Mineralization of sulfur from organic residues assessed by inverse isotope dilution

Sulfur (S) deficiency in soils is increasingly recognized in agricultural systems. The quantification of S mineralization/immobilization processes after incorporation of organic materials into soils is a key factor to predict the availability of S to growing plants. However, immobilization and mineralization occur simultaneously making the quantification of the magnitude of each process difficult. We used the inverse isotope (³⁵SO₄) dilution technique to quantify immobilization and mineralization fluxes after incorporation of two organic residues with contrasting C/S ratio's (cabbage or wheat straw) into a sandy soil in planted and unplanted soils (pot trial with ryegrass and incubation). The soil was labeled with ³⁵SO₄ and incubated for 63 days prior to the application of residues. The specific activity (SA) of soil-extractable SO₄ did not change significantly in the control soil during the subsequent experimental period despite significant net mineralization, illustrating that labile-S in soil was homogenously labeled. Application of residues decreased the SAs during the incubation due to the dilution with unlabeled-S from the residues. A three-compartment dynamic model was fitted to the SA data predicting that gross mineralization of residue-S was almost complete over 43 days incubation although this release was not matched by the increase in soil SO₄ due to immobilization reactions. Soil-extractable SO₄ was significantly increased in the cabbage-treated soil while the reverse was true in the wheat straw amended soil in which the S-immobilization was almost twice the gross mineralization of residue-S. The SA of S in ryegrass were maximally 15% lower than in corresponding soil extracts suggesting that residue mineralization was similar in planted and unplanted soils. The inverse isotope dilution method offers potential for screening S release of different residues; however the details of the dynamics of soil-S isotopes show that the individual fluxes are not constant during the incubation.     

Mineralization of organic sulfur and its importance as a reservoir of plant-available sulfur in upland soils of north China

 An open incubation technique was used to measure S mineralization in a range of upland soils of north China. Six mineralization patterns were examined, and a soil S-exhaustion experiment with ryegrass (Lolium multiflorum L.) was conducted to investigate the availability of various organic S pools to plants. For all of the 12 soils tested, the release of S as SO₄ ^2– was curvilinear with time, and during a 28-week incubation at 30  °C the amount of S mineralized ranged from 14.0 mg S kg^–1 soil to 37.4 mg S kg^–1 soil. A first-order model and Gompertz model appeared to best describe S mineralization. Examination of the soils after incubation revealed the bulk of the mineralized S was mainly derived from the C-bonded S pool, while the majority of mineralized S under soil S exhaustion by ryegrass was derived from the HI-reducible S pool. Received: 9 July 1998     

Increased degradation of phenanthrene in soil by Pseudomonas sp. GF3 in the presence of wheat

A phenanthrene-degrading bacterial strain Pseudomonas sp. GF3 was examined for plant-growth promoting effects and phenanthrene removal in soil artificially contaminated with low and high levels of phenanthrene (0, 100 and 200 mg kg⁻¹) in pot experiments. Low and high phenanthrene treatments significantly decreased the growth of wheat. Inoculation with bacterial strain Pseudomonas sp. GF3 was found to increase root and shoot growth of wheat. Strain GF3 was able to degrade phenanthrene effectively in the unplanted and planted soils. Over a period of 80 days the concentration of phenanthrene in soil in which wheat was grown was significantly lower than in unplanted soil (p<0.05). At the end of the 80-d experiments, 62.2% and 42.3% of phenanthrene had disappeared from planted soils without Pseudomonas sp. GF3 when the phenanthrene was added at 100 and 200 mg kg⁻¹ soil, respectively, but 84.8% and 70.2% of phenanthrene had disappeared from planted soils with the bacterial inoculation. The presence of vegetation significantly enhances the dissipation of phenanthrene in the soil. There was no significant difference in soil polyphenol oxidase activities among the applications of 0, 100 and 200 mg kg⁻¹ of phenanthrene. However, the enzyme activities in planted and unplanted soils inoculated with the strain Pseudomonas sp. GF3 were significantly higher than those of non-inoculation controls. The bacterial isolate was also able to colonize and develop in the rhizosphere soil of wheat after inoculation.     

