Soil‐microbial response to sugarcane filter cake and biogenic waste compost

An incubation experiment was carried out to examine the N‐immobilizing effect of sugarcane filter cake (C : N = 12.4) and to prove whether mixing it with compost (C : N = 10.5) has any synergistic effects on C and N mineralization after incorporation into the soil. Approximately 19% of the compost‐C added and 37% of the filter cake–C were evolved as CO₂, assuming that the amendments had no effects on the decomposition of soil organic C. However, only 28% of the added filter cake was lost according to the total‐C and δ¹³C values. Filter cake and compost contained initially significant concentrations of inorganic N, which was nearly completely immobilized between day 7 and 14 of the incubation in most cases. After day 14, N remineralization occurred at an average rate of 0.73 µg N (g soil)^–1 d^–1 in most amendment treatments, paralleling the N mineralization rate of the nonamended control without significant difference. No significant net N mineralization from the amendment N occurred in any of the amendment treatments in comparison to the control. The addition of compost and filter cake resulted in a linear increase in microbial biomass C with increasing amounts of C added. This increase was not affected by differences in substrate quality, especially the three times larger content of K₂SO₄‐extractable organic C in the sugarcane filter cake. In most amendment treatments, microbial biomass C and biomass N increased until the end of the incubation. No synergistic effects could be observed in the mixture treatments of compost and sugarcane filter cake.     

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.     

Carbon mineralization in saline soils as affected by residue composition and water potential

In saline soils under semi-arid climate, low matric and osmotic potential are the main stressors for microbes. But little is known about the impact of water potential (sum of matric and osmotic potential) and substrate composition on microbial activity and biomass in field collected saline soils. Three sandy loam soils with electrical conductivity of the saturated soil extract (EC_e) 3.8, 11 and 21 dS m^?1 (hereafter referred to EC3.8, EC11 and EC21) were kept at optimal water content for 14 days. After this pre-incubation, the soils were either left at optimal water content or dried to achieve water potentials of ?2.33, ?2.82, ?3.04 and ?4.04 MPa. Then, the soils were amended with 20 g?kg^?1 pea or wheat residue to increase nutrient supply. Carbon dioxide emission was measured over 14 days; microbial biomass C was measured at the end of the experiment. Cumulative respiration decreased with decreasing water potential and was significantly (P?<?0.05) lower in soils at water potential ?4 MPa than in soils at optimal water content. The effect of residue type on the response of cumulative respiration was inconsistent; with residue type having no effect in the saline soils (EC11 and EC21) whereas in the non-saline soil (EC3.8), the decrease in respiration with decreasing water potential was less with wheat than with pea residue. At a given water potential, the absolute and relative (in percentage of optimal water content) cumulative respiration was lower in the saline soils than in the non-saline soil. This can be explained by the lower osmotic potential and the smaller microbial biomass in the saline soils. However, even at a similar osmotic potential, cumulative respiration was higher in the non-saline soil. It can be concluded that high salt concentrations in the soil solution strongly reduce microbial activity even if the water content is relatively high. The stronger relative decrease in microbial activity in the saline soils at a given osmotic potential compared to the non-saline soil suggests that the small biomass in saline soils is less able to tolerate low osmotic potential. Hence, drying of soil will have a stronger negative effect on microbial activity in saline than in non-saline soils.     

Microbial use of organic amendments in saline soils monitored by changes in the C/C ratio

