The long-term fate of deposited nitrogen in temperate forest soils

Increased anthropogenic nitrogen (N) inputs can alter the N cycle and affect forest ecosystem functions. The impact of increased N deposition depends among others on the ultimate fate of N in plant and soil N pools. Short-term studies (3-18 months) have shown that the organic soil layer was the dominant sink for N. However, longer time scales are needed to investigate the long-term fate of N. Therefore, the soils of four experimental forest sites across Europe were re-sampled similar to 2 decades after labelling with(15)N. The sites covered a wide range of ambient N deposition varying from 13 to 58 kg N ha(-1)year(-1). To investigate the effects of different N loads on(15)N recovery, ambient N levels were experimentally increased or decreased. We hypothesized that: (1) the mineral soil would become the dominant(15)N sink after 2 decades, (2) long-term increased N deposition would lead to lower(15)N recovery levels in the soil and (3) variables related to C dynamics would have the largest impact on(15)N recovery in the soil. The results show that large amounts of the added(15)N remain in the soil after 2 decades and at 2 out of 4 sites the(15)N recovery levels are higher in the mineral soil than in the organic soil. The results show no clear responses of the isotopic signature to the changes in N deposition. Several environmental drivers are identified as controlling factors for long-term(15)N recovery. Most drivers that significantly contribute to(15)N recovery are strongly related to the soil organic matter (SOM) content. These findings are consistent with the idea that much of the added(15)N is immobilized in the SOM. In the organic soil layer, we identify C stock, thickness of the organic layer, N-status and mean annual temperature of the forest sites as most important controlling factors. In the mineral soil we identify C stock, C content, pH, moisture content, bulk density, temperature, precipitation and forest stand age as most important controlling factors. Overall, our results show that these temperate forests are capable of retaining long-term increased N inputs preferably when SOM availability is high and SOM turnover and N availability are low.publishedVersio

Chemical Attribution of the Homemade Explosive ETN - Part II: Isotope Ratio Mass Spectrometry Analysis of ETN and Its Precursors

In this follow-up study the collaboration between two research groups from the USA and the Netherlands was continued to expand the framework of chemical attribution for the homemade explosive erythritol tetranitrate (ETN). Isotope ratio mass spectrometry (IRMS) analysis was performed to predict possible links between ETN samples and its precursors. Carbon, nitrogen, hydrogen and oxygen isotope ratios were determined for a wide variety of precursor sources and for ETN samples that were prepared with selected precursors. The stability of isotope ratios of ETN has been demonstrated for melt-cast samples and two-year old samples, which enables sample comparison of ETN in forensic casework independent of age and appearance. Erythritol and nitric acid (or nitrate salt) are the exclusive donor of carbon and nitrogen atoms in ETN, respectively, and robust linear relationships between precursor and the end-product were observed for these isotopes. This allowed for defining isotopic enrichment ranges for carbon and nitrogen that support the hypothesis that a given erythritol or nitrate precursor was used to synthesize a specific ETN batch. The hydrogen and oxygen atoms in ETN do not originate from one exclusive donor material, making linkage prediction more difficult. However, the large negative enrichments observed for both isotopes do provide powerful information to exclude suspected precursor materials as donor of ETN. Additionally, combing the isotopic data of all elements results in a higher discrimination power for ETN samples and its precursor materials. Combining the findings of our previously reported LC–MS analysis of ETN with this IRMS study is expected to increase the robustness of the forensic comparison even further. The partially nitrated impurities can provide insight on the synthesis conditions while the isotope data contain information on the raw materials used for the production of ETN

