Immediate and long-term nitrogen oxide emissions from tropical forest soils exposed to elevated nitrogen input

Tropical nitrogen (N) deposition is projected to increase substantially within the coming decades. Increases in soil emissions of the climate‐relevant trace gases NO and N₂O are expected, but few studies address this possibility. We used N addition experiments to achieve N‐enriched conditions in contrasting montane and lowland forests and assessed changes in the timing and magnitude of soil N‐oxide emissions. We evaluated transitory effects, which occurred immediately after N addition, and long‐term effects measured at least 6 weeks after N addition. In the montane forest where stem growth was N limited, the first‐time N additions caused rapid increases in soil N‐oxide emissions. During the first 2 years of N addition, annual N‐oxide emissions were five times (transitory effect) and two times (long‐term effect) larger than controls. This contradicts the current assumption that N‐limited tropical montane forests will respond to N additions with only small and delayed increases in soil N‐oxide emissions. We attribute this fast and large response of soil N‐oxide emissions to the presence of an organic layer (a characteristic feature of this forest type) in which nitrification increased substantially following N addition. In the lowland forest where stem growth was neither N nor phosphorus (P) limited, the first‐time N additions caused only gradual and minimal increases in soil N‐oxide emissions. These first N additions were completed at the beginning of the wet season, and low soil water content may have limited nitrification. In contrast, the 9‐ and 10‐year N‐addition plots displayed instantaneous and large soil N‐oxide emissions. Annual N‐oxide emissions under chronic N addition were seven times (transitory effect) and four times (long‐term effect) larger than controls. Seasonal changes in soil water content also caused seasonal changes in soil N‐oxide emissions from the 9‐ and 10‐year N‐addition plots. This suggests that climate change scenarios, where rainfall quantity and seasonality change, will alter the relative importance of soil NO and N₂O emissions from tropical forests exposed to elevated N deposition.     

Growth of ammonia-oxidizing archaea in soil microcosms is inhibited by acetylene

Autotrophic ammonia-oxidizing bacteria were considered to be responsible for the majority of ammonia oxidation in soil until the recent discovery of the autotrophic ammonia-oxidizing archaea. To assess the relative contributions of bacterial and archaeal ammonia oxidizers to soil ammonia oxidation, their growth was analysed during active nitrification in soil microcosms incubated for 30 days at 30 °C, and the effect of an inhibitor of ammonia oxidation (acetylene) on their growth and soil nitrification kinetics was determined. Denaturing gradient gel electrophoresis (DGGE) analysis of bacterial ammonia oxidizer 16S rRNA genes did not detect any change in their community composition during incubation, and quantitative PCR (qPCR) analysis of bacterial amoA genes indicated a small decrease in abundance in control and acetylene-containing microcosms. DGGE fingerprints of archaeal amoA and 16S rRNA genes demonstrated changes in the relative abundance of specific crenarchaeal phylotypes during active nitrification. Growth was also indicated by increases in crenarchaeal amoA gene copy number, determined by qPCR. In microcosms containing acetylene, nitrification and growth of the crenarchaeal phylotypes were suppressed, suggesting that these crenarchaea are ammonia oxidizers. Growth of only archaeal but not bacterial ammonia oxidizers occurred in microcosms with active nitrification, indicating that ammonia oxidation was mostly due to archaea in the conditions of the present study.     

The Effects of Periphyton, Fish and Fertilizer Dose on Biological Processes Affecting Water Quality in Earthen Fish Ponds

The potential of periphyton-based aquaculture in South Asia is under investigation in an extensive research program. This paper is a further analysis of data from four experiments carried out in that framework, to explore periphyton, fish and fertilizer dose effects on water quality. Factor analysis and ANOVA models applied to a data matrix of water quality parameters in ponds with and without artificial substrates (bamboo poles and kanchi sticks), with and without fish (filter feeders catla and rohu, with and without bottom feeder kalbaush), and with a standard or 50% increased fertilizer dose, allowed us to identify the underlying ecological processes governing this novel periphyton-based pond system, and construct conceptual graphic models of the periphyton–environment relationships observed. We clearly established that the phosphorus flow is mainly linked to phytoplankton activity in the water column and decomposition on the pond bottom, while nitrogen flow is mainly linked to autotrophic (photosynthesis) and heterotrophic (decomposition and nitrification) processes that take place in the periphyton in addition to the water column and pond bottom. Consequently, disruption of the pond bottom by bottom feeding fish primarily promoted phosphate cycling and phytoplankton, while periphyton development on the supplied substrates and fertilization mainly improved oxygen balance and nitrogen related processes developing in the water column. The use of bamboo poles led to better results than kanchi sticks, related to the greater autotrophic periphyton development on bamboo and to the larger surface of bamboo poles that facilitate fish grazing and periphyton dislodgment that in turn have a renewal effect on periphyton. Stocking bottom feeding fish produces a fertilizing effect through the food web that benefits the filter-feeding fish and that makes it unnecessary to increase the dose of inorganic and organic fertilizers applied to the ponds. Thus, the output of this analysis will help the fish farmers in resource constrained countries to improve their production in periphyton-based ponds just by choosing bamboo substrates, stocking a bottom feeder fish together with the filter feeders, and saving money on fertilizers.     

