Response of feather moss associated N₂ fixation and litter decomposition to variations in simulated rainfall intensity and frequency

Abstract

Feather mosses in boreal forests form a dense ground-cover that is an important driver of both nutrient and carbon cycling. While moss growth is highly sensitive to moisture availability, little is known about how moss eff ects on nutrient and carbon cycling are aff ected by the dynamics of moisture input to the ecosystem. We experimentally investigated how rainfall regimes aff ected ecosystem processes driven by the dominant boreal feather moss Pleurozium schreberi by manipulating total moisture amount, frequency of moisture addition and moss presence/absence. Moisture treatments represented the range of rainfall conditions that occur in Swedish boreal forests as well as shifts in rainfall expected through climate change. We found that nitrogen (N) fi xation by cyanobacteria in feather mosses (the main biological N input to boreal forests) was strongly infl uenced by both moisture amount and frequency, and their interaction; increased frequency had greater eff ects when amounts were higher. Within a given moisture amount, N fi xation varied up to seven-fold depending on how that amount was distributed temporally. We also found that mosses promoted vascular litter decomposition rates, concentrations of litter nutrients, and active soil microbial biomass, and reduced N release into soil solution. Th ese eff ects were usually strongest under low moisture amount and/or frequency, and revealed a buff ering eff ect of mosses on the decomposer subsystem under moisture limitation. Th ese results highlight that both the amount and temporal distribution of rainfall, determine the eff ect of feather mosses on ecosystem N input and the decomposer subsystem. Th ey also emphasize the role of feather mosses in mediating moisture eff ects on decomposer processes. Finally, our results suggest that projected shifts in precipitation in the Swedish boreal forest through climate change will result in increased moss growth and N 2 fi xation but a reduced dependency of the decomposer subsystem on feather moss cover for moisture retention.

The Impact of Moss Species and Biomass on the Growth of Pinus sylvestris Tree Seedlings at Different Precipitation Frequencies

“…After pots were constructed in late November-early December, 2011, they were placed in the greenhouse to acclimatize for 12 days before newly germinated seeds were planted. Growing conditions in the greenhouse were set at a temperature of 16 ± 2 °C, 120 µmol·s radiation and an 18-hour day length, as done in a previous experiment focused on mosses collected from this region [14]. Relative humidity was kept at 70% ± 10%, which is comparable to those values that occur in the field [42], with a fine spray system.…”

“…These pots were not planted with seedlings and were instead used to study biological N 2 -fixation rates and soil nutrient fluxes to assess possible feedbacks of the moss species on the nutrient status of the pots. At the end of the experiment (i.e., after 18 weeks), we analyzed the cyanobacterial N 2 -fixation for the moss in each of these pots using the acetylene reduction method, as described in detail in previous studies focused on N 2 -fixation in mosses [1,14]. This involved random sampling of 30 shoots for P. schreberi or P. commune or 15 shoots for H. splendens or S. girgensohnii (which have larger shoots) from each pot.…”

“…Each precipitation frequency treatment received the same total quantity of water over time (i.e., 726 mL deionized water per month, equivalent to 60 mm precipitation), so that the less frequent treatment was subjected to fewer, heavier precipitation events. The ambient frequency and quantity of water added were based on observations between 2007 and 2010 at a meteorological station of the Swedish Meteorological and Hydrological Institute (SMHI) in the Arjeplog area; see also [14]. The reduced precipitation frequency treatment simulates longer periods between rainfall events and heavier individual rainfall events that are expected to occur in the boreal forests of northern Sweden as a result of climate change [34][35][36].…”

“…As such, a high moss biomass will retain water better [11,21,33], which may reduce seedling desiccation in dry periods. The moist environment within mosses has also been shown to support N 2 -fixation and N mineralization processes [14,15] that could potentially increase nutrient availability for seedlings. Those processes that can potentially stimulate tree seedlings can be negatively impacted by drought, especially when moss biomass is impaired to a level where the moss is less able to retain moisture.…”

“…Previous work has also shown that these negative effects of mosses on seedlings can vary depending on a variety of external factors, such as the degree of abiotic stress [6,9] and other site characteristics (e.g., the type of growth substrate beneath the mosses) [5,10]. Mosses may also positively affect tree seedlings indirectly by regulating moisture availability [11,12], reducing temperature fluctuations [6,13], providing nutrients through enhancing litter decomposition rates [2,14] and potentially contributing to enhanced soil nitrogen levels via symbioses with N 2 -fixing cyanobacteria [15,16]. Given that mosses can have both positive and negative effects on tree seedlings, there is a need for an improved understanding of when these different effects emerge.…”