Mineralization-immobilization Turnover Research Articles

Significance of organic nitrogen uptake from plant residues by soil microorganisms as affected by carbon and nitrogen availability

Soil microorganisms can use a wide range of nitrogen (N) compounds. When organic N sources are degraded, microorganisms can either take up simple organic molecules directly (direct route), or organic N may be mineralized first and taken up in the form of mineral N (mineralization–immobilization–turnover [MIT] route). To determine the importance of the direct route, a microcosm experiment was carried out. Two types of wheat residue were added to soil samples, including younger residue with a carbon (C) to N ratio of 12 and older residue with a C to N ratio of 29. Between days 1 and 4, the gross N mineralization rate reached 8.4 and 4.0 mg N kg −1 dry soil day −1 in the treatment with younger and older residue, respectively. During the same period, there was no difference in protease activity between the two residue amended treatments. The fact that protease activity was not related to gross N mineralization, even though the products of protease activity are the substrates for N mineralization, suggests that not all organic molecules released from residue or soil N passed through the soil mineral N pool. In fact, when leucine and glycine were added, only 10 and 53% of the amino acid-N, respectively, was mineralized. The fraction of N taken up via the direct route was estimated to be 55 and 62% for the young and older residue, respectively. After 28 days of incubation, the proportion of amino acid-N mineralized had increased especially in the soil amended with older residue, suggesting that the MIT route became increasingly important. This result is supported by an increase in the activities of enzymes responsible for the intracellular assimilation of ammonium (NH 4 +). Our results suggest that in contrast to what is proposed by many models of soil N cycling, both the direct and MIT routes were operative, with the direct route being the preferred route of residue N uptake. The direct route became less important over time and was more important in soil amended with older residue, suggesting that the direct route is favored by lower mineral N availabilities. An important implication of these findings is that when the direct route is dominant, gross N mineralization underestimates the amount of N made available from the residue.

Distribution and controls on gross N mineralization‐immobilization‐turnover in soil subjected to zero tillage

SummaryThe aim of this investigation was to provide quantitative measures of gross nitrogen (N) mineralization‐immobilization‐turnover (MIT) in soil, and of the parameters controlling MIT. This is needed to predict adequately mineral N availability in agricultural land. Respiration, MIT, soil microbial biomass (SMB)‐C, soil organic matter (SOM)‐C and dissolved organic matter (DOM)‐C were determined in soil collected over three depths at three stages in a continuous crop rotation. MIT was strongly correlated to the size and activity of SMB‐C and the size of SOM‐C but was only marginally affected by the stage in the crop rotation. The strength of the correlation of mineralization, immobilization and respiration with SOM‐C increased when analysed by curvilinear regression compared with linear regression because SOM bioavailability varied with depth. It is argued that the limiting step in the decomposition was the breakdown of SOM into DOM. The major pathway for degradation of organic‐N in this soil system appeared to be the MIT route, while only a small amount of N was likely to be assimilated in the organic form (direct route). Our findings suggest that modellers of C and N turnover in zero tillage cropping systems need to incorporate a curvilinear relationship between SOM pool size and MIT into their computer model(s).

Soil heterogeneity in lumped mineralization–immobilization models

The heterogeneous distribution of nutrient-rich and nutrient-poor patches in soils strongly affects the intensity of nitrogen cycling between organic and inorganic soil compartments. In highly heterogeneous soils, observation at the core scale of large gross mineralization and immobilization fluxes has led to the development of the mineralization–immobilization turnover (MIT) modeling scheme, which maintains that all nitrogen decomposed from organic compounds is mineralized before assimilation by the microbial biomass. This hypothesis, however, neglects the endogenous nature of the ammonification reactions at the microscopic scale, where organic N is directly assimilated by the decomposers and only the surplus is released as ammonium, as better described by the direct (DIR) pathway. Here we hypothesize that, at the micro-scale, mineralization behaves according to the DIR pathway and analyze a simple two-compartment model to simulate a heterogeneous soil with two fractions of different chemical composition (i.e., C-to-N ratio) and quality (i.e., decomposition rate). We derive the effective parameters of the aggregated model as a function of micro-scale features, and show that it represents a generalization of the parallel (PAR) scheme, which in its original form was introduced to combine DIR and MIT pathways. This physically based new parameterization improves current lumped N mineralization models.

