Mineralisation of nitrogen from an incorporated catch crop at low temperatures: experiment and simulation

The release of nitrogen from incorporated catch crop material in winter is strongly influenced by soil temperatures. A laboratory experiment was carried out to investigate this influence in the range of 1-15 °C. Samples of sandy soil or a mixture of sandy soil with rye shoots were incubated at 1-5-10-15 °C, and samples of sandy soil with rye roots were incubated at 5-10-15 °C. Concentrations of N_min (NH₄ ⁺-N and NO₃ ^--N) were measured after 0-1-2-4-7-10 weeks for the sandy soil and the sandy soil:rye shoot mixture, and after 0-2-7-10 weeks for the sandy soil:rye root mixture. At 1 °C, 20% of total organic N in the crop material had been mineralised after ten weeks, indicating that mineralisation at low temperatures is not negligible. Maximum mineralisation occurred at 15 °C; after ten weeks, it was 39% of total applied organic nitrogen from shoot and 35% from root material. The time course of mineralisation was calculated using an exponential decay function. It was found that the influence of temperature in the range 1-15 °C could be described by the Arrhenius equation, stating a linear increase of ln(k) with T^-1, k being the relative mineralisation rate in day^-1 and T the temperature (°C). A simulation model was developed in which decomposition, mineralisation and nitrification were modelled as one step processes, following first order kinetics. The relative decomposition rate was influenced by soil temperature and soil moisture content, and the mineralisation of N was calculated from the decomposition of C, the C to N ratio of the catch crop material and the C to N ratio of the microbial biomass. The model was validated first with the results of the experiment. The model was further validated with the results of an independent field experiment, with temperatures fluctuating between 3 and 20 °C. The simulated time course of mineralisation differed significantly from the experimental values, due to an underestimation of the mineralisation during the first weeks of incubation.     

Microbial degradation of geogenic organic C and N in mine spoils

Many mine spoils present at the surface of reclamation sites in the Lower Lusatian mining district are carboniferous substrates, i.e. contain geogenic organic matter. Depending on its susceptibility to microbial degradation, geogenic organic matter might influence the establishment of a carbon requiring microflora in mine spoils. As geogenic organic matter contains substantial amounts of organic nitrogen it is also a potential source for plant available N. The objective of the present study was to quantify C and N mineralisation and microbial biomass in geogenic organic matter present at reclamation sites in Lower Lusatia. We also studied, whether these properties can be influenced by raising the originally low pH to near neutral conditions. In laboratory incubation studies, the rates of CO₂ evolution and net N mineralisation were determined in geogenic organic matter and carboniferous mine spoil with and without addition of lime. At the same time, microbial biomass carbon was estimated. As a reference, soil organic matter originating from the humus layer of a 60-year-old Pinus sylvestris stand was used. As indicated by the initial rates of C mineralisation, geogenic carbon was microbially available but to a lower extent than soil organic carbon. During incubation, C mineralisation remained constant or tended to increase with time, depending on the origin of the sample, while it decreased in soil organic matter. Unlike in soil organic matter, in geogenic organic matter and carboniferous mine spoil, C mineralisation was not consistently promoted by lime addition. Prior to incubation, microbial biomass in geogenic organic matter and carboniferous mine spoil was about 10-fold lower than in soil organic matter and tended to increase with incubation time while it decreased in soil organic matter. Similar to C mineralisation, microbial biomass in geogenic organic matter increased after liming, while it declined in carboniferous mine spoil immediately after lime addition. Rates of net N mineralisation were very low in geogenic organic matter and carboniferous mine spoil regardless of the length of incubation and could not be enhanced by raising the pH. It was concluded, that in mine spoils where accumulation of soil organic matter has not yet occurred, geogenic organic matter can be favourable for the establishment of a heterotrophic microflora. However, in the short term, geogenic matter is no source for plant available N in mine spoils. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Simulating the effects of N availability, straw particle size and location in soil on C and N mineralization

