Nitrogen (N) fixation by white clover (Trifolium repens L.) was measured in the ley phases of three experimental organic ley-arable rotations at two sites, between 1997 and 2000, using the ¹⁵N dilution technique. Soil nitrate, ammonium, and total soluble N were measured at one of the sites between 1999 and 2001. Variation of soil N was assessed in different ages of ley, year, month, rotation, site and microsite type (grass or clover). Data for soil pH, organic matter, P, K, and Mg, was also compared, as was weather data for the period. Variation of N fixation, grass and clover dry matter, proportion of nitrogen derived from the atmosphere, grass and clover N concentrations, grass and clover non symbiotic N-yield, and N transfer from clover to grass were compared in different ages of ley, years, months, rotations and sites. Flowering rates and presence of clover tap-roots were also examined. Above ground N fixation varied between 26 and 75 kg ha^-1 over the course of the growing season. Estimated total N fixation ranged from 70-214 kg ha^-1. N fixation was closely related to yield of clover. There were clear relationships between soil nitrate and the behaviour of the grass and clover components of the leys. Nitrate was slightly higher under clover patches than under grass patches in mid-late summer.

White clover and perennial ryegrass were grown in circular chambers, divided radially, and their invasion into microsites with different vegetation, different management and different N treatments was observed. Ryegrass was not capable of rapidly colonising microsites by vegetative spread alone. Clover was more likely to successfully invade microsites containing grass with no added N than microsites with added N, or bare soil microsites.

I hereby declare that the work presented here is my own, with the exception of the following:

The weather data (Figures 3a-d), the ground cover data (Figure 14) and the long term soil P, K, Mg and pH values (Figures 23a-d), which were collected by the Scottish Agricultural College.

6.1.2003

This project was funded by SEERAD. Special thanks to Olive Pauline and Mary Shepherd for technical assistance, to Derek Simpson for help with fieldwork and to Catriona Brady for love and keeping me sane. I would also like to thank my parents for their financial assistance

DMC – Dry matter yield of clover

DMG – Dry matter yield of grass

FYM - Farmyard Manure

J -Joules

K – Potassium

K₂SO₄ – Potassium Sulphate

LU - Livestock Units (1 livestock unit = 1 large dairy cow)

Mg – Magnesium

%NC – percentage N (by weight) in dry clover herbage

%NG– percentage N (by weight) in dry grass herbage

%XS¹⁵NC – ¹⁵N enrichment of clover

%XS¹⁵Ng_mo– ¹⁵N enrichment of grass in monoculture

%XS¹⁵NG_mx– ¹⁵N enrichment of grass in grass-clover mixture

N - Nitrogen

N₂ – Nitrogen gas

NO – Nitric oxide

N₂O – Nitrous oxide

NO₃^- - Nitrate

NO₂ – Nitrite

NH₄⁺ - Ammonium

NfixIT - Nitrogen fixation including transfer

NfixET - Nitrogen fixation excluding transfer

P - Phosphorus

pNdfaIT - Proportion of Nitrogen in clover herbage that is derived from the atmosphere, including transfer

pNdfaET - Proportion of Nitrogen in clover herbage that is derived from the atmosphere, excluding transfer

ppm – parts per million

SON - Soluble Organic Nitrogen