Assessment of the importance of dissimilatory nitrate reduction to ammonium for the terrestrial nitrogen cycle

The nitrogen (N) cycle contains two different processes of dissimilatory nitrate (NO<sub>3</sub><sup>−</sup>) reduction, denitrification and dissimilatory NO<sub>3</sub><sup>−</sup> reduction to ammonium (DNRA). While there is general agreement that the denitrification process takes place in many soils, the occurrence and importance of DNRA is generally not considered. Two approaches have been used to investigate DNRA in soil, (1) microbiological techniques to identify soil microorganisms capable of DNRA and (2) <sup>15</sup>N tracing to elucidate the occurrence of DNRA and to quantify gross DNRA rates. There is evidence that many soil bacteria and fungi have the ability to perform DNRA. Redox status and C/NO<sub>3</sub><sup>−</sup> ratio have been identified as the most important factors regulating DNRA in soil. <sup>15</sup>N tracing studies have shown that gross DNRA rates can be a significant or even a dominant NO<sub>3</sub><sup>−</sup> consumption process in some ecosystems. Moreover, a link between heterotrophic nitrification and DNRA provides an alternative pathway of ammonium (NH<sub>4</sub><sup>+</sup>) production to mineralisation. Numerical <sup>15</sup>N tracing models are particularly useful when investigating DNRA in the context of other N cycling processes. The results of correlation and regression analyses show that highest gross DNRA rates can be expected in soils with high organic matter content in humid regions, while its relative importance is higher in temperate climates. With this review we summarise the importance and current knowledge of this often overlooked NO<sub>3</sub><sup>−</sup> consumption process within the terrestrial N cycle. We strongly encourage considering DNRA as a relevant process in future soil N cycling investigations

Mid-term Effects of Wildfire and Salvage Logging on Gross and Net Soil Nitrogen Transformation Rates in a Swedish Boreal Forest

Wildfires are natural and important disturbances of boreal forest ecosystems, and they are expected to increase in parts of the boreal zone through climate warming. There is a broad understanding of the immediate effects of fire on soil nitrogen (N) transformation rates, but less is known about these effects several years after fire. In July 2014, a large wildfire in the boreal forest zone of Central Sweden took place. Four years after the wildfire, we measured processes linked to the soil N cycle using the 15N pool dilution method (for gross N mineralization, consumption and nitrification) and the buried bags method (for net N mineralization), in soils from stands of different fire severity that had or had not been subjected to salvage logging. Gross N mineralization and consumption rates per unit carbon (C) increased by 81 % and 85 % respectively, in response to high fire severity, and nitrification rates per unit C basis decreased by 69 % in response to high fire severity, while net N mineralization was unresponsive. There was no difference in the effect of salvage logging across stands of differing fire severity on N transformation rates, although concentrations of resin adsorbed nitrate (NO3–) were overall 50 % lower in logged compared to unlogged stands. We also found that irrespective of burn severity, N immobilization rates exceeded N nitrification rates, and immobilization was therefore the dominant pathway of gross N consumption. Gross N consumption rates were higher in burned than unburned stands, despite there being a higher active microbial biomass in unburned soil, which suggests an even higher immobilization of N over time as the microbial biomass recovers following fire. Our study shows that soil N transformation rates were more affected by changes in fire severity than by salvage logging, and that four years after the fire many aspects of the N cycle did not differ between burned and unburned stands, suggesting substantial resilience of the N cycle to fire and salvage logging. However, we note that long term impact and many additional ecosystem properties or processes should be evaluated before concluding that salvage logging has no ecosystem impact. Furthermore, shortened fire regimes following climate warming accompanied with shorter intervals between salvage logging practices, could still impact the capability for the N cycle to recover after an intense fire. While wildfire in the boreal region results in a shift from nutrient conserving to nutrient demanding plant species, our results suggest this shift is dependent on a relatively short-lived pulse of higher N cycling processes that would have likely dissipated within a few years after the fire

