Earthworms increase soil microbial biomass carrying capacity and nitrogen retention in northern hardwood forests

Earthworms have been shown to produce contrasting effects on soil carbon (C) and nitrogen (N) pools and dynamics. We measured soil C and N pools and processes and traced the flow of ¹³C and ¹⁵N from sugar maple (Acer saccharum Marsh.) litter into soil microbial biomass and respirable C and mineralizable and inorganic N pools in mature northern hardwood forest plots with variable earthworm communities. Previous studies have shown that plots dominated by either Lumbricus rubellus or Lumbricus terrestris have markedly lower total soil C than uncolonized plots. Here we show that total soil N pools in earthworm colonized plots were reduced much less than C, but significantly so in plots dominated by contain L. rubellus. Pools of microbial biomass C and N were higher in earthworm-colonized (especially those dominated by L. rubellus) plots and more ¹³C and ¹⁵N were recovered in microbial biomass and less was recovered in mineralizable and inorganic N pools in these plots. These plots also had lower rates of potential net N mineralization and nitrification than uncolonized reference plots. These results suggest that earthworm stimulation of microbial biomass and activity underlie depletion of soil C and retention and maintenance of soil N pools, at least in northern hardwood forests. Earthworms increase the carrying capacity of soil for microbial biomass and facilitate the flow of N from litter into stable soil organic matter. However, declines in soil C and C:N ratio may increase the potential for hydrologic and gaseous losses in earthworm-colonized sites under changing environmental conditions.     

Recent (<4 year old) leaf litter is not a major source of microbial carbon in a temperate forest mineral soil

Microbial communities in soil A horizons derive their carbon from several potential sources: organic carbon (C) transported down from overlying litter and organic horizons, root-derived C, or soil organic matter. We took advantage of a multi-year experiment that manipulated the ¹⁴C isotope signature of surface leaf litter inputs in a temperate forest at the Oak Ridge Reservation, Tennessee, USA, to quantify the contribution of recent leaf litter C to microbial respiration and biomarkers in the underlying mineral soil. We observed no measurable difference (<∼40‰ given our current analytical methods) in the radiocarbon signatures of microbial phospholipid fatty acids (PLFA) isolated from the top 10 cm of mineral soil in plots that experienced 3 years of litterfall that differed in each year by ∼750‰ between high-¹⁴C and low-¹⁴C treatments. Assuming any difference in ¹⁴C between the high- and low-¹⁴C plots would reflect C derived from these manipulated litter additions, we estimate that <∼6% of the microbial C after 4 years was derived from the added 1-4-year-old surface litter. Large contributions of C from litter < 1 year (or >4 years) old (which fell after (or prior to) the manipulation and therefore did not differ between plots) are not supported because the ¹⁴C signatures of the PLFA compounds (averaging 200-220‰) is much higher that of the 2004-5 leaf litter (115‰) or pre-2000 litter. A mesocosm experiment further demonstrated that C leached from ¹⁴C-enriched surface litter or the O horizon was not a detectable C source in underlying mineral soil microbes during the first eight months after litter addition. Instead a decline in the ¹⁴C of PLFA over the mesocosm experiment likely reflected the loss of a pre-existing substrate not associated with added leaf litter. Measured PLFA Δ¹⁴C signatures were higher than those measured in bulk mineral soil organic matter in our experiments, but fell within the range of ¹⁴C values measured in mineral soil roots. Together, our experiments suggest that root-derived C is the major (>60%) source of C for microbes in these temperate deciduous forest soils.     

Soil fertility and bacterial community composition in a semiarid Mediterranean agricultural soil under long-term tillage management

This research attempted to investigate a part of the United Nations sustainable development goal 15, dealing with preventing land degradation and halting the loss of microorganisms’ diversity. Since soil deterioration and biodiversity loss in the Mediterranean area are occurring because of intensive management, we evaluated some biochemical and microbiological parameters and bacterial biodiversity under long-term conventional tillage (CT) and no-tillage (NT) practices, in Basilicata, a typical Region of Southern Italy, characterized by a semiarid ecosystem. The highest biological fertility index (BFI) (composed of soil organic matter, microbial biomass C, cumulative microbial respiration during 25 days of incubation, basal respiration, metabolic quotient and mineralization quotient) was determined for the 0–20 cm of NT soil (class V, high biological fertility level). The analysis of the taxonomic composition at the phylum level revealed the higher relative abundance of copiotrophic bacteria such as Proteobacteria, Actinobacteria and Bacteroidetes in the NT soil samples as compared to the CT soil. These copiotrophic phyla, more important decomposers of soil organic matter (SOM) than oligotrophic phyla, are responsible of a higher microbial C use efficiency (CUE) in tilled soil, being microbial community composition, C substrates content and CUE closely linked. The higher Chao1 and Shannon indices, under the NT management, also supported the hypothesis that the bacterial diversity and richness increased in the no-till soils. In conclusion, we can assume that the long-term no-tillage can preserve an agricultural soil in a semiarid ecosystem, enhancing soil biological fertility level and bacterial diversity.     

