Long-term impact of fertilizers on soil organic carbon pools and sequestration rates in maize–wheat–cowpea cropping system

We assessed the impact of long-term manuring and fertilization on changes in different SOC fractions over ten years period (1994–2003) in a Typic Haplustept under intensive cropping with maize ( Zea mays L.) — wheat ( Triticum aestivum L.) — cowpea ( Vigna unguiculata) in semi-arid, sub-tropical India. The application of graded doses of NPK from 50% (130 kg N, 35 kg P and 41.5 kg K ha − 1 ) to 150% (390 kg N, 105 kg P and 124 kg K ha − 1 ) in the cropping system significantly enhanced SOC, particulate organic C (POC) and KMnO 4 oxidizable C (KMnO 4–C) fractions in soil. The increase in these C fractions was greater when farmyard manure (FYM) was applied conjointly with 100% NPK (260 kg N, 70 kg P and 83 kg K ha − 1 ). This treatment showed highest amount of SOC (58.3 Mg C ha − 1 in 1994 and 72.1 Mg C ha − 1 in 2003), POC (5.30 Mg C ha − 1 in 1994 and 6.33 Mg C ha − 1 in 2003) and KMnO 4-C (10.05 Mg C ha − 1 in 1994 and 11.2 Mg C ha − 1 in 2003) in 0–45 cm soil depth. The C sequestration rate in SOC calculated over ten year period (1994–2003) was highest with 100% NPK + FYM (997 kg C ha − 1 yr − 1 ) followed by the 150% NPK (553 kg C ha − 1 yr − 1 ). It was estimated that 17.1 to 34.0% of the gross C input over ten year period contributed towards the increase in SOC content, while C sequestration efficiency (CSE) in POC (varied between 1.28 and 2.58%) was lower than KMnO 4-C (varied between 1.42 and 3.72%). The CSE was highest in 150% NPK treatment, while 100% NPK + FYM showed the lowest CSE. By applying the values of humification constant ( h) and decay constant ( k) in Jenkinson's equation, it is possible to predict SOC level in the year 2003 and the C inputs required to maintain the SOC level in the year 1994 ( A E) were calculated from Jenkinson's equation. The low k value in native SOC was responsible for lower requirements of C input required to maintain SOC in equilibrium. Thus increase in SOC concentration under long-term maize–wheat–cowpea cropping was due to the fact that annual C input by the system was higher than A E. In semi-arid sub-tropical India, continuous adoption of 100% NPK + FYM treatment in maize–wheat–cowpea cropping system might sequester 1.83 Tg C yr − 1 which corresponds to about 1% of the fossil fuel emissions by India.