How does simulated climate change affect the susceptibility of SOM to priming by LMWOS in the Subarctic?

<p>Soil organic matter (SOM) stabilization plays an important role in the long-term storage of carbon (C). However, many ecosystems are undergoing climate change, which will change the soil C balance via altered plant communities and productivity that change C inputs, and altered C losses via changes in SOM decomposition. The ongoing change of aboveground plant communities in the Subarctic (“greening”) will increase rhizosphere inputs containing low molecular weight organic substances (LMWOS), which will likely affect C-starved microbial decomposers and their subsequent contribution to SOM mineralization (priming effect).In the present study, we simulated the effects of climate change with N fertilization (simulating warming enhanced nutrient cycling) and litter additions (simulating arctic greening) in Abisko, Sweden. The 6 sampled field-treatments included a full factorial combination of 3-years of chronic N addition and litter additions, as well as, a single year of extreme climate change (3x N fertilizer or litter additions in one growth season). We found that N treatments changed plant community composition and productivityand that the associated shift in belowground LMWOS induced shifts in the soil microbial community. In the chronic N fertilization treatments, plant productivity, and therefore belowground LMWOS input, increased. This coincided with a tendency for more bacterial dominated decomposition (lower fungi/bacterial growth ratio). However, N treatments had no effect on soil C mineralization, but increased gross N mineralization.</p><p>These responses in belowground communities and processes driven by rhizosphere input prompted the next question: how does simulated climate change affect the susceptibility of SOM to priming by LMWOS? To assess this question and determine the microbial mechanisms underpinning priming of SOM mineralization, we added a factorial set of additions including <sup>13</sup>C-glucose with and without mineral N, and <sup>13</sup>C-alanine semi-continuously (every 48 hours) to simulate the effect of rhizosphere LMWOS on SOM mineralization and microbial activity. We incubated these samples for 2 weeks and assessed the priming of soil C and gross N mineralization, bacterial and fungal growth rates, PLFAs, enzyme activities, and microbial C use efficiency (CUE). We found that alanine addition primed soil C mineralization by 34%, which was higher than soil C priming induced by glucose and glucose with N. Furthermore, glucose primed fungal growth, whereas the alanine primed bacterial growth, but microbial PLFAs did not respond to either treatment. The C enzyme acquisition activity was higher than N enzyme acquisition activity in all the treatments, while P enzyme acquisition activity was higher than C for all the treatments. Surprisingly, this suggested a chronic microbial limitation by P, which was unaffected by field and lab treatments. LMWOS additions generally reduced microbial CUE. Responses of microbial mineralization of N from SOM to LMWOS suggested a directed microbial effort towards targeting resources that limited bacterial or fungal growth, suggesting that microbial SOM-use shifted to N-rich components (selective microbial “N-mining”), in contrast with enzyme results. Surprisingly, alanine primed the highest N mineralization compared other additions indicating that there was strong N-mining even if N was sufficient.</p>

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.

The biodiversity - N cycle relationship: a 15N tracer experiment with soil from plant mixtures of varying diversity to model N pool sizes and transformation rates

We conducted a 15N tracer experiment in laboratory microcosms with field-fresh soil samples from a biodiversity experiment to evaluate the relationship between grassland biodiversity and N cycling. To embrace the complexity of the N cycle, we determined N exchange between five soil N pools (labile and recalcitrant organic N, dissolved NH4+ and NO3− in soil solution, and exchangeable NH4+) and eight N transformations (gross N mineralization from labile and recalcitrant organic N, NH4+ immobilization into labile and recalcitrant organic N, autotrophic nitrification, heterotrophic nitrification, NO3− immobilization, adsorption of NH4+) expected in aerobic soils with the help of the N-cycle model Ntrace. We used grassland soil of the Jena Experiment, which includes plant mixtures with 1 to 60 species and 1 to 4 functional groups (legumes, grasses, tall herbs, small herbs). The 19 soil samples of one block of the Jena Experiment were labeled with either 15NH4+ or 15NO3- or both. In the presence of legumes, gross N mineralization and autotrophic nitrification increased significantly because of higher soil N concentrations in legume-containing plots and high microbial activity. Similarly, the presence of grasses significantly increased the soil NH4+ pool, gross N mineralization, and NH4+ immobilization, likely because of enhanced microbial biomass and activity by providing large amounts of rhizodeposits through their dense root systems. In our experiment, previously reported plant species richness effects on the N cycle, observed in a larger-scale field experiment within the Jena Experiment, were not seen. However, specific plant functional groups had a significant positive impact on the N cycling in the incubated soil samples.

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. 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. 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.

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

Monomeric organic nitrogen (N) such as free amino acids (fAA) is an important resource for both plants and soil microorganisms and is, furthermore, a source of ammonium (NH4+) via microbial fAA mineralization. We compared gross fAA dynamics with gross N mineralization in a Dutch heathland soil using 15N labelling. 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 warming and drought on these rates. The turnover of fAA in the soil was ca. 3 h, which is almost two orders of magnitude faster than that of NH4+ (i.e. ca. 4 days). This suggests that fAAs 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 eight-times smaller than the summed 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 by 34% to the gross N mineralization rate and is, thus, an important component of N mineralization. In the drought treatment, gross fAA production was reduced by 65% and 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. Warming did not significantly affect N transformations, even though that gross fAA production was more than halved. Overall our results suggest that heathland soil exposed to droughts has a shift in the composition of the SON being mineralized. Furthermore, compared to agricultural soils, fAA mineralization was relatively less important in the investigated heathland. This indicates a more complex mineralization dynamics in semi-natural ecosystems.

N₂O, NO and CH₄ exchange, and microbial N turnover over a Mediterranean pine forest soil

Trace gas exchange of N2O, NO/NO2 and CH4 between soil and the atmosphere was measured in a typical Mediterranean pine (Pinus pinaster) forest during two intensive field campaigns in spring and autumn 2003. Furthermore, gross and net turnover rates of N mineralization and nitrification as well as soil profiles of N2O and CH4 concentrations were determined. For both seasons a weak but significant N2O uptake from the atmosphere into the soil was observed. During the unusually dry and hot spring mean N2O uptake was −4.32 µg N m-2 h-1, whereas during the wet and mild autumn mean N2O uptake was −7.85 µg N m-2 h-1. The observed N2O uptake into the soil was linked to the very low availability of inorganic nitrogen at the study site. Organic layer gross N mineralization decreased from 5.06 mg N kg-1 SDW d-1 in springtime to 2.68 mg N kg-1 SDW d-1 in autumn. Mean NO emission rates were significantly higher in springtime (9.94 µg N m-2 h-1) than in autumn (1.43 µg N m-2 h-1). A significant positive correlation between NO emission rates and gross N mineralization as well as nitrification rates was found. The negative correlation between NO emissions and soil moisture was explained with a stimulation of aerobic NO uptake under N limiting conditions. Since NO2 deposition was continuously higher than NO emission rates the examined forest soil functioned as a net NOx sink. Observed mean net CH4 uptake rates were in spring significantly higher (−73.34 µg C m-2 h-1) than in autumn (−59.67 µg C m-2 h-1). Changes in CH4 uptake rates were strongly negatively correlated with changes in soil moisture. The N2O and CH4 concentrations in different soil depths revealed the organic layer and the upper 0.1 m of mineral soil as the most important soil horizons for N2O and CH4 consumption.