Priming effect of biuret addition on native soil N mineralisation under laboratory conditions

This study was carried out to quantify the priming effect of biuret on native soil nitrogen (N) mineralisation during a 112-day incubation. Addition of biuret (100 mg ¹⁵N-labelled biuret kg⁻¹ soil) increased the turnover rate constant of soil organic matter and had a positive priming effect on native soil N mineralisation in two soils. The additional mineralisation was 0.65% of the total soil N (equivalent to 47.1 kg N ha⁻¹) in a sandy loam soil and 0.62% of the soil N (equivalent to 46.5 kg N ha⁻¹) in a silt loam soil.     

Carbon and net nitrogen mineralisation in two forest soils amended with different concentrations of biuret

Biuret is a known contaminant of urea fertilisers that might be useful as a slow release N fertiliser for forestry. We studied carbon (C), net nitrogen (N) mineralisation and soil microbial biomass C and N dynamics in two forest soils (a sandy loam and a silt loam) during a 16-week long incubation following application of biuret (C 23.3%, N 40.8%, O 30.0% and H 4.9%) at concentrations of 0, 2, 10, 100 and 1000 mg kg⁻¹ (oven-dried) soil to assess the potential of biuret as a slow-release N fertiliser. Lower concentrations of biuret specifically increased C mineralisation and soil microbial biomass C in the sandy loam soil, but not in the silt loam soil. A significant decrease of microbial biomass C was found in both soils at week 16 after biuret was applied at higher concentrations. C mineralisation declined with duration of incubation in both soils due to decreased C availability. Biuret at concentrations from 10 to 100 mg kg⁻¹ soil had a significantly positive priming effect on soil organic N mineralisation in both soils. The causes for the priming effects were related to the stimulation of microbial growth and activity at an early stage of the incubation and/or the death of microbes at a later stage, which was biuret-concentration-dependent. The patterns in NH₄⁺-N accumulation differed markedly between the two soils. Net N mineralisation and nitrification were much greater in the sandy loam soil than in the silt loam soil. However, the onset of net nitrification was earlier in the silt loam soil. Biuret might be a potential slow-release N source in the silt loam soil.     

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

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

The proportional mineralisation of microbial biomass and organic matter caused by air-drying and rewetting of a grassland soil

During the first few days after rewetting of an air-dried soil (AD-RW), microbial activity increases compared to that in the original moist soil, causing increased mineralisation (a flush) of soil organic carbon (C) and other nutrients. The AD-RW flush is believed to be derived from the enhanced mineralisation of both non-biomass soil organic matter (due to its physical release and enhanced availability) and microbial biomass killed during drying and rewetting. Our aim was to determine the effects of AD-RW on the mineralisation of soil organic matter and microbial biomass during and after repeated AD-RW cycles and to quantify their proportions in the CO₂-C flushes that resulted. To do this, a UK grassland soil was amended with ¹⁴C-labelled glucose to label the biomass and then given five AD-RW cycles, each followed by 7 d incubation at 25 °C and 50% water holding capacity. Each AD-RW cycle increased the amount of CO₂-C evolved (varying from 83 to 240 μg g⁻¹ soil), compared to the control with, overall, less CO₂-C being evolved as the number of AD-RW cycles increased. In the first cycle, the amount of biomass C decreased by 44% and microbial ATP by 70% while concentrations of extractable C nearly doubled. However, all rapidly recovered and within 1.3 d after rewetting, biomass C was 87% and ATP was 78% of the initial concentrations measured prior to air-drying. Similarly, by 2 d, extractable organic C had decreased to a similar concentration to the original. After the five AD-RW cycles, the amounts of total and ¹⁴C-labelled biomass C remaining in the soil accounted for 60 and 40% of those in the similarly incubated control soil, respectively. Soil biomass ATP concentrations following the first AD-RW cycle remained remarkably constant (ranging from about 10 to 14 μmol ATP g⁻¹ biomass C) and very similar to the concentration in the fresh soil prior to air-drying. We developed a simple mathematical procedure to estimate the proportion of CO₂-C derived from biomass C and non-biomass C during AD-RW. From it, we estimate that, over the five AD-RW cycles, about 60% of the CO₂-C evolved came from mineralisation of non-biomass organic C and the remainder from the biomass C itself.     

