Soil enzymes (urease, invertase, acid and alkaline phosphatase) activity in the rhizosphere of field-grown tomato plants were used to monitor the impact of soil amendments (SA) and SA mixed with biochar on soil microbial activity four months after addition of amendments. The soil treatments were sewage sludge (SS); horse manure (HM); chicken manure (CM); vermicompost (worm castings); commercial inorganic fertilizer; commercial organic fertilizer; and no-mulch (NM) native soil used for comparison purposes. Soil treatments also were mixed with 10% (w/w) biochar to investigate the impact of biochar on soil enzymes activity. The results showed a significant increase in soil urease and invertase activities after incorporation of SA to native soil. Vermicompost and HM were superior in increasing urease and invertase activity four months after their addition to native soil. Alkaline phosphatase activity fluctuated among the soil treatments, revealing some obstruction of its activity. SS amended with biochar increased acid phosphatase activity by 115% four months after SS addition. Other than alkaline phosphatase, organic manure enhanced soil biological activity (microbial biomass and release of enzymes), indicating that the use of manures, rather than inorganic fertilizers, in crop production is an affordable and sustainable agricultural production system.

Many investigators have focused on the impact of fertilizers on crop yield and ignored fertilizers impact on the plants composition. The impact of seven types of soil treatments (sewage sludge, horse manure, chicken manure, vermicompost, elemental organic fertilizer, inorganic fertilizer, and native soil) and similar seven treatments amended with biochar on the concentrations of NH₃ and NO₃ in the roots and shoots of three commercial varieties of turnips, Brassica rapa was investigated. The three varieties (Purple Top White Globe PTWG, Scarlet Queen Red SQR, and Tokyo Cross TC) varied in concentrations of NH₃ and NO₃ levels. High levels of NO₃ in edible plants is associated with harmful effects on human health, due to the risk of creation of carcinogenic N-nitroso compounds. NO₃ in SQR roots and shoots (edible greens) was greater than varieties PTWG and TC. The concentration of NH₃ averaged 20.2, 12.8, and 8.9 µg g⁻¹ fresh turnip roots, whereas NO₃ values averaged 107.6, 64.1, and 62.9 µg g⁻¹ fresh turnip roots in varieties SQR, PTWG, and TC, respectively. Regardless of soil amendment type, the concentration of NH₃ in the shoots (44.0 µg g⁻¹) was greater than the roots (15 µg g⁻¹). On the contrary, NO₃ was higher in the roots (89.4 µg g⁻¹) compared to the shoots (67.6 µg g⁻¹ fresh tissue). Overall, biochar added to vermicompost amended soil increased NH₃ by 73% compared to vermicompost not amended with biochar. Regarding acceptable daily intake (ADI) for NO₃, none of the three varieties analyzed constitute any NO₃ adverse effects on normal human intake. Similarly, consuming turnips grown in any of the animal manures tested do not represent any hazardous issues.