Comparing differences in early-stage decay of macrophyte shoots between in the air and on the sediment surface in a temperate freshwater marsh

Litter decay is a fundamental process in ecosystem carbon flux and nutrient cycling. In wetlands, shoot litters on the sediment surface and in the air are important components of the detritus pool. We used the litterbag method to compare mass losses and nutrient dynamics of culms, sheaths, and leaves from Deyeuxia angustifolia on the sediment surface and in the air from October 2011 to October 2012 in a freshwater marsh in Northeast China. The mass losses for the three litters 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 increased nutrient (nitrogen and phosphorus) concentrations of aerial litters resulted in an approximately 10% increase in the nutrient amounts after 180 days of decay, which was greater than that of surficial corresponding litters; however, surficial litters exhibited a greater increase or a lesser decrease of nutrient amounts than did aerial corresponding litters after one year of decay. For culms, the litter nutrient amounts in the air were always more or not less than those on the sediment surface during one year of decay, although effects of the litter position on nutrient concentrations were negligible (for nitrogen) or varied over time (for phosphorus). 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.