Dynamics of 13C‐labeled mustard litter (Sinapis alba) in particle‐size and aggregate fractions in an agricultural cropland with high‐ and low‐yield areas

The application of ¹³C‐labeled litter enables to study decomposition processes as well as the allocation of litter‐derived carbon to different soil C pools. ¹³Carbon‐labeled mustard litter was used in order to compare decomposition processes in an agricultural cropland with high‐yield (HY) and low‐yield (LY) areas, the latter being characterized by a finer texture and a lower organic‐C (OC) content. After tracer application, ¹³C concentrations were monitored in topsoil samples in particulate organic matter (POM) and in fine mineral fractions (silt‐ and clay‐sized fractions). After 568 d, approximately 5% and 10% of the initial ¹³C amount were found in POM fractions of LY and HY areas, respectively. Higher amounts were found in POM occluded in aggregates than in free POM. Medium‐term (0.5–2 y) storage of the initial ¹³C in fine silt‐ and clay‐sized fractions amounts to 10% in HY and LY soils, with faster enrichment but also faster disappearance of the ¹³C signal from LY soils. Amounts of 80%–90% of the added ¹³C were mineralized or leached in the observed period. Decomposition of free POM was faster in HY than in LY areas during the first year, but the remaining ¹³C amounts in occluded‐POM fractions were higher in HY soils after 568 d. High‐yield and low‐yield areas showed different ¹³C dynamics in fine mineral fractions. In LY soils, ¹³C amounts and concentrations in mineral‐associated fractions increased within 160 d after application and decreased in the following time period. In HY areas, a significant increase in ¹³C amounts did not occur until after 568 d. The results indicate initially faster decomposition processes in HY than in LY areas due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote a faster aggregate formation and turnover, leading to a closer contact between POM and mineral surfaces in this area. This favors the OC storage in fine mineral fractions in the medium term. Lower aggregate formation and turnover in the coarser textured HY soil leads to a delayed C stabilization in silt‐ and clay‐sized fractions.     

Temporal changes in distribution and composition of N from labeled fertilizer in soil organic matter fractions

Variations in the amount and composition of immobilized nitrogen (N) in major soil organic matter fractions were investigated in a 730-day soil incubation experiment using ¹⁵N-labeled urea and ¹⁵N nuclear magnetic resonance spectroscopy with the cross polarization/magic angle spinning (¹⁵N CPMAS NMR) method. After 730 days, 24.7% of the applied N was recovered from the soil as organic N. The urea-derived N recovered from humic acids and humin decreased from 11.2 and 33.8% of the applied amount after 14 days to 1.6 and 20.4% after 730 days, respectively. When these values were corrected for the microbial biomass (MB) N, they ranged from 9.0 to 1.2% and 28 to 18%, respectively. The proportion of urea-derived N recovered from fulvic acids was low, ranging between 0.4 and 5.8% (with MB N) or 5.6% (without MB N) of the applied amount, whereas that from water-soluble nonhumic substances (WS-NHS; NHS in the fulvic acid fraction) remained high, 28–33% of the applied amount after correction for the contribution of MB N up to day 365, and decreased to 0.9% thereafter. The ¹⁵N CPMAS NMR spectra of humic acids, fulvic acids, and humin showed the largest signal at −254 to −264 ppm, corresponding to peptide/amide N. The proportions of heterocyclic, peptide/amide, guanidine/aniline, and free amino N in the urea-derived humic acid N were 3–7, 83–90, 5–7, and 2–4%, respectively. More than 80% loss of the urea-derived humic acid N did not markedly alter their composition. No time-dependent variations were also observed for the proportions of respective N functional groups in humin N, which were 3–5, 71–78, 12–17, and 6–10% in the same order as above. These results suggest the greater importance of physical stability than structural variation for the initial accumulation of organic N in soil.     