Role of soil drying in nitrogen mineralization and microbial community function in semi-arid grasslands of north-west Australia

We examined effects of wetting and then progressive drying on nitrogen (N) mineralization rates and microbial community composition, biomass and activity of soils from spinifex (Triodia R. Br.) grasslands of the semi-arid Pilbara region of northern Australia. We compared soils under and between spinifex hummocks and also examined impacts of fire history on soils over a 28 d laboratory incubation. Soil water potentials were initially adjusted to −100 kPa and monitored as soils dried. We estimated N mineralization by measuring changes in amounts of nitrate (NO₃⁻-N) and ammonium (NH₄⁺-N) over time and with change in soil water potential. Microbial activity was assessed by amounts of CO₂ respired. Phospholipid fatty acid (PLFA) analyses were used to characterize shifts in microbial community composition during soil drying. Net N mineralized under hummocks was twice that of open spaces between hummocks and mineralization rates followed first-order kinetics. An initial N mineralization flush following re-wetting accounted for more than 90% of the total amount of N mineralized during the incubation. Initial microbial biomass under hummocks was twice that of open areas between hummocks, but after 28 d microbial biomass was<2 μ g⁻¹ ninhydrin N regardless of position. Respiration of CO₂ from soils under hummocks was more than double that of soils from between hummocks. N mineralization, microbial biomass and microbial activity were negligible once soils had dried to −1000 kPa. Microbial community composition was also significantly different between 0 and 28 d of the incubation but was not influenced by burning treatment or position. Regression analysis showed that soil water potential, microbial biomass N, NO₃⁻-N, % C and δ¹⁵N all explained significant proportions of the variance in microbial community composition when modelled individually. However, sequential multiple regression analysis determined only microbial biomass was significant in explaining variance of microbial community compositions. Nitrogen mineralization rates and microbial biomass did not differ between burned and unburned sites suggesting that any effects of fire are mostly short-lived. We conclude that the highly labile nature of much of soil organic N in these semi-arid grasslands provides a ready substrate for N mineralization. However, process rates are likely to be primarily limited by the amount of substrate available as well as water availability and less so by substrate quality or microbial community composition.     

Electron donors and acceptors influence anaerobic soil organic matter mineralization in tidal marshes

Anaerobic decomposition in wetland soils is carried out by several interacting microbial processes that influence carbon storage and greenhouse gas emissions. To understand the role of wetlands in the global carbon cycle, it is critical to understand how differences in both electron donor (i.e., organic carbon) and terminal electron acceptor (TEA) availability influence anaerobic mineralization of soil organic matter. In this study we manipulated electron donors and acceptors to examine how these factors influence total rates of carbon mineralization and the pathways of microbial respiration (e.g., sulfate reduction versus methanogenesis). Using a field-based reciprocal transplant of soils from brackish and freshwater tidal marshes, in conjunction with laboratory amendments of TEAs, we examined how rates of organic carbon mineralization changed when soils with different carbon contents were exposed to different TEAs. Total mineralization (the sum of CO₂ + CH₄ produced) on a per gram soil basis was greater in the brackish marsh soils, which had higher soil organic matter content; however, on a per gram carbon basis, mineralization was greater in the freshwater soils, suggesting that the quality of carbon inputs from the freshwater plants was higher. Overall anaerobic metabolism was higher for both soil types incubated at the brackish site where SO₄²⁻ was the dominant TEA. When soils were amended with TEAs in the laboratory, more thermodynamically favorable respiration pathways typically resulted in greater organic matter mineralization (Fe(III) respiration > SO₄²⁻ reduction > methanogenesis). These results suggest that both electron donors and acceptors play important roles in regulating anaerobic microbial mineralization of soil organic matter.     

