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 KMnO4 oxidizable C (KMnO4–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 KMnO4-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 KMnO4-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 (AE) 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 AE. 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.