Decomposition of N-labelled maize leaves in soil affected by endogeic geophagous Aporrectodea caliginosa

A microcosm experiment was carried out for 56 days at 12 °C to evaluate the feeding effects of the endogeic geophagous earthworm species Aporrectodea caliginosa on the microbial use of ¹⁵N-labelled maize leaves (Zea mays) added as 5 mm particles equivalent to 1 mg C and 57 μg N g⁻¹ soil. The dry weight of A. caliginosa biomass decreased in the no-maize treatment by 10% during the incubation and increased in the maize leaf treatments by 18%. Roughly 5% and 10% of the added maize leaf-C and leaf-N, respectively, were incorporated into the biomass of A. caliginosa. About 29% and 33% of the added maize leaf-C were mineralised to CO₂ in the no-earthworm and earthworm treatments, respectively. The presence of A. caliginosa significantly increased soil-derived CO₂ production by 90 μg g⁻¹ soil in the no-maize and maize leaf treatments, but increased the maize-derived CO₂ production only by 40 μg g⁻¹ soil. About 10.5% of maize leaf-C and leaf-N was incorporated into the soil microbial biomass in the absence of earthworms, but only 6% of the maize leaf-C and 3% of the maize leaf-N in the presence of earthworms. A. caliginosa preferentially fed on N rich, maize leaf-colonizing microorganisms to meet its N demand. This led to a significantly increased C/N ratio of the unconsumed microbial biomass in soil. The ergosterol-to-microbial biomass C ratio was not significantly decreased by the presence of earthworms. A. caliginosa did not directly contribute to comminution of plant residues, as indicated by the absence of any effects on the contents of the different particulate organic matter fractions, but mainly to grazing of residue-colonizing microorganisms, increasing their turnover considerably.     

Gross N transformation rates and net N mineralisation rates related to the C and N contents of soil organic matter fractions in grassland soils of different age

We investigated the relationship between soil organic matter (SOM) content and N dynamics in three grassland soils (0-10 and 10-20 cm depth) of different age (6, 14 and 50 y-old) with sandy loam textures. To study the distribution of the total C and N content the SOM was fractionated into light, intermediate and heavy density fractions of particulate macro-organic matter (150-2000 μm) and the 50-150 μm and <50 μm size fractions. The potential gross N transformation rates (mineralisation, nitrification, NH₄⁺ and NO₃⁻ immobilization) were determined by means of short-term, fully mirrored ¹⁵N isotope dilution experiments (7-d incubations). The long-term potential net N mineralisation and gross N immobilization rates were measured in 70-d incubations. The total C and N contents mainly tended to increase in the 0-10 cm layer with increasing age of the grassland soils. Significant differences in total SOM storage were detected for the long-term (50 y-old) conversion from arable land to permanent grassland. The largest relative increase in C and N contents had occurred in the heavy density fraction of the macro-organic matter, followed by the 50-150 and <50 μm fractions. Our results suggest that the heavy density fraction of the macro-organic matter could serve as a good indicator of early SOM accumulation, induced by converting arable land to permanent grassland. Gross N mineralisation, nitrification, and (long-term) gross N immobilization rates tended to increase with increasing age of the grasslands, and showed strong, positive correlations with the total C and N contents. The calculated gross N mineralisation rates (7-d incubations) and net N mineralisation rates (70-d incubations) corresponded with a gross N mineralisation of 643, 982 and 1876 kg N ha⁻¹ y⁻¹, and a net N mineralisation of 195, 208 and 274 kg N ha⁻¹ y⁻¹ in the upper 20 cm of the 6, 14 and 50 y-old grassland soils, respectively. Linear regression analysis showed that 93% of the variability of the gross N mineralisation rates could be explained by variation in the total N contents, whereas total N contents together with the C-to-N ratios of the <50 μm fraction explained 84% of the variability of the net N mineralisation rates. The relationship between long-term net N mineralisation rates and gross N mineralisation rates could be fitted by means of a logarithmic equation (net m=0.24Ln(gross m)+0.23, R²=0.69, P<0.05), which reflects that the ratio of gross N immobilization-to-gross N mineralisation tended to increase with increasing SOM contents. Microbial demand for N tended to increase with increasing SOM content in the grassland soils, indicating that potential N retention in soils through microbial N immobilization tends to be limited by C availability.     

