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).     

Reduced net atmospheric CH4 consumption is a sustained response to elevated CO2 in a temperate forest

We compared, from 2004 through 2006, rates of soil–atmosphere CH₄ exchange at permanently established sampling sites in a temperate forest exposed to ambient (control plots; ∼380 μL L⁻¹) or elevated (ambient + 200 μL L⁻¹) CO₂ since August 1996. A total of 880 observations showed net atmospheric CH₄ consumption (flux from the atmosphere to the soil) from all static chambers most of the time at rates varying from 0.02 mg m⁻² day⁻¹ to 4.5 mg m⁻² day⁻¹. However, we infrequently found net CH₄ production (flux from the soil to the atmosphere) at lower rates, 0.01 mg m⁻² day⁻¹ to 0.08 mg m⁻² day⁻¹. For the entire study, the mean (±SEM) rate of net CH₄ consumption in control plots was higher than the mean for CO₂-enriched plots, 0.55 (0.03) versus 0.51 (0.03) mg m⁻² day⁻¹. Annual rates of 184, 196, and 197 mg m⁻² for net CH₄ consumption at control plots during the three calendar years of this study were 19, 10, and 8% higher than comparable values for CO₂ enriched plots. Differences between treatments were significant in 2004 and 2005 and nearly significant in 2006. Volumetric soil water content was consistently higher at CO₂-enriched sites and a mixed-effects model identified a significant soil moisture x CO₂ interaction on net atmospheric CH₄ consumption. Increased soil moisture at CO₂-enriched sites likely increases diffusional resistance of surface soils and the frequency of anaerobic microsites supporting methanogenesis, resulting in reduced rates of net atmospheric CH₄ consumption. Our study extends our observations of reduced net atmospheric CH₄ consumption at CO₂-enriched plots to nearly five continuous years, suggesting that this is likely a sustained negative feedback to increasing atmospheric CO₂ at this site.     

Changes in CO<Subscript>2</Subscript>, <Superscript>13</Superscript>C abundance,inorganic nitrogen, β-glucosidase,and oxidative enzyme activities of soil during the decomposition of switchgrass root carbon as affected by inorganic nitrogen additions

This study was conducted to investigate the effect of inorganic nitrogen (N) and root carbon (C) addition on decomposition of organic matter (OM). Soil was incubated for 200 days with nine treatments (three levels of N (no addition (N0) = 0, low N (NL) = 0.021, high N (NH) = 0.083 mg N g⁻¹ soil) × three levels of C (no addition (C0) = 0, low C (CL) = 5, high C (CH) = 10 mg root g⁻¹ soil)). The carbon dioxide (CO₂) efflux rates, inorganic N concentration, pH, and potential activities of β-glucosidase and oxidative enzyme were measured during incubation. At the beginning and the end of incubation, the native soil organic carbon (SOC) and root-derived SOC were quantified by using a natural labeling technique based on the differences in δ ¹³C between C3 and C4 plants. Overall, the interaction between C and N was not significant. The decomposition of OM in the NH treatment decreased. This could be attributed to the formation of recalcitrant OM by N because the potentially mineralizable C pool was significantly lower in the NH treatment (3.1 mg C g⁻¹) than in the N0 treatment (3.6 mg C  g⁻¹). In root C addition treatments, the CO₂ efflux rate was generally in order of CH > CL > C0 over the incubation period. Despite no differences in the total SOC concentration among C treatments, the native SOC in the CH treatment (18.29 mg C g⁻¹) was significantly lower than that in the C0 treatment (19.16 mg C g⁻¹).     