Predicting the C and N mineralization of straw added to soil is important for forecasting subsequent soil N availability during and between crop growth cycles. The decomposition module of the STICS model, parameterized under optimal conditions, was used to predict straw decomposition in sub-optimal conditions, i.e. when contact between soil and residue was poor (due to large size residues or surface placement) or when mineral N availability was restricted. The data used in the simulations were obtained from published studies of effects of residue size, location and N availability on C and N mineralization from straw under controlled laboratory conditions. We selected studies in which the dynamics of C and N mineralization were measured simultaneously. The dynamics of straw mineralization could be well predicted by the model under optimal conditions with standard parameter values as derived from measured C/N ratios of the residues, but not under sub-optimal conditions which required a new parameterization. A good fit could be obtained on these treatments by a marked reduction in the rate constants of residue and microbial biomass decomposition and a marked increase in the microbial biomass C/N ratio. Our results show the need to include in decomposition models routines for simulating effects of spatial heterogeneity of residue distribution, different particle sizes and limiting N availability.     

Role of soil animals in C and N mineralisation

Addition of single species of soil animals to animal-free microcosms often increases total heterotrophic respiration, but sometimes additions of microarthropods have been reported not to increase or even decrease CO₂ evolution rates. Most studies indicate that addition of soil animals increases net N mineralisation. In a study with F/H layer materials from a spruce stand in central Sweden kept at two temperatures (5 and 15°C) and three moisture levels (15, 30 and 60% of WHC), addition of a mixed fauna of soil arthropods, mainly microarthropods, could not be shown to change the CO₂ evolution rates in comparison with materials where arthropods were absent. However, addition of the arthropods significantly increased net N mineralisation for each of the temperature and moisture combinations. The increase due to the arthropods was dependent on soil temperature but not on soil moisture. Because the total net N mineralisation decreased with decreasing soil moisture, the soil arthropods had a much larger relative effect on net N mineralisation under dry than under moist conditions. It is concluded that soil arthropods are important in maintaining net N mineralisation under dry conditions when the microflora is largely inactive. The microbial/faunal release of mineral N is discussed in relation to the CN of the substrate.     

Quantifying and modelling C and N mineralization kinetics of catch crop residues in soil: parameterization of the residue decomposition module of STICS model for mature and non mature residues

C and N mineralization kinetics of 25 catch crop (CC) residues, whose organic C:N ratio varied from 9.5 to 34.0, were studied during soil incubations under controlled conditions. Decomposition rates were rather similar for the different CC residues, 59% to 68% residue-C being mineralized after 168 days incubation. C mineralized during the first weeks was mainly correlated to the soluble C content of the residue. N mineralized from CC residues was much more variable (?4.9 to +38.0 mg N g^?1 added C at day 168), and was mainly related to the organic N content in residues. C and N mineralization kinetics were simulated with STICS residue decomposition model, using the previous parameterization mostly based on mature crop residues (Nicolardot et al. Plant Soil 228:83–103, 2001). A reasonable agreement was found between measured and simulated C kinetics but N mineralization was underestimated by the model. A new parameterization was carried out to improve N predictions. The fitting procedure was first applied independently to each CC residue in order to optimise the five parameters of the model. The relationships found between each optimised parameter and the C:N ratio of CC residues were similar to those obtained previously, indicating that the same model was applicable to all residues. The parameters of these relationships were fitted on a combined dataset including CC and mature residues. The new parameterisation lead to better simulations for CC residues, the errors of prediction (RMSE) for C and N mineralization being 32 and 1.8 mg g^?1 added C, respectively. For the whole dataset (68 residues), the RMSE were 50 and 3.3 mg g^?1 added C. The prediction quality is satisfactory with respect to the model simplicity and the single criterion of residue quality (C:N ratio).     

