Can glacial flour stimulate nitrogen cycling in cropland

Abstract

Elements of contemporary agriculture threaten the resilience of future food systems through contributions to soil degradation, biodiversity loss and climate change. We propose a novel initiative which offers to transform an alluvial waste product into a renewable soil amendment, with the potential to remediate soil nutrient deficits, mitigate soil greenhouse gas fluxes and provide reliable economic value to mountain agriculture. A pot experiment was conducted to investigate the influence of Glacial Rock Flour (GRF) amendments on soil nitrogen cycling, during the cultivation of Red Clover (RC) (Trifolium pratense), a popular leguminous fodder. GRF samples from the Chhota Shigri (Himachal Pradesh Himalayas) and Sólheimajökull (Iceland) glaciers were collected from campaigns in 2017/18 & 2016 respectively, and their geochemistry analysed. RC was grown in a series of control, GRF & P/K amended pots and harvested in two cuts at 14 & 19 weeks. The 15N natural abundance method was used to estimate nitrogen derived from the atmosphere (ndfa) in RC shoot material. Greenhouse gas fluxes of pots over the growth period were monitored using sealed chambers, and microbial nitrogen cycling capacity of 15N tracer enriched post-harvest soils was assessed through the isotope dilution and 15N gas flux methods. Fixed nitrogen yield results showed that 20 T ha⁻¹ GRF amendments stimulated RC growth through supply of macronutrient base cations, particularly during nutrient deficits of perlite cultivation and secondary harvest cultivation. 20 T/ha GRF treatments (H & J) also trended towards more negative cumulative & final fortnight aggregate N₂O fluxes than the control, suggesting stimulation of rhizobial N2O reduction. 15N denitrification fluxes also indicate lower N₂O evolution in GRF amended pots and greater N₂ production, supporting N₂O chamber flux trends & stimulation of the nos reductase enzyme. GRF amendment made no significant difference to soil N-mineralisation, however it is worth noting that soil microbial activity was greater in the presence of RC. Nitrification was not quantifiable due to rapid microbial immobilisation. We conclude that RC cultivated in low fertility soils presents a tightly coupled N-cycle, highly dependent on productive symbiosis with rhizobium for both N-fixation and denitrification processes. We evidence stimulation of these processes, actuated by GRF macronutrient supply. Further investigation of geochemical weathering mechanisms and nutrient assimilation could further evidence these influences.

Date of Award	24 Jan 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Fotis Sgouridis (Supervisor), Jemma L Wadham (Supervisor) & Christopher J Williamson (Supervisor)