Kinetics of C and N mineralization, N immobilization and N volatilization of organic inputs in soil

C and N mineralization data for 17 different added organic materials (AOM) in a sandy soil were collected from an incubation experiment conducted under controlled laboratory conditions. The AOM originated from plants, animal wastes, manures, composts, and organic fertilizers. The C-to-N_AOM ratios (η_AOM) ranged from 1.1 to 27.1. Sequential fibre analyses gave C-to-N ratios of soluble (η_Sol), holocellulosic (η_Hol) and ligneous compounds (η_Lig) ranging from 1.1 to 57.2, 0.8 to 65.2, and 3.5 to 25.3, respectively. Very different patterns of net AOM-N mineralization were observed: (i) immobilization for four plant AOM; (ii) moderate mineralization (4-15% AOM-N) for composts; (iii) marked mineralization (11-27% AOM-N) for 1 animal AOM, 1 manure and 2 organic fertilizers; and (iv) high rates of transformations with possible gaseous losses for some N-rich AOM.The Transformation of Added Organics (TAO) model proposed here, described AOM-C mineralization (28 °C, 75% WHC) from three labile (L′), resistant (R) and stable (S) compartments with the sole parameters P′_L and P_S=fractions of very labile and stable compounds of AOM, respectively. Dividing the C-compartments by their C-to-N estimates supplied the remaining N_AOM fraction (RAONF). A P_im parameter split the TAO nitrogen fraction (TAONF=added N-RAONF) into two compartments, immobilized (imN) and inorganic (inorgN) N. A P_im>0 value meant that all the TAONF plus a fraction (P_im−1) of native soil inorganic N was immobilized. Additional N mineralization was predicted when necessary from imN by first order kinetics (constant k_remin). The TAO version with two parameters P_im and k_remin allowed us to predict very different patterns of N mineralization and N immobilization. In a few cases, a further first order kinetic law (constant k_v) was added to predict N volatilization from inorgN. Two hypotheses were tested: (i) η_L′, η_R, η_S (C-to-N of L′, R and S)=η_Sol, η_Hol, η_Lig, respectively, (ii) η_L′=η_R=η_S=η_AOM. The first hypothesis was validated by these data, and the second was a good approximation of the former one. In all the cases, predictions were in good agreement with measured values.     

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.     

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     

A model for calculating nitrogen fluxes in soil using N tracing

Published methods for calculating gross N rates differ in their assumptions and the method of calculation (algebrical equations or numerical methods). The calculation model presented here called FLUAZ considers the major N processes occurring in soil and enables testing of the importance of the various assumptions. It combines a numerical model for solving the mass balance equations and a non linear fitting program for optimizing the N rate parameters. It can be applied to a single or “paired” treatment(s) of an experiment in a bare soil. The model has been evaluated in two experiments made in the laboratory with wheat straw, each experiment involving two “paired” treatments. When FLUAZ was applied to the “paired” treatments, a good fit was obtained between the simulated and measured values of 10 variables (amount of NH₄⁺ and NO₃⁻, isotopic excess of NH₄⁺, NO₃⁻ and organic N). This fit validated the compartmental model and enabled calculation of six N fluxes: mineralisation (m), ammonium immobilisation (i_a), nitrate immobilisation (i_n), nitrification (n), volatilisation (v) or denitrification (d) and remineralisation of recently immobilised N (r). Sensitivity analysis indicated that the classical assumptions of exclusive ammonium immobilisation (i_n=0) and absence of N remineralisation (r=0) had to be rejected. NH₄⁺ immobilisation appeared to be dominant when ammonium and nitrate were both present, but was not exclusive: a Langmuir-type relationship could be established between the immobilisation ratio i_a/(i_a+i_n) and the molar ratio of soil N concentrations NH₄⁺/(NH₄⁺+NO₃⁻). Remineralisation of N occurred simultaneously with immobilisation during wheat straw decomposition and represented 7–18% of gross immobilisation. Taking into account small gaseous losses, volatilisation or denitrification, allowed a better fit to be obtained between observed and simulated N and pools. Nitrification was better described by first order than by zero order kinetics. The eventuality of direct assimilation of organic N by microbial biomass or N humification could not be determined but had no significant influence on the calculation of other fluxes. When FLUAZ was applied to a single treatment (NH₄⁺ labelled), it also gave a good fit but only m, i (=i_a+i_n), n, v or d could be determined. The mineralisation and immobilisation rates were slightly lower than those found with the paired treatments: this difference was mainly due to the hypothesis r=0 and disappeared when r was fixed at the value obtained with the paired treatments. The “apparent” immobilisation rates (i−r) were then similar. The model is very useful to test the consistency of measurements, estimate several N rates simultaneously and quantify the importance of various assumptions.     