Carbon, nitrogen and temperature controls on microbial activity in soils from an Antarctic dry valley

The Antarctic dry valleys are characterized by extremely low temperatures, dry conditions and lack of conspicuous terrestrial autotrophs, but the soils contain organic C, emit CO₂ and support communities of heterotrophic soil organisms. We have examined the role of modern lacustrine detritus as a driver of soil respiration in the Garwood Valley, Antarctica, by characterizing the composition and mineralization of both lacustrine detritus and soil organic matter, and relating these properties to soil respiration and the abiotic controls on soil respiration. Laboratory mineralization of organic C in soils from different, geomorphically defined, landscape elements at 10 °C was comparable with decomposition of lacustrine detritus (mean residence times between 115 and 345 d for the detritus and 410 and 1670 d for soil organic matter). The chemical composition of the detritus (C-to-N ratio=9:1-12:1 and low alkyl-C-to-O-alkyl-C ratio in solid-state ¹³C nuclear magnetic resonance spectroscopy) indicated that it was a labile, high quality resource for micro-organisms. Initial (0-6 d at 10 °C) respiratory responses to glucose, glycine and NH₄Cl addition were positive in all the soils tested, indicating both C and N limitations on soil respiration. However, over the longer term (up to 48 d at 10 °C) differential responses occurred. Glucose addition led to net C mineralization in most of the soils. In the lake shore soils, which contained accumulated lacustrine organic matter, glucose led to substantial priming of the decomposition of the indigenous organic matter, indicating a C or energetic limitation to mineralization in that soil. By contrast, over 48 d, glycine addition led to no net C mineralization in all soils except stream edge and lake shore soils, indicating either substantial assimilation of the added C (and N), or no detectable utilization of the glycine. The Q₁₀ values for basal respiration over the −0.5-20 °C temperature range were between 1.4 and 3.3 for the different soils, increasing to between 3.4 and 6.9 for glucose-induced respiration, and showed a temperature dependence with Q₁₀ increasing with declining temperature. Taken together, our results strongly support contemporaneous lacustrine detritus, blown from the lake shore, as an important driver of soil respiration in the Antarctic dry valley soils.     

Changes in mineral nitrogen, phosphorus availability and salt-extractable aluminium following the application of green manure residues in two weathered soils of South Vietnam

Phosphorus deficiency and aluminium toxicity in weathered soils can be amended by applying organic residues. Nitrogen mineralization, changes in P-availability and changes in salt-extractable Al following the incorporation of residues of various green manures (Flemingia congesta, Mucuna pruriens, Pueraria phaseoloides, Tithonia diversifolia) were quantified in a field core incubation experiment. Dried residues were added at an application rate of 45 kg P ha⁻¹ to two soils representative for the main soil groups of the South Vietnamese uplands, set up in incubation cores in an experimental field near Ho Chi Minh City, Vietnam.Decomposition of the residues proceeded at high rates. Mineralized nitrogen from the residues was recovered mainly as ammonium during the first 2 weeks of incubation. Nitrogen release from Tithonia residues with the highest lignin content and lignin:N ratio occurred more gradually compared to the three legumes. Resin-extractable P was significantly increased by residue treatments. Largest and sustained increases in resin-extractable P (0.67 and 2.06 mg P kg⁻¹ in the two soils) were observed in samples amended with Tithonia, which was related to the large P-content (0.37%) and small C:P ratio (110) of the residues. The P-concentration in the residues, rather than the total amount of P applied through the residues, affected the increase in P-availability. The increase in resin-extractable P was correlated to the P-content (R=0.64) and C:P ratio (R=−0.65) of the residues. Salt-extractable Al-concentrations were considerably reduced by the organic amendments, up to 70 and 50% in the two soils. At the rate of 45 kg P ha⁻¹, no significant differences between the residue treatments to reduce soil acidity were observed.As such, the application of high quality residues that are rich in P, in particular T. diversifolia, may enhance crop production by creating favourable soil conditions during the initial stages of plant development of the main crop.     