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

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

Gross nitrogen mineralisation rates in pastural soils and their relationships with organic nitrogen fractions, microbial biomass and protease activity under glasshouse conditions

In this study, gross nitrogen (N) mineralisation rates were determined in six pasture soils (Fleming, Kairanga, Karapoti, Lismore, Templeton and Waikoikoi) from three different regions of New Zealand. The soils were kept under controlled soil water potential (–10 to –30 kPa) and temperature (12–20°C) conditions in a glasshouse. The gross N mineralisation rates ranged from 0.76 to 5.87 g N g^–1 soil day^–1 in the six soils and were positively correlated with the amount of amino acid-N (AA-N), ammonia-N (NH₃-N), total hydrolysable-N (TH-N), microbial biomass-carbon (MB-C), microbial biomass-N (MB-N), protease activity and organic C and N. A stepwise regression was used to generate equations that could best describe gross N mineralisation rates. Microbial biomass-carbon and AA-N were included in the equation that best described the gross N mineralisation rate:

Measuring rates of gross and net mineralisation of organic phosphorus in soils

The isotopic dilution method developed by Oehl et al. [2001b. Organic phosphorus mineralisation studies using isotopic dilution techniques. Soil Science Society of America Journal 65, 780-787] to measure gross mineralisation of soil organic phosphorus (P) was tested on a range of low-P sorbing soils. This isotopic dilution method relies on accurate prediction of radiolabel behaviour due to soil physicochemical processes. Based on experimental validation of the extrapolation for isotopic dilution due to physicochemical processes using autoclaved soils, a simple power function was used for extrapolation rather than the more complex equation used in the original method. For several soils, however, a potential overestimation of gross mineralisation by 0.1-2.0 mg P kg⁻¹ d⁻¹ was revealed. In addition, the detection limit of P mineralisation ranged between 0.6 and 2.6 mg P kg⁻¹ d⁻¹. The method is likely to be at the detection limit for soils that are high in available P and low in biological activity. The method was modified with respect to the extrapolation and successfully applied to a soil with relatively high microbial P (18 mg P kg⁻¹) and soil respiration rates (29 mg C kg⁻¹ d⁻¹), revealing gross mineralisation rates of organic P of 0.9-1.2 mg P kg⁻¹ d⁻¹. Measurement of uptake of ³²P by the microbial biomass allowed derivation of a net organic P mineralisation rate of 0.5-0.9 mg P kg⁻¹ d⁻¹.     

Sulphur mineralisation in some New Zealand soils

Summary An open incubation technique was used to measure S mineralisation in a range of New Zealand soils. For most of the soils studied, the release of S as sulphate was curvilinear with time, and during a 10-week incubation, the amounts of S mineralised ranged from less than 3 g S g^-1 soil to more than 26 g S g^-1 soil. The best predictor of mineralised S appeared to be the amount of C-bonded S in the soil (explaining 59% of the variation in mineralised S between soils). Examination of the soils after incubation also revealed that the bulk of the mineralised S was derived from the C-bonded S pool. Hydriodic acid-reducible forms of organic S appeared to make little contribution to mineralised S.Attempts were made to predict total potentially mineralisable S (S _o) from incubation data using an exponential equation and a reciprocal-plot technique. However, the dependence of estimated values of S _o on the length and temperature of incubation cast doubts on the validity of this approach.     

Microbial biomass dynamics in recently air-dried and rewetted soils compared to others stored air-dry for up to 103 years

