Japanese miso made with barley koji, earthy and mellow.
Substitutes
Doenjang (Korean Soybean Paste) — 1:1 (doenjang is saltier and more pungent, reduce by 10%). Korean doenjang and Japanese barley miso (mugi miso) are both fermented soybean pastes with deep umami. Per 'The Oxford Companion to Food' (Davidson), mugi miso uses barley koji for fermentation giving a milder, sweeter character, while doenjang uses meju (soybean blocks) for a more intense, pungent flavor. Both are rich in glutamic acid. Doenjang works well in soups and stews where barley miso's subtlety would be lost in other flavors.
Miso — 1:1 (taste and adjust saltiness). Any miso substitutes for barley miso (mugi miso), which has a slightly nuttier flavor from barley koji. Miso fermentation by aspergillus oryzae produces glutamic acid at 200-700mg/100g, ensuring consistent umami across varieties [McGee]. Standard miso is very close.
Red Miso — 1:1 (reduce added salt by 1/4 tsp per tbsp). Red miso has similar intensity to barley miso but is slightly saltier due to longer fermentation — typically 1-3 years vs mugi miso's shorter cycle. Both develop umami through aspergillus oryzae converting proteins into free glutamic acid [McGee]. Reduce other salt.