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

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

Differences in fungal and bacterial physiology alter soil carbon and nitrogen cycling: insights from meta‐analysis and theoretical models

Since fungi and bacteria are the dominant decomposers in soil, their distinct physiologies are likely to differentially influence rates of ecosystem carbon (C) and nitrogen (N) cycling. We used meta‐analysis and an enzyme‐driven biogeochemical model to explore the drivers and biogeochemical consequences of changes in the fungal‐to‐bacterial ratio (F : B). In our meta‐analysis data set, F : B increased with soil C : N ratio (R² = 0.224, P < 0.001), a relationship predicted by our model. We found that differences in biomass turnover rates influenced F : B under conditions of C limitation, while differences in biomass stoichiometry set the upper bounds on F : B once a nutrient limitation threshold was reached. Ecological interactions between the two groups shifted along a gradient of resource stoichiometry. At intermediate substrate C : N, fungal N mineralisation fuelled bacterial growth, increasing total microbial biomass and decreasing net N mineralisation. Therefore, we conclude that differences in bacterial and fungal physiology may have large consequences for ecosystem‐scale C and N cycling.     

Influence of biochemical quality on C and N mineralisation from a broad variety of plant materials in soil

We studied C and N mineralisation patterns from a large number of plant materials (76 samples, covering 37 species and several plant parts), and quantified how these patterns related to biological origin and selected indicators of chemical composition. We determined C and N contents of whole plant material, in water soluble material and in fractions (neutral detergent soluble material, cellulose, hemicellulose and lignin) obtained by stepwise chemical digestion (modified van Soest method). Plant materials were incubated in a sandy soil under standardised conditions (15 °C, optimal water content, no N limitation) for 217days, and CO₂ evolution and soil mineral N contents were monitored regularly. The chemical composition of the plant materials was very diverse, as indicated by total N ranging from 2 to 59 mg N g^–1, (i.e. C/N-ratios between 7 and 227). Few materials were lignified (median lignin=4% of total C). A large proportion of plant N was found in the neutral detergent soluble (NDS) fraction (average 84%) but less of the plant C (average 46%). Over the entire incubation period, holocellulose C content was the single factor that best explained the variability of C mineralisation (r=–0.73 to –0.82). Overall, lignin C explained only a small proportion of the variability in C mineralisation (r=–0.44 to –0.51), but the higher the lignin content, the narrower the range of cumulative C mineralisation. Initial net N mineralisation rate was most closely correlated (r=0.76) to water soluble N content of the plant materials, but from Day 22, net N mineralisation was most closely correlated to total plant N and NDS-N contents (r varied between 0.90 and 0.94). The NDS-N content could thus be used to roughly categorise the net N mineralisation patterns into (i) sustained net N immobilisation for several months; (ii) initial net N immobilisation, followed by some re-mineralisation; and (iii) initially rapid and substantial net N mineralisation. Contrary to other studies, we did not find plant residue C/N or lignin/N-ratio to be closely correlated to decomposition and N mineralisation.     

