Effects of plants on net mineralization of nitrogen in forest soil microcosms

Summary The effects of plant roots on net N mineralization were examined by comparing soil microcosms with and without plants. Additionally, inorganic N amendments were used to test for competition for N between plants and microorganisms. Daily watering and the application of suction to microcosms eliminated the effects of transpiration on soil moisture content. Monthly litter collections reduced the influence of the aboveground portions of plants. Plants decreased net N mineralization by 23% during days 0–114 and then increased net mineralization by the same amount during days 144–124. Root-free soil collected from with-plant microcosms on day 244 evolved 24% more CO₂ in laboratory incubations than soil from without-plant microcosms. This indicates that plants had increased substrate availability to soil microorganisms. Inorganic N amendments had no significant effects on the microcosms or on laboratory soil incubations. Evidence is most consistent with the hypothesis that plant roots increased microbial activity due to the increased substrate availability. Different net N mineralization rates probably resulted from changes in the substrate C : N ratio.     

A new approach to trenching experiments for measuring root-rhizosphere respiration in a lowland tropical forest

Soil respiration in tropical forests is a major source of atmospheric CO₂. The ability to partition soil respiration into its individual components is becoming increasingly important to predict the effects of disturbance on CO₂ efflux from the soil as the responses of heterotrophic and autotrophic respiration to change are likely to differ. However, current field methods to partition respiration suffer from various methodological artefacts; root-rhizosphere respiration is particularly difficult to estimate. We used trenched subplots to estimate root-rhizosphere respiration in large-scale litter addition (L+), litter removal (L−) and control (CT) plots in a lowland tropical semi-evergreen forest in Panama. We took a new approach to trenching by making measurements immediately before-and-after trenching and comparing them to biweekly measurements made over one year. Root-rhizosphere respiration was estimated to be 38%, 17% and 27% in the CT, L+, and L− plots, respectively, from the measurements taken immediately before and one day after trenching in May-June 2007. Biweekly measurements over the following year provided no estimates of root-rhizosphere respiration for the first seven months due to decomposition of decaying roots. We were also unable to estimate root-rhizosphere respiration during the dry season due to differences in soil water content between trenched and untrenched soil. However, biweekly measurements taken during the early rainy season one year after trenching (May-June 2008) provided estimates of root-rhizosphere respiration of 39%, 24% and 36% in the CT, L+, and L− plots, respectively, which are very similar to those obtained during the first day after trenching. We suggest that measurements taken immediately before and one day after root excision are a viable method for a rapid estimation of root-rhizosphere respiration without the methodological artefacts usually associated with trenching experiments.     

Interactions between plants, litter and microbes in cycling of nitrogen and phosphorus in the arctic

Estimated nutrient mineralization in northern nutrient-poor ecosystems, measured as differences in soil inorganic nutrients before and after a period of soil incubation in the absence of plants and litter, usually shows a discrepancy of much lower rates than plant nutrient uptake rates. In plots that had been pre-treated by 12 year of warming and fertilizer addition, we incubated soils together with litter and plants added and examined whether the absence of plants and litter in ‘traditional’ incubations could explain the discrepancy. The pre-treatment had no effect on nitrogen (N) mineralization but increased phosphorus (P) mineralization, while litter addition decreased N and increased P mineralization but without any effect on plant and microbial N and P sequestration. Incubations of soils with plants increased N mobilization to the soil inorganic plus plant pools several-fold as compared to the net mineralization in soils without plants. Hence, the presence of plants stimulated mobilization of the growth-limiting N. The growth-sufficient P was not affected by the presence of plants, however. Furthermore, increased plant and microbial N uptake correlated positively, which speaks against competition for plant available N from soil microbes in N-constrained ecosystems, at least during the time-span of 10 weeks the experiment lasted, and instead suggests facilitation.     