Our aim was to compare the soil microbial biomass concentration and its activity (measured as CO₂-C evolved) following the rewetting and aerobic incubation of soils which have previously been stored air-dry for different periods. Some of the soils have been stored in the Rothamsted sample archive for 103 years, others were comparable freshly sampled soils following air-drying and rewetting and other soils were stored air-dry for 2 years then rewetted for the work described here. Following air-drying, soil ATP concentrations were variable in recently air-dried soil, comprising about 10-35% of the initial ATP concentrations in fresh soil. Following rewetting, the percentage recovery of ATP increased in all soils by 7 days, then declined to between 73% and 87% of the original ATP concentration in the air-dried soils by day 12. Storage of air-dried soils decreased the ability of the microbial biomass to restore its ATP concentrations. For example, the ATP concentration in a soil sampled from stubbed (i.e. tree seedling, saplings and bushes cut frequently to ground level) grassland of the Broadbalk continuous wheat experiment at Rothamsted then air-dried for 2 years was only about 14% of that in the fresh soil at 2 days after rewetting. In other soils from the Hoosfield Barley Experiment, also at Rothamsted, previously given NPK or FYM since 1852, and sampled then stored air-dry for between 13 and 83 years, from 52% to 57% of the ATP in the comparable fresh soils was measured at two days after rewetting. The soil ATP concentration then changed little more up to 12 days. One of the most interesting findings was that while the microbial biomass ATP concentration in the above NPK soils only ranged from about 2 to 4 μmol ATP g⁻¹ biomass C, in the FYM soil the microbial biomass ATP concentrations (range 11.5-13.6 μmol ATP g⁻¹ biomass C) were the same as we repeatedly measure in fresh moist aerobic soil. We do not yet know the reasons for this. More than twice as much CO₂-C was evolved from the long-term stored soils than from freshly sampled ones. However, the specific respiration of the microbial biomass did not change much after the first 12 years of storage, indicating that loss of viability mainly occurred in the earlier years.     

Soil biochemical activity and phosphorus transformations and losses from acidified forest soils

Three Bohemian Forest catchments, Plešné, ?erné and ?ertovo, were studied. These catchments have similar climatic conditions, relief and vegetation, but differ in their bedrock composition. The granitic bedrock in the Plešné catchment was more susceptible to phosphorus (P) leaching under acid conditions than was the mica schist bedrock in the other catchments. The goal of this study was to determine if higher P leaching from the Plešné catchment was associated with differences in microbial P transformations and enzymatic P hydrolysis. Phosphorus and nitrogen contents in soil microbial biomass (P_MB, N_MB; chloroform fumigation), C mineralisation rate (C_min; CO₂ production by GC) and phosphatase activity (MUF-phosphate), were measured in three successive years. Phosphatase activity, P_MB, and C_min were used to characterise the enzymatic hydrolysis of organic P, microbial P accumulation, and microbial mineralisation rates of organic compounds, respectively. Soil chemical properties were characterised by C, N and P content, pH, and by oxalate-extractable P, Fe and Al. Spatial variability in N_MB, P_MB, C_min and phosphatase activity within the catchment was higher (coefficient of variation, CV<50%) than their temporal variability (CV<30%). Multivariate analysis revealed a significant soil layer effect but not that of catchment. When soil layers were evaluated separately, a difference between the Plešné and ?erné or ?ertovo catchments was found in litter and mineral layers, even though the variability within one catchment was high. Within soil profile, phosphatase activity was positively correlated with C_tot, N_MB and C_min (r²=0.89-0.92) being very correlated with P_MB (r²=0.99). Phosphatase activity was higher in the litter (14.0 nmol g⁻¹ h⁻¹) and humus (8.65 nmol g⁻¹ h⁻¹) layers of Plešné than in the same layers of the ?erné (9.65 and 6.40 nmol g⁻¹ h⁻¹) and ?ertovo (12.8 and 6.0 nmol g⁻¹ h⁻¹) soils. Similarly, P_MB in the litter and humus layers of Plešné soil (161 and 93 μg g⁻¹) was higher than P_MB of the same layers of the ?erné (120 and 66 μg g⁻¹) and ?ertovo (148 and 89 μg g⁻¹) soils. High MUFP hydrolysis rate: C_min molar ratio (0.16-1.17 M of P per 1 M of respired C) indicated that potential enzymatic P hydrolysis exceeded estimated microbial P demand (0.034 M of P per 1 M of respired C) in all catchments. The results suggest that higher microbial P transformations and enzymatic P hydrolysis could contribute to enhanced P leaching from the Plešné catchment, which could be enhanced by the lower Fe content in the soil of this catchment as compared to the ?erné and ?ertovo catchments.     