Effects of long‐term annual inputs of straw and organic manure on plant N uptake and soil N fluxes

AbstractThis study investigated the effects of long‐term annual inputs of animal manure and straw on the rate of gross nitrogen (N) mineralization–immobilization turnover (MIT), net N mineralization and potential nitrification, and examined how these N transformation rates affect plant N availability. The experiment was conducted during May–June 2001 in long‐term field experiments in Askov, Denmark, where organic manure and barley straw had been applied annually for 11 and 20 years prior to the year 2000, respectively. Thus, any differences could be attributed to residual effects from the previous years of application. Inputs of straw and organic manure to soil increased soil organic matter (SOM)‐N content in soil in the order: without straw, without manure < without straw, with manure < with straw, without manure < with straw, with manure. The inputs did not change net N mineralization in the soil. There was a distinct but non‐significant trend towards higher gross N mineralization with increasing SOM‐N. Gross N immobilization was enhanced by straw inputs and to a lesser extent by organic manure inputs, while potential nitrification was enhanced by both amendments. The results show that long‐term annual inputs of straw and organic manure can increase MIT rate and potential nitrification rate without influencing net N mineralization rate. MIT and potential nitrification explained 23–31% of the variation in plant N uptake, while net N mineralization rate only explained 1%. Plant N uptake therefore seems to be more influenced by MIT rate and potential nitrification rate than by net mineralization rate, presumably because mineral N in the transition between gross N mineralization and gross N immobilization is available for assimilation by plants.

Manipulating the N release from 15N-labelled celery residues by using straw and vinasses in Flanders (Belgium)

The effect of straw and vinasses on the N mineralization–immobilization turnover of celery residues was investigated in a field experiment in Flanders (Belgium). The field was laid out in raised beds on a loamy sand soil. At the start of the experiment, 15N-labelled celery residues (4 t dry matter (DM) ha −1) were mixed with straw (12 t DM ha −1) in order to immobilize the released celery-N followed by an incorporation of vinasses (4 t DM ha −1) after 200 days aiming to remineralize the immobilized N. Total N, mineral N and their 15N enrichments as well as microbial biomass N were determined at regular time intervals. During the first 62 days after the incorporation, straw immobilized the celery derived 15N in the microbial biomass and reduced the total celery- 15N losses from the top 25 cm by 38%. However, after 62 days, the N immobilization ceased due to low temperatures (<10 °C) in the raised beds, and was followed by natural remineralization of immobilized celery- 15N (without addition of vinasses) at a moment when the risk of nitrate leaching was still high. Hence, straw was not able to reduce the celery-N losses during the complete winter period. The addition of vinasses in spring caused no real positive priming effect, although it did increase the amount of remineralized celery- 15N (7.6% of celery derived 15N) compared to the straw treatment without vinasses (1.7% of celery derived 15N) probably due to an apparent added nitrogen interaction caused by displacement reactions with the soil microbial biomass. In conclusion, in raised beds, it is not possible to reduce nitrate leaching and to achieve a synchronization between the N release from crop residues and N demand by the following crop by using straw and vinasses to manipulate the N release from crop residues.

