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

Precipitation legacies amplify ecosystem nitrogen losses from nitric oxide emissions in a Pinyon-Juniper dryland

Climate change is increasing the variability of precipitation, altering the frequency of soil drying-wetting events and the distribution of seasonal precipitation. These changes in precipitation can alter nitrogen (N) cycling and stimulate nitric oxide (NO) emissions (an air pollutant at high concentrations), which may vary according to legacies of past precipitation and represent a pathway for ecosystem N loss. To identify whether precipitation legacies affect NO emissions, we excluded or added precipitation during the winter growing season in a Pinyon-Juniper dryland and measured in situ NO emissions following experimental wetting. We found that the legacy of both excluding and adding winter precipitation increased NO emissions early in the following summer; cumulative NO emissions from the winter precipitation exclusion plots (2750 ± 972 μg N-NO m^-2 ) and winter water addition plots (2449 ± 408 μg N-NO m^-2 ) were higher than control plots (1506 ± 397 μg N-NO m^-2 ). The increase in NO emissions with previous precipitation exclusion was associated with inorganic N accumulation, while the increase in NO emissions with previous water addition was associated with an upward trend in microbial biomass. Precipitation legacies can accelerate soil NO emissions and may amplify ecosystem N loss in response to more variable precipitation.

A Drying-Rewetting Cycle Imposes More Important Shifts on Soil Microbial Communities than Does Reduced Precipitation

Global changes will result in altered precipitation patterns, among which the increasing frequency of drought events has the highest deleterious potential for agriculture. Soil microbes have shown some promise to help crops adapt to drought events, but it is uncertain how crop-associated microorganisms will respond to altered precipitation patterns. To investigate this matter, we conducted a field experiment where we seeded two wheat cultivars (one resistant to water stress and the other sensitive) that were subjected to four precipitation exclusion (PE) regimes (0%, 25%, 50%, and 75% exclusion). These cultivars were sampled seven times (every 2 weeks, from May to August) within one growing season to investigate short-term microbiome responses to altered precipitation regimes and seasonality using 16S rRNA gene and internal transcribed spacer (ITS) region amplicon sequencing. One of the most striking features of the data set was the dramatic shift in microbial community diversity, structure, and composition together with a doubling of the relative abundance of the archaeal ammonia oxidizer genus Nitrososphaera following an important drying-rewetting event. Comparatively small but significant effects of PE and wheat cultivar on microbial community diversity, composition, and structure were observed. Taken together, our results demonstrate an uneven response of microbial taxa to decreasing soil water content, which was dwarfed by drying-rewetting events, to which soil bacteria and archaea were more sensitive than fungi. Importantly, our study showed that an increase in drying-rewetting cycles will cause larger shifts in soil microbial communities than a decrease in total precipitation, suggesting that under climate changes, the distribution of precipitation will be more important than small variations in the total quantity of precipitation. IMPORTANCE Climate change will have a profound effect on the precipitation patterns of global terrestrial ecosystems. Seasonal and interannual uneven distributions of precipitation will lead to increasing frequencies and intensities of extreme drought and rainfall events, which will affect crop productivity and nutrient contents in various agroecosystems. However, we still lack knowledge about the responses of soil microbial communities to reduced precipitation and drying-rewetting events in agroecosystems. Our results demonstrated an uneven response of the soil microbiome and a dramatic shift in microbial community diversity and structure to a significant drying-rewetting event with a large increase in the relative abundance of archaeal ammonia oxidizers. These findings highlight the larger importance of rewetting of dry soils on microbial communities, as compared to decreased precipitation, with potential for changes in the soil nitrogen cycling.

