annual biomass Growth of spring annual plants was estimated by harvesting total herbaceous aboveground biomass at peak production. Four one square meter quadrats were identified and permanently marked within each plot, two under L. tridentata shrubs and two in inter-plant spaces. Total herbaceous biomass was harvested from one quarter of each one square meter quadrat (0.25 square meter sub-plot) in March 2009. The location of the 0.25 square meter sub-plot is rotated within the permanent one square meter quadrat to avoid the area harvested the previous year. Biomass from each of the sub-plots is dried at 60 degrees Celsius and averaged (2 0.25 square meter sub-plots per one square meter quadrat; 2 quadrats per patch type; 2 patch types per plot).

stem growth Seasonal growth of the perennial shrubs L. tridentata and Ambrosia spp. was estimated by measuring elongation of stems. Stems were measured in April to estimate growth from October-April (winter/spring), and in October to estimate growth from April-October (summer/fall). One apical stem was selected from each of four cardinal directions on each of five individual plants per plot. Each stem was marked with tape several centimeters from the tip and measured at the beginning of the growth interval. Stem elongation was calculated as the difference in stem length from the beginning to the end of the measurement interval.

infiltration Infiltration rate was estimated as the field-saturated hydraulic conductivity, Kfs (centimeter per minute), using a modified single-ring infiltrometer method at two locations in each site. Briefly, a brass ring (7.5-centimeter diameter; 16-centimeter height) was inserted into the soil to a depth of 5 centimeter, then a 5-centimeter ponding depth was maintained using a Mariotte reservoir. Infiltration was allowed to proceed until a steady rate of vertical hydrological transport was achieved. The time and water height in the Mariotte reservoir were recorded for 10 minutes to determine the infiltration rate (centimeter per minute) upon reaching a steady rate of flow. The ponding depth was then set to 10 centimeter, and the process repeated. Kfs was calculated using both single- and dual- head approaches. There were problems reaching equilibrium at two sites (MVP, EMW), and only single- head Kfs values were calculated. Single- and dual-head approaches generated estimates that were highly correlated (Spearman r = 0.88, P < 0.001). As such, single-head Kfs values were used for statistical analyses because values were available for all sites.

surface soil collection Soil sampling was stratified by patch type. Three soil cores (8-centimeter diameter; 2-centimeter depth) per patch type were taken and homogenized per plot. Soil moisture, water-holding capacity, inorganic N and P concentration, soil organic matter [SOM] content, and potential rates of net N mineralization and nitrification for all soils in every season were measured. Soils from control plots were sampled in March 2006 while samples from all treatments were collected in October 2006 for total C and N. Soil pH from all treatments was measured in spring 2008 and 2009. Soils were stored on ice (10-12 degrees C) for up to 48 hours after collection, after which they were sieved (2 millimeter), and processed immediately for time-sensitive microbial properties and processes: inorganic N and P concentration, net potential N transformations, and soil moisture. Soil pH was measured on air-dried soils. Other soil properties (total soil C and N, inorganic C, SOM, and texture) were measured on oven-dried soils.

net potential N transformations, and inorganic N and P concentration Net potential N mineralization and net potential nitrification were measured in soils incubated at 60 percent water-holding capacity over 30 days to assess relative differences in C and N demand by microorganisms between nutrient treatments and across similar soils in locations differentially exposed to the urban atmosphere. The laboratory incubations used in this assay were not intended to assess actual N mineralization or nitrification rates experienced in the field, but to identify relative differences in microbial N transformation potential. One 10-gram subsample of soil was immediately shaken for 1 hour in 50 ml 2N KCl, filtered through pre-leached Whatman #42 filters, and then frozen for later analysis. Another 10-gram subsample was placed in a small, capped cup, its water content raised to 60 percent water-holding capacity with deionized water, and incubated in the dark for 30 days at 24 degrees C. Soils were extracted as described above after the incubation. KCl extracts were analyzed colorimetrically for NH4+-N and NO2-+NO3--N using a Lachat Quikchem 8000 autoanalyzer (Loveland, CO). Net potential N mineralization was calculated as the difference between the sum of NH4+-N and NO3--N concentration before and after each incubation. Net potential nitrification was calculated as the difference between NO3--N concentration before and after each incubation. Bicarbonate extractable soil P pools were measured by shaking 2-gram of soil in a 0.5 N bicarbonate solution for 1 hour. Soils were filtered as above and extracts were frozen until analysis on a Bran-Luebbe (now Seal Analytical) TRAACS 800 autoanalyzer (Mequon, WI).

other soil properties Gravimetric soil water was determined by drying soil subsamples for 24 hours at 105 degrees Celsius. Water-holding capacity was estimated as the gravimetric water content 24 hrs after soils were saturated and allowed to drain. SOM was estimated gravimetrically as mass lost following combustion for 4 h at 550 degrees Celsius. Inorganic C was determined on a 1-gram sample of ball-milled soil using the modified calcimeter method. Soil pH was determined by adding one 15-gram subsample to 30 ml deionized water (in 2008) or 0.01M CaCl2 (after 2008). Samples were gently stirred and allowed to stand for 30 minutes before analyses.

plant tissue chemistry Leaves from the perennial shrubs L. tridentata, Ambrosia spp., and aboveground leaves and stems from the dominant annual forbs Pectocarya recurvata and Pectocarya platycarpa were collected in the field and analyzed for foliar C, N. Pectocarya platycarpa was present across sites at low abundance and was not differentiated from Pectocarya recurvata for tissue nutrient analysis. Two mature, undamaged leaves from the most recent growing season were sampled on each individual shrub from each of four cardinal directions. For Pectocarya spp., both green stems and leaves from at least five individuals were sampled after air-drying. Plant tissue from all species was dried at 60 degrees Celsius, ground on a ball mill, and analyzed for C and N on a Perkin-Elmer 2400 Series II CHNS/O elemental analyzer. Tissue P was measured using persulfate/sulfuric acid digestion followed by colorimetric analysis (ascorbic acid analysis method). One 2-mg subsample from each foliar sample was placed in a 5 percent peroxodisulfate and 30 percent sulfuric acid solution and digested in an autoclave for 90 minutes. Digests were analyzed using a modified ascorbic acid colorimetric method using a Thermo Scientific Genesys Bio spectrophotometer set to 880 nm.

rainfall Precipitation data were harvested from the Maricopa County Flood Control District.