Assessment of carbon carrying capacities of Alfisols and Vertisols under long-term manuring and fertilization

Carbon (C) sequestration in soil is limited by the soil's maximum C carrying capacity (C-m) and C saturation deficit (S-d). These two parameters have enormous implication for choosing appropriate management strategies to increase a soil's C sequestration potential (CSP). The present study aims to estimate the C-m and S-d of two Alfisols from Bhubaneswar (Alfisol-BHNS) and Pattambi (Alfisol-PTMB) and two Vertisols from Parbhani (Vertisol-PRBN) and Junagadh (Vertisol-JNGD) in India with a history of long-term manuring and fertilization practices. Both the Alfisols maintained a rice-rice cropping system whereas the Vertisols had a soybean-safflower and groundnut-wheat cropping system, respectively. Irrespective of soil types and cropping systems, the soil organic C (SOC) content and stock were the highest in the treatment receiving 100% of recommended dose of NPK combined with 10 Mg ha(-1) well decomposed farmyard manure (100%NPK+FYM). The mean annual estimated return of C (MAERC) across the soil types followed the order 100%NPK + FYM> 150%NPK = 100%NPK +Lime (Agricultural grade, purity 80%)> 100%NPK > 50%NPK > control. The C-m at 0-30 cm soil depth was 39.3, 21.7, 47.6 and 25.0 Mg ha(-1), respectively for Alfisol-PTMB, Alfisol-BHNS, Vertisol-JNGD and Vertisol-PRBN. The S-d value was the highest in the control treatment and lowest in100% NPK + FYM treatment, signifying a higher CSP of the former soil than the later. Therefore, 100%NPK +FYM treatment would require a minimum threshold level of C addition to maintain the SOC at equilibrium. The estimated minimum threshold limit of annual C input (A(E)) to preserve the existing SOC level was 1.84, 0.39, 1.57 and 0.90 Mg ha(-1) yr(-1) for Alfisol-PTMB, Alfisol-BHNS, Vertisol-JNGD and Vertisol-PRBN, respectively. Alfisol-PTMB and Vertisol-JNGD showed a higher CSP than Alfisol-BHNS and Vertisol- JNGD because the former two soils had higher S-d values than the later soils. The MAERC, climate, soil texture and clay mineralogy largely influenced the SOC sequestration and C-m values. An integrated application of 100%NPK with either lime or FYM is recommended to maintain the sequestration and stabilization of SOC in the studied soils. The CSP of the soils could be further increased via innovative approaches such as increasing soil's reactive mineral contents to lock additional C.