John M. Stark

Water pulses and biogeochemical cycles in arid and semiarid ecosystems

The episodic nature of water availability in arid and semiarid ecosystems has significant consequences on belowground carbon and nutrient cycling. Pulsed water events directly control belowground processes through soil wet-dry cycles. Rapid soil microbial response to incident moisture availability often results in almost instantaneous C and N mineralization, followed by shifts in C/N of microbially available substrate, and an offset in the balance between nutrient immobilization and mineralization. Nitrogen inputs from biological soil crusts are also highly sensitive to pulsed rain events, and nitrogen losses, particularly gaseous losses due to denitrification and nitrate leaching, are tightly linked to pulses of water availability. The magnitude of the effect of water pulses on carbon and nutrient pools, however, depends on the distribution of resource availability and soil organisms, both of which are strongly affected by the spatial and temporal heterogeneity of vegetation cover, topographic position and soil texture. The ‘inverse texture hypothesis’ for net primary production in water-limited ecosystems suggests that coarse-textured soils have higher NPP than fine-textured soils in very arid zones due to reduced evaporative losses, while NPP is greater in fine-textured soils in higher rainfall ecosystems due to increased water-holding capacity. With respect to belowground processes, fine-textured soils tend to have higher water-holding capacity and labile C and N pools than coarse-textured soils, and often show a much greater flush of N mineralization. The result of the interaction of texture and pulsed rainfall events suggests a corollary hypothesis for nutrient turnover in arid and semiarid ecosystems with a linear increase of N mineralization in coarse-textured soils, but a saturating response for fine-textured soils due to the importance of soil C and N pools. Seasonal distribution of water pulses can lead to the accumulation of mineral N in the dry season, decoupling resource supply and microbial and plant demand, and resulting in increased losses via other pathways and reduction in overall soil nutrient pools. The asynchrony of resource availability, particularly nitrogen versus water due to pulsed water events, may be central to understanding the consequences for ecosystem nutrient retention and long-term effects on carbon and nutrient pools. Finally, global change effects due to changes in the nature and size of pulsed water events and increased asynchrony of water availability and growing season will likely have impacts on biogeochemical cycling in water-limited ecosystems.

CONTROLS ON NITROGEN CYCLING IN TERRESTRIAL ECOSYSTEMS: A SYNTHETIC ANALYSIS OF LITERATURE DATA

Isotope pool dilution studies are increasingly reported in the soils and ecology literature as a means of measuring gross rates of nitrogen (N) mineralization, nitrification, and inorganic N assimilation in soils. We assembled data on soil characteristics and gross rates from 100 studies conducted in forest, shrubland, grassland, and agricultural systems to answer the following questions: What factors appear to be the major drivers for production and consumption of inorganic N as measured by isotope dilution studies? Do rates or the relationships between drivers and rates differ among ecosystem types? Across a wide range of ecosystems, gross N mineralization is positively correlated with microbial biomass and soil C and N concentrations, while soil C:N ratio exerts a negative effect on N mineralization only after adjusting for differences in soil C. Nitrification is a log-linear function of N mineralization, increasing rapidly at low mineralization rates but changing only slightly at high mineralization rates. In contrast, NH 4 1 assimilation by soil microbes increases nearly linearly over the full range of mineralization rates. As a result, nitrification is proportionately more important as a fate for NH4 1 at low mineralization rates than at high mineralization rates. Gross nitrification rates show no relationship to soil pH, with some of the fastest nitrification rates occurring below pH 5 in soils with high N mineralization rates. Differences in soil organic matter (SOM) composition and concentration among ecosystem types in- fluence the production and fate of mineralized N. Soil organic matter from grasslands appears to be inherently more productive of ammonium than SOM from wooded sites, and SOM from deciduous forests is more so than SOM in coniferous forests, but differences appear to result primarily from differing C:N ratios of organic matter. Because of the central importance of SOM characteristics and concentrations in regulating rates, soil organic matter depletion in agricultural systems appears to be an important determinant of gross process rates and the proportion of NH4 1 that is nitrified. Addition of 15 N appears to stimulate NH4 1 consumption more than NO3 2 consumption processes; however, the magnitude of the stim- ulation may provide useful information regarding the factors limiting microbial N trans- formations.

Plant species effects and carbon and nitrogen cycling in a sagebrush–crested wheatgrass soil