Microbial responses and nitrous oxide emissions during wetting and drying of organically and conventionally managed soil under tomatoes

The types and amounts of carbon (C) and nitrogen (N) inputs, as well as irrigation management are likely to influence gaseous emissions and microbial ecology of agricultural soil. Carbon dioxide (CO₂) and nitrous oxide (N₂O) efflux, with and without acetylene inhibition, inorganic N, and microbial biomass C were measured after irrigation or simulated rainfall in two agricultural fields under tomatoes (Lycopersicon esculentum). The two fields, located in the California Central Valley, had either a history of high organic matter (OM) inputs (“organic” management) or one of low OM and inorganic fertilizer inputs (“conventional” management). In microcosms, where short-term microbial responses to wetting and drying were studied, the highest CO₂ efflux took place at about 60% water-filled pore space (WFPS). At this moisture level, phospholipid fatty acids (PLFA) indicative of microbial nutrient availability were elevated and a PLFA stress indicator was depressed, suggesting peak microbial activity. The highest N₂O efflux in the organically managed soil (0.94 mg N₂O-N m⁻² h⁻¹) occurred after manure and legume cover crop incorporation, and in the conventionally managed soil (2.12 mg N₂O-N m⁻² h⁻¹) after inorganic N fertilizer inputs. Elevated N₂O emissions occurred at a WFPS >60% and lasted <2 days after wetting, probably because the top layer (0–150 mm) of this silt loam soil dried quickly. Therefore, in these cropping systems, irrigation management might control the duration of elevated N₂O efflux, even when C and inorganic N availability are high, whereas inorganic N concentrations should be kept low during times when soil moisture cannot be controlled.     

Spatial variability and biophysicochemical controls on N<Subscript>2</Subscript>O emissions from differently tilled arable soils

Nitrous oxide (N₂O) emissions, soil microbial community structure, bulk density, total pore volume, total C and N, aggregate mean weight diameter and stability index were determined in arable soils under three different types of tillage: reduced tillage (RT), no tillage (NT) and conventional tillage (CT). Thirty intact soil cores, each in a 25 × 25-m² grid, were collected to a depth of 10 cm at the seedling stage of winter wheat in February 2008 from Maulde (50°3′ N, 3°43′ W), Belgium. Two additional soil samples adjacent to each soil core were taken to measure the spatial variance in biotic and physicochemical conditions. The microbial community structure was evaluated by means of phospholipid fatty acids analysis. Soil cores were amended with 15 kg NO₃⁻-N ha⁻¹, 15 kg NH₄⁺-N ha⁻¹ and 30 kg ha⁻¹ urea-N ha⁻¹ and then brought to 65% water-filled pore space and incubated for 21 days at 15°C, with regular monitoring of N₂O emissions. The N₂O fluxes showed a log-normal distribution with mean coefficients of variance (CV) of 122%, 78% and 90% in RT, NT and CT, respectively, indicating a high spatial variation. However, this variability of N₂O emissions did not show plot scale spatial dependence. The N₂O emissions from RT were higher (p < 0.01) than from CT and NT. Multivariate analysis of soil properties showed that PC1 of principal component analysis had highest loadings for aggregate mean weight diameter, total C and fungi/bacteria ratio. Stepwise multiple regression based on soil properties explained 72% (p < 0.01) of the variance of N₂O emissions. Spatial distributions of soil properties controlling N₂O emissions were different in three different tillages with CV ranked as RT > CT > NT.     

Nitrogen mineralization and CO2 and N2O emissions in a sandy soil amended with original or acidified pig slurries or with the relative fractions

The following six pig slurries obtained after acidification and/or solid/liquid separation were used in the research: original (S) and acidified (AS) pig slurry, nonacidified (LF) and acidified (ALF) pig slurry liquid fraction, and nonacidified (SF) and acidified (ASF) pig slurry solid fraction. Laboratory incubations were performed to assess the effect of the application of these slurries on N mineralization and CO₂ and N₂O emissions from a sandy soil. Acidification maintained higher NH₄ ⁺-N contents in soil particularly in the ALF-treated soil where NH₄ ⁺-N contents were two times higher than in LF-treated soil during the 55–171-day interval. At the end of the incubation (171 days), 32.9 and 24.2 mg N kg⁻¹ dry soil were mineralized in the ASF- and SF-treated soils, respectively, but no mineralization occurred in LF- and S-treated soils, although acidification decreased N immobilization in ALF- (−25.3 mg N kg⁻¹ soil) and AS- (−12.7 mg N kg⁻¹ soil) compared to LF- (−34.4 mg N kg⁻¹ soil) and S-treated (−18.6 mg N kg⁻¹ soil) soils, respectively. Most of the dissolved CO₂ was lost during the acidification process. More than 90% of the applied C in the LF-treated soil was lost during the incubation, indicating a high availability of the added organic compounds. Nitrous oxide emissions occurred only after day 12 and at a lower rate in soils treated with acidified than nonacidified slurries. However, during the first 61 days of incubation, 1,157 μg N kg⁻¹ soil was lost as N₂O in the AS-treated soil and only 937 in the S-treated soil.     

