Microbial Biomass Turnover Research Articles

Microbial biomass turnover times and clues to cellular protein repair in energy-limited deep Baltic Sea sediments.

The discovery of active microbial life deeply buried beneath the seafloor has opened important questions: how do microorganisms cope with extreme energy limitation, what is their metabolic activity, and how do they repair damages to essential biomolecules? We used a D:L-amino acid model to calculate microbial biomass turnover times. We used a metagenome and metatranscriptome analysis to investigate the distribution of the gene that encodes Protein-L-iso aspartate(D-aspartate) O-methyltransferase (PCMT), an enzyme which recognizes damaged L-isoapartyl and D-aspartyl residues in proteins and catalyzes their repair. Sediment was retrieved during the Integrated Ocean Drilling Program (IODP) Expedition 347 from Landsort Deep and the Little Belt in the Baltic Sea. The study covers the period from the Baltic Ice Lake ca. 13 000 years ago to the present. Our results provide new knowledge on microbial biomass turnover times and protein repair in relation to different regimes of organic matter input. For the first time, we show that the PCMT gene was widely distributed and expressed among phylogenetically diverse groups of microorganisms. Our findings suggest that microbial communities are capable of repairing D-amino acids within proteins using energy obtained from the degradation of a mixture of labile compounds in microbial necromass and more recalcitrant organic matter.

Variation in rhizosphere priming and microbial growth and carbon use efficiency caused by wheat genotypes and temperatures

Living roots can influence microbial decomposition of soil organic matter, which has been referred to as the rhizosphere priming effect (RPE). Both microbial carbon efficiency (CUE) and microbial growth and turnover rates are associated with microbial decomposition and respiration of soil-derived C, but their linkage to the RPE remains poorly understood. Here we used a natural 13C tracer method to determine the RPE in soils planted with two wheat genotypes (249 or IAW2013) grown at high (30/24 °C during day/night) and low temperature (25/17 °C during day/night). We also determined microbial CUE, growth and biomass turnover rate using a substrate-independent H218O labeling method. The RPE varied from −2 to +455%, with significant effects of genotype, sampling date and their interaction with temperature. Compared to the unplanted control, microbial biomass C and growth/turnover rate were both enhanced in planted pots, with an average increase of 17% and 70%, respectively. Microbial CUE was lowest in pots planted with IAW2013 at low temperature, but there were no significant main effects of planting and temperature. Microbial biomass growth/turnover rate together with CUE accounted for 83% of the variation in soil-derived CO2, with a relatively larger contribution of microbial biomass growth/turnover rate (52%) than CUE (31%). Furthermore, using linear regression, we demonstrated that the RPE was significantly positively related to microbial biomass growth/turnover rate. No net soil organic C (SOC) loss or gain was detected, indicating that any increase in SOC due to increased microbial growth/turnover was counteracted by C loss caused by a higher RPE during the relatively short time of planting. These findings suggest that microbial biomass turnover associated with growth could control the loss of SOC with planting. We highlight the importance of plant-induced changes in microbial CUE and biomass growth/turnover for long-term soil C dynamics.

Microbial C:N:P stoichiometry and turnover depend on nutrients availability in soil: A 14C, 15N and 33P triple labelling study

