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Agricultural grasslands comprise over 50% of the total land area of the UK and provide important ecosystem services that include livestock and forage production. These services are rarely measured against the effects that key management practices might have on the long-term ability of grassland soils to cycle and store carbon (C). The common management practice of re-seeding (i.e. the ploughing and seeding of grasslands with more productive grass cultivars) can cause significant soil disturbance; yet the net long-term effects of re-seeding on soil C gains and losses in permanent grasslands are poorly understood. Here, we selected a chronosequence of 45 permanent grasslands across Northern Ireland with a well-documented history of single re-seeding events over the last 50 years. Second, we asked whether and how soil C cycling and storage might differ between recently re-seeded ‘young’ grasslands and increasingly ‘older’ (or never re-seeded) grasslands. We measured (1) soil CO2 fluxes, (2) soil C stocks, (3) the C content of different soil aggregate fractions, and (4) root C stocks. We found that soil CO2 fluxes were significantly higher in recently re-seeded, ‘young’, grasslands. However, total soil C stocks (0–20 cm depth) did not increase in ‘older’ grasslands despite these grasslands showing greater root C stocks. Instead, soil C stocks significantly decreased with increases in soil bulk density. Higher soil bulk density was also associated with lower C content in smaller organo-mineral aggregate sizes (i.e. more recalcitrant C pools) regardless of grassland age (time since re-seeding). Overall, our results suggest that management-induced effects on key soil physical properties, i.e. bulk density, may have significantly greater implications for C sequestration in permanent grassland soils than high disturbance, but infrequent, re-seeding events.

Grasslands cover more than a third of the European agricultural area and are often intensively managed to support the livestock-farming sector. Despite grassland intensification can greatly influence soil carbon (C) cycling, changes in soil C stocks both across years and at increasing soil depths remain difficult to quantify simultaneously. Here we measured spatial and temporal changes in soil C stocks to assess C stocks’ response to intensive grassland management. We measured soil C stocks (a) in a long-term nutrient fertilization experiment on permanent grassland (established in 1970), and (b) in 126 grassland fields distributed across 11 lowland farms in Northern Ireland (UK), which are associated with different frequencies of soil tillage. Using 45 years of data from the plot-scale grassland experiment we found evidence that significant changes in soil C stocks mainly occurred in the soil top 20 cm (not in deeper soils) and only between ‘extreme’ nutrient treatments (i.e. unfertilized vs. highly fertilized soils). Soil physical fractionation, radiocarbon and stable nitrogen isotope analyses all suggest that new C has accumulated in these soils but perhaps not fast enough to affect C stocks in deeper soil layers. Results at the field-scale from intensively managed grasslands show how the frequency of soil tillage neither affected soil C stocks between 0–20, 20–40 or 40–60 cm depth layers nor the C pool of different soil physical fractions at increasing soil depths. Our findings demonstrate how the response of soil C stocks to grassland intensification (i.e. C stocks response to different rates of nutrient fertilization or frequency of soil tillage) can be very slow both in time and space under cool and humid climate conditions. We suggest how the persistence of these soil C stocks under intensive grassland management offers the unique opportunity to improve nutrient use efficiency and cycling thus promoting the delivery of multiple ecosystem processes.

Two primary approaches to perennial biofuel crop production studied so far are fertilized grassmonocultures and low-input high-diversity grasslands. While high-yielding perennial grass varieties arebeing developed in fertilized monocultures, breeding for yield in low-input high-diversity systemswould be difficult. Before initiating breeding for low-input systems, it is therefore important to know theminimum number of functional groups and species required for maximum biomass harvest from lowinputgrasslands. We controlled the number of perennial grassland species in 168 plots in Minnesota,USA. Species were selected at random from a pool of 18, and 1, 2, 4, 8, or 16 were planted in each plot.Aboveground biomass was measured annually, and the plots were burned each spring. We found astrongly positive log-linear relationship between average annual aboveground biomass and plantedspecies number, but a large proportion of plot-to-plot variability remained unexplained. Weperformed aconditional analysis of the aboveground biomass data to determine whether considering species identitywould reduce the minimum number of species necessary in order to achieve yields similar to the highestdiversity treatments. A model that accounted for the presence of legumes in general, and for the presenceof the legume species Lupinus perennis in particular, showed no increase in biomass yield with increasedspecies number. Over 11 years, average yields of L. perennis/C4 grass bicultures were similar to those of16-species (maximum diversity) plots, and both were >200% greater than the average of monocultures.Thus, under low-input conditions, the choice of the appropriate few perennial plant species for eachlocation might result in systems with biomass yields similar to those from high-diversity systems.Because breeding biofuel crops in diverse mixtures would introduce complexity that is unwarranted interms of maximum biomass yield, the first biofuel crop breeding programs for low-input systems arelikely to accelerate progress by focusing on grass–legume bicultures.