Role of organic fractions on C decomposition and N mineralization of animal wastes in soil

The relative contributions of water-soluble, water-non-soluble, Van Soest-soluble, and neutral detergent fiber (NDF) fractions of pig slurry (PS), cattle slurry (CS), cattle farmyard manure (FYM), and composted cattle farmyard manure (CFYM) to the overall C and N mineralization of the raw wastes were studied by incubating treated soil for 107 days at 15°C under non-limiting N conditions. The C or N mineralization of soluble fractions was calculated from the difference between C or N mineralization of the raw and non-soluble fractions. The organic N content of raw wastes ranged from 15 to 32 mg N g⁻¹ dry matter and organic C to organic N ratio from 13 to 29. The water-soluble fraction (SOL_W) was close to 100 mg C g⁻¹ raw waste C for CS, FYM, and CFYM but reached 200 mg C g⁻¹ for PS. The Van Soest-soluble fraction (SOL_VS) was the main fraction for PS, CS, and CFYM (>500 mg C g⁻¹ raw waste C) but only 303 mg C g⁻¹ raw waste C for FYM. Both soluble and non-soluble fractions contributed to C decomposition of slurries, with half to more than half of the decomposed C derived from the degradation of soluble compounds. Most of the C decomposed from FYM was derived from the large NDF fraction, but the contribution from the water-soluble C to the decomposition was also significant. Carbon mineralization of CFYM was due to the degradation of the NDF fraction, whereas soluble C did not contribute. Amounts of N mineralized or immobilized by raw wastes and non-soluble fractions at the end of incubation were significantly correlated (P < 0.01) with their organic C to organic N ratio. The contribution of the Van Soest-soluble fraction to N mineralization varied greatly between the four wastes. Finally, large differences in the C degradability and N availability of the water and Van Soest-soluble fractions were demonstrated.     

Dairy manure N mineralization estimates from incubations and litterbags

A laboratory incubation trial and a field litterbag study were conducted to determine the rate and magnitude of mineralization of dairy manure N components in a south central Wisconsin silt loam. Dairy manure components (urine, feces, or bedding, each ¹⁵N-labeled and the other components left unlabeled) were incubated in soil at 11, 18, or 25°C. Samples were taken at 14, 21, 42, 84, and 168 days and analyzed for mineralized N ( and ) and ¹⁵N abundance in the inorganic and organic fraction (at day 168 only). In the field study, nylon mesh (38 μm) litterbags filled with ¹⁵N-labeled manure (2000) or unlabeled manure (2000 and 2002) were placed 7.5 cm below the surface and excavated at 7, 14, 21, 28, 35 (2000 only), 42, 56, 84, 98, and 126 days after burial and at corn (Zea mays L.) harvest, after 142 days in 2002 and 154 days in 2000. In the incubation study, 50−60% of applied urine N was mineralized showing the importance of this manure N component as a source of plant available N. About 14−19% of applied N was mineralized from the fecal and bedding components. In the litterbag experiment, approximately 70% of the dry mass and 67% of the N was mineralized from the litterbags with similar amounts measured using either labeled or unlabeled N. Rates of manure organic matter decomposition and N mineralization were best predicted using single exponential models for both years with most of the release occurring during the first 21 days.     

C and N mineralization of undisrupted and disrupted soil from different structural zones of conventional tillage and no-tillage systems in northern France