Nitrifying activity monitoring and kinetic parameters determination in a biofilm airlift reactor by respirometry

The applicability of batch respirometry, as a simple technique for monitoring off-line nitrifying activity and kinetic parameters, was evaluated using two sets of ammonia and nitrite concentrations. The O₂ uptake rate (OUR) profiles obtained from the assays were adjusted to a substrate inhibition model. The maximum specific ammonia-oxidizing biomass activity (r_Smax) was 0.079 g N-NH₄ ⁺ g VSS^–1 d^–1 with a half saturation coefficient (K_S) of 11 mg N-NH₄ ⁺ l^–1 and an inhibition coefficient (K_i) of 3300 mg N-NH₄ ⁺ l^–1. Besides, the maximum specific value of nitrite-oxidizing activity was 0.082 g N-NO₂ ^– g VSS^–1 d^–1 with a K_S of 4.1 mg N-NO₂ ^– l^–1 and K_i of 1400 mg N-NO₂ ^– l^–1.     

Isotopic signatures of N2O produced by ammonia-oxidizing archaea from soils

N₂O gas is involved in global warming and ozone depletion. The major sources of N₂O are soil microbial processes. Anthropogenic inputs into the nitrogen cycle have exacerbated these microbial processes, including nitrification. Ammonia-oxidizing archaea (AOA) are major members of the pool of soil ammonia-oxidizing microorganisms. This study investigated the isotopic signatures of N₂O produced by soil AOA and associated N₂O production processes. All five AOA strains (I.1a, I.1a-associated and I.1b clades of Thaumarchaeota) from soil produced N₂O and their yields were comparable to those of ammonia-oxidizing bacteria (AOB). The levels of site preference (SP), δ¹⁵N^bulk and δ¹⁸O -N₂O of soil AOA strains were 13–30%, −13 to −35% and 22–36%, respectively, and strains MY1–3 and other soil AOA strains had distinct isotopic signatures. A ¹⁵N-NH₄⁺-labeling experiment indicated that N₂O originated from two different production pathways (that is, ammonia oxidation and nitrifier denitrification), which suggests that the isotopic signatures of N₂O from AOA may be attributable to the relative contributions of these two processes. The highest N₂O production yield and lowest site preference of acidophilic strain CS may be related to enhanced nitrifier denitrification for detoxifying nitrite. Previously, it was not possible to detect N₂O from soil AOA because of similarities between its isotopic signatures and those from AOB. Given the predominance of AOA over AOB in most soils, a significant proportion of the total N₂O emissions from soil nitrification may be attributable to AOA.     

Winter climate change affects growing‐season soil microbial biomass and activity in northern hardwood forests

Understanding the responses of terrestrial ecosystems to global change remains a major challenge of ecological research. We exploited a natural elevation gradient in a northern hardwood forest to determine how reductions in snow accumulation, expected with climate change, directly affect dynamics of soil winter frost, and indirectly soil microbial biomass and activity during the growing season. Soils from lower elevation plots, which accumulated less snow and experienced more soil temperature variability during the winter (and likely more freeze/thaw events), had less extractable inorganic nitrogen (N), lower rates of microbial N production via potential net N mineralization and nitrification, and higher potential microbial respiration during the growing season. Potential nitrate production rates during the growing season were particularly sensitive to changes in winter snow pack accumulation and winter soil temperature variability, especially in spring. Effects of elevation and winter conditions on N transformation rates differed from those on potential microbial respiration, suggesting that N‐related processes might respond differently to winter climate change in northern hardwood forests than C‐related processes.     

Improved nitrogen removal by application of new nitrogen-cycle bacteria

In order to meet increasingly stringentEuropean discharge standards, new applicationsand control strategies for the sustainableremoval of ammonia from wastewater have to beimplemented. In this paper we discuss anitrogen removal system based on the processesof partial nitrification and anoxic ammoniaoxidation (anammox). The anammox process offersgreat opportunities to remove ammonia in fullyautotrophic systems with biomass retention. Noorganic carbon is needed in such nitrogenremoval system, since ammonia is used aselectron donor for nitrite reduction. Thenitrite can be produced from ammonia inoxygen-limited biofilm systems or in continuousprocesses without biomass retention. Forsuccessful implementation of the combinedprocesses, accurate biosensors for measuringammonia and nitrite concentrations, insight inthe complex microbial communities involved, andnew control strategies have to be developed andevaluated.