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

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

Soluble organic nitrogen in temperate forest soils

Very few studies have been related to soluble organic nitrogen (SON) in forest soils. However, this nitrogen pool could be a sensitive indicator to evaluate the soil nitrogen status. The current study was conducted in temperate forests of Thuringia, Germany, where soils had SON (extracted in 0.5 M K₂SO₄) varying from 0.3 to 2.2% of total N, which was about one-third of the soil microbial biomass N by CFE. SON in study soils were positively correlated to microbial biomass N and soil total N. Multiple regression analysis also showed that mineral N negatively affected SON pool. The dynamics of the SON was significantly affected by mineralization and immobilization. During the 2 months of aerobic incubation, the SON were significantly correlated with net N mineralization and microbial biomass N. SON extracted by two different salt solution (i.e. 1 M KCl and 0.5 M K₂SO₄₎ were highly correlated. In mineral soil, SON concentrations extracted by 1 M KCl and 0.5 M K₂SO₄ solutions were similar. In contrast, in organic soil layer the amount of KCl-extractable SON was about 1.2-1.4 times higher than the K₂SO₄-extractable SON. Further studies such as the differences of organic N form and pool size between SON and dissolved organic N (DON) are recommended.     

Quantification and bioavailability of scyllo-inositol hexakisphosphate in pasture soils

The recent identification of scyllo-inositol hexakisphosphate in alkaline soil extracts by solution ³¹P NMR spectroscopy allowed us to investigate this compound in soils by re-analyzing spectra from two previously published studies. Concentrations of scyllo-inositol hexakisphosphate in 29 temperate pasture soils from England and Wales ranged between 11 and 130 mg P kg⁻¹ soil and accounted for between 4 and 15% of the soil organic phosphorus. The ratio of scyllo-inositol hexakisphosphate to myo-inositol hexakisphosphate ranged between 0.29 and 0.79. In a 10 month pot experiment with six grassland soils from New Zealand, growth of pine seedlings (Pinus radiata D. Don) decreased scyllo-inositol hexakisphosphate concentrations by between 10 and 46%. Growth of ryegrass (Lolium perenne L.) decreased scyllo-inositol hexakisphosphate in three low-nutrient soils by 5-21%, but increased it in three other soils by 11-16%. We conclude that scyllo-inositol hexakisphosphate is an important component of soil organic phosphorus with potential ecological significance.     

Fate of gram-negative bacterial biomass in soil—mineralization and contribution to SOM

Soil microorganisms contribute to the formation of non-living soil organic matter (SOM) by metabolic transformation of plant-derived material. After cell death, their biomass components with a specific molecular character become incorporated into SOM imprinting its chemical properties, although this process has not yet been quantified. In order to elucidate the contribution to SOM formation, we investigated the fate of gram-negative bacterial model biomass (Escherichia coli usually introduced into soil with manure or feces) during incubation of soil with isotopically (¹³C) and genetically (lux gene) labeled cells. The decline of living cells was monitored by the loss of bioluminescence. The carbon turnover and mineralization was balanced by bulk soil stable isotope analysis, and the persistence of nucleic acids was investigated by PCR amplification of the lux gene. During incubation, the number of viable E. coli cells decreased rapidly (99.9% within the first 42 d) serving as substrate for other microorganisms or for the formation of SOM, and bioluminescent cells could only be detected during the first 56 d. However, the lux gene was still detected after 224 d, which indicates stabilization of DNA in SOM. Although the survival of E. coli in soil is limited, only about 65% of the added labeled biomass carbon was mineralized to ¹³CO₂ and 51% remained in soil after 224 d with an average ¹³C recovery of 117%. The amount of ¹³C found in the PLFA representative of living cells had decreased to 25% of the initial value, suggesting a proportional decrease of the ¹³C in the soil microbial biomass. The extent of this decrease is higher than the mineralization of the bulk E. coli C and thus the difference of around 25% has to be stabilized as metabolites, or in non-living SOM. The data provide evidence that the genetic information and a considerable part of the carbon from dying bacterial biomass were retained in both the soil microbial food web and in non-living SOM.     