Differences in soil structure created by tillage systems are often believed to have large impacts on C and N mineralization, in turn influencing total soil C and N stocks, CO₂ emissions and soil mineral N supply. The objectives of our work were therefore (i) to study C and N mineralization in undisrupted fresh soils from long-term conventional till (CT) and no-till (NT) systems in northern France and (ii) to evaluate at which scale soil structure plays a significant role in protecting organic matter against C and N mineralization. The in situ heterogeneity of soil structure was taken into account during sampling. Two megastructure zones induced by tillage and compaction were identified in the ploughed layer of CT: zones with loose structure (CT_Loose) and clods with dense structure (CT_Dense). The soil samples in NT were taken from layers that differed in both structure and organic matter content (NT_0-5 and NT_5-20). Soil from the two zones of different megastructure in CT showed similar levels of protection and similar C and N mineralization. Undisrupted soil from NT_0-5 showed greater absolute and specific C and N mineralization than CT_Loose, CT_Dense and NT_5-20. Limited soil structure destruction (sieving through 2 mm) had no effect on C and N mineralization. Increased disturbance (sieving down to 250 μm) only induced a significant increase of both C and N mineralization in the 5-20 cm layer of NT. Further disruption of soil structures (sieving through 50 μm) resulted in greater C and N mineralization for all treatments except C mineralization in the upper layer of NT. Protection in the four structural zones in CT and NT was, in general, greatest in the NT deeper layer and least in the NT upper layer. Our results therefore suggest that physical protection in the 5-20 cm soil layer can partly account for larger C and N stocks in NT, but that the large C and N concentrations in the 0-5 cm soil layer are determined by mechanisms other than physical OM protection.     

Changes in the soil C and N contents,C decomposition and N mineralization potentials in a rice paddy after long-term application of inorganic fertilizers and organic matter

A long-term experiment on combined inorganic fertilizers and organic matter in paddy rice (Oryza sativa L.) cultivation began in May 1982 in Yamagata, northeastern Japan. In 2012, after the 31^st harvest, soil samples were collected from five fertilizer treatments [(1) PK, (2) NPK, (3) NPK + 6 Mg ha^?1 rice straw (RS), (4) NPK + 10 Mg ha^?1 rice straw compost (CM1), and (5) NPK + 30 Mg ha^?1 rice straw compost (CM3)], at five soil depths (0–5, 5–10, 10–15, 15–20 and 20–25 cm), to assess the changes in soil organic carbon (SOC) content and carbon (C) decomposition potential, total nitrogen (TN) content and nitrogen (N) mineralization potential resulting from long-term organic matter addition. The C decomposition potential was assessed based on the methane (CH₄) and carbon dioxide (CO₂) produced, while the N mineralization potential was determined from the potassium chloride (KCl)-extractable ammonium-nitrogen (NH₄⁺-N), after 2, 4, 6 and 8 weeks of anaerobic incubation at 30°C in the laboratory. Compared to NPK treatment, SOC in the total 0–25 cm layer increased by 67.3, 21.0 and10.8%, and TN increased by 64.2, 19.7 and 10.6%, in CM3, RS and CM1, respectively, and SOC and TN showed a slight reduction in the PK treatment by 5.2 and 5.7%, respectively. Applying rice straw compost (10 Mg ha^?1) instead of rice straw (6 Mg ha^?1) to rice paddies reduced methane production by about 19% after the soils were measured under 8 weeks of anaerobic incubation at 30°C. Soil carbon decomposition potential (Co) and nitrogen mineralization potential (No) were highly correlated with the SOC and TN contents. The mean ratio of Co/No was 4.49, lower than the mean ratio of SOC/TN (13.49) for all treatments, which indicated that the easily decomposed organic matter was from soil microbial biomass and soil proteins.     

Kinetics of C and N mineralization, N immobilization and N volatilization of organic inputs in soil