Abstract Shifts in plant community structure in shrub and grass-dominated ecosystems are occurring over large land areas in the western US. It is not clear what effect this vegetative change will have on rates of carbon and nitrogen cycling, and thus long-term ecosystem productivity. To study the effect of different plant species on the decomposability of soil organic substrates and rates of C- and N-cycling, we conducted laboratory incubations of soils from a 15-yr-old experimental plot where big sagebrush (Artemisia tridentata Nutt.) and crested wheatgrass (Agropyron desertorum [Fisch.] Schult.) plants had been planted in a grid pattern. Soil samples collected from beneath crested wheatgrass had significantly greater total N and NO3− concentrations and lower C-to-N ratios than samples collected beneath sagebrush plants or from interspaces. Soil carbonate concentrations beneath crested wheatgrass were intermediate between sagebrush soils and interspace soils. During the 16-week laboratory incubation, soil C mineralization rates, gross and net N mineralization rates, gross NH4+ consumption rates, net nitrification rates and soil NO3− concentrations were significantly different, with slightly higher values in the crested wheatgrass soil and lower values in the interspace soil. Gross NH4+ assimilation, gross nitrification, gross NO3− assimilation, microbial biomass C and N and microbial growth efficiency showed no significant differences. During the 16-week incubation, microbial biomass C, microbial respiration and gross N assimilation rates declined by more than 50%, suggesting that the microbial biomass was C-limited. However, addition of NH4+ appeared to stimulate NH4+ assimilation. In addition, the form of N assimilated by micro-organisms shifted from predominantly NH4+ (95%) when NO3− was relatively unavailable at the beginning of the incubation, to predominantly NO3− (88–96%) as NO3− concentrations increased. Both of these latter observations suggest that microbes were NH4+-limited. Either co-limitation by C and NH4+ or the presence of separate C-limited and NH4+-limited microsites could explain these results. Nitrifying bacteria consumed an increasing proportion of the NH4+ pool as the incubation progressed, suggesting that increased microsite structure was responsible for shifts in C and N dynamics over time. The results of this study demonstrate that different plant species can significantly influence soil C and N cycling rates; however, after 15 yr the magnitude of the effect was still fairly small.

Kinetic characteristics of ammonium-oxidizer communities in a California oak woodland-annual grassland

Abstract We must have information on the kinetic characteristics of soil biological processes if we are accurately to predict how environmental change will affect soil nutrient cycles. Little information is available on the characteristics of NH 4 + -oxidizing bacteria in uncultivated soils. In addition, little is known about how much physiological diversity occurs in NH 4 + -oxidizer communities within a single ecosystem. Therefore, we evaluated how NH 4 + -oxidizer communities from an oak woodland-annual grass ecosystem in north-eastern California respond to changes in temperature, osmotic potential and substrate concentrations. We used nitrification potential assays to determine the effects of temperature and osmotic potential on rates of NH 4 + oxidation in two soil depths, under two types of vegetative cover and during two seasons. We determined the kinetics of substrate utilization using soil slurries and enrichment cultures. In slurries, NH 4 + oxidation rates were measured using 15 N-isotope dilution to avoid confounding the effects of NO 3 − consumption with NO 3 − production. Ammonium-oxidizer communities beneath the canopies of oaks had lower temperature optima, greater activities and greater seasonal fluctuations in activity than communities in open grassy interspaces. Temperature optima of communities beneath oak canopies (31.8°C) and in open grassy interspaces (35.9°C) were as different as those reported for communities from tropical and temperate climatic zones. Ammonium-oxidizer communities from beneath oak canopies and open interspaces showed no difference in tolerance of low osmotic potential. The effect of substrate concentration on NH 4 + oxidation rates in slurries were best described by the Michaelis-Menten equation. Rates in liquid cultures were best described by the Haldane equation because of substrate inhibition. The half-saturation constant ( K m ) for NH 4 + oxidation in these soils averaged 15 μ m NH 4 + (=0.012 μ m NH 3 ), which is substantially lower than values reported in the literature for agricultural soils, sediments and sewage sludge. Enrichment cultures were inhibited by lower substrate concentrations (1600 μ m NH 4 + or 1.3 μ m NH 3 ) than reported for isolates from sewage systems. These results suggest that NH 4 + -oxidizer communities in uncultivated soils are more oligotrophic in nature, and thus kinetic parameters reported in the literature for agricultural soils, sediments and sewage sludge are not appropriate for describing NH 4 + oxidation rates in these soils.

Modeling the temperature response of nitrification

To model nitrification rates in soils, it is necessary to have equations that accurately describe the effect of environmental variables on nitrification rates. A variety of equations have been used previously to describe the effect of temperature on rates of microbial processes. It is not clear which of these best describes the influence of temperature on nitrification rates in soil. I compared five equations for describing the effects of temperature on nitrification in two soils with very different temperature optima from a California oak woodland-annual grassland. The most appropriate equation depended on the range of temperatures being evaluated. A generalized Poisson density function best described temperature effects on nitrification rates in both soils over the range of 5 to 50 °C; however, the Arrhenius equation best described temperature effects over the narrower range of soil temperatures that normally occurs in the ecosystem (5 to 28 °C). Temperature optima for nitrification in most of the soils were greater than even the highest soil temperatures recorded at the sites. A model accounting for increased maintenance energy requirements at higher temperatures demonstrates how net energy production, rather than the gross energy production from nitrification, is maximized during adaptation by nitrifier populations to soil temperatures.

