Microbial Populations, Activity and Gene Abundance in Tropical Vertisols Under Intensive Chemical Farming

There are increasing concerns on the environmental impacts of intensive chemical agriculture. The effect of high agrochemical inputs used in intensive chemical farming was assessed on soil microbiological, molecular and biochemical properties in tropical Vertisols in India. Farm field sites under normal cultivation of arable crops using high inputs of fertilizers and pesticides in chili (Capsicum annum L., 5.0× dose for fertilizers and 1.5× dose for pesticides over normal inputs) and black gram (Vigna mungo L. Hepper, 2.2× dose for fertilizers and 2.3× dose for pesticides over normal inputs) were compared with adjacent sites using normal recommended doses. Organic carbon and basal respiration showed no response to high inputs of fertilizers and pesticides in soils of both crops. Labile carbon decreased by 10% in chili soils and increased by 24% in black gram soils under high input farming system. The proportion of soil labile carbon as a fraction of soil organic carbon was unaffected by high inputs. The labile carbon mineralization coefficient (qMLC) increased by 50.0% in chili soils, indicating that the soil microorganisms were under stress due to high agochemical inputs, whereas qMLC decreased by 36.4% in black gram soils. Copiotrophs increased due to high inputs in soils of both chili (63.1%) and black gram (47.1%). Oligotrophs increased by 10.8% in black gram soils but not in chili soils. The abundance of amoA gene reduced by 39.3% in chili soils due to high inputs and increased significantly by 110.8% in black gram soils. β-Glucosidase also increased by 27.2% and 325.0%, respectively. Acid phosphatase activity reduced by 29.2% due to high inputs in chili soils and increased by 105.0% in black gram soils. The use of high agrochemical inputs thus had adverse consequences on biological health in chili but not in black gram soils. In soils cultivated with black gram, the moderating effect of cultivating legumes and their beneficial effect on soil health were evident from the increase in soil labile carbon, lower qMLC, higher amoA gene and enzyme activities. Overall results showed that cultivation of legumes permits intensive chemical farming without deteriorating soil biological health.