Dynamics of 15N-labeled ammonium sulfate in various inorganic and organic soil fractions of wetland rice soils

Summary The dynamics of basally applied ¹⁵N-labeled ammonium sulfate in inorganic and organic soil fractions of five wetland rice soils of the Philippines was studied in a greenhouse experiment. Soil and plant samples were collected and analyzed for ¹⁵N at various growth stages. Exchangeable NH₄ ⁺ depletion continued after 40 days after transplanting (DAT) and corresponded with increased nitrogen uptake by rice plants. Part of the applied fertilizer was fixed by 2:1 clay minerals, especially in Maligaya silty clay loam, which contained beidellite as the dominant clay mineral. After the initial fixation, nonexchangeable ¹⁵N was released from 20 DAT in Maligaya silty clay loam, but fixation delayed fertilizer N uptake from the soil. Part of the applied N was immobilized into the organic fraction. In Guadalupe clay and Maligaya silty clay loam, immobilization increased with time while the three other soils showed significant release of fertilizer N from the organic fraction during crop growth. Most of the immobilized fertilizer N was recovered in the nondistillable acid soluble (alpha-amino acid + hydrolyzable unknown-N) fraction at crop maturity. Between 61% and 66% of applied N was recovered from the plant in four soils while 52% of fertilizer N was recovered from the plant in Maligaya silty loam. Only 20% – 30% of the total N uptake at maturity was derived from fertilizer N. Nmin (mineral N) content of the soil before transplanting significantly correlated with N uptake. Twenty-two to 34% of applied N was unaccounted for possibly due to denitrification and ammonia volatilization.     

Natural N abundance of plant and soil inorganic-N as evidence for over-fertilization with compost

To test the hypothesis that N isotope composition can be used as evidence of excessive compost application, we measured variation in patterns of N concentrations and corresponding δ¹⁵N values of plants and soil after compost application. To do so, a pot experiment with Chinese cabbage (Brassica campestris L. cv. Maeryok) was conducted for 42 days. Compost was applied at rates of 0 (SC0), 500 (SC1), 1000 (SC2), and 1500 mg N kg⁻¹ soil (SC3). Plant-N uptake linearly increased with compost application (r² = 0.956, P < 0.05) with an uptake efficiency of 76 g N kg⁻¹ of compost-N at 42 days after application, while dry-mass accumulation did not show such linear increases. Net N mineralized from compost-N increased linearly (r² = 0.998, P < 0.01) with a slope of 122 g N kg⁻¹ of compost-N. Plant-δ¹⁵N increased curvilinearly with increasing compost application, but this increase was insignificant between SC2 and SC3 treatments. The δ¹⁵N of soil inorganic-N (particularly NO₃⁻-N) increased with compost application. We found that plant-δ¹⁵N reflected the N isotope signal of soil NO₃⁻-N at each measurement during plant growth, and that δ¹⁵N of inner leaves and soil NO₃⁻-N was similar when initial NO₃⁻ in the compost was abundant. Therefore, we concluded that δ¹⁵N of whole plant (more obviously in newer plant parts) and soil NO₃⁻-N could reveal whether compost application was excessive, suggesting a possible use of δ¹⁵N in plants and soil as evidence of excess compost application.     

Kinetics of 15N -labelled nitrogen from co-compost made from cattle manure and chemical fertilizer in a paddy field

In order to produce an effective organic fertilizer, cattle manure was cocomposted with chemical fertilizer. And the kinetics of nitrogen uptake by rice plants from the co-compost was investigated using the ¹⁵N labelled co-composts on either cattle manure or chemical fertilizer. As a control, nitrogen kinetics from the mixture of cattle manure and chemical fertilizer without co-composting was investigated. At the early stage, rice growth may have been promoted by co-composting, while, it may have been promoted by the larger N-content of cattle manure at the harvesting stage. The ratios of nitrogen uptake by rice plants and residual nitrogen in soil from the cattle manure and chemical fertilizer were determined by measuring ¹⁵N -atom%. The N -uptake ratios by rice plants from the cattle manure in the co-composted plot were about 2–4 times higher than those from the cattle manure without co-composting. However, the N -uptake ratios from the chemical fertilizer in the co-composted plot were lower than those from the chemical fertilizer without co-composting. The N -content of the rice plants derived from chemical fertilizer without co-composting decreased consistently after 28 d. The nitrogen from chemical fertilizer in the co-compost was absorbed again in the latter period of rice growth. The total nitrogen uptake by rice plants from cattle manure and chemical fertilizer was similar regardless of co-composting. However, co-composting would be advantageous at least· in terms of the following aspects: increase of the N -uptake by rice plants from cattle manure, slow-release ability of nitrogen from chemical fertilizer, decrease of nitrogen loss by denitrification.     