Soil organic N dynamics and stand quality in Pinus radiata pinewoods of the temperate humid region

Soil organic-N dynamics, its controlling factors and its relationships with stand quality were studied in the 0-15 cm soil layer of 24 pinewoods with contrasting age, productivity and parent material (granite; acid schists), searching for N variables useful to predict stand growth and site quality. No significant differences were found between young and old stands for any of the N variables considered, nor two- or three-order interactions among stand age, site quality and parent material. The soil total-N content, which was correlated positively with the Al oxides content (a soil organic matter (SOM) stabilizing agent), did not vary significantly according to parent material, but it was lower (P≤0.02) in stands with high than with low site index (2.68±1.11 and 3.97±1.13 g N kg⁻¹ soil, respectively). The soil δ¹⁵N ranged from +3.5 to +6.5 δ, without significant differences among stand groups, and it was negatively correlated with water holding capacity, exchangeable bases, Al oxides and N content, suggesting that: (i) N losses by NO₃⁻ leaching are the most important controlling factor of δ¹⁵N in these temperate humid region soils; and (ii) soil N richness is related with limited N losses, which discriminate against ¹⁵N. At any incubation time, no significant differences were found in soil inorganic-N content among stand groups (7.78±4.57, 39.33±16.20 and 67.80±26.50 mg N kg⁻¹ soil at 0, 42 and 84 d, respectively). During the incubation, the relative importance of ammonification decreased and that of the nitrification increased. The net N mineralization rate (NNMR, in percentage of organic N) was significantly higher in granite than in schists soils at both 42 d (1.24±0.34 and 0.75±0.37%, respectively) and 84 d (2.18±0.56 and 1.53±0.66%, respectively). In high quality pinewoods, the NNMR at 42 and 84 d (1.16±0.45 and 2.12±0.79%, respectively) were significantly higher than in low quality stands (0.83±0.35 and 1.59±0.45%, respectively). This result, together with those on soil total-N and inorganic-N supply, suggests that soil N dynamics in low and high quality stands are different: in the former there is a bigger N pool with a slower turnover, whereas in the latter there is a smaller N pool with a faster turnover, both factors being nearly compensated, making the soil available N supply in both types of stand similar. After 42 and 84 d of incubation, the NNMR and the nitrification rates were higher in the coarse textured soils, likely due to the low physical and chemical protection of their SOM; both rates were positively correlated with available P, exchangeable K⁺ and CEC base saturation, suggesting strong relationships among the availabilities of the main plant nutrients, and they increased with SOM quality (low C-to-N ratio). The strong negative correlation of site index with soil total-N (r=−0.707; P≤0.005), and its positive correlations with NNMR after 42 and 84 d of incubation, suggested that site quality and potential productivity are closely related to soil organic-N dynamics. Half of the site index variation in the stands studied could be predicted with a cheap and easy analysis of soil N content, the prediction being slightly improved if soil δ¹⁵N is included and, more significantly, by including N mineralization measurements.     

Biodegradation of estrone and 17 β-estradiol in grassland soils amended with animal wastes

The release of endocrine disrupting chemicals into the environment is of increasing concern due to the formation of an intersex state in freshwater organisms and potential risks to human health. The aim of this study was to investigate the persistence of the naturally occurring hormones, estrone and 17 β-estradiol in three agricultural grassland soils in the presence and absence of cattle and sheep wastes (urine and manure). Biodegradation was investigated using ¹⁴C-labelled hormones which were applied to soil in three different solvents (water, artificial urine and natural sheep urine). When applied directly to soil the two hormones degraded at a similar rate, however, the speed of mineralization was soil type and solvent dependant. The half-life (t_1/2) of the hormones in soils ranged from 5 to 25 d. The hormones were also applied to the soils in sheep and cattle manure of different ages (7 d to 2 yr). Generally, the rate of degradation in the animal manure amended soils was more rapid than in the unamended soils (t_1/2=1-9 d), with mineralization being largely independent of manure age and type. We conclude that in comparison to many xenobiotics, estrogens are not persistent in agricultural soils. However, our calculations suggest that if they are lost to freshwater via runoff or leaching then they may have an appreciable effect on freshwater organisms. Assuming normal landspreading rates our results suggest that the risk of estrogen contamination of freshwater associated with manure spreading is very low.     