Microbial biomass turnover and the associated recycling of carbon (Cmic), nitrogen (Nmic) and phosphorus (Pmic) depend on their stoichiometric relationships and plays a pivotal role for soil fertility. This study examines the responses of Cmic, Nmic, Pmic, the microbial respiration rate (CO2 efflux), and the total DNA content to C and nutrient addition in forest soils with very low (Low-P) and high P (High-P) contents. Both the Low-P and High-P soils were treated with a low and high level of C, N and P (5% and 200% of Cmic, Nmic and Pmic). Phosphorus (33P) was added before the addition of C (14C) and N (15N) to investigate the potential P limitation. We hypothesized two modes of microbial biomass C and nutrient turnover: 1) maintenance through intracellular metabolisms and/or 2) microbial growth and death through necromass reutilization. In Low-P soil, the 2-day-sooner increase of Cmic and Pmic compared to the increase of CO2 efflux and DNA content after high CN input showed the rapid initial uptake of C and limiting nutrients into microbial cells. It also demonstrated a lag period before microbial growth commenced. In High-P soil, however, the CO2 efflux and DNA content increased simultaneously with increases in microbial biomass, reflecting the microbial capacity for immediate growth. Afterwards, CO2 efflux and DNA content dropped to the level before CNP addition, with a decline of Cmic and Pmic in Low-P soil and a decline of Nmic in High-P soil, suggesting a C and P limitation in Low-P soil and N limitation in High-P soil. Under low CNP addition, the microorganisms in High-P soil are ready to grow, while those in Low-P soil are mainly in maintenance mode. The microorganisms under maintenance in low-P soil can switch to growth/death mode after removing the nutrient limitation. High CNP input caused a non-homeostatic response of Cmic: Nmic: Pmic stoichiometry from 691:105:1 to 33:1:1 in Low-P soil, mainly resulting from a higher storage of the limiting elements (C and P) in microbial biomass. The ratio remained stable under low CNP addition due to the endogenous metabolism of C and nutrient at maintenance. The C and nutrient were turnovered much faster by microorganisms in the growth/death mode, confirming a key principle of ecology: the stronger the limitation by an element, the more efficiently that element is retained within an organism, and the more intensively it is reused. The triple labeling approach linked with Cmic: Nmic: Pmic stoichiometry helped to identify the dominant maintenance and growth/death modes of microbial biomass CNP turnover in nutrient-limited and -unlimited soil.

Growth explains microbial carbon use efficiency across soils differing in land use and geology

The ratio of carbon (C) that is invested into microbial growth to organic C taken up is known as microbial carbon use efficiency (CUE), which is influenced by environmental factors such as soil temperature and soil moisture. How microbes will physiologically react to short-term environmental changes is not well understood, primarily due to methodological restrictions. Here we report on two independent laboratory experiments to explore short-term temperature and soil moisture effects on soil microbial physiology (i.e. respiration, growth, CUE, and microbial biomass turnover): (i) a temperature experiment with 1-day pre-incubation at 5, 15 and 25 °C at 60% water holding capacity (WHC), and (ii) a soil moisture/oxygen (O2) experiment with 7-day pre-incubation at 20 °C at 30%, 60% WHC (both at 21% O2) and 90% WHC at 1% O2. Experiments were conducted with soils from arable, pasture and forest sites derived from both silicate and limestone bedrocks. We found that microbial CUE responded heterogeneously though overall positively to short-term temperature changes, and decreased significantly under high moisture level (90% WHC)/suboxic conditions due to strong decreases in microbial growth. Microbial biomass turnover time decreased dramatically with increasing temperature, and increased significantly at high moisture level (90% WHC)/suboxic conditions. Our findings reveal that the responses of microbial CUE and microbial biomass turnover to short-term temperature and moisture/O2 changes depended mainly on microbial growth responses and less on respiration responses to the environmental cues, which were consistent across soils differing in land use and geology.

Contrasting effects of organic and mineral nitrogen challenge the N-Mining Hypothesis for soil organic matter priming

