Nitrogen-fixing bacterial communities differ between perennial agroecosystem crops.

Biocrusts, common in natural ecosystems, are specific assemblages of microorganisms at or on the soil surface with associated microorganisms extending into the top centimeter of soil. Agroecosystem biocrusts have similar rates of nitrogen (N) fixation as those in natural ecosystems, but it is unclear how agricultural management influences their composition and function. This study examined the total bacterial and diazotrophic communities of biocrusts in a citrus orchard and a vineyard that shared a similar climate and soil type but differed in management. To contrast climate and soil type, these biocrusts were also compared with those from an apple orchard. Unlike natural ecosystem biocrusts, these agroecosystem biocrusts were dominated by proteobacteria and had a lower abundance of cyanobacteria. All of the examined agroecosystem biocrust diazotroph communities were dominated by N-fixing cyanobacteria from the Nostocales order, similar to natural ecosystem cyanobacterial biocrusts. Lower irrigation and fertilizer in the vineyard compared with the citrus orchard could have contributed to biocrust microbial composition, whereas soil type and climate could have differentiated the apple orchard biocrust. Season did not influence the bacterial and diazotrophic community composition of any of these agroecosystem biocrusts. Overall, agricultural management and climatic and edaphic factors potentially influenced the community composition and function of these biocrusts.

Contrasting seasonal patterns and factors regulating biocrust N2-fixation in two Florida agroecosystems.

Biocrusts are communities of microorganisms within the top centimeter of soil, often dominated by phototrophic dinitrogen-fixing (N2-fixing) organisms. They are common globally in arid ecosystems and have recently been identified in agroecosystems. However, unlike natural ecosystem biocrusts, agroecosystem biocrusts receive regular fertilizer and irrigation inputs. These inputs could influence seasonal biocrust N2-fixation and their relationship with soil nutrients in perennial agroecosystems, which is of particular interest given crop management requirements. In this study, biocrust and adjacent bare soil N2-fixation activity was measured in the field during the summer, fall, spring, and winter seasons in a Florida citrus orchard and vineyard using both acetylene reduction assays and 15N2 incubations. Samples were analyzed for microbial and extractable carbon (MBC, EC), nitrogen (MBN, EN), and phosphorus (MBP, EP). In both agroecosystems, biocrusts had greater microbial biomass and extractable nutrients compared to bare soil. The citrus and grape biocrusts were both actively fixing N2, despite crop fertilization, with rates similar to those found in natural arid and mesic systems, from 0.1 to 142 nmol of C2H4 g-1 of biocrust dry weight h-1 (equivalent to 1-401 µmol m-2h-1). Lower soil temperatures and higher EC:EN ratios were associated with higher N2-fixation rates in citrus biocrusts, while higher soil moisture and higher EP were associated with higher N2-fixation rates in grape biocrusts. The N2-fixation activity of these agroecosystem biocrusts indicates the possibility of biocrusts to enhance N cycling in perennial agroecosystems, with potential benefits for crop production.

Organic nitrogen in residential stormwater runoff: Implications for stormwater management in urban watersheds.

