Cumulative Microbial Respiration Research Articles

Transformation of Biochar from Plant Biomass in Gray Forest Soil: Evaluation by Isotopic Labeling Method

Pyrolysis is considered to be one of promising methods for processing agricultural waste and for producing fertilizers. The efficiency of the resulting biochar as a fertilizer has been proven, but the preferential way of decomposition of organic substances in it—biotic or abiotic—is still open to argument. The ways of transformation of biochar obtained from corn (a plant of the C4 type of photosynthesis with an increased 13C content) were assessed in this work, using the solid-phase CP/MAS 13C NMR spectroscopy. Biochar was placed into the top layer of a monolith of gray forest soil, and the precipitation regime characteristic of Central Russia was simulated for 90 days. The peak at 129 ppm typical for aromatic compounds increased during the experiment in the obtained NMR spectra of soil samples with biochar in the upper soil layer, but not in other layers. This testifies that biochar particles do not migrate down the soil profile during one season. The intensity of cumulative microbial respiration in the presence of biochar increases from 85.0 g CO2 kg–1 in the control sample to 201.4 g CO2 kg–1 in the sample with biochar (the topsoil). According to the NMR spectra of the salt formed during mineralization of carbon dioxide released from the soil, it contains labeled carbon: there is a peak at 169 ppm characteristic of carbonates. The cumulative volume of CO2 released from the soil with biochar is 1.9 times greater as compared to the control soil. The addition of microorganisms-decomposers caused an additional increase in the CO2 volume: 2.4 times relative to the control, which indicates the role of microorganisms in the destruction of soil organic matter and of biochar. However, based on the stability of the total carbon content in the soil, it can be concluded that only a small proportion of biochar components is susceptible to biotic decomposition.

Ecotoxicological impact of butisanstar and clopyralid herbicides on soil microbial respiration and the enzymatic activities

The application of herbicides in soil has been noted for its detrimental effect on the soil microbial community, crucial for various biochemical processes. This study provides a comprehensive assessment of the impact of butisanstar and clopyralid herbicides, both individually and in combination at different dosage (recommended field dose (RFD), ½, 2 and 5-times RFD). The assessment focuses on soil basal respiration (SBR), cumulative microbial respiration (CMR), and the activities dehydrogenase (DH), catalase (CAT), urease, acid and alkaline phosphatases (Ac–P and Alk-P) enzymes, along with their variations on days 10, 30, 60, and 90 post-herbicide application. Results indicate that, although herbicides, even at lower doses of RFD, demonstrate inhibitory effects on DH, CAT, and microbial respiration, they paradoxically lead to a significant enhancement in urease and phosphatase activities, even at higher doses. The inhibitory/enhancing intensity varies based on herbicide type, incubation period, and dosage. Co-application of herbicides manifests synergistic effects compared to individual applications. The most notable inhibitory effects on DH, CAT, and SBR are observed on the 30th day, coinciding with the highest activities of urease and phosphatases on the same day. The persistent inability to restore respiration and enzyme activities to initial soil (control) levels emphasizes the lasting adverse and inhibitory effects of herbicides, especially clopyralid, over the long term. It becomes apparent that soil microorganisms require an extended duration to decompose and acclimate to the presence of herbicides. Consequently, these agrochemical compounds pose a potential risk to crucial biochemical processes, such as nutrient cycling, ultimately impacting crop production.