Regulation of microbial carbon,nitrogen, and phosphorus transformations by temperature and moisture during decomposition of <Emphasis Type="Italic">Calluna vulgaris</Emphasis> litter

To evaluate the effect of climate change on ecosystem functioning, the temperature and moisture response of microbial C, N, and P transformations during decomposition of Calluna vulgaris (L.) Hull. litter was studied in a laboratory incubation experiment. The litter originated from a dry heathland in the Netherlands where P limited vegetation growth. Fresh litter was incubated at 5, 10, 15, or 20°C and at a moisture content of 50, 100, or 200% in a full factorial design. Microbial nutrient transformations and activity were evaluated during two successive periods: an initial period of 48 days characterized by microbial growth and a second period from 48 to 206 days in which microbial growth declined significantly. Temperature and moisture response of respiration rate, the metabolic quotient (qCO₂), C, N, and P immobilization, net N and P mineralization and nitrification rates were evaluated by performing linear regressions. Microbial nutrient transformations and microbial activity depended both on temperature and moisture. In the first period, the respiration rate, qCO₂, microbial C and N immobilization, net P mineralization, net N mineralization and net nitrification rates were more strongly affected by temperature, while the microbial P immobilization rate was more strongly affected by moisture. The respiration rate, qCO₂, P immobilization rate, net P and N mineralization rate, and nitrification rate increased with temperature and moisture, while the C and N immobilization rate decreased with increasing temperature and increased with moisture. In the second period, C, N, and P immobilization and net N and P mineralization rates were significantly lower. The respiration rate and qCO₂ continued to increase with temperature and moisture, but C and N immobilization rates increased with temperature and declined with increasing moisture. Net P mineralization rate decreased at higher temperature and moisture, and nitrification rate declined with increasing temperature and increased with moisture. It was concluded that plant growth in these P-limited systems is very sensitive to climate change as it strongly relies on the competition for P with microbes, and temperature and moisture have a large effect on the immobilization rate of available P.     

Episodic rewetting enhances carbon and nitrogen release from chaparral soils

The short-term pulse of carbon (C) and nitrogen (N) mineralization that accompanies the wetting of dry soils may dominate annual C and N production in many arid and semi-arid environments characterized by seasonal transitions. We used a laboratory incubation to evaluate the impact of short-term fluctuations in soil moisture on long-term carbon and nitrogen dynamics, and the degree to which rewetting enhances C and N release. Following repeated drying and rewetting of chaparral soils, cumulative CO₂ release in rewet soils was 2.2-3.7 times greater than from soils maintained at equivalent mean soil moisture and represented 12-18% of the total soil C pool. Rewetting frequency did not affect cumulative CO₂ release but did enhance N turnover, and net N mineralization and nitrification increased with rewetting in spite of significant reductions in nitrification potential. Litter addition decreased inorganic N release but enhanced dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) from dry soils, indicating the potential importance of a litter-derived pulse to short-term nutrient dynamics.     

Subtropical plantations are large carbon sinks: Evidence from two monoculture plantations in South China

Quantifying the net carbon (C) storage of forest plantations is required to assess their potential to offset fossil fuel emissions. In this study, a biometric approach was used to estimate net ecosystem productivity (NEP) for two monoculture plantations in South China: Acacia crassicarpa and Eucalyptus urophylla. This approach was based on stand-level net primary productivity (NPP, based on direct biometric inventory) and heterotrophic respiration (R_h). In comparisons of R_h determination based on trenching vs. tree girdling, both trenching and tree girdling changed soil temperature and soil moisture relative to undisturbed control plots, and we assess the effects of corrections for disturbances of soil moisture and soil moisture on the estimation of soil CO₂ efflux partitioning. Soil microbial biomass and dissolved organic carbon were significantly lower in trenched plots than in tree girdled plots for both plantations. Annual soil CO₂ flux in trenched plots (R_h-t) was significantly lower than in tree-girdled plots (R_h-g) in both plantations. The estimates of R_h-t and R_h-g, expressed as a percentage of total soil respiration, were 58 ± 4% and 74 ± 6%, respectively, for A. crassicarpa, and 64 ± 3% and 78 ± 5%, respectively, for E. urophylla. By the end of experiment, the difference in soil CO₂ efflux between the trenched plots and tree-girdled plots had become small for both plantations. Annual R_h (mean of the annual R_h-t and R_h-g) and net primary production (NPP) were 470 ± 25 and 800 ± 118 g C m⁻² yr⁻¹, respectively, for A. crassicarpa, and 420 ± 35 and 2380 ± 187 g C m⁻² yr⁻², respectively, for E. urophylla. The two plantations in the developmental stage were large carbon sinks: NEP was 330 ± 76 C m⁻² yr⁻¹ for A. crassicarpa and 1960 ± 178 g C m⁻² yr⁻¹ for E. urophylla.     

