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Biological nitrogen fixation (BNF) represents the natural pathway by which mosses meet their demands for bioavailable/reactive nitrogen (Nr) in peatlands. However, following intensification of nitrogen fertilizer and fossil fuel use, atmospheric Nr deposition has increased exposing peatlands to Nr loading often above the ecological threshold. As BNF is energy intensive, therefore, it is unclear whether BNF shuts down when Nr availability is no longer a rarity. We studied the response of BNF under a gradient of Nr deposition extending over decades in three peatlands in the U.K., and at a background deposition peatland in Sweden. Experimental nitrogen fertilization plots in the Swedish site were also evaluated for BNF activity. In situ BNF activity of peatlands receiving Nr deposition of 6, 17, and 27 kg N ha–1 yr–1 was not shut down but rather suppressed by 54, 69, and 74%, respectively, compared to the rates under background Nr deposition of ∼2 kg N ha–1 yr–1. These findings were corroborated by similar BNF suppression at the fertilization plots in Sweden. Therefore, contribution of BNF in peatlands exposed to chronic Nr deposition needs accounting when modeling peatland’s nitrogen pools, given that nitrogen availability exerts a key control on the carbon capture of peatlands, globally.

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<p>Population keeps growing so as the need for food production. The increase in energy prices is putting a lot of pressure in energy intensive industrial processes such as the production of fertilizers. Farmers need to fine-tune the amount of fertilizer needed by the soil, so that they do not add in excess, elevating costs and polluting the environment, or do not fall short, suffering sub-optimal crop yields.</p><p>This work reports the fabrication and characterization of a low-cost device for the continuous monitoring of the concentration of plant nutrients based on ion-selective electrodes and textile threads that work in direct contact with soils. Here, as proof of concept, we developed a thread-based, microfluidic sensor platform. We utilized traditional polymer membrane-based ion-selective electrodes (ISEs) for potassium, nitrate, ammonium and pH were drop-casted directly on top of a miniaturized, 3D-printed holder. Electrical contact is established via graphite-based contacts link to the electrochemical signal reader via electrical wires. The sensor platform was enhanced by the addition of five 30 cm long textile threads connected to an absorption pad on the opposite side. This is the key innovation as these threads mimic the roots and via capillary action wick the moisture from the soil to the sensing area. The entire sensor platform contained 4 ISEs for each chemical species and one reference electrode and was encased into a 3D printed housing. The device is placed next to the soil that is going to be analysed inserting the threads in the soil sampling area.</p><p>Preliminary results show that thread-based sensor system is reproducible and consistently provides a near-Nernstian sensitivity of 55±5 and 50±3 for potassium, -58±1 and -63±2 for nitrate, and 60±1 and 60±12 (mV/decade) for ammonium between 2.8x10<sup>-6</sup> and 1.3x10<sup>-2</sup> M without (directly in solution) and with textile threads respectively. Analysis of soil samples with different soil moisture content (100%, 75%, 50% and 40%) using our low cost device gave a correlation coefficient of R<sup>2</sup> = 0.91 for potassium and R<sup>2</sup> = 0.92 for ammonium when compared to the values measured using traditional methods such as inductively coupled plasma optical emission spectroscopy (ICP-OES) and flow injection analysis (FIA), respectively. The promising performance of this low-cost device is encouraging towards its use as an extended network to measure soil ion concentration at high temporal and spatial resolution.</p>