We evaluated the abiotic formation of dissolved organic nitrogen (DON) by the fast reaction of iron (Fe) with nitrate (NO3 −) in the dissolved organic matter (DOM) of volcanic soils in a temperate rainforest (>5000 mm precipitation per year). During five days, the educts and products of abiotic reactions under anoxic conditions were measured in a microcosm experiment depending on the Fe and NO3 − concentrations. A control zero-Fe was not used because there was no chemical reaction with nitrate addition. Using a novel technique of automated sample preparation for inorganic N (SPIN) attached to a membrane inlet quadrupole mass spectrometry (MIMS), the 15N abundances and inorganic N concentrations were determined directly in aqueous solutions. The results were explained in the context of the Ferrous Wheel Hypothesis which states that Fe(II) is utilized to reduce NO3 − to nitrite (NO2 −) that is incorporated into DOM. Fe(II) is regenerated from Fe(III) in anaerobic soil microsites. Here we tested one part of this hypothesis, the processes occurring in DOM (instead of soil organic matter). Using the SPIN-MIMS technique, we could overcome Ferrous Wheel Hypothesis criticism regarding possible Fe interference during NO3 − analysis. The total recovery of 15N added as NO3 − fluctuated between 63 and 101%, and the remaining 15N was measured as gaseous N2O. The 15N-labelled NO3 − added decreased immediately after 15 min of incubation. After five days of incubation, approximately 25% of the labelled NO3 − (e− acceptors) added was transformed to DON in the presence of a high amount of Fe(II) (e− donors). Small amounts of N2O and CO2 provided further evidence of NO3 − reduction and DOM oxidation, respectively. From these results, we propose a new theoretical model that includes the Ferrous Wheel Hypothesis, where only the transformation of NO3 − to DON was proven. The present results explain the high retention of NO3 − in DOM from volcanic soils in ecosystems with high precipitation.
No abstract is provided for this article.
. Microorganisms regulate the carbon (C) cycle in soil, controlling the utilization and recycling of organic substances. To reveal the contribution of particular microbial groups to C utilization and C turnover within the microbial cells, fate of 13C-labeled glucose was studied under field conditions. The 13C was traced in cytosolic substances, amino sugars and phospholipid fatty acids (PLFA) at intervals of 3, 10 and 50 days after glucose addition. 13C enrichment into PLFA (~1.5 % of PLFA C at day 3) was one order of magnitude greater than into the cytosol, showing the importance of cell membranes for initial C utilization. 13C enrichment of amino sugars in living microorganisms at day 3 accounted for 0.57 %, resulting that the turnover of cell wall components is two times slower than that of cell membranes. Turnover time of C in the cytosol (150 days) was three times longer than in PLFAs (47 days). Consequently, despite the lability of cytosol pool and expected fast turnover rates, intensive recycling of cytosol components, within the living cells, leads to a longer turnover time. Amino sugars originate mainly from microbial residues, thus longer experimental periods are required for estimation of their turnover times. Both PLFA and amino sugar profiles indicated that glucose C was preferentially used by bacteria. The 13C incorporated into bacterial cell membrane components decreased with time, but it remained constant or even increased for filamentous microorganisms. Hence, over a short period, bacteria contribute more to the utilization of low molecular weight organic substances, whereas filamentous microorganisms are responsible for further C transformations. Thus, tracing 13C in cellular compounds with contrasting turnover rates elucidated the role of microbial groups and their cellular compartments in C utilization and recycling in soil. This information is especially important for assessing C fluxes in soil and the contribution of C from microbial residues to soil organic matter.
<p>Hotspots in agricultural soils, which include rhizosphere, detritusphere and drilosphere, are characterized by strongly different dynamics than those of natural ecosystems. This involves hotspot properties such as element cycling intensity, microbial activation, lifetime or spatial extension. Evidence from studies around the globe suggests that key hotspot characteristics intensify or increase under strongly limiting cropping conditions e.g. low-input agriculture: i) nutrient mining is more intensive around roots in infertile soils, ii) root exudates decompose more slowly under water limitation, and iii) the rhizo-hyphosphere forms a more spatially extended hyphal network under P deficiency. These examples suggest that smart management of hotspots might be a sustainable strategy to overcome soil limitations, not only for crop production on marginal soils but also as a strategy to save resources for future agriculture.</p><p>Here, we will present a set of studies applying management strategies, which actively modify hotspot intensity, lifetime or spatial extension with the aim to manipulate biogeochemical cycles of the respective agroecosystem. Most traditional, tillage enlarges the topsoil detritusphere or moves it to lower soil depths. Rather novel but increasingly studied approaches seek to modify rhizosphere properties: applying genotypes with i) specific root traits such as an optimized root morphology (e.g. modified root hairs or deeper fine root system) or ii) modified root exudate compositions and resulting rhizosphere microbiomes. Such approaches need to be applied site- and agroecosystem-specifically to optimize resource utilization. Moreover, as agroecosystems are under long-term controls, hotspot management strategies are not limited to one growing season but can stretch over years of cultivation. The generation of specific biopores – the root channels - created by e.g. tap-rooted or deep-rooting cover crops is a management practise inducing a rhizosphere-detritusphere-rhizosphere transition over time. ‘Re-activated’ hotspots feature unique biogeochemical conditions for young roots as well as microbial communities. Such ‘highways to subsoil’ foster rhizosphere establishment in subsoils, where i) hotspots remain moist and thus active under drought and ii) where gradients from hotspots to bulk soils are for magnitudes higher compared to topsoils. All these aspects present a unique, however largely unexploited potential for future agriculture, yet.</p><p>By a novel set of methodological approaches and their combinations, comprising multi-isotope applications, in-situ imaging techniques, biomarkers and microbial activity measures with high spatial resolution, we will provide new insights into the potential of hotspot management in agroecosystems. We will discuss implications for crop production under resource limitation up to the potential for a sustainable development of future agricultural production systems especially in the face of projected climate change.</p>