A theoretical analysis of nonlinearities and feedbacks in soil carbon and nitrogen cycles

Analytical formulations of soil carbon and nitrogen cycles are used to explore the effects of model nonlinearities and feedbacks on the resulting dynamics. Two particular aspects are addressed: (i) nonlinearities in the decomposition rate of soil organic matter and (ii) nitrogen limitation feedbacks on microbial activity and plant–microbe competition. Although linear models of decomposition are more typical in the literature, nonlinear models accounting for the coupling between microbial biomass and its substrate have also been proposed. In deterministic conditions, linear models behave like exponential decay functions, while nonlinear models may also show fluctuating behavior, with dynamic bifurcations between stable-node and stable-focus equilibria as a function of the climatic parameters (e.g., soil moisture and temperature). Both data-model comparison and linear stability analysis support the conclusion that linear models are less suited to describe the soil fluctuating dynamics that arise under certain conditions. A second strong nonlinearity appears when the nitrogen-limitation feedback on decomposition is analyzed. Nitrogen limitation is often established when the substrate of the microbes is N-poor, and/or the competition with plants for mineral N is strong. Such conditions mainly occur when a large fraction of the microbial community cannot meet its nitrogen demand through organic N assimilation, so that mineral N is used. On the contrary, when the microbial community predominantly assimilates organic nitrogen, the occurrence of nitrogen-limitation is less likely and mineralization is given by microbial N surplus. The first case is traditionally modeled by the mineralization–immobilization turnover (MIT) scheme, while the second by the direct assimilation (DIR) scheme. However, since organic N assimilation and mineral N immobilization likely occur simultaneously because of soil heterogeneity and coexistence of different microbial communities, the two schemes only represent extreme cases. Thus, we combine them in a flexible model framework (parallel scheme) and explore how different efficiencies of organic nitrogen assimilation, mineral nitrogen availability and climatic factors control the outcome of plant–microbe competition. We conclude that models accounting only for the DIR pathway implicitly assume that microbes are superior competitors over plants, while models implementing only the MIT pathway might be too sensitive to N-limitation.

Manipulating the N release from 15N labelled celery residues by using straw and vinasses

The aim of this laboratory study was to investigate the effect of straw and vinasses on the nitrogen (N) mineralization–immobilization turnover of celery residues during two periods (each simulating a time period from autumn till spring) under laboratory conditions. During the first period (1–198 d), 15N-labelled celery residues (1.1 g dry matter (DM) kg −1 soil) were incubated together with straw (8.1 g DM kg −1 soil), aiming to immobilize the N released from celery residues, followed by an incorporation of vinasses (1.9 g DM kg −1 soil) after 84 d, with a view to remineralizing the immobilized celery–N. During the second period (198–380 d), the experimental set-up was repeated, except that non-labelled celery residues were used. Total N, mineral N and their 15N enrichments as well as microbial biomass N were determined at regular time intervals. During both periods, mixing celery residues with straw significantly increased microbial biomass N (90.5 and 40.5 mg N kg −1 extra compared to celery only treatment) and decreased the amount of mineral N (reduction of 56.1 and 45.9 mg N kg −1 soil compared to celery only treatment) and the celery-derived mineral 15N (0% of mineral celery-derived 15N in straw treatment compared to 35% of mineral celery-derived 15N in celery only treatment). After maximum immobilization, a natural remineralization (without addition of vinasses) of 32.2 (at day 198) and 11.1 mg N kg −1 soil (at day 380) occurred in the straw treatment, but the mineral N content remained significantly lower than in the celery only treatment during the complete experiment, and the amount of remineralized celery– 15N was very low (5.4% of celery-derived 15N after 380 d). Vinasses caused no real priming effect, although it did slightly increase the amount of remineralized celery– 15N (+6.4% of celery-derived 15N at day 380 compared to the straw treatment), probably due an apparent added N interaction caused by displacement reactions with the soil microbial biomass.