C and N mineralization data for 17 different added organic materials (AOM) in a sandy soil were collected from an incubation experiment conducted under controlled laboratory conditions. The AOM originated from plants, animal wastes, manures, composts, and organic fertilizers. The C-to-N_AOM ratios (η_AOM) ranged from 1.1 to 27.1. Sequential fibre analyses gave C-to-N ratios of soluble (η_Sol), holocellulosic (η_Hol) and ligneous compounds (η_Lig) ranging from 1.1 to 57.2, 0.8 to 65.2, and 3.5 to 25.3, respectively. Very different patterns of net AOM-N mineralization were observed: (i) immobilization for four plant AOM; (ii) moderate mineralization (4-15% AOM-N) for composts; (iii) marked mineralization (11-27% AOM-N) for 1 animal AOM, 1 manure and 2 organic fertilizers; and (iv) high rates of transformations with possible gaseous losses for some N-rich AOM.The Transformation of Added Organics (TAO) model proposed here, described AOM-C mineralization (28 °C, 75% WHC) from three labile (L′), resistant (R) and stable (S) compartments with the sole parameters P′_L and P_S=fractions of very labile and stable compounds of AOM, respectively. Dividing the C-compartments by their C-to-N estimates supplied the remaining N_AOM fraction (RAONF). A P_im parameter split the TAO nitrogen fraction (TAONF=added N-RAONF) into two compartments, immobilized (imN) and inorganic (inorgN) N. A P_im>0 value meant that all the TAONF plus a fraction (P_im−1) of native soil inorganic N was immobilized. Additional N mineralization was predicted when necessary from imN by first order kinetics (constant k_remin). The TAO version with two parameters P_im and k_remin allowed us to predict very different patterns of N mineralization and N immobilization. In a few cases, a further first order kinetic law (constant k_v) was added to predict N volatilization from inorgN. Two hypotheses were tested: (i) η_L′, η_R, η_S (C-to-N of L′, R and S)=η_Sol, η_Hol, η_Lig, respectively, (ii) η_L′=η_R=η_S=η_AOM. The first hypothesis was validated by these data, and the second was a good approximation of the former one. In all the cases, predictions were in good agreement with measured values.     

Influence of the N and P status of plant material and of added N and P on the mineralization of C from 14C-labelled ryegrass in soil

Ryegrass was grown under conditions of low N, low P, or high N and P nutrient supply in an atmosphere containing ¹⁴CO₂ and then incubated in soil supplemented with or without N or P fertilizer. Determined in fresh plant tissue, the persistency of residual labelled C after 6 months was in the order low-N plants>low-P plants>high-N and-P plants. The addition of N conserved C, particularly when there was additional P present. Hydrolysable labelled C (12M/0.5M H₂SO₄) showed similar trends. In analyses of freeze-dried plant tissue, the main effect was also the increased persistency of C from low-N plants compared to high-N plants. The addition of N fertilizer increased the persistence of plant residue C, but only with grass containing low P. The addition of P fertilizer had no effect. In freeze-dried low-P plant tissue, sampled after 1.5, 6, and 12 months, the conserving effect of adding fertilizer N was confirmed. The addition of P, in contrast, enhanced the rate of decomposition. After 6 months, about a third of the C remained, and after 12 months, about one-quarter. It is concluded that P, whether intrinsic or added, can increase the rate of decomposition of organic residues in soil, but there is a strong interaction with N, which has a predominant influence. The effects of N depend on the form it is in. Increased intrinsic tissue N can increase the rate of C loss, whereas added inorganic N can decrease the rate of C loss during decomposition.     

外加碳、氮对黄绵土有机质矿化与激发效应的影响

应用14C标记的葡萄糖和麦秸,15N标记的(NH4)2SO4和Ca(NO3)2对生黄绵土、菜园黄绵土土壤有机质的矿化与激发效应进行了研究。结果表明,外加有机质,特别是外加易分解的葡萄糖,和外加氮源,特别是外加(NH4)2SO4,对两种黄绵土土壤的有机质矿化与激发效应都有明显的促进作用,土壤有机质的矿化是高肥力菜园黄绵土高于低肥力生黄绵土,而有机质矿化的激发效应却是低肥力生黄绵土高于高肥力菜园黄绵土。外加有机质与外加N同时施入土壤时,外加N对外加有机质的矿化与激发效应同样有明显的促进作用,并发现外加有机质与外加N在促进土壤有机质矿化与激发效应过程中表现出正交互作用。激发效应对土壤肥力的更新和培养有积极作用。     