Nitrogen-limited mangrove ecosystems conserve N through dissimilatory nitrate reduction to ammonium

Earlier observations in mangrove sediments of Goa, India have shown denitrification to be a major pathway for N loss1. However, percentage of total nitrate transformed through complete denitrification accounted for <0–72% of the pore water nitrate reduced. Here, we show that up to 99% of nitrate removal in mangrove sediments is routed through dissimilatory nitrate reduction to ammonium (DNRA). The DNRA process was 2x higher at the relatively pristine site Tuvem compared to the anthropogenically-influenced Divar mangrove ecosystem. In systems receiving low extraneous nutrient inputs, this mechanism effectively conserves and re-circulates N minimizing nutrient loss that would otherwise occur through denitrification. In a global context, the occurrence of DNRA in mangroves has important implications for maintaining N levels and sustaining ecosystem productivity. For the first time, this study also highlights the significance of DNRA in buffering the climate by modulating the production of the greenhouse gas nitrous oxide

Nitrogen mineralization, not N₂ fixation, alleviates progressive nitrogen limitation – Comment on “Processes regulating progressive nitrogen limitation under elevated carbon dioxide: a meta-analysis” by Liang et al. (2016)

No abstract available

Effects of a high-severity wildfire and post-fire straw mulching on gross nitrogen dynamics in Mediterranean shrubland soil.

Little is known about the combined impacts of fire and straw mulching, a widely used post‐fire emergency measure, on the soil nitrogen (N) cycle. Unburnt (US) and severely‐burnt soils without (BS) and with straw mulching (BSM) were preincubated (3 and 6 months) in the laboratory before fire and mulching effects on gross N transformations were investigated with a paired 15N‐labelling experiment. The ammonium‐to‐nitrate (NH4 +/NO3 ‐) ratio of burnt soils decreased with preincubation time from 21 to 1.3, consistent with a shift of the N cycle towards net nitrification. After 3 months of preincubation, gross mineralisation (MSON) and gross NH4 + immobilisation (INH4) in BS more than doubled compared to US, in the latter being MSON 4.82 mg N kg‐1 day‐1 and INH4 3.01 mg N kg‐1 day‐1. Mulching partly mitigated this stimulation in the mineralisation‐immobilisation turnover (MIT). After 6 months, MIT differences among treatments disappeared and gross rates approached those in US after 3 months. After three months, autotrophic nitrification (NH4 + oxidation) in all treatments was 0.41‐0.52 N kg‐1 day‐1, while after 6 months it remained similar in US but increased 8‐fold in burnt soils. Heterotrophic nitrification of organic N only occurred in burnt soils, and its importance was similar to autotrophic nitrification after 3 months, but around 4‐fold lower after 6 months. To conclude, burning opened up the N cycle and NO3 ‐ accumulated, increasing the potential for ecosystem N losses. In the short term, straw mulching slightly mitigates the effects of fire on the N cycle.Peer reviewe

Amino acid and N mineralization dynamics in heathland soil after long-term warming and repetitive drought

