Biological indicators of soil quality in a long-term rice-wheat system on the Indo-Gangetic plain: combined effect of tillagewater- nutrient management

The assessment of soil quality is essential to track changes in soils as a result of management practices. Although a range of soil physical and chemical properties have been used internationally to track change in soil quality, work to evaluate a range of soil biological indicators as a means of monitoring soil quality has been more limited. In order to identify key biological indicators of soil quality, a long-term field trial at the Indian Agricultural Research Institute, New Delhi, was conducted which included tillage (main plots), water (subplots) and nutrient (sub-subplots) treatments for both rice and wheat. Here the combined influence of tillage, water and nutrient management after eight cropping cycles of rice-wheat, on selected soil microbial properties, was assessed. Results showed that non-puddling significantly enhanced dehydrogenase activity (5%), microbial biomass carbon (3%) and potentially mineralizable nitrogen (5%) over puddling, whereas the latter treatment hugely benefited soil respiration (48%) and metabolic quotient (41%) in rice. No-tillage resulted in higher values of soil biological indicators under wheat cultivation. Partial substitution of fertilizer N by farmyard manure, sewage sludge and a combination of (FYM + biofertilizer + crop residues/green manure) increased indicators at higher magnitudes, like dehydrogenase activity (36%), microbial biomass carbon (33%) and potentially mineralizable nitrogen (57%), but reduced the metabolic quotient which implied an accumulation of stable organic C under organic nutrient management uniformly after both the crops. The drainage of irrigation water in rice also increased dehydrogenase activity and microbial biomass carbon. In contrast more frequent (five times and three times) irrigations in wheat significantly increased these indicators. Principal component analysis revealed that both microbial metabolic quotient and dehydrogenase activity were the most promising indicators of soil biological quality in the present experimental setup.