Plant community responses to nitrogen fertilization along a semiarid grassland transition zone

Background/Question/Methods   Human alterations to the nitrogen cycle and increased nitrogen deposition have ignited many studies of the effects of nitrogen fertilization on plant community dynamics. Increased nitrogen can alter biomass, lower species richness, and modify community dominance in ecosystems worldwide. Although semiarid regions are primarily limited by water, previous studies indicated nitrogen was frequently co-limited with water. Although impacts of nitrogen additions on semiarid ecosystems have been examined, nitrogen effects in arid communities remain unclear. To determine if community responses vary in an ecotone, we examined community response to nitrogen addition along the transition zone between Chihuahuan Desert grassland and shortgrass steppe in central New Mexico. The site is composed of a mixture of plant species from each community. Since 1995 5 g N m-2 was added in spring and summer each year. Aboveground net primary production (ANPP) was calculated with allometric conversions from field surveys in the spring and fall. Since 2004, belowground production (BNPP) has been measured within root donuts once each year. Detailed meteorological data from the site were used to relate community variables to seasonal and annual patterns of rainfall and soil moisture.

Results/Conclusions   Aboveground NPP was related to the amount of summer precipitation and the greatest total ANPP occurred in years with wet summers. While forbs generally had higher species richness than grasses, the majority of total biomass was from grasses. One exception was the very wet summer monsoon in 2006 that resulted in exceptionally high biomass of forbs. Belowground NPP was also related to summer precipitation and greatest root growth occurred the year following the wettest summer. Following peak biomass both above and belowground NPP, especially forbs, steadily decreased. No nitrogen treatment effect was observed for belowground growth. Differences in ANPP between fertilized and control treatments were restricted to years with greater than average precipitation. Presumably nitrogen only became limiting once water stress was alleviated. Species richness increased with greater summer precipitation but did not vary between nitrogen treatments. When water was not limited, biomass increased but species richness did not change with nitrogen additions. This is contrary to most nitrogen addition studies that find increased biomass and decreased diversity with nitrogen fertilization.