Dynamics of dissolved and extractable organic nitrogen upon soil amendment with crop residues

Dissolved organic nitrogen (DON) is increasingly recognized as a pivotal pool in the soil nitrogen (N) cycle. Numerous devices and sampling procedures have been used to estimate its size, varying from in situ collection of soil solution to extraction of dried soil with salt solutions. Extractable organic N (EON) not only consists of DON but contains also compounds released from soil biomass and desorbed organic matter. There is no consensus whether DON or EON primarily regulates N mineralisation in soil, and their contribution to N mineralisation has not been quantified simultaneously. We evaluated three sampling procedures on their ability to determine the dynamic of dissolved organic N pools. The three procedures were the determination of DON in 1) soil solution collected by centrifugation, and the determination of EON in 2) a 0.01 M CaCl₂ extract of field moist or 3) dried soil. We added unlabeled leek and ¹⁵N-labeled ryegrass residues to a loamy sandy soil to create a temporarily increase in DON and EON, to stimulate microbial activity, and to test whether the source and dynamics of the three pools differ. We also tested whether the flow of N through DON or EON was associated with the production of inorganic N using ¹⁵N isotope tracing. Sampling procedures significantly affected the amount, but not the dynamics and origin of the three organic N pools. DON and EON (determined on field-moist and dried soils) showed all a significant increase upon crop amendment and returned to their background concentrations within 10 to 30 days. The fraction of DON and EON originating from the crop residue slightly decreased over 138 days and was not different for DON and EON. Field moist extraction of a loamy sandy soil with 0.01 M CaCl₂ gave a reliable estimate of the concentration of in situ dissolved organic N. In contrast, extraction of dried soil significantly increased EON compared to DON. The agreement in dynamics, ¹⁵N enrichment and C-to-N ratio’s indicate that dissolved and extracted organic N have a similar role in N mineralisation. Our results also suggest that they make a minor contribution to N mineralisation; changes in the turnover rate of EON were not associated with changes in the net N mineralisation rate.     

Low soil temperature effects on short-term gross N mineralisation–immobilisation turnover after incorporation of a green manure

The decomposition of Italian ryegrass (Lolium multiflorum L.) was studied over 37 days in an incubation experiment, at constant temperatures of 3, 9 and 15°C. With the use of a ¹⁵N label in the form of (¹⁵NH₄)₂SO₄ and employing the pool dilution principle, the short-term dynamics of the gross N transformation rates were estimated using both the classical analytical equations formulated by Kirkham and Bartholomew (Soil Sci. Soc. Am. Proc., 18 (1954) 33) and the numerical model FLUAZ (Mary et al., Soil Biol. Biochem., 30 (1998) 1963). The assumptions of the pool-dilution method concerning homogeneity of labelling, disturbance of processes upon labelling and reliability of measurements were evaluated. Gross transformation rates calculated with the numerical model FLUAZ were considered superior to those calculated analytically, because with the FLUAZ model data variability could be taken into account, statistical measures corresponding to calculated rates were given, and nitrate immobilisation and nitrification kinetics were considered. The effect of temperature on the C mineralisation and gross N transformation rates was clear, all rates increasing with increasing temperature. Initially, there was high microbial activity in the Italian ryegrass treatment, followed by a decline in the second half of the incubation, reflecting changes in the quality of substrate being decomposed. The Q₁₀ relationship was used to shed light on this effect and a comparison of Q₁₀ values indicated that the breakdown of recalcitrant substances was more limited at low temperature than that of the more easily degradable substances. Decreases in the gross N mineralisation-to-immobilisation ratio with increasing temperature suggested that gross N immobilisation may be more sensitive to low temperatures than gross N mineralisation. That this may be the case was indicated by a positive net mineralisation rate for Italian ryegrass at 3°C, versus a net immobilisation in the short term at 9 and 15°C, as would also normally be expected for a green manure material with a C-to-N ratio above 20 such as the Italian ryegrass used in this study.     