Mineralization of carbon and nitrogen from fresh and anaerobically stored sheep manure in soils of different texture

A sandy loam soil was mixed with three different amounts of quartz sand and incubated with (¹⁵NH₄)₂SO₄ (60 g N g^-1 soil) and fresh or anaerobically stored sheep manure (60 g g^-1 soil). The mineralization-immobilization of N and the mineralization of C were studied during 84 days of incubation at 20°C. After 7 days, the amount of unlabelled inorganic N in the manure-treated soils was 6–10 g N g^-1 soil higher than in soils amended with only (¹⁵NH₄)₂SO₄. However, due to immobilization of labelled inorganic N, the resulting net mineralization of N from manure was insignificant or slightly negative in the three soil-sand mixtures (100% soil+0% quartz sand; 50% soil+50% quartz sand; 25% soil+75% quartz sand). After 84 days, the cumulative CO₂ evolution and the net mineralization of N from the fresh manure were highest in the soil-sand mixutre with the lowest clay content (4% clay); 28% fo the manure C and 18% of the manure N were net mineralized. There was no significant difference between the soil-sand mixtures containing 8% and 16% clay, in which 24% of the manure C and -1% to 4% of the manure N were net mineralized. The higher net mineralization of N in the soil-sand mixture with the lowest clay content was probably caused by a higher remineralization of immobilized N in this soil-sand mixture. Anaerobic storage of the manure reduced the CO₂ evolution rates from the manure C in the three soil-sand mixtures during the initial weeks of decomposition. However, there was no effect of storage on net mineralization of N at the end of the incubation period. Hence, there was no apparent relationship between net mineralization of manure N and C.     

Effects of land-use type and nitrogen addition on nitrous oxide and carbon dioxide production potentials in Japanese Andosols

Land-use type and nitrogen (N) addition strongly affect nitrous oxide (N₂O) and carbon dioxide (CO₂) production, but the impacts of their interaction and the controlling factors remain unclear. The aim of this study was to evaluate the effect of both factors simultaneously on N₂O and CO₂ production and associated soil chemical and biological properties. Surface soils (0–10 cm) from three adjacent lands (apple orchard, grassland and deciduous forest) in central Japan were selected and incubated aerobically for 12 weeks with addition of 0, 30 or 150 kg N ha^–1 yr^–1. Land-use type had a significant (p < 0.001) impact on the cumulative N₂O and CO₂ production. Soils from the apple orchard had higher N₂O and CO₂ production potentials than those from the grassland and forest soils. Soil net N mineralization rate had a positive correlation with both soil N₂O and CO₂ production rates. Furthermore, the N₂O production rate was positively correlated with the CO₂ production rate. In the soils with no N addition, the dominant soil properties influencing N₂O production were found to be the ammonium-N content and the ratio of soil microbial biomass carbon to nitrogen (MBC/MBN), while those for CO₂ production were the content of nitrate-N and soluble organic carbon. N₂O production increased with the increase in added N doses for the three land-use types and depended on the status of the initial soil available N. The effect of N addition on CO₂ production varied with land use type; with the increase of N addition doses, it decreased for the apple orchard and forest soils but increased for the grassland soils. This difference might be due to the differences in microbial flora as indicated by the MBC/MBN ratio. Soil N mineralization was the major process controlling N₂O and CO₂ production in the examined soils under aerobic incubation conditions.     