Seasonal soil moisture thresholds inhibit bacterial activity and decomposition during drought in a tallgrass prairie

Soil moisture reductions during drought often inhibit soil microbial activity and inhibit decomposition rates by reducing microbial biomass or by altering microbial communities. Evidence suggests that soil water must drop below a critical threshold to inhibit microbial activity. Thus, it is likely that the seasonal timing of drought will determine the extent to which belowground processes are adversely impacted by drought. Specifically, the effects of drought might be minimal during cool, wet periods typical of late spring but dramatic during hot summer months with high evapotranspiration rates that lower soil moisture levels below the critical threshold. Here, we present results from a study designed to quantify the effect of drought on soil microbial abundance, community composition, and soil water diffusion across four months, and to then assess how drought impacts the microbial decomposition of leaf matter. We imposed a season‐long drought in a Wisconsin tallgrass prairie and measured soil moisture, bacterial composition and abundance, microbial respiration, and decomposition rates throughout the growing season. Bacterial communities varied considerably among dates, but drought did not affect either bacterial abundance or community composition. Microbial respiration declined significantly during periods of drought when soil pores likely became hydrologically isolated, ultimately reducing cumulative microbial respiration by 10%. The reduction in microbial activity in drought treatments caused a 50% decline in the decomposition of refractory material. Our study highlights that sublethal effects of drought on microbial communities, occurring only when soil moisture declined below a tolerance threshold, can have large impacts on microbial carbon release or decomposition, highlighting the need to incorporate such measures into future studies.

Assessment of cumulative microbial respiration and their ameliorative role in sustaining maize growth under salt stress

Cumulative microbial respiration reflects microbial activities and their potential to support plant growth, where salt tolerant rhizobacteria can optimize their respiration, and ensure plant survival under salt stress. We evaluated cumulative microbial respiration of different salt tolerant rhizobacterial strains at different salinity levels, and checked their ability to sustain plant growth under natural saline conditions by using maize as test crop. Our results revealed that at the highest EC level (10 dS m−1), strain ‘SUA-14’ performed significantly better, and exhibited the greatest cumulative respiration (4.2 fold) followed by SHM-13 (3.8 fold), as compared to un-inoculated control. Moreover, results of the field trial indicated a similar trend, where significant improvements in shoot fresh weight (59%), root fresh weight (80%), shoot dry weight (56%), root dry weight (1.4 fold), leaf area (1.9 fold), straw yield (41%), cob diameter (33%), SPAD value (84%), yield (99%), relative water contents (91%), flavonoid (55%), 1000 grain weight (∼100%), soluble sugars (41%) and soluble proteins (45%) were observed due to inoculation of strain ‘SUA-14’ as compared to un-inoculated control. Similarly, substantial decline in leaf Na+ (34%), Na+/K+ ratio (69%), electrolyte leakage (8%), catalase (54%), peroxidase (73%), and H2O2 (50%) activities were observed after inoculation of ‘SUA-14’ with a concomitant increment in the leaf K+ contents (70%) under salinity stress than un-inoculated control. Hence, among all the tested rhizobacterial isolates, ‘SUA-14’ served as the most efficient strain in alleviating the detrimental impacts of salinity on maize growth and yield. The 16S rRNA sequencing identified it as Acinetobacter johnsonii.

Ecological restoration of degraded lands with alternate land use systems improves soil functionality in semiarid tropical India

AbstractTo test the applicability of alternate land use systems for improving soil functionality in restored ecologies, soils were sampled from 0–15, 15–30 and 30–45 cm deep layers of Leucaena leucocephala (LL), Hardwickia binata (HB), Emblica officinalis (EO), Azadiracta indica (AI)‐based silviculture systems; Acacia nilotica‐based silvipasture systems (AN); and natural grassland (NG). These were compared with samples from fallow land (F). They were evaluated for their carbon management index (CMI), nutrient supply capacity (NSC), soil functionality (SF), ecorestoration efficiency (ERE) and 21‐day cumulative microbial respiration (CO2‐21) to assess their applicability in semiarid India. Soil functionality and functional diversity as impacted by restoration have remained largely overlooked. The LL had ~12, 7 and 11% higher CMI than fallow in sampled soil layers. ERE of LL was ~ 55, 65 and 79% higher than fallow land in sampled soil layers, respectively. However, ERE in surface layer was poorer than subsequent soil layers for all systems. The LL, HB and AN improved NSC and SF by: a) ~ 2.5‐, 2.2‐ and 1.6‐times; and b) 9.3‐, 5.3‐ and 5.1‐times over fallow land in the surface soil layer. A similar trend was observed for SF in lower layers. However, the topsoil layer had >16% mean SF values than subsequent layers. LL, HB and AN systems had ~4.2‐, 2.3‐ and 1.9‐times higher CO2‐21 than fallow land in the top layer. CO2‐21 was positively correlated with NSC and SF but could not predict ERE well. Hence, legume tree‐based restoration tactics might be useful for improving land restoration and soil functionality in semiarid regions.