An incubation experiment was carried out to investigate whether salinity at high pH has negative effects on microbial substrate use, i.e. the mineralization of the amendment to CO₂ and inorganic N and the incorporation of amendment C into microbial biomass C. In order to exploit natural differences in the ¹³C/¹²C ratio, substrate from two C₄ plants, i.e. highly decomposed and N-rich sugarcane filter cake and less decomposed N-poor maize leaf straw, were added to two alkaline Pakistani soils differing in salinity, which had previously been cultivated with C₃ plants. In soil 1, the additional CO₂ evolution was equivalent to 65% of the added amount in the maize straw treatment and to 35% in the filter cake treatment. In the more saline soil 2, the respective figures were 56% and 32%. The maize straw amendment led to an identical immobilization of approximately 48 μg N g⁻¹ soil over the 56-day incubation in both soils compared with the control soils. In the filter cake treatment, the amount of inorganic N immobilized was 8.5 μg N g⁻¹ higher in soil 1 than in soil 2 compared with the control soils. In the control treatment, the content of microbial biomass C₃-C in soil 1 was twice that in soil 2 throughout the incubation. This fraction declined by about 30% during the incubation in both soils. The two amendments replaced initially similar absolute amounts of the autochthonous microbial biomass C, i.e. 50% of the original microbial biomass C in soil 1 and almost 90% in soil 2. The highest contents of microbial biomass C₄-C were equivalent to 7% (filter cake) and 11% (maize straw) of the added C. In soil 2, the corresponding values were 14% lower. Increasing salinity had no direct negative effects on microbial substrate use in the present two soils. Consequently, the differences in soil microbial biomass contents are most likely caused indirectly by salinity-induced reduction in plant growth rather than directly by negative effects of salinity on soil microorganisms.     

Response of microbial activity and biomass to increasing salinity depends on the final salinity,not the original salinity

Salinization is a global land degradation issue which inhibits microbial activity and plant growth. The effect of salinity on microbial activity and biomass has been studied extensively, but little is known about the response of microbes from different soils to increasing salinity although soil salinity may fluctuate in the field, for example, depending on the quality of the irrigation water or seasonally. An incubation experiment with five soils (one non-saline, four saline with electrical conductivity (EC_e) ranging from 1 to 50 dS m⁻¹) was conducted in which the EC was increased to 37 EC_e levels (from 3 to 119 dS m⁻¹) by adding NaCl. After amendment with 2% (w/w) pea straw to provide a nutrient source, the soils were incubated at optimal water content for 15 days, microbial respiration was measured continuously and chloroform-labile C was determined every three days. Both cumulative respiration and microbial biomass (indicated by chloroform-labile C) were negatively correlated with EC. Irrespective of the original soil EC, cumulative respiration at a given adjusted EC was similar. Thus, microorganisms from previously saline soils were not more tolerant to a given adjusted EC than those in originally non-saline soil. Microbial biomass in all soils increased from day 0 to day 3, then decreased. The relative increase was greater in soils which had a lower microbial biomass on day 0 (which were more saline). Therefore the relative increase in microbial biomass appears to be a function of the biomass on day 0 rather than the EC. Hence, the results suggest that microbes from originally saline soils are not more tolerant to increases in salinity than those from originally non-saline soils. The strong increase in microbial biomass upon pea straw addition suggests that there is a subset of microbes in all soils that can respond to increased substrate availability even in highly saline environments.     

Reversing agriculture from intensive to sustainable improves soil quality in a semiarid South Italian soil

Intensive agriculture (IA) is widespread in South Italy, although it requires frequent tillage, large amounts of fertilizers and irrigation water. We have assessed the efficacy of reversing IA to sustainable agriculture (SA) in recovering quality of a typical South Italy soil (Lithic Haploxeralf). This reversion, lasting from 2000 to 2007, replaced 75% of nutrients formerly supplied inorganically by farmyard manuring and reduced the tillage frequency. Several chemical and biochemical properties, functionally related to C and N mineralisation–immobilisation processes and to P and S nutrient cycles, were monitored annually from 2005 to 2007 in the spring. Reversing IA to SA decreased soil bulk density, almost doubled the soil organic matter (SOM) as favoured the immobilisation of C and N, increased most soil microbial indicators but decreased contents of nitrate, mineral N and K₂SO₄-extractable C. The K₂SO₄-extractable C/K₂SO₄-extractable organic N ratio suggested that substrate quality rather than the mass of readily available C and N affected biomass and activity of soil microflora. Also, the largely higher 10-day-evolved CO₂–C-to-inorganic N ratio under SA than IA indicated that higher C mineralisation, associated with higher microbial biomass N immobilisation, occurred under SA than IA. Decreases in most soil enzyme activities under IA, compared to SA, were much higher than concomitant decreases in SOM content. Soil salinity and sodicity were always higher in IA than SA soil, although not critically high, likely due to the intensive inorganic fertilisation as irrigation waters were qualitatively and quantitatively the same between the two soils. Thus, we suggest that the cumulative small but long-term saline (osmotic) and sodic (dispersing) effects in IA soil decreased the microbial variables more than total organic C and increased soil bulk density.     

