Sewage Sludge and Inorganic Chemical Fertilization Modulate Rhizosphere Microbial Activity and Diversity in Kale and Spinach Cropping Systems

Abstract

Sewage sludge is widely explored as an organic nutrient source, but its ecological effects on soil microbial communities remain context dependent. This study evaluated the effects of sewage sludge and NPK fertilizers on soil microbial populations associated with kale (Brassica oleracea L. cv. ‘GM Dari’) and spinach (Spinacia oleracea L. cv. ‘Shalimar Green’). A two-season field experiment (rabi and kharif) was conducted at the Experimental Farm, Wadura Campus, Sher-e-Kashmir University of Agricultural Sciences and Technology of Kashmir, using a randomized complete block design with nine treatment combinations and three replications. Results showed that sewage sludge amendments significantly increased the abundance and diversity of soil microorganisms compared to control and solely chemical fertilizer treatments. Rhizospheric soils of kale and spinach contained Flavobacterium, Bacillus, and Alcaligenes species, while fungal genera such as Aspergillus, Trichoderma, Mucor, and Paecilomyces were predominant in sludge-amended soils. The highest bacterial counts (39.30 × 10x CFU g-1 for kale and 39.25 × 10x CFU g-1 for spinach) and fungal counts (16.70 × 10t CFU g-1 for kale and 16.39 × 10t CFU g-1 for spinach) were recorded under 100% sewage sludge, followed closely by the combined treatment of 75% sludge + 25% NPK, whereas the control plots exhibited the lowest microbial abundance. The findings demonstrate that sewage sludge effectively enhances soil microbial activity and diversity, improving soil biological fertility. Partial substitution of mineral fertilizers with sludge (75% sludge + 25% NPK) appears optimal for maintaining microbial health and supporting sustainable vegetable production in the Kashmir Valley.