Estimation of Gross Nitrogen Transformations and Nitrogen Retention in Grassland Soils Using FLUAZ

The flux of N through mineralization–immobilization turnover (MIT) in grassland soils is a major determinant for plant N uptake and for N loss processes. In this study we investigated the dynamics of gross N transformation rates and potential N retention on mineral fertilizer addition in three permanent grassland soils of varying texture (loamy sand, loam, and clay loam). Gross N transformation rates were calculated with the 15N‐tracing model FLUAZ. Differentially 15N‐labeled NH4NO3 (at a rate of 100 mg N kg−1 soil) was added to the soils in paired laboratory incubation experiments. Size and 15N enrichment of the NH4+, NO3−, and soil organic N pools were measured at 0, 1, 3, 7, 14, and 30 d after NH4NO3 addition. The accuracy of the simulations of the data using FLUAZ were robust, but tended to decrease (i) with increasing incubation times, (ii) with increasing duration of the time intervals considered, and (iii) with increasing experimental variability. The proportion of the initial N content mineralized on incubation was largest in the loamy sand soil (2.5%), followed by the clay loam soil (1.2%) and the loam soil (0.8%). The actual gross nitrification and N immobilization activity followed the same trend. The loam soil showed the lowest relative N retention (ratio N immobilization over [gross N mineralization + gross nitrification]), which was attributed to its low C availability. In general, the 15N retention after addition of 15NH414NO3 was approximately five times larger than after addition of 14NH415NO3

Temperature and stubble management influence microbial CO 2–C evolution and gross N transformation rates

Few studies have examined the kinetics of gross nitrogen (N) mineralization, immobilization, and nitrification rates in soil at temperatures above 15 °C. In this study, 15N isotopic pool dilution was used to evaluate the influence of retaining standing crop residues after harvest versus burning crop residues on short-term gross N transformation rates at constant temperatures of 5, 10, 15, 20, 30, and 40 °C. Gross N mineralization rates calculated per unit soil organic carbon were between 1 and 7 times lower in stubble burnt treatments than in stubble retained treatments. In addition, significant declines in soil microbial biomass ( P=0.05) and CO 2–C evolution ( P<0.001) were associated with stubble burning. Immobilization rates were of similar magnitude to gross N mineralization rates in stubble retained and burnt treatments incubated between 5 and 20 °C, but demonstrated significant divergence from gross N mineralization rates at temperatures between 20 and 40 °C. Separation in the mineralization immobilization turnover (MIT) in soil at high temperatures was not due to a lack of available C substrate, as glucose-C was added to one treatment to test this assumption. Nitrification increased linearly with temperature ( P<0.001) and dominated over immobilization for available ammonium in soil incubated at 5 °C, and above 20 °C indicating that nitrification is often the principal process controlling NH 4 + consumption in a semi-arid soil. These findings illustrate that the MIT at soil temperatures above 20 °C is not tightly coupled, and consequently that the potential for loss of N (as nitrate) is considerably greater due to increased nitrification.

Gross N mineralization?immobilization rates in heterogeneous intact soil cores can be estimated without marked error

Gross rates of N mineralization–immobilization turnover (MIT) and gross nitrification in soil can be determined by use of a 15NH4+ pool dilution technique, under the assumption that native 14NH4+, applied 14NH4+ and microbial processes are uniformly distributed in the soil. In a laboratory investigation we compared gross N fluxes obtained from two labelling techniques applied to an arable sandy loam: (1) injection of 15NH4+ solution into intact soil cores, and (2) mixing 15NH4+ solution into disturbed soil. It was assumed that MIT obtained with the mixing technique reflected the “true” rates, since the assumption of uniformity was thought to be satisfied by this technique. MIT from the two techniques were not significantly different, thus non-uniform distribution of native 14NH4+, injected 15NH4+ and microbial processes in the intact-core technique did not cause a marked error in the MIT rates. In contrast the gross nitrification rates were twofold higher with the mixing technique than with the intact-core technique. Gross nitrification rates are likely to increase with the mixing technique because mixing redistributes nitrifiers and added 15NH4+, and thereby increases the contact between NH4+ and nitrifiers.