Differential Organic Carbon Mineralization Responses to Soil Moisture in Three Different Soil Orders Under Mixed Forested System

Soil microbial respiration is one of the largest sources of carbon (C) emissions to the atmosphere in terrestrial ecosystems, which is strongly dependent on multiple environmental variables including soil moisture. Soil moisture content is strongly dependent on soil texture, and the combined effects of texture and moisture on microbial respiration are complex and less explored. Therefore, this study examines the effects of soil moisture on the mineralization of soil organic C Soil organic carbon in three different soils, Ultisol, Alfisol and Vertisol, collected from mixed forests of Georgia, Missouri, and Texas, United States , respectively. A laboratory microcosm experiment was conducted for 90 days under different moisture regimes. Soil respiration was measured weekly, and destructive harvests were conducted at 1, 15, 60, and 90 days after incubation to determine extractable organic C (EOC), phospholipid fatty acid based microbial community, and C-acquiring hydrolytic extracellular enzyme activities (EEA). The highest cumulative respiration in Ultisol was observed at 50% water holding capacity (WHC), in Alfisol at 100% water holding capacity, and in Vertisol at 175% WHC. The trends in Extractable Organic Carbon were opposite to that of cumulative microbial respiration as the moisture levels showing the highest respiration showed the lowest EOC concentration in all soil types. Also, extracellular enzyme activities increased with increase in soil moisture in all soils, however, respiration and EEA showed a decoupled relationship in Ultisol and Alfisol soils. Soil moisture differences did not influence microbial community composition.

Degradation Rate of Bio-based Agricultural Mulch is Influenced by Mulch Composition and Biostimulant Application

Spunbond polylactic acid (PLA) based agricultural mulches provide several benefits including environmental sustainability, durability, weed control, and soil moisture conservation. Large-scale adoption of PLA mulches in organic and conventional agricultural systems has been limited due to slow to biodegradation and persistence in soil. A 16-week microcosm study was conducted to assess the effects of four commercially available biostimulants (Biocat 1000, Extract PBA, Custom GP, and Environoc 501), a compost extract, and distilled water, urea, and sucrose controls on biodegradation and microbial respiration of bio-based mulch in soil. Mulch treatments included a spunbond PLA mulch, two novel composite PLA mulches with alfalfa (PLA-A) and soy (PLA-S) particles embedded in the fabric, paper mulch, and bio-based plastic film. After 16 weeks, the PLA-A and PLA-S mulches lost 43% and 48% more mass than the PLA mulch. Cumulative microbial respiration in the PLA-A and PLA-S mulch microcosms was 245% and 239% greater than respiration in PLA mulch microcosms. The effects of biostimulants on biodegradation and microbial respiration were inconsistent. Our results suggest that composite spunbond PLA mulches containing plant-based materials degrade more quickly than pure PLA mulches in soil, and certain biostimulant products may accelerate biodegradation.

