Organic N deposition favours soil C sequestration by decreasing priming effect

Plant and Soil - Understanding seed-soil dynamics is important for improving plant emergence and growth. The objectives of this study were to develop a Seed-Soil model to simulate the dynamic...     

Microclimate and forest management alter fungal-to-bacterial ratio and N<Subscript>2</Subscript>O-emission during rewetting in the forest floor and mineral soil of mountainous beech forests

The effects of site exposure (microclimate) and forest management (thinning) on fungal-to-bacterial (F:B) respiratory ratio and N₂O emission from forest floor and Ah layer samples were studied at untreated and thinned beech forests. Microclimate effects were studied by selecting sites facing north-east (NE) or south-west (SW). The F:B respiratory ratio was estimated using substrate-induced respiration in combination with inhibitors either affecting fungi or bacteria. N₂O production was evaluated after moistening samples initially pre-incubated at different moisture levels to 100% of the water holding capacity (WHC). F:B respiratory ratios were significantly affected by microclimate and thinning, with site exposure having the strongest effect on fungal-to-bacterial ratio and N₂O production both for the forest floor and the Ah layer. Significantly more N₂O was produced from soils pre-incubated under low (15% WHC) moisture conditions as compared to soils pre-incubated under air dry (5% WHC) or wet conditions (30–60% WHC). A positive correlation between N₂O emission and F:B respiratory ratio for Ah layer samples and a negative correlation between bacterial substrate induced respiration (SIR) and N₂O emission for both Ah layer and forest floor samples indicated that net N₂O production was the result of predominantly fungal N₂O production and predominantly bacterial N₂O consumption. The latter hypothesis was further supported by increased N₂O emission from samples treated with bacterial inhibitor.     

Nitrogen uptake and utilisation as a competition factor between invasive Duchesnea indica and native Fragaria vesca

The Indian mock strawberry [Duchesnea indica (Andrews) Focke] is an invasive plant in several regions of central Europe and Germany. In order to explore its competitive ability, we compared it with the native woodland strawberry (Fragaria vesca L.) by growing it alone as well as in intra- or inter-specific competition in a pot experiment under greenhouse conditions. Nutrient solution was added several times at two nitrogen (N) levels. One addition involved ¹⁵N labelling to determine whether the competition of both plant species depends on their ability to acquire N from soil. Duchesnea had a higher biomass production than Fragaria when grown in nutrient-rich soil, both in competition and as a solitary plant. Under N-poor conditions, root interference could change this superiority due to limited soil space. After 65 days of growth, total plant dry weight, total N content and ¹⁵N content in the plant tissues were determined. The results show that the predominance of Duchesnea in biomass production was confirmed at high, but not at low N availability. The assimilate partitioning strategy of Duchesnea differs from that of Fragaria: the former generally had a higher shoot-to-root ratio. The N content in shoots and roots was affected only by N addition but not by competition or species. Duchesnea allocated more N to the leaves, Fragaria to the roots. The amount of ¹⁵N taken up was nearly equal for both species. In relation to root biomass, Duchesnea had a higher specific uptake rate at low N addition because of the higher root biomass in Fragaria. The roots of Fragaria and Duchesnea did not affect each other when grown together. We conclude that the invasive potential of Duchesnea is only poorly related to the N uptake rate or to better root competition for N. In N-rich environments, however, Duchesnea is highly competitive because of the preferred investment in shoot biomass. Therefore, environments with increased N deposition, i.e. from anthropogenic sources, could promote the invasive potential of Duchesnea.     

Microbial biomass and growth kinetics of microorganisms in chernozem soils under different land use modes

The carbon content of microbial biomass and the kinetic characteristics of microbial respiration response to substrate addition have been estimated for chernozem soils under different land use: arable lands used for 10, 46, and 76 years, mowed meadow, natural forest, and forest shelter belt. Microbial biomass and the content of microbial carbon in humus (C_mic /C_org) decreased in the following order: soils under forest cenoses—mowed meadow—10-year arable land—46- and 75-year arable land. The amount of microbial carbon in the long-plowed horizon was 40% of its content in the upper horizon of natural forest. Arable soils were characterized by a lower metabolic diversity of microbial community and by the highest portion of microorganisms able to grow directly on glucose introduced into soil. The effects of different scenarios of carbon sequestration in soil on the amounts and activity of microbial biomass are discussed.     

