Long-term effects of NPK fertilizers and organic manures on carbon stabilization and management index under rice-wheat cropping system

- A 15 years effects of NPK fertilizers and organic manures on soil organic carbon.
- Labile fractions of soil organic carbon were more sensitive to fertilizer managements.
- Type of organic manures influenced carbon stabilization in soil.
- Build-up of soil organic carbon under long-term cultivation of rice-wheat and integrated use of NPK and organic manures.
- Carbon management index was a good indicator to study the effect of managements on soil organic carbon.

The potential of storing carbon (C) in soil and offsetting the atmospheric CO2 depends on the management practices. This study investigated the long-term (15 years) effects of using nitrogen: phosphorous: potassium (NPK) fertilizers with and without organic manures namely farmyard manure (FYM), green manure (GM), and straw incorporation (SI) on soil C stabilization, and C management index (CMI) in intensive rice-wheat system in Typic Ustochrepts soil of a semiarid region in Punjab, India. The long-term effects of different fertilizer treatments as control, NPK, NPK + GM, NPK + SI, and NPK + FYM on soil organic carbon (SOC), labile fractions of SOC (LFSOC), C stabilization, C sequestration, and C management index (CMI) were determined. Prolonged application of NPK fertilizers alone or in combination with organic manures significantly decreased soil bulk density (from 7.3 to 16.6%) and increased total soil porosity of (from 11.7 to 26.2%) as compared to the control in surface soil (0-15 cm). Fertilizations had positive and variable effects on different LFSOC, namely water soluble C (WSC), labile carbon (LC), particulate organic C (POC), hot water soluble carbon (HWSC), and total organic carbon (TOC). Long-term intensive cultivation and fertilization significantly increased the SOC in surface soil (0-15 cm) up to 5.16 g kg−1 as compared with its antecedent value of 2.42 g kg−1 in 1999. The sensitivity index indicated that among LFSOC, LC (51-85%), WSC (67-131%), POC (64-159%), and HWSC (38-131%) were more sensitive compared with SOC (29-59%). NPK + SI (5.06 Mg C ha−1) or NPK + FYM (5.31 Mg C ha−1) along with NPK sequestered significantly higher C than the application of NPK (3.57 Mg C ha−1) or NPK + GM (4.10 Mg C ha−1). The fraction of C in stable fractions (Cfrac3 + Cfract4) increased from 7 to 29% with application of NPK fertilizers and organic manures compared with that of the control. The CMI indicated that the use of NPK + SI or NPK + FYM was statistically superior to the use of NPK + GM for improving the SOC status. The study concluded that a relatively high sensitivity index of LFSOC may be used as an early indicator for determining the potential of different management practices for improving SOC on short-term basis. The results indicated that the stability of SOC varies with the nature of the added organic manures, suggesting that the selection of organic manure is important for long-term C sequestration.