Fire Alters Soil Properties and Vegetation in a Coniferous–Broadleaf Mixed Forest in Central China

Fire is the predominant natural disturbance that influences the community structure as well as ecosystem function in forests. This study was conducted to examine the soil properties, loss of aboveground biomass, and understory plant community in response to an anthropogenic fire in a coniferous (Pinus massoniana Lamb.) and broadleaf (Quercus acutissima Carruth.) mixed forest in a subtropical–temperate climatic transition zone in Central China. The results showed that soil pH, NO3−-N concentration, and microbial biomass carbon (C) increased three months after the fire; however, there were no significant differences in soil organic C, total nitrogen (N), NH4+-N concentration, or microbial biomass N between the burned and unburned observed plots. The total aboveground biomass was 39.0% lower in the burned than unburned plots four weeks after fire. Direct biomass combustion (19.15 t ha−1, including understory shrubs and litters) was lower than dead wood biomass loss (23.69 t ha−1) caused by the fire. The declining trends of tree mortality with increasing diameter at breast height for both pine and oak trees suggest that small trees are more likely to die during and after fires due to the thinner bark of small trees and tree and branch fall. In addition, burning significantly stimulated the density of shrub (160.9%) and herb (88.0%), but it also affected the richness of shrub and herb compared with that in the unburned plots two months after the fire. The rapid recovery of understory plants after fires suggest that the diversity of understory species could benefit from low-severity fires. Our findings highlight that the decomposition of dead wood and understory community recovery should be considered for offsetting C emissions after fires for further research.

The Growth and N Retention of Two Annual Desert Plants Varied Under Different Nitrogen Deposition Rates

Nitrogen (N) partitioning between plant and soil pools is closely related to biomass accumulation and allocation, and is of great importance for quantifying the biomass dynamics and N fluxes of ecosystems, especially in low N-availability desert ecosystems. However, partitioning can differ among species even when growing in the same habitat. To better understand the variation of plant biomass allocation and N retention within ephemeral and annual species we studied the responses of Malcolmia Africana (an ephemeral) and Salsola affinis (an annual) to N addition, including plant growth, N retention by the plant and soil, and N lost to the environment using ¹⁵N (double-labeled ¹⁵NH₄ ¹⁵NO₃ (5.16% abundance) added at 0, 0.8, 1.6, 3.2, and 6.4 g pot^-1, equivalent to 0, 15, 30, 60, and 120 kg N ha^-1) in a pot experiment. Higher N addition (N120) inhibited plant growth and biomass accumulation of the ephemeral but not the annual. In addition, the aboveground:belowground partitioning of N (the R:S ratio) of the ephemeral decreased with increasing N addition, but that of the annual increased. The N input corresponding to maximum biomass and ¹⁵N retention of the ephemeral was significantly less than that of the annual. The aboveground and belowground retention of N in the ephemeral were significantly less than those of the annual, except at low N rates. The average plant-soil system recovery of added ¹⁵N by the ephemeral was 70%, significantly higher than that of the annual with an average of 50%. Although the whole plant-soil ¹⁵N recovery of this desert ecosystem decreased with increasing N deposition, our results suggested that it may vary with species composition and community change under future climate and elevated N deposition.

Future N deposition and precipitation changes will be beneficial for the growth of Haloxylon ammodendron in Gurbantunggut Desert, northwest China

Evaluation of precipitation and nitrogen (N) deposition in desert ecosystems helps to elucidate the reaction of desert ecosystems to future environmental changes. An in-situ field experiment was established to examine the influence of a long-term enhanced precipitation and N deposition on the photosynthetic traits and physiological characteristics of Haloxylon ammodendron in the Gurbantunggut Desert, northwest China, throughout the growing season in 2014-2016. Results showed a significant interaction between precipitation and N applications. Increased precipitation and N deposition and their coupling could significantly improve photosynthetic capacity, alter the variability in amplitude of water potential and change the content of substances regulating osmotic pressure in H. ammodendron. According to the comprehensive evaluation of H. ammodendron's adaptability using six different water and N coupling models, a combination of a 30% increase in precipitation and a 30 kg N ha^-1 yr^-1 addition in nitrogen deposition, or the addition of N at a concentration of 60 kg N ha^-1 yr^-1 with natural precipitation were beneficial to H. ammodendron growth and development. Hence, changes in the future global environment can be anticipated to be beneficial to H. ammodendron growth.