Comparison of nitrogen mineralisation patterns from root residues of<Emphasis Type="Italic"> Trifolium subterraneum</Emphasis> and<Emphasis Type="Italic"> Medicago sativa</Emphasis>

Crops grown in the first or second year after Medicago sativa L. (lucerne or alfalfa) in southern Australia have sometimes yielded less than crops grown after Trifolium subterraneum L. (subterranean clover), despite the greater annual accumulation of legume nitrogen (N) and enhanced N₂ fixation in lucerne-based pastures. To understand why, we studied the N mineralisation patterns of root residues collected from 16- or 20-week-old plants using incubation assays, in two separate experiments with contrasting soil types (a red loam and a grey clay loam). Fine roots of both species added to soil produced more mineral N than the control soil with no root residues added. In contrast, less mineral N accumulated in the presence of coarse roots than in the control soil. These patterns were not explained by differences in physical size or surface-area, but appeared to be more related to the carbon to nitrogen ratio (C:N) of fine and coarse roots. Fine roots of both species had a C:N of about 11, while the C:N of coarse roots ranged from 28 to 37. Subterranean clover had a high proportion of fine roots giving a weighted average C:N of 19 for the whole root system, and could be expected to result in a rapid net mineralisation of N. The root systems of lucerne on the other hand, consisted mainly of coarse roots giving an average C:N of 26, and would be likely to induce a transient net immobilisation of N and a delay in net mineralisation. The same general patterns of N mineralisation/immobilisation were observed in the second experiment. Tissue chemical analyses suggested that even where the total C:N of subterranean clover and lucerne residues were similar, the amounts of C and N in the soluble fraction of the residues and the C:N of that soluble fraction could largely account for the observed differences in N mineralisation/immobilisation.     

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.     

30 Years of N Fertilisation in a Forest Ecosystem - The Fate of Added N and Effects on N Fluxes

We investigated the fate of added N and its effect on N fluxes in a long-term nitrogen fertilisation experiment. Ammonium nitrate was added annually (30 years) at mean rates of 0 (N0), 35 (N1), 73 (N2) and 108 (N3) kg N ha^?1 yr^?1 to a spruce forest in Sweden, which initially showed signs of N deficiency. Net N mineralisation and N leaching were measured in situ together with soil N pools. We used the PnET-CN model to model the maximum sustainable net N mineralisation rate. The short-term fate of added N was studied by addition of ¹⁵NH₄Cl. In N1 and N2 most of the added N (80–120%) was retained in the system, compared to 45% in N3. A major fraction was retained in the organic horizons (58–79%). The internal N fluxes had increased considerably as a result of the N additions. Net N mineralisation in N1 had increased by a factor 10 and litterfall N flux by a factor 4. The PnET-CN model could not mimic the fast changes in tree growth and N mineralisation, but the maximum N mineralisation rate seems realistic. The ratio of actual to maximum mineralisation rate indicates that the N1 treatment now is close to N saturation, and nitrate was occasionally found in soil solution from the B-horizon in N1. The N retained was probably to a great extent immobilised directly by mycorrhizal fungi, as indicated by the high amounts of ¹⁵N found in the L and F layers and by the great fraction of ¹⁵N found in amino sugars compared to amino acids.     

