Nitrogen dynamics in temporary multi-species grasslands

Summary

Plant diversity is often low in high-yielding temporary grasslands in Europe, with grass-clover being the traditional dominant mixture. Non-leguminous dicot forage herbs have potential as complementary species to traditional grassland species. Chicory (Cichorium intybus L.), ribwort plantain (Plantago lanceolata L.) and caraway (Carum carvi L.) are the three non-leguminous dicot forage herbs (hereafter called forbs) that have attracted most attention. Another form of grassland production of increasing interest is the practice of growing species mixtures containing only forage legumes for low-input production of biomass with high protein content. In this study, grassland nitrogen (N) dynamics were studied in a sward that included the forbs chicory, plantain and caraway in a traditional mixture of perennial ryegrass (Lolium perenne L.)and red clover (Trifolium pratense L.), with fertilizer application as an additional factor, and in a sward containing species mixtures of only forage legumes red clover, white clover (Trifolium repens L.) and lucerne (Medicago sativa L.). The objectives of the study were to determine how inclusion of forbs in ryegrass-red clover mixtures would influence sward production and red clover biological N2 fixation (BNF), red clover N transfer to the companion non-legumes and residual soil N fertility, and to investigate how species mixtures of only forage legumes would influence sward production and BNF. The BNF and N transfer studies were performed in situ using the 15N isotope dilution and 15N leaf-labelling method, respectively. The residual N effect was determined in terms of soil initial inorganic N and mineralizable organic N contents, and biomass production and N uptake in the subsequent spring barley test crop in a pot experiment.
Low-input swards containing forbs in ryegrass-red clover or only forage legumes were highly productive with seasonal herbage dry-matter (DM) yields of up to 17 t ha-1. Neither the inclusion of forbs in the ryegrass-red clover, nor the cultivation of only forage legumes in species mixtures demonstrated any species-diversity effect on percentage of legume N derived from BNF (%Ndfa). The three forbs displayed different competitive strengths when included in the ryegrass-red clover,where chicory and plantain were more competitive than caraway. Plantain and caraway appeared more compatible with ryegrass-red clover, where the sward DM yield and BNF of ryegrass-red clover-plantain or caraway were comparable to ryegrass-red clover. A high seeding proportion of chicory did, however reduce the biomass proportion of red clover in the mixture, thus suppressing the seasonal amount of BNF by up to 60% and decreasing sward DM production in unfertilized plots by 20%. Fertilization did not affect %Ndfa in the mixtures, indicating a low level of soil N or vi non-legume competition for plant-available soil N, but tended to decrease the amount of BNF by up to 35%, suppressing red clover growth to the benefit of grass. Red clover transferred up to 15% of its N to the companion ryegrass and forbs. Forbs included in the ryegrass-red clover relied much less on N transferred from red clover – 0.6 to 4% of the red clover N – than the ryegrass, and showed no effect on red clover N transferred to co-existing non-legume species or on N rhizodeposition in the soil. In addition, including forbs in the ryegrass-red clover did not change the residual N fertility in the subsequent spring barley crop. In the sward containing only forage legumes, red clover appeared to be the strongest species. Mixtures containing red clover showed a similar potential of sward production and input of N from BNF to red clover pure stand. The herbage DM yield, N accumulation and amount of BNF increased by 25 to 50%, 11 to 34%, and 7- 36%, respectively, in the mixtures containing red clover compared to white clover or lucerne in pure stands, or a white clover-lucerne mixture. The study concludes that when used in right proportions, the non-leguminous forbs chicory, ribwort plantain or caraway can be included in the traditional perennial ryegrass-red clover mixture with no negative effects on herbage production, red clover BNF, red clover N transfer to non-legumes or short-term residual soil N fertility. However, to achieve a balance between legume and nonlegume species in the sward, it is important that the mixture does not include a high seeding proportion of chicory. Similarly, the forage legumes red clover, white clover and lucerne can be grown in mixtures without compromising herbage production or input of N from BNF compared to the pure stand of red clover, as long as red clover is included in the mixture. Thus, the study has provided new insights into the design and implementation of multi-species temporary grasslands for increasing biodiversity and improving N self-sufficiency and productivity of low-input grassarable cropping systems.