Compared effects of long-term pig slurry applications and mineral fertilization on soil denitrification and its end products (N2O, N2)

The long-term (9 years) effect of pig slurry applications vs mineral fertilization on denitrifying activity, N₂O production and soil organic carbon (C) (extractable C, microbial biomass C and total organic C) was compared at three soil depths of adjacent plots. The denitrifying activities were measured on undisturbed soil cores and on sieved soil samples with acetylene method to estimate denitrification rates under field or potential conditions. Pig slurry applications had a moderate impact on the C pools. Total organic C was increased by +6.5% and microbial biomass C by ≥25%. The potential denitrifying activity on soil suspension was stimulated (×1.8, P<0.05) 12 days after the last slurry application. This stimulation was still apparent, but not significant, 10 months later and, according to both methods of denitrifying activity measurement (r ²=0.916, P<0.01 on sieved soil; r ²=0.845, P<0.001 on soil cores), was associated with an increase in microbial biomass C above a threshold of about 105 mg kg⁻¹. The effect of pig slurry on denitrification and N₂O reduction rates was detected on the surface layer (0–20 cm) only. However, no pig slurry effect could be detected on soil cores at field conditions or after NO₃ ⁻ enrichments at 20°C. Although the potential denitrifying activity in sieved soil samples was stimulated, the N₂O production was lower (P<0.03) in the plot fertilized with pig slurry, indicating a lower N₂O/(N₂O + N₂) ratio of the released gases. The pig-slurry-fertilized plot also showed a higher N₂O reduction activity, which is coherent with the lower N₂O production in anaerobiosis.     

Immobilization and mineralization of nitrogen in a saline and alkaline soil during microbial use of sugarcane filter cake amended with glucose

A 42-day incubation was conducted to study the effect of glucose and ammonium addition adjusted to a C/N ratio of 12.5 on sugarcane filter cake decomposition and on the release of inorganic N from microbial residues formed initially. The CO₂ evolved increased in comparison with the non-amended control from 35% of the added C with pure +5 mg g⁻¹ soil filter cake amendment to 41% with +5 mg g⁻¹ soil filter cake +2.5 mg g⁻¹ soil glucose amendment to 48% with 5 mg g⁻¹ soil filter cake +5 mg g⁻¹ soil glucose amendment. The different amendments increased microbial biomass C and microbial biomass N within 6 h and such an increase persisted. The fungal cell-membrane component ergosterol initially showed a disproportionate increase in relation to microbial biomass C, which completely disappeared by the end of the incubation. The cellulase activity showed a 5-fold increase after filter cake addition, which was not further increased by the additional glucose amendment. The cellulase activity showed an exponential decline to values around 4% of the initial value in all treatments. The amount of inorganic N immobilized from day 0 to day 14 increased with increasing amount of C added, in contrast to the control treatment. After day 14, the immobilized N was re-mineralized at rates between 1.3 and 1.5 μg N g⁻¹ soil d⁻¹ in the treatments being more than twice as high as in the control treatment. This means that the re-mineralization rate is independent of the actual size of the microbial residues pool and also independent of the size of the soil microbial biomass.     

Earthworm (<Emphasis Type="Italic">Metaphire guillelmi</Emphasis>) effects on rice photosynthates distribution in the plant–soil system

An experiment was conducted to investigate the effects of earthworm (Metaphire guillelmi) activities on rice photosynthates distribution in plant–soil system through ¹⁴C pulse-labelling method. Rice was planted in pots, and maize straw was mulched on the surface with or without earthworms. Rice plants at tillering stage or heading stage were labelled with ¹⁴CO₂. Plant and soil were sampled 15 days after labelling at the tillering or heading stage and at harvest. Rice growth was inhibited by earthworms (M. guillelmi) at early stage, but the inhibition disappeared at later stage. Earthworms significantly (P < 0.01) increased the ¹⁴C percentage in root at day 15 after tillering stage labelling, but the effect disappeared at harvest. Earthworms (M. guillelmi) significantly (P < 0.01) increased the ¹⁴C percentage in root at day 15 after heading stage labelling and increased ¹⁴C percentage in soil at harvest. Earthworms decreased the percentages of total organic ¹⁴C (TO¹⁴C) present as microbial biomass ¹⁴C (MB¹⁴C) and increased the percentages of total organic ¹⁴C present as dissolved organic ¹⁴C (DO¹⁴C) at all sampling times. It is suggested that earthworms might alter the transfer of plant photosynthates from the aboveground to the belowground, and thus, soil active C pool. However, these data should be also confirmed in the field.     

