Straw amendment decreased Cd accumulation in Solanum lycopersicum due to enhanced root functional traits under low-P supply.

Manipulating root functional traits related to phosphorus (P) mobilization and acquisition by using the optimum rate of synthetic P fertilizer coupled with straw addition is a promising option for improving nutrient-use efficiency in agriculture. How such practices influence soil cadmium (Cd) fractions and plant Cd accumulation remains unknown. We conducted two field trials: exp. 1 with varied P fertilizer rates [control without P, reduced rates of P fertilization at 100 and 160 kg P ha as well as the standard P fertilization rate (200 kg P ha) used by farmers], and exp. 2 with reduced P fertilization at 100 and 160 kg P ha without or with straw addition (10 t ha) to investigate soil Cd fractionations and S. lycopersicum Cd uptake as influenced by root morphological and exudation traits related to P acquisition. In experiment 1, reduced P rates (100 and 160 kg P ha) increased the concentration of exchangeable Cd in soil by 6.4 %-77.1 %, which corresponded to a 12.6 %-18.4 % increase in Cd concentration in S. lycopersicum fruits. These reduced rates of P fertilization induced root proliferation and rhizosphere carboxylate exudation, increasing the relative proportion of exchangeable Cd in the soil solution and enhancing Cd uptake, especially from 30th to 45th day of S. lycopersicum growth. By contrast, the straw addition (exp 2) increased soil organic matter in soil by 7.19 %-15.8 % and decreased both rhizosphere carboxylate content by 6.47 %-35.5 % and soil exchangeable Cd content irrespective of P treatments. Consequently, with straw addition, fruit Cd content decreased by 26.5 % and 26.4 %, respectively, at 100 and 160 kg P ha. In summary, the P-responsive root functional traits influenced soil Cd fractionation (via carboxylate exudation) and mediated Cd accumulation (via root proliferation). Straw amendment diminished these P-responsive root traits, thus decreasing Cd accumulation by S. lycopersicum.