Effects of long-term exposure to enriched CO<Subscript>2</Subscript> on the nutrient-supplying capacity of a grassland soil

Altered soil nutrient cycling under future climate scenarios may affect pasture production and fertilizer management. We conducted a controlled-environment study to test the hypothesis that long-term exposure of pasture to enriched carbon dioxide (CO₂) would lower soil nutrient availability. Perennial ryegrass was grown for 9 weeks under ambient and enriched (ambient + 120 ppm) CO₂ concentrations in soil collected from an 11.5-year free air CO₂ enrichment experiment in a grazed pasture in New Zealand. Nitrogen (N) and phosphorus (P) fertilizers were applied in a full factorial design at rates of 0, 12.5, 25 or 50 kg N ha⁻¹ and 0, 17.5 or 35 kg P ha⁻¹. Compared to ambient CO₂, under enriched CO₂ without P fertilizer, total plant biomass did not respond to N fertilizer, and tissue N/P ratio was increased indicating that P was co-limiting. This limitation was alleviated with the lowest rate of P fertilizer (17.5 kg P ha⁻¹). Plant biomass in both CO₂ treatments increased with increasing N fertilizer when sufficient P was available. Greater inputs of P fertilizer may be required to prevent yield suppression under enriched CO₂ and to stimulate any response to N.     

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.     

Comparison of two versions of the acetylene inhibition/soil core method for measuring denitrification loss from an irrigated wheat field

 Two versions of the acetylene inhibition (AI)/soil core method were compared for the measurement of denitrification loss from an irrigated wheat field receiving urea-N at a rate of 100 kg ha^–1. With AI/soil core method A, the denitrification rate was measured by analysing the headspace N₂O, followed by estimation of N₂O dissolved in the solution phase using Bunsen absorption coefficients. With AI/soil core method B, N₂O entrapped in the soil was measured in addition to that released from soil cores into the headspace of incubation vessels. In addition, the two methods were also compared for measurement of the soil respiration rate. Of the total N₂O produced, 6–77% (average 40%) remained entrapped in the soil, whereas for CO₂, the corresponding figures ranged from 12–65% (average 44%). The amount of the entrapped N₂O was significantly correlated with the water-filled pore space (WFPS) and with the N₂O concentration in the headspace, whereas CO₂ entrapment was dependent on the headspace CO₂ concentration but not on the WFPS. Due to the entrapment of N₂O and CO₂ in soil, the denitrification rate on several (18 of the 41) sampling dates, and soil respiration rate on almost all (27 of the 30) sampling dates were significantly higher with method B compared to method A. Averaged across sampling dates, the denitrification rate measured with method B (0.30 kg N ha^–1 day^–1) was twice the rate measured with method A, whereas the soil respiration rate measured with method B (34.9 kg C ha^–1 day^–1) was 1.6 times the rate measured with method A. Results of this study suggest that the N₂O and CO₂ entrapped in soil should also be measured to ensure the recovery of the gaseous products of denitrification by the soil core method. Received: 12 May 1998     