Effects of Cd,Zn, or both on soil microbial biomass and activity in a clay loam soil

We investigated Cd, Zn, and Cd + Zn toxicity to soil microbial biomass and activity, and indigenous Rhizobium leguminosarum biovar trifolii, in two near neutral pH clay loam soils, under long-term arable and grassland management, in a 6-month laboratory incubation, with a view to determining the causative metal. Both soils were amended with Cd- or Zn-enriched sewage sludge, to produce soils with total Cd concentrations at four times (12 mg Cd g⁻¹ soil), and total Zn concentrations (300 mg Zn kg⁻¹ soil) at the EU upper permitted limit. The additive effects of Cd plus Zn at these soil concentrations were also investigated. There were no significant differences in microbial biomass C (B _C), biomass ninhydrin N (B _N), ATP, or microbial respiration between the different treatments. Microbial metabolic quotient (defined as qCO₂ = units of CO₂–C evolved unit⁻¹ biomass C unit⁻¹ time) also did not differ significantly between treatments. However, the microbial maintenance energy (in this study defined as qCO₂-to-μ ratio value, where μ is the growth rate) indicated that more energy was required for microbial synthesis in metal-rich sludge-treated soils (especially Zn) than in control sludge-treated soils. Indigenous R. leguminosarum bv. trifolii numbers were not significantly different between untreated and sludge-treated grassland soils after 24 weeks regardless of metal or metal concentrations. However, rhizobial numbers in the arable soils treated with metal-contaminated sludges decreased significantly (P < 0.05) compared to the untreated control and uncontaminated sludge-treated soils after 24 weeks. The order of decreasing toxicity to rhizobia in the arable soils was Zn > Cd > Cd + Zn.     

Effects of cow manure and sewage sludge on the activity and kinetics of <Emphasis Type="SmallCaps">l</Emphasis>-glutaminase in soil

A study was conducted to investigate the effects of cow manure and sewage sludge application on the activity and kinetics of soil l-glutaminase. Soil samples were collected from a farm experiment in which 0, 25, and 100 Mg ha⁻¹ of either cow manure or sewage sludge had been applied annually for 4 consecutive years to a clay loam soil (Typic Haplargid). A chemical fertilizer treatment had also been applied. Results indicated that the effects of chemical fertilizer and the solid waste application on pH in the 18 surface soil (0–15 cm) samples were not significant. The organic C content, however, was affected significantly by the different treatments, being the greatest in soils treated with 100 Mg ha⁻¹ cow manure, and the least in the control treatment. l-Glutaminase activity was generally greater in solid-waste applied soils and was significantly correlated (r = 0.939, P < 0.001) with organic C content of soils. The values of l-glutaminase maximum velocity (Vmax) ranged from 331 to 1,389 mg NH₄ ⁺–N kg⁻¹ 2 h⁻¹. Values of the Michaelis constant (K _m) ranged from 35.1 to 71.7 mM. Organic C content of the soils were significantly correlated with V _max (r = 0.919, P < 0.001) and K _m (r = 0.763, P < 0.001) values. These results demonstrate the considerable influence that solid waste application has on this enzymatic reaction involved in N mineralization in soil.     