Simulation of nitrogen rhizodeposition and assimilation back into corn (Z ea mays L.) roots

The objectives were to quantify the dynamics of N exchange between corn root and soil under field conditions and to compute the amount of N-rhizodeposition that is recycled back into corn by maturity. In a previous publication the simulation model NCSWAP/NCSOIL was used to quantify the release of organic-C from corn and its incorporation into soil organic matter. In this publication, results from the same simulations but pertinent to N are presented. The model was calibrated against measured N concentration in corn and soil and 15N enrichment data obtained from a long-term field experiment located near St Paul, MN. Field management included rototillage, the removal of stover-residue and grain and 4 fertilizer N treatments: 200 and 20 Kg N ha −1 added yearly from 1980 to 1992, with 15N added with the fertilizer from 1980 until either 1985 or 1992. The same total amount of 15N was added with the 200 and 20 Kg N ha −1 fertilizer: 4 and 40 15N at.%, respectively. Corn roots released 24% of total N uptake. This loss was mitigated by the recycling of 14% of N-rhizodeposition into corn by maturity. 15N enrichment in corn and soil was higher for the 200 than the 20 Kg N ha −1 treatment. This resulted from the rapid N mineralization–immobilization turnover that channelled N through the inorganic N pool whose 15N enrichment was fixed yearly to that of the fertilizer. Tracer N enrichment decreased more rapidly in corn than in soil from 1986 to 1992 when tracer N was no longer added with the fertilizer, and by 1992, 15N was localized in the stable pool and flushed from the more labile pools.

A comparison of the isotope-dilution and the difference method for estimating fertilizer nitrogen recovery fractions in crops. III. Experimental

Fertilizer nitrogen (N) recovery fractions may be calculated by the difference or the isotope-dilution method. With the first method an apparent recovery fraction (ARF) and with the latter method a 15N recovery fraction ( 15NRF) is calculated. The two methods may give different results, ARF generally being higher than 15NRF, particularly in the higher range of ARF-values. Fertilizer N recovery fractions in crops calculated by the two methods were compared using experimental data derived from the literature. The experimental results could be largely explained on the basis of theoretical relationships between 15NRF and ARF, based on the assumptions of (1) complete and instantaneous mixing of initial soil mineral N and fertilizer N, and (2) zero order kinetics for plant uptake, mineralization and immobilization reactions. In the lower range of ARF-values, values of 15NRF exceeded those of ARF, presumably because the plant derives its N from fertilizer as well as soil N, even if there is no crop response to applied N. In the higher range of ARF-values, 15NRF tended to be lower than ARF, presumably because of the occurrence of mineralization-immobilization turnover, which causes the 15N content of the mineral N pool to decrease. This phenomenon, referred to as ‘pool substitution’, may be enhanced by increased uptake efficiency of soil N by fertilized crops or by increased mineralization in fertilized treatments. From an agronomic point of view, the use of ARF would be preferred, as this quantity accurately reflects the overall effect of fertilizer application on crop N uptake. 15NRF is a meaningful quantity in rebalance studies. It accurately estimates the fertilizer N recovery in the crop, provided 4N released in mineralization-immobilization turnover in soil is not considered fertilizer N. The theory developed in the present series of papers may contribute to the understanding of the dynamics and transformations of fertilizer N in soil-crop systems and thus help to develop criteria for efficient and effective fertilizer N management.

A comparison of the isotope-dilution and the difference method for estimating fertilizer nitrogen recovery fractions in crops. I. Plant uptake and loss of nitrogen

