Long-term manuring and fertilization effects on soil organic carbon pools in a Typic Haplustept of semi-arid sub-tropical India

Soil is a potential C sink and could offset rising atmospheric CO 2. The capacity of soils to store and sequester C will depend on the rate of C inputs from plant productivity relative to C exports controlled by microbial decomposition. Management practices, such as no-tillage and high intensity cropping sequences, have the potential to enhance C and N sequestration in agricultural soils. An investigation was carried out to study the influence of long-term applications of fertilizers and manures on different organic C fractions in a Typic Haplustept under intensive sequence of cropping with maize–wheat–cowpea in a semi-arid sub-tropic of India. In 0–15 cm, the bulk density was lowest (1.52 Mg m −3) in plots treated with 100% NPK + FYM, while the control treatment showed the highest value (1.67 Mg m −3). Balanced application of NPK (100% NPK) showed significantly lower bulk density (1.56 Mg m −3) over either 100% N (1.67 Mg m −3) or 100% NP (1.61 Mg m −3) in surface soils. The application of super-optimal dose of NPK (150% NPK) showed higher total organic C (TOC) (12.9 g C kg −1) over either 50% NPK (9.3 g C kg −1) or 100% NPK (10.0 g C kg −1) in 0–15 cm soil layer. There was an improvement in TOC in 100% NPK or 100% NP (9.3 g C kg −1) over 100% N (8.7 g C kg −1) in the same depth. The application of FYM with 100% NPK showed 15.2, 9.9 and 5.2 g C kg −1 in 0–15, 15–30 and 30–45 cm, respectively. Application of graded doses of NPK from 50 to 150% of recommendation NPK significantly enhanced other organic C fractions like, microbial biomass C (MBC), particulate organic C (POC) and KMnO 4 oxidizable C (KMnO 4–C) in all the three soil depths. The TOC in 0–45 cm soil depth in 150% NPK (63.5 Mg C ha −1) was increased by 39% over that in 50% NPK treatment (51.5 Mg C ha −1) and 29% over that in 100% NPK treatment (54.1 Mg C ha −1). Integrated use of farmyard manure with 100% NPK (100% NPK + FYM) emerged as the most efficient management system in accumulating largest amount of organic C (72.1 Mg C ha −1) in soil. Nevertheless, this treatment also sequestered highest amount of organic C (731 kg C ha −1 year −1). Particulate organic carbon, a physically protected carbon pool in soil, could well be protected in sub-surface soil layers than in surface soil layer as a means of carbon aggradations. Microbial metabolic quotient (qCO 2) was significantly lower in 100% NPK + FYM over other treatments to indicate this to be the most efficient manuring practice to preserve orga