C and N natural abundance of the soil microbial biomass

Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is ¹³C, and especially ¹⁵N-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was ¹⁵N-enriched relative to the total (3.2 ‰) and extractable N pools (3.7 ‰), and ¹³C-enriched relative to the extractable C pool (2.5 ‰). The microbial biomass was also ¹³C-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 ‰), but ¹³C-depleted for soils with a C4 signature (−1.1 ‰). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation.     

Impact of tillage intensity on carbon and nitrogen pools in surface and sub‐surface soils of three long‐term field experiments

Management options such as the intensity of tillage are known to influence the turnover dynamics of soil organic matter. However, less information is available about the influence of the tillage intensity on individual soil organic matter pools with different turnover dynamics in surface as compared with sub‐surface soils. This study aimed to analyse the impact of no tillage (NT), reduced tillage (RT) and conventional tillage (CT) on labile, intermediate and stable carbon (C) and nitrogen (N) pools in surface and sub‐surface soils. We took surface and sub‐surface soil samples from the three tillage systems in three long‐term field experiments in Germany. The labile, intermediate and stable C and N pool sizes were determined by using the combined application of a decomposition experiment and a physical‐chemical separation procedure. For the surface soils, we found larger stocks of the labile C and N pool under NT and RT (C, 1.7 and 1.3 t ha^?1; N, 180 and 160 kg ha^?1) than with CT (C, 0.5 t ha^?1; N, 60 kg ha^?1). In contrast, we found significantly larger stocks of the labile C pool under CT (2.7 t ha^?1) than with NT and RT (2 t ha^?1) for the sub‐surface soils. The intermediate pool accounted for 75–84% of the soil organic C and total N stocks. However, the stocks of the intermediate N and C pools were only distinctly larger for NT than for CT in the surface soils. The stocks of the stable C and N pools were not affected by the tillage intensity but were positively correlated with the stocks of the clay‐size fraction and oxalate soluble aluminum, indicating a strong influence of site‐specific mineral characteristics on the size of these pools. Our results indicate soil depth‐specific variations in the response of organic matter pools to tillage of different intensity. This means that the potential benefits of decreasing tillage intensity with respect to soil functions that are closely related to organic matter dynamics have to be evaluated separately for surface and sub‐surface soils.     

Drivers of microbial respiration and net N mineralization at the continental scale

The dominant pools of C and N in the terrestrial biosphere are in soils, and understanding what factors control the rates at which these pools cycle is essential in understanding soil CO₂ production and N availability. Many previous studies have examined large scale patterns in decomposition of C and N in plant litter and organic soils, but few have done so in mineral soils, and fewer have looked beyond ecosystem specific, regional, or gradient-specific drivers. In this study, we examined the rates of microbial respiration and net N mineralization in 84 distinct mineral soils in static laboratory incubations. We examined patterns in C and N pool sizes, microbial biomass, and process rates by vegetation type (grassland, shrubland, coniferous forest, and deciduous/broadleaf forest). We also modeled microbial respiration and net N mineralization in relation to soil and site characteristics using structural equation modeling to identify potential process drivers across soils. While we did not explicitly investigate the influence of soil organic matter quality, microbial community composition, or clay mineralogy on microbial process rates in this study, our models allow us to put boundaries on the unique explanatory power these characteristics could potentially provide in predicting respiration and net N mineralization. Mean annual temperature and precipitation, soil C concentration, microbial biomass, and clay content predicted 78% of the variance in microbial respiration, with 61% explained by microbial biomass alone. For net N mineralization, only 33% of the variance was explained, with mean annual precipitation, soil C and N concentration, and clay content as the potential drivers. We suggest that the high R² for respiration suggests that soil organic matter quality, microbial community composition, and clay mineralogy explain at most 22% of the variance in respiration, while they could explain up to 67% of the variance in net N mineralization.     