Short-term N availability in response to dissolved-organic-carbon from poultry manure, alone or in combination with cellulose

 Short-term changes in N availability in a sandy soil in response to the dissolved organic carbon (DOC) from a poultry manure (application rate equivalent to approximately 250 kg N ha^–1) were evaluated in a 44-day aerobic incubation experiment. The treatments included poultry manure alone and two treatments in which an extra source of C, of low water solubility, was added with the poultry manure in the form of a low (1.05 g kg^–1) and a high (4.22 g kg^–1) amount of cellulose. All treatments were fertilised with the equivalent of 60 kg N ha^–1 of (¹⁵NH₄)₂SO₄ in solution. A control treatment consisted of sieved field-moist soil plus 60 kg N ha^–1 of (¹⁵NH₄)₂SO₄ in solution. Measurements were made of N₂O and CO₂ emissions, inorganic N, DOC, biomass N, biomass C and labelled N contained in the inorganic N and biomass N pools. The dynamics of N turnover in this study were driven mainly by processes of mineralisation–immobilisation with little significant loss of N by volatilisation or denitrification. The DOC supplied with the poultry manure played a more important role in N₂O emissions than differences in C/N ratio. Changes in DOC and cumulative CO₂-C production during the first 11 days were also highly correlated (R ²=0.88–0.66, P<0.01). An initial net immobilisation of N, with significant increases in biomass C and biomass N (P<0.05) for all treatments over the control at day 11, indicated a high availability of C from the DOC fraction. The presence of additional C from the applied cellulose did not enable a massive N immobilisation. Total inorganic N and unlabelled inorganic N concentrations were highest in soils treated with poultry manure alone (P<0.05), indicating that an active gross mineralisation of the added poultry manure and a possible positive priming effect were taking place during the incubation. Received: 29 May 1998     

Gross fluxes of nitrogen in grassland soil exposed to elevated atmospheric pCO2 for seven years

Plant response to increasing atmospheric CO₂ partial pressure (pCO₂) depends on several factors, one of which is mineral nitrogen availability facilitated by the mineralisation of organic N. Gross rates of N mineralisation were examined in grassland soils exposed to ambient (36 Pa) and elevated (60 Pa) atmospheric pCO₂ for 7 years in the Swiss Free Air Carbon dioxide Enrichment experiment. It was hypothesized that increased below-ground translocation of photoassimilates at elevated pCO₂ would lead to an increase in immobilisation of N due to an excess supply of energy to the roots and rhizosphere. Intact soil cores were sampled from Lolium perenne and Trifolium repens swards in May and September, 2000. The rates of gross N mineralisation (m) and NH₄⁺ consumption (c) were determined using ¹⁵N isotopic dilution during a 51-h period of incubation. The rates of N immobilisation were estimated either as the difference between m and the net N mineralisation rate or as the amount of ¹⁵N released from the microbial biomass after chloroform fumigation. Soil samples from both swards showed that the rates of gross N mineralisation and NH₄⁺ consumption did not change significantly under elevated pCO₂. The lack of a significant effect of elevated pCO₂ on organic N turnover was consistent with the similar size of the microbial biomass and similar immobilisation of applied ¹⁵N in the microbial N pool under ambient and elevated pCO₂. Rates of m and c, and microbial ¹⁵N did not differ significantly between the two sward types although a weak (p<0.1) pCO₂ by sward interaction occurred. A significantly larger amount of NO₃⁻ was recovered at the end of the incubation in soil taken from T. repens swards compared to that from L. perenne swards. Eleven percent of the added ¹⁵N were recovered in the roots in the cores sampled under L. perenne, while only 5% were recovered in roots of T. repens. These results demonstrate that roots remained a considerable sink despite the shoots being cut at ground level prior to incubation and suggest that the calculation of N immobilisation from gross and net rates of mineralisation in soils with a high root biomass does not reflect the actual immobilisation of N in the microbial biomass. The results of this study did not support the initial hypothesis and indicate that below-ground turnover of N, as well as N availability, measured in short-term experiments are not strongly affected by long-term exposure to elevated pCO₂. It is suggested that differences in plant N demand, rather than major changes in soil N mineralisation/immobilisation, are the long-term driving factors for N dynamics in these grassland systems.     