Quantifying atmospheric N deposition in dryland ecosystems: A test of the Integrated Total Nitrogen Input (ITNI) method

Estimating nitrogen (N) deposition to terrestrial ecosystems is complicated by the multiple forms and routes of N loading from the atmosphere. We used the integrated total nitrogen input (ITNI) method, which is based on the principle of isotope dilution within a plant-liquid-sand system, to quantify N inputs to coastal sage scrub ecosystems in Riverside, California. Using the ITNI method, we measured atmospheric N deposition of 29.3 kg N ha^-1 yr^-1 over a range of aboveground plant biomass of 228 to 424 g m^-2. From 85 to 96% of the atmospheric N inputs were taken up by plants in the ITNI modules with most of the assimilation mediated by, and stored in, aboveground biomass. Parallel measurements using conventional approaches yielded deposition rates of 25.2 kg N ha^-1 yr^-1 when using the inferential method and 4.8 kg N ha^-1 yr^-1 using throughfall collectors. The relatively low throughfall estimates were attributed to canopy retention of inorganic N, low rainfall, and to the fact that the throughfall flux data did not include organic N and stomatal uptake of N gases. Also, during dry periods, frequent watering of ITNI modules may have increased stomatal conductance and led to overestimates of N deposition. Across published studies that used the ITNI method, areal N deposition rates varied by ~40-fold, were positively correlated with plant biomass and 90% of the variability in measured deposition rates can be explained by plant biomass production. The ITNI method offers a holistic approach to measuring atmospheric N deposition in arid ecosystems, although more study is needed to understand how watering rates effect N deposition measurements.

Drought increases the frequencies of fungal functional genes related to carbon and nitrogen acquisition

Although water is a critical resource for organisms, microbially-mediated processes such as decomposition and nitrogen (N) transformations can endure within ecosystems even when water is scarce. To identify underlying mechanisms, we examined the genetic potential for fungi to contribute to specific aspects of carbon (C) and N cycling in a drought manipulation in Southern California grassland. In particular, we measured the frequency of fungal functional genes encoding enzymes that break down cellulose and chitin, and take up ammonium and amino acids, in decomposing litter. Furthermore, we used "microbial cages" to reciprocally transplant litter and microbes between control and drought plots. This approach allowed us to distinguish direct effects of drought in the plot environment versus indirect effects via shifts in the microbial community or changes in litter chemistry. For every fungal functional gene we examined, the frequency of that gene within the microbial community increased significantly in drought plots compared to control plots. In contrast, when plot environment was held constant, frequencies of these fungal functional genes did not differ significantly between control-derived microbes versus drought-derived microbes, or between control-derived litter versus drought-derived litter. It appears that drought directly selects for fungi with the genetic capacity to acquire these specific C- and N-containing compounds. This genetic trait may allow fungi to take advantage of ephemeral water supplies. Altogether, proliferation of fungi with the genetic capacity for C and N acquisition may contribute to the maintenance of biogeochemical cycling under drought.

Life in Dry Soils: Effects of Drought on Soil Microbial Communities and Processes

Throughout Earth's history, drought has been a common crisis in terrestrial ecosystems; in human societies, it can cause famine, one of the Four Horsemen of the apocalypse. As the global hydrological cycle intensifies with global warming, deeper droughts and rewetting will alter, and possibly transform, ecosystems. Soil communities, however, seem more tolerant than plants or animals are to water stress—the main effects, in fact, on soil processes appear to be limited diffusion and the limited supply of resources to soil organisms. Thus, the rains that end a drought not only release soil microbes from stress but also create a resource pulse that fuels soil microbial activity. It remains unclear whether the effects of drought on soil processes result from drying or rewetting. It is also unclear whether the flush of activity on rewetting is driven by microbial growth or by the physical/chemical processes that mobilize organic matter. In this review, I discuss how soil water, and the lack of it, regulates microbial life and biogeochemical processes. I first focus on organismal-level responses and then consider how these influence whole-soil organic matter dynamics. A final focus is on how to incorporate these effects into Earth System models that can effectively capture dry–wet cycling.

Effects of altered dry season length and plant inputs on soluble soil carbon

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^-1 ·h^-1 and far exceeded the rates of reducing sugar accumulation (~0.001 μmol·g^-1 ·h^-1 ). 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.