Increased moisture and methanogenesis contribute to reduced methane oxidation in elevated CO2 soils

Awareness of global warming has stimulated research on environmental controls of soil methane (CH₄) consumption and the effects of increasing atmospheric carbon dioxide (CO₂) on the terrestrial CH₄ sink. In this study, factors impacting soil CH₄ consumption were investigated using laboratory incubations of soils collected at the Free Air Carbon Transfer and Storage I site in the Duke Forest, NC, where plots have been exposed to ambient (370 μL L⁻¹) or elevated (ambient + 200 μL L⁻¹) CO₂ since August 1996. Over 1 year, nearly 90% of the 360 incubations showed net CH₄ consumption, confirming that CH₄-oxidizing (methanotrophic) bacteria were active. Soil moisture was significantly (p < 0.01) higher in the 25–30 cm layer of elevated CO₂ soils over the length of the study, but soil moisture was equal between CO₂ treatments in shallower soils. The increased soil moisture corresponded to decreased net CH₄ oxidation, as elevated CO₂ soils also oxidized 70% less CH₄ at the 25–30 cm depth compared to ambient CO₂ soils, while CH₄ consumption was equal between treatments in shallower soils. Soil moisture content predicted (p < 0.05) CH₄ consumption in upper layers of ambient CO₂ soils, but this relationship was not significant in elevated CO₂ soils at any depth, suggesting that environmental factors in addition to moisture were influencing net CH₄ oxidation under elevated CO₂. More than 6% of the activity assays showed net CH₄ production, and of these, 80% contained soils from elevated CO₂ plots. In addition, more than 50% of the CH₄-producing flasks from elevated CO₂ sites contained deeper (25–30 cm) soils. These results indicate that subsurface (25 cm+) CH₄ production contributes to decreased net CH₄ consumption under elevated CO₂ in otherwise aerobic soils.     

Effect of soil characteristics on N mineralization capacity in 112 native and agricultural soils from the northwest of Spain

N mineralization capacity and its main controlling factors were studied in a large variety (n=112) of native (forest, bush) and agricultural (pasture, cultivated) soils from several climatic zones in Spain. The available inorganic N content, net N mineralization, and net N mineralization rate were determined after 6 weeks of aerobic incubation. NH _inf4 ^sup+ –N largely predominated over NO _inf3 ^sup- -N (ratio near 10:1) except in some agricultural soils. Net N mineralization predominated (83% of soils) over net N immobilization, which was more frequent in agricultural soils (25%) than in native soils (9%). In forest soils, both net N mineralization and the net N mineralization rate were significantly higher than in the other soil groups. The net N mineralization rate of pasture and cultivated soils was similar to that of bush soils, but available inorganic N was lower. The net N mineralization rate decreased in the order: soils over acid rocks>soils over sediments>soils over basic rocks or limestone; moreover, the highest net N mineralization and available inorganic N were found in soils over acid rocks. The highest N mineralization was found in soils with low C and N contents, particularly in the native soils, in which N mineralization increased as the C:N ratio increased. N mineralization was higher in soils with a low pH and base saturation than in soils with high pH and base saturation values, which sometimes favoured N immobilization. Soils with an Al gel content of >1% showed lower net N mineralization rates than soils with Al gel contents of <1%, although net N mineralization and available inorganic N did not differ between these groups. The net N mineralization rate in silty soils was significantly lower than in sandy and clayey soils, although soil texture only explained a low proportion of the differences in N mineralization between soils.     

Effects of liming on carbon and nitrogen mineralization in coniferous forests

A temporary decline in tree growth has often been observed after liming in coniferous forests poor in N but seldom in forests rich in N. To test the hypothesis that the decline was caused by decreases in N supply, C and N mineralization were estimated in incubated soil: (1) after liming in the laboratory, and (2) after earlier liming in the field. Liming increased the C mineralization rate in needle litter, nor humus and 0 to 5 cm mineral soil for a period of 40 to 100 days at 15°C. After that period, liming had no effect on the CO₂ evolution rate in materials poor in N (C:N ratios 30 to 62) but increased the CO₂ evolution rate in materials rich in N (C:N ratios 24 to 28). When liming induced nitrification, the CO₂ evolution rate was reduced. Liming resulted in lower net N mineralization rate in needle litter and mor humus. The reduction was more pronounced when NH₄ ⁺ was the only inorganic form than when NO₃ ^? was the predominant form. The reason is probably that chemical fixation of NH₃ and amino compounds increases with increasing pH. Because of the fixation, the incubation technique most likely underestimated the mineralized N available to the roots. Taking this underestimation into consideration, liming initially reduced the N release in the litter layer. In the other soil layers, liming increased the N release in soils rich in N and had only small effects in soils poor in N. For the total N supply to the roots in the litter, humus and 0 to 5 cm mineral soil layers, liming caused a slight reduction in soils poor in N and a slight increase in soils rich in N. Data on tree growth corresponded with these results.The hypotheses that tree growth depressions can be caused by reduced N supply after liming and that tree growth increases can be caused by increased N supply after liming thus seem reasonable.     