Modelling the effect of active roots on soil organic matter turnover

The aim of this experiment was to study the effect of living roots on soil carbon metabolism at different decomposition stages during a long-term incubation. Plant material labelled with ¹⁴C and ¹⁵N was incubated in two contrasting soils under controlled laboratory conditions, over two years. Half the samples were cropped with wheat (Triticum aestivum) 11 times in succession. At earing time the wheat was harvested, the roots were extracted from the soil and a new crop was started. Thus the soils were continuously occupied by active root systems. The other half of the samples was maintained bare, without plants under the same conditions. Over the 2 years, pairs of cropped and bare soils were analysed at eight sampling occasions (total-, plant debris-, and microbial biomass-C and -¹⁴C). A five compartment (labile and recalcitrant plant residues, labile microbial metabolites, microbial biomass and stabilised humified compounds) decomposition model was fitted to the labelled and soil native organic matter data of the bare and cropped soils. Two different phases in the decomposition processes showed a different plant effect. (1) During the initial fast decomposition stage, labile ¹⁴C-material stimulated microbial activities and N immobilisation, increasing the ¹⁴C-microbial biomass. In the presence of living roots, competition between micro-organisms and plants for inorganic N weakly lowered the measured and predicted total-¹⁴C mineralisation and resulted in a lower plant productivity compared to subsequent growths. (2) In contrast, beyond 3–6 months, when the labile material was exhausted, during the slow decomposition stage, the presence of living roots stimulated the mineralisation of the recalcitrant plant residue-¹⁴C in the sandy soil and of the humified-¹⁴C in the clay soil. In the sandy soil, the presence of roots also substantially stimulated decomposition of old soil native humus compounds. During this slow decomposition stage, the measured and predicted plant induced decrease in total-¹⁴C and -C was essentially explained by the predicted decrease in humus-¹⁴C and -C. The ¹⁴C-microbial biomass (MB) partly decayed or became inactive in the bare soils, whereas in the rooted soils, the labelled MB turnover was accelerated: the MB-¹⁴C was replaced by unlabelled-C from C derived from living roots. At the end of experiment, the MB-C in the cropped soils was 2.5–3 times higher than in the bare soils. To sustain this biomass and activity, the model predicted a daily root derived C input (rhizodeposition), amounting to 5.4 and 3.2% of the plant biomass-C or estimated at 46 and 41% of the daily net assimilated C (shoot + root + rhizodeposition C) in the clay and sandy soil, respectively. This revised version was published online in June 2006 with corrections to the Cover Date.     

植物残体对青藏高原高寒草甸土壤、微生物和胞外酶C∶N∶P化学计量特征的影响

植物残体是引起土壤、微生物和胞外酶C∶N∶P改变的关键因素,但是其作用机理尚不明确。本研究以青藏高原东缘高寒草甸为对象,通过测定土壤、微生物生物量和胞外酶活性等指标,探究移除地上植物或根系及植物残体添加对土壤、微生物和胞外酶C∶N∶P的影响。结果表明: 与无人为扰动草甸相比,移除地上植物显著降低了土壤C∶N(变幅为-23.7%,下同)、C∶P(-14.7%)、微生物生物生物量C∶P、N∶P,显著提高了微生物生物量C∶N、胞外酶C∶N∶P。与移除地上植物相比,移除地上植物和根系显著降低了土壤C∶N(-11.6%)、C∶P(-24.0%)、N∶P(-23.3%)和微生物生物量C∶N,显著提高了微生物生物量N∶P和胞外酶N∶P;移除地上植物后添加植物残体显著提高了微生物生物量C∶N、C∶P和胞外酶C∶N,显著降低了胞外酶N∶P。与移除地上植物和根系相比,移除地上植物和根系后添加植物残体显著降低了土壤C∶N(-16.4%)、微生物生物量C∶P、N∶P和胞外酶N∶P,显著提高了胞外酶C∶N。综上可知,去除植物显著影响土壤、微生物和胞外酶的C∶N∶P,微生物生物量和胞外酶C∶N∶P对植物残体的响应更为敏感。有无根系是添加植物残体时土壤、微生物和胞外酶的生态化学计量稳定性强弱的关键所在。添加植物残体的措施适用于植物根系尚且完好的草甸,有利于高寒草甸土壤碳固存,对没有根系的草甸土壤可能不适用,会增加土壤CO₂排放。     

Nitrogen released from different plant residues of the Loess Plateau and their additions on contents of microbial biomass carbon, nitrogen in soil