Stormwater runoff containing organic nitrogen (N) is a source of potentially bioavailable N in water bodies. Characterization and concentrations of dissolved organic N (DON) and particulate organic N (PON) in urban stormwater runoff are rarely reported and considered in stormwater management. Our objectives were to (1) characterize the organic (DON, PON) and inorganic (NO3- and NH4+) N pools in residential stormwater runoff and (2) determine the rainfall driven landscape sources of runoff PON using an isotopic mixing model with 13C and 15N during a wet season (June-September). We instrumented a 13 ha (0.13 km2) residential catchment located in Florida, United States with an ISCO autosampler and collected stormwater runoff samples (n = 52) over 11 individual stormwater runoff events. Mean concentration of total N in runoff during the wet season was 1.61 mg L-1, of which 37% was DON and 25% was PON. A strong seasonal first flush of PON, giving rise to a large PON:TN ratio, was observed as the wet season progressed from June (PON:TN = 0.39;) to September (PON:TN = 0.12), whereas DON did not display any seasonal variability (mean: 0.66 mg L-1). The isotope mixing model estimated that 76% of PON in the runoff originated from oak detritus (leaves: 50%, acorns: 26%) and the remaining 24% from lawn grass clippings. The dominance of organic N fractions in the urban stormwater runoff suggests that landscape controls on PON and DON are needed to reduce N loading in the urban stormwater runoff. The seasonal first flush of PON indicates that monitoring strategies should focus on how nutrient concentrations in runoff may respond to seasonal drivers such as leaf litterfall and that there may be optimal times for N management, such as after a prolonged dry season in which materials accumulate and pose the risk for later mobilization.

Characterization of dissolved organic nitrogen in leachate from a newly established and fertilized turfgrass.

Understanding the mechanisms of nitrogen (N) retention and loss from fertilized urban turfgrass is critical to develop practices that mitigate N transport and protect water quality in urban ecosystems. We investigated the fate of N in lysimeters sodded with St. Augustine turfgrass and amended with labeled 15N from either ammonium sulfate or urea. Fourier transform ion cyclotron resonance mass spectroscopy (FTICR-MS) was employed to identify various biomolecular classes in the leached dissolved organic N (DON) from one lysimeter for each treatment and the control. Mean DON concentrations, over 92 days, were 88, 94, and 94% of total N in the leachate from the control, urea, and ammonium sulfate treatments, respectively. Isotopic analysis showed that <3% of N in the leachate originated from newly applied N fertilizer, suggesting that the remainder of the N in the leachate was derived from the lysimeter soil or sod biomass pools. The 15N fertilizer recovery was greatest in soil (44-48%), followed by sod+thatch (18-33%), grass clippings (10-13%), and leachate (<3%). Despite isotopic evidence of little contribution of N from fertilizers in the leachate, a fraction of ammonium sulfate fertilizer was recovered as DON in the leachate, likely after uptake and conversion of inorganic fertilizer to organic plant exudates and/or microbial byproducts. FTICR-MS identified N-bearing organic molecular formulas in the leachate from urea and ammonium sulfate treatments, providing evidence of N leaching from newly established turfgrass of DON compounds in a range of biomolecular compositions such as lipid-, protein-, carbohydrate-, and lignin-like molecules.

Rate of warming affects temperature sensitivity of anaerobic peat decomposition and greenhouse gas production.

Temperature sensitivity of anaerobic carbon mineralization in wetlands remains poorly represented in most climate models and is especially unconstrained for warmer subtropical and tropical systems which account for a large proportion of global methane emissions. Several studies of experimental warming have documented thermal acclimation of soil respiration involving adjustments in microbial physiology or carbon use efficiency (CUE), with an initial decline in CUE with warming followed by a partial recovery in CUE at a later stage. The variable CUE implies that the rate of warming may impact microbial acclimation and the rate of carbon-dioxide (CO2 ) and methane (CH4 ) production. Here, we assessed the effects of warming rate on the decomposition of subtropical peats, by applying either a large single-step (10°C within a day) or a slow ramping (0.1°C/day for 100 days) temperature increase. The extent of thermal acclimation was tested by monitoring CO2 and CH4 production, CUE, and microbial biomass. Total gaseous C loss, CUE, and MBC were greater in the slow (ramp) warming treatment. However, greater values of CH4 -C:CO2 -C ratios lead to a greater global warming potential in the fast (step) warming treatment. The effect of gradual warming on decomposition was more pronounced in recalcitrant and nutrient-limited soils. Stable carbon isotopes of CH4 and CO2 further indicated the possibility of different carbon processing pathways under the contrasting warming rates. Different responses in fast vs. slow warming treatment combined with different endpoints may indicate alternate pathways with long-term consequences. Incorporations of experimental results into organic matter decomposition models suggest that parameter uncertainties in CUE and CH4 -C:CO2 -C ratios have a larger impact on long-term soil organic carbon and global warming potential than uncertainty in model structure, and shows that particular rates of warming are central to understand the response of wetland soils to global climate change.