Monomeric organic nitrogen (N) compounds such as free amino acids (FAAs) are an important resource for both plants and soil microorganisms and a source of ammonium (NH₄⁺) via microbial FAA mineralization. We compared gross FAA dynamics with gross N mineralization in a Dutch heathland soil using a ¹⁵N tracing technique. A special focus was made on the effects of climate change factors warming and drought, followed by rewetting. Our aims were to (1) compare FAA mineralization (NH₄⁺ production from FAAs) with gross N mineralization, (2) assess gross FAA production rate (depolymerization) and turnover time relative to gross N mineralization rate, and (3) assess the effects of a 14 years of warming and drought treatment on these rates. <br><br> The turnover of FAA in the soil was ca. 3 h, which is almost 2 orders of magnitude faster than that of NH₄⁺ (i.e. ca. 4 days). This suggests that FAA is an extensively used resource by soil microorganisms. In control soil (i.e. no climatic treatment), the gross N mineralization rate (10 ± 2.9 μg N g⁻¹ day⁻¹) was 8 times smaller than the total gross FAA production rate of five AAs (alanine, valine, leucine, isoleucine, proline: 127.4 to 25.0 μg N g⁻¹ day⁻¹). Gross FAA mineralization (3.4 ± 0.2 μg N g⁻¹ day⁻¹) contributed 34% to the gross N mineralization rate and is therefore an important component of N mineralization. In the drought treatment, a 6–29% reduction in annual precipitation caused a decrease of gross FAA production by 65% and of gross FAA mineralization by 41% compared to control. On the other hand, gross N mineralization was unaffected by drought, indicating an increased mineralization of other soil organic nitrogen (SON) components. A 0.5–1.5 °C warming did not significantly affect N transformations, even though gross FAA production declined. <br><br> Overall our results suggest that in heathland soil exposed to droughts a different type of SON pool is mineralized. Furthermore, compared to agricultural soils, FAA mineralization was relatively less important in the investigated heathland. This indicates more complex mineralization dynamics in semi-natural ecosystems

Hemiparasitic litter additions alter gross nitrogen turnover in temperate semi-natural grassland soils

Hemiparasitic plants accumulate nutrients in their leaves and therefore produce high-quality litter with faster decomposition and nutrient release rates compared to non-parasitic litter. Higher levels of plantavailable nitrogen (N) in the presence of hemiparasitic plants have been attributed to this ‘litter effect’, but effects on N dynamics in the soil remain unstudied. We tested the hypothesis that litter of Rhinanthus angustifolius and Pedicularis sylvatica increases N transformation rates in the soil more than non-parasitic litter of a species mix from the same communities. We expected the litter effect to be higher in the oligotrophic Pedicularis soil compared to the mesotrophic Rhinanthus soil. Gross N transformation rates were quantified using a 15N tracing modeling approach. Differentially 15N labeled NH4Cl þ KNO3 was added to two soils with three treatments (control, soil amended with non-parasitic litter, soil amended with Rhinanthus or Pedicularis litter) in a laboratory ncubation experiment. The concentration and 15N enrichment of NH4 þ and NO3 in the soil were measured at six time points within one or two weeks (depending on the soil) after label addition. Hemiparasitic litter addition increased the overall cycling of N more compared to the addition of non-parasitic litter. Relative to the non-parasitic litter, addition of Rhinanthus litter increased the net flux from organic N to NH4 þ by 61% and net (autotrophic) nitrification by 80%. Addition of Pedicularis litter increased the net flux from organic N to NH4 þ by 28% relative to addition of non-parasitic litter, while there was no effect on nitrification. Surprisingly, gross mineralization of organic N to NH4 þ decreased with litter addition for the Rhinanthus soil (control soil > nonparasitic litter > Rhinanthus litter), while it increased with litter addition in the Pedicularis soil (control soil < non-parasitic litter < Pedicularis litter). Our results support the hypothesis that litter from hemiparasitic plants increases soil N availability more than non-parasitic litter, but contradicts the expectation that the hemiparasitic litter effect would be more pronounced in an oligotrophic as compared to a mesotrophic system. This litter-induced augmentation in soil fertility provides e in addition to the parasitic suppression of hosts e a second potentially important pathway by which hemiparasitic plants impact on plant community composition. However, future research on P and K return via hemiparasitic litter should be consideredpublisher: Elsevier articletitle: Hemiparasitic litter additions alter gross nitrogen turnover in temperate semi-natural grassland soils journaltitle: Soil Biology and Biochemistry articlelink: http://dx.doi.org/10.1016/j.soilbio.2013.10.025 content_type: article copyright: Copyright © 2013 Elsevier Ltd. All rights reserved.status: publishe