The apparent recovery fraction (ARF) of applied nitrogen (N) by a crop is calculated as the difference between the total N uptake by crops from fertilized and unfertilized treatments per unit N applied. The 15N recovery fraction ( 15SNRF) is calculated as the amount of 15W-labeled N recovered in fertilized crops per unit 15SN-Iabeled N applied. The relationship between ARF and 15NRF is discussed on the basis of a complete-mixing model for the distribution of 15N-labeled N over different N pools in the soil-crop system. Mineralization-immobilization turnover in soil is not considered in the model. It is shown that in the lower range of ARF values, i.e., on soils high in available N, values of 15NRF are likely to exceed those of ARF. This is because the fertilizer N mixes with the soil mineral N pool and thus the plant derives its N from applied as well as soil N, even if there is little or no crop response to applied N. In the higher range of ARF values, i.e., in N-deficient soils, values of 15SNRF may be lower than those of ARF due to an increased uptake efficiency of soil N in fertilized treatments. Loss of N, either from the fertilizer or from the mixed soil mineral N pool, reduces the range of values of 15WRF and ARF. From an agronomic point of view, ARF is a meaningful quantity as it accurately reflects the overall effect of fertilizer application on crop N uptake, whereas 15SNRF is a meaningful quantity in 15N tracer studies on N fertilizer use efficiency and N balances in soil-crop systems. In the absence of mineralization-immobilization turnover in soil, the fertilizer N recovery in the crop is accurately estimated by 15NRF.

Influence of urease and nitrification inhibitors on N losses from soils fertilized with urea

The aim of this study was to evaluate how the N losses through volatilization and leaching from soils fertilized with urea can be affected by the application of a urease inhibitor or a urease plus a nitrification inhibitor. The experiment was carried out using lysimeters with 15N-labelled urea and N-(n-butyl) thiophosphoric triamide (NBPT) as urease inhibitor and dicyandiamide (DCD) as nitrification inhibitor, comparing three different treatments: urea alone (U), urea + NBPT (UN) and urea + NBPT + DCD (UND). Both volatilization and leaching were significantly different in the soils used, according to their physico-chemical characteristics. However, the pattern of the loss was similar: the volatilization was significantly reduced by NBPT (UN), but the presence of DCD (UND) significantly increased the loss, with respect to UN. Considering leaching, the highest amount of NO3 – was lost with UND, the lowest with U. The greatest amount of N lost by leaching was soil-derived N produced by the N mineralization-immobilization turnover. We suggest that, by maintaining the NH4 + in the soils, the inhibitors, in particular DCD, caused a priming effect with a subsequent increase in the rate of soil organic matter mineralization and an extra release of soil organic N. The priming effect was real in the sandy loam (SL) soil where a net N release was observed, whereas in the clay loam (CL) soil the effect of the inhibitors was less pronounced and an apparent priming effect was observed; however, a real priming effect also cannot be excluded in this soil.

The influence of plants grown under elevated CO2 and N fertilization on soil nitrogen dynamics

We investigated the effects of spring barley growth on nitrogen (N) transformations and rhizosphere microbial processes in a controlled system under elevated carbon dioxide (CO2) at two levels of N fertilization (applied with 15N labelling). After 25 d, elevated CO2 (twice ambient) increased plant growth (dry weight, DW) by 141% at low-N fertilization and by 60% at high-N fertilization, but its positive effect on the root-to-shoot ratio was only significant at low-N input. As a result of this plant response, elevated CO2 caused a greater soil CO2 efflux, rhizosphere soil DW, and soil microbial biomass under N-limiting conditions than under high N availability. Elevated CO2 also caused a significant (P < 0.001) increase in the N recovered by the plant from both the labelled (Nf) and unlabelled (Ns + Nuf) N pools. The dynamics of N in the system as affected by elevated CO2 were driven principally by mineralization–immobilization turnover, with little loss by denitrification. Under N-limiting conditions, there is evidence to suggest enhanced nutrient release from soil organic matter (SOM) pools—a process which could be defined as priming. The results of our experiment did not indicate a direct plant-mediated effect of elevated CO2 on nitrous oxide (N2O) fluxes or denitrification activity.

Limitations of using delta 18O for the source identification of nitrate in agricultural soils.