Effect of drying and rewetting on mineralization and distribution of bacterial constituents in soil fractions

Summary Mineralization of ¹⁴C- and ¹⁵N-labelled whole bacteria, cytoplasm, and cell walls and their distribution in different soil fractions were studied during 211 days of incubation including two drying and rewetting cycles. With any of these three soil amendments, almost 60% of C derived from cellular constituents was released as CO₂, 15% was incorporated into the living microbial biomass and 25% was distributed into protected microbial metabolites or recalcitrant microbial products. The distribution of C and N derived from the amendments in the different soil fractions showed that constituents adsorbed on fine clay (<0.2 m were more rapidly decomposed than those adsorbed on silt (50-2 ) and coarse clay (2–0.2 ), indicating a faster organic matter turnover in fine clay than in silt and coarse clay. Although alternate soil drying and rewetting cycles did not significantly affect the mineralization of bacterial constituents, the cycles did have an important effect on the size and specific activities of newly formed microbial biomass. This suggests the presence of an active and a dormant fraction of soil biomass.     

Comparison of biological and chemical methods to predict nitrogen mineralization in animal wastes

Summary Biological and chemical methods to predict the level of plant-available N in animal manure were investigated under laboratory and growth-chamber conditions. Two biological methods (maize cultivation in pots and incubation of soil-waste mixtures) and four chemical methods (N extraction by autoclaving, 0.5 N KMnO₄, pepsin, and 6 N HCl) were compared for their accuracy in determining the availability of N in 10 samples of animal manure applied to soil. The autoclaving, permanganate, and pepsin methods were able to predict N availability in this group of wastes. Total N mineralized in a soil amended with different samples of animal manure ranged from 0 to 311 mg N kg^-1 soil. Expressed as a percentage of organic N added to the soil, mineralized N range from 0 to 39%. The poultry manure samples gave higher mineralization rates than the other types of manure tested. In general, mineralized N became immobilized during the incubation process, except for the second poultry manure sample, which showed an initially rapid then a a slow release of mineral N, and pig manure sample 2, which showed a slow initial rate, followed by a rapid increase, and then a slow rate of N release. The first-order exponential model used was able to describe the pattern of N mineralization in pig manure sample 2, poultry manure sample 2 but not the other samples.     

Assessment of N Mineralization and N Leaching in Soil Using a New in situ Incubation Method

Field incubations have been pointed out as the more realistic method to provide estimates of nitrogen (N) mineralization. The aim of this study is to evaluate the quality of the results obtained in a field incubation using an open reactor to estimate net N mineralization and N leaching. The incubation experiment was initiated with 24 reactors. At each date, the reactors were destructed and mineral N in the soil and adsorbed on the exchange resins was determined. Net N mineralization and N leaching were estimated as 23.3 and 18.0 mg/kg, respectively. The results revealed an acceptable repeatability, with coefficient of variation (CV) of 9.0%, a significant adjustment (r² = 0.991), and a low root mean square error (S_yx = 4.2) for the regression model used. The use of this type of reactor may be considered as a reliable alternative to assess N mineralization kinetics from native organic matter and probably for organic residues applied to soils in field studies.     

Carbon and nitrogen mineralization of non-composted and composted municipal solid waste in sandy soils

A sterilized, but undecomposed, organic by-product of municipal waste processing was incubated in sandy soils to compare C and N mineralization with mature municipal waste compost. Waste products were added to two soils at rates of 17.9, 35.8, 71.6, and dry weight and incubated at for 90 d. Every 30 d, nitrate and ammonium concentrations were analyzed and C mineralization was measured as total CO₂-C evolved and added total organic C. Carbon mineralization of the undecomposed waste decreased over time, was directly related to application rate and soil nutrient status, and was significantly higher than C mineralization of the compost, in which C evolution was relatively unaffected across time, soils, and application rates. Carbon mineralization, measured as percentage C added by the wastes, also indicated no differences between composted waste treatments. However, mineralization as a percentage of C added in the undecomposed waste treatments was inversely related to application rate in the more productive soil, and no rate differences were observed in the highly degraded soil. Total inorganic N concentrations were much higher in the compost- and un-amended soils than in undecomposed waste treatments. Significant N immobilization occurred in all undecomposed waste treatments. Because C mineralization of the undecomposed waste was dependant on soil nutrient status and led to significant immobilization of N, this material appears to be best suited for highly degraded soils low in organic matter where restoration of vegetation adapted to nutrient poor soils is desired.     

