Above- and belowground responses to nitrogen addition in a Chihuahuan Desert grassland

Abstract

Increased available soil nitrogen can increase biomass, lower species richness, alter soil chemistry and modify community structure in herbaceous ecosystems worldwide. Although increased nitrogen availability typically increases aboveground production and decreases species richness in mesic systems, the impacts of nitrogen additions on semiarid ecosystems remain unclear. To determine how a semiarid grassland responds to increased nitrogen availability, we examined plant community structure and above- and belowground net primary production in response to long-term nitrogen addition in a desert grassland in central New Mexico, USA. Plots were fertilized annually (10 g N m(-2)) since 1995 and NPP measured from 2004 to 2009. Differences in aboveground NPP between fertilized and control treatments occurred in 2004 following a prescribed fire and in 2006 when precipitation was double the long-term average during the summer monsoon. Presumably, nitrogen only became limiting once drought stress was alleviated. Belowground NPP was also related to precipitation, and greatest root growth occurred the year following the wettest summer, decreasing gradually thereafter. Belowground production was unrelated to aboveground production within years and unrelated to nitrogen enrichment. Species richness changed between years in response to seasonal precipitation variability, but was not altered by nitrogen addition. Community structure did respond to nitrogen fertilization primarily through increased abundance of two dominant perennial grasses. These results were contrary to most nitrogen addition studies that find increased biomass and decreased species richness with nitrogen fertilization. Therefore, factors other than nitrogen deposition, such as fire or drought, may play a stronger role in shaping semiarid grassland communities than soil fertility.

Effects of fire on belowground biomass in Chihuahuan desert grassland

“…This production response is in stark contrast to aboveground patterns in which fire dramatically alters species composition and abundance, and greatly reduces aboveground production, which may take nearly a decade to recover to pre-fire levels. Together these patterns also demonstrate that rates of above-and belowground production are decoupled in this ecosystem (Ladwig et al 2012). That is, aboveground and belowground production fluctuate over multi-year cycles that are weakly linked with growing season precipitation (Muldavin et al 2008).…”

“…Following harvest, the volume of collected soils was measured by depth, roots were sifted (2 mm) and floated out of each sample, collected, dried at 608C for 48 hours, and weighed. BNPP is expressed as g m À2 yr À1 (see Ladwig et al 2012). Aboveground net primary production (ANPP) was measured in 30 permanently located 1-m 2 quadrats in burned and unburned grassland adjacent to the root donuts.…”

“…Grasslands dominated by B. eriopoda occupy large areas throughout the northern Chihuahuan Desert where they intergrade with shrub-dominated vegetation and may co-dominate with Bouteloua gracilis (Jameson 1962, Dick-Peddie 1993, Brown and Smith 2000, Kroël-Dulay et al 2004. Aboveground production of B. eriopoda grasslands is strongly driven by seasonal (monsoon) precipitation and temperature (Gosz and Gosz 1996, Muldavin et al 2008), but unlike aboveground responses the relationship between belowground production, climate and disturbance remains poorly understood (Ladwig et al 2012).…”

Impact of Precipitation Fluctuation on Desert-Grassland ANPP

“…This is different from some results in meadow steppes and alpine grasslands. The high confidence model in meadow steppes was a power function in others' research [26], and in the alpine grasslands the model between precipitation and he ANPP were more complex [30][31][32][33]. This is because the precipitation-ANPP model for the meadow steppe and alpine grassland is determined not only by the factor of precipitation, but also by other factors like heat.…”

“…This is because the precipitation-ANPP model for the meadow steppe and alpine grassland is determined not only by the factor of precipitation, but also by other factors like heat. So they need a simple multi-equation or a differential equation, which are established between heat, precipitation and ANPP, to explain the impact of meteorological factors on the ANPP [27,[31][32][33]. For a desert steppe in arid and semi-arid regions, because the most important factor that influence ANPP is precipitation and the non-decisive factor is heat, using precipitation without heat to build model is decisive.…”