Decomposition dynamics of plant materials in relation to nitrogen availability and biochemistry determined by NMR and wet-chemical analysis

Improved understanding of the interactive relationships of plant material decomposition kinetics to biochemical characteristics and nitrogen availability is required for terrestrial C accounting and sustainable land management. In this study, 15 typical and/or native Australian plant materials were finely ground and incubated with a sandy soil at 25 °C and 55% water holding capacity without nitrogen (−N) or with nitrogen (+N) addition (77 mg N kg⁻¹ soil as KNO₃). The C mineralisation dynamics were monitored for 356 days and the initial biochemical characteristics of the plant materials were determined by NMR and wet-chemical analyses.Under the −N treatment, C mineralisation rates of the plant materials were positively correlated with their initial N contents during the first several weeks, and then negatively correlated with lignin and polyphenols contents during the late stages of incubation. Thus the ratios of lignin/N, polyphenols/N and (lignin+polyphenols)/N had more consistent correlation with the cumulative amounts of C mineralised throughout the incubation than did any single component. In terms of the C types determined by NMR analysis, the C mineralisation rates were initially related positively to carbonyl C contents, and then negatively to aryl and O-aryl C contents from day 3 onwards.Addition of NO₃⁻-N accelerated C mineralisation during the early stages, but resulted in lower cumulative C mineralisation at the end of the incubation for most plant materials. Under the +N treatment, the decomposition rates were correlated with the contents of lignin and the sum of cellulose+acid detergent-extractable non-phenolic compounds, or with aryl, O-aryl and N-alkyl+methoxyl C contents. Regardless of the N treatment, the ratios of aryl/carbonyl, O-aryl/carbonyl and (aryl+O-aryl)/carbonyl C had the closest and most consistent correlations with the cumulative C mineralisation among all biochemical indices examined.A double exponential model with defined mineralisation rate constants for the active and slow pools was used to describe the C mineralisation dynamics. The biological meanings of the kinetically estimated active and slow pool sizes are interpreted and their relationships to the initial chemical/biochemical composition of the plant materials are explored.     

Gross nitrogen transformations in adjacent native and plantation forests of subtropical Australia

The impact of land-use change on soil nitrogen (N) transformations was investigated in adjacent native forest (NF), 53 y-old first rotation (1R) and 5 y-old second rotation (2R) hoop pine (Araucaia cunninghamii) plantations. The ¹⁵N isotope dilution method was used to quantify gross rates of N transformations in aerobic and anaerobic laboratory incubations. Results showed that the land-use change had a significant impact on the soil N transformations. Gross ammonification rates in the aerobic incubation ranged between 0.62 and 1.78 mg N kg⁻¹ d⁻¹, while gross nitrification rates ranged between 2.1 and 6.6 mg N kg⁻¹ d⁻¹. Gross ammonification rates were significantly lower in the NF and the 1R soils than in the 2R soils, however gross nitrification rates were significantly higher in the NF soils than in the plantation soils. The greater rates of gross nitrification found in the NF soil compared to the plantation soils, were related to lower soil C:N ratios (i.e. more labile soil N under NF). Nitrification was found to be the dominant soil N transformation process in the contrasting forest ecosystems. This might be attributed to certain site conditions which may favour the nitrifying community, such as the dry climate and tree species. There was some evidence to suggest that heterotrophic nitrifiers may undertake a significant portion of nitrification.     

Carbon pulses but not phosphorus pulses are related to decreases in microbial biomass during repeated drying and rewetting of soils

Drying and rewetting cycles are known to be important for the turnover of carbon (C) in soil, but less is known about the turnover of phosphorus (P) and its relation to C cycling. In this study the effects of repeated drying-rewetting (DRW) cycles on phosphorus (P) and carbon (C) pulses and microbial biomass were investigated. Soil (Chromic Luvisol) was amended with different C substrates (glucose, cellulose, starch; 2.5 g C kg⁻¹) to manipulate the size and community composition of the microbial biomass, thereby altering P mineralisation and immobilisation and the forms and availability of P. Subsequently, soils were either subjected to three DRW cycles (1 week dry/1 week moist) or incubated at constant water content (70% water filled pore space). Rewetting dry soil always produced an immediate pulse in respiration, between 2 and 10 times the basal rates of the moist incubated controls, but respiration pulses decreased with consecutive DRW cycles. DRW increased total CO₂ production in glucose and starch amended and non-amended soils, but decreased it in cellulose amended soil. Large differences between the soils persisted when respiration was expressed per unit of microbial biomass. In all soils, a large reduction in microbial biomass (C and P) occurred after the first DRW event, and microbial C and P remained lower than in the moist control. Pulses in extractable organic C (EOC) after rewetting were related to changes in microbial C only during the first DRW cycle; EOC concentrations were similar in all soils despite large differences in microbial C and respiration rates. Up to 7 mg kg⁻¹ of resin extractable P (P_resin) was released after rewetting, representing a 35-40% increase in P availability. However, the pulse in P_resin had disappeared after 7 d of moist incubation. Unlike respiration and reductions in microbial P due to DRW, pulses in P_resin increased during subsequent DRW cycles, indicating that the source of the P pulse was probably not the microbial biomass. Microbial community composition as indicated by fatty acid methyl ester (FAME) analysis showed that in amended soils, DRW resulted in a reduction in fungi and an increase in Gram-positive bacteria. In contrast, the microbial community in the non-amended soil was not altered by DRW. The non-selective reduction in the microbial community in the non-amended soil suggests that indigenous microbial communities may be more resilient to DRW. In conclusion, DRW cycles result in C and P pulses and alter the microbial community composition. Carbon pulses but not phosphorus pulses are related to changes in microbial biomass. The transient pulses in available P could be important for P availability in soils under Mediterranean climates.     