Extractability of labelled microbial biomass N by electro-ultrafiltration and CaCl2 extraction

A laboratory soil incubation and a pot experiment with ryegrass were carried out in order to examine the extractability of microbial biomass N by using either 10-mM CaCl₂ extraction or the electro-ultrafiltration (EUF) method. The aim of the experiment was to test the hypothesis whether the organic N (Norg) extracted by EUF or CaCl₂ from dried soil samples represents a part of the microbial biomass. For the laboratory incubation a ¹⁵N-labelled Escherichia coli suspension was mixed with the soil. For the pot experiment a suspension of ¹⁵N-labelled bacteria was applied which had previously been isolated from the soil used. Soil samples of both treatments, with and without applied bacterial suspension, were extracted by EUF and CaCl₂. The extractability of applied microbial biomass was estimated from the difference in extractable Norg between the two treatments. In addition, the N isotopic composition in the upper plant matter, in the soil, and in organic and inorganic N fractions of EUF and CaCl₂ extracts was analysed. Both experiments showed that the applied microbial biomass was highly accessible to mineralization and thus represented potentially mineralizable N. However, this mineralizable N was not extractable by CaCl₂ or by the EUF method. It was, therefore, concluded that the organic N released on soil drying and which was thus extractable was derived from the non-biomass soil organic matter. The result suggests that both extraction methods may provide a suitable index for mineralizable N only in cases where the decomposable organic substrates are derived mainly from sources other than the living soil biota.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

Organic matter turnover in soil physical fractions following woody plant invasion of grassland: Evidence from natural C and N

Soil physical structure causes differential accessibility of soil organic carbon (SOC) to decomposer organisms and is an important determinant of SOC storage and turnover. Techniques for physical fractionation of soil organic matter in conjunction with isotopic analyses (δ¹³C, δ¹⁵N) of those soil fractions have been used previously to (a) determine where organic C is stored relative to aggregate structure, (b) identify sources of SOC, (c) quantify turnover rates of SOC in specific soil fractions, and (d) evaluate organic matter quality. We used these two complementary approaches to characterize soil C storage and dynamics in the Rio Grande Plains of southern Texas where C₃ trees/shrubs (δ¹³C=−27‰) have largely replaced C₄ grasslands (δ¹³C=−14‰) over the past 100-200 years. Using a chronosequence approach, soils were collected from remnant grasslands (Time 0) and from woody plant stands ranging in age from 10 to 130 years. We separated soil organic matter into specific size/density fractions and determined their C and N concentrations and natural δ¹³C and δ¹⁵N values. Mean residence times (MRTs) of soil fractions were calculated based on changes in their δ¹³C with time after woody encroachment. The shortest MRTs (average=30 years) were associated with all particulate organic matter (POM) fractions not protected within aggregates. Fine POM (53-250 μm) within macro- and microaggregates was relatively more protected from decay, with an average MRT of 60 years. All silt+clay fractions had the longest MRTs (average=360 years) regardless of whether they were found inside or outside of aggregate structure. δ¹⁵N values of soil physical fractions were positively correlated with MRTs of the same fractions, suggesting that higher δ¹⁵N values reflect an increased degree of humification. Increased soil C and N pools in wooded areas were due to both the retention of older C₄-derived organic matter by protection within microaggregates and association with silt+clay, and the accumulation of new C₃-derived organic matter in macroaggregates and POM fractions.     