Increase in soil pH due to Ca-rich organic matter application causes suppression of the clubroot disease of crucifers

Clubroot disease of cruciferous plants caused by the soil-borne pathogen Plasmodiophora brassicae is difficult to control because the pathogen survives for a long time in soil as resting spores. Disease-suppressive and conducive soils were found during the long-term experiment on the impact of organic matter application to arable fields and have been studied to clarify the biotic and abiotic factors involved in the disease suppression. The fact that a large amount of organic matter, 400 t ha⁻¹ yr⁻¹ farmyard manure (FYM) or 100 t ha⁻¹ yr⁻¹ food factory sludge compost (FSC), had been incorporated for more than 15 yr in the suppressive soils and these soils showed higher pH and Ca concentration than the disease conducive soil led us to hypothesize that an increase in soil pH due to the long-term incorporation of Ca-rich organic matter might be the primary cause of the disease suppression. We have designed a highly reproducible bioassay system to examine this hypothesis. The suppressive and conducive soils were mixed with the resting spores of P. brassicae at a rate of 10⁶ spore g⁻¹ soil, and Brassica campestris was grown in a growth chamber for 8 d. The number of root hair infections was assessed on a microscope. It was found that the incorporation of FYM and FSC at 2.5% (w/w) to the conducive soil suppressed the infection and that the finer particles (?5 mm) of FSC inhibited the infection and increased soil pH more effectively. Neutralization of the conducive soil by Ca(OH)₂, CaCO₃ and KOH suppressed the infection, but the effectiveness of KOH was less than those of Ca(OH)₂ and CaCO₃. Acidification of the suppressive soils by H₂SO₄, promoted the infection. The involvement of soil biota in the disease suppression was investigated using the sterilized (γ-ray irradiation) suppressive soils with respect to soil pH. The γ-ray irradiation promoted the infection at pH 5.5, but no infection was observed at pH 7.4 irrespective of the sterilization status. All these observations suggest that soil pH is a major factor in disease suppression by organic matter application and that Ca and soil biota play certain roles in the suppression under the influence of soil pH.     

Heterotrophic and autotrophic nitrification in two acid pasture soils

Laboratory incubation experiments, using ¹⁵N-labeling techniques and simple analytical models, were conducted to measure heterotrophic and autotrophic nitrification rates in two acid soils (pH 4.8-5.3; 1/5 in H₂O) with high organic carbon contents (6.2-6.8% in top 5 cm soil). The soils were from pastures located near Maindample and Ruffy in the Northeast Victoria, Australia. Gross rates of N mineralization, nitrification and immobilization were measured. The gross rates of autotrophic nitrification were 0.157 and 0.119 μg N g⁻¹ h⁻¹ and heterotrophic nitrification rates were 0.036 and 0.009 μg N g⁻¹ h⁻¹ for the Maindample and Ruffy soils, respectively. Heterotrophic nitrification accounted for 19% and 7% of the total nitrification in the Maindample and Ruffy soils, respectively. The heterotrophic nitrifiers used organic N compounds and no as the substrate for nitrification.     