Bulk soil C to N ratio as a simple measure of net N mineralization from stabilized soil organic matter in sandy arable soils

Investigations of 23 northwestern German sandy Ap horizons (mean clay content 35 g kg⁻¹), that had higher organic matter (OM) levels than expected for sands, showed that the bulk soil C to N ratio reliably indicated the release of N from stabilized OM. Soils were incubated at 35 °C for 200 days under aerobic conditions. Cumulative N release curves were split into N released from fresh materials (N_fast) and N released from the larger pool of stabilized, older OM (N_slow rates, 0.06-0.77 μg N g⁻¹ soil d⁻¹, or 0.7-49 μg N g⁻¹ OM). Correlating the N_slow rates with total N contents of soils yielded no satisfactory relationships while their relationship with C to N ratios was very close (negative exponential, R²=0.88). Low rates of N release (N_slow) per unit of OM occurred if C to N exceeded 15. This was associated with historical factors like podzolization, calluna heathland, plaggen fertilization or a combination of these.     

中国亚热带稻田土壤碳氮含量及矿化动态

Dynamics of soil organic matter in a cultivation chronosequence of paddy fields were studied in subtropical China. Mineralization of soil organic matter was determined by measuring CO2 evolution from soil during 20 days of laboratory incubation. In the first 30 years of cultivation, soil organic C and N contents increased rapidly. After 30 years, 0-10 cm soil contained 19.6 g kg^-1 organic C and 1.62 g kg^-1 total N, with the corresponding values of 18.1 g kg^-1 and 1.50 g kg^-1 for 10-20 cm, and then remained stable even after 80 years of rice cultivation. During 20 days incubation the mineralization rates of organic C and N in surface soil （0-10 cm） ranged from 2.2% to 3.3% and from 2.8% to 6.7%, respectively, of organic C and total N contents. Biologically active C size generally increased with increasing soil organic C and N contents. Soil dissolved organic C decreased after cultivation of wasteland to 10 years paddy field and then increased. Soil microbial biomass C increased with number of years under cultivation, while soil microbial biomass N increased during the first 30 years of cultivation and then stabilized. After 30 years of cultivation surface soil （0-10 cm） contained 332.8 mg kg^-1 of microbial biomass C and 23.85 mg kg^-1 of microbial biomass N, which were 111% and 47% higher than those in soil cultivated for 3 years. It was suggested that surface soil with 30 years of rice cultivation in subtropical China would have attained a steady state of organic C content, being about 19 g kg^-1.     

土地利用方式对田间植物残体中C、N矿化的影响

Four kinds of plant materials(astragalus,azolla,rice straw and water hyacinth) were allowed to decom-pose for 10 years in two soils with different mineralogical characteristics in fields under upland and submerged conditions.Greater amounts of C and N from azolla were retained in soils throughout the 10-year experi-mental period compared to those from the other plant materials.The residual C of all the plant materials in the two soils under upland conditions mineralized at rates corresponding to half-lives between 4.4-6.6 years,while the corresponding figures for those under submerge conditions were between 6.5-13.1 years,Minera-liztion of residual organic N followed the same pattern as residual C.Compared to residual C,however,the mineralization rates of residual organic N in most cases were significantly lower and the percentages of added N retained in soils were higher.More N from plant materials was retained in the yellow-brown soil than in the red soil,but no consistent differences in the amounts of C from plant materials and in the mineraliztion rates of both residual C and residual organic N between the wto soils could be folund.     