Seasonal patterns of nitrogen cycling in subtropical short-hydroperiod wetlands: Effects of precipitation and restoration.

In the event of increased frequency of extreme wet or dry events resulting from climate change, it becomes more important to understand the temporal dynamics of soil nitrogen (N) processes in ecosystems. Here, seasonal patterns of N cycling were characterized in subtropical wetlands in Everglades National Park, Florida, USA. Two restored sites and one reference site with different nutrient status, soil depth, and vegetation communities, were selected. Soil available N, microbial biomass, potential N mineralization and denitrification rates, enzyme activities of leucine aminopeptidase (LAP) and N-acetyl-ß-d-glucosaminidase (NAG) were measured across the wet and dry seasons from 2010 to 2011. In general, most N processes were significantly correlated with soil water contents (P<0.05) which reflected the precipitation regime. The lower elevation and shallower soil (2-3cm depth) at the restored site may contribute to their higher soil water contents compared to the reference site with ~10cm soil depth, which further led to the earlier peaks of microbial biomass at the two restored sites. Potential N mineralization was positively correlated with LAP at the restored sites whereas with NAG at the reference site (P<0.05), implying that different vegetation composition may provide varying substrates for soil microbes. The build-up of nitrate in the dry spring of 2011 induced a pulse of denitrification after rewetting by a sudden rainfall, implying the presence of a hot moment of denitrification during the dry-rewetting transition period. The decrease of MBC:MBN ratio from dry to wet season indicates a possible microbial composition shift from fungi to bacteria, shedding lights on the potential contribution of fugal groups to denitrification in the dry season. Our study highlight that even under the same climate regime, the small-scale variations could affect the seasonal patterns of N cycling.

Nitrous oxide production and consumption by denitrification in a grassland: Effects of grazing and hydrology.

Denitrification is generally recognized as a major mechanism contributing to nitrous oxide (N2O) production, and is the only known biological process for N2O consumption. Understanding factors controlling N2O production and consumption during denitrification will provide insights into N2O emission variability, and potentially predict capacity of soils to serve as sinks or sources of N2O. This study investigated the effects of hydrology and grazing on N2O production and consumption in a grassland based agricultural watershed. A batch incubation study was conducted on soils (0-10 cm) collected along a hydrological gradient representing isolated wetland (Center), transient zone (Edge) and pasture upland (Upland), from both grazed and ungrazed areas. Production and consumption potentials of N2O were quantified on soils under four treatments, including (i) ambient condition, and amended with (ii) NO3(-), (iii) glucose-C, and (iv) NO3(-) +glucose-C. The impacts of grazing on N2O production and consumption were not observed. Soils in hydrologically distinct zones responded differently to N2O production and consumption. Under ambient conditions, both production and consumption rates of Edge soils were higher than those observed for Center and Upland soils. Results of amended incubations suggested NO3(-) was a key factor limiting N2O production and consumption rates in all hydrological zones. Over 5-d incubation with NO3(-) amendment, cumulative production and consumption of N2O for Center soils were 1.6 and 3.3 times higher than Edge soils, and 3.6 and 7.6 times higher than Upland soils, respectively. However, cumulative N2O net production for Edge soils was the highest, with 2 to 3 times higher than Upland and Center soils. Our results suggest that the transient areas between wetland and upland are likely to be "hot spots" of N2O emissions in this ecosystem. Wetlands within agricultural landscapes can potentially function to reduce both NO3(-) leaching and N2O emissions.

Wading bird guano enrichment of soil nutrients in tree islands of the Florida Everglades.