Simulation of net N immobilisation and mineralisation in substrate-amended soils by the NCSOIL computer model

 The capability of the NCSOIL computer model to simulate the effects of residue fractions on mineralisation-immobilisation turnover was evaluated. Heterogeneous organic substrates were represented in the model by three Van Soest pools, decomposing at different rates. Dried and ground wheat straw, sunflower stalks, wheat stubble and sheep manure (5.22 g kg^–1 soil) were respectively added to a Chromic Calcixerert and aerobically incubated for 224 days at 22±2  °C and 75% field capacity. The CO₂ evolution rates peaked shortly after the C amendments were added, with the highest rate in the sunflower- stalk-amended soils. The addition of organic substrates induced rapid N immobilisation. Net mineralisation was detected earliest in the sunflower-stalk treatment (day 14), while soils with the other amendments showed no net N mineralisation until day 52. The NCSOIL model was calibrated for this soil with CO₂ and inorganic N data from the control soil, yielding a χ² value of 0.011. The overestimation by the model of the C mineralisation data in the case of C-amended soils clearly showed that the concept of three Van Soest pools, decomposing independently at a specific rate constant, is not valid. A retardation factor, that was related to the lignin content of the decomposing material, was introduced into the model. After its introduction the model satisfactorily simulated the C mineralisation rates. However, for all plant residues, N mineralisation was underestimated towards the end of the incubation period. In the case of the soil amended with sheep manure, there was a large discrepancy between simulated and experimental N mineralisation-immobilisation kinetics, suggesting a different allocation of N in animal manure to N-containing fractions compared to that of plant residues. The results indicated that a N fractionation procedure for organic residues should be tested and incorporated into the model. Received: 9 January 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.     

Correlation between mite community structure and gross N fluxes

Net nitrogen (N) mineralisation rate is the result of two independent processes: gross N mineralisation and N immobilisation. Techniques for determining these rates have only been developed recently and no studies have examined the relationship between soil fauna and these two components of net N mineralisation. We present data which demonstrates that mite community structure, described using the Shannon index, is correlated with gross N immobilisation rates in the Western Australian wheat belt. These results suggest that examining the impacts of mites on gross N fluxes is warranted.     

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.     

Maize straw is more effective than maize straw biochar at improving soil N availability and gross N transformation rate

Soil nitrogen (N) transformation is vital in determining farmland N availability. Although many studies have investigated the effect of biochar on N retention and loss via leaching and gaseous emissions, few have determined the dynamics of gross N transformation during crop growth in long-term biochar-amended soils and compared the effect of the biochar with that of its feedstock. In this study, we conducted a five-time field measurement of soil gross N turnover rates via ¹⁵N isotope pool dilution during maize growth in 2021. Three treatments were employed, including no amendment, biochar and straw applied annually at rates of 2.63 and 7.50 t ha⁻¹, respectively, since 2013. The results showed that biochar did not change the rate of gross N mineralisation when compared with no amendment, but straw increased it by 139% in August, resulting in significantly higher cumulative gross N mineralisation than biochar and no amendment (701 vs 489 and 499 mg kg⁻¹ in 200 d). The inconsistent influence was attributed to the fact that inherent biochar-N was recalcitrant and could not be mineralized like the straw. The gross nitrification rate was decreased by 72.9% and 77.4% by biochar and straw application, respectively, in June relative to no amendment, but then it increased from July to August in the straw treatment as a result of the elevated gross N mineralisation rate. The decreased nitrification in the biochar treatment was an outcome of the synergetic effect of a low ammonium pool (−59.4%) and a high gross ammonium immobilisation rate (+263%), which was likely due to excessive fertilizer N loss and abiotic adsorption to biochar. Meanwhile, biochar amendment inhibited bacterial 16S and fungal ITS genes, as well as ureC and bacterial and archaea-amoA gene copies. In conclusion, straw is more effective than biochar at improving soil N transformation and availability in the long term.     