Effects of filter type and extraction efficiency on nitrogen mineralization measurements using the aerobic leaching soil incubation method

Aerobic incubation of soils with sequential leachings to extract mineralized N is often used to determine N mineralization potential and N availability in the laboratory. This study used tropical forest soils with differing mineralogy and texture to address: (1) the effects of filter type and equilibration time on soil moisture and N mineralization and (2) the N extraction efficiency of 0.01 M CaCl₂, minus-N nutrient solution (containing 0.004 M CaCl₂) and 2 M KCl. Use of glass microfiber filters compared to cellulose acetate or polyethersulfone membrane filters resulted in a lower moisture content for both low-and high-clay soils. However, filter type did not affect N mineralization. Under 47 kPa suction, soil moisture equilibration occurred between 240 and 360 min regardless of filter type. Extraction efficiency for mineralized N using 0.01 M CaCl₂ or minus-N nutrient solution was lower in forest soils of smectitic mineralogy and soils with a higher proportion of macroaggregates. However, with the exception of allophanic soils, the cumulative amount of N mineralized measured in a long-term incubation for approximately 1 year was not different when either a leaching or an unleached incubation method was used. These results indicate that researchers may wish to conduct preliminary evaluations to determine whether their incubation method will achieve a desired uniform moisture level and N extraction efficiency.     

秸秆还田施氮调节碳氮比对土壤无机氮、 酶活性及作物产量的影响

秸秆的质量,特别是C/N是影响秸秆分解速率和养分释放的重要因素。在秸秆还田条件下,如何科学合理地施用氮肥是秸秆利用和优化施肥研究的关键问题。本研究以秸秆还田施入碳氮的C/N为切入点,于2012—2013年通过田间试验(设秸秆不还田不施肥、秸秆还田不施氮、秸秆还田施用无机氮肥调节C/N为10∶1、16∶1和25∶1以及秸秆还田施用有机氮肥调节C/N为25∶1处理),研究秸秆还田不同氮输入对小麦-玉米轮作田土壤无机氮、土壤微生物量氮、酶活性以及作物产量的影响。结果表明:1)在C/N为25∶1下,施用有机氮肥和无机氮肥对土壤无机氮含量无显著影响;在施用无机氮肥的情况下,C/N越低土壤无机氮含量越高。2)秸秆还田施氮提高了土壤微生物量氮含量,但是各秸秆还田施氮处理之间差异不显著;秸秆还田不同施氮处理对脲酶活性无显著影响;秸秆还田施氮提高了FDA水解酶活性,并随C/N降低呈升高趋势,施用无机氮肥的效果强于施用有机氮肥的。3)秸秆还田施用无机氮肥显著提高了小麦和玉米地上部生物量,施用无机氮肥调节C/N为10∶1和16∶1相比于C/N为25∶1提高了小麦和玉米的苗期和成熟期地上部生物量;施用有机氮肥调节C/N为25∶1相比秸秆还田不施氮对地上部生物量无显著影响。秸秆还田施用无机氮肥提高了作物产量,施用无机氮肥调节C/N为16∶1产量最高,而施用有机氮肥调节C/N为25∶1有降低作物产量的趋势。综合以上结果来看,施用无机氮肥调节C/N为16∶1较为合理。     