An incubation method was used to investigate the nitrogen release characteristics from the residue of ten plant species which commonly grow in the northern part of the Loess Plateau. The effect of the residue on soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN) was also determined. There were significant differences in the total N content and the C/N ratios among the different types of plant residue. The total N content of the residues ranged from 6.61 to 32.78 g kg^?1. The C/N ratio of the residue ranged from 14 to 65. There was an immediate increase in soil N after alfalfa, erect milkvetch, and korshinsk peashrub residue was added to the soil. In contrast, soil N decreased after elm, sea buckthorn, and wild peach residue was added to the soil. The soil N content remained relatively low for 14–34 days and then increased. This indicated that N immobilization occurred during the early portion of the incubation period when elm, sea buckthorn and wild peach residue was added to the soil. Soil N levels were low during the entire incubation period when simon poplar, locust, Stipa bungeana, and old world bluestem residue were added to the soil. The addition of plant residue significantly increased SMBC and SMBN in all treatments. The SMBC and SMBN values were greatest in treatments containing plant residue with high total N content and low C/N ratios. The C/N ratios of korshinsk peashrub, sea buckthorn, and wild peach residues were similar, but the amount of N released from these residues and the effects of the residue on SMBC and SMBN in soil were significantly different. This indicates that not only the C/N ratio but also the chemical composition of the plant residue affected decomposition. It is important to consider C and N release characteristics from plant residue in order to adjust the C and N balance of soil when revegetating degraded ecosystems.     

Effect of tissue phosphorus concentration on the mineralisation of nitrogen from stylo and cowpea residues

Laboratory studies were conducted to investigate the effect of phosphorus concentration in residues of cowpea (t Vigna unguiculata, L. Walp) and stylo (t Stylosanthes hamata, L., cv Verano) on their rate of nitrogen mineralisation when incubated in a soil whose P status was deficient for plant growth. Residues with a range of P concentrations were obtained by applying varying rates of P to soil in which the plants were grown in the field or the glasshouse. Variations in P concentration of field- or glasshouse-grown residues were not accompanied by variations in other chemical components (C:N ratio, lignin and polyphenol concentrations). Both lignin and polyphenol concentrations were higher in the field-grown than in the glasshouse-grown residues. Lignin concentration was greater in cowpea than in stylo, but polyphenols were higher in stylo. Cowpea residues mineralised N less rapidly than stylo. N mineralisation from residues with low P concentration was consistently less than from those of higher P concentration; reduced mineralisation was observed for P concentration in the residues below 1.6 g kg^–1. When inorganic P was added to the residue-soil systems, N mineralisation from the residues was increased, though no interaction between the effects of adding inorganic P and P concentration in the residues was observed.     

C and N mineralisation in the decomposer food webs of a European forest transect

Belowground processes are essential for the overall carbon and nitrogen fluxes in forests. Neither the functioning of the soil food web mediating these fluxes, nor its modulation by environmental factors is sufficiently understood. In this study the belowground carbon and nitrogen mineralisation of four European coniferous forest sites (northern Sweden to north‐east France) with different climate and N depositional inputs was analysed by investigating the soil food webs using field observations and modelling. The soil fauna directly contributed 7–13% to C mineralisation, among which the testate amoebae (Protozoa) made the largest contribution. Microbial grazing was suggested to have an important indirect effect by stimulating bacterial turnover. Due to relatively high C:N ratios of their substrate, bacteria immobilized N, while the fauna i.e. testate amoebae, nematodes, microarthropods and enchytraeids, counteracted this N immobilisation. Despite similar food web biomass, the sites differed with respect to food web structure and C and N flows. Model calculations suggested a significant influence of food web structure on soil ecosystem processes in addition to environmental factors and resource quality. Mineralisation rates were lowest at the low N input boreal site with a food web dominated by fungal pathways. Further south, as N availability increased, bacterial pathways became more important and the cycling of C and N was faster. The bioavailability of degradable C sources is suggested to be a limiting factor for microbial activity and overall mineralisation rates. In this respect, above‐ and belowground interactions e.g. transfers of labile C sources from the vegetation to the decomposer system deserve further attention. Our study revealed the combined effects of climate and nutrient inputs to ecosystems and the subsequent changes in the structure and functioning of the systems. If decomposition, and therefore carbon loss, is stimulated as a consequence of structural and/or nutritional changes, resulting for example from continuous industrial N emission, the storage capacity of forest ecosystems could be altered.     