Analysis of manure and soil nitrogen mineralization during incubation

Understanding the N-cycling processes that ensue after manuring soil is essential in order to estimate the value of manure as an N fertilizer. A laboratory incubation of manured soil was carried out in order to study N mineralization, gas fluxes, denitrification, and microbial N immobilization after manure application. Four different manures were enclosed in mesh bags to allow for the separate analysis of manure and soil. The soils received 0.15 mg manure N g^–1 soil, and the microcosms were incubated aerobically and sampled throughout a 10-week period. Manure addition resulted in initial NH₄-N concentrations of 22.1 to 36.6 mg kg^–1 in the microcosms. All manured microcosms had net declines in soil mineral N. Denitrification resulted in the loss of 14.7 to 39.2% of the added manure N, and the largest N losses occurred in manures with high NH₄-N content. Increased soil microbial biomass N amounted to 6.0 to 8.6% of the added manure N. While the microcosms as a whole had negative N mineralization, all microcosms had positive net nitrification within the manure bags. Gas fluxes of N₂O and CO₂ increased in all manured soils relative to the controls. Our results show that measurement of microbial biomass N and denitrification is important to understand the fate of manure N upon soil application.     

Dissolved organic matter and inorganic N jointly regulate greenhouse gases fluxes from forest soils with different moistures during a freeze-thaw period

ABSTRACT

Antecedent soil moisture before freezing can affect greenhouse gases (GHG) fluxes from soils during thaw, but their critical threshold values for GHG fluxes and the underlying mechanisms are still not clear. By using packed soil-core incubation experiments, we have studied nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) fluxes from a mature broadleaf and Korean pine-mixed forest soil and an adjacent white birch forest soil with nine levels of soil moisture ranging from 10 to 90% water-filled pore space (WFPS) during a 2-month freezing at ?8°C and the following 10-day thaw at 10°C. The threshold values of soil moisture ranged from 50 to 70% WFPS for CH₄ uptake and from 70 to 90% WFPS for N₂O and CO₂ emissions from the two soils during the freeze-thaw period. Under the optimum soil moisture condition, fulvic-like compounds with high bioavailability contributed more than 60% of dissolved organic matter (DOM) in the soil. Cumulative N₂O emissions from forest soils during the freeze-thaw period were greatest when the concentration ratio of nitrate-N to dissolved organic carbon (DOC) was 0.04 g N g^?1 C. Cumulative soil CO₂ emissions and CH₄ uptake during the freeze-thaw period were both regulated by the interaction between soil DOC and net N mineralization. The activities of β-1,4-glucosidase and β-1,4-N-acetyl-glucosaminidase, microbial biomass C and N, and the microbial biomass C-to-N ratios, were all significantly correlated to the soil N₂O, CO₂, and CH₄ fluxes. Overall, upon a freeze-thaw period with different soil moistures, GHG fluxes from forest soils were jointly regulated by inorganic N and DOC concentrations, and related to the labile components of DOM released into the soil, which could be strictly controlled by the related microbial properties.     

Biological and chemical properties of arable soils affected by long-term organic and inorganic fertilizer applications