Measuring microbial uptake of nitrogen in nutrient-amended sandy soils—A mass-balance based approach

Soil microbial biomass N is commonly determined through fumigation-extraction (FE), and a conversion factor (K_EN) is necessary to convert extractable N to actual soil biomass N. Estimation of K_EN has been constrained by various uncertainties including potential microbial immobilisation. We developed a mass-balance approach to quantify changes in microbial N storage during nutrient-amended incubation, in which microbial uptake is determined as the residual in a ‘mass-balance’ based on soil-water N before and after amended incubation. The approach was applied to three sandy soils of southwestern Australia, to determine microbial N immobilisation during 5-day incubation in response to supply of 2.323 mg C g⁻¹, 100 μg N g⁻¹ and 20 μg P g⁻¹. The net N immobilisation was estimated to be 95-114 μg N g⁻¹ in the three soils, equivalent to 82.7-85.1% of soil-water N following the amendment. Such estimation for microbial uptake does not depend on fumigation and K_EN conversion, but for comparison purposes we estimated ‘nominal’ K_EN values (0.11-0.14) for the three soils, which were comparable to previously reported K_EN from soils receiving C and N amendment. The accuracy of our approach depends on the mass-balance equation and the integrated measurement errors of the multiple N pools, and was assessed practically through recoveries of added-N when microbial uptake can be minimised. Near-satisfactory recoveries were achieved under such conditions. Our mass-balance approach provides information not only about changes in the microbial biomass nitrogen storage, but also major N-pools and their fluxes in regulating soil N concentrations under substrate and nutrient amended conditions.     

Release of carbon and nitrogen from fodder radish (Raphanus sativus) shoots and roots incubated in soils with different management history

Plant roots are generally considered to decompose slower than shoots and contribute more to accumulation of soil organic matter, and management history is expected to shape the structure and function of decomposer communities in soil. Here we study the effect of chemical characteristics of shoots and roots from fodder radish (Raphanus sativus oleiformis L.), a widely used cover crop, on the release of their C and N after addition to soil. Shoots and roots were incubated for 180?d at 20°C using four soils with different management histories (organic versus mineral fertiliser, with and without use of cover crops), and the release of CO₂ and extractable mineral N was determined. More shoot C than root C was mineralised during the first 10?d of incubation. After 180?d, 58% of the C input was mineralised with no difference between shoots and roots. At the end of incubation, shoots had released more N (42% of shoot N) than roots (28% of root N). Moreover, management history did not affect net mineralisation of added plant C. Residues incubated in soil with a management history involving cover crops showed an enhanced net N mineralisation. Therefore, long-term decomposition of C added in radish shoots and roots is unaffected by differences in chemical characteristics or soil management history. However, the net mineralisation of N in shoots is faster than for N in roots, and net N mineralisation of added materials is higher in soil with than without a history of cover crops.