Carbon and nitrogen mineralization in acidic, limed and calcareous agricultural soils: Apparent and actual effects

Soil pH and calcium carbonate contents are often hypothesized to be important factors controlling organic matter turnover in agricultural soils. The aim of this study was to differentiate the effects of soil pH from those related to carbonate equilibrium on C and N dynamics. The relative contributions of organic and inorganic carbon in the CO₂ produced during laboratory incubations were assessed. Five agricultural soils were compared: calcareous (74% CaCO₃), loess (0.2% CaCO₃) and an acidic soil which had received different rates of lime 20 years ago (0, 18 or 50 t ha⁻¹). Soil aggregates were incubated with or without rape residues under aerobic conditions for 91 days at 15 °C. The C and N mineralized, soil pH, O₂ consumption and respiratory quotient (RQ=ΔCO₂/ΔO₂) were monitored, as well as the δ¹³C composition of the evolved CO₂ to determine its origin (mineral or organic). Results showed that in non-amended soils, the cumulative CO₂ produced was significantly greater in the limed soil with a pH>7 than in the same soil with less or no lime added, whereas there was no difference in N mineralization or in O₂ consumption kinetics. We found an exponential relationship between RQ values and soil pH, suggesting an excess production of CO₂ in alkaline soils. This CO₂ excess was not related to changes in substrate utilization by the microbial biomass but rather to carbonates equilibrium. The δ¹³C signatures confirmed that the CO₂ produced in soils with pH>7 originated from both organic and mineral sources. The contribution of soil carbonates to CO₂ production led to an overestimation of organic C mineralization (up to 35%), the extent of which depended on the nature of soil carbonates but not on the amount. The actual C mineralization (derived from organic C) was similar in limed and unlimed soil. The amount of C mineralized in the residue-amended soils was ten times greater than in the basal soil, thus masking the soil carbonate contribution. Residue decomposition resulted in a significant increase in soil pH in all soils. This increase is attributed to the alkalinity and/or decarboxylation of organic anions in the plant residue and/or to the immobilization of nitrate by the microbial biomass and the corresponding release of hydroxyl ions. A theoretical composition (C, O, H, N) of residue and soil organic matter is proposed to explain the RQ measured. It emphasizes the need to take microbial biomass metabolism, O₂ consumption due to nitrification and carbon assimilation yield into account when interpreting RQ data.     

Carbon mineralization in subtropical alluvial arable soils amended with sugarcane bagasse and rice husk biochars

Subtropical recent alluvial soils are low in organic carbon (C). Thus, increasing organic C is a major challenge to sustain soil fertility. Biochar amendment could be an option as biochar is a C-rich pyrolyzed material, which is slowly decomposed in soil. We investigated C mineralization (CO₂-C evolution) in two types of soils (recent and old alluvial soils) amended with two feedstocks (sugarcane bagasse and rice husk) (1%, weight/weight), as well as their biochars and aged biochars under a controlled environment (25 ±2 ℃) over 85 d. For the recent alluvial soil (charland soil), the highest absolute cumulative CO₂-C evolution was observed in the sugarcane bagasse treatment (1 140 mg CO₂-C kg^-1 soil) followed by the rice husk treatment (1 090 mg CO₂-C kg^-1 soil); the lowest amount (150 mg CO₂-C kg^-1 soil) was observed in the aged rice husk biochar treatment. Similarly, for the old alluvial soil (farmland soil), the highest absolute cumulative CO₂-C evolution (1 290 mg CO₂-C kg^-1 soil) was observed in the sugarcane bagasse treatment and then in the rice husk treatment (1 270 mg CO₂-C kg^-1 soil); the lowest amount (200 mg CO₂-C kg^-1 soil) was in the aged rice husk biochar treatment. Aged sugarcane bagasse and rice husk biochar treatments reduced absolute cumulative CO₂-C evolution by 10% and 36%, respectively, compared with unamended recent alluvial soil, and by 10% and 18%, respectively, compared with unamended old alluvial soil. Both absolute and normalized C mineralization were similar between the sugarcane bagasse and rice husk treatments, between the biochar treatments, and between the aged biochar treatments. In both soils, the feedstock treatments resulted in the highest cumulative CO₂-C evolution, followed by the biochar treatments and then the aged biochar treatments. The absolute and normalized CO₂-C evolution and the mineralization rate constant of the stable C pool (K_s) were lower in the recent alluvial soil compared with those in the old alluvial soil. The biochars and aged biochars had a negative priming effect in both soils, but the effect was more prominent in the recent alluvial soil. These results would have good implications for improving organic matter content in organic C-poor alluvial soils.     