Microbial dormancy promotes microbial biomass and respiration across pulses of drying-wetting stress

Recent work suggests that metabolic activation and deactivation of microbes in soil strongly influences soil carbon (C) dynamics and climate feedbacks. However, few soil C models consider these transitions. We hypothesized that microbes’ capacity to enter and exit dormancy in response to unfavorable and favorable environmental conditions decreases the sensitivity of microbial biomass and cumulative respiration to environmental stress. To test this hypothesis, we collected data from a rewetting experiment and used it to design and parameterize dormancy in an existing microbe-based soil C model. Then we compared predictions of microbial biomass and soil heterotrophic respiration (RH) under simulated cycles of stressful (dryness) and favorable (wet pulses) conditions. Because the influence of moisture on microbial processes in soil generally depends on temperature, we collected data and tested predictions at different temperatures. When dormancy was not taken into account, simulated microbial biomass and cumulative microbial respiration over five years were lower and decreased faster under lengthening drying-wetting cycles. Differences due to dormancy increased with temperature and with the length of the dry periods between wetting events. We conclude that ignoring both the capacity of microbes to enter and exit dormancy in response to the environment and the consequences of these metabolic responses for soil C cycling results in predictions of unrealistically low RH under warming and drying-wetting cycles.

Responses of microbial performance and community to corn biochar in calcareous sandy and clayey soils

Biochar can be used as an organic amendment to improve soil physical and chemical attributes, with a potentially significant influence on soil microbial performance. However, this effect of biochar is poorly understood for arid soils with low organic matter content. The main objective of this study was to quantify the response of microbial attributes to corn biochar in two calcareous soils with different texture. Three slow pyrolysis biochars were prepared at 200, 400 and 600°C from corn feedstocks. The biochars were added to sandy and clayey soils at 0.5 and 1% (w/w) and the mixtures were incubated for 90days under standard laboratory conditions (25±1°C and 70% of soil field capacity). Soils amended with raw (uncharred) feedstock and unamended (without biochar and raw residue) as the control were also considered in the experiment. The soil properties measured included microbial respiration during 60days, microbial biomass carbon (C), substrate-induced respiration (SIR), fungal (FR) and (BR) bacterial respiration at the end of the incubation. Compared with the unamended control, the addition of corn raw feedstock or its biochar significantly increased cumulative microbial respiration (62–462%), MBC (66–169%), SIR (50–216%), BR (129–308%) and FR (42–200%), but tended to decrease FR/BR ratio (5–90%), depending largely upon its production temperature and application rate as well as soil texture. The positive effects of biochar addition on increasing microbial properties were more pronounced at 1% than 0.5% application rates for all the attributes and in sandy than clayey soils for MBC, SIR and FR attributes. Overall, the measured microbial attributes were all greater in uncharred than charred feedstock treatments and tended to decline with increasing pyrolysis temperature. The relative abundance of soil bacteria increased with biochar addition and pyrolysis temperature, while that of soil fungi decreased. Biochar addition increased microbial performance potentially due to an increase in soil C content while increasing pyrolysis temperature altered biochar chemistry and properties which may have contributed to the decreased microbial performance. It is concluded that although biochar application may improve microbial processes and attributes in calcareous soils with low organic matter content, its effects on microbiological properties are mainly meditated by soil texture, pyrolysis temperature for biochar production and application rate. The greatest response by the microbial indicators can occur when corn biochars produced at low temperatures were added to less fertile sandy soils at 1% addition rate. The study provided clear evidence that application of low temperature corn biochars at 45–50tha−1 to calcareous soils may have a great potential for improvements in the microbial indicators of soil quality.

Responses of soil microbial respiration to plantations depend on soil properties in subtropical China

