Isolation of Potential Photosynthetic N\u3csub\u3e2\u3c/sub\u3e-Fixing Microbes from Topsoil of Native Grasslands in South Dakota

Nitrogen fertilizer is one of the most limiting factors and costly inputs in agriculture production. Current fossil fuel-dependent ammonia production is both energy intensive and environmentally damaging. An economically practical and environmentally friendly solution for the production of ammonia is urgently needed. Solar-powered N2-fixing cyanobacteria provide a unique opportunity and promise for applications in agriculture compared to all other N2-fixing bacteria that cannot use solar energy. Isolation of nitrogen-fixing microbes from the topsoil of native grasslands may have the potential to use them in crop fields as living ammonia factories. This may be a mechanism to free farmers from heavy reliance on fossil fuels-dependent chemical nitrogen fertilizers and to improve soil health for sustainable agriculture. To screen for solar-powered N2-fixing cyanobacteria in topsoil of native grasslands in South Dakota, we collected 144 topsoil samples from several native grasslands. Six photosynthetic microbial strains were isolated that are capable of growing well autotrophically in a nitrogen-free medium, suggesting that these six microbial strains have the ability to fix N2. They were assigned the names: Xu15, Xu81, Xu86, Xu111, Xu141, and WW3. Based on cell morphology and its 18S rRNA gene sequence that we obtained, strain Xu15 was reassigned as Chloroidium saccharophilum Xu15, a common terrestrial coccoid green alga. An acetylene reduction assay detected substantial ethylene production, suggesting nitrogenase activity occurrences in cultures Xu81 and Xu15. The other four are in the process of purification for testing their nitrogenase activity. Xu81, Xu111 and Xu141 are probably unicellular microalga, while WW3 and Xu86 are likely filamentous cyanobacteria. Future research will focus on developing these validated N2-fixing microbes as in situ living ammonia factories in crop fields

Inorganic and organic nitrogen acquisition by a fern Dicranopteris dichotoma in a subtropical forest in South China.

The fern Dicranopteris dichotoma is an important pioneer species of the understory in Masson pine (Pinus massoniana) forests growing on acidic soils in the subtropical and tropical China. To improve our understanding of the role of D. dichotoma in nitrogen (N) uptake of these forests, a short-term (15)N experiment was conducted at mountain ridge (MR, with low N level) and mountain foot (MF, with high N level). We injected (15)N tracers as (15)NH4, (15)NO3 or (15)N-glycine into the soil surrounding each plant at both MR and MF sites. Three hours after tracer injection, the fern D. dichotoma took up 15NH4+ significantly faster at MF than at MR, but it showed significantly slower uptake of (15)NO3- at MF than at MR. Consequently, (15)NO3- made greater contribution to the total N uptake (50% to the total N uptake) at MR than at MF, but (15)N-glycine only contributed around 11% at both sites. Twenty-four hours after tracer injection, D. dichotoma preferred (15)NH4+ (63%) at MR, whereas it preferred (15)NO3- (47%) at MF. We concluded that the D. dichotoma responds distinctly in its uptake pattern for three available N species over temporal and spatial scales, but mainly relies on inorganic N species in the subtropical forest. This suggests that the fern employs different strategies to acquire available N which depends on N levels and time

Results of multifactorial ANOVA for the effects of site, N species, time, and their interactions on N uptake rates of <i>D. dichotoma</i> in a subtropical forest.

<p>Results of multifactorial ANOVA for the effects of site, N species, time, and their interactions on N uptake rates of <i>D. dichotoma</i> in a subtropical forest.</p

Chemical niche shifts in terms of N acquisition by the fern <i>D. dichotoma</i> under two available N levels.

<p>The area enclosed by the solid line refers to MR site (low N availability), whereas the dashed line encloses MF site data (high N availability). Values are means (±1SE) of 6 replicates. Asterisks indicate significant differences between mountain ridge and mountain foot MR and MF at <i>P<</i>0.05.</p

The two sites located along the mountain slope: (a) mountain ridge (MR), (b) mountain foot (MF).

<p>The two sites located along the mountain slope: (a) mountain ridge (MR), (b) mountain foot (MF).</p

Characteristics of topsoil (0–10 cm) of the mountain ridge (MR) and the mountain foot (MF) sites where <i>Dicranopteris dichotoma</i> grows.

<p>Means (±1SE) of six replicates are presented (n = 6). Asterisks indicate significant differences between two sites at <i>P<</i>0.05.</p