Microbial Growth and Carbon Use Efficiency in the Rhizosphere and Root-Free Soil

Plant-microbial interactions alter C and N balance in the rhizosphere and affect the microbial carbon use efficiency (CUE)–the fundamental characteristic of microbial metabolism. Estimation of CUE in microbial hotspots with high dynamics of activity and changes of microbial physiological state from dormancy to activity is a challenge in soil microbiology. We analyzed respiratory activity, microbial DNA content and CUE by manipulation the C and nutrients availability in the soil under Beta vulgaris. All measurements were done in root-free and rhizosphere soil under steady-state conditions and during microbial growth induced by addition of glucose. Microorganisms in the rhizosphere and root-free soil differed in their CUE dynamics due to varying time delays between respiration burst and DNA increase. Constant CUE in an exponentially-growing microbial community in rhizosphere demonstrated the balanced growth. In contrast, the CUE in the root-free soil increased more than three times at the end of exponential growth and was 1.5 times higher than in the rhizosphere. Plants alter the dynamics of microbial CUE by balancing the catabolic and anabolic processes, which were decoupled in the root-free soil. The effects of N and C availability on CUE in rhizosphere and root-free soil are discussed.     

Growth rates of rhizosphere microorganisms depend on competitive abilities of plants and N supply

Abstract

Plant‐microbial interactions under N‐limiting conditions are governed by competitive abilities of plants for N. Our study aimed to examine how two plant species of strawberry, Fragaria vesca L. (native species) and Duchesnea indica (Andrews) Focke (an invasive plant in central Europe), growing in intra‐specific and inter‐specific competition alter the functions of rhizosphere microorganisms in dependence on N availability. By intra‐specific competition at low N level, a 2.4‐fold slower microbial‐specific growth rate was observed under D. indica characterized by smaller root biomass and lower N content in roots compared with F. vesca. By inter‐specific competition of both plants at low N level, microbial growth rates were similar to those for D. indica indicating that plants with stronger competitive abilities for N controls microbial community in the rhizosphere. Since a high N level smoothed the differences between plant species in root and microbial biomass as well as in microbial growth rates under both intra‐specific and inter‐specific competition, we conclude that competitive abilities of plant species were crucial for microbial growth in the rhizosphere only under N imitation.     

The Antibiotic-Aided Distinguishing of Fungal and Bacterial Substrate-Induced Respiration in Various Soil Ecosystems

Fungal and bacterial substrate-induced respiration have been distinguished in gray forest and chestnut soils in various ecosystems (forest, grassland, arable soil, fallow land, and shelterbelt) using the antibiotics cycloheximide and streptomycin. The optimal inhibitory concentrations of the antibiotics, added separately and in combination; the preincubation time of the antibiotics with the soil before glucose addition; and the mass of added inert material (talc) have been determined. The inhibitor additivity ratio (IAR) has been calculated for the antibiotics. With the IAR differing from 1.0 by a value of more than 5%, the fungal and bacterial substrate-induced respiration cannot be distinguished reliably. Respiration measurements show that the microbial communities of natural ecosystems are dominated by fungi (81–95% on average). The smallest amount of fungi (54–59%) is found in the arable soil ecosystem.__________Translated from Mikrobiologiya, Vol. 74, No. 3, 2005, pp. 394–400.Original Russian Text Copyright © 2005 by Susyan, Ananyeva, Blagodatskaya.     