Assessing the impact of plantation on microbial respiration (MR) is vitally important to understand the interactions between belowground metabolism and land use change. In this study, cumulative MR was determined by alkali absorption method in 1, 3, 7, 14, 21, 28, 35, 42, 49, and 56 days from the soil in a representative plantations in the subtropical region of China. The treatment of plantations contained no plant (CK), orange trees (Citrus reticulata)+Bahia grass (Paspalum notatum) (GB), orange trees (C. reticulata)+Bahia grass (P. notatum)+soybean (Giycine max (L.) Merrill) (GBH). Results showed that plantation had significant effects on microbial respiration and the responses of microbial respiration to plantation from different soil layers and topographies were different: in 0–20 cm in uphill: GB>GBH>CK; in 20–40 cm in uphill: GBH>CK>GB; in 0–20 cm in downhill: GBH>CK>GB; in 20–40 cm in downhill: GB>CK>GBH. Furthermore, plantation also altered the relationships between MR and soil properties. In CK, microbial respiration was positively correlated with NH4+ and soil total N, and negatively correlated with soil moisture, pH, NO3−, and microbial biomass carbon (MBC). In GB, microbial respiration under GB significantly negatively correlated with dissolved organic carbon (DOC). In GBH, microbial respiration under GBH was positively correlated with NH4+, MBC, total soil carbon (TC), and total soil nitrogen (TN), and negatively correlated with soil moisture (SM), pH, NO3−, and DOC. The underlying mechanisms could be attributed to soil heterogeneity and the effects of plantation on soil properties. Our results also showed that plantation significantly increased soil C storage, which suggested plantation is a key measure to enhance soil C sequestration and mitigate global CO2 emission, especially for the soil with low initial soil carbon content or bared soil.

Spartina alterniflora invasion alters soil microbial community composition and microbial respiration following invasion chronosequence in a coastal wetland of China.

The role of exotic plants in regulating soil microbial community structure and activity following invasion chronosequence remains unclear. We investigated soil microbial community structure and microbial respiration following Spartina alterniflora invasion in a chronosequence of 6-, 10-, 17-, and 20-year-old by comparing with bare flat in a coastal wetland of China. S. alterniflora invasion significantly increased soil moisture and salinity, the concentrations of soil water-soluble organic carbon and microbial biomass carbon (MBC), the quantities of total and various types of phospholipid fatty acids (PLFAs), the fungal:bacterial PLFAs ratio and cumulative microbial respiration compared with bare flat. The highest MBC, gram-negative bacterial and saturated straight-chain PLFAs were found in 10-year-old S. alterniflora soil, while the greatest total PLFAs, bacterial and gram-positive bacterial PLFAs were found in 10- and 17-year-old S. alterniflora soils. The monounsaturated:branched PLFAs ratio declined, and cumulative microbial respiration on a per-unit-PLFAs increased following S. alterniflora invasion in the chronosequence. Our results suggest that S. alterniflora invasion significantly increased the biomass of soil various microbial groups and microbial respiration compared to bare flat soil by increasing soil available substrate, and modifying soil physiochemical properties. Soil microbial community reached the most enriched condition in the 10-year-old S. alterniflora community.

Lack of functional redundancy in the relationship between microbial diversity and ecosystem functioning

Summary Biodiversity is declining world‐wide with detrimental effects on ecosystems. However, we lack a quantitative understanding of the shape of the relationship between microbial biodiversity and ecosystem function (BEF). This limits our understanding of how microbial diversity depletion can impact key functions for human well‐being, including pollutant detoxification. Three independent microcosm experiments were conducted to evaluate the direction (i.e. positive, negative or null) and the shape of the relationships between bacterial diversity and both broad (i.e. microbial respiration) and specialized (i.e. toxin degradation) functions in five Australian and two UK freshwater ecosystems using next‐generation sequencing platforms. Reduced bacterial diversity, even after accounting for biomass, caused a decrease in broad (i.e. cumulative microbial respiration) and specialized (biodegradation of two important toxins) functions in all cases. Unlike the positive but decelerating BEF relationship observed most frequently in plants and animals, most evaluated functional measurements were related to bacterial diversity in a non‐redundant fashion (e.g. exponentially and/or linearly). Synthesis. Our results suggest that there is a lack of functional redundancy in the relationship between bacterial diversity and ecosystem functioning; thus, the consequences of declining microbial diversity on ecosystem functioning and human welfare have likely been considerably underestimated.