Influence of soil phosphorus status and nitrogen addition on carbon mineralization from 14C-labelled glucose in pasture soils

 This study examines the effect of soil P status and N addition on the decomposition of ¹⁴C-labelled glucose to assess the consequences of reduced fertilizer inputs on the functioning of pastoral systems. A contrast in soil P fertility was obtained by selecting two hill pasture soils with different fertilizer history. At the two selected sites, representing low (LF) and high (HF) fertility status, total P concentrations were 640 and 820 mg kg^–1 and annual pasture production was 4,868 and 14,120 kg DM ha^–1 respectively. Soils were amended with ¹⁴C-labelled glucose (2,076 mg C kg^–1 soil), with and without the addition of N (207 mg kg^–1 soil), and incubated for 168 days. During incubation, the amounts of ¹⁴CO₂ respired, microbial biomass C and ¹⁴C, microbial biomass P, extractable inorganic P (P_i) and net N mineralization were determined periodically. Carbon turnover was greatly influenced by nutrient P availability. The amount of glucose-derived ¹⁴CO₂ production was high (72%) in the HF and low (67%) in the LF soil, as were microbial biomass C and P concentrations. The ¹⁴C that remained in the microbial biomass at the end of the 6-month incubation was higher in the LF soil (15%) than in the HF soil (11%). Fluctuations in P_i in the LF soil during incubation were small compared with those in HF soil, suggesting that P was cycling through microbial biomass. The concentrations of P_i were significantly greater in the HF samples throughout the incubation than in the LF samples. Net N mineralization and nitrification rates were also low in the LF soils, indicating a slow turnover of microorganisms under limited nutrient supply. Addition of N had little effect on biomass ¹⁴C and glucose utilization. This suggests that, at limiting P fertility, C turnover is retarded because microbial biomass becomes less efficient in the utilization of substrates. Received: 18 October 1999     

Biological nitrogen fixation by common beans (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) increases with bio-char additions

This study examines the potential, magnitude, and causes of enhanced biological N₂ fixation (BNF) by common beans (Phaseolus vulgaris L.) through bio-char additions (charcoal, biomass-derived black carbon). Bio-char was added at 0, 30, 60, and 90 g kg⁻¹ soil, and BNF was determined using the isotope dilution method after adding ¹⁵N-enriched ammonium sulfate to a Typic Haplustox cropped to a potentially nodulating bean variety (CIAT BAT 477) in comparison to its non-nodulating isoline (BAT 477NN), both inoculated with effective Rhizobium strains. The proportion of fixed N increased from 50% without bio-char additions to 72% with 90 g kg⁻¹ bio-char added. While total N derived from the atmosphere (NdfA) significantly increased by 49 and 78% with 30 and 60 g kg⁻¹ bio-char added to soil, respectively, NdfA decreased to 30% above the control with 90 g kg⁻¹ due to low total biomass production and N uptake. The primary reason for the higher BNF with bio-char additions was the greater B and Mo availability, whereas greater K, Ca, and P availability, as well as higher pH and lower N availability and Al saturation, may have contributed to a lesser extent. Enhanced mycorrhizal infections of roots were not found to contribute to better nutrient uptake and BNF. Bean yield increased by 46% and biomass production by 39% over the control at 90 and 60 g kg⁻¹ bio-char, respectively. However, biomass production and total N uptake decreased when bio-char applications were increased to 90 g kg⁻¹. Soil N uptake by N-fixing beans decreased by 14, 17, and 50% when 30, 60, and 90 g kg⁻¹ bio-char were added to soil, whereas the C/N ratios increased from 16 to 23.7, 28, and 35, respectively. Results demonstrate the potential of bio-char applications to improve N input into agroecosystems while pointing out the needs for long-term field studies to better understand the effects of bio-char on BNF.     

Impact of growing maize (<Emphasis Type="Italic">Zea mays</Emphasis>) on the decomposition of incorporated fresh alfalfa residues