The stable isotopic composition (delta 15N and delta 18O) of nitrate was analyzed in two lysimeter field experiments in order to identify the conditions under which the dual isotope approach can be applied to identify the main source of nitrate in agricultural soils. The first field experiment involved six lysimeters beneath fields that had been fertilized for 10 yr with the same type of fertilizer (NH4NO3; delta 15N = +1.2@1000, delta 18O = +18.6@1000). The isotope ratios of NO3- in the leachate (delta 15N approximately 0@1000; delta 18O approximately +2@1000) could not be interpreted in a conventional way with either fertilizer or soil organic nitrogen as main sources. These results provided clear evidence for the microbial immobilization and subsequent mineralization and nitrification to NO3- (mineralization-immobilization turnover concept). This process masked the original oxygen isotope ratio of the fertilizer source during the summer when microbial activity was high. A second experiment involving the application of Ca(NO3)2 to three lysimeters during the winter confirmed that the dual isotope approach remains valid for the source identification of nitrate under conditions of low microbial activity. The study reveals the limitation of the dual isotope approach to characterize nitrate sources under biologically active conditions and the ability to quantify microbial processes when the main sources can be controlled.

Effects of Soil Structure Disturbance on Mineralization of Organic Soil Nitrogen

Disturbance of soil structure by tillage operations is thought to make soil organic N accessible for mineralization which was otherwise protected from degradation. The origin of N released by disturbance of soil structure is, however, poorly understood and needs to be related to microbial activity. This study was performed to investigate the effect of soil structure disturbance on the release of active or protected organic N pools in surface soils (0–2 cm) under plow‐ (PT) or no‐tillage (NT) management. Active soil N was defined as the pool participating in mineralization–immobilization turnover during short‐term incubation (6 d) while protected pools were considered inactive during this period. The active pool of soil N was labelled with 15N in intact samples of PT and NT soils. The samples were either kept intact or sieved and repacked, and then leached weekly during a 35‐d incubation period. The disturbance of soil structure increased mineral N release from 6 to 15 mg kg−1 in the NT soil within the first week after disturbance. This release was found to originate from both active and physically protected N pools as could be assessed by the relative differences in 15N content of mineralized N by intact and disturbed soil samples. In contrast, the release from the PT soil was 7 to 9 mg N kg−1 after disturbance, with only a minor contribution from protected N pools. These results support the theory that disturbance of soil structure by tillage may destabilize and release protected pools of soil N. Over the entire period of incubation, protected N accounted for 27% of total N release in the NT soil and 12% in that of PT. The calculation of availability ratios, defined as the ratio between the 15N enrichment of mineralized N and that of total soil N, showed that recently added 15N was less available for mineralization in the NT soil as compared to that of PT. The probable cause for this difference was the higher C/N ratio of organic matter in NT surface soil indicating more nonhumified organic matter when compared to PT organic matter.

Mineralization and immobilization of nitrogen in heath soil under intact Calluna, after heather beetle infestation and nitrogen fertilization

The maintenance of low availability of mineral N in heath soils is thought to be a key factor for the stability of heathland ecosystems. We investigated the turnover of NH 4 + and NO 3 − in the organic surface layer of soils from a Danish heathland using 15N isotope techniques in laboratory incubations. The soils were sampled under intact and dead Calluna vegetation. The dead Calluna vegetation had been fertilized at rates of 0, 15 or 35 kg N ha −1 per year and the death of vegetation had been caused by a naturally occurring heather beetle infestation. In the soil under intact Calluna, the NH 4 + pool was very low with no net mineralization, while a substantial mineralization-immobilization turnover of NH 4 + was found with a large capacity for short term net NH 4 + immobilization (36 μg N g −1 during 1 h; 135 μg N g −1 during 24 h). The metabolic inhibitor mercury chloride completely inhibited assimilation of NH 4 + indicating the process was biological. The immobilization of NH 4 + had no short or long-term (38 days) effect on soil respiration while NH 4 + immobilization stimulated net mineralization of soil N during long-term incubation. The soils sampled under dead and dead/fertilized Calluna had large pools and high net mineralization rates of NH 4 + with a decrease of gross NH 4 + immobilization relative to the soil under intact Calluna. Neither net nor gross nitrification activity could be detected in any of the soils. The results indicate that the effects of an increased atmospheric N deposition to the heathland may be delayed because of the tight cycling of NH 4 + and the storage capacity for N in the soil and vegetation. The ecosystem may, however, be susceptible to disruption of the tight NH 4 + cycling because of the limited capacity of the ecosystem to remove excess mineral N from the soil. This may increase the risk of conversion of the heath into grassland.