Abbreviations: CC: cover crop; IF: inorganic fertilizer; M: manure     

Response of microbial community composition and activity in agricultural and grassland soils after a simulated rainfall

Rainfall in Mediterranean climates may affect soil microbial processes and communities differently in agricultural vs. grassland soils. We explored the hypothesis that land use intensification decreases the resistance of microbial community composition and activity to perturbation. Soil carbon (C) and nitrogen (N) dynamics and microbial responses to a simulated Spring rainfall were measured in grassland and agricultural ecosystems. The California ecosystems consisted of two paired sets: annual vegetable crops and annual grassland in Salinas Valley, and perennial grass agriculture and native perennial grassland in Carmel Valley. Soil types of the respective ecosystem pairs were derived from granitic parent material and had sandy loam textures. Intact cores (30 cm deep) were collected in March 1999. After equilibration, dry soil cores (approx. −1 to −2 MPa) were exposed to a simulated Spring rainfall of 2.4 cm, and then were measured at 0, 6, 24, and 120 h after rewetting. Microbial biomass C (MBC) and inorganic N did not respond to rewetting. N₂O and CO₂ efflux and respiration increased after rewetting in all soils, with larger responses in the grassland than in the agricultural soils. Phospholipid fatty acid (PLFA) profiles indicated that changes in microbial community composition after rewetting were most pronounced in intensive vegetable production, followed by the relict perennial grassland. Changes in specific PLFA markers were not consistent across all sites. There were more similarities among microbial groups associated with PLFA markers in agricultural ecosystems than grassland ecosystems. Differences in responses of microbial communities may be related to the different plant species composition of the grasslands. Agricultural intensification appeared to decrease microbial diversity, as estimated from numbers of individual PLFA identified for each ecosystem, and reduce resistance to change in microbial community composition after rewetting. In the agricultural systems, reductions in both the measures of microbial diversity and the resistance of the microbial community composition to change after a perturbation were associated with lower ecosystem function, i.e. lower microbial responses to increased moisture availability.     

Rhizosphere priming can promote mobilisation of N-rich compounds from soil organic matter

Soil organic matter (SOM) is the dominant store of nutrients required for plant growth, but the availability of these nutrients is dependent on transformations mediated by the microbial biomass. The addition of labile C to soil is known to alter SOM turnover (priming effect, PE), but understanding of this is limited, particularly with respect to impact on gross nitrogen (N) fluxes. Here we examined relationships between C and N fluxes from SOM under primed and non-primed conditions in two soils. Stable isotopes (¹³C and ¹⁵N) were used to measure gross C and N fluxes from SOM and to differentiate between SOM mineralised due to priming and that from basal mineralisation. ¹³C-glucose was added daily to simulate the effect of addition of labile C on SOM-C and –N mineralisation within the rhizosphere. Addition of glucose increased both gross N and C mineralisation from SOM. However, the C-to-N ratio of the mineralised flux from ‘primed’ SOM was 5:1, whereas the C-to-N ratio of the basal mineralised flux was 20:1 indicating that priming acted on specific organic matter pools. This result is consistent with the concept that priming is a distinct N-mining response of the microbial biomass, as opposed to an acceleration of the basal flux. Our data suggest that C and N fluxes are not directly linked through their gross stoichiometry in SOM. This is due to the heterogeneity and overall passiveness of OM relative to the dynamic nature of mineralisation fluxes and source pools, and in primed systems the mineralisation of N-rich compounds.     

Reaction of microorganisms to rewetting in continuous cereal and legume rotation soils of semi-arid Sub-Saharan Africa

A 20-day incubation experiment with continuous cereal (CC) versus cereal legume (CL) rotation soils of two semi-arid Sub-Saharan sites (Fada-Kouaré in Burkina Faso, F, and Koukombo in Togo, K) were carried out to investigate the effects of rewetting on soil microbial properties. Site- and system-specific reactions of soil microorganisms were observed on cumulative CO₂ production, adenylates (ATP, ADP, and AMP), microbial biomass C and N, ergosterol, muramic acid and glucosamine. Higher values of all parameters were found in the CL rotation soils and in both soils from Fada-Kouaré. While the inorganic N concentration showed only a system-specific response to rewetting, the adenylate energy charge (AEC) showed only a site-specific response. ATP recovered within 6 h after rewetting from ADP and AMP due to rehydration of microorganisms and not due to microbial growth. Consequently, no N seemed to be immobilized by microorganisms and all NO₃ in the soil was immediately available to the plants. The fungal cell-membrane component ergosterol was three (CC) and five (CL) times larger at Fada than in the respective soils at Koukombo. The concentrations of the bacterial cell-wall component muramic acid were by 20% and of mainly fungal glucosamine by 10% larger in the CL rotation soils than in the CC soils. This indicates long-shifts in the microbial community structure.     