Addition of easily available organic substances to soil often increases the CO2 efflux from pre-existing soil carbon (C). This phenomenon is often explained in terms of the Nitrogen (N)-Mining Hypothesis. According to this proposed – but never conclusively proven – mechanism, increased C availability induces N limitation in microbes, which then access N by degrading soil organic matter (SOM) – a priming effect. This is supported by some experiments demonstrating reduced CO2 efflux after mineral N addition. However, amino acids cause priming, despite their very low C:N ratios and rapid deamination to mineral N. To explore this contradiction, we applied 14C- and 15N-labelled C and N sources (glucose, alanine and ammonium sulfate) to rigorously test two key predictions of the N-Mining Hypothesis: (i) an amino acid should stimulate much less priming than glucose, and (ii) priming should be similarly suppressed for an amino acid or a stoichiometrically equivalent addition of glucose plus mineral N. Both of these key predictions of the N-Mining Hypothesis failed. Efflux of CO2 from native C was essentially determined by the type and amount of C added, with alanine stimulating more priming than glucose (16–50% cumulative increase relative to control, versus 0–25%, respectively). Higher C additions caused more priming than low additions. Mineral N reduced native-C-derived CO2 efflux when added alone or with organic substrates, but this effect was independent of the organic C additions and did not influence C-induced priming. These results were inconsistent with the hypothesized role of N mining in priming. We conclude that the N-Mining Hypothesis, at least in its current form, is not a universal explanation for observed priming phenomena.Instead, we observed a strong correlation between the rates of priming and the mineralization of the added substrates, especially during the first 8 days. This indicated that priming was best explained by energy-induced synthesis of SOM-degrading exoenzymes, possibly in combination with apparent priming from accelerated turnover of microbial biomass.

Intensive fertilization (N, P, K, Ca, and S) decreases organic matter decomposition in paddy soil

Paddy soils have experienced intensive fertilization in recent decades. However, our understanding of the effects of fertilization on the carbon (C) cycle remains incomplete. In the present study, we investigated soil organic matter (SOM) decomposition in a 60-day incubation in response to N, P, K, Ca, and S addition to nutrient-limited paddy soil at three low and three high concentrations. High levels of nutrient addition decreased CO2 emission, qCO2, and microbial biomass. CO2 emissions increased (12–17%) owing to low levels of nutrient addition, whereas it decreased (3–21%) in response to high levels of nutrient addition. Microbial biomass and nutrient turnover rates increased after low levels of nutrient addition. Positive priming effect occurs under nutrient-limited conditions owing to the stimulation of microbial biomass production after low amount of exogenous nutrient input. In contrast, high levels of nutrient addition decreased microbial biomass and net N mineralization. This high N, P, K, Ca, and S addition could satisfy the needs of microbial growth, thereby decreasing the dependency of the organisms on the original nutrients from SOM decomposition. Therefore, negative priming was observed after high-level nutrient addition. In conclusion, intensive fertilization (with N, P, K, Ca, and S) reduces SOM decomposition through increased microbial turnover in paddy soils, which might positively affect C sequestration.

NH3 Volatilization, N2O Emission and Microbial Biomass Turnover from 15N-Labeled Manure Under Laboratory Conditions

ABSTRACTOn irrigated agricultural soils from semi-arid and arid regions, ammonia (NH3) volatilization and nitrous oxide (N2O) emission can be a considerable source of N losses. This study was designed to test the capture of 15N loss as NH3 and N2O from previous and recent manure application using a sandy, calcareous soil from Oman amended one or two times with 15N labeled manure to elucidate microbial turnover processes under laboratory conditions. The system allowed to detect 15N enrichments in evolved N2O-N and NH3-N of up to 17% and 9%, respectively, and total N, K2SO4 extractable N and microbial N pools from previous and recent 15N labeled manure applications of up to 7%, 8%, and 15%. One time manured soil had higher cumulative N2O-N emissions (141 µg kg−1) than repeatedly manured soil with 43 µg kg−1 of which only 22% derived from recent manure application indicating a priming effect.

The tree species matters: Belowground carbon input and utilization in the myco-rhizosphere

