Water level drawdown and perennial vegetation impact litter decomposition in the sediment of a eutrophic wetland in the Netherlands

Managed wetlands often lack natural dynamics, such as water level fluctuations that normally cause dry-wet cycles. Several dry years result in a (partial) water level drawdown, after which an excess of rainfall creates the wet phase. To mimic these natural cycles, a water level drawdown can be manually induced. However, this will have major consequences for the cycling of carbon and nutrients. Yet, it is hereto unknown how water level drawdown, and the associated changes in vegetation composition and environmental conditions, affect plant litter decomposition. In the sediment of a eutrophic wetland, we buried green and rooibos tea bags, and local reed litter within a full-factorial design of water level drawdown (yes or no) and perennial vegetation (yes or no) (n=9). Inundated conditions had lower temperature and higher  conductivity and moisture content in the sediment.  We found lower tea decay in the water level drawdown area, indicating more tea mass remaining after 90 days. The decomposition rate of tea was higher in the areas with perennial vegetation, pointing towards faster breakdown of organic matter. Results on remaining reed litter mass were in line with those from the tea-bags; increasing with a water level drawdown. Our results suggest that a multi-year water level drawdown reduces the decomposition rate in the sediment, while increasing the potential for more carbon sequestration. After a drawdown, establishment of perennial vegetation and re-inundation could speed up decomposition rates in the sediment, showing the importance of allowing water level dynamics to maintain high productivity.  

Methods
To study the impact of changing environmental conditions during a water level drawdown on organic matter breakdown, tea bags and reed litter bags were buried in a full-factorial design, covering the water level drawdown management conditions (yes or no) and the presence of perennial reed vegetation (yes or no), resulting in four location types. The use of both tea bags and reed litter bags allows for the comparison with other areas that have used the tea bag index and for a standardized comparison between the drawdown and non-drawdown area. The added value of the reed litter bags is that this is area-specific material and may thus respond differently to the local conditions. Discrepancies between the two methods may indicate that material from the area itself is more adapted to the disturbances that occur. Exclosures were built to protect the buried litter bags against herbivory and bioturbation at the locations with no perennial vegetation, that are prone to disturbance. In the water level drawdown area the exclosures are 1.5 meter in diameter and 2 meter in height to protect against herbivory by red deer (Cervus elaphus) and geese (Anser spp.). In the non-water level drawdown area – under inundated conditions – we employed small wire cages of 1 meter in diameter and 1.5 meter in height to prevent fish from disturbing the study. Since the litter bags in the perennial vegetation were less visible and less prone to disturbance by grazers, no exclosures were used for these locations. Each of the four environmental conditions consisted of 9 locations distributed over the area, resulting in a total of 36 locations. For two locations in the area with a water level drawdown and with perennial vegetation, we were unable to relocate the tea bags and the reed litter bags (n=7).