Salinity reduces the ability of soil microbes to utilise cellulose

An incubation experiment was conducted to determine the response of soil microbial biomass and activity to salinity when supplied with two different carbon forms. One nonsaline and three saline soils of similar texture (sandy clay loam) with electrical conductivities of the saturation extract (EC_e) of 1, 11, 24 and 43 dS m^?1 were used. Carbon was added at 2.5 and 5 g C kg^?1 (2.5C, 5C) as glucose or cellulose; soluble N and P were added to achieve a C/N ratio of 20 and C/P ratio of 200. Soil microbial activity was assessed by measuring CO₂ evolution continuously for 3 weeks; microbial biomass C and available N and P were determined on days 2, 7, 14 and 21. In all soils, cumulative respiration was higher with 5C than with 2.5C and higher with glucose than with cellulose. Cumulative respiration was highest in the nonsaline soil and decreased with increasing EC, whereas the decrease was gradual with glucose, there was a sharp drop in cumulative respiration with cellulose from the nonsaline soil to soil with EC11 with little further decrease at higher ECs. Microbial biomass C and available N and P concentrations were highest in the nonsaline soil but did not differ among the saline soils. Microbial biomass C was higher and available N was lower with 5C than with 2.5C. The C form affected the temporal changes of microbial biomass and available nutrients differentially. With glucose, microbial biomass was highest on day 2 and then decreased, whereas available N showed the opposite pattern, being lowest on day 2 and then increasing. With cellulose, microbial biomass C increased gradually over time, and available N decreased gradually. It is concluded that salinity reduced the ability of microbes to decompose cellulose more than that of glucose.     

Interactions of mustard plants and soil microorganisms after application of sugarcane filter cake and pea residues to an Andosol

In a pot experiment using a strongly P‐fixing Andosol from Nicaragua, the effects of sugarcane–filter cake application on the growth of white mustard (Sinapis alba L.) were compared with those of ¹³C‐labeled pea residues. The application of pea residues led to a 50% increase and the application of filter cake to a 30% decrease in soil organic matter–derived microbial biomass C compared with the control. In contrast, the application of filter cake resulted in a four times higher content of substrate‐derived microbial biomass C than that of pea residues. The application of organic substrates generally increased microbial biomass N. Mustard growth led to significant increases in microbial biomass P in the control, but also in the organic‐amendment treatments, which always resulted in decreased microbial biomass C : P ratios. Mustard growth also led to increased contents of Bray‐1‐extractable P, but this increase was only significant in the filter cake treatment. The application of pea residues had no effect on the yield of shoot C, but a positive effect on the yield of root C in comparison with the nonamended control. In contrast, the application of filter cake significantly depressed yields of shoot C and root C, due to N immobilization, presumably due to the high concentration of lignin.     

