Soil biotest results using Azotobacter bacteria: interpretation problems and possible solutions

The aim of the study. The Azotobacter genus is a well-known bioassay for testing the soil environment quality. A large number of these bacteria is considered as evidence of the ecological well-being of a soil. However, a high number of these microorganisms was found in disturbed ecosystems, which means there is a problem of how to interpret the results of the biotest. Therefore the aim of the study was to clarify the causes of this problem and suggest a possible way to solve it. Location and objects of the study. The study was conducted in West Siberia (Russia) in the Priobskoe plateau (54°53'13.5"N, 82°59'36.7E"). Leached сhernozem with different content of organic matter (mortmass) was studied under the following treatments: 1) permanent fallow, 2) wheat cultivation, annual removal of straw from the field + summer fallow, 3) wheat + left straw + summer fallow, 4) wheat +left straw + green manure fallow, 5) grassland. The content of N-NO3, respectively, was equal to 50, 10, 15, 5, 0 mg/kg. Another object was a site at the mining and processing plant "Denisovsky" in South Yakutia (Russia) (56°46'20.23"N, 124°51'06.95"E). Abandoned for a long time (30 years) after coal mining spoils were studied in two variants: without plants and with well-developed vegetation cover. Total SOC content was 1.8 and 5.7%, N – 0.3 and 0.4 %, respectively. Methodology. The direct sowing of single soil aggregates onto the N-free medium containing glucose as C-source was used to cultivate Azotobacter. Glucose (10 mg/g soil) was added to the soil to activate Azotobacter growth. A live culture of bacteria was introduced into the soil at a dose of 1 ml/g to check the viability of Azotobacter in experimental soils. Main results. The status of the microbial community in situ was observed on microbial landscapes obtained by exposing slides in the undisturbed soil for 30 days. The overgrowth of soil lumps in the specified range of options was 0–80–40–0–0% and after glucose addition – 100–80–80–0%. The activation of Azotobacter growth after glucose addition was inversely proportional to the C: N ratio (between the mortmass and the mineral nitrogen). Live Azotobacter culture under grassland developed 2.5 times slower in comparison with the fallow. Similar patterns were found in the study of the soils developed on the coal mining spoils. Activation of Azotobacter growth by glucose (response to stress) was more pronounced in soils with apparently less favorable environment for bacteria. Conclusion. The reason behind misleading interpretation of Azotobacter biotest results was that the original purpose of the test was to assess fertility of arable soils. This role of the indicator bacterium was previously underestimated. It is known that the arable soil belongs to the category of disturbed ones, and the abundance of Azotobacter may indicate instability in the microbial community of the soil. To expand the capabilities of the biotest, the authors propose to supplement the test with a procedure for evaluating the Azotobacter growth response to experimental stress, e.g. C-substrate addition.

The Decline In Summer Fallow In The Northern Plains Cooled Near-Surface Climate But Had Minimal Impacts On Precipitation

Land management strategies can moderate or intensify the impacts of a warming atmosphere. Since the early 1980s, nearly 116,000 km2 of crop land that was once held in fallow during the summer is now planted in the northern North American Great Plains. To simulate the impacts of this substantial land cover change on regional climate processes, convection-permitting model experiments using the Weather Research and Forecasting (WRF) model were performed to simulate modern and historical amounts of summer fallow, and were extensively validated using multiple observational data products as well as eddy covariance tower observations. Results of these simulations show that the transition from summer fallow to modern land cover lead to ~1.5 °C cooler temperatures and decreased vapor pressure deficit by ~0.15 kPa during the growing season, which is consistent with observed cooling trends. The cooler and wetter land surface with vegetation leads to a shallower planetary boundary layer and lower lifted condensation level, creating conditions more conducive to convective cloud formation and precipitation. Our model simulations however show little widespread evidence of land surface changes effects on precipitation. The observed precipitation increase in this region is more likely related to increased moisture transport by way of the Great Plains Low Level Jet as suggested by the ERA5 reanalysis. Our results demonstrate that land cover change is consistent with observed regional cooling in the northern North American Great Plains but changes in precipitation cannot be explained by land management alone.

