Many regions on Earth are expected to become drier with climate change, which may impact nitrogen (N) cycling rates and availability. We used a meta‐analytical approach on the results of field experiments that reduced precipitation and measured N supply (i.e., indices of N mineralization), soil microbial biomass, inorganic N pools (ammonium (NH4+) and nitrate (NO3−)), and nitrous oxide (N2O) emissions. We hypothesized that N supply and N2O emissions would be relatively insensitive to precipitation reduction and that reducing precipitation would increase extractable NH4+ and NO3− concentrations because microbial processes continue, whereas plant N uptake diminishes with drought. In support of this hypothesis, extractable NH4+ increased by 25% overall with precipitation reduction; NH4+ also increased significantly with increasing magnitude of precipitation reduction. In contrast, N supply and extractable NO3− did not change and N2O emissions decreased with reduced precipitation. Across studies microbial biomass appeared unchanged, yet from the diversity of studies, it was clear that proportionally smaller precipitation reductions increased microbial biomass, whereas larger proportional reductions in rainfall reduced microbial biomass; there was a positive intercept (P = 0.005) and a significant negative slope (P = 0.0002) for the regression of microbial biomass versus % precipitation reduction (LnR = −0.009 × (% precipitation reduction) + 0.4021). Our analyses imply that relative to other N variables, N supply is less sensitive to reduced precipitation, whereas processes producing N2O decline. Drought intensity and duration, through sustained N supply, may control how much N becomes vulnerable to loss via hydrologic and gaseous pathways upon rewetting dry soils.

Nitric oxide (NO) is an important trace gas and regulator of atmospheric photochemistry. Theory suggests moist soils optimize NO emissions, whereas wet or dry soils constrain them. In drylands, however, NO emissions can be greatest in dry soils and when dry soils are rewet. To understand how aridity and vegetation interact to generate this pattern, we measured NO fluxes in a California grassland, where we manipulated vegetation cover and the length of the dry season and measured [δ 15 -N]NO and [δ 18 -O]NO following rewetting with 15 N-labeled substrates. Plant N uptake reduced NO emissions by limiting N availability. In the absence of plants, soil N pools increased and NO emissions more than doubled. In dry soils, NO-producing substrates concentrated in hydrologically disconnected microsites. Upon rewetting, these concentrated N pools underwent rapid abiotic reaction, producing large NO pulses. Biological processes did not substantially contribute to the initial NO pulse but governed NO emissions within 24 h postwetting. Plants acted as an N sink, limiting NO emissions under optimal soil moisture. When soils were dry, however, the shutdown in plant N uptake, along with the activation of chemical mechanisms and the resuscitation of soil microbial processes upon rewetting, governed N loss. Aridity and vegetation interact to maintain a leaky N cycle during periods when plant N uptake is low, and hydrologically disconnected soils favor both microbial and abiotic NO-producing mechanisms. Under increasing rates of atmospheric N deposition and intensifying droughts, NO gas evasion may become an increasingly important pathway for ecosystem N loss in drylands.nitric oxide | chemodenitrification | drylands | NO pulses | N cycling N itric oxide (NO) is an important trace gas; it regulates the oxidative capacity of the atmosphere and indirectly influences Earth's climate (1). In the troposphere, NO catalyzes the production of hydroxyl radical, a powerful oxidant and cleanser of atmospheric contaminants. High concentrations of NO also favor the production of ozone (O 3 ), an urban pollutant and contributor to radiative forcing (1). Globally, fossil fuel combustion and biomass burning are major sources of NO, but soils are also a substantial source (2). Roughly 25-60% of terrestrial NO emissions originate from drylands (arid and semiarid environments) (3), which cover one-third of the terrestrial land surface (4), suggesting arid environments are important to global NO production. Climate models predict an expansion of drylands and intensifying droughts in existing arid regions (5), and when coupled with increasing rates of nitrogen (N) deposition and changes in the magnitude and frequency of precipitation events (6), increased soil NO emissions are possible (7). Paradoxically, arid soils are often described as infertile because biological processes are limited by water and N (8), raising the questions of why drylands are NO emission "hotspots."NO is produced in soils through both abiotic and biotic processes (2). Chemodeni...

