Most studies on plant nutrition tend to focus on the topsoil (plough layer) and frequently neglect subsoil processes. However, cereal roots can potentially acquire nutrients including organic and inorganic nitrogen (N) from deep in the soil profile. Greater knowledge on the interaction of plants and microbes in subsoil environments is required to evaluate whether deep rooting traits in cereals will achieve greater nutrient use efficiency and greater soil carbon (C) storage in cropping systems. This study aimed to evaluate the relationship between root distribution, organic and inorganic N availability and potential N supply at the critical growth period during the wheat cropping cycle in a sand textured Eutric Cambisol. Our results provide evidence of significant microbial capacity in the subsoil. The rate of plant residue turnover and the mineralization of organic C and N substrates (glucose, amino acids, peptides, protein) declined slightly with increasing soil depth; however, these rates were not correlated with basal soil respiration, microbial biomass or community structure. This suggests that the microbial population in subsoil is more C limited but that its activity can be readily stimulated upon C substrate addition. A significant potential for organic and inorganic N turnover was also demonstrated at depth with a similar abundance of ammonifiers and ammonia oxidizing bacteria (AOB) and archaea (AOA) throughout the soil profile. Again, N mineralization in subsoils appears to be substrate limited. Root density declined rapidly down the soil profile with few roots present past 50 cm; suggesting that this is the major factor limiting C recharge of soil organic matter and microbial activity in subsoils. Greater root proliferation at depth could allow greater capture of water and the recapture of N lost by leaching; however, our results suggest that plant-microbial competition for C and N is as intense in the subsoil as in the topsoil. We conclude that while deeper rooting may improve nutrient and water use efficiency it may not lead to much greater C sequestration in subsoils, at least in the short term.

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Microbial competition for nitrogen and carbon is as intense in the subsoil as in the topsoil

Summary � The ability of plants to compete effectively for nitrogen (N) resources is critical to plant survival. However, controversy surrounds the importance of organic and inorganic sources of N in plant nutrition because of our poor ability to visualize and understand processes happening at the root–microbial–soil interface. � Using high-resolution nano-scale secondary ion mass spectrometry stable isotope imaging (NanoSIMS-SII), we quantified the fate of 15 N over both space and time within the rhizosphere. We pulse-labelled the soil surrounding wheat (Triticum aestivum) roots with either 15 NH þ 4 or 15 N-glutamate and traced the movement of 15 N over 24 h. � Imaging revealed that glutamate was rapidly depleted from the rhizosphere and that most 15 N was captured by rhizobacteria, leading to very high 15 N microbial enrichment. After microbial capture, approximately half of the 15 N-glutamate was rapidly mineralized, leading to the excretion of NH þ , which became available for plant capture. Roots proved to be poor competitors for 15 N-glutamate and took up N mainly as 15 NH þ . Spatial mapping of 15 N revealed differential patterns of 15 N uptake within bacteria and the rapid uptake and redistribution of 15 N within roots. � In conclusion, we demonstrate the rapid cycling and transformation of N at the soil–root interface and that wheat capture of organic N is low in comparison to inorganic N under the conditions tested.

/pdf/competition-between-plant-and-bacterial-cells-at-the-3getunioyg.pdf

Competition between plant and bacterial cells at the microscale regulates the dynamics of nitrogen acquisition in wheat (Triticum aestivum)

Mosses, among all types of terrestrial vegetation, are excellent scavengers of anthropogenic nitrogen (N), but their utilization of dissolved organic N (DON) and their reliance on atmospheric N remain uncharacterized in natural environments, which obscures their roles in N cycles. Natural (15) N abundance of N sources (nitrate (NO(3)(-)), ammonium (NH(4)(+)) and DON in deposition and soil) for epilithic and terricolous mosses was analyzed at sites with different N depositions at Guiyang, China. Moss NO(3)(-) assimilation was inhibited substantially by the high supply of NH(4)(+) and DON. Therefore, contributions of NH(4)(+) and DON to moss N were partitioned using isotopic mass-balance methods. The N contributions averaged 56% and 46% from atmospheric NH(4)(+), and 44% and 17% from atmospheric DON in epilithic and terricolous mosses, respectively. In terricolous mosses, soil NH(4)(+) and soil DON accounted for 16% and 21% of bulk N, which are higher than current estimations obtained using (15) N-labeling methods. Moreover, anthropogenic NH(4)(+) deposition suppressed utilization of DON and soil N because of the preference of moss for NH(4)(+) under elevated NH(4)(+) deposition. These results underscore the dominance of, and preference for, atmospheric NH(4)(+) in moss N utilization, and highlight the importance of considering DON and soil N sources when estimating moss N sequestration and the impacts of N deposition on mosses.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.12284

