Complex sentence: He is evaluating the effects of growth stage, wounding, method of auxin delivery, and auxin concentration on the rooting of softwood stem cuttings, in addition to the effects of nitrogen form and solution pH on the growth of A. spicata plants grown from seed.
Split into simpler sentences: He is evaluating the effects of growth stage, wounding, method of auxin delivery, and auxin concentration on the rooting of softwood stem cuttings. In addition, he is investigating the effects of nitrogen form and solution pH on the growth of "A. spicata" plants grown from seed.