The combined effect of fertiliser nitrogen and phosphorus on herbage yield and change in soil nutrients of a grass/clover and grass-only sward

The combined effect of reduced nitrogen ( N ) and phosphorus ( P ) application on the production of grass- only and grass/ clover swards was studied in a five- year cutting experiment on a marine clay soil, established on newly sown swards. Furthermore, changes in soil N, P and carbon ( C ) were measured. Treatments included four P ( 0, 35, 70 and 105 kg P ha(-1) year(-1)) and three N levels (0, 190 and 380 N kg ha(-1) year(-1) ) and two sward types ( grass- only and grass/ clover ). Nitrogen was the main factor determining the yield and quality of the harvested herbage. On the grass- only swards, N application increased the DM yield with 28 or 22 kg DM kg N-1, at 190 or 380 kg N ha(-1) year(-1), respectively. The average apparent N recovery was 0.78 kg kg(-1). On the grass/ clover swards, N application of 190 ha(-1) year(-1) increased grass production at the cost of white clover, which decreased from 41 to 16%. Phosphorus application increased grass yields, but did not increase clover yields. A positive interaction between N and P applications was observed. However, the consequences of this interaction for the optimal N application were only minor, and of little practical relevance. Both the P- AL- value and total soil P showed a positive response to P application and a negative response to N application. Furthermore, the positive effect of P application decreased with increasing N application. The annual changes in P- AL- value and total soil P were closely related to the soil surface surplus, which in turn was determined by the level of N and P application and their interaction. The accumulation of soil N was similar on both sward types, but within the grass-only swards soil N was positively affected by N application. The accumulation of organic C was unaffected by N or P application, but was lower under grass/ clover than under grass- only.