In this study, the effects of growing maize plants on the microbial decomposition of easily degradable plant residues were investigated in a 90-day pot experiment using a sandy arable soil. Four treatments were carried out: (1) untreated control, (2) with freshly chopped alfalfa residues (Medicago sativa L.) incorporated into soil, (3) with growing maize plants (Zea mays L.), and (4) with growing maize plants and freshly chopped alfalfa residues incorporated into soil. The amount of alfalfa residues was equivalent to 1.5 mg C g⁻¹ soil and 120 μg N g⁻¹ soil. At the end of the experiment, only the combination of growing maize plants and alfalfa residues significantly increased the contents of microbial biomass C, microbial biomass N, and ergosterol in soil compared to the non-amended control. The dry weight of the maize shoot material was more than doubled in the treatment with alfalfa residues than without. In treatment (2), 6% of the alfalfa residues could be recovered as plant remains >2 mm. In treatment (4), this fraction contained 14.7% alfalfa residues and 85.3% maize root remains, calculated on the basis of δ ¹³C values. This means that 60% more alfalfa-C was recovered than in treatment (2). The reasons for the retardation in the breakdown of alfalfa residues might be water deficiency of soil microorganisms in the increased presence of maize roots. Assuming that the addition of alfalfa residues did not affect the decomposition of native soil organic matter, only 23% of the alfalfa residues were found as CO₂ monitored with a portable gas analyzer with a dynamic chamber. The discrepancy is probably due to problems in measuring peak concentrations of CO₂ evolution in the two alfalfa treatments at the beginning of the experiment and in the two maize treatments at the end, especially in treatment (4).     

CO2 efflux by rapid decomposition of low molecular organic substances in soils

Decomposition rates of the [2-¹⁴C]-glucose and [2-¹⁴C]-glycine in four different soils of the long-term field trial of Moscow were investigated in a 3-months laboratory experiment in which ¹⁴CO₂ respiration was measured. A model with three decomposition components and two distribution parameters was developed and validated with the data of the experiment. The decay rate constants of free [2-¹⁴C]-glucose (4–32 day^-1) were slower than those of [2-¹⁴C]-glycine (16–44 day^-1). The calculated use efficiency for microbial biosynthesis of the second carbon atom was 47% for glucose and 31% for glycine. The potential half-life of labelled carbon in the microbial soil biomass ranged from 0.6 to 4.4 days, depending on the soil type and the initial amount of added substrate. The calculated total utilisation of carbon by the soil biomass from glycine was about 2–5 times lower than that of glucose.The modelled ¹⁴C incorporation into the microbial soil biomass reached its maximum on the first day of the incubation experiment and did not exceed 22% of the ¹⁴C input. Both of the investigated substances decomposed most rapidly in the soil samples from sites that have not being fertilised with organic or mineral fertilisers during an 81-years period.     

N<Subscript>2</Subscript> fixation by faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.) in a gypsum-amended sodic soil

The response of faba bean to the application of four rates of gypsum (0, 2.5, 5.0, 10.0 t ha⁻¹) to a non-saline, alkaline sodic soil was measured in terms of grain yield, dry matter (DM) production, N accumulation and the proportional dependence of the legume on symbiotic N₂ fixation (P _atm). A yield-independent, time-integrated ¹⁵N-dilution model was used to estimate symbiotic dependence. A significant decrease in the exchangeable sodium percentage and significant increases in exchangeable Ca⁺⁺ and the Ca⁺⁺:Mg⁺⁺ ratio in the 0–10-cm soil layer were measured 30 months after application of 10 t ha⁻¹ gypsum. Despite low and erratic rainfall during crop growth, faba bean DM and N uptake responded positively to gypsum application. The symbiotic dependence of the legume at physiological maturity was little affected by sodicity (P _atm = 0.74 at zero gypsum and 0.81–0.82 at 2.5–10 t ha⁻¹ gypsum). The increase in fixed N due to gypsum application was mainly due to increases in legume DM and total N uptake. At 10 t ha⁻¹ of gypsum, faba bean fixed more than 200 kg N ha⁻¹ in above-ground biomass.     

Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N₂O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization–immobilization and N₂O emission. Residues were added at the rate of 3 t C ha⁻¹ to soil with, and without, 150 kg urea N ha⁻¹. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P < 0.05) lower NO₃⁻–N, which reached minimum values of 2.8 mg N kg⁻¹ for sugarcane (at day 28), 10.3 mg N kg⁻¹ for maize (day 7), and 5.9 mg N kg⁻¹ for sorghum (day 7), compared to 22.7 mg N kg⁻¹ for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg⁻¹ in sugarcane, 34 mg kg⁻¹ in maize, 29 mg kg⁻¹ in sorghum, and 16 mg kg⁻¹ in cotton amended soil compared to soil + N fertilizer, although soil NO₃⁻–N increased by 7 mg kg⁻¹ in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N₂O from crop residue amended soils occurred in the first 4–5 days of incubation. Overall, after 84 days of incubation, the cumulative N₂O emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative N₂O emission was significantly and positively correlated with NO₃⁻–N (r = 0.92, P < 0.01) and total mineral N (r = 0.93, P < 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO₂ in the first 3 and 5 days of incubation (r = 0.59, P < 0.05).     