Split-root labelling to investigate 15

We investigated the transfer of 15N into the soil via 15N uptake and release by tree roots, which involves the principles of the split-root technique. One half of the root system received an injection of (15NH4)2SO4 and the other half equivalent amounts of (NH4)2SO4 at 15N natural abundance level. 15N was transferred from one side of the root system (15N side) to the other side (14N side) and released into the soil. The method was conducted with Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies [L.] Karst). Two concentration levels of (NH4)2SO4 were used, corresponding with annual N deposition in the Netherlands (30 kg N ha–1) and a twelfth of that (2.5 kg N ha−1). Samples were taken 3 and 6 weeks after labelling and divided into needles + stem, roots, rhizosphere and bulk soil. Already 3 weeks after labelling, Scots pine took up 23.7 % of the low and 9.1 % of the high amounts of 15N, while Norway spruce took up 21.5 and 32.1 %, respectively. Both species transported proportions of 15N to the rhizosphere (0.1–0.2 %) and bulk soil (0.3–0.9 %). The method is a useful tool to investigate the fate of root-derived N in soils, for example, for the formation of stable forms of soil organic matter

Colloidal catchment response to snowmelt and precipitation events differs in a forested headwater catchment

Climate change affects the occurrence of high-discharge (HD) events and associated nutrient exports in catchment stream water. Information on colloidal events-based losses of important nutrients, such as organic C(Corg), N, P, and S, remain relatively scarce. We hypothesized that contributions of colloidal exported N, S, and P due to differing hydrological mechanisms vary between HD events in late winter and spring. We examined one combined snowmelt and rainfall event (March 2018) with one rainfall event (May 2018) for temporal Corg, N, P, and S dynamics. The catchment exports of colloids and their subset nanoparticles were analyzed by asymmetric-flow field flow fractionation (P) and a filtration cascade (N and S). The Corg source in both events was assessed by δ13C composition of the stream water in relation to that of the soil. In winter, 0.1 μm), but this was 29–64% in spring and was associated with Corg, Fe, and Al. Colloidal N and particulate S (>1 μm) were higher during both events, but the majority of losses were dissolved (<0.1 μm). The δ13C values of dissolved organic matter (13CDOM) showed that in winter, most Corg was exported from the hydrologically connected hillslopes by water flowing through mineral horizons, due to snowmelt. During and after the rainfall events, export from organic horizons dominated the nutrient losses as particulates, including colloids. These events highlight the need for a better quantification of often underreported particulate, colloid, and nanoparticle contributions to weather-driven nutrient losses from catchments

The long-term fate of deposited nitrogen in temperate forest soils

Increased anthropogenic nitrogen (N) inputs can alter the N cycle and affect forest ecosystem functions. The impact of increased N deposition depends among others on the ultimate fate of N in plant and soil N pools. Short-term studies (3-18 months) have shown that the organic soil layer was the dominant sink for N. However, longer time scales are needed to investigate the long-term fate of N. Therefore, the soils of four experimental forest sites across Europe were re-sampled similar to 2 decades after labelling with(15)N. The sites covered a wide range of ambient N deposition varying from 13 to 58 kg N ha(-1)year(-1). To investigate the effects of different N loads on(15)N recovery, ambient N levels were experimentally increased or decreased. We hypothesized that: (1) the mineral soil would become the dominant(15)N sink after 2 decades, (2) long-term increased N deposition would lead to lower(15)N recovery levels in the soil and (3) variables related to C dynamics would have the largest impact on(15)N recovery in the soil. The results show that large amounts of the added(15)N remain in the soil after 2 decades and at 2 out of 4 sites the(15)N recovery levels are higher in the mineral soil than in the organic soil. The results show no clear responses of the isotopic signature to the changes in N deposition. Several environmental drivers are identified as controlling factors for long-term(15)N recovery. Most drivers that significantly contribute to(15)N recovery are strongly related to the soil organic matter (SOM) content. These findings are consistent with the idea that much of the added(15)N is immobilized in the SOM. In the organic soil layer, we identify C stock, thickness of the organic layer, N-status and mean annual temperature of the forest sites as most important controlling factors. In the mineral soil we identify C stock, C content, pH, moisture content, bulk density, temperature, precipitation and forest stand age as most important controlling factors. Overall, our results show that these temperate forests are capable of retaining long-term increased N inputs preferably when SOM availability is high and SOM turnover and N availability are low