C and N natural abundance of the soil microbial biomass

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

Elevated CO2, but not defoliation, enhances N cycling and increases short-term soil N immobilization regardless of N addition in a semiarid grassland

Elevated CO₂ and defoliation effects on nitrogen (N) cycling in rangeland soils remain poorly understood. Here we tested whether effects of elevated CO₂ (720 μl L⁻¹) and defoliation (clipping to 2.5 cm height) on N cycling depended on soil N availability (addition of 1 vs. 11 g N m⁻²) in intact mesocosms extracted from a semiarid grassland. Mesocosms were kept inside growth chambers for one growing season, and the experiment was repeated the next year. We added ¹⁵N (1 g m⁻²) to all mesocosms at the start of the growing season. We measured total N and ¹⁵N in plant, soil inorganic, microbial and soil organic pools at different times of the growing season. We combined the plant, soil inorganic, and microbial N pools into one pool (PIM-N pool) to separate biotic + inorganic from abiotic N residing in soil organic matter (SOM). With the ¹⁵N measurements we were then able to calculate transfer rates of N from the active PIM-N pool into SOM (soil N immobilization) and vice versa (soil N mobilization) throughout the growing season. We observed significant interactive effects of elevated CO₂ with N addition and defoliation with N addition on soil N mobilization and immobilization. However, no interactive effects were observed for net transfer rates. Net N transfer from the PIM-N pool into SOM increased under elevated CO₂, but was unaffected by defoliation. Elevated CO₂ and defoliation effects on the net transfer of N into SOM may not depend on soil N availability in semiarid grasslands, but may depend on the balance of root litter production affecting soil N immobilization and root exudation affecting soil N mobilization. We observed no interactive effects of elevated CO₂ with defoliation. We conclude that elevated CO₂, but not defoliation, may limit plant productivity in the long-term through increased soil N immobilization.     

Variability of soil physical quality and erodibility in a water-eroded cropland

Physical degradation of the soil increases its susceptibility to erosion by water action. However, relatively few studies have evaluated the opposite, i.e., the impact of water erosion on soil erodibility. This study was conducted in a corn field in Ohio. Some sites within the field have experienced water-induced soil erosion following heavy rainstorms. Physical characteristics of the soil were compared between eroded (ER) and un-eroded sites (UN). Compared with ER, the soil in UN had lower penetration resistance (4.87 vs. 4.53 MPa), bulk density (1.45 vs. 1.33 Mg m^?3), and sand content (17.4 vs. 14.2%), and higher shear strength (80.1 vs. 125.3 KPa), hydraulic conductivity (3.0 vs. 3.4 cm h^?1), intrinsic permeability (31.9 vs. 36.4 × 10^?10 cm²), and contents of soil organic carbon (36.1 vs. 32.1 g kg^?1), total nitrogen (3.3 vs. 3.1 g kg^?1), clay (25.2 vs. 24.2%), silt (60.5 vs. 58.4%), and very fine sand (3.4 vs. 1.1%). Also Munsell's variables differed between ER and UN (1.24 vs. 0.54 for hue, 4.59 vs. 4.35 for value, and 1.99 vs. 1.79 for chroma, respectively). The erodibility factor (K) was lower in UN than in ER (0.00327 vs. 0.00354 Mg ha h ha^?1 MJ^?1 mm^?1, respectively). Hence, it is suggested the ER sites within the corn field agroecosystem are more susceptible to accelerated erosion as compared with UN sites.     

Effects of tillage, organic resources and nitrogen fertiliser on soil carbon dynamics and crop nitrogen uptake in semi-arid West Africa