Fungal contributions to carbon flow and nutrient cycling during decomposition of standing Typha domingensis leaves in a subtropical freshwater marsh

Summary Despite the well‐known occurrence of ‘standing‐dead’ emergent plant litter in freshwater marshes, the role of fungi in its decomposition is poorly known. Here, we quantified the growth and biomass dynamics of fungi associated with standing‐dead Typha domingensis leaves, estimated the contribution of fungi to carbon flow during decomposition and assessed their contribution to nutrient (nitrogen and phosphorus) cycling. In a subtropical freshwater marsh, standing leaves of T. domingensis were sampled in August while living (green) and then monthly during leaf senescence and standing‐dead decomposition for 1 year. Leaf samples were analysed for mass loss, fungal biomass (ergosterol), rates of fungal production (14C‐acetate incorporation) and microbial respiration (CO2 evolution), and for litter chitin (glucosamine), carbon, N and P concentrations. Losses in T. domingensis leaf carbon (37%) occurred during senescence and standing decomposition. During this time, increases in ergosterol and chitin concentrations were observed in the standing litter, indicating the rapid colonisation of decaying Typha leaves by fungi. Estimated fungal biomass (from ergosterol) reached a maximum of 37 mg C g−1 detrital C. Over the entire study period, estimated cumulative fungal production in standing Typha litter was 39 mg C g−1 initial detrital C, indicating that 11% of leaf C was converted to fungal C. The corresponding estimate of cumulative microbial respiration was 136 mg C g−1 initial detrital C, indicating that 37% of Typha leaf litter C was mineralised by microorganisms (bacteria and fungi) during decomposition. Fungi also immobilised up to c.27% and c.55% of the total detrital N and P, respectively. Fungi play an important role in the cycling of C and nutrients in freshwater marshes, and this should be integrated into current models that describe major biogeochemical pathways.

Aggregation stability and microbial activity of China's black soils under different long‐term fertilisation regimes

Black soils (Mollisols) are distributed in the Songnen Plain in the north‐east of China and underpin a large agricultural production base. Over the years, these highly fertile and productive soils have suffered a deterioration of properties under intensive cultivation, including a loss of soil organic carbon, which has reduced their yield potential. To develop a more sustainable agriculture and to help maintain continuing high grain yields, a long‐term fertilisation experiment was conducted at the Key Observation Station of the Harbin Black Soil Ecology, Ministry of Agriculture, Heilongjiang province, China. Four simple fertiliser treatments have been maintained for 28 years (since 1979), these being: no fertiliser (CK), simple chemical fertiliser (NPK), simple organic fertiliser (M), and chemical plus organic fertiliser (NPKM). These treatments were investigated for their effects on soil aggregation stability and microbial activity. Organic fertiliser application (NPKM and M) increased organic carbon (OC) concentration in whole soil and in some of the aggregate size fractions, promoting macro‐aggregate formation, particularly in the larger (1–2 mm) fractions. Microbial activity (quantified as cumulative microbial respiration) increased larger fractions (>0.5 mm), but decreased the smaller fractions (<0.5 mm). Microbial synthesis products serve as binding agents for aggregate formation. The combination of chemical and organic fertilisers was most effective for increasing macro‐aggregate stability.

Rewetting and litter addition influence mineralisation and microbial communities in soils from a semi-arid intermittent stream

