Long-Term Nitrogen Fertilization and Liming Effect on Carbon Storage in Soil Water-Stable Aggregates in a Semi-Arid Grassland

Abstract

Grasslands cover up to 40.5% of the world’s landmass and could store a tremendous amount of carbon (C). Various practices, including mineral fertilization and liming, are used to manage these ecosystems with potential long-term effects on the size and distribution of soil aggregates and inevitably carbon dynamics. The objective of this study was to examine the long-term effects of nitrogen fertilizers and liming on soil organic carbon storage and its dynamics in water-stable aggregates of a semi-arid grassland. Soil samples (0-10 cm) were collected from Ukulinga long-term grassland trial in Pietermaritzburg, South Africa where nitrogen fertilizers have been applied annually and lime every five years for 70 years. Ten treatments were studied including the control (C; 0 kg/ha-unfertilized and unlimed), lime at 2250 kg ha -1 (L), ammonium sulphate at 70 kg ha -1 (AS70) and 211 kg ha -1 (AS211); ammonium nitrate at 70 kg ha -1 (AN70) and 211 kg ha -1 (AN211); AS70 + lime (AS70L); AS211 + lime (AS211L); AN70+ lime (AN70L) and AN211+ lime (AN211L). Nitrogen or lime application had no significant effects on soil C concentration and C stocks. Nitrogen fertilizers significantly reduced soil pH and increased total soil N. Nitrogen application decreased MWD, macroaggregates, and increased C associated with microaggregates while liming increased C stored in macroaggregates. Soil pH played an important role in explaining aggregate stability rather than soil carbon. The findings of this study suggest the importance of soil aggregation, pH, and plant detritus composition in mediating carbon dynamics under chronic N fertilization and liming at the Ukulinga long-term grassland experiment.