Microbial immobilisation of C rhizodeposits in rhizosphere and root-free soil under continuous C labelling of oats

A greenhouse experiment was conducted by growing oats (Avenasativa L.) in a continuously ¹³CO₂ labeled atmosphere. The allocation of ¹³C-labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO₂ emissions were examined over the plant's life cycle. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 μm mesh) filled with soil. Two peaks of ¹³C in rhizosphere pools of microbial biomass and dissolved organic carbon (DOC), as well as in CO₂ emissions at the earing and ripeness stages were revealed. These ¹³C maxima corresponded to: (i) the end of rapid root growth and (ii) beginning of root decomposition, respectively. The δ¹³C values of microbial biomass were higher than those of DOC and of soil organic matter (SOM). The microbial biomass C accounted for up to 56 and 39% of ¹³C recovered in the rhizosphere and root-free soil, respectively. Between 4 and 28% of ¹³C assimilated was recovered in the root-free soil. Depending on the phenological stage, the contribution of root-derived C to total CO₂ emission from soil varied from 61 to 92% of total CO₂ evolved, including 4-23% attributed to rhizomicrobial respiration. While 81-91% of C substrates used for microbial growth in the root-free soil and rhizosphere came from SOM, the remaining 9-19% of C substrates utilized by the microbial biomass was attributable to rhizodeposition. The use of continuous isotopic labelling and physical separation of root-free and rhizosphere soil, combined with natural ¹³C abundance were effective in gaining new insight on soil and rhizosphere C-cycling.     

Carbon flow into microbial and fungal biomass as a basis for the belowground food web of agroecosystems

The origin and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. The present study aimed to elucidate and quantify the carbon (C) flow from both root and shoot litter residues into soil organic, extractable, microbial and fungal C pools. Using the shift in C stable isotope values associated with replacing C3 by C4 plants we followed root- vs. shoot litter-derived C resources into different soil C pools. We established the following treatments: Corn Maize (CM), Fodder Maize (FM), Wheat + maize Litter (WL) and Wheat (W) as reference. The Corn Maize treatment provided root- as well as shoot litter-derived C (without corn cobs) whereas Fodder Maize (FM) provided only root-derived C (aboveground shoot material was removed). Maize shoot litter was applied on the Wheat + maize Litter (WL) plots to trace the incorporation of C4 litter C into soil microorganisms. Soil samples were taken three times per year (summer, autumn, winter) over two growing seasons. Maize-derived C signal was detectable after three to six months in the following pools: soil organic C (C_org), extractable organic C (EOC), microbial biomass (C_mic) and fungal biomass (ergosterol). In spite of the lower amounts of root- than of shoot litter-derived C inputs, similar amounts were incorporated into each of the C pools in the FM and WL treatments, indicating greater importance of the root- than shoot litter-derived resources for the soil microorganisms as a basis for the belowground food web. In the CM plots twice as much maize-derived C was incorporated into the pools. After two years, maize-derived C in the CM treatment contributed 14.1, 24.7, 46.6 and 76.2% to C_org, EOC, C_mic and ergosterol pools, respectively. Fungi incorporated maize-derived C to a greater extent than did total soil microbial biomass.     

Characterizing the relationships between soil organic matter components and microbial function and composition along a tillage disturbance gradient

