Catalyzes the hydrolysis of L-arginine to urea and L-ornithine. The latter can be utilized in the urea cycle or as a precursor for the synthesis of both polyamines and proline (By similarity). Possesses agmatinase activity. Catalyzes the formation of putrescine from agmatine (PubMed:28716421). H2O + L-arginine = L-ornithine + urea agmatine + H2O = putrescine + urea Binds 2 manganese ions per subunit. Nitrogen metabolism; urea cycle; L-ornithine and urea from L-arginine: step 1/1. Amine and polyamine biosynthesis; putrescine biosynthesis via agmatine pathway; putrescine from agmatine: step 1/1. A number of isoforms are produced. According to EST sequences. Expressed in vasculature of roots, root tips, leaves and cotyledons. By methyl jasmonate and infection with the fungal pathogen B.cinerea. Increased formation of lateral and adventitious roots and increased production of NO in roots. Plants over-expressing ARGAH2 have decreased susceptibility to the fungal pathogen B.cinerea. Belongs to the arginase family.