Where is the N? Measuring N-pools in a Chihuahuan Desert grassland after 25-years of fertilization

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Violeta Mendoza Martinez, University of Texas at El Paso, Jennie R. McLaren, Biological Sciences, University of Texas at El Paso, El Paso, TX and Scott Collins, Department of Biology, University of New Mexico, Albuquerque, NM

Background/Question/Methods
Increased nitrogen (N) supply has been shown to increase plant biomass and alter soil chemistry in grasslands across the globe. However, the majority of these studies have focused on mesic grasslands, and the fate of added N in arid and semi-arid ecosystems remains unclear. To study the impacts of long-term increased N availability, we sampled soils from a 25-year-long nitrogen addition experiment in the northern Chihuahuan Desert at the Sevilleta National Wildlife Refuge (SNWR) in New Mexico. In this experiment, ten plots have received yearly N-enrichment of 10 g N m-2 as NH4NO3 and ten plots serve as ambient controls. To determine the fate of the added nitrogen, we collected soils from 0-10 cm depth three times during the growing season: late June prior to the monsoon, late July during the monsoon and early October after the monsoon. Samples from the fertilized and control plots were analyzed for N-content in the soil, microbial, and plant pools to determine 1) the effect of N-enrichment on the allocation of the additional N across these different pools (soil NO3-N and NH4-N, microbial biomass N and aboveground plant biomass) and 2) how the distribution of N varies across season.
Results/Conclusions
We determined that despite over two decades of nutrient addition, pools of N in the soil, microbial, and plant components of the ecosystem did not differ substantially between treatment and control plots. Soil ammonium and nitrate pools were elevated by fertilization immediately after the fertilizer treatments were applied, but this effect was transient. However, there were seasonal effects on both soil N and microbial N pools. Soil nitrate was highest before the monsoon, whereas soil ammonium was consistently low throughout the season. Soil microbial biomass (carbon and N) peaked during the monsoon and remained elevated for the post-monsoon sampling. Given the lack of effect of N-additions on the soil and microbial pools, and the predominance of small rain events in this system, we conclude that the majority of the N-additions are being released back into the atmosphere because water limitation in the system has led to low retention and use of additional N.