Root-induced nitrogen mineralisation: A theoretical analysis

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of inorganic nitrogen (N) to plants by stimulating microbial activity. The release of soluble C compounds from roots is assumed to occur by one of two general processes: cortical cell death or exudation from intact cells. On the basis of several assumptions chosen to allow maximal amounts of N mineralisation to be calculated, greater amounts of net N mineralisation are theoretically possible at realistic soil C:N ratios of bacteria are grazed by predators such as protozoa, than if bacteria alone are active. More N is mineralised when the substrate released from the root has a high C:N ratio (as in cell death) than when it is relatively N-rich. The amounts of N that a root might realistically cause to be mineralised are unlikely to account entirely for high nitrate inflow rates that have been measured experimentally. However there are circumstances in which the loss of C from roots is essential if any N is to be mineralised and obtained by plants.     

The biogeochemical consequences of litter transformation by insect herbivory in the Subarctic: a microcosm simulation experiment

Warming may increase the extent and intensity of insect defoliations within Arctic ecosystems. A thorough understanding of the implications of this for litter decomposition is essential to make predictions of soil-atmosphere carbon (C) feedbacks. Soil nitrogen (N) and C cycles naturally are interlinked, but we lack a detailed understanding of how insect herbivores impact these cycles. In a laboratory microcosm study, we investigated the growth responses of heterotrophic soil fungi and bacteria as well as C and N mineralisation to simulated defoliator outbreaks (frass addition), long-term increased insect herbivory (litter addition at higher background N-level) and non-outbreak conditions (litter addition only) in soils from a Subarctic birch forest. Larger amounts of the added organic matter were mineralised in the outbreak simulations compared to a normal year; yet, the fungal and bacterial growth rates and biomass were not significantly different. In the simulation of long-term increased herbivory, less litter C was respired per unit mineralised N (C:N of mineralisation decreased to 20?±?1 from 38?±?3 for pure litter), which suggests a directed microbial mining for N-rich substrates. This was accompanied by higher fungal dominance relative to bacteria and lower total microbial biomass. In conclusion, while a higher fraction of foliar C will be respired by insects and microbes during outbreak years, predicted long-term increases in herbivory linked to climate change may facilitate soil C-accumulation, as less foliar C is respired per unit mineralised N. Further work elucidating animal-plant-soil interactions is needed to improve model predictions of C-sink capacity in high latitude forest ecosystems.     

根系分泌物C∶N对刺槐林地土壤理化特征和土壤呼吸的影响

为探究根系分泌物C∶N对土壤养分循环及微生物活性的影响,本研究以黄土高原人工刺槐林为对象,在生境条件基本一致的15、25、35、45 a刺槐林地取原位土壤,通过模拟不同C∶N的根系分泌物(只添加N、C∶N=10、C∶N=50、C∶N=100和只添加C)添加至土壤,以去离子水作为对照,分析根系分泌物C∶N对土壤碳、氮、磷、pH值等理化特征和土壤呼吸的影响。结果表明: 1)有机碳含量与根系分泌物C∶N呈正相关,根系分泌物C∶N=10时土壤有机碳(SOC)分解较快,高根系分泌物C∶N(C∶N=100)能延缓SOC分解,而只添加C处理对SOC无显著影响。2)不同C∶N根系分泌物处理对全氮的影响不明显,碳添加能促进微生物对铵态氮的吸收,氮添加能促进铵态氮的硝化,随着根系分泌物C∶N增加,土壤中铵态氮含量下降。3)氮添加会导致土壤pH值下降,增加土壤全磷含量。4)刺槐林地土壤呼吸值与根系分泌物C∶N呈正相关,随着C∶N增加,根系分泌物对25和35 a人工刺槐林土壤呼吸的促进作用更显著。综上,根系分泌物C∶N值越高,对人工刺槐林土壤呼吸的促进作用越显著。研究结果进一步加深了对森林根系-土壤-微生物互作过程的认识。     

