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

•Long-term grazing, liming and nutrient fertilization effects on soil aggregates•Liming significantly increased C and N pools of small soil aggregate fractions•Liming effects were significantly explained by increases in soil pH.•Evidence of positive liming effects on the C balance of managed grasslands

Common grassland management 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 few studies 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 fertilization might influence C and N content (%) as well as C and N pools of different soil aggregate fractions in a long-term 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: Small Macro Aggregate (SMA, 250 μm-2 mm), Micro Aggregate (MiA, 53-250 μm), and Silt Clay Aggregate (SCA < 53 μm). 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 how agricultural 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.

Graphical abstractSignificant negative (−) and positive (+) effects of 23 years of grassland management on the carbon pool of different soil aggregate fractions: Large Macro Aggregate (LMA > 2 mm), Small Macro Aggregate (SMA 250 μm-2 mm), Micro Aggregates (MiA 53-250 μm), Silt Clay Aggregates (SCA < 53 μm). Note: N.S. = Not significant.Download high-res image (50KB)Download full-size image