The effect of reduced tillage on nitrous oxide emissions of silt loam soils

The effect of reduced tillage (RT) on nitrous oxide (N₂O) emissions of soils from fields with root crops under a temperate climate was studied. Three silt loam fields under RT agriculture were compared with their respective conventional tillage (CT) field with comparable crop rotation and manure application. Undisturbed soil samples taken in September 2005 and February 2006 were incubated under laboratory conditions for 10 days. The N₂O emission of soils taken in September 2005 varied from 50 to 1,095 μg N kg⁻¹ dry soil. The N₂O emissions of soils from the RT fields taken in September 2005 were statistically (P < 0.05) higher or comparable than the N₂O emissions from their respective CT soil. The N₂O emission of soils taken in February 2006 varied from 0 to 233 μg N kg⁻¹ dry soil. The N₂O emissions of soils from the RT fields taken in February 2006 tended to be higher than the N₂O emissions from their respective CT soil. A positive and significant Pearson correlation of the N₂O–N emissions with nitrate nitrogen (NO₃ ⁻–N) content in the soil was found (P < 0.01). Leaving the straw on the field, a typical feature of RT, decreased NO₃ ⁻–N content of the soil and reduced N₂O emissions from RT soils.     

Short-term effects of tillage systems on active soil microbial biomass

 Conservation tillage, and especially no-tillage, induce changes in the distribution of organic pools in the soil profile. In long-term field experiments, marked stratification of the total soil microbial biomass and its activity have been observed as consequence of the application of no-tillage to previously tilled soils. Our objective was to study the evolution of the total and active soil microbial biomass and mineralized C in vitro during the first crop after the introduction of no-tillage to an agricultural soil. The experiment was performed on a Typic Hapludoll from the Argentinean Pampa. Remaining plant residues, total and active microbial biomass and mineralized C were determined at 0–5 cm and 5–15 cm depths, at three sampling times: wheat tilling, silking and maturity. The introduction of no-tillage produced an accumulation of plant residues in the soil surface layer (0–5 cm), showing stratification with depth at all sampling dates. Active microbial biomass and C mineralization were higher under no-tillage than under conventional tillage in the top 5 cm of the profile. The total soil microbial biomass did not differ between treatments. The active soil biomass was highly and positive correlated with plant residues (r ²=0.617;P<0.01) and with mineralized C (r ²=0.732;P<0.01). Consequently, the active microbial biomass and mineralized C reflected immediately the changes in residue management, whereas the total microbial biomass seemed not to be an early indicator of the introduction of a new form of soil management in our experiment. Received: 23 February 1999     

Effects of long-term mineral fertilization on microbial biomass,microbial activity,and the presence of <Emphasis Type="Italic">r</Emphasis>- and <Emphasis Type="Italic">K</Emphasis>-strategists in soil

Long-term effects of mineral fertilization on microbial biomass C (MBC), basal respiration (R _B), substrate-induced respiration (R _S), β-glucosidase activity, and the r–K-growth strategy of soil microflora were investigated using a field trial on grassland established in 1969. The experimental plots were fertilized at three rates of mineral N (0, 80, and 160 kg ha⁻¹ year⁻¹) with 32 kg P ha⁻¹ year⁻¹ and 100 kg K ha⁻¹ year⁻¹. No fertilizer was applied on the control plots (C). The application of a mineral fertilizer led to lower values of the MBC and R _B, probably as a result of fast mineralization of available substrate after an input of the mineral fertilizer. The application of mineral N decreased the content of C extracted by 0.5 M K₂SO₄ (C _ex). A positive correlation was found between pH and the proportion of active microflora (R _S/MBC). The specific growth rate (μ) of soil heterotrophs was higher in the fertilized than in unfertilized soils, suggesting the stimulation of r-strategists, probably as the result of the presence of available P and rhizodepositions. The cessation of fertilization with 320 kg N ha⁻¹ year⁻¹ (NF) in 1989 also stimulated r-strategists compared to C soil, probably as the result of the higher content of available P in the NF soil than in the C soil.     

