Transfer of N from red clover to perennial ryegrass in mixed stands under different cutting strategies

Mixed ley crops including legumes are important in agriculture, giving high quality forage as well as adding N to the cropping system. These leys are also increasingly being used as green manure crops. Due to the large N(2) fixing capacity of the leguminous crops the N input is large, causing large internal N flows in the ley systems. Nitrogen budgets are thus intricate but important for evaluation of the N efficiency and environmental footprint of the production systems including mixed leys. Three field experiments were established on sandy soils during three consecutive years. Total quantities of N fixed symbiotically by red clover (Trifolium pratense L.) in pure and mixed (with perennial ryegrass (Lolium perenne L.)) stands, including N transferred to the grass, were determined under different cutting regimes. Estimates of N transfer from red clover to ryegrass were derived through (a) dilution of (15)N label in microplots of immobilised (15)N-labelled soil, and (b) recovery of (15)N label by direct transfer from leaf-labelled clover plants in mezotrons. The estimated all-season amount of N transferred from red clover to ryegrass determined from isotope dilution was 0.7-6.8 g m(-2), with no significant differences between intact and harvested stands, but mulched stands had more transferred N than harvested stands during the last growth period. All-season N transfer corresponded to an average of 30 and 57% of grass N (in harvested and intact stands, respectively) and 13 and 26% of clover N (in harvested and intact stands, respectively) with an increasing proportion being transferred successively during the growing season. The estimated amounts of N transferred from leaf-labelled clover to the companion ryegrass were 0.4-4.7 g m(-2), i.e. somewhat lower than the isotope dilution estimates. This N corresponded to approximately 16% of grass N and 15% of clover N, irrespective of cutting strategy. The discrepancy between results obtained by the two methods was attributed to leaf fall being prevented in the direct transfer plots, but not in the dilution-based estimates. In addition, the mulch in plots for direct transfer estimates was replaced with non-labelled mulch, whereas the 'native' mulch was left in the plots for estimation through dilution. The N transfer contributed strongly to the N budget of the companion ryegrass, especially in the stands where leaf fall contributed to the transfer. The uptake of clover-derived N by a companion grass may have implications for ley composition and feeding value as well as for the effects of mixed green manure and catch crops in agriculture. (C) 2010 Elsevier B.V. All rights reserved.