Rhizodeposits act as major carbon (C) source for microbial communities and rhizosphere-driven effects on forest C cycling receive increasing attention for maintaining soil biodiversity and ecosystem functions. By in situ13CO2 pulse labeling we investigated C input and microbial utilization of rhizodeposits by analyzing 13C incorporation into phospholipid fatty acids (PLFA) of beech- (Fagus sylvatica) and ash-associated (Fraxinus excelsior) rhizomicrobial communities. Plant compartments and soil samples were analyzed to quantify the allocation of assimilates. For 1 m high trees, ash assimilated more of the applied 13CO2 (31%) than beech (21%), and ash allocated twice as much 13C belowground until day 20. Approximately 0.01% of the applied 13C was incorporated into total PLFAs, but incorporation varied significantly between microbial groups. Saprotrophic and ectomycorrhizal fungi under beech and ash, but also arbuscular mycorrhizal fungi and Gram negative bacteria under ash, incorporated most 13C. PLFA allowed differentiation of C fluxes from tree roots into mycorrhiza: twice as much 13C was incorporated into the fungal biomarker 18:2ω6,9 under beech than under ash. Within 5 days, 30% of the fungal PLFA-C was replaced by rhizodeposit-derived 13C under beech but only 10% under ash. None of the other microbial groups reached such high C replacement, suggesting direct C allocation via ectomycorrhizal symbioses dominates the C flux under beech. Based on 13CO2 labeling and 13C tracing in PLFA we conclude that ash allocated more C belowground and has faster microbial biomass turnover in the rhizosphere compared to beech.

Microbial biomass phosphorus and C/N/P stoichiometry in forest floor and A horizons as affected by tree species

Forest floor horizons contain significant total P stocks, but information on the contribution of microbial biomass P (PMB) and on the controlling factors of this pool is limited. Slightly modified fumigation extraction procedures were used to investigate the stoichiometric relationships of PMB to microbial biomass C (CMB) and microbial biomass N (NMB) in the forest floor (L, F, H, and A horizons) at five sites, differing in P availability to trees, under adjacent spruce (Picea abies) and beech (Fagus sylvatica) stands. CMB, NMB, and PMB contents were higher in forest floors under beech than under spruce. Mean stocks of PMB and total P were roughly 27 and 100 kg ha−1 in the forest floor, respectively, but did not differ between the tree species, due to an increased organic matter accumulation in the forest floor under spruce. This reveals the importance of forest floor horizons and microbial biomass turnover for P nutrition of trees in acidic soils with the humus form moder. C/PMB ratios declined from roughly 26 in L to 13 in F and H horizons, followed by an increase to roughly 17 in A horizons. The range of C/PMB ratios was small at all sites in relation to the wide SOC/total P ratios of the litter used as microbial substrate, indicating a relatively strict homeostatic regulation of the forest floor microbial, mainly fungal biomass stoichiometry.

Nitrogen fertilization increases rhizodeposit incorporation into microbial biomass and reduces soil organic matter losses

Agricultural soils receive large amounts of anthropogenic nitrogen (N), which directly and indirectly affect soil organic matter (SOM) stocks and CO2 fluxes. However, our current understanding of mechanisms on how N fertilization affects SOM pools of various ages and turnover remains poor. The δ13C values of SOM after wheat (C3)-maize (C4) vegetation change were used to calculate the contribution of C4-derived rhizodeposited C (rhizo-C) and C3-derived SOM pools, i.e., rhizo-C and SOM. Soil (Ap from Haplic Luvisol) sampled from maize rhizosphere was incubated over 56 days with increasing N fertilization (four levels up to 300 kg N ha−1), and CO2 efflux and its δ13C were measured. Nitrogen fertilization decreased CO2 efflux by 27–42% as compared to unfertilized soil. This CO2 decrease was mainly caused by the retardation of SOM (C3) mineralization. Microbial availability of rhizo-C (released by maize roots within 4 weeks) was about 10 times higher than that of SOM (older than 4 weeks). Microbial biomass and dissolved organic C remained at the same level with increasing N. However, N fertilization increased the relative contribution of rhizo-C to microbial biomass by two to five times and to CO2 for about two times. This increased contribution of rhizo-C reflects strongly accelerated microbial biomass turnover by N addition. The decomposition rate of rhizo-C was 3.7 times faster than that of SOM, and it increased additionally by 6.5 times under 300 kg N ha−1 N fertilization. This is the first report estimating the turnover and incorporation of very recent rhizo-C (4 weeks old) into soil C pools and shows that the turnover of rhizo-C was much faster than that of SOM. We conclude that the contribution of rhizo-C to CO2 and to microbial biomass is highly dependent on N fertilization. Despite acceleration of rhizo-C turnover, the increased N fertilization facilitates C sequestration by decreasing SOM decomposition.