Root-induced nitrogen mineralisation: A nitrogen balance model

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of nitrogen (N) to plants by stimulating microbial activity. Two models are described, both of which assume that C released from roots is used by bacteria to mineralise and immobilise soil organic N and that immobilised N released when bacteria are grazed by bacterial-feeding nematodes or protozoa is taken up by the plant. The first model simulates the individual transformations of C and N and indicates that root-induced N mineralisation could supply only up to 10% of the plant's requirement, even if unrealistically ideal conditions are assumed. The other model is based on evidence that about 40% of immobilised N is subsequently taken up by the plant. A small net gain of N by the plant is shown (i.e. the plant takes up more N than it loses through exudation), although with exudate of up to C:N 33:1 less than 6% of the plant's requirement is supplied by root-induced N mineralisation. It is argued, however, that rhizosphere bacteria do not use plant-derived C to mineralise soil organic N to any great extent and that in reality root-induced N mineralisation is even less important than these models indicate.     

长期施肥下农田土壤-有机质-微生物的碳氮磷化学计量学特征

探讨外源养分的输入对土壤系统内碳、氮、磷化学计量特征的影响,对于深刻认识农田土壤有机碳(C)和养分循环及其相互作用过程具有重要意义。以26年的农田长期定位施肥试验为平台,分析长期不同施肥条件下土壤、有机态及微生物生物量碳、氮、磷含量及其化学计量学特征,并根据内稳性模型y=c x~(1/H)计算其化学计量内稳性指数H。结果表明:与长期撂荒处理(CK_0)相比,种植作物条件下26年化肥配施有机肥处理(MNPK和1.5MNPK)显著降低微生物生物量氮含量,但显著提高了微生物生物量磷的含量。相对于撂荒处理,即使长期配施化肥磷处理(NP、PK、NPK),其土壤有机磷降低显著。对于C∶N比而言,化肥配施有机物料处理(秸秆或有机肥)的土壤C∶N比、有机质C∶N及微生物生物量C∶N比均显著低于化肥处理(N、NP、PK和NPK)。对于C∶P比而言,相对于撂荒处理,26年施用磷肥(化肥磷或有机磷)显著降低了土壤C∶P比和微生物生物量C∶P比,而CK和偏施化肥处理(N、NP和PK)显著降低了土壤有机质C∶P比。对于土壤N∶P比而言,撂荒处理土壤N∶P比显著高于其他处理,而撂荒处理土壤有机质N∶P比显著高于CK和化肥处理,表明不施肥或化肥条件下作物种植加剧了土壤有机质中氮素的消耗。微生物生物量C∶N、C∶P、N∶P比的内稳性指数H分别为0.24、0.75、0.64,不具有内稳性特征。微生物生物量C∶N、C∶P、N∶P比分别与土壤C∶N、C∶P、N∶P比呈显著正相关关系,但与土壤有机质碳氮磷化学计量比之间无显著相关性。表明土壤碳、氮、磷元素的改变会直接导致微生物生物量碳、氮、磷化学计量比的改变,但微生物生物量碳氮磷化学计量比对土壤有机质碳氮磷化学计量比无显著影响,土壤有机质的碳氮磷计量比可能更多是受到作物和施肥等养分管理措施的影响。     

15N enrichment as an integrator of the effects of C and N on microbial metabolism and ecosystem function

Organic carbon (C) and nitrogen (N) are essential for heterotrophic soil microorganisms, and their bioavailability strongly influences ecosystem C and N cycling. We show here that the natural ¹⁵N abundance of the soil microbial biomass is affected by both the availability of C and N and ecosystem N processing. Microbial ¹⁵N enrichment correlated negatively with the C : N ratio of the soil soluble fraction and positively with net N mineralization for ecosystems spanning semiarid, temperate and tropical climates, grassland and forests, and over four million years of ecosystem development. In addition, during soil incubation, large increases in microbial ¹⁵N enrichment corresponded to high net N mineralization rates. These results support the idea that the N isotope composition of an organism is determined by the balance between N assimilation and dissimilation. Thus, ¹⁵N enrichment of the soil microbial biomass integrates the effects of C and N availability on microbial metabolism and ecosystem processes.