Production of root-derived material and associated microbial growth in soil at different nutrient levels

Summary Maize plants were grown for 42 days in a sandy soil at two different mineral nutrient levels, in an atmosphere containing ¹⁴CO₂. The ¹⁴C and total carbon contents of shoots, roots, soil and soil microbial biomass were measured 28, 35 and 42 days after germination. Relative growth rates of shoots and roots decreased after 35 days at the lower nutrient level, but were relatively constant at the higher nutrient level. In the former treatment, 2% of the total ¹⁴C fixed was retained as a residue in soil at all harvests while at the higher nutrient level up to 4% was retained after 42 days. Incorporation of ¹⁴C into the soil microbial biomass was close to its maximum after 35 days at the lower nutrient level, but continued to increase at the higher level. Generally a good agreement existed between microbial biomass, ¹⁴C contents and numbers of fluorescent pseudomonads in the rhizosphere. Numbers of fluorescent pseudomonads in the rhizosphere were maximal after 35 days at the lower nutrient level and continued to increase at the higher nutrient level. The proportions of the residual ¹⁴C in soil, incorporated in the soil microbial biomass, were 28% to 41% at the lower nutrient level and 20%6 – 30% at the higher nutrient level. From the lower nutrient soil 18%6 – 52%6 of the residual soil ¹⁴C could be extracted with 0.5 N K₂SO₄, versus 14%6 – 16% from the higher nutrient soil.Microbial growth in the rhizosphere seemed directly affected by the depletion of mineral nutrients while plant growth and the related production of root-derived materials continued.     

Short-term partitioning of <Superscript>14</Superscript>C-[U]-glucose in the soil microbial pool under varied aeration status

The effect of soil aeration status on carbon partitioning of a labelled organic substrate (¹⁴C-[U]-glucose) into CO₂, microbial biomass, and extra-cellular metabolites is described. The soil was incubated in a continuous flow incubation apparatus under four different aeration conditions: (1) permanently aerobic, (2) permanently anaerobic, (3) shifted from anaerobic to aerobic, and (4) shifted from aerobic to anaerobic. The soil was pre-incubated for 10 days either under aerobic or under anaerobic conditions. Afterwards, glucose was added (315 g C g^–1) and the soils were incubated for 72 h according to four treatments: aerobic or anaerobic conditions maintained, aerobic conditions shifted to anaerobic conditions and anaerobic conditions shifted to aerobic conditions. Carbon partitioning was measured 0, 8, 16, 24, 48 and 72 h after the glucose addition. In permanently aerobic conditions, the largest part of the consumed glucose was built into microbial biomass (72%), much less was mineralised to CO₂ (27%), and only a negligible portion was transformed to soluble extra-cellular metabolites. Microbial metabolism was strongly inhibited when aeration conditions were changed from aerobic to anaerobic, with only about 35% of the added glucose consumed during the incubation. The consumed glucose was transformed proportionally to microbial biomass and CO₂. In permanently anaerobic conditions, 42% of the consumed glucose was transformed into microbial biomass, 30% to CO₂, and 28% to extra-cellular metabolites. After a shift of anaerobic to aerobic conditions, microbial metabolism was not suppressed and the consumed glucose was transformed mainly to microbial biomass (75%) and CO₂ (23%). Concomitant mineralisation of soil organic carbon was always lower in anaerobic than in aerobic conditions.     

