Professor Arizona State University Tempe, Arizona, United States
After water, nitrogen (N) availability is the most important limiting factor for aboveground net primary productivity. Surprisingly, after experimentally supplementing a semiarid grassland with water and N for over a decade, we observed no effect on aboveground net primary production. In this mixed mesquite-grassland system, we hypothesized that N limitation does not occur because plants in experimental rainfall and N fertilization plots access different nitrogen pools, resulting in a non-nitrogen effect on primary productivity even under irrigation treatments. To address our hypothesis, we manipulated incoming precipitation at the Jornada Basin LTER (NM, USA) by ± 80% and fertilized half of the plots with ammonium nitrate (+10 g N m-2 yr-1) for 15 years (n = 3). Leaves of the dominant plant species, Bouteloua eriopoda and Prosopis glandulosa, and soils (0-50 cm, in 10 cm increments) were analyzed for percent nitrogen, C:N, and δ15N.
Nitrogen fertilization had strong effects on the total N in surface soils (0-10 cm) that was modified by water availability, where %N was highest in drought plots (Water effects: F160,2=5.169, p< 0.05; Fertilizer effects: F160,1=32.112, p< 0.05; Interaction: ns). At soil depths of 30+ cm, however, %N was highest in irrigated plots; this effect was due to water treatment alone (F65,2=5.203, p< 0.05). Isotopic analyses indicated that vertical movement of N to deeper soils is from the existing inorganic soil pool, not fertilizer. The addition of nitrogen fertilizer alone strongly increased foliar %N for both of our study species (Fertilizer effects on Bouteloua: F107,1=29.184, p< 0.05; Prosopis: F153,1=9.364, p< 0.05), but the effect of water treatment was not significant. These results suggest that the differential rooting profile among plant species and locations of N source result in 1) N accumulation in surface soils when water is largely unavailable to facilitate plant nutrient uptake, 2) vertical movement of N to deeper soils when uptake rates cannot match the supply, and 3) the “switching” of N sources by Bouteloua species to N made available by the fertilizer with subsequent loss of existing inorganic N in the soil pool to deeper soil depths.