Carbon sequestration and soil carbon pools in a rice–wheat cropping system: Effect of long-term use of inorganic fertilizers and organic manure

Agricultural soil is a potential sink for atmospheric carbon as soil organic carbon. The carbon sequestration is affected by cropping system and management practices adopted. Rice–wheat is a dominant cropping system in the Indo-Gangetic plains. Previous studies done by different research workers revealed both its positive as well as negative impacts on carbon sequestration. The objective of this study was to determine C sequestration after nine year's rice (Oryza sativa L.)–wheat (Triticum aestivum L.) cropping under an ongoing experiment at Punjab Agricultural University, Ludhiana, Punjab (India). This study was based on five treatments (100%N, 100%NP, 100%NPK, 100%NPK + FYM and the control). In the surface soil layer (0–15 cm), soil organic carbon (SOC) increased from the initial status of 2.42 to 3.26 g kg−1 in the control, which significantly increased with the application of 100%NPK (4.11 g kg−1) and 100%NPK + FYM (4.55 g kg−1). The rice–wheat cropping even without any fertilization (control) contributed toward carbon sequestration (1.94 Mg C ha−1) with soil organic carbon pools and carbon sequestration rate of 7.84 Mg C ha−1 and 0.22 Mg C ha−1 yr−1, respectively. The soil organic carbon pools, carbon sequestration and rate of carbon sequestration as observed in treatment of balanced fertilization (100%NPK) were significantly increased from 9.19 to 9.99 Mg C ha−1, 3.30 to 4.10 Mg C ha−1 and 0.37 to 0.46 Mg C ha−1 yr−1, respectively when farmyard manure was applied in conjunction with 100%NPK. The application of 100%NPK and 100%NPK + FYM significantly increased the soil labile carbon (1378 and 1578 mg kg−1, respectively), water soluble carbon (35.3 and 37.2 mg kg−1, respectively) and water soluble carbohydrates (526 and 538 mg kg−1, respectively) as compared to the control, where the corresponding values were 898, 16.8 and 464 mg kg−1. The content of water stable aggregates organic carbon also increased with fertilization especially in combination with farmyard manure, whereas bulk density of soil was significantly reduced in the treatment of 100%NPK (1.49 Mg m−3) and 100%NPK + FYM (1.46 Mg m−3) over the control (1.60 Mg m−3). The fertilizer treatments (100%N, 100%NP and 100%NPK) made a positive influence on soil organic carbon content in subsurface layers (15–60 cm) also and it was more so in the treatment of 100%NPK + FYM as compared to the control, although contents did not differ significantly. Balanced fertilization (100%NPK) with and without FYM significantly improved the labile C content of soil (up to depth of 60 cm) over the control. Balanced fertilization in combination with FYM significantly increased the water soluble carbon content of soil in comparison to the control (up to depth of 60 cm). Bulk density of sub surface soil (15–60 cm) was reduced in all the treatments as compared to the control although the treatment effect was non-significant. The rice–wheat cropping sequence thus, showed the potential of mitigating atmospheric carbon load through its sequestration and integrated nutrient management may further enhance this potential.

► Nine years changes in SOC pools and its sequestration in soil under rice–wheat.
► Rice–wheat cropping without fertilization contributed toward C sequestration.
► This effect was further accelerated with the application of fertilizer.
► Effect was highest in balanced use of chemical fertilizers with and without manure.
► The beneficial effect of NPK + FYM was more pronounced in the surface layer of soil.