Multi-objective optimisation of a model of the decomposition of animal slurry in soil: Tradeoffs between simulated C and N dynamics

To formulate best management practices for animal slurry, it is important to understand and predict its decomposition in the soil. Slurry decomposition dynamics can be studied by measuring CO₂ fluxes and soil mineral nitrogen concentration during laboratory incubations and subsequently calibrating a simulation model. Carbon and nitrogen dynamics are linked and both should be properly simulated. In this work we wanted to identify the tradeoffs between errors in the simulation of C respiration and of soil inorganic N concentration.We optimised six parameters of CN-SIM (a mechanistic dynamic simulation model), using data of respired C and soil inorganic N measured during a 180-day laboratory incubation of five dairy slurries on three soils. Optimisation was carried out with a multi-objective genetic algorithm (NSGA-II), by minimising the Relative Root Mean Squared Error (RRMSE) between observations and simulations.The simulation of C respiration was frequently conflicting with the simulation of inorganic N, i.e. low RRMSE–CO₂ corresponded with high RRMSE–N and vice versa. When minimising RRMSE–CO₂ a set of parameters was obtained that enhanced microbial N immobilisation and reduced the turnover of the organic pools, to match the observed decrease of inorganic N in the 28 days after slurry addition to soil. Remineralisation occurring in the following 150 days caused a marked overestimation of inorganic N. When minimising RRMSE–N, the optimisation provided parameters that strongly reduced remineralisation of immobilised N by markedly diminishing C respiration, with a consequent underestimation of CO₂ emission. A modified version of the model, containing a simple implementation of denitrification and of clay fixation/release of ammonium, performed better than the original model for most treatments.We conclude that the mineralisation/immobilisation turnover in the model is not fully adequate to represent C and N dynamics. We also discuss the implementation of changes (time-varying microbial efficiency and C to N ratio; simulation of ammonium clay fixation and emissions of N₂/N₂O) to improve model performance.     

Biological and chemical properties of arable soils affected by long-term organic and inorganic fertilizer applications

 Using soils from field plots in four different arable crop experiments that have received combinations of manure, lime and inorganic N, P and K for up to 20 years, the effects of these fertilizers on soil chemical properties and estimates of soil microbial community size and activity were studied. The soil pH was increased or unaffected by the addition of organic manure plus inorganic fertilizers applied in conjunction with lime, but decreased in the absence of liming. The soil C and N contents were greater for all fertilized treatments compared to the control, yet in all cases the soil samples from fertilized plots had smaller C:N ratios than soil from the unfertilized plots. The soil concentrations of all the other inorganic nutrients measured were greater following fertilizer applications compared with the unfertilized plots, and this effect was most marked for P and K in soils from plots that had received the largest amounts of these nutrients as fertilizers. Both biomass C determined by chloroform fumigation and glucose-induced respiration tended to increase as a result of manure and inorganic fertilizer applications, although soils which received the largest additions of inorganic fertilizers in the absence of lime contained less biomass C than those to which lime had been added. Dehydrogenase activity was lower in soils that had received the largest amounts of fertilizers, and was further decreased in the absence of lime. This suggests that dehydrogenase activity was highly sensitive to the inhibitory effects associated with large fertilizer additions. Potential denitrification and anaerobic respiration determined in one soil were increased by fertilizer application but, as with both the microbial biomass and dehydrogenase activity, there were significant reductions in both N₂O and CO₂ production in soils which received the largest additions of inorganic fertilizers in the absence of lime. In contrast, the size of the denitrifying component of the soil microbial community, as indicated by denitrifying enzyme activity, was unaffected by the absence of lime at the largest rate of inorganic fertilizer applications. The results indicated differences in the composition or function of microbial communities in the soils in response to long-term organic and inorganic fertilization, especially when the soils were not limited. Received: 10 March 1998     

Biochar suppresses N2O emissions while maintaining N availability in a sandy loam soil