Secondary salinity effects on soil microbial biomass

Secondary soil salinilization is a big problem in irrigated agriculture. We have studied the effects of irrigation-induced salinity on microbial biomass of soil under traditional cotton (Gossypium hirsutum L.) monoculture in Sayhunobod district of the Syr-Darya province of northwest Uzbekistan. Composite samples were randomly collected at 0–30 cm depth from weakly saline (2.3 ± 0.3 dS m⁻¹), moderately saline (5.6 ± 0.6 dS m⁻¹), and strongly saline (7.1 ± 0.6 dS m⁻¹) replicated fields, 2-mm sieved, and analyzed for pH, electrical conductivity, total C, organic C (C_Org), and extractable C, total N and P, and exchangeable ions (Ca²⁺, Mg²⁺, K⁺, Na⁺, Cl⁻, and CO₃²⁻), microbial biomass (C_mic). The Na⁺ and Cl⁻ concentrations were 36-80% higher in strongly saline compared to weakly saline soil. The C_Org concentration was decreased by 10% and C_Ext by 40% by increasing soil salinity, whereas decrease in C_mic ranged from 18-42% and the percentage of C_Org present as C_mic from 8% to 26%. We conclude that irrigation-induced secondary salinity significantly affects soil chemical properties and the size of soil microflora.     

Effects of biochar addition on N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> emissions from two paddy soils

Impacts of biochar addition on nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions from paddy soils are not well documented. Here, we have hypothesized that N₂O emissions from paddy soils could be depressed by biochar incorporation during the upland crop season without any effect on CO₂ emissions. Therefore, we have carried out the 60-day aerobic incubation experiment to investigate the influences of rice husk biochar incorporation (50 t ha⁻¹) into two typical paddy soils with or without nitrogen (N) fertilizer on N₂O and CO₂ evolution from soil. Biochar addition significantly decreased N₂O emissions during the 60-day period by 73.1% as an average value while the inhibition ranged from 51.4% to 93.5% (P < 0.05–0.01) in terms of cumulative emissions. Significant interactions were observed between biochar, N fertilizer, and soil type indicating that the effect of biochar addition on N₂O emissions was influenced by soil type. Moreover, biochar addition did not increase CO₂ emissions from both paddy soils (P > 0.05) in terms of cumulative emissions. Therefore, biochar can be added to paddy fields during the upland crop growing season to mitigate N₂O evolution and thus global warming.     

Nitrous oxide emission from feedlot manure and green waste compost applied to Vertisols

Application of feedlot manure (FLM) to cropping and grazing soils could provide a valuable N nutrient resource. However, because of its high but variable N concentration, FLM has the potential for environmental pollution of water bodies and N₂O emission to the atmosphere. As a potential management tool, we utilised the low-nutrient green waste compost (GWC) to assess its effectiveness in regulating N release and the amount of N₂O emission from two Vertisols when both FLM and GWC were applied together. Cumulative soil N₂O emission over 32 weeks at 24°C and field capacity (70% water-filled pore space) for a black Vertisol (Udic Paleustert) was 45 mg N₂O m⁻² from unamended soil. This increased to 274 mg N₂O m⁻² when FLM was applied at 1 kg m⁻² and to 403 mg N₂O m⁻² at 2 kg m⁻². In contrast, the emissions of 60 mg N₂O m⁻² when the soil was amended with GWC 1 kg m⁻² and 48 mg N₂O m⁻² at 2 kg m⁻² were not significantly greater than the unamended soil. Emission from a mixture of FLM and GWC applied in equal amounts (0.5 kg m⁻²) was 106 mg N₂O m⁻² and FLM applied at 0.5 kg m⁻² and GWC at 1.5 kg GWC m⁻² was 117 mg N₂O m⁻². Although cumulative N₂O emissions from an unamended grey Vertisol (Typic Chromustert) were only slightly higher than black Vertisol (57 mg N₂O m⁻²), FLM application at 1 kg m⁻² increased N₂O emissions by 14 times (792 mg N₂O m⁻²) and at 2 kg m⁻² application by 22 times (1260 mg N₂O m^-2). Application of GWC did not significantly increase N₂O emission (99 mg N₂O m⁻² at 1 kg m⁻² and 65 mg N₂O m⁻² at 2 kg m⁻²) above the unamended soil. As observed for the black Vertisol, a mixture of FLM (0.5 kg m⁻²) and GWC (0.5 or 1.5 kg m⁻²) reduced N₂O emission by >50% of that from the FLM alone, most likely by reducing the amount of mineral N (NH₄⁺–N and NO₃⁻–N) in the soil, as mineral N in soil and the N₂O emission were closely correlated.     

