Agriculture Environmental Stewardship Act of 2021 This bill allows energy tax credits through 2023 for investments in (1) qualified biogas property, or (2) qualified manure resource recovery property. Qualified biogas property comprises a system that (1) uses anaerobic digesters or other specified processes to convert biomass into a gas which is at least 52% methane, and (2) captures the gas for use as a fuel. The term includes property that cleans and conditions the gas for use as a fuel. Qualified manure resource recovery property comprises a system that uses specified processes to recover the nutrients nitrogen and phosphorus from a non-treated digestate or animal manure by reducing or separating at least 50% of the nutrients, excluding any reductions during the incineration, storage, composting, or field application of the non-treated digestate or animal manure. The term also includes certain processing equipment. The Department of the Treasury must enter into an agreement with the National Renewable Energy Laboratory for a study of biogas and report to Congress on the study.