Effect of Rhizobium inoculation and phosphorus application on native Texas legumes grown in local soil

Greenhouse experiments were conducted to determine the effects of Rhizobium inoculation and addition of phosphorus to soil on the productivity and quality of the Texas range legumes Desmanthus illinoiensis (Michx.) MacMill. ex B.L. Rob. & Fernald, Desmanthus velutinus Scheele, Desmanthus virgatus (L.) Willd., and Neptunia lutea (Leavenw.) Benth. Phosphorus (P) fertilization was applied at 0, 40, 80, and 120 kg P ha-1 in Experiment 1 with half the pots also receiving D522-1/2 (a Desmanthus isolate). Experiment 2 consisted only the Desmanthus species and tested four different inoculant treatments (uninoculated control, inoculated with commercial cowpea Rhizobia, D522-1/2 from Desmanthus isolates, and L1145, a Leucaena isolate). Two soil treatments were used in Experiment 2, gamma radiation sterilized and unsterilized soil, to determine whether introduced Rhizobia were as effective as native populations. Shoot dry matter (DM) yield, root DM weight, and shoot nitrogen (N) and P concentration increased with increasing P application levels up to 80 kg P ha-1. Rhizobium strain D522-1/2 was an effective inoculant for all legumes when applied in combination with P fertilizer (Experiment 1). In sterilized P-deficient soil (Experiment 2), inoculation improved the parameters measured. In unsterilized soil, however, only D522-1/2 and L1145 improved shoot nitrogen concentration while the cowpea Rhizobium strain was detrimental in the case of D. velutinus. The low concentration of available soil P and the scarcity of active Rhizobium in the soil might be the cause of the slow establishment of native legume species in disturbed soils without native Rhizobia populations.