Coupling of microbial nitrogen transformations and climate in sclerophyll forest soils from the Mediterranean Region of central Chile

The Mediterranean region of central Chile is experiencing extensive "mega-droughts" with detrimental effects for the environment and economy of the region. In the northern hemisphere, nitrogen (N) limitation of Mediterranean ecosystems has been explained by the decoupling between N inputs and plant uptake during the dormant season. In central Chile, soils have often been considered N-rich in comparison to other Mediterranean ecosystems of the world, yet the impacts of expected intensification of seasonal drought remain unknown. In this work, we seek to disentangle patterns of microbial N transformations and their seasonal coupling with climate in the Chilean sclerophyll forest-type. We aim to assess how water limitation affects microbial N transformations, thus addressing the impact of ongoing regional climate trends on soil N status. We studied four stands of the sclerophyll forest-type in Chile. Field measurements in surface soils showed a 67% decline of free-living diazotrophic activity (DA) and 59% decrease of net N mineralization rates during the summer rainless and dormant season, accompanied by a stimulation of in-situ denitrification rates to values 70% higher than in wetter winter. Higher rates of both free-living DA and net N mineralization found during spring, provided evidence for strong coupling of these two processes during the growing season. Overall, the experimental addition of water in the field to litter samples almost doubled DA but had no effect on denitrification rates. We conclude that coupling of microbial mediated soil N transformations during the wetter growing season explains the N enrichment of sclerophyll forest soils. Expected increases in the length and intensity of the dry period, according to climate change models, reflected in the current mega-droughts may drastically reduce biological N fixation and net N mineralization, increasing at the same time denitrification rates, thereby potentially reducing long-term soil N capital.

Impacts of water and nitrogen addition on nitrogen recovery in Haloxylon ammodendron dominated desert ecosystems

Desert ecosystems are likely to change in response to global climate change and nitrogen (N) deposition. The effects of increased precipitation and N deposition on plant growth and the N cycle largely depend on N allocation and N recovery efficiency in the plant-soil ecosystem, but there is limited research on this in desert ecosystems. Here we report results using double-labeled ¹⁵NH₄¹⁵NO₃ (30 and 60kgNha^-1yr^-1) as a tracer under ambient (no additional water addition) and enhanced precipitation (60mm water addition) in a Haloxylon ammodendron dominated ecosystem in the Gurbantunggut Desert of Northwest China. Herbaceous plants were a significantly larger sink for added ¹⁵N than the H. ammodendron trees, and N retention varied with water and N addition, relative to growing season precipitation. The retention of added ¹⁵N varied within the components of H. ammodendron, with the stems retaining most, followed by the assimilation branches. Soil was the dominant sink for added ¹⁵N, in which the topsoil and subsoil respond differently to water and N addition over the two-year period. Nitrogen relative recovery percentage in the whole ecosystem ranged from 43% to 61%, lower than average recovery rate in temperate forests; N tracer recovery percentage significantly increased with water addition but decreased with enhanced N deposition. Future N cycling in central Asian deserts will depend on changes in precipitation.

Acidity and organic matter promote abiotic nitric oxide production in drying soils

Soils are an important source of NO, particularly in dry lands because of trade-offs that develop between biotic and abiotic NO-producing processes when soils dry out. Understanding how drier climates may offset the balance of these trade-offs as soils transition toward more arid states is, therefore, critical to estimating global NO budgets, especially because drylands are expected to increase in size. We measured NO emission pulses after wetting soils from similar lithologies along an altitudinal gradient in the Sierra Nevada, CA, where mean annual precipitation varied from 670 to 1500 mm. Along the gradient, we measured field NO emissions, and used chloroform in the laboratory to reduce microbial activity and partition between biotic and abiotic NO-producing processes (i.e., chemodenitrification). Field NO emission pulses were lowest in the acidic and SOM-rich soils (4-72 ng NO-N m^-2 s^-1 ), but were highest in the high-elevation barren site (~560 ng NO-N m^-2 s^-1 ). In the laboratory, NO emission pulses were up to 19× greater in chloroform-treated soils than in the controls, and these abiotic pulses increased with elevation as pH decreased (6.2-4.4) and soil organic matter (SOM) increased (18-157 mg C g^-1 ). Drought can shift the balance between the biotic and abiotic processes that produce NO, favoring chemodenitrification during periods when biological processes become stressed. Acidic and SOM-rich soils, which typically develop under mesic conditions, are most vulnerable to N loss via NO as interactions between pH, SOM, and drought stimulate chemodenitrification.

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

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.