Nitrogen amendments and woody encroachment increase enzymatic activity driving decomposition in a mesic grassland

Background/Question/Methods

The link between rising atmospheric CO₂ concentrations and global climate change makes it increasingly important that we determine under what conditions ecosystems act as carbon (C) sources or sinks. The capacity for soils to store C is determined by complex interactions between the quality of plant litter inputs, nitrogen (N) availability and the microbial communities that control decomposition rates. This study explores these complex interactions controlling the release and storage of soil C in a mesic grassland that has experienced experimental manipulations of C and N availability through fertilization and haying (7 years) and woody encroachment (24 years). We measured a suite of soil parameters, including inorganic N, dissolved organic C (DOC), and soil moisture in addition to extracellular enzyme activity (EEA) to determine how the activity and proxies for the efficiency of microbial decomposers are affected by varying levels of substrate C and N quality and quantity.

Results/Conclusions

Fertilization and woody encroachment have promoted increases in litter quality and labile soil C stocks with concurrent increases in microbial EEA promoting the break-down of C rich substrates such as cellulose, starch, and lignin, such that EEA was 48% higher with fertilization and 64% higher with woody encroachment compared to unfertilized ‘old field’ and hayed grasslands. Grasslands that were both hayed and fertilized had lower litter quality and levels of inorganic N and DOC, coupled with EEA approximately 78% of that in fertilized, un-hayed grasslands. When enzyme activity was normalized by microbial biomass, trends of increasing EEA with fertilization and woody encroachment remained, suggesting enzyme production by microbial communities has not resulted in increased community size under those land management regimes. Microbial biomass produced per unit of available C or N substrate exhibited declines with fertilization and woody encroachment. Our results suggest possible shifts in microbial community structure favoring less efficient, fast growing, low growth yield, copitrophic organisms when these grassland systems are fertilized or allowed to undergo woody succession. The combination of higher plant litter quality, larger pools of labile soil organic C, and the observed increases in enzyme activity corresponding to the breakdown of both recalcitrant and labile soil organic carbon with fertilization and woody encroachment suggests that these land management practices may have negative rather than positive effects on long term soil C sequestration in these grassland systems.