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     

Influence of soluble carbohydrates,free amino acids,and protein content on the decomposition of Lolium multiflorum shoots

The aim of the present investigation was to determine how the chemical composition of L. multiflorum (var. Imperial) shoots influences the initial dynamic decomposition phase. Decomposition in soil was monitored by taking semicontinuous respiration measurements with a resolution of 1 h. Shoots with six different total N contents(2.0–5.2%) were compared. The carbohydrate content of the shoots decreased with increasing total N content, whereas the free amino acid and protein contents increased. Two respiration peaks were observed during the 1-week-long incubation. Comparisons of C mineralisation curves for water-extracted and whole shoots showed that the first peak was derived from water-soluble plant components. Lengths of lag phases and values of specific microbial growth rates obtained from respiration curves of sugars and amino acids implied that these substances were mineralised during the time of the first respiration peak. Amounts of CO₂-C evolved from the shoots during the first peak increased linearly as a function of the sum of the content of glucose, fructose, sucrose, fructans, and free amino acids in the shoots. The shoots with two highest N contents showed net N mineralisation during the first respiration peak, whereas at a lower N content there was net immobilisation. It was concluded that soluble carbohydrates and free amino acids were important C and energy sources for the decomposers during the time encompassed by the first respiration peak. The second peak was derived from both water-soluble and water-insoluble plant components, and the amount of C respired increased with an increasing protein content.     

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.     

Carbon respiration and nitrogen dynamics in Corsican pine litter amended with aluminium and tannins

We investigated the carbon (C) mineralisation and nitrogen (N) dynamics in litter from a Corsican pine forest in response to individual and combined additions of aluminium (Al), condensed tannin (extracted from fresh Corsican pine needles) and hydrolysable tannin (commercial tannic acid). Production rates of CO₂, NH₄⁺ and NO₃⁻ concentrations, tannin concentrations and Al speciation were determined at various time intervals during a 28-day incubation experiment. The addition of Al decreased CO₂ production and shut down nitrification. Exchangeable NH₄⁺ strongly increased in the Al-amended litter, likely due to (i) decreased microbial uptake of NH₄⁺, (ii) the inhibition of nitrification and (iii) competition for soil organic matter (SOM) binding sites by Al. Both tannin species affected C mineralisation and/or N dynamics, be it in different ways. Addition of tannic acid led to a strong increase of the C mineralisation rate and microbial uptake of N, caused by rapid degradation of this labile tannin and subsequent increased microbial nutrient demand. Net immobilisation of N occurred as long as one week after addition. Condensed tannin was not consumed but probably strongly bound to (nitrogenous) SOM compounds, forming recalcitrant complexes and decreasing net N mineralisation. Complexation of Al by tannins in solution before addition to the litter mitigated the Al-induced release of exchangeable NH₄⁺. In the case of condensed tannin with complexed Al, this was due to detoxification of Al through complexation. Increased microbial demand for N likely played a major role in decreased NH₄⁺ accumulation in the samples to which tannic acid with complexed Al was added. Nitrification was shut down despite of the complexation of Al by either condensed tannin or tannic acid.     

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.     

Effect of cattle faeces with different microbial biomass content on soil properties, gaseous emissions and plant growth

A study was carried out to investigate the effects of different diets for heifers, low- and high-yielding cows on the microbial composition of their faeces and subsequently the impacts of these faeces on CO₂ and N₂O emissions, N mineralisation and plant N uptake. A diet low in N and high in acid detergent fibre offered to heifers resulted in faeces dominated by fungi. These faeces were characterised by a low content in microbial biomass C and N and a high ergosterol concentration in comparison to the faeces of high-yielding cows. Added to soil, faeces of heifers led to lower emission and stronger N immobilisation during a 14-day incubation in comparison to the faeces of high-yielding cows. Total N₂O emission was significantly (P?<?0.05) correlated with faecal microbial biomass N. Rye grass yield and N uptake were lowest in the soil supplemented with faeces from heifers in a 62-day pot experiment. Plant N uptake was influenced by the faecal microbial biomass C/N ratio and the fungal C to bacterial C ratio. In conclusion, the faecal microbial biomass was affected to a high degree by the feeding regime and faecal microbial characteristics revealed higher impacts on plant N uptake than soil microbial properties.