Nitrogen (N) and carbon (C) mineralisation are triggered by pulses of water availability in arid and semi-arid systems. Intermittent streams and their associated riparian communities are obvious ‘hot spots’ for biogeochemical processes in arid landscapes where water and often C are limiting. Stream landscapes are characterized by highly heterogeneous soils that may respond variably to rewetting. We used a laboratory incubation to quantify how N and C mineralisation in rewetted soils and sediments from an intermittent stream in the semi-arid Pilbara region of north-west Australia varied with saturation level and substrate addition (as ground Eucalyptus litter). Full (100%) saturation was defined as the maximum gravimetric moisture content (%) achieved in free-draining soils and sediments after rewetting, with 50% saturation defined as half this value. We estimated rates and amounts of N mineralised from changes in inorganic N and microbial respiration as CO 2 efflux throughout the incubation. In soils and sediments subject to 50% saturation, >90% of N mineralised accumulated within the first 7 d of incubation, compared to only 48% when soils were fully saturated (100% saturation). Mineralisation rates and microbial respiration were similar in riparian and floodplain soils, and channel sediments. N mineralisation rates in litter-amended soils and sediments (0.73 mg N kg −1 d −1) were only one-third that of unamended samples (3.04 mg N kg −1 d −1), while cumulative microbial respiration was doubled in litter-amended soils, suggesting N was more rapidly immobilized. Landscape position was less important in controlling microbial activity than soil saturation when water-filled pore space (% WFPS) was greater than 40%. Our results suggest that large pulses of water availability resulting in full soil saturation cause a slower release of mineralisation products, compared to small pulse events that stimulate a rapid cycle of C and N mineralisation–immobilization.

Effect of elevated atmospheric CO2 concentration on soil and root respiration in winter wheat by using a respiration partitioning chamber

Soil respiration in a cropland is the sum of heterotrophic (mainly microorganisms) and autotrophic (root) respiration. The contribution of both these types to soil respiration needs to be understood to evaluate the effects of environmental change on soil carbon cycling and sequestration. In this paper, the effects of free-air CO2 enrichment (FACE) on hetero- and autotrophic respiration in a wheat field were differentiated and evaluated by a novel split-root growth and gas collection system. Elevated atmospheric pCO2 of approximately 200 μmol mol−1 above the ambient pCO2 significantly increased soil respiration by 15.1 and 14.8% at high nitrogen (HN) and low nitrogen (LN) application rates, respectively. The effect of elevated atmospheric pCO2 on root respiration was not consistent across the wheat growth stages. Elevated pCO2 significantly increased and decreased root respiration at the booting-heading stage (middle stage) and the late-filling stage (late stage), respectively, in HN and LN treatments; however, no significant effect was found at the jointing stage (early stage). Thus, the effect of increased pCO2 on cumulative root respiration for the entire wheat growing season was not significant. Cumulative root respiration accounted for approximately 25–30% of cumulative soil respiration in the entire wheat growing season. Consequently, cumulative microbial respiration (soil respiration minus root respiration) increased by 22.5 and 21.1% due to elevated pCO2 in HN and LN, respectively. High nitrogen application significantly increased root respiration at the late stage under both elevated pCO2 and ambient pCO2; however, no significant effects were found on cumulative soil respiration, root respiration, and microbial respiration. These findings suggest that heterotrophic respiration, which is influenced by increased substrate supplies from the plant to the soil, is the key process to determine C emission from agro-ecosystems with regard to future scenarios of enriched pCO2.

Chronic experimental [formula omitted] deposition reduces the retention of leaf litter DOC in a northern hardwood forest soil

