Bioinputs with phosphorus fertilization atered sugarcane yield but not organic carbon after four years

Bioinputs are often proposed as tools to build soil organic carbon (SOC), but SOC can respond slowly in sugarcane systems. We tested two bioinputs, a biological conditioner (BC) and phosphate solubilizing bacteria (PSB), applied annually with P fertilization for four consecutive years at three sites (one clayey and two sandy soils). Treatments included control; P; P + BC and P + PSB in randomized complete blocks with five blocks per site. We measured SOC at 0 to 10 cm by colorimetry method and cane yield as tons of cane per hectare (TCH). We evaluated planned contrasts at α = 0.05. Across sites, SOC did not differ among treatments in any cycle. SOC means remained close among treatments within each cycle and ranged from 6.77 to 7.44 g kg^-1 (cane plant), 7.07 to 7.36 g kg^-1 (first ratoon), 7.95 to 8.44 g kg^-1 (second ratoon), and 7.77 to 8.48 g kg^-1 (third ratoon). Yield responses occurred without SOC gains. P increased TCH in cane plant by 13.2 t ha^-1 relative to the control and in third ratoon by 4.0 t ha^-1. BC increased TCH in second ratoon by 6.4 t ha^-1 relative to P. In third ratoon, BC and PSB reduced TCH relative to P by 8.1 and 7.3 t ha^-1. Colorimetry method may underestimate small SOC differences relative to dry combustion. Overall,  four years of bioinputs did not increase SOC at 0 - 10 cm, which matches the short evaluation window and method limitations.