Frankia inoculation,soil biota,and host tissue amendment influence Casuarina nodulation capacity of a tropical soil

The effects of soil biota, Frankia inoculation and tissue amendment on nodulation capacity of a soil was investigated in a factorial study using bulked soil from beneath a Casuarina cunninghamiana tree and bioassays with C. cunninghamiana seedlings as capture plants. Nodulation capacities were determined from soils incubated in sterile jars at 21 °C for 1, 7, and 28 days, after receiving all combinations of the following treatments: ± steam pasteurization, ± inoculation with Frankia isolate CjI82001, and ± amendment with different concentrations of Casuarina cladode extracts. Soil respiration within sealed containers was determined periodically during the incubation period as a measure of overall microbial activity. Soil respiration, and thus overall microbial activity, was positively correlated with increasing concentrations of Casuarina cladode extracts. The nodulation capacity of soils inoculated with Frankia strain Cj82001 decreased over time, while those of unpasteurized soils without inoculation either increased or remained unaffected. The mean nodulation capacity of unpasteurized soil inoculated with Frankia CjI82001 was two to three times greater than the sum of values for unpasteurized and inoculated pasteurized soils. Our results suggest a positive synergism between soil biota as a whole and Frankia inoculum with respect to host infection.     

Seasonal flooding regimes influence survival,nitrogen fixation,and the partitioning of nitrogen and biomass in Alnus incana ssp. rugosa

Alteration of natural flooding regimes can expose lowlands to waterlogged soil conditions during any month of the year. The seasonality of flooding may have profound effects on the carbon and nitrogen budgets of N-fixing alders (Alnus spp.), and in turn, may impact the C and N economy of extensive alder-dominated, wetland ecosystems, including those dominated by speckled alder (Alnus incana ssp. rugosa). To better understand this process, two-year-old, nodulated seedlings of speckled alder were subjected to late spring (May 10 – July 10), summer (July 10 – September 8), and fall (September 8 – November 8) flooding treatments. Alders were root-flooded outdoors in tanks containing an N-free nutrient solution and compared with unflooded alders at the experimental site. Flooding arrested N fixation, photosynthesis, and growth of alders without recovery in all flooding treatments for the remainder of the growing season. Late spring and summer flooding resulted in complete mortality of alders while all seedlings survived flooding in the fall. Fall flooding increased foliar N resorption by 140% over unflooded seedlings. Eighty-seven percent of the total N fixed and 89% of biomass accumulation for the entire growing season occurred in unflooded alders after July 10. In unflooded alders, nitrogen fixation rates per unit mass declined by 63% for nodules, 28% for leaves, and 48% for whole seedlings during the fall, while total N fixed per plant in the fall was similar to that fixed in the summer. The majority of newly fixed N in unflooded alders was allocated to leaves before September 8 and to roots/nodules combined after September 8. In unflooded plants, the greatest proportion of new biomass was partitioned to leaves before July 10, to stems between July 10 and September 8, and equally to stems and roots/nodules after September 8. Fall-flooded alders did not increase root or nodule biomass. Proportional allocation of plant resources were such that the ratio of N fixed to seedling growth of unflooded alders decreased by 19% during summer before rebounding by 6% in fall. Seasonality of flooding alters seedling survival, growth, and resource allocation, and may be a critical determinant of speckled alder recruitment and occurrence in wetlands.