Short-term effects of olive mill waste water (OMW) on chemical and biochemical properties of a semiarid Mediterranean soil

Olive mill waste water (OMW), a by-product of the olive mill industry, is produced in large amounts in Mediterranean countries. Olive mill waste water contains a high organic load, substantial amounts of plant nutrients but also several compounds with recognized toxicity towards living organisms. Moreover, OMW may represent a low cost source of water. Thus, the use of OMW for soil fertigation is a valuable option for its disposal, provided that its impact on soil chemical and biochemical properties is established. Investigations were performed on the short-term influence of OMW on several chemical and biochemical properties of a soil from a continental semi-arid Mediterranean region (Morocco). The soil was amended with 0, 18 and 36 ml 100 g⁻¹ soil of OMW (corresponding to a field rate of 0, 40 and 80 m³ ha⁻¹, respectively) and changes in various functionally related properties such as microbial biomass, basal respiration, extractable C and N, and soil hydrolases and oxido-reductases activities were measured over time. The variations of the main physical and chemical properties as well as the residual phytotoxicity of OMW amended and non-amended soils as assessed by tomato seed germination tests were also monitored. Temporary and permanent changes in several chemical and biochemical soil properties occurred following OMW application, thus being these properties varied in sensitivity to the applied disturbance. A sudden increase of total organic C, extractable N and C, available P and extractable Mn and Fe contents were measured. Simultaneously, a rapid increase of soil respiration, dehydrogenase and urease activities and microbial biomass (at 14 day incubation) of OMW amended soils occurred. In contrast, the activities of phosphatase, β-glucosidase, nitrate reductase and diphenol oxidase decreased markedly. The soil became highly phytotoxic after OMW addition (large decline of soil germination capability), mainly at 80 m³ ha⁻¹ OMW. After 42 days' incubation, however, a complete recovery of the soil germination capability and a residual phytotoxicity of about 30% were observed with 40 and 80 m³ ha⁻¹ OMW, respectively. These findings indicate that the impact of OMW on soil properties was the result of opposite effects, depending on the relative amounts of beneficial and toxic organic and inorganic compounds present. The toxic compounds contained in OMW most likely counteracted the beneficial effect of organic substrates provided, which promoted the growth and activity of indigenous microorganisms.     

Turnover of grain legume N rhizodeposits and effect of rhizodeposition on the turnover of crop residues

The turnover of N derived from rhizodeposition of faba bean (Vicia faba L.), pea (Pisum sativum L.) and white lupin (Lupinus albus L.) and the effects of the rhizodeposition on the subsequent C and N turnover of its crop residues were investigated in an incubation experiment (168 days, 15 °C). A sandy loam soil for the experiment was either stored at 6 °C or planted with the respective grain legume in pots. Legumes were in situ ¹⁵N stem labelled during growth and visible roots were removed at maturity. The remaining plant-derived N in soil was defined as N rhizodeposition. In the experiment the turnover of C and N was compared in soils with and without previous growth of three legumes and with and without incorporation of crop residues. After 168 days, 21% (lupin), 26% (faba bean) and 27% (pea) of rhizodeposition N was mineralised in the treatments without crop residues. A smaller amount of 15–17% was present as microbial biomass and between 30 and 55% of mineralised rhizodeposition N was present as microbial residue pool, which consists of microbial exoenzymes, mucous substances and dead microbial biomass. The effect of rhizodeposition on the C and N turnover of crop residues was inconsistent. Rhizodeposition increased the crop residue C mineralisation only in the lupin treatment; a similar pattern was found for microbial C, whereas the microbial N was increased by rhizodeposition in all treatments. The recovery of residual ¹⁵N in the microbial and mineral N pool was similar between the treatments containing only labelled crop residues and labelled crop residues + labelled rhizodeposits. This indicates a similar decomposability of both rhizodeposition N and crop residue N and may be attributable to an immobilisation of both N sources (rhizodeposits and crop residues) as microbial residues and a subsequent remineralisation mainly from this pool.Abbreviations C or N_dec C or N decomposed from residues - C or N_mic microbial C or N - C or N_micres microbial residue C or N - C or N_min mineralised C or N - C or N_input added C or N as crop residues and/or rhizodeposits - dfr derived from residues - dfR derived from rhizodeposition - Ndfr N derived from residues - NdfR N derived from rhizodeposition - N_loss losses of N derived from residues - SOM soil organic matter - WHC water holding capacity