Yield decline or stagnation under long-term cultivation and its relationship with soil organic matter fractions are rarely considered. To understand this phenomenon, soil organic matter fractions and soil aggregate size distribution were studied in a long-term experiment at Akola, in a Vertisol in a semiarid tropical environment. For 14 years, the following fertiliser treatments were compared with undisturbed fallow plots: unfertilised (control), 100% recommended rates of N, NP, NPK (N : P : K ratios of 100 : 21.8 : 18.2 and 120 : 26.2 : 50 kg/ha for sorghum and wheat, respectively) and 100% NPK plus farmyard manure (FYM) and continuous cropping with a sorghum (Sorghum bicolor L. Moench) and wheat (Triticum aestivum L.) system during 1988–2001. The significant negative yield trend was observed in unbalanced use of inorganic N application for both crops. However, yields were maintained when NPK and NPK + FYM were applied. Results showed that soil organic C and total N in the unfertilised plot decreased by 21.7 and 18.2%, compared to the initial value, at a depth of 0–15 cm. Depletion of large macroaggregates (>2 mm) accounted for 22–81% of the total mass of aggregates in N, NP and unfertilised control plots compared to fallow plots. Irrespective of treatments, small macroaggregates (0.25–2 mm) dominated aggregate size distribution (56–71%), followed by microaggregates (0.053–0.25 mm, 18–37%). Active fractions, such as microbial biomass C, microbial biomass N, hot water soluble C and N, and acid hydrolysable carbohydrates were greater in NPK and NPK + FYM treatments than in the control. Carbon and N mineralisation were greater in small macroaggregates than microaggregates. Particulate organic matter C (POMC) and N (POMN) were significantly correlated (P < 0.01) with water-stable aggregate C and N (0.25–2 mm size classes), respectively. It was further observed that POMC and POMN were significantly greater in NPK and NPK + FYM plots than N and NP treated plots. Microbial biomass C was positively correlated with acid-hydrolysable carbohydrates (r = 0.79, P < 0.05). Continuous cropping and fertiliser use also influenced humic acid C and fulvic acid C fractions of the soil organic matter. Acid-hydrolysable N proportion in humic acid was greater than fulvic acid and it was greatest in NPK + FYM treatments. Continuous application of 100% NPK + FYM could restore soil organic carbon (SOC) to a new equilibrium level much earlier (t = 1/k, 2.4 years) than N (t = 1/k, 25.7 years), NP (t = 1/k, 8.1 years) and NPK (t = 1/k, 5.02 years). In conclusion, integrated use of NPK with FYM would be vital to obtain sustainable yields without deteriorating soil quality.
Soil organic matter assessments in a long-term cropping system study