Carbon and nitrogen mineralization dynamics in different soils of the tropics amended with legume residues and contrasting soil moisture contents

Seasonal drought in tropical agroecosystems may affect C and N mineralization of organic residues. To understand this effect, C and N mineralization dynamics in three tropical soils (Af, An₁, and An₂) amended with haricot bean (HB; Phaseolus vulgaris L.) and pigeon pea (PP; Cajanus cajan L.) residues (each at 5 mg g⁻¹ dry soil) at two contrasting soil moisture contents (pF2.5 and pF3.9) were investigated under laboratory incubation for 100–135 days. The legume residues markedly enhanced the net cumulative CO₂–C flux and its rate throughout the incubation period. The cumulative CO₂–C fluxes and their rates were lower at pF3.9 than at pF2.5 with control soils and also relatively lower with HB-treated than PP-treated soil samples. After 100 days of incubation, 32–42% of the amended C of residues was recovered as CO₂–C. In one of the three soils (An₁), the results revealed that the decomposition of the recalcitrant fraction was more inhibited by drought stress than easily degradable fraction, suggesting further studies of moisture stress and litter quality interactions. Significantly (p < 0.05) greater NH₄⁺–N and NO₃⁻–N were produced with PP-treated (C/N ratio, 20.4) than HB-treated (C/N ratio, 40.6) soil samples. Greater net N mineralization or lower immobilization was displayed at pF2.5 than at pF3.9 with all soil samples. Strikingly, N was immobilized equivocally in both NH₄⁺–N and NO₃⁻–N forms, challenging the paradigm that ammonium is the preferred N source for microorganisms. The results strongly exhibited altered C/N stoichiometry due to drought stress substantially affecting the active microbial functional groups, fungi being dominant over bacteria. Interestingly, the results showed that legume residues can be potential fertilizer sources for nutrient-depleted tropical soils. In addition, application of plant residue can help to counter the N loss caused by leaching. It can also synchronize crop N uptake and N release from soil by utilizing microbes as an ephemeral nutrient pool during the early crop growth period.     

Influence of different agricultural practices (type of crop, form of N-fertilizer) on soil nitrous oxide emissions

 N₂O emissions were periodically measured using the static chamber method over a 1-year period in a cultivated field subjected to different agricultural practices including the type of N fertilizer (NH₄NO₃, (NH₄)₂SO₄, CO(NH₂)₂ or KNO₃ and the type of crop (rapeseed and winter wheat). N₂O emissions exhibited the same seasonal pattern whatever the treatment, with emissions between 1.5 and 15 g N ha^–1 day^–1 during the autumn, 16–56 g N ha^–1 day^–1 in winter after a lengthy period of freezing, 0.5–70 g N ha^–1 day^–1 during the spring and lower emissions during the summer. The type of crop had little impact on the level of N₂O emission. These emissions were a little higher under wheat during the autumn in relation to an higher soil NO₃ ^– content, but the level of emissions was similar over a 7-month period (2163 and 2093 g N ha^–1 for rape and wheat, respectively). The form of N fertilizer affected N₂O emissions during the month following fertilizer application, with higher emissions in the case of NH₄NO₃ and (NH₄)₂SO₄, and a different temporal pattern of emissions after CO(NH₂)₂ application. The proportion of applied N lost as N₂O varied from 0.42% to 0.55% with the form of N applied, suggesting that controlling this agricultural factor would not be an efficient way of limiting N₂O emissions under certain climatic and pedological situations. Received: 1 December 1997     

Impact of elevated CO<Subscript>2</Subscript>, flooding,and temperature interaction on heterotrophic nitrogen fixation in tropical rice soils