In situ mineral <Superscript>15</Superscript>N dynamics and fate of added <Superscript>15</Superscript>NH<Subscript>4</Subscript><Superscript>+</Superscript> in hoop pine plantation and adjacent native forest in subtropical Australia

Background, aim, and scope  Hoop pine (Araucaria cunninghamii) is a nitrogen (N) demanding indigenous Australia softwood species with plantations in Southeast Queensland, Australia. Soil fertility has declined with increasing rotations and comparison study of N cycling between hoop pine plantations, and adjacent native forest (NF) is required to develop effective forest management for enhancing sustainable forest production and promoting environmental benefits. Field in situ mineral ¹⁵N transformations in these two forest ecosystems have not been studied. Hence, the present study was to compare the differences in soil nutrients, N transformations, ¹⁵N fluxes, and fate between the hoop pine plantation and the adjacent native forest. Materials and methods  The study sites were in Yarraman State Forest (26°52′ S, 151°51′ E), Southeastern Queensland, Australia. The in situ core incubation method was used in the field experiments. Mineral N was determined using a LACHAT Quickchem Automated Ion Analyzer. ¹⁵N were performed using an isotope ratio mass spectrometer with a Eurovector elemental analyzer. All statistical tests were carried out by the SPSS 11.0 for Windows statistical software package. Results  Soil total C and N were significantly higher in the NF than in the 53-year-old hoop pine plantation. Concentrations of NO₃ ^– were significantly higher in the NF soil than in the plantation soil. The plantation soil had significantly higher ¹⁵N and ¹³C natural abundances than the NF soil. The NF soil had significantly lower C/N ratios than the plantation soil. NO₃ ^––N was dominated in mineral N pools in both NF and plantation soils, accounting for 91.6% and 70.3% of the total mineral N pools, respectively. Rates of net nitrification and net N mineralization were, respectively, four and three times higher in the NF soil than in the plantation soil. The ¹⁵NO₃ ^––N and mineral ¹⁵N were significantly higher in the NF soil than in the plantation soil. Significant difference in ¹⁵NH₄ ⁺–N was found in the NF soil before and after the incubation. Discussion  The NF soil had significantly higher NO₃ ^––N, mineral N, total N and C but lower δ¹⁵N, δ¹³C, and C/N ratios than the plantation soil. Moreover, the rates of soil net N mineralization and nitrification were significantly higher, but ammonification rate was lower in the NF than in the plantation. The NF soil had many more dynamic N transformations than the plantation soil due to the combination of multiple species and layers and, thus, stimulation of microbial activity and alteration of C and N pool sizes in favor of the N transformations by soil microbes. The net rate of N and ¹⁵N transformation demonstrated differences in N dynamic related to the variation in tree species between the two ecosystems. Conclusions  The change of land use and trees species had significant impacts on soil nutrients and N cycling processes. The plantation had larger losses of N than the NF. The NO₃ ^––N and ¹⁵NO₃ ^––N dominated in the mineral N and ¹⁵N pools in both forest ecosystems. Recommendations and perspectives  Native forest soil had strong N dynamic compared with the plantation soil. Composition of multiple tree species with different ecological niches in the plantation could promote the soil ecosystem sustainability. The ¹⁵N isotope dilution technique in the field can be quite useful for studying in situ mineral ¹⁵N transformations and fate to further understand actual N dynamics in natural forest soils.     

Evaluation of rice–legume–rice cropping system on grain yield,nutrient uptake,nitrogen fixation,and chemical,physical, and biological properties of soil