Tillage, organic resources and fertiliser effects on soil carbon (C) dynamics were investigated in 2000 and 2001 in Burkina Faso (West Africa). A split plot design with four replications was laid-out on a loamy-sand Ferric Lixisol with till and no-till as main treatments and fertiliser types as sub-treatments. Soil was fractionated physically into coarse (0.250–2 mm), medium (0.053–0.250 mm) and fine fractions (< 0.053 mm). Particulate organic carbon (POC) accounted for 47–53% of total soil organic carbon (SOC) concentration and particulate organic nitrogen (PON) for 30–37% of total soil nitrogen concentration. The POC decreased from 53% of total SOC in 2000 to 47% of total SOC in 2001. Tillage increased the contribution of POC to SOC. No-till led to the lowest loss in SOC in the fine fraction compared to tilled plots. Well-decomposed compost and single urea application in tilled as well as in no-till plots induced loss in POC. Crop N uptake was enhanced in tilled plots and may be up to 226 kg N ha⁻¹ against a maximum of 146 kg N ha⁻¹ in no-till plots. Combining crop residues and urea enhanced incorporation of new organic matter in the coarse fraction and the reduction of soil carbon mineralisation from the fine fraction. The PON and crop N uptake are strongly correlated in both till and no-till plots. Mineral-associated N is more correlated to N uptake by crop in tilled than in no-till plots. Combining recalcitrant organic resources and nitrogen fertiliser is the best option for sustaining crop production and reducing soil carbon decline in the more stabilised soil fraction in the semi-arid West Africa.     

Effects of annual grass senescence on NH4 and N-glycine uptake by blue oak (Quercus douglasii) seedlings and soil microorganisms in California oak woodland

Annual grasses are stronger competitors for available soil N than blue oak seedlings and soil microorganisms. However, little is known about the dynamics of N competition during annual grass senescence. We conducted a field experiment in a California oak woodland to study effects of annual grass senescence on N uptake by grasses, blue oak seedlings and soil microorganisms. Labeled N was applied at the beginning of April, May and of June in the form of ¹⁵NH₄⁺ or ¹⁵N-glycine. Plants and soils were harvested after 5 days (¹⁵NH₄⁺ and ¹⁵N-glycine treatments) and after 26 days (¹⁵NH₄⁺ treatment only). We evaluated effects of N form, season and labeling period on N competition among oak seedlings, annual grasses and soil microorganisms. N forms did not affect competition among grasses, oak seedlings and soil microorganisms, but more ¹⁵N was incorporated into the soil organic N pool in the ¹⁵N-glycine treatments than in the ¹⁵NH₄⁺ treatments. There were no seasonal (May vs June) effects on ¹⁵N recovery in blue oak seedlings and soil microorganisms. Plant samples from April harvest were lost. In June, when grasses were senescing, more ¹⁵N was found in the soil inorganic pool than in May. Extremely dry soils in June may have limited inorganic N availability to oak seedlings and soil microorganisms. After 26-day labeling period, ¹⁵N recovery in blue oak seedlings and the soil organic N pool significantly increased, while ¹⁵N recovery in both the soil microbial and inorganic N pools decreased compared to the 5-day labeling period. Although blue oak seedling biomass changed little from early May to late June, N concentrations in oak roots increased 53%. In contrast, annual grass biomass peaked in May, and then decreased rapidly. Our results suggest that blue oak seedlings and annual grasses have different temporal competitive abilities. Blue oak seedlings appear to have a long-term strategy for N competition. Blue oaks take up N slowly but steadily, increasing N uptake from 5 to 26 days. This extended time period has a greater positive effect on N uptake than does reduced grass uptake caused by senescence.     

Seasonal Changes of Shoot Nitrogen Concentrations and 15N/14N Ratios in Common Reed in a Constructed Wetland

Abstract

Nitrogen (N) concentrations and stable N isotope abundances (δ¹⁵N) of common reed (Phragmites australis) planted in a constructed wetland were measured periodically between July 2001 and May 2002 to examine their seasonal variations in relation to N uptake and N translocation within common reed. Nitrogen concentrations in P. australis shoots were higher in the growing stage (7.5 to 24.8 g N kg^?1) than in the senescence stage (4.2 to 6.8 g N kg^?1), indicating N translocation from shoots to rhizomes. Meanwhile, the corresponding δ¹⁵N values were higher in the senescence stage (+12.2 to +22.4‰) than in the growing stage (+5.1 to +11.3‰). Coupled with the negative correlation (R²=0.24, P<0.05, n=18) between N concentrations and δ¹⁵N values of shoots in the senescence stage, our results suggested that shoot N became enriched in ¹⁵N due to N isotopic fractionation (with an isotopic fractionation factor, α_s/p, of 1.012) during N translocation to rhizomes. However, the positive correlation between N concentrations and δ¹⁵N values in the growing stage (R²=0.19, P<0.001, n=54) suggested that P. australis relies on N re‐translocated from rhizome in the early growing stage and on mineral N in the sediment during the active growing stage. Therefore, seasonal δ¹⁵N variations provide N‐isotopic evidence of N translocation within and N uptake from external N sources by common reed.     