We studied the effect of no-till (disc seeder), conventional-till (tine scarifier+disc seeder) and rotary-till (rotary hoe+disc seeder) management on soil organic matter (SOM) components, rates of carbon (C) and nitrogen (N) cycling, substrate utilization and microbial community composition. We hypothesized that labile SOM fractions are sensitive to changes in tillage techniques and, in turn mediate any tillage-induced changes in microbial function and composition. A replicated field site was established in May 1998 in the semi-arid agricultural region of Western Australia and soils were collected in September 2004. We found soil pH varied between different tillage techniques as an initial lime application was mixed to deeper soil depths in rotary-till soil than no-till and conventional-till soil. Total-C was greater in surface soil and lower in subsurface soil from no-till and conventional-till plots than from rotary-till plots, but there was no effect of tillage technique on total-C when averaged across soil depths. Light (specific density <1.0 g cm^?3) fraction organic matter (LFOM), dissolved organic matter (DOM) and microbial biomass (MB) C and N pools, and rates of C and N cycling all tended to decrease with soil depth. In general, LFOM-C and N, dissolved organic C (DOC) and microbial biomass carbon (MB-C), soil respiration, cellulase activity, gross immobilization rates were positively correlated (r>0.50) and were greater in no-till and conventional-till soil than rotary-till soil both within, and across soil depths. These soil variables generally increased (r>0.5) with increasing soil pH. Dissolved organic N and gross N mineralization were positively correlated (r>0.90) but neither was affected by tillage techniques. No-till soil had greater utilization of carboxylic acids and lower utilization of amino acids and carbohydrates than conventional-till and rotary-till soil; surface soil also had greater utilization of carboxylic acids than subsurface soil. In turn, substrate utilization differed between soil depths, and between no-till soil and conventional-till and rotary-till soil; these differences were correlated with soil pH, total-N, DOC, LFOM-N and microbial biomass nitrogen (MB-N). Bacterial and fungal biomasses generally decreased with soil depth and were greater in no-till and conventional-till soil than rotary-till soil. Microbial community composition differed between all tillage techniques and soil depths; these differences were correlated with soil textural classes, soil pH, and total, LFOM, DOM and microbial C and N pools. These results indicate that most tillage-induced changes to soil properties were associated with the greater soil disturbance under rotary-till than under no-till or conventional-till management. Our results indicate that tillage-induced changes to soil pH, and LFOM, DOM and microbial biomass pools are likely to be important regulators of the rates of C and N cycling, substrate utilization and microbial community composition in this coarse textured soil.     

Seasonal dynamics of leaf- and root-derived C in arctic tundra mesocosms

We investigated contributions of leaf litter, root litter and root-derived organic material to tundra soil carbon (C) storage and transformations. ¹⁴C-labeled materials were incubated for 32 weeks in moist tussock tundra soil cores under controlled climate conditions in growth chambers, which simulated arctic fall, winter, spring and summer temperatures and photoperiods. In addition, we tested whether the presence of living plants altered litter and soil organic matter (SOM) decomposition by planting shoots of the sedge Eriophorum vaginatum in half of the cores. Our results suggest that root litter accounted for the greatest C input and storage in these tundra soils, while leaf litter was rapidly decomposed and much of the C lost to respiration. We observed transformations of ¹⁴C between fractions even when total C appeared unchanged, allowing us to elucidate sources and sinks of C used by soil microorganisms. Initial sources of C included both water soluble (WS) and acid-soluble (AS) fractions, primarily comprised of carbohydrates and cellulose, respectively. The acid-insoluble (AIS) fraction appeared to be a sink for C when conditions were favorable for plant growth. However, decreases in ¹⁴C activity from the AIS fraction between the fall and spring harvests in all treatments indicated that microorganisms consumed recalcitrant C compounds when soil temperatures were below 0 °C. In planted leaf litter cores and in both planted and unplanted SOM cores, the greatest amounts of ¹⁴C at the end of the experiment were found in the AIS fraction, suggesting a high rate of humification or accumulation of decay-resistant plant tissues. In unplanted leaf litter cores and planted and unplanted root litter cores most of the ¹⁴C remaining at the end of the experiment was in the AS fraction suggesting less extensive humification of leaf and root detritus. Overall, the presence of living plants stimulated decomposition of leaf litter by creating favorable conditions for microbial activity at the soil surface. In contrast, plants appeared to inhibit decomposition of root litter and SOM, perhaps because of microbial preferences for newer, more labile inputs from live roots.     

Root-derived carbon in soil respiration and microbial biomass determined by 14C and 13C