Utilization of the neighborhood design to evaluate suitable cover crops and their density for Echinochloa colona management

Summer weed species, including Echinochloa colona, are becoming problematic in the eastern grain region of Australia, but cover crops can be useful to suppress weeds during the summer fallow period. The present study evaluated the growth and seed production of E. colona grown alone or with four and eight cover crop plants per pot (i.e., 80 and 160 plants m-2). Four legume (cowpea, lablab, pigeonpea, and soybean) and two grass (forage sorghum and Japanese millet) cover crops were used. Interference by cover crops reduced the height, the number of leaves and tillers, inflorescence number, seed production, and biomass of this weed than when it was grown alone. Cover crops differed in their ability to suppress the growth and seed production of E. colona. The effect of cover crop density on the studied attributes was non-significant in most cases. Pigeonpea as a cover crop was the least effective in suppressing the growth and seed production of E. colona. In general, leguminous cover crops exhibited less suppression of E. colona than grasses. Forage sorghum was most efficient in reducing the growth of this weed. Forage sorghum and Japanese millet reduced E. colona leaf and tiller numbers per plant by 90 and 87%, respectively. These cover crops reduced E. colona leaf number to only 17 per plant as against 160 per plant recorded without cover crops. Inflorescence number per E. colona plant growing alone was as high as 48. However, it was reduced by 20–92% when this weed was grown with cover crop plants. E. colona’s seed production was significantly suppressed by all the cover crops, except pigeonpea. Biomass of E. colona was suppressed largely by forage sorghum and Japanese millet compared to other cover crops. Among the cover crops, pigeonpea produced the lowest biomass of 11 g pot-1, and the highest biomass (114 g pot-1) was produced by forage sorghum. The study demonstrated the usefulness of cover crops, especially forage sorghum and Japanese millet, to suppress the growth and seed output of E. colona.

Basmati Rice Quality Enhancement by Zinc Fertilization and Green Manuring on a Sub-tropical Inceptisol in Indo-Gangetic Plains of India

Basmati (aromatic) rice is premier rice grown in north-western India and Pakistan. This rice is preferred for their long and slender kernels which expand 3-4 times in length and remain fluffy and are well known all over the world, especially in the Middle East and South Asia for their long fluffy grains on cooking. Paddy soils are usually deficient in organic matter because of high temperature and moisture, which causes rapid decomposition of organic matter. The importance of leguminous green manure crops in improving soil fertility, and soil physical properties received increasing attention. Also, the zinc (Zn) deficiency in soils is prevalent worldwide, especially in high pH calcareous soils. No reports were available on combining green manuring crops and Zn fertilization on productivity, Zn content and kernel quality of Basmati rice. Therefore, the current investigation was undertaken to quantify the combined effects of summer green manuring crops and zinc fertilization on productivity, Zn content and kernel quality of Basmati rice in summer green manuring-Basmati rice cropping system. A field study was therefore conducted for two years (2009 and 2010) on a sandy clay-loam soil (typic Ustochrept) at the research farm of the ICAR-Indian Agricultural Research Institute, New Delhi, India. The experiments were conducted in split plot design, keeping three green manuring crops viz. Sesbania aculeata (Dhaincha), Crotalaria juncea (Sunhemp), and Vigna unguiculata (Cowpea) and one summer fallow treatment as main-plot treatments and six Zn sources viz. control (no Zn application), ZnSO4∙7H2O (21% Zn), ZnSO4∙H2O (33% Zn), ZnO (82% Zn), ZnSO4∙7H2O + ZnO (50% + 50%) and EDTA-chelated Zn (12% Zn) in sub-plots and was replicated thrice. The experiments in both the years were conducted with a fixed lay-out plan on the same site. The results showed that incorporation of green manures along with zinc (Zn) fertilization increased grain and straw yield, enhanced Zn concentrations and improved the kernel quality before and after cooking in Basmati rice ‘Pusa Basmati 1’. The application of EDTA-chelated Zn (12% Zn) was the best in terms of grain and straw yield and Zn concentrations in grain and straw and kernel quality before and after cooking Basmati rice. Application of ZnSO4∙7H2O (21% Zn) was the second-best treatment followed by ZnSO4∙H2O (33% Zn) and ZnSO4∙7H2O + ZnO (50% + 50%). Application of ZnO (82% Zn) had least effect in increasing the studied parameters. The lowest values were observed with control (no Zn application). Among the summer green manuring crops, incorporation of Sesbania aculeata (Dhaincha) was found to be the best over Crotalaria juncea (Sunhemp), Vigna unguiculata (Cowpea) and summer fallow in terms of grain and straw yield, Zn concentrations in grain and straw and kernel quality before and after cooking in Basmati rice. Zn fertilization with EDTA-chelated Zn (12% Zn) lead to 25.91 and 21.26% higher grain yield; 60.66 and 82.14% Zn-denser grains; with 13.33 and 10.92% increase in head rice recovery in Basmati rice over control (no Zn application) during 2009 and 2010, respectively.