Nitrous oxide (N₂O) from agricultural soil is a significant source of greenhouse gas emissions. Biochar amendment can contribute to climate change mitigation by suppressing emissions of N₂O from soil, although the mechanisms underlying this effect are poorly understood. We investigated the effect of biochar on soil N₂O emissions and N cycling processes by quantifying soil N immobilisation, denitrification, nitrification and mineralisation rates using ¹⁵N pool dilution techniques and the FLUAZ numerical calculation model. We then examined whether biochar amendment affected N₂O emissions and the availability and transformations of N in soils.Our results show that biochar suppressed cumulative soil N₂O production by 91% in near-saturated, fertilised soils. Cumulative denitrification was reduced by 37%, which accounted for 85–95 % of soil N₂O emissions. We also found that physical/chemical and biological ammonium (NH₄⁺) immobilisation increased with biochar amendment but that nitrate (NO₃⁻) immobilisation decreased. We concluded that this immobilisation was insignificant compared to total soil inorganic N content. In contrast, soil N mineralisation significantly increased by 269% and nitrification by 34% in biochar-amended soil.These findings demonstrate that biochar amendment did not limit inorganic N availability to nitrifiers and denitrifiers, therefore limitations in soil NH₄⁺ and NO₃⁻ supply cannot explain the suppression of N₂O emissions. These results support the concept that biochar application to soil could significantly mitigate agricultural N₂O emissions through altering N transformations, and underpin efforts to develop climate-friendly agricultural management techniques.     

Elevated atmospheric CO2 changes phosphorus fractions in soils under a short rotation poplar plantation (EuroFACE)

Future high levels of atmospheric carbon dioxide (CO₂) may increase biomass production of terrestrial plants and hence plant requirements for soil mineral nutrients to sustain a greater biomass production. Phosphorus (P), an element essential for plant growth, is found in soils both in inorganic and in organic forms. In this work, three genotypes of Populus were grown under ambient and elevated atmospheric CO₂ concentrations (FACE) for 5 years. An N fertilisation treatment was added in years 4 and 5 after planting. Using a fractionation scheme, total P was sequentially extracted using H₂O, NaOH, HCl and HNO₃, and P determined as both molybdate (Mo) reactive and total P. Molybdate-reactive P is defined as mainly inorganic but also some labile organic P which is determined by Vanado-molybdophosphoric acid colorimetric methods. Organic P was also measured to assess all plant available and weatherable P pools. We tested the hypotheses that higher P demand due to increased growth is met by a depletion of easily weatherable soil P pools, and that increased biomass inputs increases the amount of organic P in the soil. The concentration of organic P increased under FACE, but was associated with a decrease in total soil organic matter. The greatest increase in the soil P due to elevated CO₂ was found in the HCl-extractable P fraction in the non-fertilised treatment. In the NaOH-extractable fraction the Mo-reactive P increased under FACE, but total P did not differ between ambient and FACE. The increase in both the NaOH- and HCl-extractable fractions was smaller after N addition. The results showed that elevated atmospheric CO₂ has a positive effect on soil P availability rather than leading to depletion. We suggest that the increase in the NaOH- and HCl-extractable fractions is biologically driven by organic matter mineralization, weathering and mycorrhizal hyphal turnover.     

Impact of organic manures with and without mineral fertilizers on soil chemical and biological properties under tropical conditions

Soils receiving organic manures with and without chemical fertilizers for the last 7 yr with pearlmillet–wheat cropping sequence were compared for soil chemical and biological properties. The application of farmyard manure, poultry manure, and sugarcane filter cake alone or in combination with chemical fertilizers improved the soil organic C, total N, P, and K status. The increase in soil microbial‐biomass C and N was observed in soils receiving organic manures only or with the combined application of organic manures and chemical fertilizers compared to soils receiving chemical fertilizers only. Basal and glucose‐induced respiration, potentially mineralizable N, and arginine ammonification were higher in soils amended with organic manures with or without chemical fertilizers, indicating that more active microflora is associated with organic and integrated system using organic manures and chemical fertilizers together which is important for nutrient cycling.     

Microbial biomass and activity in salt affected soils under arid conditions

The effects of salinity on the size, activity and community structure of soil microorganisms in salt affected arid soils were investigated in Shuangta region of west central Anxi County, Gansu Province, China. Eleven soils were selected which had an electrical conductivity (EC) gradient of 0.32–23.05 mS cm⁻¹. There was a significant negative exponential relationship between EC and microbial biomass C, the percentage of soil organic C present as microbial biomass C, microbial biomass N, microbial biomass N to total N ratio, basal soil respiration, fluorescein diacetate (FDA) hydrolysis rate, arginine ammonification rate and potentially mineralizable N. The exponential relationships with EC demonstrate the highly detrimental effect that soil salinity had on the microbial community. In contrast, the metabolic quotient (qCO₂) was positively correlated with EC, and a quadratic relationship between qCO₂ and EC was observed. There was an inverse relationship between qCO₂ and microbial biomass C. These results indicate that higher salinity resulted in a smaller, more stressed microbial community which was less metabolically efficient. The biomass C to biomass N ratio tended to be lower in soils with higher salinity, reflecting the bacterial dominance in microbial biomass in saline soils. Consequently, our data suggest that salinity is a stressful environment for soil microorganisms.     

