition Of Macrophyte Litters In The Air In Freshwater Marshes Of The Sanjiang Plain, Northeast China

Plant litter decay is an important component of global C budget and a major determinant of nutrient cycles. In wetlands, little of plant biomass is consumed during the growing season and most ends up in the detritus food web. Thus, the decaying plant litters representing important energy input, and continued nutrient availability mainly depends on decomposing organic matters. In most wetlands, a significant quantity of emergent macrophyte shoots do not collapse onto the sediment surface immediately after senescence, but remain standing and can decompose in a standing position. Similar to the litters on the sediment surface, the standing litters are also important components of the detritus pool. In this study, we carried out studies in the freshwater marsh in the Sanjiang Plain by combining field control experiments and observations. The aims of this study were to clarify the decay rates, nutrient dynamics and CO2 emission flux of the standing litters, reveal the effects of litter positions(in the air or on the sediment surface) on early stage of litter decay, and explore the responses of litter decay processes in the air on the changes of nutrient availability in wetland ecosystems. The results were showed as follows:(1) In the freshwater marsh, the leaf and culm mass losses were 19.3 ~ 45.1 % and 14.3 ~ 23.1 %, respectively, following one year of aerial decomposition. Litter mass loss was closely related to microbial respiration rates and initial ratios of C:N and C:P. The litter N amount increased during aerial decomposition, but there was a net release of P from the standing litters after one year of decay. Therefore, the decomposition of the standing litter contributes markedly to overall litter decay, and thus is a key component of C and nutrient cycles in temperate wetlands.(2) In a swamp meadow, the mass losses for three litters(i.e., leaves, sheaths and culms) from Deyeuxia angustifolia in the air were only approximately 3% and less than that on the sediment surface after 180 days of decay, but increased to 16 ~ 44% after one year of decay and thus exceeded or approached that of surficial corresponding litters. For leaves and sheaths, the nutrient amounts of aerial litters were more than those of surficial corresponding litters after 180 days of decay; however, such patter concerted after one year of decay. For culms, the litter nutrient amounts in the air were often more than those on the sediment surface during one year of decay. Overall, the litter position significantly affected the decay processes in this marsh. Therefore, it is essential to consider litter mass and decay processes at different positions to comprehensively understand carbon cycle and nutrient turnover in temperate freshwater marshes.(3) In a D. angustifolia-dominated swamp meadow, CO2 evolution from the standing litter exhibited a pronounced diel pattern, with high evolution rates concurring with increased litter water potential due to the increase of relative humidity at night. In the laboratory, rates of CO2 evolution from wetted culms and leaves increased exponentially with increasing temperature. The calculated annual CO2 flux(culms, 3.07 g C m-2 yr-1; leaves, 6.81 g C m-2 yr-1) was equivalent to a mean of 2.32%(culms) and 6.94%(leaves) of annual net aboveground production, but only 1.12% of the ecosystem respiration. Therefore, the aerial decay plays an important role in litter decay and C cycling in wetland ecosystem, but the CO2 flux from standing litter could be neglected in assessing the greenhouse gas flux at the ecosystem level in the freshwater marshes in this area.(4) In a swamp meadow, N input exerted significant effects on the initial nutrient concentrations and stoichiometrical properties of leaf litters, and the effects varied with N input rates. The litter decay and the associated microbial evolution often were accelerated by the higher rather than the lower N input rate. In the standing-dead phase, N input decreased the N accumulation in decaying litters, while the effects of N input on dynamics of P in decaying litters varied with N input rates. In addition, litter nutrient dynamics during the aerial decay were closely correlated with its initial nutrient concentrations. In the Sanjiang Plain, therefore, the increase of N availability in freshwater marshes resulted from human activities and climate change would exert substantial effects on the litter decay processes in the air by changing litter quality, and thus the ecosystem C cycling and nutrient turnover.(5) In a wet meadow, P input accelerated nutrient concentrations of leaf litters, and decreased nutrient accumulation in decaying leaves in the air, but had no significant effects on litter quality of culms and the nutrient dynamics of decaying culms in the standing-dead phase. In addition, there was no significant effect of P input on decay rates and associated microbial respiration rates of the standing litters. In the Sanjiang Plain, therefore, the increase of P availability would speed up the turnover rates of nutrients in decaying aerial litters, and thus substantially influence the nutrient cycles in freshwater marshes, but have no significant effects on C release at the standing-dead stage.