Subsoiling during summer fallow in rainfed winter-wheat fields enhances soil organic carbon sequestration on the Loess Plateau in China

Scientific management of the soil organic carbon (SOC) pool, e.g., through a reasonable tillage system, is a potential way to mitigate global climate change. There is scarce information about the effect of tillage during the summer fallow period on the SOC pool in rainfed winter-wheat fields. The present study was designed to evaluate the effects of tillage practices, i.e., plow tillage (PTF), subsoiling (STF) and no tillage (NTF), during the summer fallow period on SOC sequestration in winter-wheat fields in the rainfed area of the eastern Loess Plateau of China. The SOC, mineral-associated organic carbon (MOC), permanganate-oxidizable organic carbon (POxC) and particulate organic carbon (POC) concentrations were determined after four years of tillage implementation during the summer fallow period. Our results showed that in comparison to the adoption of NTF, the adoption of STF significantly increased POxC, POC and MOC concentrations by 56.6–111.2%, 45.7–118.7% and 26.2–29.4%, respectively, at the 10–20 and 30–40 cm soil depths before sowing (P < 0.05). The POxC and MOC concentrations under STF at depths of 0–10, 10–20, 20–30 and 30–50 cm were significantly greater than those under PTF and NTF after harvesting (P < 0.05). In addition, the SOC concentration and SOC stock under STF were significantly greater than those under NTF at the 0–10, 10–20, 20–30 and 30–40 cm soil depths before sowing and after harvesting (P < 0.05). Furthermore, in comparison to PTF and NTF, STF resulted in significantly higher SOC stocks by 12.0–25.3% and 7.1–19.2% than PTF and NTF, respectively, in the 0–10, 0–20, 0–30, 0–40 and 0–50 cm soil profiles at harvesting (P < 0.05). In summary, the adoption of STF could be beneficial to the management of the SOC pool in the 0–50 cm soil profile in the rainfed area of winter-wheat on the Loess Plateau of China.

Winter Triticale: A Long-Term Cropping Systems Experiment in a Dry Mediterranean Climate

Triticale (X Triticosecale Wittmack) is a cereal feed grain grown annually worldwide on 4.2 million ha. Washington is the leading state for rainfed (i.e., non-irrigated) triticale production in the USA. A 9-year dryland cropping systems project was conducted from 2011 to 2019 near Ritzville, WA to compare winter triticale (WT) with winter wheat (Triticum aestivum L.) (WW) grown in (i) a 3-year rotation of WT-spring wheat (SW) -no-till summer fallow (NTF) (ii) a 3-year rotation of WW-SW-undercutter tillage summer fallow (UTF) and (iii) a 2-year WW-UTF rotation, We measured grain yield, grain yield components, straw production, soil water dynamics, and effect on the subsequent SW wheat crop (in the two 3-year rotations). Enterprise budgets were constructed to evaluate the production costs and profitability. Grain yields averaged over the years were 5816, 5087, and 4689 kg/ha for WT, 3-year WW, and 2-year WW, respectively (p < 0.001). Winter triticale used slightly less water than WW (p = 0.019). Contrary to numerous reports in the literature, WT never produced more straw dry biomass than WW. Winter wheat produced many more stems than WT (p < 0.001), but this was compensated by individual stem weight of WT being 60% heavier than that of WW (p < 0.001). Spring wheat yield averaged 2451 vs. 2322 kg/ha after WT and WW, respectively (p = 0.022). The market price for triticale grain was always lower than that for wheat. Winter triticale produced an average of 14 and 24% more grain than 3-year and 2-year WW, respectively, provided foliar fungal disease control, risk reduction, and other rotation benefits, but was not economically competitive with WW. A 15–21% increase in WT price or grain yield would be necessary for the WT rotation to be as profitable as the 3-year and 2-year WW rotations, respectively.