Nitrogen (N) is the element limiting primary production in most terrestrial ecosystems [Vitousek and Howarth, 1991]. Fertilizer use and fossil fuel combustion have more than doubled the input of biologically available N into ecosystems, mainly in the northern hemisphere [Vitousek et al., 1997]. This generally leads to an enhancement of aboveground plant productivity; however, at very high levels, N deposition can lead to N saturation, characterized by the alleviation of N limitations on primary productivity, soil acidification, and increased N losses from the ecosystem [Aber et al., 1989]. Enhanced N deposition is widespread [Galloway et al., 1995], and N saturation has been observed in numerous temperate forest ecosystems in North America (summarized by Fenn et al. ).
 N deposition has also been demonstrated to affect belowground carbon cycling, although the net result on soil carbon (C) storage is unclear. Changes in litter quality and quantity, as well as in microbial community composition and activity can all affect decomposition and soil respiration rates. The degree to which each of these changes is expressed will vary with climate. Most results to date explore changes in mesic, temperate systems, but in xeric or cryic ecosystems, where decomposition rates are limited by lack of water or freezing temperatures, the rate of response to N deposition may be dramatic, as evidenced by rapid loss of centuries old C in tundra soils exposed to N + P addition [Mack et al., 2004; Nowinski et al., 2008].
 As aboveground primary production generally increases with N deposition, leaf litter inputs to the soil surface are higher [Vitousek and Howarth, 1991]. Litter quality (as measured by C:N, lignin:N or N content of litter) also increases with N deposition. Higher quality litter is associated with higher rates of decomposition [Swift et al., 1979; Aber and Melillo, 1982; Melillo et al., 1982] and decomposition increases in response to N addition [Knorr et al., 2005], at least in the short term. Most sequential coring studies suggest that fine root production does not change or decreases with N addition, although C and N budget-based methods infer increased root N inputs [Nadelhoffer, 2000]. Plants may allocate a smaller proportion of C to the rhizosphere due to decreasing nutrient needs, but with increased N concentrations in roots, the overall effect of changing belowground litter quality and quantity is unclear [Nadelhoffer, 2000].
 The makeup of the microbial community is also altered by N addition or deposition. Numerous enzyme assay studies have found that cellulolytic enzymes are more active with greater N, while lignin-oxidizing enzymes are less active with N enrichment [Carreiro et al., 2000; Frey et al., 2004; Saiya-Cork et al., 2002; Sinsabaugh et al., 2002]. The decrease in lignin degradation may reflect reduced activity of microorganisms capable of degrading lignin, particularly white-rot fungi, and/or the N-mediated enhancement of recalcitrance, while cellulose degradation may be enhanced because of reduced N limitation of some soil organisms that degrade cellulose [Fog, 1988].
 In short-term experiments, the enhancement of cellulose decomposition, which occurs early in the decomposition process, generally dominates the response to N additions, while in long-term experiments, the depression of lignin degradation becomes more important as cellulose has largely been lost [Fog, 1988]. These factors can contribute to differing responses to N amendment across ecosystems and soil fractions depending on the amount of labile C. For example, Neff et al.  found that in alpine tundra soils where decomposition rates are low most of the year due to cold temperatures, N addition accelerated decomposition in the light fraction where largely labile C resides, while having little effect on the heavy fraction where more recalcitrant C resides. Consequently, long-term N addition in xeric or cryic ecosystems might not result in lower decomposition, as frequently seen in mesic systems, because undecomposed litter and consequently, cellulose stores, are likely to be more abundant.
 This study explores the effects of N amendment on two xeric sites in the San Bernardino Mountains of southern California that vary in background N deposition rate and pollution. Owing to high rates of fossil fuel combustion, nitrogen deposition rates in southern California are the highest in the United States, with up to 70 kg N ha−1 being deposited annually. Nitrogen saturation symptoms have been observed at the polluted end-member site and over the western end of the San Bernardino Mountains [Fenn et al., 1996].
 Previous studies at these sites have shown a wide variety of changes associated with the pollution gradient. The high pollution site is highly N enriched exhibiting severe symptoms of N saturation, as demonstrated by high nitrate concentrations in soil and in stream water runoff as well as N enrichment of vegetation. In contrast, N cycling remains conservative at the low pollution site [Fenn and Poth, 1999b; Fenn et al., 1996, 2008]. In addition to these N eutrophication effects, the high pollution site has experienced severe soil acidification and decreasing base saturation as a result of N deposition over the past half century [Wood et al., 2007]. These forests are less likely to decline from soil acidification effects due to relatively high base saturation, even in high deposition sites. However, forest health is affected by air pollution in the San Bernardino Mountains because the combined effects of ozone and N deposition increase susceptibility to bark beetles, reduce root biomass, and increase drought stress and risk of severe fire occurrence [Jones et al., 2004; Grulke et al., 2008].
 Ozone is also present at high levels in the polluted site (Table 1) and has several effects on ecosystem functioning [Fenn and Bytnerowicz, 1993]. Ozone damage to ponderosa pine needles at the high pollution site causes them to be replaced more frequently, resulting in increased allocation to aboveground plant tissues [Grulke and Balduman, 1999]. The variation in δ13C signatures and N contents of ponderosa pine is consistent with depressed photosynthesis and higher N availability in the more polluted end of the gradient. Consequently, litter inputs of C and N are much higher on the polluted end of the gradient, which ultimately results in extremely thick litter layers [Grulke et al., 1998; Grulke and Balduman, 1999].
|Camp Osceola||Camp Paivika|
|Mean annual temperature (°C)||10.6||12.9|
|Mean annual precipitation (cm)||90||98|
|Ozone concentration (ppb h−1)||62||80|
|N deposition (kg ha a−1)||7.5||71.1|
|pH (A horizon)||5.77||3.98|
 Changes in decomposition and respiration observed along the gradient appear to differ from those resulting from N amendment in more mesic environments. Fenn and Dunn  found that heterotrophic respiration rates and fungal diversity in recently fallen litter increased with increasing pollution inputs. CO2 evolution rates were positively correlated with litter N concentration and negatively correlated with litter Ca concentration, both of which would be associated with younger needles and consequently may result from ozone damage more than increased soil N availability [Fenn and Dunn, 1989]. It is difficult to separate the effects of N from the effects of ozone and litter inputs in a gradient study. Therefore, N addition treatments were begun at two of the end-member sites in 1997, and we sampled the different levels of N amendment at both sites in 2006. We combined radiocarbon measurements of organic matter and microbially respired CO2 with measures of C stocks and fluxes to quantify differences in C cycling with 9 years of N amendment in sites representing high and low ends of the pollution gradient.