Rhizosphere priming of soil organic matter by bacterial groups in a grassland soil

Plants often impact the rate of native soil organic matter turnover through root interactions with soil organisms; however the role of root-microbial interactions in mediation of the “priming effect” is not well understood. We examined the effects of living plant roots and N fertilization on belowground C dynamics in a California annual grassland soil (Haploxeralf) during a two-year greenhouse study. The fate of ¹³C-labeled belowground C (roots and organic matter) was followed under planted (Avena barbata) and unplanted conditions, and with and without supplemental N (20 kg N ha⁻¹ season⁻¹) over two periods of plant growth, each followed by a dry, fallow period of 120 d. Turnover of belowground ¹³C SOM was followed using ¹³C-phospholipid fatty acid (PLFA) biomarkers. Living roots increased the turnover and loss of belowground ¹³C compared with unplanted soils. Planted soils had 20% less belowground ¹³C present than in unplanted soils after 2 cycles of planting and fallow. After 2 treatment cycles, unlabeled soil C was 4.8% higher in planted soils than unplanted. The addition of N to soils decreased the turnover of enriched belowground ¹³C during the first treatment season in both planted and unplanted soils, however no effect of N was observed thereafter. Our findings suggest that A. barbata may increase soil C levels over time because root and exudate C inputs are significant, but that increase will be moderated by an overall faster C mineralization rate of belowground C. N addition may slow soil C losses; however, the effect was minor and transient in this system. The labeled root-derived ¹³C was initially recovered in gram negative (highest enrichment), gram positive, and fungal biomarkers. With successive growing seasons, the labeled C in the gram negative and fungal markers declined, while gram positive markers continued to accumulate labeled belowground C. The rhizosphere of A. barbata shifted the microbial community composition, resulting in greater abundances of gram negative markers and lower abundances of gram positive, actinobacteria and cyclopropyl PLFA markers compared to unplanted soil. However, the longer-term utilization of labeled belowground C by gram positive bacteria was enhanced in the rhizosphere microbial community compared with unplanted soils. We suggest that the activities of gram positive bacteria may be major controllers of multi-year rhizosphere-related priming of SOM decomposition.     

Gross N mineralization rates after application of composted grape marc to soil

Grape marc is a common waste product of the wine production industry. When partially composted and applied to soil it may contain enough N to affect vine growth and hence wine quality. Yet little is known about the quantity and timing of N release from composted grape marc. A laboratory incubation was conducted where composted grape marc amended and non-amended soils were periodically sampled over 148 days at 15 °C for gross N mineralization rates, C mineralization and microbial biomass-C. Gross N mineralization rates were determined by ¹⁵N pool dilution using both analytical equations and the numerical model FLUAZ (Mary, B., Recous, S., Robin, D., 1998. A model for calculating nitrogen fluxes in soil using 15N tracing. Soil Biology & Biochemistry 30, 1963-1979.). Both analytical and FLUAZ determined gross N mineralization rates were in close agreement in the control soil. However, in composted grape marc amended soils there was a discrepancy between the two solutions. Findings indicate that composted grape marc caused a net immobilization of N for the first 50-days of incubation, after which enough N was released to require consideration in fertilizer-N strategies.     

Community composition and activities of denitrifying bacteria from adjacent agricultural soil, riparian soil, and creek sediment in Oregon, USA

We examined denitrifying bacteria from wet soils and creek sediment in an agroecosystem in Oregon, USA that received inputs of nitrogen (N) fertilizer. Our objective was to determine the variation in denitrifying community composition and activities across three adjacent habitats: a fertilized agricultural field planted to perennial ryegrass, a naturally vegetated riparian area, and creek sediment. Using C₂H₂ inhibition, denitrifying enzyme and N₂O-reductase activities were determined in short-term incubations of anaerobic slurries. A key gene in the denitrification pathway, N₂O reductase (nosZ), served as a marker for denitrifiers. Mean denitrifying enzyme activity (DEA) was similar among habitats, ranging from 0.5 to 1.8 μg N g⁻¹ dry soil h⁻¹. However, the ratio of N₂O production, without C₂H₂, to DEA was substantially higher in riparian soil (0.64±0.02; mean±standard error, n=12) than in agricultural soil (0.19±0.02) or creek sediment (0.32±0.03). Mean N₂O-reductase activity ranged from 0.5 to 3.2 μg N g⁻¹ dry soil h⁻¹, with greater activity in agricultural soil than in riparian soil. Denitrifying community composition differed significantly among habitats based on nosZ terminal-restriction fragment length polymorphisms. The creek sediment community was unique. Communities in the agricultural and riparian soil were more closely related but distinct. A number of unique nosZ genotypes were detected in creek sediment. Sequences of nosZ obtained from riparian soil were closely related to nosZ from Bradyrhizobium japonicum. Although nosZ distribution and N₂O-reductase activity differed among habitats, relationships between activity and community composition appeared uncoupled across the agroecosystem.     

