Grassland legacy effects on yield of a follow-on crop in rotation strongly influenced by legume proportion and moderately by drought

Abstract

We investigated the degree to which plant species diversity, drought and fertiliser level in a grassland ley can affect performance of a follow-on crop in a rotation. Grassland species and functional group diversity (grasses, legumes and herbs) were manipulated from monocultures to six-species mixtures in the grassland ley phase. A simulated two-month summer drought treatment was compared to a ‘rainfed’ control. Plots received 150 kg ha−1 yr−1 of nitrogen (N) fertiliser; additional replicates of L. perenne monoculture received 300 kg ha−1 yr−1 of N fertiliser. After two years, grassland communities were terminated, and each plot reseeded with an Italian ryegrass (Lolium multiflorum) model crop; its yield indicated the relative legacy effect of the preceding treatments (plant diversity, drought, N input). There was a modest but constant negative effect of drought on dry matter (−0.36 ± 0.091 t ha−1) and nitrogen yield (DMY and NY respectively) of the subsequent crop of L. multiflorum, across all plant communities. There were strong differences among the identity effects of the six former grassland species on DMY and NY of L. multiflorum. Legume species had the strongest effects on DMY of L. multiflorum (6.09 t ha−1 for the former T. pratense monoculture and 6.54 t ha−1 for T. repens). The lowest crop yield was from the former low-diversity high-input replicates (4.16 t ha−1 for former L. perenne monoculture with 300 kg ha−1 yr−1). There was no evidence that interspecific interactions in the grassland phase affected yield of the follow-on crop. Thus, the legacy effect of grassland mixtures was estimated by the identities and proportions of the species sown in the mixture. Similar patterns were obtained for NY. High-diversity, low-input grassland yielded more DMY and NY than low-diversity, high-input grassland (across both ley and follow-on crop phases). However, a legume proportion in the grassland ley of at least 33% is required to achieve high forage and crop performance in a grassland-crop rotation.