Availability of Nitrogen in 15N‐Labeled Ruminant Manure Components to Successively Grown Crops

AbstractTo improve crop N use efficiency of animal manures, the availability of N in individual manure components must be better understood. This microplot field study quantifies crop offtakes of N in four similar batches of ruminant manure containing 15N‐labeled urine, feces, or straw, or unlabeled components only. The urine and feces were from a sheep first fed unlabeled hay and then 15N‐labeled hay. Manures (≈ 19 g total N m‐2) were incorporated into two coarse‐textured soils before planting to spring barley (Hordeum vulgare L.) undersown with perennial ryegrass (Lolium perenne L.). Manures with one 15N‐labeled component were supplemented with unlabeled NH4NO3 (7.3 g N m‐2), while unlabeled manure was given 15NH415NO3. Labeled and unlabeled N were determined in the spring barley at maturity and in six cuts of ryegrass taken during the succeeding 2.5 yr. The homogeneity of feces and urine 15N‐labeling was high. Dry matter yields and crop N offtakes were similar in all treatments. Barley (grain and straw) recovered 40, 26, 10, and 6%, respectively of 15N added with mineral fertilizer, urine, straw, and feces. Weighted mean recovery of the combined manure and fertilizer dressing was 22% of the added 15N. Crop recovery of urine and feces 15N was smaller and that of straw 15N higher than reported in previous studies on individual components, indicating that the N mineralization‐immobilization turnover (MIT) of the manure components interacted. In the second and third growth seasons, 2.7 to 4.4% and 1.1 to 2.0% of the 15N was recovered in grass cuts, respectively. Total recovery ranged from 84 to 95% of the added 15N, suggesting small N losses from this cropping system.

Gross nitrogen transformations in grassland soils as affected by previous management intensity

Gross transformations of nitrogen were studied in a grassland soil which had received N inputs of 100, 200, 300, 400 or 500 kg N ha −1 y −1 for the past 7 y. Paired soil incubations were undertaken in January 1996 using differentially 15N-labelled NH 4NO 3 applied in solution at a rate of 57 μmol N g −1 oven-dried soil and at an enrichment of 10 atom % excess. The change in concentration and enrichment of the NH 4 +-N, NO 3 −-N and N 2O pools was determined at 0, 1, 3, 7, 14 and 21 d. Using 15N pool dilution gross rates of nitrification, mineralization and immobilization were calculated. The results showed that the mineralization–immobilization turnover in January was similar on all soils, despite their different management histories. There was little net mineralization of organic N throughout the incubation. Changes in the 15N enrichment of the NO 3 −-N pool showed that there was substantial nitrification during the incubation and that it was greater in soils with a history of high N inputs compared to those receiving low N inputs. Cumulative gross nitrification after 21 d was equivalent to 44.0, 84.2, 106.8, 165.7 and 181.7 kg N ha −1 for the soils with a history of inputs of 100, 200, 300, 400 and 500 kg N ha −1 y −1, respectively. The N 2O and N 2 concentrations were low in all soils. The average daily N 2O flux over the incubation was 0.38. 0.27, 0.38, 0.64 and 1.07 mg N 2O-N kg −1 d −1 for soils previously receiving 100, 200, 300, 400 and 500 kg N ha −1 y −1, respectively. The N 2 flux was lower than that of N 2O. Evidence suggested that the N 2O produced was solely from denitrification which was proportionately greater on soils with a history of high N applications. Nitrification and denitrification in grassland soils is, therefore, highly influenced by previous management intensity.