Extractable Microbial DNA Pool and Microbial Activity in Paleosols of Southern Urals

An evaluation of microbial DNA pools was performed using direct quantitative isolation of DNA from contemporary soils of Southern Urals and paleosols sealed under burial mounds early in the Bronze Age more than 5000 years B.P. Significant regression dependence was found between the biomass and DNA contents in these soils (R ²= 0.97). Activity and dominant ecological strategies of microbial communities of paleosols and contemporary southern black soil were compared from growth parameters obtained by analysis of respiratory curves. The ratio of maximum specific growth rates of soil microorganisms on glucose and on yeast extract was shown to provide an auxotrophy index for soil microbial communities.     

Labile carbon retention compensates for CO2 released by priming in forest soils

Increase of belowground C allocation by plants under global warming or elevated CO₂ may promote decomposition of soil organic carbon (SOC) by priming and strongly affects SOC dynamics. The specific effects by priming of SOC depend on the amount and frequency of C inputs. Most previous priming studies have investigated single C additions, but they are not very representative for litterfall and root exudation in many terrestrial ecosystems. We evaluated effects of ¹³C‐labeled glucose added to soil in three temporal patterns: single, repeated, and continuous on dynamics of CO₂ and priming of SOC decomposition over 6 months. Total and ¹³C labeled CO₂ were monitored to analyze priming dynamics and net C balance between SOC loss caused by priming and the retention of added glucose‐C. Cumulative priming ranged from 1.3 to 5.5 mg C g^?1 SOC in the subtropical, and from ?0.6 to 5.5 mg C g^?1 SOC in the tropical soils. Single addition induced more priming than repeated and continuous inputs. Therefore, single additions of high substrate amounts may overestimate priming effects over the short term. The amount of added glucose C remaining in soil after 6 months (subtropical: 8.1–11.2 mg C g^?1 SOC or 41‐56% of added glucose; tropical: 8.7–15.0 mg C g^?1 SOC or 43–75% of glucose) was substantially higher than the net C loss due to SOC decomposition including priming effect. This overcompensation of C losses was highest with continuous inputs and lowest with single inputs. Therefore, raised labile organic C input to soils by higher plant productivity will increase SOC content even though priming accelerates decomposition of native SOC. Consequently, higher continuous input of C belowground by plants under warming or elevated CO₂ can increase C stocks in soil despite accelerated C cycling by priming in soils.     

Soil C and N availability determine the priming effect: microbial N mining and stoichiometric decomposition theories

The increasing input of anthropogenically derived nitrogen (N) to ecosystems raises a crucial question: how does available N modify the decomposer community and thus affects the mineralization of soil organic matter (SOM). Moreover, N input modifies the priming effect (PE), that is, the effect of fresh organics on the microbial decomposition of SOM. We studied the interactive effects of C and N on SOM mineralization (by natural ¹³C labelling adding C₄‐sucrose or C₄‐maize straw to C₃‐soil) in relation to microbial growth kinetics and to the activities of five hydrolytic enzymes. This encompasses the groups of parameters governing two mechanisms of priming effects – microbial N mining and stoichiometric decomposition theories. In sole C treatments, positive PE was accompanied by a decrease in specific microbial growth rates, confirming a greater contribution of K‐strategists to the decomposition of native SOM. Sucrose addition with N significantly accelerated mineralization of native SOM, whereas mineral N added with plant residues accelerated decomposition of plant residues. This supports the microbial mining theory in terms of N limitation. Sucrose addition with N was accompanied by accelerated microbial growth, increased activities of β‐glucosidase and cellobiohydrolase, and decreased activities of xylanase and leucine amino peptidase. This indicated an increased contribution of r‐strategists to the PE and to decomposition of cellulose but the decreased hemicellulolytic and proteolytic activities. Thus, the acceleration of the C cycle was primed by exogenous organic C and was controlled by N. This confirms the stoichiometric decomposition theory. Both K‐ and r‐strategists were beneficial for priming effects, with an increasing contribution of K‐selected species under N limitation. Thus, the priming phenomenon described in ‘microbial N mining’ theory can be ascribed to K‐strategists. In contrast, ‘stoichiometric decomposition’ theory, that is, accelerated OM mineralization due to balanced microbial growth, is explained by domination of r‐strategists.