The response of legumes to inoculation with rhizobia can be affected by many factors. Little work has been undertaken to examine how indigenous populations of rhizobia affect this response. We conducted a series of inoculation trials in four Hawaiian soils with six legume species (Glycine max, Vigna unguiculata, Phaseolus lunatus, Leucaena leucocephala, Arachis hypogaea, and Phaseolus vulgaris) and characterized the native rhizobial populations for each species in terms of the number and effectiveness of the population for a particular host. Inoculated plants had, on average, 76% of the nodules formed by the inoculum strain, which effectively eliminated competition from native strains as a variable between soils. Rhizobia populations ranged from less than 6 x 100/g of soil to 1 x 104/g of soil. The concentration of nitrogen in shoots of inoculated plants was not higher than that in uninoculated controls when the most probable number MPN counts of rhizobia were at or above 2 x 10'/g of soil unless the native population was completely ineffective. Tests of random isolates from nodules of uninoculated plants revealed that within most soil populations there was a wide range of effectiveness for N2 fixation. All populations had isolates that were ineffective in fixing N2. The inoculum strains generally did not fix more N2 than the average isolate from the soil population in single-isolate tests. Even when the inoculum strain proved to be a better symbiont than the soil rhizobia, there was no response to inoculation. Enhanced N2 fixation after inoculation was related to increased nodule dry weights. Although inoculation generally increased nodule number when there were less than 1 x 102 rhizobia per g of soil, there was no corresponding increase in nodule dry weight when native populations were effective. Most species compensated for reduced nodulation in soils with few rhizobia by increasing the size of nodules and therefore maintaining a nodule dry weight similar to that of inoculated plants with more nodules. Even when competition by native soil strains was overcome with a selected inoculum strain, it was not always possible to enhance N2 fixation when soil populations were above a threshold number and had some effective strains.

We examined the relationship between P nutrition of soybean [Glycine max (L.) Merr. cv. ‘Davis’] and N2 fixation by five strains of Rhizobium japonicum ranging from an ineffective (SM‐5) to a highly effective (USDA 110) symbiosis with soybean. Phosphorus at 0, 50, 125, and 400 mg P kg−1 soil (P0, P50, P125, P400) as Ca(H2PO4)2 · H2O was applied to pots containing 2.7 kg of a P‐fixing humoxic tropohumult free of R. japonicum. Treatments consisting of + N and five strains of R. japonicum and the four P treatments were arranged in a complete factorial replicated three times. With 400 mg P kg−1 soil treatments ranked + N > USDA 110 > USDA 31> USDA 123> USDA 33>SM‐5 for shoot dry weight and accumulation of N in the shoot. There were no significant strain effects for shoot dry weight, shoot N or nodule activity at Po. Strain rankings were the same at the intermediate levels of P but differences were smaller. Although nodule number was somewhat enhanced by P, it bore no relationship to strain effectiveness and was not a factor limiting N2 fixation at low P. Nodule dry weight and nitrogenase activities significantly increased with P additions. The concentrations of N and P in the shoot also increased with P additions. The concentrations of shoot P and N were higher in plants provided mineral N at the low and intermediate P levels than in the inoculated plants. The results show that P nutrition is important in interpreting the N2 fixation capability of strains of Rhizobium. Synergism between P and Rhizobium inputs requires that to obtain maximal response to the application of one input necessitates employing the other at maximum levels.

In 2013 and 2014, different mulching systems were investigated for their potential to suppress above and below ground pests, and enhance beneficial arthropods and soil health in green onion (Allium cepa) plantings. Three types of mulch systems (organic, living and solarization mulch) were evaluated and compared. Green onions were grown in four pre-plant treatments: 1) organic mulch generated by flail mowing a sunn hemp (Crotalaria juncea) cover crop (SH) followed by a no-till cropping system, 2) soil solarization (Sol), 3) sunn hemp-solarization mulch in which the sunn hemp was incorporated into the soil followed by soil solarization (SHSol), and 4) bare ground with insecticides (BG). Sol, SHSol and BG were practiced in conventional tilled system. SH (in 2013 and 2014) and SHSol (in 2014) treatments included strips of buckwheat and cowpea along the crop border to serve as living mulch or insectary plants. Green onion plants in SH had lower thrips and leaf miners damage than other treatments in both study years. When insectary borders were added to SHSol in 2014, thrips and leaf miners damages were also reduced in SHSol compared to Sol and BG. Incidence of purple blotch was reduced in SH and SHSol treatments in 2014. The SH treatment contained an increased abundance and richness of detritivores, predatory arthropods, parasitoids and beneficial free-living nematodes (particularly bacterivores and fungivores). Although weeds and initial populations of plant-parasitic nematodes were lower in solarization (Sol, SHSol) than other treatments, green onion yield was greatest in SH organic mulch systems in both study years. Overall, the integration of surface organic, living and solarization mulches into a green onion agroecosystem provided multiple ecosystem services including suppression of above and below ground pest organisms.

No abstract