4.1. Physical and chemical properties of soils
Sugarcane mono-sequencing over the years degraded agro-ecosystem due to alteration of physical, chemical and biological properties of soil, and destroy self-adjustment capability of the ecosystem (Lin et al., 2012). Magarey (1996); Pankhurst et al., (2003) and Lin et al. (2012) revealed that several factors like as imbalance fertilization and excessive use of agrochemicals, tillage, mechanical harvesting and haul-out with heavy machinery are accountable for the degrading physical, chemical and biological attributes of soil under long-term sugarcane monoculture. The MC, soil pH, SOC, and micro-nutrients (Zn, Cu, Fe and Mn) were higher in EZ soils than CZ and WZ due to low-lying areas which hold more moisture content. Quietly higher soil pH in EZ could be related to accumulate more basic cations in upper surface of soil (Jenny, 1941). The SOC was increased by 14.1 and 11.8% in EZ soils than CZ and WZ, respectively due to more organic matter added through newly developed roots in low lying areas. Hidaka et al. (2007) also suggested that newly developed cane roots shared more in dry matter accumulation under flooding condition. Reversely, decreased SOC in WZ and CZ soils than EZ was associated with excess management practices followed in soil preparation (chiseling, plowing and harrowing) resulting exposed soil micro-aggregates favours C mineralization, and increasing CO2 emissions (Moscatelli et al., 2007). Naranjo et al. (2006) supported our results as 17% decline in total soil carbon over the 30 years of sugarcane monoculture compared to natural land uses (Naranjo et al., 2006).
The low Na, Pa, Ka and Sa were in EZ soil might be due to lower doses of mineral fertilizers were applied even though recommended dose of mineral fertilizers (150, 60 and 60 kg N, P2O5and K2O ha-1). It mismatches between nutrient applied and removal (205, 53 and 280 kg N, P and K for the production of 100 Mg cane yield ha-1) over the decades caused nutrient imbalances (Shukla et al., 2017). Moreover, low lying area led to decrease Na through leaching and denitrification. Similarly, Pa also decreased in EZ soil through surface overflow and greater soil pH. Hart, Quin, & Nguyen, (2004) reported P loss via runoff, which ranged between 0.7 and 42% of the fertilizer applied. Hence, low Pa in EZ soil related to higher soil pH and moisture content as significant negative correlation existed among them (Table 3S). The lowest Sain EZ due to gradual removal from the soil as leaching, overflow and through plant removal, but it could not applied as mineral fertilizers regularly (Shukla & Lal, 2004). The low-lying areas that waterlogged during monsoon season caused favorable condition to reduce Cu+, Fe2+ and Mn+resulting to increase its availability in EZ sols. Contrary to that, decrease Zn, Cu, Fe and Mn availability in WZ soil due to greater redox potential liable to converted Cu+ to Cu+2, Fe+2 to Fe+3and Mn+2 to Mn+4 (Pavanasasivam & Axley, 1980). Further, harvesting more cane yield in WZ zone thereby reduced its availability due to its greater mining through crop removal.