How do grassland ecosystems respond to long-term N enrichment? Meta-data analysis of global nitrogen addition experiments and data-model inter-comparison.

Abstract:

Grasslands are an important ecosystem type, covering 40% of the Earth’s land surface and storing about one third of terrestrial ecosystem carbon. They are prone to invasion by introduced species and rapid conversion to agricultural lands and pasture lands. There has been a substantial increase in the Nitrogen (N) deposition over the globe owing to anthropogenic activities like fossil fuel combustion and high-intensity agricultural practices. Though N is one of the essential nutrients to stimulate plant growth, excessive availability of N could have detrimental effects. Previous measurement and modeling studies have explored the impact of N on aboveground biomass of grassland. However, the effect of N addition on different functional and provenance types of grassland vegetation remains poorly understood, and existing modeling work is less constrained by long-term N addition experiments.

The goal of this study is to understand how long-term nitrogen addition affects different plant functional types as well as provenance categories in a grassland ecosystem. The Nutrient Network (NutNet) is a highly coordinated global effort of measuring the effect of N addition in grassland sites. The data from NutNet was analyzed to understand the biomass response and N use efficiency of the vegetation to N addition. We further used the meta-data analysis to benchmark the global estimates from the MsTMIP, a multi-scale synthesis project which contains standardized simulations and inter-comparison of over 20 terrestrial models with prescribed environmental and meteorological drivers.

Our analysis shows that the two major functional types in grasslands, grasses and forbs, differ in their response to N addition over a long-term period. This pattern is further highlighted when considering the proportional provenance of vegetation. The weather and edaphic characteristics of the site are also important to understand these dynamics. Thus, the vegetation structure of a grassland plays a vital role in its responses to N addition. To compare the MsTMIP results with experimental evidence from the corresponding sites in the NutNet helps us to constrain the modeled ecosystem responses to N addition across the globe, and to better inform the future projections under increasing anthropogenic N input.