Two approaches to quantitatively estimating root-derived carbon in soil CO₂ efflux and in microbial biomass were compared under controlled conditions. In the ¹⁴C labelling approach, maize (Zea mays) was pulse labelled and the tracer was chased in plant and soil compartments. Root-derived carbon in CO₂ efflux and in microbial biomass was estimated based on a linear relationship between the plant shoots and the below-ground compartment. Since the maize plants were grown on C₃ soil, in a second approach the differences in ¹³C natural abundance between C₃ and C₄ plants were used to calculate root-derived carbon in the CO₂ efflux and in the microbial biomass. The root-derived carbon in the total CO₂ efflux was between 69% and 94% using the ¹⁴C labelling approach and between 86% and 94% in the natural ¹³C labelling approach. At a ¹³C fractionation measured to be 5.2‰ between soil organic matter (SOM) and CO₂, the root-derived contribution to CO₂ ranged from 70% to 88% and was much closer to the results of the ¹⁴C labelling approach. Root-derived contributions to the microbial biomass carbon ranged from 2% to 9% using ¹⁴C labelling and from 16% to 36% using natural ¹³C labelling. At a 3.2‰ ¹³C fractionation between SOM and microbial biomass, both labelling approaches yielded an equal contribution of root-derived C in the microbial biomass. Both approaches may therefore be used to partition CO₂ efflux and to quantify the C sources of microbial biomass. However, the assumed ¹³C fractionation strongly affects the contributions of individual C sources.     

No-till soil management increases microbial biomass and alters community profiles in soil aggregates

Aggregation is important for soil functioning, providing physical protection of organic matter and microbial inhabitants. Tillage disrupts aggregates, increases wind and water erosion of soils and exposes formerly protected organic matter to decomposition and losses. Microbial biomass and community dynamics in dry-sieved aggregate-size classes from long-term no-till (NT) and conventionally tilled (CT) soils were examined using phospholipid fatty acid analysis (PLFA). Bacterial, fungal, and total biomass were up to 32% greater in NT compared to CT aggregates. Aggregate size also affected microbial biomass, which was highest in the 1–2 mm size class. Arbuscular mycorrhizal fungi (AMF) were particularly affected by tillage disturbance with increases of 40–60% among aggregate-size classes in NT vs. CT, but glomalin related soil protein concentration was not different between tillage treatments or among aggregate-size classes. Bacterial stress biomarkers were higher in CT than NT aggregates but were not significantly correlated with total C, total N or C:N ratio, indicating that the physiological status of bacteria within aggregates was not simply governed by the quantity of available resources. Ordination analysis of PLFA profiles demonstrated a shift in microbial community structure between NT and CT aggregates, correlated with AMF abundance in NT aggregates and increased bacterial stress biomarkers in CT aggregates. Our results demonstrated greater microbial biomass and altered microbial community structure in NT vs. CT aggregates. This work demonstrates that tillage management influences microbial community structure within aggregates and may provide a potential explanation for differences in process rates observed in NT vs. CT soils. Further research into the processes that govern community structure in aggregates from NT and tilled soils is needed to better understand how the interaction of microorganisms with their physical environment affects nutrient turnover and availability.     

Impacts of organic residue management on the soil C dynamics in a tropical eucalypt plantation on a nutrient-poor sandy soil after three rotations

Harvest residue management is a key issue for the sustainability of short rotation plantations of fast growing tree species established on poor soils and for potential carbon (C) accretion in many soils. By measuring the C isotope composition (δ¹³C) of different soil organic matter fractions and microbial respiration, we examined the effects of organic residue management at harvest on soil C dynamics in a tropical eucalypt plantation established on a savannah at the end of the third rotation on three treatments repeated at each harvest. We compared plots where the standard harvesting method in Congolese commercial plantations was applied (SWH; only removing the debarked commercial-sized boles) with plots where all the aboveground biomass and the litter layer from the previous rotations were removed at harvest (R) and plots where the residues from a whole tree harvest treatment were added (double slash, DS). Organic residue removal decreased the accretion of eucalypt-derived C in coarse and fine particulate organic matter (POM) fractions and in the organo-mineral fraction, resulting in 44% less total soil C in the top soil (0–0.05 m) but did not affect the amount of savannah-derived C in any SOM fraction. In contrast, increasing the amount of harvest residue by 36% in comparison with the reference practice did not increase the amount of C stored in the soil nor the accretion of eucalypt-derived C in any soil organic matter fractions, but the amount of savannah-derived C remaining in the coarse POM fraction was higher. We concluded that carbon accretion may be limited by the low C saturation level of these sandy-structured soils but that higher rates of residue retention may reduce priming on older savannah-derived C.     

