Demand of all living organisms on the same nutrients forms the basis for interspecific competition between plants and microorganisms in soils. This competition is especially strong in the rhizosphere. To evaluate competitive and mutualistic interactions between plants and microorganisms and to analyse ecological consequences of these interactions, we analysed 424 data pairs from 41 (15)N-labelling studies that investigated (15)N redistribution between roots and microorganisms. Calculated Michaelis-Menten kinetics based on K(m) (Michaelis constant) and V(max) (maximum uptake capacity) values from 77 studies on the uptake of nitrate, ammonia, and amino acids by roots and microorganisms clearly showed that, shortly after nitrogen (N) mobilization from soil organic matter and litter, microorganisms take up most N. Lower K(m) values of microorganisms suggest that they are especially efficient at low N concentrations, but can also acquire more N at higher N concentrations (V(max)) compared with roots. Because of the unidirectional flow of nutrients from soil to roots, plants are the winners for N acquisition in the long run. Therefore, despite strong competition between roots and microorganisms for N, a temporal niche differentiation reflecting their generation times leads to mutualistic relationships in the rhizosphere. This temporal niche differentiation is highly relevant ecologically because it: protects ecosystems from N losses by leaching during periods of slow or no root uptake; continuously provides roots with available N according to plant demand; and contributes to the evolutionary development of mutualistic interactions between roots and microorganisms.
Fruit carbonate of Buglossoides arvensis (syn. Lithospermum arvense ) is a valuable dating and paleoenvironmental proxy for late Quaternary deposits and cultural layers because CaCO 3 in fruit is assumed to be accumulated from photosynthetic carbon (C). However, considering the uptake of HCO 3 – by roots from soil solution, the estimated age could be too old depending on the source of HCO 3 – allocated in fruit carbonate. Until now, no studies have assessed the contributions of photosynthetic and soil C to the fruit carbonate. To evaluate this, the allocation of photo-assimilated carbon and root uptake of HCO 3 – was examined by radiocarbon ( 14 C) labeling and tracing. B. arvensis was grown in carbonate-free and carbonate-containing soils (sand and loess, respectively), where 14 C was provided as (1) 14 CO 2 in the atmosphere (5 times shoot pulse labeling), or (2) Na 2 14 CO 3 in soil solution (root-labeling; 5 times by injecting labeled solution into the soil) during one month of fruit development. Distinctly different patterns of 14 C distribution in plant organs after root- and shoot labeling showed the ability of B. arvensis to take up HCO 3 – from soil solution. The highest 14 C activity from root labeling was recovered in roots, followed by shoots, fruit organics, and fruit carbonate. In contrast, 14 C activity after shoot labeling was the highest in shoots, followed by fruit organics, roots and fruit carbonate. Total photo-assimilated C incorporated via shoot labeling in loess-grown plants was 1.51 mg lower than in sand, reflecting the presence of dissolved carbonate (i.e. CaCO 3 ) in loess. Loess carbonate dissolution and root-respired CO 2 in soil solution are both sources of HCO 3 – for root uptake. Considering this dilution effect by carbonates, the total incorporated HCO 3 – comprised 0.15% of C in fruit carbonate after 10 hr of shoot labeling. However, if the incorporated HCO 3 – during 10 hr of shoot labeling is extrapolated for the whole month of fruit development (i.e. 420-hr photoperiod), fruit carbonate in loess-grown plants incorporated approximately 6.3% more HCO 3 – than in sand. Therefore, fruit carbonates from plants grown on calcareous soils may yield overestimated 14 C ages around 500 yr because of a few percentage uptake of HCO 3 – by roots. However, the age overestimation because of HCO 3 – uptake becomes insignificant in fruits older than approximately 11,000 yr due to increasing uncertainties in age determination.
Dataset for the article: Mason-Jones, K., Schmücker, N., Kuzyakov, Y. (2018) Contrasting Effects of Organic and Mineral Nitrogen Challenge the N-Mining Hypothesis for Soil Organic Matter Priming. Soil Biology and Biochemistry 124, 38-46, https://doi.org/10.1016/j.soilbio.2018.05.024
Application of an antibiotic (streptomycin) and a fungicide (benomyl) in order to separate root respiration and microbial respiration of exudates was tested using two plant species (ryegrass and spring wheat) growing on a Haplic Luvisol. Both xenobiotics were added to the soil with growing plants either separately or in combination. Plants were 14C-pulse labelled and both labelled and total CO2 emission from soil were measured. After seven days plants were harvested, dried and total carbon and C content in shoots and roots were determined. Growing plants increased the total CO2 emission from soil by about 2.5 times in comparison to unplanted soil. The temporal pattern of 14CO2 evolution in control treatments was similar as reported in the literature with maximum emission rates (0.55 % and 0.15 % of assimilated C per hour for wheat and ryegrass, respectively) during the first day after labelling. It was affected by both xenobiotics used but in different ways. With ryegrass streptomycin decreased CO2 emission rates during the first day after labelling, while it left the second phase of tracer emission unchanged; with spring wheat the 14CO2 evolution rates were reduced on the second day. Applied xenobiotics did not change the 14C content in the shoots, but smaller tracer amounts were found in the roots and in soil-derived CO2. This was more pronounced for ryegrass than for spring wheat plants.
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