Root Production, Standing Root Biomass Accumulation and Carbon Sequestration of Ryegrass-White Clover Swards Affected By Grassland Age and Slurry Application.

Renovation of old grassland and slurry application are common practices in Northwestern Europe. Renovation inevitably leads to soil organic matter (SOM) loss hereafter followed by the SOM restoration phase. Such scenario would be further complicated by fertilization regarding to its potential in modifying belowground productivity and / or belowground allocation. However, that to what extent and time root production, standing root biomass as well as litter degradation process would vary with sward aging, is far from clear. A two years’ field experiment in perennial ryegrass (Lolium perenne L.) - white clover (Trifolium repens L.) swards at four ages (1-year, 2-years, 5-years and 17/18-years after renovation) and two slurry levels (0 or 240 kg N ha^-1) showed that while slurry improved root production in all swards, the root fraction (f_BNPP) was significantly higher in 1-year-old sward than the others. Significant interaction of sward age and slurry was found, that the f_BNPP was reduced by slurry in 1-year-old sward whereas it was increased in all established swards. Further, both the new root turnover and specific root length were reduced by slurry to a larger extent in older swards. Surprisingly, despite of these positive aspects of slurry on root-derived C residence, mean belowground biomass (BGB) only increased along with sward age. It has been found that the SOM restoration to a large extent commenced in the first 2 years; differences of soil Corg between 5-years- and 17/18-years-old swards was negligible. Our results highlighted that merely 2 years after renovation the stimulated soil respiration and degradation of old root litter/particulate organic matter (i.e. an enlarged labile C pool) brought by slurry became beyond its initial effect in increasing C influx to soil. To what extent the managed grassland could act as C pool should be more finely evaluated along a time scale.