Soil microbial response to tillage systems and fertilization in a sunflower rhizosphere

Tillage systems and fertilization have important effects on soil microorganism activity. Information regarding the simultaneous evaluation of long-term tillage and fertilization on soil microbial traits in sunflower fields is not available. Therefore, this study was conducted to determine the best tillage and fertilization system for soil microbial parameters. The experimental design was a split plot based on a randomized complete block design with three replications. Main plots consisted of the long-term tillage systems (1999–2011) including: no tillage (NT), minimum tillage (MT) and conventional tillage (CT). Six methods of fertilization, including farmyard manure (N1), compost (N2), chemical fertilizers (N3), farmyard manure + compost (N4); farmyard manure + compost + chemical fertilizers (N5), and control (N6) were arranged in subplots. Results showed that the highest amount of microbial biomass was observed in treatment NTN4. The highest and lowest values of enzyme activities (acid, alkaline phosphatase, urease, dehydrogenase and protease) were found in organic fertilizers + NT and chemical fertilizers + CT plots, respectively. Highest basal and induced respiration values were found for NTN4 treatment. Correlation coefficients between enzyme activity, respiration and microbial biomass carbon were significant.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Root-derived respiration and non-structural carbon of rice seedlings

Various methods have been suggested to separate root and microbial contributions to soil respiration. However, to date there is no ideal approach available to partition below-ground CO₂ fluxes in its components although the combination of traditional methods with approaches based on isotopes seems especially promising for the future improvement of estimates. Here we provide evidence for the applicability of a new approach based on the hypothesis that root-derived (rhizomicrobial) respiration, including root respiration and CO₂ derived from microbial activity in the immediate vicinity of the root, is proportional to non-structural carbon contents (sugars and organic acids) of plant tissues. We examined relationships between root-derived CO₂ and non-structural carbon of rice (Oryza sativa) seedlings using ¹⁴C pulse labelling techniques, which partitioned the ¹⁴C fixed by photosynthesis into root-derived ¹⁴CO₂, and ¹⁴C in sugars and organic acids of roots and shoots. After the ¹⁴C pulse ¹⁴C in both sugars and organic acids of plant tissues decreased steeply during the first 12 h, and then decreased at a lower rate during the remaining 60 h. Soil ¹⁴CO₂ efflux and soil CO₂ efflux strongly depended on ¹⁴C pools in non-structural carbon of the plant tissues. Based on the linear regression between root-derived respiration and total non-structural carbon (sugars and organic acids) of roots, non-rhizomicrobial respiration (SOM-derived) was estimated to be 0.25 mg C g⁻¹ root d.w. h⁻¹. Assuming the value was constant, root-derived respiration contributed 85–92% to bulk soil respiration.     

Micro-scale modelling of carbon turnover driven by microbial succession at a biogeochemical interface

The detritusphere is a very thin but microbiological highly active zone in soil. To trace the fate of litter carbon in the detritusphere we developed a new 1D dynamic mechanistic model. In a microcosm experiment soil cores were incubated with ¹³C labelled rye residues (δ¹³C=299‰), which were placed on the surface. Microcosms were sampled after 3, 7, 14, 28, 56 and 84 days and soil cores were separated into layers of increasing distance to the litter. Gradients in soil organic carbon (TOC), dissolved organic carbon (DOC), microbial biomass and activity were detected over a distance of 3 mm from the litter layer. The newly developed 1D model simulates both the total carbon and the ¹³C carbon pools and fluxes, so that it was possible to include the ¹³C data in model optimisation. The special feature of the model is that it operates with two decomposer populations; the first one is assumed to be dominated by bacteria (initial-stage decomposer) and second one by fungi (late-stage decomposer). Moreover, in the model the DOC pool is divided into two sub pools. Each DOC pool is consumed by one of the decomposer populations. After parameter optimisation the model was well suited to simulate the experimental data. The model explained 92% of the observed variance. The model output provides a comprehensive insight into the carbon cycling within the detritusphere. The simulation results showed among others that after 84 days about 10% of total litter C was transferred to the soil organic matter (SOM) pool. Only 3% was located in the microbial biomass. From the evolved CO₂ 71% was litter-derived and 29% was soil-derived. From the litter-derived CO₂, 69% was directly formed in the litter layer. The remaining 31% was transported to soil before mineralisation. Our study shows that a combination of experimental work and mathematical modelling is a powerful approach to provide a comprehensive insight into the small-scale carbon turnover in soil.     

