Leela E. Rao 1 , David R. Parker 1 , Andrzej Bytnerowicz 2 , and Edith B. Allen 3

1. Nitrogen Mineralization Across an Atmospheric Nitrogen Deposition Gradient in Southern California Deserts Leela E. Rao1, David R. Parker1, Andrzej Bytnerowicz2, and Edith B. Allen3 1Dept. of Environmental Sciences, University of California, Riverside 2U.S. Forest Service Fire Laboratory, Riverside, CA 3Dept. of Botany and Plant Sciences and Center for Conservation Biology, University of California, Riverside, CA

2. Background • N deposition has been shown to affect natural areas in the greater Los Angeles area • Altered soil processes (Fenn et al. 1996) • Mycorrhizae (Egerton-Warburton and Allen 2000) • Enhanced invasive grass growth (Allen et al. 1998) • Primary source of anthropogenic nitrogen in the region are cars and trucks producing NOx. Atmospheric NH3 is produced from catalytic converters and agriculture. • Joshua Tree National Park is located 140 miles east of Los Angeles and is under a gradient of N deposition.

3. July 2005 HNO3 (mg/m3) 2.7 - 3.7 3.8 - 5.0 5.1 - 6.0 6.1 - 7.1 7.2 - 10.1 10.2 - 15.3 July 2005 NH3 (mg/m3) 3.9 - 5.0 5.1 - 6.0 6.1 - 7.0 7.1 - 10.3 10.4 - 13.5 13.6 - 19.9

4. March 2005 HNO3 (mg/m3) 0 - 0.75 0.76 - 1.5 1.6 - 2.0 2.1 - 2.5 2.6 - 3.25 3.26 - 5.75 March 2005 NH3 (mg/m3) 0 - 0.99 1.0 - 1.45 1.46 - 1.75 1.76 - 2.25 2.26 - 3.0 3.1 - 6.4

5. Nitrogen Mineralization • N mineralization may be an indicator of N deposition and used to define critical loads for deposition effects on soils • Mineralization is known to be affected by: • Environmental conditions (soil temperature, moisture) • Litter quality (carbon and nitrogen content) • Soil texture (water holding capacity, OM sorption) • N deposition has been found to increase nitrification rates and increase nitrogen content of litter

6. Objectives • Examine potential nitrogen mineralization across a nitrogen gradient to determine effects of increased nitrogen inputs on nitrogen dynamics • Determine the extent to which nitrogen supply through mineralization is correlated with exotic annual grass cover • Examine the extent to which other soil factors may be influencing nitrogen dynamics and exotic grass cover in this region Hypothesis Because microbial processes in arid systems are often limited in both carbon and nitrogen (Schaeffer et al. 2003) we hypothesize that nitrogen deposition should increase mineralization by increasing both total production of herbaceous annuals (Allen et al. 2007) and the nitrogen content of the annuals (Sirulnik et al. 2007).

7. Methods • Soils were collected in July from sixteen sites in and around Joshua Tree Park and brought to the laboratory for a 14 week mineralization incubation experiment • Soils were brought to 60% WFPS and incubated at 25°C • Three replicate samples were extracted for inorganic N at regular intervals • Percent cover of native and exotic vegetation was recorded at each site • Percent rock cover, bulk density, pH, total C, organic C, and total N of the soils was also determined for each site

8. Results: Mineralization Incubation

9. SHBD DP SLBD Results: Soil Properties High BD, High pH Measured soil properties (bulk density, pH, % sand-silt-clay, percent cover of rocks) were correlated. Principal components analysis performed on correlations to create two, uncorrelated soil characteristic variables. Low BD, Low pH Very sandy More silts/clays DP: desert pavement, SHBD: sandy with high bulk density, SLBD: sandy with low bulk density

10. N-Mineralization & Soil Properties • No effect of soil grouping on mineralization rate observed • Total amount of N mineralized was significantly lower in the desert pavement sites compared to the sandy, low bulk density sites (ANOVA P = 0.014) DP: desert pavement, SHBD: sandy with high bulk density, SLBD: sandy with low bulk density

11. C:N Ratio • No relationship between C:N and soil groupings (P = 0.14) • No effect of C:N on mineralization rate (R2 = 0.13) • Total N mineralized increased as C:N decreased (R2= 0.43; P = 0.006)

12. Annual Vegetation • No relationship between native or exotic cover and nitrogen supply (R2 = 0.18; P = 0.105) or the C:N (R2 = 0.18; P = 0.102) • Major controlling factor on exotic cover in this region appears to be presence of desert pavement (P=0.037) DP: desert pavement, SHBD: sandy with high bulk density, SLBD: sandy with low bulk density

13. N-Deposition • Strong correlation between July atmospheric HNO3 and extractable soil N (R2=0.75; P<0.0001) but no grouping of soil along HNO3 gradient (P = 0.107) • Expected soil N from deposition calculated using monthly deposition velocities, seasonal averages of HNO3 and NH3, and soil bulk densities • Predicted soil N and initial soil N were also correlated (R2=0.74; P < 0.0001) DP: desert pavement, SHBD: sandy with high bulk density, SLBD: sandy with low bulk density

14. N-Mineralization and Deposition • Inorganic nitrogen from soils collected in July are directly correlated with the N- deposition gradient. • Amount of nitrogen mineralized is also directly correlated with the initial amount of extractable nitrogen (R2 = 0.76; P<0.0001)

15. Conclusions • Nitrogen deposition is affecting the amount of nitrogen mineralized but not the rate of the mineralization in this region. • Mineralization is increased by improved SOM quality (decreased C:N) and is decreased in desert pavement areas. • Although nitrogen supply is increased in high deposition areas, exotic species cover is not affected by deposition. • Exotic species seem to be excluded from desert pavement areas, which may become a refuge for native species if invasion increases.

16. Acknowledgements Field Assistance: Robert Steers, Robin Marushia, Chris True, Greg Miller Lab Assistance: Catrina Paez Funding Sources: U.C. IPM program, Joshua Tree National Park, and NSF grants