Drying and rewetting of soils triggers a cascade of physical, chemical, and biological processes; understanding these responses to varying moisture levels becomes increasingly important in the context of changing precipitation patterns. When soils dry and water content decreases, diffusion is limited and substrates can accumulate. Upon rewetting, these substrates are mobilized and can energize hot moments of intense biogeochemical cycling, leading to pulses of trace gas emissions. Until recently, it was difficult to follow the rewetting dynamics of nutrient cycling in the field without physically disturbing the soil. Here

No 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.

a b s t r a c tSoil nitric oxide (NO) emissions are variable in both space and time, and are important pathways for N loss in seasonally dry ecosystems that undergo abrupt transitions from dry-to-wet soil conditions. We measured soil NO emissions from a chaparral catchment to characterize seasonal variability of, and triggers for enhanced NO losses. Pulses in NO emissions were observed in the summer and autumn when dry soils (soil water content (q) < 6%) were wetted naturally and artificially (range: 97e513 ng NOeN m À2 s À1). The rapidity and magnitude of these pulses suggest that abiotic processes may influence NO emissions. Outside of the observed pulses, NO emissions were highest during the dry season (q < 6%; dry season mean ¼ 3.4 ng NOeN m À2 s À1) and lowest during the winter wet season (q > 20%; wet season mean ¼ 0.14 ng NOeN m À2 s À1). These observed seasonal patterns contrast with previous DAYCENT simulations of NO emissions in our catchment, which predicted higher NO emissions during the wet season. Our field observations are consistent with sustained rates of nitrification, reduced plant N uptake, and high soil gas diffusivity observed during the dry season in arid environments.

Soil moisture controls microbial activity and soil carbon cycling. Because microbial activity decreases as soils dry, decomposition of soil organic matter (SOM) is thought to decrease with increasing drought length. Yet, microbial biomass and a pool of water-extractable organic carbon (WEOC) can increase as soils dry, perhaps implying microbes may continue to break down SOM even if drought stressed. Here, we test the hypothesis that WEOC increases as soils dry because exoenzymes continue to break down litter, while their products accumulate because they cannot diffuse to microbes. To test this hypothesis, we manipulated field plots by cutting off litter inputs and by irrigating and excluding precipitation inputs to extend or shorten the length of the dry season. We expected that the longer the soils would remain dry, the more WEOC would accumulate in the presence of litter, whereas shortening the length of the dry season, or cutting off litter inputs, would reduce WEOC accumulation. Lastly, we incubated grass roots in the laboratory and measured the concentration of reducing sugars and potential hydrolytic enzyme activities, strictly to understand the mechanisms whereby exoenzymes break down litter over the dry season. As expected, extending dry season length increased WEOC concentrations by 30% above the 108 μg C/g measured in untreated plots, whereas keeping soils moist prevented WEOC from accumulating. Contrary to our hypothesis, excluding plant litter inputs actually increased WEOC concentrations by 40% above the 105 μg C/g measured in plots with plants. Reducing sugars did not accumulate in dry senesced roots in our laboratory incubation. Potential rates of reducing sugar production by hydrolytic enzymes ranged from 0.7 to 10 μmol·g ·h and far exceeded the rates of reducing sugar accumulation (~0.001 μmol·g ·h ). Our observations do not support the hypothesis that exoenzymes continue to break down litter to produce WEOC in dry soils. Instead, we develop the argument that physical processes are more likely to govern short-term WEOC dynamics via slaking of microaggregates that stabilize SOM and through WEOC redistribution when soils wet up, as well as through less understood effects of drought on the soil mineral matrix.

Nitrogen (N) trace gas emission pulses produced after wetting dry soils may be important pathways of ecosystem N loss. However, the rates and mechanisms controlling these emissions remain unclear. We tested whether changes in microbial community structure and increased rates of atmospheric N deposition could explain N emissions at two desert sites differing in atmospheric N deposition by * six fold. We measured peak NO x (sum of nitric oxide and nitrogen dioxide) emissions 12 h postwetting. NO x emissions remained elevated over 24 h and increased after adding N. In contrast, we measured the highest nitrous oxide (N 2 O) emissions within only 15 min post-wetting. N 2 O emissions decreased within Keywords Nitrous oxide Á Nitric oxide Á NO x Á N 2 O Á Drying-rewetting Á Nitrogen deposition