Interactions of NaCl and Na2SO4 on soil organic C mineralization after addition of maize straws

NaCl and Na₂SO₄ often dominate salt compositions in saline soils. While either salt alone affects soil organic matter mineralization, their interactions on soil organic matter dynamics are unknown. This study aimed to investigate interactive effects of the two salts on organic C mineralization and microbial biomass C of the saline soils after addition of maize straws. Both NaCl and Na₂SO₄ were applied at 0, 40 and 80 mmol Na kg⁻¹ soil and the incubation was undertaken at soil water content of 15% and 20% (w/w) in dark at 28.5 °C for 70 days. The study found significant interactions of NaCl and Na₂SO₄ on CO₂-C evolution during the early incubation periods—a suppressing effect at days 1-2 but a stimulating effect at days 6-8 and 17-20, and thereafter the salt interactions were influenced by water content. The interactions of water content with NaCl or Na₂SO₄ on CO₂-C evolution were observed through the incubation periods except days 1-2, showing that the salt effects were dependent on water content. Total CO₂ evolution over the 70-day-long incubation decreased with increasing NaCl but increased with increasing Na₂SO₄ compared to the nil-salted treatment. Salt interactions on soil microbial biomass C were observed at days 7, 21, but not at day 49. Microbial biomass C increased at day 7 in the soils treated with either NaCl or Na₂SO₄ but decreased where the two salts were combined. At day 21, microbial biomass C increased with NaCl but decreased with Na₂SO₄ regardless whether the counterpart salt was added. The results suggest that soil organic C mineralization can be affected by the interactions of NaCl and Na₂SO₄, possibly through the salt-induced changes in microbial biomass community structure.     

Symbiotically-fixed and soil-derived nitrogen in legumes grown in pots in soils with different amounts of available nitrate

Symbiotically-fixed and soil-derived nitrogen have been measured in pot experiments for Medicago littoralis (medic), grown alone or with Lolium multiflorum (ryegrass) and for Pisum sativum (field pea). The four soils used contained organic matter labelled with ¹⁵N, and differed in their capacities to release available N. During a 4–12-week incubation each released inorganic N (NO^?₃) of approximately constant ¹⁵N atom% enrichment. In one soil, the mineralized N was supplemented by ¹⁵NO^?₃ of similar ¹⁵N atom% enrichment. Incubation of soils under intermittently moist and dry conditions increased N mineralization rates, but did not affect the ¹⁵N atom% enrichments of the released N.For all soils and treatments the amounts of soil-derived N taken up by plants equalled the amounts of available N in moist incubated, unplanted soils. The enrichment of ryegrass root N grown alone or with medic was slightly but consistently less than that of top N. Nitrogen of the legume nodules and pods (peas) was least enriched, followed by N of legume stems, leaves and roots; the ¹⁵N atom% enrichments of root N were 4–5 times those of nodule N.Peas generally outyielded and fixed more N than medic grown alone. Medic grown with ryegrass yielded least and fixed least N.For unamended soils, yields of legume dry matter and amounts of N fixed were greatest in Roseworthy or Avon sandy loam soils and least in Northfield clay loam. Addition of ¹⁵NO^?₃ to Avon soil decreased N fixed by peas and by medic grown alone or with ryegrass. For this soil, soil-derived N of plant tops exceeded fixed N of roots, even for unamended soil where fixation by legumes was relatively high. Thus, complete removal of plant tops would have produced a net loss of N from the soil, the net loss increasing with increasing amounts of ¹⁵NO^?₃ added.