Long-term fertilization effects on carbon pools and carbon management index of loamy soil under grass–forage legumes mixture in semi-arid environment

Soil organic carbon (SOC) is a key component for sustaining crop production. A field experiment was conducted during 2004–2018 to assess the changes in soil carbon fractions under different fertilization practices in grass-legumes mixture. The result indicates that application of farmyard manure (FYM) at 80 Mg ha–1 has increased SOC concentration leading to carbon sequestration rate of 4.2 Mg ha–1 year–1. Further, it has increased the proportion of labile carbon in the total SOC and have accumulated 126, 60, 83 and 95% higher very labile, labile, less labile and non-labile C stock than that of control plot, respectively, in top 30 cm soil layer. Inorganic fertilization and FYM 20 Mg ha–1 influenced SOC concentration, SOC stock and C sequestration rate similarly. The highest carbon management index (264) was found in the treatment receiving FYM 80 Mg ha–1 and it was positively correlated with SOC (r = 0.84**). The sensitivity index of the SOC varied from 26 to 152% and the differences were greatest in FYM treatments. The result indicates that grass-legumes mixture build-up the SOC in long term and the addition of FYM further increases it.