Nutrient Context and Litter Composition Control Wetland Plant Root Decomposition

Abstract

Anthropogenic factors such as deforestation and agriculture have both a direct and indirect impact on nutrient availability and carbon (C) storage potential. Deposition of nutrients can modify plant uptake of nutrients, plant composition (quality of litter), soil microbial communities, plant-microbe associations and availability of organic C and nitrogen. Decomposition rates are dependent on a few of these factors such as plant composition, moisture availability, and concentrations of carbon to nitrogen (C:N) ratio in soils and litter. In this study, I addressed how nutrient enrichment influences the plant root decomposition rate and C storage potential. I hypothesized that if plant species vary in growth response to different nutrient context histories, then root decomposition rates will depend on nutrient context (long-term fertilization vs. ambient) and plant functional group (grass vs. shrub). I tested this hypothesis by conducting a long-term wetland ecology experiment at East Carolina University’s West Research Campus in Greenville, North Carolina by measuring mass loss of different plant litters (grass Chasmanthium laxum and shrub Rhus copallinum) sourced from different soil nutrient histories and buried in different soil environmental conditions and associated microbial communities. Additionally, I analyzed C:N ratios for both plant litter and soil collected from mowed fertilized and mowed unfertilized plots. Results revealed that plant root litter C:N ratios were different based on the interaction between plant type (grass vs shrub) and fertilization (fertilized vs unfertilized). Soil C:N ratio showed a difference across fertilization. The variation in fertilization effects on soil C:N ratio and root litter properties influenced plant root litter decomposition to different degrees. Percent mass loss of grass roots was similar across all main effects of fertilization, plant nutrient history, and mesh size, while there was a difference across buried plots and mesh size (µm) for the shrub roots. Shrub roots buried in fertilized plots within 200 µm nylon mesh bags had higher percent mass loss than those buried in unfertilized plots within 20 µm nylon mesh bags due to the access of macrofauna allowed with the larger mesh size. Additionally, shrub roots buried in fertilized plots within 200 µm nylon mesh bags had higher percent mass loss compared to unfertilized plots. The soil C:N ratio combination with differences in root litter C:N ratio influenced rates of decomposition where shrub litter with high C:N ratios had faster decomposition rates when buried in fertilized soils (with lower soil C:N ratio), while grass litter with low C:N ratio had similar effects on both fertilized and unfertilized plots. This study provided insight into how nutrient enrichment influences native plant root decomposition rates and C storage potential by measuring mass loss of different plant litter sourced from different soil nutrient histories and showed higher mass loss rates led to greater soil C stocks. Since long-term fertilization increases the relative abundance of woody species such as Rhus copallinum over time, the higher mass loss of these roots could negate carbon gains from higher biomass due to root turnover outweighing the plant biomass inputs. Therefore, understanding the relationship between long-term fertilization and plant functional groups is important for evaluating plant root composition and its influence on microbial activity and the role wetlands can play in climate change mitigation.