Effects of long-term grassland management on the carbon and nitrogen pools of different soil aggregate fractions

Common grasslandmanagement practices include animal grazing and the repeated addition of lime and nutrient fertilizers to soils. These practices can greatly influence the size and distribution of different soil aggregate fractions, thus altering the cycling and storage of carbon (C) and nitrogen (N) in grassland soils. So far, very fewstudies have simultaneously addressed the potential long-term effect that multiple management practices might have on soil physical aggregation. Here we specifically ask whether and how grazing, liming and nutrient fertilizationmight influence C and N content (%) aswell as C and N pools of different soil aggregate fractions in a longterm grassland experiment established in 1991 at Silwood Park, Berkshire, UK. We found that repeated liming applications over 23 years significantly decreased the C pool (i.e. g C Kg(-1) soil) of Large Macro Aggregate (LMA > 2 mm) fractions and increased C pools within three smaller soil aggregate fractions: SmallMacro Aggregate (SMA, 250 mu m-2mm), Micro Aggregate (MiA, 53-250 mu m), and Silt Clay Aggregate (SCA < 53 mu m). Soil C (and N) accrual in smaller fractions was mainly caused by positive liming effects on aggregate fraction mass rather than on changes in soil C (and N) content (%). Liming effects could be explained by increases in soil pH, as this factor was significantly positively related to greater soil C and N pools of smaller aggregate fractions. Long-term grazing and inorganic nutrient fertilization had much weaker effects on both soil aggregate-fraction mass and on soil C and N concentrations, however, our evidence is that these practices could also contribute to greater C and N pools of smaller soil fractions. Overall our study demonstrates howagricultural liming can contribute to increase C pools of small (more stable) soil fractions with potential significant benefits for the long-term C balance of human-managed grassland soils. (C) 2017 Elsevier B.V. All rights reserved.