Biochar-phytagel hydrogel enhances growth, nutrient uptake, and drought resilience of lettuce microgreens

Abstract

Soil-free cultivation systems require substrates that can provide both moisture and nutrients to sustain plant growth. Biochar, a carbon-rich product of biomass pyrolysis, is well known for improving soil water retention and nutrient availability, yet its role in hydrogel-based substrates remains under-explored. In this study, corn-derived biochar (1–10 % w/w) and biochars from cocoa husk, nutshell, bamboo, and poultry litter (5 % w/w each) were incorporated into phytagel hydrogels for lettuce (Lactuca sativa) microgreens. Germination rates were comparable across treatments (72–74 %), although emergence was modestly delayed in biochar composites. At 70 % relative humidity (RH), biochar amendments enhanced shoot elongation (up to 12.2 ± 0.3 cm vs. 9.8 ± 0.4 cm in the control), root growth (6.0 ± 0.4 cm vs. 4.0 ± 0.4 cm), and fresh biomass (2.4 ± 0.03 g vs. 1.6 ± 0.1 g). Under reduced humidity (50 % and 30 % RH), biochar-phytagel substrates buffered moisture loss, preserved chlorophyll contents, and maintained leaf water above 90 %, whereas the control fell below 88 %. Nutrient profiling further revealed significant increases in phosphorus, potassium, calcium, magnesium, and micronutrients (Fe, Zn, Cu, Mn, etc), particularly in 5–10 wt % corn and poultry litter biochars. These results demonstrate that biochar-hydrogel composites enhance both the resilience and nutritional quality of microgreens under moisture limitation, offering promising substrates for sustainable controlled environment agriculture.