To achieve higher yields and better soil quality under rice–legume–rice (RLR) rotation in a rainfed production system, we formulated integrated nutrient management (INM) comprised of Azospirillum (Azo), Rhizobium (Rh), and phosphate-solubilizing bacteria (PSB) with phosphate rock (PR), compost, and muriate of potash (MOP). Performance of bacterial bioinoculants was evaluated by determining grain yield, nitrogenase activity, uptake and balance of N, P, and Zn, changes in water stability and distribution of soil aggregates, soil organic C and pH, fungal/bacterial biomass C ratio, casting activities of earthworms, and bacterial community composition using denaturing gradient gel electrophoresis (DGGE) fingerprinting. The performance comparison was made against the prevailing farmers’ nutrient management practices [N/P₂O₅/K₂O at 40:20:20 kg ha⁻¹ for rice and 20:30:20 kg ha⁻¹ for legume as urea/single super-phosphate/MOP (urea/SSP/MOP)]. Cumulative grain yields of crops increased by 7–16% per RLR rotation and removal of N and P by six crops of 2 years rotation increased significantly (P < 0.05) in bacterial bioinoculants-based INM plots over that in compost alone or urea/SSP/MOP plots. Apparent loss of soil total N and P at 0–15 cm soil depth was minimum and apparent N gain at 15–30 cm depth was maximum in Azo/Rh plus PSB dual INM plots. Zinc uptake by rice crop and diethylenetriaminepentaacetate-extractable Zn content in soil increased significantly (P < 0.05) in bacterial bioinoculants-based INM plots compared to other nutrient management plots. Total organic C content in soil declined at 0–15 cm depth and increased at 15–30 cm depth in all nutrient management plots after a 2-year crop cycle; however, bacterial bioinoculants-based INM plots showed minimum loss and maximum gain of total organic C content in the corresponding soil depths. Water-stable aggregation and distribution of soil aggregates in 53–250- and 250–2,000 μm classes increased significantly (P < 0.05) in bacterial bioinoculants-based INM plots compared to other nutrient management plots. Fungal/bacterial biomass C ratio seems to be a more reliable indicator of C and N dynamics in acidic soils than total microbial biomass C. Compost alone or Azo/Rh plus PSB dual INM plots showed significantly (P < 0.05) higher numbers of earthworms’ casts compared to urea/SSP/MOP alone and bacterial bioinoculants with urea or SSP-applied plots. Hierarchical cluster analysis based on similarity matrix of DGGE profiles revealed changes in bacterial community composition in soils due to differences in nutrient management, and these changes were seen to occur according to the states of C and N dynamics in acidic soil under RLR rotation.     

Plant canopy effects on litter accumulation and soil microbial biomass in two temperate forests

The objective of this study was to determine whether differences in canopy structure and litter composition affect soil characteristics and microbial activity in oak versus mixed fir-beech stands. Mean litter biomass was greater in mixed fir-beech stands (51.9t ha⁻¹) compared to oak stands (15.7t ha⁻¹). Canopy leaf area was also significantly larger in mixed stands (1.96m² m⁻²) than in oak stands (1.73m² m⁻²). Soil organic carbon (C _org) and moisture were greater in mixed fir-beech stands, probably as a result of increased cover. Soil microbial biomass carbon (C _mic), nitrogen (N _mic), and total soil nitrogen (N _tot) increased slightly in the mixed stand, although this difference was not significant. Overall, mixed stands showed a higher mean C _org/N _tot ratio (22.73) compared to oak stands (16.39), indicating relatively low rate of carbon mineralization. In addition, the percentage of organic C present as C _mic in the surface soil decreased from 3.17% in the oak stand to 2.26% in the mixed stand, suggesting that fir-beech litter may be less suitable as a microbial substrate than oak litter.     

Microbial biomass and C mineralization in agricultural soils as affected by atrazine addition

This study examines the effects of atrazine on both microbial biomass C and C mineralization dynamics in two contrasting agricultural soils (organic C, texture, and atrazine application history) located at Galicia (NW Spain). Atrazine was added to soils, a Humic Cambisol (H) and a Gleyic Cambisol (G), at a recommended agronomic dose and C mineralization (CO₂ evolved), and microbial biomass measurements were made in non-treated and atrazine-treated samples at different time intervals during a 12-week aerobic incubation. The cumulative curves of CO₂–C evolved over time fit the simple first-order kinetic model [Ct = Co (1 − e ^−kt )], whose kinetic parameters were quantified. Differences in these parameters were observed between the two soils studied; the G soil, with a higher content in organic matter and microbial biomass C and lower atrazine application history, exhibited higher values of the total C mineralization and the potentially mineralizable labile C pool than those for the H soil. The addition of atrazine modified the kinetic parameters and increased notably the C mineralized; by the end of the incubation the cumulative CO₂–C values were 33–41% higher than those in the corresponding non-added soils. In contrast, a variable effect or even no effect was observed on the soil microbial biomass following atrazine addition. The data clearly showed that atrazine application at normal agricultural rates may have important implications in the C cycling of these two contrasting acid soils.