Influence of integrated nutrient management on carbon sequestration and its relationship with productivity of pearlmillet (Pennisetum glaucum)-mustard (Brassica juncea) cropping sequence

A field experiment was conducted from rainy (kharif) season of 2010–11 to winter (rabi) season of 2011–12 under pearlmillet [Pennisetum glaucum (L.) R. Br. emend. stuntz]-mustard [Brassica juncea (L.) Czernj. Cosson] sequence cropping at Gwalior to evaluate the impact of different levels of N,P and K alone or in combination with farmyard manure (FYM) and/or biofertilizers on total carbon sequestration (TCSQ) and dynamics of various pools of oxidizable organic carbon in relation to crop productivity. The experiment having 16 fertility treatments with 3 replications was conducted in randomized block design. The highest (0.93 Mg/ha) build up of very labile carbon (CVL) was recorded in 100% recommended dose of NPK + FYM @10 t/ha/year. The treatment 100% RDNPK + FYM @10 t/ha/year + Azotobacter + PSB showed regular build up of CVL (0.54 Mg/ha), non-labile carbon (CNL; 0.57 Mg/ha) and maintained labile carbon pool (CL). Both treatments were dominant contributors (54 and 58%) to active pool (ACT) over the passive pool (PSV) and provided the highest value of active: passive pool ratio (1.35 and 1.17), respectively, over sub-optimal dose, imbalanced fertility treatments and control. The change in less-labile carbon (CL) pool was not identical. Higher contribution to passive pool (PSV) in total organic carbon (TOC) was recorded for the control (80%), followed by imbalanced fertility treatments 100% N alone (70%) and 100% NP (71%). The continuous supply of optimal dose of N,P and K and/or biofertilizers in combination with FYM @10 t/ha/year, and super-optimal dose (150% RDNPK) treatments showed dissolution of carbonates (CaCO3) as soil inorganic carbon (SIC) and regular build up of TOC, crop biomass carbon (CBC) and TCSQ of system over the control, suboptimal dose and imbalanced fertility treatments. The combined use of optimal dose of chemical fertilizers, FYM @10 t/ha/year and biofertilizers significantly increased seed and straw yields of pearlmillet and mustard, respectively over other fertility treatments and control. The highly significant and positive correlation of total productivity was observed with pearlmillet-equivalent yield, TOC, CVL, CLL and ACT and negative with CNL, while moder-ately positive significant correlation was observed with CL and negative with PSV and SIC.