Using litter chemistry controls on microbial processes to partition litter carbon fluxes with the Litter Decomposition and Leaching (LIDEL) model

New understanding of the connection between dynamic microbial carbon use efficiency (CUE), litter decomposition products, and pathways of soil organic carbon (SOC) formation have not been fully integrated into current generalizable litter decomposition models. We developed a new approach, the Litter Decomposition and Leaching (LIDEL) model, that: 1) includes leaching and formation of dissolved organic carbon (DOC), important components of vertical C movement and SOC inputs into deeper soil layers, and 2) uses widely available litter chemistry data to drive the simulation of microbial processes that partition litter during decomposition, through affecting rates of CO2 respiration versus formation of microbial biomass and microbial products. Two ecologically important but poorly understood processes explored in this analysis include 1) the relationship between litter nitrogen (N) availability and rates of microbial decay and assimilation, and 2) the efficiency of DOC generation from the decomposition and leaching of soluble-versus cellulose-dominated plant litter fractions. We tested multiple hypothesis-driven model formulations, and for each estimated initial conditions and parameters using hierarchical Bayesian approaches. We combined data from experimental results and literature review for five types of litter that vary by initial lignin and N content. Our analyses showed the LIDEL model formulations with a logistic N limitation curve gave better predictions than model formulations using a linear N limitation curve. Model formulations with higher DOC generation efficiency from the soluble litter pool yielded more variable predictions and parameter estimations (shown by consistently wider 95% Bayesian credible intervals), but may have better simulated large DOC leaching events in early decomposition. Our analyses highlight a need for targeted studies clarifying measures of soluble litter and the generation of DOC during early litter decomposition, as well as rates of microbial biomass turnover and the flux of soluble versus non-soluble microbial products. Overall, the LIDEL model provides a robust generalizable framework to express and test hypotheses connecting litter chemistry and dynamic microbial CUE with the generation of DOC and microbial products during litter decomposition.

Soil microbial carbon use efficiency and biomass turnover in a long-term fertilization experiment in a temperate grassland

Soil microbial carbon use efficiency (CUE), defined as the ratio of organic C allocated to growth over organic C taken up, strongly affects soil carbon (C) cycling. Despite the importance of the microbial CUE for the terrestrial C cycle, very little is known about how it is affected by nutrient availability. Therefore, we studied microbial CUE and microbial biomass turnover time in soils of a long-term fertilization experiment in a temperate grassland comprising five treatments (control, PK, NK, NP, NPK). Microbial CUE and the turnover of microbial biomass were determined using a novel substrate-independent method based on incorporation of 18O from labeled water into microbial DNA. Microbial respiration was 28–37% smaller in all three N treatments (NK, NP, and NPK) compared to the control, whereas the PK treatment did not affect microbial respiration. N-fertilization decreased microbial C uptake, while the microbial growth rate was not affected. Microbial CUE ranged between 0.31 and 0.45, and was 1.3- to 1.4-fold higher in the N-fertilized soils than in the control. The turnover time ranged between 80 and 113 days and was not significantly affected by fertilization. Net primary production (NPP) and the abundance of legumes differed strongly across the treatments, and the fungal:bacterial ratio was very low in all treatments. Structural equation modeling revealed that microbial CUE was exclusively controlled by N fertilization and that neither the abundance of legumes (as a proxy for the quality of the organic matter inputs) nor NPP (as a proxy for C inputs) had an effect on microbial CUE. Our results show that N fertilization did not only decrease microbial respiration, but also microbial C uptake, indicating that less C was intracellularly processed in the N fertilized soils. The reason for reduced C uptake and increased CUE in the N-fertilization treatments is likely an inhibition of oxidative enzymes involved in the degradation of aromatic compounds by N in combination with a reduced energy requirement for microbial N acquisition in the fertilized soils. In conclusion, the study shows that N availability can control soil C cycling by affecting microbial CUE, while plant community-mediated changes in organic matter inputs and P and K availability played no important role for C partitioning of the microbial community in this temperate grassland.