长期稻秆还田对土壤微生物量及C、N动力学的影响

A study was performed on the long-term effect of straw incorporation on soil microbial biomass C contents, C and N dynamics in both Rothamsted and Woburn soils. The results showed that for both soils, the microbial biomass C contents were significantly different among all the treatments, and followed the sequence in treatments of straw chopped and incorporated into 10 cm (CI10) > straw burnt and incorporated into 10 cm (BI10) > straw chopped and incorporated into 20 cm (CI20) > straw burnt and incorporated into 20 cm (BI20). Laboratory incubation of soils showed that the cumulative CO₂ evolution was closely related to the soil microbial biomass C content. Carbon dioxide evolution rates (CO₂-C, μg (g•d)^-1) decreased rapidly in the first two weeks' incubation, then decreased more slowly. The initial K₂SO₄-extractable NH₄-N and NO₃-N contents were low and similar in all the treatments, and all increased gradually with the incubation time. However, net N immobilization was observed in chopped treatments for Rothamsted soils during the first 4 weeks. Nevertheless, more N mineralization occurred in Treatment CI10 than any other treatment at the end of incubation for both soils. The Woburn soils could more easily suffer from the leaching of nitrate because the soils were more permeable and more N was mineralized during the incubation compared to the Rothamsted soils.     

高寒草甸土CO2流通和氮矿化之间的关系以及对外加碳或氮的响应

In nutrient-limited alpine meadows,nitrogen(N) mineralization is prior to soil microbial immobilization;therefore,increased mineral N supply would be most likely immobilized by soil microbes due to nutrient shortage in alpine soils.In addition,low temperature in alpine meadows might be one of the primary factors limiting soil organic matter decomposition and thus N mineralization.A laboratory incubation experiment was performed using an alpine meadow soil from the Tibetan Plateau.Two levels of NH4NO3(N) or glucose(C) were added,with a blank without addition of C or N as the control,before incubation at 5,15,or 25 ℃ for 28 d.CO2 efflux was measured during the 28-d incubation,and the mineral N was measured at the beginning and end of the incubation,in order to test two hypotheses:1) net N mineralization is negatively correlated with CO2 efflux for the control and 2) the external labile N or C supply will shift the negative correlation to positive.The results showed a negative correlation between CO2 efflux and net N immobilization in the control.External inorganic N supply did not change the negative correlation.The external labile C supply shifted the linear correlation from negative to positive under the low C addition level.However,under the high C level,no correlation was found.These suggested that the correlation of CO2 efflux to net N mineralization strongly depend on soil labile C and C:N ratio regardless of temperatures.Further research should focus on the effects of the types and the amount of litter components on interactions of C and N during soil organic matter decomposition.     

Increased N deposition retards mineralization of old soil organic matter

The aim of this study was to investigate the effects of increased N deposition on new and old pools of soil organic matter (SOM). We made use of a 4-yr experiment, where spruce and beech growing on an acidic loam and a calcareous sand were exposed to increased N deposition (7 vs. 70 kg N ha⁻¹ yr⁻¹) and to elevated atmospheric CO₂. The added CO₂ was depleted in ¹³C, which enabled us to distinguish between old and new C in SOM-pools fractionated into particle sizes. Elevated N deposition for 4 yr increased significantly the contents of total SOM in 0-10 cm depth of the acidic loam (+9%), but not in the calcareous sand. Down to 25 cm soil depth, C storage in the acidic loam was between 100 and 300 g C m⁻² larger under high than under low N additions. However, this increase was small as compared with the SOM losses of 600-700 g C g C 0.25 m⁻¹ m⁻² from the calcareous sand resulting from the disturbance of soils during setting up of the experiment. The amounts of new, less than 4 yr old SOM in the sand fractions of both soils were greater under high N deposition, showing that C inputs from trees into soils increased. Root biomass in the acidic loam was larger under N additions (+25%). Contents of old, more than 4 yr old C in the clay and silt fractions of both soils were significantly greater under high than under low N deposition. Since clay- and silt-bound SOM consists of humified compounds, this indicates that N additions retarded mineralization of old and humified SOM. The retardation of C mineralization in the clay and silt fraction accounted for 60-80 g C m⁻² 4 yr⁻¹, which corresponds to about 40% of the old SOM mineralized in these fraction. As a consequence, preservation of old and humified SOM under elevated N deposition might be a process that could lead to an increased soil C storage in the long-term.