Soil health indicators as affected by diverse forage species and mixtures in semi-arid pastures

‘WW-B.Dahl’ Old World bluestem [OWB, Bothriochloa bladhii (Retz) S.T. Blake] is a persistent pasture grass in the semi-arid Texas High Plains. Some growers are transitioning their irrigated continuous cotton (Gossypium hirsutum L.) land to dryland or low-irrigation production of WW-B.Dahl owing to diminished water supply. This grass strongly deters soil-dwelling ants (Hymenoptera: Formicidae), presumably via release of unidentified phytochemical repellants in the soil. Our aim was to determine whether the inhibitory effects of OWB extend to soil microbial community structure and function in relation to other adapted forages. We compared soil chemical and microbial properties in a Pullman clay-loam soil for OWB relative to OWB-alfalfa (Medicago sativa L.), alfalfa, and native mixed-grass pastures at 0–5 cm and 10–15 cm depth in June and December of 2016. The pastures ranged in age from 7 to 17 yr. Most measurements of organic forms of C and N showed greatest amounts in OWB-alfalfa; however, not always significantly. Enzyme activities linked to C, N, and S transformations were greatest (P ≤ 0.03) with OWB-alfalfa. This trend was similar for alkaline phosphatase activity, involved in P cycling, but not always significantly. Soil organic C in OWB was 1.2–1.4 fold greater than in the native mix depending on soil depth and sampling date, whereas SOC in OWB-alfalfa was 1.4–1.6 fold greater than in the native mix. Enzyme activities and microbial biomass N (MBN) were likewise greater in OWB and OWB-alfalfa than in the native mix, likely because the latter received no irrigation or N inputs. The microbial community structure demonstrated greater total FAMEs, bacterial markers for Gram+, Gram−, and actinomycetes in OWB-alfalfa compared to OWB, alfalfa, and native mix. Similar results were found in fungal markers for arbuscular mycorrhizal fungi (AMF) and saprophytic fungi. Protozoan indicators were found only in December, of which the native mix had the highest levels in the 0–5 cm depth. In general, selected soil properties, soil microbial biomass, community and enzyme activities were greater in December than in June except AMF, which was greater in June. The stimulating effect of OWB growing with alfalfa on the soil microbial community suggested this forage combination as favorable for soil health in relation to OWB, alfalfa alone, and native mixed-grass pastures. The previously reported inhibitory effect of OWB on soil-dwelling ants did not carry over to a depressing effect on the soil microbial component.