Microbial carbon use efficiency and biomass turnover times depending on soil depth – Implications for carbon cycling

Processing of organic carbon (C) by soil microorganisms is a key process of terrestrial C cycling. For this reason we studied (i) microbial carbon use efficiency (CUE) defined as C allocated to growth over organic C taken up by the microbial community, and (ii) the turnover time of microbial biomass in a pasture and in two forest soils. We hypothesized that microbial CUE decreases in mineral soils with depth from the topsoil to the subsoil, while the turnover time of the microbial biomass increases due to energetic constrains. We determined microbial CUE and turnover of microbial biomass C using a novel substrate-independent method based on incorporation of 18O from labeled water into microbial DNA with concurrent measurements of basal respiration. Microorganisms showed decreasing C uptake rates with decreasing C contents in the deeper soil layers. In the forest soils, no adaptation of microbial CUE with soil depth took place, i.e., microbes in the forest topsoil used C at the same efficiency as microbes in the subsoil. However, in the pasture soil, microbial CUE decreased in the lower soil layers compared to the topsoil, indicating that microorganisms in the deeper soil layers allocated relatively more C to respiration. In the organic soil layer, microorganisms respired more per unit microbial biomass C than in the subsoil, but had a similar CUE despite the high C-to-nitrogen and C-to-phosphorus ratios of the litter layers. The turnover time of microbial biomass increased with soil depth in the two forest soils. Thus, in the forest soils, a lower microbial C uptake rate in the deeper soil layers was partially compensated by a longer turnover time of microbial biomass C. In conclusion, our findings emphasize that in addition to microbial CUE, the turnover time of the microbial biomass strongly affects soil C cycling.

Viral activities and life cycles in deep subseafloor sediments

Viruses are highly abundant in marine subsurface sediments and can even exceed the number of prokaryotes. However, their activity and quantitative impact on microbial populations are still poorly understood. Here, we use gene expression data from published continental margin subseafloor metatranscriptomes to qualitatively assess viral diversity and activity in sediments up to 159 metres below seafloor (mbsf). Mining of the metatranscriptomic data revealed 4651 representative viral homologues (RVHs), representing 2.2% of all metatranscriptome sequence reads, which have close translated homology (average 77%, range 60-97% amino acid identity) to viral proteins. Archaea-infecting RVHs are exclusively detected in the upper 30 mbsf, whereas RVHs for filamentous inoviruses predominate in the deepest sediment layers. RVHs indicative of lysogenic phage-host interactions and lytic activity, notably cell lysis, are detected at all analysed depths and suggest a dynamic virus-host association in the marine deep biosphere studied here. Ongoing lytic viral activity is further indicated by the expression of clustered, regularly interspaced, short palindromic repeat-associated cascade genes involved in cellular defence against viral attacks. The data indicate the activity of viruses in subsurface sediment of the Peruvian margin and suggest that viruses indeed cause cell mortality and may play an important role in the turnover of subseafloor microbial biomass.

Stoichiometric controls upon low molecular weight carbon decomposition

Soil carbon (C) and nitrogen (N) cycles are inextricably linked, yet the impacts of N availability upon soil C sequestration and turnover are poorly understood. According to stoichiometric theory, in the absence of nutrient limitation substrate decomposition will reach maximum rates, with C assimilated into microbial biomass at the expense of CO2 production. In this study, we added a 14C labelled low molecular weight substrate (glucose) to a sandy soil along with eleven increasing levels of N, phosphorus (P), and sulphur (S) in relative proportions as required for microbial biomass production. Adding a simple soluble substrate allowed us to explicitly examine changes in microbial transformations of added C, rather than changes resulting from extracellular enzyme activity or the extent of substrate decomposition. We hypothesized that as nutrient addition increased, an increasing proportion of the glucose-C provided would be incorporated into microbial biomass at the expense of CO2 production and stabilized as soil organic carbon (SOC). Instead, CO2 production from glucose-C increased significantly with nutrient addition without measurable changes in glucose-derived microbial biomass or SOC. This suggests that if there was greater glucose-derived microbial biomass produced under higher nutrient addition it was offset by a higher rate of microbial biomass turnover. We also found greater soil-derived microbial biomass at lower nutrient addition levels, potentially supporting the concept of microbial mining of soil organic matter (SOM) for nutrients under low nutrient availability. In conclusion, our data suggest that in a sandy soil with low capacity for physical protection of SOM, nutrient addition does not immediately promote C sequestration in the soil microbial community, and that the interaction between C stabilization and nutrient addition requires further work, especially for predicting ecosystem responses.