Plant biomass influences rhizosphere priming effects on soil organic matter decomposition in two differently managed soils

We used a continuous labeling method of naturally ¹³C-depleted CO₂ in a growth chamber to test for rhizosphere effects on soil organic matter (SOM) decomposition. Two C3 plant species, soybean (Glycine max) and sunflower (Helianthus annus), were grown in two previously differently managed soils, an organically farmed soil and a soil from an annual grassland. We maintained a constant atmospheric CO₂ concentration at 400±5 ppm and δ¹³C signature at −24.4‰ by regulating the flow of naturally ¹³C-depleted CO₂ and CO₂-free air into the growth chamber, which allowed us to separate new plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration. Rhizosphere priming effects on SOM decomposition, i.e., differences in soil-derived CO₂-C between planted and non-planted treatments, were significantly different between the two soils, but not between the two plant species. Soil-derived CO₂-C efflux in the organically farmed soil increased up to 61% compared to the no-plant control, while the annual grassland soil showed a negligible increase (up to 5% increase), despite an overall larger efflux of soil-derived CO₂-C and total soil C content. Differences in rhizosphere priming effects on SOM decomposition between the two soils could be largely explained by differences in plant biomass, and in particular leaf biomass, explaining 49% and 74% of the variation in primed soil C among soils and plant species, respectively. Nitrogen uptake rates by soybean and sunflower was relatively high compared to soil C respiration and associated N mineralization, while inorganic N pools were significantly depleted in the organic farm soil by the end of the experiment. Despite relatively large increases in SOM decomposition caused by rhizosphere effects in the organic farm soil, the fast-growing soybean and sunflower plants gained little extra N from the increase in SOM decomposition caused by rhizosphere effects. We conclude that rhizosphere priming effects of annual plants on SOM decomposition are largely driven by plant biomass, especially in soils of high fertility that can sustain high plant productivity.     

Effects of land-use type and nitrogen addition on nitrous oxide and carbon dioxide production potentials in Japanese Andosols

Land-use type and nitrogen (N) addition strongly affect nitrous oxide (N₂O) and carbon dioxide (CO₂) production, but the impacts of their interaction and the controlling factors remain unclear. The aim of this study was to evaluate the effect of both factors simultaneously on N₂O and CO₂ production and associated soil chemical and biological properties. Surface soils (0–10 cm) from three adjacent lands (apple orchard, grassland and deciduous forest) in central Japan were selected and incubated aerobically for 12 weeks with addition of 0, 30 or 150 kg N ha^–1 yr^–1. Land-use type had a significant (p < 0.001) impact on the cumulative N₂O and CO₂ production. Soils from the apple orchard had higher N₂O and CO₂ production potentials than those from the grassland and forest soils. Soil net N mineralization rate had a positive correlation with both soil N₂O and CO₂ production rates. Furthermore, the N₂O production rate was positively correlated with the CO₂ production rate. In the soils with no N addition, the dominant soil properties influencing N₂O production were found to be the ammonium-N content and the ratio of soil microbial biomass carbon to nitrogen (MBC/MBN), while those for CO₂ production were the content of nitrate-N and soluble organic carbon. N₂O production increased with the increase in added N doses for the three land-use types and depended on the status of the initial soil available N. The effect of N addition on CO₂ production varied with land use type; with the increase of N addition doses, it decreased for the apple orchard and forest soils but increased for the grassland soils. This difference might be due to the differences in microbial flora as indicated by the MBC/MBN ratio. Soil N mineralization was the major process controlling N₂O and CO₂ production in the examined soils under aerobic incubation conditions.