What limits soil N-cycling and potential losses under grassland at Hob Moor, York, UK?

- Measuring mineral-N in incubated vegetation-free soil shows trends in soil-N supply.
- Limited biodegradable litter supply limits mineral-N supplies under grass in summer.
- Ammonium-N can be highly mobile within and from soil horizons.
- Soil N supply, not just plant N uptake, limits summer leaching to surface waters.

It is important to understand what limits mineral-N losses to ground waters and surface waters from soils under diverse vegetation types. Therefore a field experiment was conducted to test two hypotheses. The first is that, in summer months, low supply of fresh, readily biodegradable litter in soil under grassland limits the availability of ammonium-N and nitrate N, and hence potential nitrate leaching to surface waters and ground waters. The second was that ammonium-N mobility is important to the distribution and redistribution of soil mineral-N in soil profiles. Water-soluble and KCl-extractable ammonium and nitrate concentrations were measured approximately monthly in soils from 0 to 10 and 10 to 20 cm depths in both fresh, field-moist soils and in soils incubated for 7 days at ambient outdoor temperatures after plant shoot and root removal. The results confirmed that appreciable amounts of ammonium-N are water soluble and hence potentially mobile between soil depths. Nitrate leaching from soils under grassland is shown to be low in summer largely because of the lower availability of readily mineralizable plant litter of shoot and root origins, and not just because of plant N uptake as widely supposed.