 Using soils from field plots in four different arable crop experiments that have received combinations of manure, lime and inorganic N, P and K for up to 20 years, the effects of these fertilizers on soil chemical properties and estimates of soil microbial community size and activity were studied. The soil pH was increased or unaffected by the addition of organic manure plus inorganic fertilizers applied in conjunction with lime, but decreased in the absence of liming. The soil C and N contents were greater for all fertilized treatments compared to the control, yet in all cases the soil samples from fertilized plots had smaller C:N ratios than soil from the unfertilized plots. The soil concentrations of all the other inorganic nutrients measured were greater following fertilizer applications compared with the unfertilized plots, and this effect was most marked for P and K in soils from plots that had received the largest amounts of these nutrients as fertilizers. Both biomass C determined by chloroform fumigation and glucose-induced respiration tended to increase as a result of manure and inorganic fertilizer applications, although soils which received the largest additions of inorganic fertilizers in the absence of lime contained less biomass C than those to which lime had been added. Dehydrogenase activity was lower in soils that had received the largest amounts of fertilizers, and was further decreased in the absence of lime. This suggests that dehydrogenase activity was highly sensitive to the inhibitory effects associated with large fertilizer additions. Potential denitrification and anaerobic respiration determined in one soil were increased by fertilizer application but, as with both the microbial biomass and dehydrogenase activity, there were significant reductions in both N₂O and CO₂ production in soils which received the largest additions of inorganic fertilizers in the absence of lime. In contrast, the size of the denitrifying component of the soil microbial community, as indicated by denitrifying enzyme activity, was unaffected by the absence of lime at the largest rate of inorganic fertilizer applications. The results indicated differences in the composition or function of microbial communities in the soils in response to long-term organic and inorganic fertilization, especially when the soils were not limited. Received: 10 March 1998     

Short-term dynamics of carbon and nitrogen after tillage in a freshwater marsh of northeast China

The Sanjiang Plain, one of the largest freshwater marshes in China, has experienced intensive cultivation over the past 50 years. However, there were few reports of short-term dynamics of soil carbon and nitrogen and CO₂ emission after tillage. In this paper, we studied the short-term dynamics of carbon and nitrogen after tillage in a freshwater marsh of northeast China. The results showed that response of carbon and nitrogen dynamic to tillage was different for intact wetland and soil cultivated for 10 years. Tillage was followed by immediate and significant increases in CO₂ efflux, which peaked at 0.25 h after tillage, four times higher than control in the wetland soils; while, only 2.5 times higher than control in the cultivated soils. Although, dissolved organic C (DOC) increased, the relative stability of microbial biomass C (MBC) pools together with the decreased respiration in the wetland soil suggested that the tillage did not lead to a burst in microbial activity and growth. Other factors such as moisture content before and after tillage may play an important role in determining microbial activity in the intact wetland. On the contrary, although dissolved organic C did not change, MBC pools, and soil respiration increase after tillage, suggesting tillage led to an increase in microbial activity and growth in the cultivated soil. Tillage initiated changes in soil aeration that was an important factor affecting soil microbiology in the long history of cultivation. Net N mineralization and nitrification occurred in both wetland and cultivated soils, but at different rates after tillage that in the intact wetland soil was higher than cultivated soil. Macroaggregates in the wetland soil would be expected to contain larger amounts of organic matter, and thus release a larger source of newly available substrate for microbes after tillage. In the intact wetland soil, ammonium, nitrate, and dissolved organic N (DON) concentrations were significantly negatively correlated to soil moisture (p < 0.01), suggesting high soil moisture in the natural wetland was not in favor of N mineralization.     

Assessment of 10 years of CO2 fumigation on soil microbial communities and function in a sweetgum plantation

Increased vegetative growth and soil carbon (C) storage under elevated carbon dioxide concentration ([CO₂]) has been demonstrated in a number of experiments. However, the ability of ecosystems, either above- or belowground, to maintain increased C storage relies on the response of soil processes, such as those that control nitrogen (N) mineralization, to climatic change. These soil processes are mediated by microbial communities whose activity and structure may also respond to increasing atmospheric [CO₂]. We took advantage of a long-term (ca 10 y) CO₂ enrichment experiment in a sweetgum plantation located in the southeastern United States to test the hypothesis that observed increases in root production in elevated relative to ambient CO₂ plots would alter microbial community structure, increase microbial activity, and increase soil nutrient cycling. We found that elevated [CO₂] had no detectable effect on microbial community structure using 16S rRNA gene clone libraries, on microbial activity measured with extracellular enzyme activity, or on potential soil N mineralization and nitrification rates. These results support findings at other forested Free Air [CO₂] Enrichment (FACE) sites.     

