dry matter accumulation and increased '^N fertilizer uptake, but less nitrogen fixed on the mineral upland site than on the peat soil.Whether ''N was given as ammonium or nitrate made little difference in these experiments.Grass and clover production and nitrogen cycling were compared in 1983 and 1984 at three sites: an upland peaty gley and upland and lowland brown earths. The clover varieties Olwen and SI84 were compared in 1983 and SIOO and S184 in 1984. Ammonium and nitrate sources of '^N were used to measure nitrogen recovery from fertilizer and soil, nitrogen fixation and nitrogen transfer from clover to grass. Acetylene reduction was measured once, in 1983, but isotope dilution was used in both years.Olwen clover produced more dry matter and took up more '^N than SI84. Olwen fixed more nitrogen than SI84 over the whole season, as measured by '^N isotope dilution. Companion grass took up more soil nitrogen when growing with SI84 than with Olwen. The clover variety SIOO, tested at the lowland site in 1984, caused no significant variations in dry matter accumulation or N fixation.In the dry 1984 season, grass dry matter accumulation and "N uptake were less than in 1983, and plants actually lost total nitrogen from their roots to the soil. Fixation rate varied more in 1984 than in 1983. Nitrogen transfer from clover to grass was detected by isotope ratio differences in 1983, and by total N differences in 1984.Site differences were dominated by the greater dry matter accumulation of Olwen in the lowland in 1983, but in that year there was also increased

The impact of photoperiod on the rate and magnitude of N remobilization relative to uptake of inorganic N during the recovery of shoot growth after a severe defoliation was compared over 18 days in two temperate grass species, timothy (Phleum pratense L. cv. Bodin) and meadow fescue (Festuca pratensis Huds. cv. Salten). Plants were grown in flowing solution culture with N supplied as 20 mM NH4NO3 and pre‐treated by extending the 11 h photosynthetically significant light period either by 1 h (short‐day or SD plants) or 7 h (long‐day or LD plants) of very low light intensity, during the 10 days prior to defoliation. Following a single severe defoliation, 15N‐labelled NH4+ or NH4++ NO3− was supplied over a 20‐day recovery period under the same SD and LD conditions. Changes in the relative contributions of remobilized N and newly acquired mineral N to shoot regrowth were assessed by sequential harvests. Both absolute and relative rates of N remobilization from root and stubble fractions were higher in LD than SD plants of both species, with the enhancement more acute but of shorter duration in timothy than fescue. Remobilized N was the predominant source of N for shoot regrowth in all treatments between days 0 and 8 after cutting; on average more so for fescue than timothy, because the presence of NO3− reduced the proportional contribution of remobilized N to the regrowth of timothy but not of fescue. Net uptake of mineral N began to recover between days 4 and 6 after cutting, with NO3− uptake restarting 1 or 2 days earlier than NH4+ uptake, even when NH4+ was the only form of N supply. LD timothy plants supplied solely with NH4+ were slowest to resume uptake of mineral N. Supplying NO3− in addition to NH4+ after defoliation promoted shoot regrowth rate but not remobilization of N. Rates of regrowth (shoot dry weight production per plant) were not correlated with rates of N remobilization from stubble either over the short‐term (days 0–8) or longer term (days 0–18), interpreted as evidence against a causal dependence of regrowth rate on N remobilization under these conditions. The results are discussed in relation to hypotheses for source/sink‐driven rates of N remobilization and their interactions with mineral N uptake following defoliation.

S U M M A R YLolium perenne cv. S.23, L. multijlorum cv. RvP, and Trifolium repens cvs S. 184 and Olwen, were grown in mixed sward and monoculture during 1979. Whereas in mixtures grass roots absorbed more 32P than clover roots, in monoculture clover generally absorbed more 32P than grass roots. This showed that grass was a very strong competitor for uptake in mixed swards.Clover and grass monocultures absorbed most 32P from 10 or 15 cm depth in the soil, while grass in mixtures absorbed most 32P at 22.5 cm depth.Comparing varieties,. in monocultures in June, Olwen was most active in absorbing 32P at 15 cm. In August, Olwen absorbed more at 15 cm and 22.5 cm than S.184 or the grass varieties. Differences in absorption depth between varieties were less in mixtures than in monocultures. S.23 absorbed more 32P in the late season than RvP, both in monoculture and in mixtures. Thus Olwen differed from S. 184 in depth and timing of uptake, whilst S.23 differed from RvP in time of uptake. Such varietal differences could be exploited by manipulation of depth and timing of fertiliser application to increase the precision of sward management.0 1982 Association of Applied Biologists

SUMMARY Lolium perenne cv. S.23 and Trifolium repens cv. Olwen were sown together in 1975, fertilised then and in 1976, and finally given nitrogen doses of either 50, 100, 200 or 400 kg/ha (as N) combined with 0.64 times as much potassium (as K2O) in 1977. As nitrogen increased, grass yield increased, but clover decreased. Grass roots absorbed more 32P than clover roots, and nitrogen increased this difference. Grass roots bore more mycorrhiza than clover roots. The difference in 32P uptake between grass and clover was less in June and July than in August. Clover roots took up most phosphate from the upper layers of soil, while grass absorbed 32P rather uniformly down to 25 cm. It was concluded that optimum fertiliser placement for clover growth was a surface dressing in the early season.

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