Environmental conditions

The environmental conditions were measured for all four location types at the start (day 0), after 44 days and at the end (day 90) of the study. At each location, the vegetation community was monitored in a plot of 1x1 meter at the start of the study (day 0) by identifying each plant to species level and estimating the cover of each species. To quantify the abiotic conditions, two sediment samples of 10 centimeters depth (d = 3.5 cm) were taken to determine organic matter content at the start of the study and after 44 and 90 days. The two cores were taken opposite of each other and in between the buried tea- and reed litter bags. Per location, the sediment samples were pooled in one bag and stored at 5 ⁰C until analysis. After thoroughly mixing the sediment sample, a subsample of 20 mL was taken and weighted. The subsample was dried at 70 ⁰C for 48 hours and weighted, after which the subsample was combusted at 550 ⁰C for 4 hours and weighted again. These weights were used to calculate moisture content (%) and loss of ignition, that we used as a proxy for organic matter content (%). In addition to the sediment samples, we used the W.E.T. sensor kit (Delta-T Devices Ltd) to measure sediment temperature (⁰C) and sediment conductivity (mS/m) in triplo at each location at day 0, day 44 and day 90. At inundated locations (no water level drawdown, no perennial vegetation), probes were used to measure water temperature (⁰C) and water conductivity (mS/m) at day 44 and day 90 (Hach HQ40D). Some data on temperature and conductivity in the non-drawdown area is missing on either the locations with perennial vegetation (Day 44, 90) or without perennial vegetation (Day 0). We dealt with these missing data by testing for overall differences (irrespective of measurement time). The average water depth was 24.7 ± 11.7 cm with pH of 8.8 ± 0.3 and oxygen concentration of 10.8 ± 0.7 mg/L (pH and oxygen measured with Hach HQ40D). During the study period the average air temperature was 17.8 ⁰C with a maximum temperature of 31.8 ⁰C and a minimum temperature of 4.9 ⁰C (measurements KNMI; weather station in Lelystad). The average amount of sun per day was 8.3 hours and there was rainfall on 57% of the days with on average 2.3 mm rain per day and a maximum of 20.2 mm rain on the 31st of July 2024. 

Tea bag study

The tea-bag index (TBI) (www.teatime4science.org) was used to determine the ability to break down organic matter in a standardized way. The TBI allows the calculation of the relative decomposition rate (k) and an organic matter stabilization factor (S) using two types of tea, green tea (higher portion of labile litter) and rooibos tea (high portion of recalcitrant litter), as organic matter substitutes (Keuskamp et al., 2013). The difference in decomposition rate between the two types, fast for green tea and slow for rooibos, allows a single measurement effort in time.

At the end of May 2023, thee replicates of a paired green tea bag and rooibos tea bag were buried at each location, resulting in a total of 216 buried tea bags. Tea bags were buried at a depth of 8 cm and a distance of ± 1 meter was adhered between pairs of bags. After approximately 90 days, following Keuskamp et al. (2013), the tea bags were retrieved from the sediment. The possible organic material adhering to the tea bags was removed and the tea bags were dried in the oven at 70⁰C for 48 hours and weighted (precision scale: 0.001g). To correct for the small clay particles adhering to the tea bags, ash-free dry mass was determined through combustion of the tea bags at 550 ⁰C for four hours  Ash-free dry weight of the green tea and rooibos tea was used to calculate the decomposition rate (k), the weight of the green tea was used to calculate the stabilization factor (S) (Keuskamp et al., 2013). The stabilization factor quantifies to which degree the labile fraction is not broken down but stabilizes. A negative s indicates that also part of the recalcitrant fraction was decomposed.

Reed litter bags

In addition to the TBI, local plant litter consisting of dry reed (Phragmites australis) leaves (3.1 ± 0.1 g) was buried to get a better understanding of area-specific decomposition rates. In April 2023, dry reed leaves were collected from both locations with and without water level drawdown in the field, mixed and airdried for 48 hours in the lab before use in the litter bags. PVC-coated glass-fiber mesh was used to create non-decomposable bags of 14.6 cm by 7.3 cm by sowing them with a nylon wire. The mesh size was 1.2 x 1.2 mm to also allow the entering of microfauna.

At the end of May 2023, three reed litter bags were buried at each location in the Oostvaardersplassen, resulting in a total of 108 buried reed bags. At three points in time (after 44, 70 and 90 days), one of the bags was removed from each location. This allowed the construction of a decomposition curve over time for each location type. In the water level drawdown area with perennial vegetation, we lost 5, 5 and 4 litter bags respectively at each collection point in time, either, due to the inability to relocate the litter bags in the dense vegetation or because the litter bags were uprooted, possibly by red deer.  After collecting the reed litter bags, the protocol was similar to that described in section 2.4 on the analysis of the tea bags. Ash-free dry weight was additionally corrected for the amount of reed remaining after combusting reed leaves, to avoid an overestimation of the amount of reed mass remaining.