Assessing Nitrogen-Saturation in a Seasonally Dry Chaparral Watershed: Limitations of Traditional Indicators of N-Saturation

Abstract

To evaluate nitrogen (N) saturation in xeric environments, we measured hydrologic N losses, soil N pools, and microbial processes, and developed an N-budget for a chaparral catchment (Sierra Nevada, California) exposed to atmospheric N inputs of approximately 8.5 kg N ha -1 y -1. Dual-isotopic techniques were used to trace the sources and processes controlling nitrate (NO 3 -) losses. The majority of N inputs occurred as ammonium. At the onset of the wet season (November to April), we observed elevated streamwater NO 3 -concentrations (up to 520 lmol l -1 ), concomitant with the period of highest gaseous N-loss (up to 500 ng N m -2 s -1 ) and suggesting N-saturation. Stream NO 3 -d 15N and d 18O and soil N measurements indicate that nitrification controlled NO 3 -losses and that less than 1% of the loss was of atmospheric origin. During the late wet season, stream NO 3 -concentrations decreased (to <2 lmol l -1 ) as did gaseous N emissions, together suggesting conditions no longer indicative of N-saturation. We propose that chaparral catchments are temporarily N-saturated at £ 8.5 kg N ha -1 y -1 , but that N-saturation may be difficult to reach in ecosystems that inherently leak N, thereby confounding the application of N-saturation indicators and annual N-budgets. We propose that activation of N sinks during the typically rainy winter growing season should be incorporated into the assessment of ecosystem response to N deposition. Specifically, the N-saturation status of chaparral may be better assessed by how rapidly catchments transition from N-loss to N-retention.

Aridity and plant uptake interact to make dryland soils hotspots for nitric oxide (NO) emissions

“…WEON decreased in step with irrigation frequency producing smaller NO pulses upon rewetting dry soils at the onset of the wet season. In support of NH 4 + oxidation, we observed high NO 3 − concentrations and nitrification rates upon rewetting soils-a pattern observed in our study site and other drylands (26,32)-and note that a 31‰ δ-15 N fractionation effect is consistent with nitrification (32, 45). Although we cannot conclusively differentiate between nitrification or nitrifier denitrification as the source of NO, nitrifier denitrification can account for a substantial fraction of NO emissions (11,46) and may best explain the [*δ- 18 O]NO isotopic trends we observed.…”

“…Based on these effects, we expected plant phenology would help explain seasonal NO emission patterns at our site [i.e., explain why NO emissions increased as soils dried and plants senesced, similar to observations in another dryland study (18)]. In support of plants controlling NO emissions, differences in plant species composition explained differences in NO flux between two forested sites (33), whereas other studies have suggested that plant N uptake controls hydrologic N loss (26,34) and, therefore, plants should also be capable of influencing gaseous N export (2,18,26). Thus, during the dry season, NO emissions likely increased because the shutdown in plant N uptake increased N supply to NOproducing processes and reactions.…”

“…Although nitrifiers are sensitive to drought (35), they can remain active in thin water films formed as soils dry (23,36), possibly explaining how interactions between vegetation and aridity caused NO emissions to increase during summer. In these diffusion-limited environments, the accumulation of microbial metabolic products (e.g., NH 4 + , NO 2 − , and NO 3 − ) may maintain a flux of NO even as soils dry (23,26,32,37). NO 2 − , in particular, is chemically reactive (38) and, in dry soils, may be less likely to be oxidized by nitrifiers but may still chemodenitrify (10,29,39).…”

“…Aridity, on the other hand, may influence NO emissions by favoring NO-producing mechanisms when dry soils are rewet (23)(24)(25). In drylands, asynchrony between N availability and plant demand (26) and between soil C mineralization and nitrification (27) limit N retention and could cause drylands to operate as NO hotspots.…”

“…Because aridity controls vegetation and water availability (52), N cycling in arid systems is less influenced by plants and more so controlled by interactions between microbial and abiotic processes (51). Building on this understanding, it is thought that in arid systems, microbial processes controlling N gas evasion may regulate the fate of soil δ-15 N (51), because these emissions can represent a substantial fraction of ecosystem N loss (18,26), while strongly discriminating against 15 …”