Regulation and Measurement of Nitrification in Terrestrial Systems

Understanding nitrification rates and their regulation continues as a key area of research for assessing humans increasing impact on the terrestrial N cycle. We review the organisms and processes responsible for nitrification in terrestrial systems. The control of nitrification by substrate availability is discussed with particular attention to the factors affecting ammonia/ammonium availability. The effects on nitrification rates of environmental controls including oxygen, water potential, temperature and pH are described. With this general understanding of the factors affecting nitrification rates as a basis, we present an in depth analysis of methods used to measure nitrification in terrestrial systems. Net, gross and potential nitrification rate measurements are explained including the use of isotopes and inhibitors to measure rates in soils. Methods for the estimation of nitrification kinetics and modeling are briefly described. Future challenges will require understanding the factors controlling nitrification across spatial scales from ecosystems to soil microsites if we are to sustainably manage reactive nitrogen in terrestrial environments.

Inorganic N turnover and availability in annual- and perennial-dominated soils in a northern Utah shrub-steppe ecosystem

The exotic annual grass Bromus tectorum has replaced thousands of hectares of native perennial vegetation in semi-arid ecosystems of the western United States. Inorganic N availability and production were compared in soil from monodominant patches of Bromus tectorum, the perennial bunchgrass Elymus elymoides, and the shrub Artemisia tridentata, in Curlew Valley, a salt-desert shrub site in Northern Utah. Bromus-dominated soil had greater %N in the top 10 cm than Artemisia or Elymus-dominated soils. As determined by spring isotope-dilution assays, gross mineralization and nitrification rates were higher in Bromus-dominated than Artemisia-dominated soils, but gross rates of NH4+ and NO3− consumption were also higher. Litterbags had greater mass loss and N mineralization when buried in Bromus stands than in Artemisia stands, indicating the soil environment under the annual grass promotes decomposition. As determined by nitrification potential assays, nitrifier populations were higher under Bromus than under Artemisia and Elymus. Soil inorganic N concentrations were similar among vegetation types in the spring, but NO3− accumulated under Bromus once it had senesced. An in situ net mineralization assay conducted in autumn indicated that germinating Bromus seedlings are a strong sink for soil NO3−, and that net nitrification is inherently low in soils under Artemisia and Elymus. Results of the study suggest that differences in plant uptake and the soil environment promote greater inorganic N availability under Bromus than under perennial species at the site.

Nitrogen Limitation of the Microbial Biomass in an Old-Growth Forest Soil

Abstract:We assessed whether the microbial biomass of an old-growth mixed-conifer forest soil was N limited by applying the equivalent of 450 kg N/ha as (NH4)2SO4 at a 2-cm depth to mineral soil co...

Influences of Chloroform Exposure Time and Soil Water Content on C and N Release in Forest Soils

We investigated the influence of fumigation conditions of the chloroform fumigation‐ extraction method on the release of extractable carbon (C) and nitrogen (N) from organic and mineral soil horizons of 11 mature forests. Soil samples were fumigated with chloroform vapor for 1, 3, 5, or 10 d at two different water contents: field moist or field capacity (233 kPa matric potential). We found that for approximately half our soils, 0.5 M K2SO4-extractable N and C reached a maximum after 1 d of fumigation. The effect of soil wetting on C and N flushes from O horizons was variable but in mineral soils, increasing the soil water content generally resulted in more extractable C and N following fumigation. For the majority of soils assessed, increasing the soil water content did not change the fumigation time necessary to generate the maximum C or N flush. Observed changes in the C/N ratios of the fumigation flush with changes in fumigation time and the sensitivity of this C/N ratio to changes in soil water content suggest that different fumigation conditions may result in different organic pools being extracted. These effects appear to be extremely soil specific. We recommend that the effects of fumigation time and water content be evaluated before the C and N flushes from the fumigation extraction method are used to assess differences in microbial C and N among contrasting forest soils. q 2002 Elsevier Science Ltd. All rights reserved.

Resource limitations to nitric oxide emissions from a sagebrush-steppe ecosystem

We monitored soil emissions of NO, NO2, N2O, and CO2 throughout the summer dry season at a remote North American sagebrush-steppe ecosystem following application of several resources, including water, NH4+, NO3− and sucrose. Despite low levels of soil NH4+ (5.60±0.95 mg NH4+-N per kg soil, mean±S.E.), and NO3−-N (1.34±0.20 mg NO3−-N per kg soil), NO emissions ranged from about 0.2 to 2.8 ng NO-N m−2 s−1, comparable to rates measured from many agricultural, tropical, and other undisturbed ecosystems. Soil wetting increased NO emissions as much as 400-fold when initial gravimetric soil moisture contents were less than about 50 mg kgsoil−1, and soil temperature was greater than or equal to 20 °C. Wetting treatments with 20 mg NH4+-N kgsoil−1 raised NO emission rates to a level that was nearly an order of magnitude higher than that observed after water addition alone. Wetting treatments with 20 mg NO3−-N kgsoil−1, 240 mg sucrose-C kgsoil−1, or NO3−plus sucrose had no statistically significant effect upon NO emissions. Soil denitrifying enzyme activity was low at this site, and N2O emissions in the field were below detection limits. Soil nitrifying enzyme activity was extremely high at this site, indicating that the NH4+ released by ammonification would be consumed at least once every 1.7 days. These observations indicate that NO emissions from this undisturbed ecosystem were likely a consequence of high nitrification activity, and that sagebrush-steppe ecosystems may be a more important NO source than has been previously assumed.