Long-term fertilization, manure and liming effects on soil organic matter and crop yields

Yield decline or stagnation and its relationship with soil organic matter fractions in soybean (Glycine max L.)-wheat (Triticum aestivum L.) cropping system under long-term fertilizer use are not well understood. To understand this phenomenon, soil organic matter fractions and soil aggregate size distribution were studied in an Alfisol (Typic Haplustalf) at a long-term experiment at Birsa Agricultural University, Ranchi, India. For 30 years, the following fertilizer treatments were compared with undisturbed fallow plots (without crop and fertilizer management): unfertilized (control), 100% recommended rate of N, NP, NPK, NPK+ farmyard manure (FYM) and NPK + lime. Yield declined with time for soybean in control (30 kg ha(-1) yr(-1)) and NP (21 kg ha(-1) yr(-1)) treatments and for wheat in control (46 kg ha(-1) yr(-1)) and N (25 kg ha(-1) yr(-1)) treatments. However, yield increased with time for NPK + FYM and NPK + lime treatments in wheat. At a depth of 0-15 cm, small macroaggregates (0.25-2 mm) dominated soil (4361 %) followed by microaggregates (0.053-0.25 mm) with 13-28%. Soil microbial biomass carbon (SMBC), nitrogen (SMBN) and acid hydrolysable carbohydrates (HCH) were greater in NPK + FYM and NPK + lime as compared to other treatments. With three decades of cultivation, C and N mineralization were greater in microaggregates than in small macroaggregates and relatively resistant mineral associated organic matter (silt + clay fraction). Particulate organic carbon (POC) and nitrogen (PON) decreased significantly in control, N and NP application over fallow. Results suggest that continuous use of NPK + FYM or NPK + lime would sustain yield in a soybean-wheat system without deteriorating soil quality. (C) 2006 Published by Elsevier B.V.