Abstract
A laboratory incubation study was conducted on a temperate grassland soil to quantify the main mineral nitrogen (N) transformation rates and pathways via a15N tracing approach. Soil samples were taken from a long-term phosphorus (P) trial to investigate the effects on gross N-transformations under high and low phosphorus amendment. The soils were incubated over a 2-week period and treated with ammonium-nitrate (NH4NO3) which was applied to the soil both with and without a glucose amendment and labelled with 15N either on the ammonium (NH4+) or nitrate (NO3−) moiety at 50% atom enrichment. The results showed immobilisation to greatly outweigh mineralisation and that NO3− was predominantly produced via heterotrophic nitrification. Individual pathways for NO3− production were quantified including oxidation of NH4+, recalcitrant and labile organic N. Oxidation of labile organic N to NO3−, a newly considered pathway, accounted for between 63 and 83% of total NO3− production across the various treatments and P levels. This process was significantly higher in the low-P rather than the high-P soils (p < 0.05), highlighting the effect of soil P on the microbial community.
Highlights
Nitrogen (N) and phosphorus (P) are essential for biological prolif eration and are the dominant rate-limiting nutrients in most natural systems, as such they are major constituents of agrochemical fertilisers (Stark and Richards, 2008; Guignard et al, 2017)
All plots received 40 kg N ha− 1 as calcium ammonium nitrate (CAN), and potassium was applied as a muriate of potash (KCl) at a rate of 125 kg K ha− 1 year− 1 to compensate for potassium removal (Massey, 2012; Randall et al, 2019)
The model was extended by an additional heterotrophic nitrification pathway from labile organic N to NO3− (ONlab) which satisfactorily simulated the observed data
Summary
Nitrogen (N) and phosphorus (P) are essential for biological prolif eration and are the dominant rate-limiting nutrients in most natural systems, as such they are major constituents of agrochemical fertilisers (Stark and Richards, 2008; Guignard et al, 2017). Improved grassland ecosystems are a major contributor to N-pollution; comprising approximately 25% of the Earth’s terrestrial surface, they are intensively managed for grazing, which results in significant N-in puts via both animal excreta and inorganic fertiliser (Oenema et al, 2005; Saggar et al, 2013). These N-inputs undergo a series of trans formations in the soil; the drivers and interactions of which are often unclear.
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