Response of N₂ fixation to elevated CO₂ would be modified by changes in temperature and soil moisture because CO₂ and temperature or water availability has generally opposing effects on N₂ fixation. In this study, we assessed the impacts of elevated CO₂ and temperature interactions on nitrogenase activities, readily mineralizable C (RMC), readily available N (NRN) contents in an alluvial and a laterite rice soil of tropical origin. Soil samples were incubated at ambient (370 μmol mol^-1) and elevated (600 μmol mol^-1) CO₂ concentration at 25oC, 35oC, and 45oC under non-flooded and flooded conditions for 60 days. Elevated CO₂ significantly increased nitrogenase activities and readily mineralizable C in both alluvial and laterite soils. All these activities were further stimulated at higher temperatures. Increases in nitrogenase activity as a result of CO₂ enrichment effect over control were 16.2%, 31.2%, and 66.4% and those of NRN content were 2.0%, 1.8%, and 0.5% at 25oC, 35oC and 45oC, respectively. Increases in RMC contents were 7.7%, 10.0%, and 10.6% at 25°C, 35°C and 45°C, respectively. Soil flooding resulted in a more clear impact of CO₂ enrichment than the non-flooded soil. The results suggest that in tropical rice soils, elevated CO₂ increased readily available C content in the soil, which probably stimulates growth of diazotrophic bacteria with enhanced N₂ fixation and thereby higher available N.     

Effects of rainfall pattern on carbon and nitrogen dynamics in soil amended with biogas slurry and composted cattle manure

Soil moisture affects the degradation of organic fertilizers in soils considerably, but less is known about the importance of rainfall pattern on the turnover of C and N. The objective of this study was to determine the effects of different rainfall patterns on C and N dynamics in soil amended with either biogas slurry (BS) or composted cattle manure (CM). Undisturbed soil cores without (control) or with BS or CM, which were incorporated at a rate of 100 kg N ha^–1, were incubated for 140 d at 13.5°C. Irrigation treatments were (1) continuous irrigation (cont_irr; 3 mm d^–1); (2) partial drying and stronger irrigation (part_dry; no irrigation for 3 weeks, 1 week with 13.5 mm d^–1), and (3) periodic heavy rainfall (hvy_rain; 24 mm d^–1 every 3 weeks for 1 d and 2 mm d^–1 for the other days). The average irrigation was 3 mm d^–1 in each treatment. Cumulative emissions of CO₂ and N₂O from soils amended with BS were 92.8 g CO₂‐C m^–2 and 162.4 mg N₂O‐N m^–2, respectively, whereas emissions from soils amended with CM were 87.8 g CO₂‐C m^–2 and only 38.9 mg N₂O‐N m^–2. While both organic fertilizers significantly increased CO₂ production compared to the control, N₂O emissions were only significantly increased in the BS‐amended soil. Under the conditions of the experiment, the rainfall pattern affected the temporal production of CO₂ and N₂O, but not the cumulative emissions. Cumulative NO leaching was highest in the BS‐amended soils (9.2 g NO ‐N m^–2) followed by the CM‐amended soil (6.1 g NO ‐N m^–2) and lowest in the control (4.7 g NO ‐N m^–2). Nitrate leaching was also independent of the rainfall pattern. Our study shows that rainfall pattern may not affect CO₂ and N₂O emissions and NO leaching markedly provided that the soil does not completely dry out.     

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.     

Soil N<Subscript>2</Subscript>O emissions and N<Subscript>2</Subscript>O/(N<Subscript>2</Subscript>O+N<Subscript>2</Subscript>) ratio as affected by different fertilization practices and soil moisture

The objective of this work was to evaluate the effect of the chemical nature and application frequency of N fertilizers at different moisture contents on soil N₂O emissions and N₂O/(N₂O+N₂) ratio. The research was based on five fertilization treatments: unfertilized control, a single application of 80 kg ha⁻¹ N-urea, five split applications of 16 kg ha⁻¹ N-urea, a single application of 80 kg ha⁻¹ N–KNO₃, five split applications of 16 kg ha⁻¹ N–KNO₃. Cumulative N₂O emissions for 22 days were unaffected by fertilization treatments at 32% water-filled pore space (WFPS). At 100% and 120% WFPS, cumulative N₂O emissions were highest from soil fertilized with KNO₃. The split application of N fertilizers decreased N₂O emissions compared to a single initial application only when KNO₃ was applied to a saturated soil, at 100% WFPS. Emissions of N₂O were very low after the application of urea, similar to those found at unfertilized soil. Average N₂O/(N₂O+N₂) ratio values were significantly affected by moisture levels (p = 0.015), being the lowest at 120% WFPS. The N₂O/(N₂O+N₂) ratio averaged 0.2 in unfertilized soil and 0.5 in fertilized soil, although these differences were not statistically significant.     