In forests of the Great Lakes region, experimental NO 3 − deposition has suppressed soil respiration and enhanced DOC export. Reasons for these responses are unknown, but they could arise via two alternatives: (i) direct suppression of microbial activity by NO 3 − or (ii) indirect suppression of the microbial community via changes in litter biochemistry in response to greater N availability. To test the second alternative, we conducted a controlled laboratory experiment to examine how chronic experimental NO 3 − deposition affects the contributions of fresh leaf litter to microbial respiration and DOC export. The study reported here used manipulations of mineral soil and fresh leaf litter from a previously studied northern hardwood forest stand in northern Lower Michigan that has received 9 years of ambient and experimental (three times ambient) atmospheric NO 3 − deposition. We found that cumulative microbial respiration over the 6-week incubation was substantially greater in fresh litter plus mineral soil (20.2–13.4 mg C) versus mineral soil alone (4.4–4.1 mg C); however, experimental NO 3 − deposition had no effect on microbial respiration across the litter–mineral soil manipulations. DOC production (∼75%) was primarily associated with leaching from fresh litter. In contrast, mineral soil was a significant sink for litter-derived DOC. Significantly, the mineral soil sink was less pronounced in soil receiving experimental NO 3 − deposition in which ∼30% more DOC was leached compared to the ambient NO 3 − deposition treatment. Furthermore, mineral soil was also both a source and sink for soluble phenolics; however, NO 3 − deposition suppressed a mineral–soil sink for phenolics derived from fresh leaf litter. These results suggest that increases in DOC export and declines in soil respiration in response to NO 3 − deposition in the field are not related to obvious changes in litter biochemistry or to the microbial metabolism of this material. Alternatively, these patterns may be linked to decreased abiotic sinks for litter-derived DOC in mineral soil, an unexpected ecosystem consequence of increased anthropogenic ( NO 3 − ) deposition.

Fate of nitrogen from crop residues as affected by biochemical quality and the microbial biomass

Net mineralization of N from a range of shoot and root materials was determined over a period of 6 months following incorporation into a sandy-loam soil under controlled environment conditions. Biochemical “quality” components of the materials showed better correlation with net N mineralization than did gross measures of the respiration and N content of the soil microbial community during decomposition. The quality components controlling net N mineralization changed during decomposition, with water-soluble phenolic content significantly correlated with net N mineralization at early stages, and water-soluble N, followed by cellulose at later stages. C-to-N and total N were correlated with net N mineralization towards the end of the incubation only. Cumulative microbial respiration during the early stages of decomposition was correlated with net N mineralization measured after 2 months, at which time maximum net N mineralization was recorded for most residues. However, there was no relationship between microbial-N and net N mineralization. Biochemical quality factors controlling the C and N content of the residue remaining at the end of the incubation as light fraction organic matter (LFOM) were also investigated. Both C and N content of LFOM derived from the residues were correlated with residue cellulose content, and the chemical characteristics of LFOM were highly correlated with those of the original plant material. Incorporation of low cellulose, high water-soluble N-containing shoot residues resulted in more N becoming mineralized than had been added in the residues, demonstrating that net mineralization of native soil organic matter had occurred. Large amounts of N were lost from the mineral-N pool during the incubation, which could not be accounted for by microbial immobilization.

Microbial colonization, respiration, and breakdown of maple leaves along a stream-marsh continuum

Breakdown rates, macroinvertebrate and bacterial colonization, and microbial respiration were measured on decaying maple (Acer saccharum) leaves at three sites along a stream-marsh continuum. Breakdown rates (−k ±SE) were 0.0284±0.0045 d−1 for leaves in a high-gradient, non-tidal stream; 0.0112±0.0019 d−1 for leaves at the confluence of the stream with a tidal, freshwater marsh; and 0.0062±0.0009 d2−1 for leaves in the tidal, freshwater marsh. Breakdown rates were significantly faster (ANCOVA, F<0.008) at the high-gradient, non-tidal stream site and at the tidal stream site than in the tidal marsh. Macroinvertebrate density on decaying leaves was low at all sites (<7 organisms g−1 AFDM leaf mass) and was dominated by chironomids and amphipods. Bacterial density on decaying leaves ranged from 8.56 × 108 CFU g−1 AFDM leaf mass to 13.38 × 108 CFU g−1 AFDM. Cumulative microbial respiration, calculated as the product of mean respiration on a sampling date, days in the interval preceding the sampling date, and hours per day, accounted for 34.3±6.0%, 53.0±4.8%, and 51.5±7.9% of the leaf mass loss (as carbon) at these sites. Although the breakdown rate was fastest at the non-tidal stream site, significantly less leaf mass was lost through microbial respiration. Most mass loss from leaves at this site was probably due to physical processing associated with stream habitats.