Priming depletes soil carbon and releases nitrogen in a scrub-oak ecosystem exposed to elevated CO2

Elevated atmospheric CO₂ tends to stimulate plant productivity, which could either stimulate or suppress the processing of soil carbon, thereby feeding back to atmospheric CO₂ concentrations. We employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO₂. We found that soil carbon concentration in the slow pool was 27% lower in elevated than ambient CO₂ plots at 0-10 cm depth. The difference in carbon mass was equivalent to roughly one-third of the increase in plant biomass that occurred in the same experiment. These results concur with previous reports from this ecosystem that elevated CO₂ stimulates microbial degradation of relatively stable soil organic carbon pools. Accordingly, elevated CO₂ increased net N mineralization in the 10-30 cm depth, which may increase N availability, thereby allowing for continued stimulation of plant productivity by elevated CO₂. Our findings suggest that soil texture and climate may explain the differential response of soil carbon among various long-term, field-based CO₂ studies. Increased mineralization of stable soil organic carbon by a CO₂-induced priming effect may diminish the terrestrial carbon sink globally.     

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.     

Effects of liming on carbon and nitrogen mineralization in coniferous forests

A temporary decline in tree growth has often been observed after liming in coniferous forests poor in N but seldom in forests rich in N. To test the hypothesis that the decline was caused by decreases in N supply, C and N mineralization were estimated in incubated soil: (1) after liming in the laboratory, and (2) after earlier liming in the field. Liming increased the C mineralization rate in needle litter, mor humus and 0 to 5 cm mineral soil for a period of 40 to 100 days at 15°C. After that period, liming had no effect on the CO₂ evolution rate in materials poor in N (C:N ratios 30 to 62) but increased the CO₂ evolution rate in materials rich in N (C:N ratios 24 to 28). When liming induced nitrification, the CO₂ evolution rate was reduced. Liming resulted in lower net N mineralization rate in needle litter and mor humus. The reduction was more pronounced when NH ₄ ⁺ was the only inorganic form than when NO ₃ ^? was the predominant form. The reason is probably that chemical fixation of NH₃ and amino compounds increases with increasing pH. Because of the fixation, the incubation technique most likely underestimated the mineralized N available to the roots. Taking this underestimation into consideration, liming initially reduced the N release in the litter layer. In the other soil layers, liming increased the N release in soils rich in N and had only small effects in soils poor in N. For the total N supply to the roots in the litter, humus and 0 to 5 cm mineral soil layers, liming caused a slight reduction in soils poor in N and a slight increase in soils rich in N. Data on tree growth corresponded with these results. The hypotheses that tree growth depressions can be caused by reduced N supply after liming and that tree growth increases can be caused by increased N supply after liming thus seem reasonable.     

Changes in soil microbial biomass carbon and enzyme activities under elevated CO2 affect fine root decomposition processes in a Mongolian oak ecosystem

The relationships between soil microbial properties and fine root decomposition processes under elevated CO₂ are poorly understood. To address this question, we determined soil microbial biomass carbon (SMB-C) and nitrogen (SMB-N), enzymes related to soil carbon (C) and nitrogen (N) cycling, the abundance of cultivable N-fixing bacteria and cellulolytic fungi, fine root organic matter, lignin and holocellulose decomposition, and N mineralization from 2006 to 2007 in a Mongolian oak (Quercus mongolica Fischer ex Ledebour) ecosystem in northeastern China. The experiment consisted of three treatments: elevated CO₂ chambers, ambient CO₂ chambers, and chamberless plots. Fine roots had significantly greater organic matter decomposition rates under elevated CO₂. This corresponded with significantly greater SMB-C. Changes in the activities of protease and phenol oxidase under elevated CO₂ could not explain the changes in fine root N release and lignin decomposition rates, respectively, while holocellulose decomposition rate had the same response to experimental treatments as did cellulase activity. Changes in cultivable N-fixing bacterial and cellulolytic fungal abundances in response to experimental treatments were identical to those of N mineralization and lignin decomposition rates, respectively, suggesting that the two indices were closely related to fine root N mineralization and lignin decomposition. Our results showed that the increased fine root organic matter, lignin and holocellulose decomposition, and N mineralization rates under elevated CO₂ could be explained by shifts in SMB-C and the abundance of cellulolytic fungi and N-fixing bacteria. Enzyme activities are not reliable for the assessment of fine root decomposition and more attention should be given to the measurement of specific bacterial and fungal communities.