Influence of soil tillage systems on aggregate stability and the distribution of C and N in different aggregate fractions

Soil aggregation is influenced by the tillage system used, which in turn affects the amount of C and N in the different aggregate fractions. This study assessed the impact of different tillage systems on soil aggregates by measuring the aggregate stability, the organic carbon (C_org) and the total nitrogen (N_tot) contents within different aggregate fractions, and their release of dissolved organic carbon (DOC). Soil samples were collected from the top 0 to 10 cm of a long-term tillage experiment at Fuchsenbigl (Marchfeld, Austria) where conventional tillage (CT), reduced tillage (RT), and minimum tillage (MT) treatments were applied to a Chernozem fine sandy loam. The stable aggregates (1000–2000 μm) were subject to dispersion by the soil aggregate stability (SAS or wet sieving) method after Kemper and Rosenau (1986), and the ultrasonic method of Mayer et al. (2002). Chemical analysis of the soil was obtained for the aggregate fractions 630–1000, 250–630 and 63–250 μm gathered from the ultrasonic method. Using the SAS method, CT and RT had the least amounts of stable aggregates (18.2% and 18.9%, respectively), whereas MT had twice as much stable aggregates (37.6%). Using the ultrasonic method, MT also had the highest amount of water stable aggregates in all three fractions (1.5%, 3.7%, and 35%, respectively), followed by RT (1%, 2.3%, 32.3%), and CT (0.8%, 1.7%, 29.1%). For comparison, a reference soil, EUROSOIL 7 (ES-7) was also analysed (40%, 6.7%, and 12.1%). The highest amounts of C_org and N_tot were measured under MT in all three fractions, with 8.9%, 3.8%, and 1.3% for C_org, and 0.4%, 0.3%, and 0.1% for N_tot. Apart from the fraction 630–1000 μm, the aggregates of RT and CT contained <50% of the C_org and N_tot values of MT. The C/N ratio was least favourable for CT (42.6) in the aggregate fraction 630–1000 μm. The DOC release from stable aggregates after 10 min of ultrasonic dispersion was highest from MT soil (86.7 mg l⁻¹). The values for RT and CT were 21% and 25% below this value. The results demonstrate that tillage type influences both aggregate stability and aggregate chemical composition. This research confirms that CT interferes more with the natural soil properties than RT and MT. Furthermore, MT has the highest potential to sequester C and N in this agriculturally used soil.     

Changes in forest soil organic matter pools after a decade of elevated CO2 and O3

The impact of rising atmospheric carbon dioxide (CO₂) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O₃) or vary based on species-specific growth responses to elevated CO₂. To understand how projected increases in atmospheric CO₂ and O₃ alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO₂-O₃ Enrichment (FACE) experiment. Tracer amounts of ¹⁵NH₄⁺ were applied to the forest floor of Populus tremuloides, P. tremuloides-Betula papyrifera and P. tremuloides-Acer saccharum communities exposed to factorial CO₂ and O₃ treatments. The ¹⁵N tracer and strongly depleted ¹³C-CO₂ were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO₂ relative to ambient CO₂. As main effects, neither CO₂ nor O₃ significantly altered ¹⁵N recovery in SOM. Elevated CO₂ significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO₂ has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O₃ had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO₂ can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response.     

