Simultaneous quantification of depolymerization and mineralization rates by a novel 15N tracing model

The depolymerization of soil organic matter, such as proteins and (oligo-)peptides, into monomers (e.g. amino acids) is currently considered to be the rate-limiting step for nitrogen (N) availability in terrestrial ecosystems. The mineralization of free amino acids (FAAs), liberated by the depolymerization of peptides, is an important fraction of the total mineralization of organic N. Hence, the accurate assessment of peptide depolymerization and FAA mineralization rates is important in order to gain a better process-based understanding of the soil N cycle. In this paper, we present an extended numerical 15N tracing model Ntrace, which incorporates the FAA pool and related N processes in order to provide a more robust and simultaneous quantification of depolymerization and gross mineralization rates of FAAs and soil organic N. We discuss analytical and numerical approaches for two forest soils, suggest improvements of the experimental work for future studies, and conclude that (i) when about half of all depolymerized peptide N is directly mineralized, FAA mineralization can be as important a rate-limiting step for total gross N mineralization as peptide depolymerization rate; (ii) gross FAA mineralization and FAA immobilization rates can be used to develop FAA use efficiency (NUEFAA), which can reveal microbial N or carbon (C) limitation

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

Advances in N-15-tracing experiments: new labelling and data analysis approaches

To obtain an in-depth understanding of soil nitrogen dynamics, it is necessary to quantify a variety of simultaneously occurring gross nitrogen transformation processes. In order to do so, most studies apply N-15 in a disturbed soil-microbial-root system and quantify gross rates based on the principles of N-15 isotope dilution. However, this approach has several shortcomings. First, studying disturbed soil provides only limited information on in situ soil nitrogen dynamics. Secondly, the analytical data analysis allows the quantification of total production and consumption rates of the labelled pool, but does not provide information on process-specific transformation rates. Combining in situ N-15 isotope labelling over 1-2 weeks with numerical data analysis allows determining process-specific gross nitrogen transformations in undisturbed soils under field conditions in the presence of live roots and their associated microbial communities. This has the potential to increase our understanding of nitrogen dynamics in the soil environment

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

Process rates of nitrogen cycle in uppermost topsoil after harvesting in no-tilled and ploughed agricultural clay soil

No-till is considered an agricultural practice beneficial for the environment as soil erosion is decreased compared to ploughed soils. For on overall evaluation of the benefits and disadvantages of this crop production method, understanding the soil nutrient cycle is also of importance. The study was designed to obtain information about gross soil nitrogen (N) process rates in boreal no-tilled and mouldboard ploughed spring barley (Hordeum vulgare L.) fields after autumn harvesting. In situ soil gross N transformation process rates were quantified for the 5 cm topsoil in 9 days' incubation experiment using N-15 pool dilution and tracing techniques and a numerical N-15 tracing model. Gross N mineralization into ammonium (NH4+) and NH4+ immobilization were the most important N transformation processes in the soils. The gross mineralization rate was 14% and NH4+ immobilization rate 64% higher in no-till than in ploughing. Regardless of the faster mineralization, the gross rate of NH4+ oxidation into nitrate (NO3-) in no-till was one order of magnitude lower compared the ploughing. The results indicate that the no-tilled soils have the potential to decrease the risk for NO3- leaching due to slower NH4+ oxidation.Peer reviewe

Nitrogen dynamics after two years of elevated CO2 in phosphorus limited Eucalyptus woodland

It is uncertain how the predicted further rise of atmospheric carbon dioxide (CO2) concentration will affect plant nutrient availability in the future through indirect effects on the gross rates of nitrogen (N) mineralization (production of ammonium) and depolymerization (production of free amino acids) in soil. The response of soil nutrient availability to increasing atmospheric CO2 is particularly important for nutrient poor ecosystems. Within a FACE (Free-Air Carbon dioxide Enrichment) experiment in a native, nutrient poor Eucalyptus woodland (EucFACE) with low soil organic matter (≤ 3%), our results suggested there was no shortage of N. Despite this, microbial N use efficiency was high (c. 90%). The free amino acid (FAA) pool had a fast turnover time (4 h) compared to that of ammonium (NH4+) which was 11 h. Both NH4-N and FAA-N were important N pools; however, protein depolymerization rate was three times faster than gross N mineralization rates, indicating that organic N is directly important in the internal ecosystem N cycle. Hence, the depolymerization was the major provider of plant available N, while the gross N mineralization rate was the constraining factor for inorganic N. After two years of elevated CO2, no major effects on the pools and rates of the soil N cycle were found in spring (November) or at the end of summer (March). The limited response of N pools or N transformation rates to elevated CO2 suggest that N availability was not the limiting factor behind the lack of plant growth response to elevated CO2, previously observed at the site

Sources of nitrous oxide and fate of mineral nitrogen in sub-Arctic permafrost peat soils

Nitrous oxide (N2O) emissions from permafrost-affected terrestrial ecosystems have received little attention, largely because they have been thought to be negligible. Recent studies, however, have shown that there are habitats in the subarctic tundra emitting N2O at high rates, such as bare peat (BP) surfaces on permafrost peatlands. Nevertheless, the processes behind N2O production in these high-emission habitats are poorly understood. In this study, we established an in situ 15N-labeling experiment with two main objectives: (1) to partition the microbial sources of N2O emitted from BP surfaces on permafrost peatlands and (2) to study the fate of ammonium and nitrate in these soils and in adjacent vegetated peat (VP) surfaces showing low N2O emissions. Our results confirm the hypothesis that denitrification is mostly responsible for the high N2O emissions from BP. During the study period, denitrification contributed ∼ 79 % of the total N2O emissions from BP, whereas the contribution from ammonia oxidation was less (about 19 %). Both gross N mineralization and gross nitrification rates were higher in BP than in VP, with high C/N ratios and a low water content likely limiting N transformation processes and, consequently, N2O production in the latter soil type. Our results show that multiple factors contribute to high N2O production in BP surfaces on permafrost peatlands, with the most important factors being the absence of plants, an intermediate to high water content and a low C/N ratio, which all affect the mineral-N availability for soil microbes, including those producing N2O. The process understanding produced here is important for the development of process models that can be used to evaluate future permafrost–N feedbacks to the climate system.peerReviewe