Differential distribution of nutrients within an ecosystem can offer insight of ecological and physical processes that are otherwise unclear. This study was conducted to determine if enrichment of phosphorus (P) in tree island soils of the Florida Everglades can be explained by bird guano deposition. Concentrations of total carbon, nitrogen (N), and P, and N stable isotope ratio (δ(15)N) were determined on soil samples from 46 tree islands. Total elemental concentrations and δ(15)N were determined on wading bird guano. Sequential chemical extraction of P pools was also performed on guano. Guano contained between 53.1 and 123.7 g-N kg(-1) and 20.7 and 56.7 g-P kg(-1). Most of the P present in guano was extractable by HCl, which ranged from 82 to 97% of the total P. Total P of tree islands classified as having low or high P soils averaged 0.71 and 40.6 g kg(-1), respectively. Tree island soil with high total P concentration was found to have a similar δ(15)N signature and total P concentration as bird guano. Phosphorus concentrations and δ(15)N were positively correlated in tree island soils (r = 0.83, p< 0.0001). Potential input of guano with elevated concentrations of N and P, and (15)N enriched N, relative to other sources suggests that guano deposition in tree island soils is a mechanism contributing to this pattern.

Nutrient release from combustion residues of two contrasting herbaceous vegetation types.

Fire is a critical regulator of biogeochemical cycles in approximately 40% of the earth's land surface. However, little is known about nutrient release from combustion residues (ash and char) from herbaceous or grassland fires of varying intensity. Much of our knowledge in this area is derived from muffle furnace temperature gradient experiments. Therefore, we used two approaches (muffle and flame burning) to combust herbaceous biomass from contrasting nutrient level sites to estimate the forms and availability of nutrients after fire. Clear differences were measured in total and extractable nutrient concentrations in combustion residues of different plant types, with most carbon (C) and nitrogen (N) being volatilized (>99%), while P remained in high concentrations in the residues. Different combustion methods yielded contrasting results, where temperatures greatly affected nutrient quantity and form in muffle furnace residues, while relatively similar residues resulted from flame combustion at varying intensities. It was also found that only 5% of N and 50% of P remaining in flame combustion residues were extractable. Flame residues appeared to be composed of mixtures of materials (ash and char) created at low (<350 °C) muffle temperatures (extractable P forms), and high (>450 °C) muffle temperatures (pH, extractable potassium (K), and extractable NH(4)-N). We attribute dissimilar results of the combustion methods to heterogeneity of combustion (zones of low oxygen availability) and short duration (<300 s) of combustion characterizing natural fires in herbaceous, grassland systems. These results can be adapted to ecosystem level models to better predict nutrient changes that may occur after a fire event.

Phosphorus transformations during decomposition of wetland macrophytes.

The microbially mediated transformation of detrital P entering wetlands has important implications for the cycling and long-term sequestration of P in wetland soils. We investigated changes in P forms in sawgrass (Cladium jamaicense Crantz) and cattail (Typha domingensis Pers.) leaf litter during 15 months of decomposition at two sites of markedly different nutrient status within a hard-water subtropical wetland (Water Conservation Area 2A, Florida). Leaf litter decomposition at the nutrient enriched site resulted in net sequestration of P from the environment in forms characteristic of microbial cells (i.e., phosphodiesters and pyrophosphate). In contrast, low P concentrations at the unenriched site resulted in little or no net sequestration of P, with changes in P forms limited to the loss of compounds present in the initial leaf litter. We conclude that under nutrient-rich conditions, P sequestration occurs through the accumulation of microbially derived compounds and the presumed concentration of endogenous macrophyte P. Under nutrient-poor conditions, standing P pools within wetland soils appear to be independent of the heterotrophic decomposition of macrophyte leaf litter. These conclusions have important implications for our ability to predict the nature, stability, and rates of P sequestration in wetlands in response to changes in nutrient loading.