Evaluation of biological and chemical nitrogen indices for predicting nitrogen-supplying capacity of paddy soils in the Taihu Lake region,China

Simple and rapid chemical indices of soil nitrogen (N)-supplying capacity are necessary for fertilizer recommendations. In this study, pot experiment involving rice, anaerobic incubation, and chemical analysis were conducted for paddy soils collected from nine locations in the Taihu Lake region of China. The paddy soils showed large variability in N-supplying capacity as indicated by the total N uptake (TNU) by rice plants in a pot experiment, which ranged from 639.7 to 1,046.2 mg N pot⁻¹ at maturity stage, representing 5.8% of the total soil N on average. Anaerobic incubation for 3, 14, 28, and 112 days all resulted in a significant (P < 0.01) correlation between cumulative mineral NH₄⁺-N and TNU, but generally better correlations were obtained with increasing incubation time. Soil organic C, total soil N, microbial C, and ultraviolet absorbance of NaHCO₃ extract at 205 and 260 nm revealed no clear relationship with TNU or cumulative mineral NH₄⁺-N. Soil C/N ratio, acid KMnO₄-NH₄⁺-N, alkaline KMnO₄-NH₄⁺-N, phosphate–borate buffer extractable NH₄⁺-N (PB-NH₄⁺-N), phosphate–borate buffer hydrolyzable NH₄⁺-N (PB_HYDR-NH₄⁺-N) and hot KCl extractable NH₄⁺-N (H_KCl−NH₄⁺-N) were all significantly (P < 0.05) related to TNU and cumulative mineral NH₄⁺-N of long-term incubation (>28 days). However, the best chemical index of soil N-supplying capacity was the soil C/N ratio, which showed the highest correlation with TNU at maturity stage (R = −0.929, P < 0.001) and cumulative mineral NH₄⁺-N (R = −0.971, P < 0.001). Acid KMnO₄-NH₄⁺-N plus native soil NH₄⁺-N produced similar, but slightly worse predictions of soil N-supplying capacity than the soil C/N ratio.     

Denitrification from the horticultural peats: effects of pH,nitrogen, carbon,and moisture contents

Denitrification plays an important role in N-cycling. However, information on the rates of denitrification from horticultural growing media is rare in literature. In this study, the effects of pH, N, C, and moisture contents on denitrification were investigated using four moderately decomposed peat types (oligotrophic, mesotrophic, eutrophic, and transitional). Basal and potential denitrification rates (20°C, 18 h) from the unlimed peat samples varied widely from 2.0 to 21.8 and from 118.9 to 306.6 μg (N₂O + N₂)–N L⁻¹ dry peat h⁻¹, respectively, with the highest rates from the eutrophic peat and the lowest from the transitional one. Both basal and potential denitrification rates were substantially increased by 3.6–14- and 1.4–2.3-fold, respectively, when the initial pH (4.3–4.8) was raised to 5.9–6.5 units. Emissions of (N₂O + N₂)–N from oligotrophic, mesotrophic, and transitional peats were markedly increased by the addition of 0.15 g NO₃–N L⁻¹ dry peat but further additions had no effect. Denitrification rates were increased by increasing glucose concentration suggesting that the activity of denitrifiers in all peat types was limited by the low availability of easily decomposable C source. Increasing moisture contents of all peats from 40 to 50% water-filled pore space (WFPS) did not significantly (p > 0.05) increase (N₂O + N₂)–N emissions. However, a positive effect was observed when the moisture contents were increased from 60% to 70% WFPS in the eutrophic peat, from 70% to 80% in the transitional, from 80% to 90% in the oligotrophic and from 70% to 90% in the mesotrophic peat. It can be concluded that liming, N-fertilization, availability of easily decomposable C, and moist condition above 60% WFPS could encourage denitrification from peats although the rates are greatly influenced by the peat-forming environments (eutrophic > mesotrophic > oligotrophic > transitional types).