Pak. J. Bot., 46(6): 2157-2162, 2014. IMPACT OF SOLE CROPPING AND MULTIPLE CROPPING ON SOIL HUMIFIED CARBON FRACTIONS IRFAN AZIZ 1 , TARIQ MAHMOOD 2* AND KHANDAKAR RAFIQ ISLAM 3 1 Department of Agronomy, PAMS Arid Agriculture University, Rawalpindi, Pakistan, 2 Department of Environmental Sciences, PMS Arid Agriculture University, Rawalpindi, Pakistan, 3 Soil and Water Resources, Ohio State University South Centers, Piketon, Ohio, USA, and Soil Drainage Research, USDA-ARS, Columbus, Ohio, USA * Corresponding author e-mail: tamahmood@hec.gov.pk Abstract The present study was planned to improve our understanding how crop rotation can enhance humified C fractions. A long term experiment was conducted on Vanmeter farm of the Ohio State University South Centers at Piketon Ohio, USA from 2002 to 2007. Crop rotation treatments included were continuous corn (CC), corn-soybean (CS) and corn–soybean- wheat-cowpea (CSW) rotations. Randomized complete block design with 6 replications was used under natural field conditions. The findings of this long-term study revealed that multiple cropping had significantly improved humified carbon fractions compared to mono-cropping system. Although total humified carbon (THOC), sugar free humified carbon (HOC) concentration were non-significant however, humin (NH) contents, humic (HA), fulvic acids (FA), humic and fulvic acid associated glucose (HA-NH and FA-NH) were significantly affected by various crop rotations within five years. The soil under CC had 22-52% significantly greater NH concentration than CSW and CS rotations respectively. Similarly all crop rotations had shown 5–16 increase in HA and 5-17% decreased in FA over time. Likewise soil under CC had 16 and 54% greater HA-NH concentration as compared to CSW and CS rotations. The FA-NH concentration increased significantly by 27– 51% in soil under all treatments over time. The soil under CSW had greater HA/FA (1.6) fallowed by CC (1.4) and CS (1.1). Soils under CSW had significantly greater HA/HOC (12–18%) as compare to CC and CS respectively. Conversely, the value of FA/HOC decreased (1–23%) in soil under all crop rotation treatments within five years. Degree of humification (DH) had shown a significant increase (7–12%) in soil under all treatments as compared to 2002. Irrespective of crop rotation THOC, HOC, NH, humin, HA, HR and FA/HOC concentration decreased significantly with increase in soil depth. While fulvic acid concentration HA/HOC in all crop rotation increased with increase in soil depth. The effect of crop rotation on humified C fractions could be because of variations in type, amount and quality of C returned by different plants into the soil. So replacing mono-cropping with multiple cropping can enhance humified C fractions and can improve soil functional properties. Key words: Sole Cropping and Multiple Cropping, Total Humified Carbon, Humin, Humic Acid, Fulvic acid Introduction Humus constitutes the bulk of agricultural soil organic matter (Stevenson, 1982) and plays a significant role in nutrients recycling of soil ecosystems, regulate carbon cycling and interact with inorganic components and pesticides (Tan 1994; Lal 2004). As soil organic matter is thermodynamically unstable with respect to CO 2 , it is part of balance among the natural processes of primary production, decomposition and transformation which largely contribute to the formation of humified C fractions (e.g. humic, fulvic, and humin) in soil (Wander & Traina 1996). Moreover, the quantity and quality of humus is as well affected by cultural operations (Slepetiene, 2001; Gonzalez et al., 2003). Several researchers investigated that crop rotation has probably influence on quality of soil organic matter (Wood & Edwards, 1992). The use of traditional plowing and sole- cropping has resulted in reduction OM contents, which ultimately resulted in decline soil quality (Wander et al., 1994). Wood & Edwards, (1992) also reported that changes in soil C quality were more in continuous corn as compared to corn-soybean rotation due to greater C sequestration than continuous soybean. Therefore, replacing mono-cropping with multiple cropping and less-disruptive systems like no-till could enhance humic C fractions and improve soil functional properties. Higher fulvic acid contents were reported in soil having low HA/FA ratio probably due to management strategies. Moreover, wheat-wheat had favorable environment for humification process than wheat- grass as result of variations in bulk densities (Andriulo et al., 1990) might be the result of decreased HA: FA in former. Conversely, the greater stable fraction of HS (humic substances), humin, was reduced under wheat- grass, most likely due to variations in physical conditions of by conservation tillage. Doane et al. (2003) reported that HS pools have significant turnover of C, verify the idea that each humic fraction possibly have unique role in C cycling based on substrate and environments. The variation might be recognized to soil physical properties where humification occurs. Greater bulk density and variations in porosity of grazed lands may decrease microbial activity due to less space for microbes (Andriulo et al., 1990; Iglesias et al., 1996). It is believed that functionality of humic substances is because of their physical structure and chemical composition (Hayes, 1989). There are continues processes undergoing in SOM by soil biota and chemical reactions and during humification process labile pools are converted into recalcitrant fractions (Zech et al., 1997). The remainder is a non-humic discrete material composed of mainly of polysaccharide and protein-like structures (Flaig et al., 1975).