Ammonium first: natural mosses prefer atmospheric ammonium but vary utilization of dissolved organic nitrogen depending on habitat and nitrogen deposition.

Soil extractable organic nitrogen plays an important role in nitrogen transformation and migration in many ecosystems. However, it is generally ignored due to its low content in agricultural soils. The objective of this study was to evaluate the leaching risk of soil extractable organic nitrogen affected by manure application in an intensively irrigated greenhouse vegetable cropping system through investigating its spatial (vertical profile) and temporal dynamics. Results showed that extractable organic nitrogen was present in appreciable quantities, despite that nitrate was the main soluble nitrogen form in 0–60 cm soil profile. Both the extractable organic and inorganic nitrogen were enriched in the surface and subsurface soils, and showed a high temporal variability throughout the cucumber growing season. Manure application increased the stocks of extractable organic nitrogen significantly in the soil profile. Average extractable organic nitrogen reserves in 0–60 cm layer were 47, 71, and 131 kg ha−1 f...

Manure increase the leaching risk of soil extractable organic nitrogen in intensively irrigated greenhouse vegetable cropping systems

The objective of this study was to evaluate the nitrification rate in soil polluted with zinc. The experimental protocol was as follows: soil (sandy loam) collected from the 0-20 cm layer of a cropped field was passed through a 2 mm mesh sieve, placed in 150 cm 3 glass beakers, 100 g of soil in each, and polluted with the following doses of Zn2+ per 1 kg d.m. of soil: 0, 300, 600, 1 200 and 2 400. Zinc was applied in the form of ZnCl 2 aqueous solution. Afterwards, ammonia nitrogen as (NH 4 ) 2 SO 4 was added to the soil material in two doses: 0 and 240 mg N kg -1 d.m. Once zinc chloride and ammonium sulphate had been thoroughly mixed with the soil, water was added until the soil moisture content reached 50% of capillary water holding capacity and then the beakers were placed in a laboratory incubator at 25oC. After 10, 20, 30 and 40 days, the incubated soil was tested to determine the content of N-NH 4 and N-NH 3 . Additionally, after 10 and 40 days of incubation, the most probable counts of nitrifying bacteria involved in the first and second step of the nitrification process were determined. The experiment was run with three replicates for each day. Two determinations of each parameter were performed in the soil samples placed in beakers. In total, 6 results were obtained for each experimental variant. Based on the determinations, the amounts of nitrified and immobilized nitrogen were calculated and the resistance (RS) and resilience (RL) of the nitrification process and nitrifying bacteria to the contamination of soil with zinc were expressed. It has been experimentally demonstrated that excess zinc in soil significantly disturbs the nitrification rate. As little as 300 mg Zn 2+ kg -1 d.m. of soil significantly inhibits nitrification. Zinc contamination interferes with nitrification and other metabolic process which affect soil nitrogen, which is confirmed by depressed nitrogen immobilization at higher rates of soil contamination with this element. The adverse effect of zinc on nitrification is primarily due to the negative impact of this element in the soil environment on nitrifying bacteria. Zinc more strongly inhibits the first than the second step nitrification bacteria, but ammonia-oxidizing bacteria recover more quickly than nitrate forms. The RS parameters for the nitrification process towards zinc pollution were on a low level and tended to decrease as the degree of zinc contamination rose. The resistance of nitrifying bacteria to zinc decreased parallel to the increasing amounts of zinc in soil.

/pdf/pressure-exerted-by-zinc-on-the-nitrification-process-4b4sbhyxa2.pdf

P. W. H. ll

Papers

Microbial activity differentially regulates the vertical mobility of nitrogen compounds in soil