Carbon and nitrogen stocks in relation to organic matter fractions, aggregation and pore size distribution in no-tillage and conventional tillage in northern France

The magnitude of and mechanisms for long‐term differences in soil organic matter stocks under no‐tillage and conventional tillage are still relatively poorly known. We quantified differences in total C and N stocks after 32 years of no‐tillage (NT) and conventional tillage (CT) in plots with a long‐term cultivation history before differentiation and the same annual C and N returns to the soil. The role of physical protection of organic matter (OM) in these stock differences was further investigated by examining the changes at different levels of structural complexity, i.e. organic matter fractions, aggregation and pore‐size distribution. Four structural zones were sampled: loose and dense soil zones under CT and the 0–5 cm (rich in OM) and 5–20 cm (massive structure) soil layers under NT. The C and N stocks, calculated for an equivalent mass of dry soil, were only 10–15% larger under NT than under CT. Mineral‐associated N and particulate organic matter accounted for about 50% of the difference in N stocks. However, 66% of the total difference in C stocks was due to differences in the particulate organic matter (58%) and free residues (8%) fractions. The additional C and N under NT were almost exclusively situated in aggregates larger than 250 μm in diameter. Our results suggest that physical protection of OM under NT contributes significantly to the differences in C and N stocks between NT and CT by (i) enhanced macroaggregate formation in the 0–5 cm layer due to greater microbial activity and OM content and (ii) a better protection of soil organic matter in the 5–20 cm layer due to the presence of small pores and lack of soil disruption by tillage or climate.     

Soil carbon and nitrogen dynamics as affected by long-term tillage and nitrogen fertilization

Quantifying seasonal dynamics of active soil C and N pools is important for understanding how production systems can be better managed to sustain long-term soil productivity especially in warm subhumid climates. Our objectives were to determine seasonal dynamics of inorganic soil N, potential C and N mineralization, soil microbial biomass C (SMBC), and the metabolic quotient of microbial biomass in continuous corn (Zea mays L.) under conventional (CT), moldboard (MB), chisel (CH), minimum tillage (MT), and no-tillage (NT) with low (45kgNha^–1) and high (90kgNha^–1) N fertilization. An Orelia sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) in south Texas, United States, was sampled before corn planting in February, during pollination in May, and following harvest in July. Soil inorganic N, SMBC, and potential C and N mineralization were usually highest in soils under NT, whereas these characteristics were consistently lower throughout the growing season in soils receiving MB tillage. Nitrogen fertilization had little effect on soil inorganic N, SMBC, and potential C and N mineralization. The metabolic quotient of microbial biomass exhibited seasonal patterns inverse to that of SMBC. Seasonal changes in SMBC, inorganic N, and mineralizable C and N indicated the dependence of seasonal C and N dynamics on long-term substrate availability from crop residues. Long-term reduced tillage increased soil organic matter (SOM), SMBC, inorganic N, and labile C and N pools as compared with plowed systems and may be more sustainable over the long term. Seasonal changes in active soil C and N pools were affected more by tillage than by N fertilization in this subhumid climate. Received: 20 September 1996     

Land use change and soil nutrient transformations in the Los Haitises region of the Dominican Republic

We characterized soil cation, carbon (C) and nitrogen (N) transformations within a variety of land use types in the karst region of the northeastern Dominican Republic. We examined a range of soil pools and fluxes during the wet and dry seasons in undisturbed forest, regenerating forest and active agricultural sites within and directly adjacent to Los Haitises National Park. Soil moisture, soil organic matter (SOM), soil cations, leaf litter C and pH were significantly greater in regenerating forest sites than agricultural sites, while bulk density was greater in active agricultural sites. Potential denitrification, microbial biomass C and N, and microbial respiration g⁻¹ dry soil were significantly greater in the regenerating forest sites than in the active agricultural sites. However, net mineralization, net nitrification, microbial biomass C, and microbial respiration were all significantly greater in the agricultural sites on g⁻¹ SOM basis. These results suggest that land use is indirectly affecting microbial activity and C storage through its effect on SOM quality and quantity. While agriculture can significantly decrease soil fertility, it appears that the trend can begin to rapidly reverse with the abandonment of agriculture and the subsequent regeneration of forest. The regenerating forest soils were taken out of agricultural use only 5-7 years before our study and already have soil properties and processes similar to an undisturbed old forest site. Compared to undisturbed mogote forest sites, regenerating sites had smaller amounts of SOM and microbial biomass N, as well as lower rates of microbial respiration, mineralization and nitrification g⁻¹ SOM. Initial recovery of soil pools and processes appeared to be rapid, but additional research must be done to address the long-term rate of recovery in these forest stands.