Comparison of NH4 pool dilution techniques to measure gross N fluxes in a coarse textured soil

We compared gross N fluxes by ¹⁵N pool dilution in a coarse-textured agricultural soil when ¹⁵N was applied to the soil NH₄⁺ pool by either: (i) mixing a ¹⁵NH₄NO₃ solution into disturbed soil or (ii) injection of ¹⁵NH₃ gas into intact soil cores. The two techniques produced similar results for gross N mineralization rates indicating that NH₄⁺ production in soil was not altered by soil disturbance, method of application (gas vs. solution), or amount of N applied. This was not the case for immobilization rates, which were twofold higher when ¹⁵N label was applied to the soil NH₄⁺ pool with the mixing technique compared to the injection technique. This was attributed to the fact that more NH₄⁺ was applied with the mixing technique. Estimates of gross nitrification were accompanied by large error terms meaning differences between ¹⁵N labeling methods could not be accurately assessed for this process rate.     

A call to investigate drivers of soil organic matter retention vs. mineralization in a high CO2 world

Understanding how elevated atmospheric CO₂ alters the formation and decomposition of soil organic carbon (SOC) is important but challenging. If elevated CO₂ induces even small changes in rates of formation or decay of SOC, there could be substantial feedbacks on the atmosphere's concentration of CO₂. However, the long turnover times of many SOC pools - decades to centuries - make the detection of changes in the soil's pool size difficult. Long-term CO₂ enrichment experiments have offered unprecedented opportunities to explore these issues in intact ecosystems for more than a decade. Increased NPP with elevated CO₂ has prompted the hypothesis that SOC may increase at the same time that increased vegetation nitrogen (N) uptake and accumulation indicates probable declines in SON. Varying investigators thus have hypothesized that SOC will increase and SON will decline to explain increased NPP with elevated CO₂; researchers also invoke biogeochemical theory and stoichiometric constraints to argue for strong limitations on the co-occurrence of these phenomena. We call for researchers to investigate two broad research questions to elucidate the drivers of these processes. First, we ask how elevated CO₂ influences compound structure and stoichiometry of that proportion of NPP retained by soil profiles for relatively long time periods. We also call for investigations of the mechanisms underlying the decomposition of mineralizable organic matter with elevated CO₂. Specifically, we need to understand how elevated CO₂ influences microbial priming (driven by enhanced microbial energy needs associated with increases in biomass or activity) and microbial mining of N (driven by enhanced microbial N demand associated with greater vegetative N uptake), two processes that necessarily will be constrained by the stoichiometry of both substrates and microbial demands. Applying technologies such as nuclear magnetic resonance and the detection of biomarkers that reveal organic matter structure and origins, and studying microbial stoichiometric constraints, will dramatically improve our ability to predict future patterns of ecosystem C and N cycling.     

Turnover of organic matter in a Miscanthus field: effect of time in Miscanthus cultivation and inorganic nitrogen supply

To accurately predict the potential environmental benefits of energy crops, the sequestration of carbon in soil needs to be quantified. The aim of this study was to investigate the mineralisation rate of the perennial C₄ grass Miscanthus giganteus and Miscanthus-derived soil organic matter under contrasting nitrogen supply. Soils were collected from sites where Miscanthus had been grown for 11 and 18 years, respectively, and where a C₃-grass (Lolium spp.) had been grown for 7 years. The soils were incubated for 4 months at two levels of soil inorganic nitrogen with or without dead root material of Miscanthus.Addition of root material (residues) increased carbon mineralisation of indigenous organic matter when no nitrogen was added. Added inorganic nitrogen decreased carbon mineralisation in all soils. Nitrogen addition did not affect carbon mineralisation of the residues. Using the ¹³C fraction to calculate the proportion of respiratory CO₂ derived from Miscanthus showed that nitrogen addition decreased carbon mineralisation in soils, but it did not affect carbon mineralisation of the residues. Nitrogen mineralisation was highest in the C₃ grass soil without added residues. Nitrification decreased pH, especially in the treatments where nitrogen was added. The Miscanthus-derived organic matter is at least as stable as C₃ grassland-derived organic matter. Furthermore, the turnover time of the organic matter increases with time under Miscanthus cultivation.The CENTURY soil organic matter sub-model was used to simulate the organic matter decomposition in the experiment. Carbon mineralisation was accurately simulated but there were unexplained discrepancies in the simulation of the δ¹³C in the respiration from the treatment with residues. The δ¹³C in respiration did not decrease with time as predicted, indicating that lignin accumulation did not influence the measurements.