Short-term effects of prescribed burning on phosphorus availability in a suburban native forest of subtropical Australia

Prescribed burning can alter nutrient availability to plants. Plant growth in tropical and subtropical forests is frequently phosphorus (P) limited. Soil P availability is influenced by a combination of multiple factors including soil chemical and biological properties. The aims of this study were to investigate the short-term effects of prescribed burning on soil P status and to evaluate the key drivers responsible for the variation in soil P fractions. Soil samples were collected at a depth of 0–10 cm at two sites in a suburban native forest. One site (the burnt site) was burned on 11 August 2011. The other site (the control site) was not burned but served as a reference. Sampling was conducted at four times: before burning, 12 days after burning (T1), 1 week after T1 (T2), and 1 month after T2 (T3). Soil pH, P fractions, microbial biomass carbon (C) and P, and activities of acid and alkaline phosphatase were measured. Total P was relatively low at both sites compared with other subtropical forests. Microbial biomass P accounted for approximately 10 % of soil total P at the two sites, suggesting that the turnover of microbial biomass is critical for soil P availability. Soil properties at the control site remained unchanged over the time. Soil organic forms of P at the burnt site were decreased by the prescribed burning, and the greatest reduction was found in moderately labile organic P (e.g., NaOH-extractable fractions). Soil inorganic forms of P, however, were not correspondingly increased by the prescribed burning. Microbial biomass P was closely related to the shifts in P fractions. These effects were only detected immediately after the fire. Microbial biomass could serve as a sink of P in P-impoverished soils and play an important role in soil P transformation. Our results indicate that microbial biomass is an important factor that governs P status after prescribed burning. The rapid recovery of microbial biomass P could be beneficial to the P requirement for plant regrowth after prescribed burning.

Microbial biomass, nutrient availability and nutrient uptake by wheat in two soils with organic amendments

A 72-day greenhouse pot experiment was conducted with a sandy loam or a silt loam soil to examine the effects of farmyard manure (FYM), poultry litter (PL) and biogenic waste compost (BWC) at 10 g dw kg-1 soil on microbial biomass and activity and growth and nutrient uptake by wheat. Soil samples were collected at days 0, 14, 28, 42, 56 and 72 after planting. Growth and nutrient uptake by wheat were determined on day 72. All three amendments increased microbial biomass C, N and P, dehydrogenase activity, plant growth and nutrient uptake with a greater effect by FYM and PL than by BWC. All amendments increased microbial biomass C, N and P and enzyme activity particularly on day 0. These microbial parameters decreased after day 0 indicating microbial biomass turnover. All amendments increased plant growth and nutrient uptake. It is concluded that organicamendments can stimulate microbial growth and nutrient uptake as well as plant growth and nutrient uptake. Microbes can increase plant nutrient availability by nutrient mobilisation but also because nutrients taken up by the microbial biomass initially could become available to plants when the microbial biomass turns over as the easily available C is depleted.

