Ammonia oxidation rates in soil are higher under legumes than under long-term, experimentally fertilized non-legume shrubs in the Sonoran Desert

Background/Question/Methods

Ammonia oxidation (AO) is a key biogeochemical pathway in the nitrogen (N) cycle, catalyzed primarily by autotrophic microorganisms.  In low-productivity ecosystems like deserts, the rate of soil AO is thought to be limited by low availability of ammonium (NH₄⁺) mineralized from organic matter.  However, NH₄⁺ in soils is also produced by N-fixing microorganisms in legume root nodules and from atmospheric N deposition.  We asked, what is the relationship between the activity of ammonia oxidizers and NH₄⁺ availability under different plant patch types in the Sonoran Desert?  We measured AO rates in surface soils from under three different patch types within desert preserves near the Phoenix metropolitan area, including under 1) leguminous trees (Prosopis), 2) non-legume shrubs (Ambrosia deltoidea), and 3) non-legume shrubs (A. deltoidea) in plots that have been fertilized with N for 6 years (60 kg N as NH₄NO₃ ha^-1 yr^-1).  We hypothesized that AO rates are positively related to soil NH₄⁺ concentration across patch types, regardless of the source (legumes or fertilizer).  Thus, we expected higher AO rates from soils under non-legume shrubs in N fertilized plots followed by soils under legumes and, finally from soils under non-fertilized, non-legume shrubs. 

Results/Conclusions

Contrary to expectations, AO rates were higher in soils under legumes (0.86 ± 0.14 μg NO₂^- g dry soil^-1 hr^-1) compared to soils under non-fertilized and fertilized non-legume shrubs (0.46 ± 0.07 μg NO₂^- g^-1 hr^-1, 0.31 ± 0.05 μg NO₂^- g^-1 hr^-1, respectively; ANOVA, p = 0.001).  In contrast, soil NH₄⁺ concentrations were highest and pH was lowest under N-fertilized shrubs.  In a multiple regression, AO rates were strongly and positively related to soil organic matter (SOM; std β = 0.8) but negatively related to soil NH₄⁺ (std β = -0.3). In this arid system, N fertilization negatively affects nitrification rates relative to controls, perhaps due to the negative effects of pH on microbial activity.  Soils under legumes have large pools of high-quality SOM that likely support steady rates of N mineralization and water availability relative to soils under less productive non-legume shrubs.  The results from this study suggest that soil N enrichment from atmospheric deposition in deserts may not lead to higher rates of ammonia oxidation like in forests and other mesic ecosystems.