Preferential use of root litter compared to leaf litter by beech seedlings and soil microorganisms

Litter decomposition is regulated by e.g. substrate quality and environmental factors, particularly water availability. The partitioning of nutrients released from litter between vegetation and soil microorganisms may, therefore, be affected by changing climate. This study aimed to elucidate the impact of litter type and drought on the fate of litter-derived N in beech seedlings and soil microbes. We quantified 15N recovery rates in plant and soil N pools by adding 15N-labelled leaf and/or root litter under controlled conditions. Root litter was favoured over leaf litter for N acquisition by beech seedlings and soil microorganisms. Drought reduced 15N recovery from litter in seedlings thereby affecting root N nutrition. 15N accumulated in seedlings in different sinks depending on litter type. Root turnover appears to influence (a) N availability in the soil for plants and soil microbes and (b) N acquisition and retention despite a presumably extremely dynamic turnover of microbial biomass. Compared to soil microorganisms, beech seedlings represent a very minor short-term N sink, despite a potentially high N residence time. Furthermore, soil microbes constitute a significant N pool that can be released in the long term and, thus, may become available for N nutrition of plants.

Microbial responses to carbon and nitrogen supplementation in an Antarctic dry valley soil

AbstractThe soils of the McMurdo Dry Valleys are exposed to extremely dry and cold conditions. Nevertheless, they contain active biological communities that contribute to the biogeochemical processes. We have used ester-linked fatty acid (ELFA) analysis to investigate the effects of additions of carbon and nitrogen in glucose and ammonium chloride, respectively, on the soil microbial community in a field experiment lasting three years in the Garwood Valley. In the control treatment, the total ELFA concentration was small by comparison with temperate soils, but very large when expressed relative to the soil organic carbon concentration, indicating efficient conversion of soil organic carbon into microbial biomass and rapid turnover of soil organic carbon. The ELFA concentrations increased significantly in response to carbon additions, indicating that carbon supply was the main constraint to microbial activity. The large ELFA concentrations relative to soil organic carbon and the increases in ELFA response to organic carbon addition are both interpreted as evidence for the soil microbial community containing organisms with efficient scavenging mechanisms for carbon. The diversity of the ELFA profiles declined in response to organic carbon addition, suggesting the responses were driven by a portion of the community increasing in dominance whilst others declined.

Seasonality of soil microbial nitrogen turnover in continental steppe soils of Inner Mongolia

Annual estimates and seasonal patterns of gross nitrogen turnover in terrestrial soils are poorly understood due to the lack of experimental evidence. Based on year round sampling in wintergrazed and ungrazed steppe soils of Inner Mongolia, we show that measurements of net rates of ammonification (−9 to −6 kg N ha−1 year−1) or nitrification (19 to 31 kg N ha−1 year−1) do not at all reflect the pronounced dynamics of gross rates of ammonification (215–240 kg N ha−1 year−1) or nitrification (362–417 kg N ha−1 year−1).Four different seasons with characteristic functional patterns of N turnover were identified: (1) Growing season dynamics as characterized by drying/rewetting cycles and negatively correlated temporal courses of net microbial growth and periods with intensive gross ammonification, contributing 40–52% and 29–32% to cumulative annual gross ammonification and nitrification, respectively. Net N mineralization was almost exclusively observed during the growing season. (2) Microbial N dynamics during the autumn freeze‐thaw period was characterized by a sharp decline in microbial biomass in conjunction with a peak of gross nitrification contributing 19–36% to cumulative annual fluxes. (3) During winter at constantly frozen soil, a net build‐up of microbial biomass was observed, whereas gross N turnover rates were low, contributing 7–10% and 6–11% to cumulative annual gross ammonification or gross nitrification, respectively. (4) The spring freeze‐thaw period showed extremely dynamic changes in gross N turnover and soil nitrate concentrations. This period contributed 34–44% and 21–46% to cumulative annual gross ammonification and nitrification, respectively. This study highlights that freeze‐thaw cycles are key periods for understanding patterns and magnitudes of gross N turnover in semi‐arid continental steppe ecosystems. The results further imply that the observed patterns of microbial biomass and gross N turnover dynamics are likely the consequence of a seasonal succession of microbial communities and turnover of microbial biomass. Our findings emphasize the necessity for high resolution studies on gross N turnover as a prerequisite to infer functioning and annual budgets of ecosystem N cycling.