Predictors of gross N mineralization and immobilization during decomposition of stabilized organic matter in agricultural soil

Long-term additions of different types of organic amendments affect the amount of soil organic matter. Less is known about how this in turn affects carbon (C) and nitrogen (N) mineralization from the pool of stabilized soil organic matter, or the extent to which gross N immobilization influences the net amount of N mineralized. Soils, differing in the quantity and quality of organic matter inputs they had received since 1956, were sampled approximately 6 or 18 months after the most recent applications of organic amendments. Two laboratory experiments were carried out to: (i) evaluate if, and how, the organic amendments had affected C mineralization, gross and net N mineralization; (ii) examine the relation between gross N immobilized and free-light fraction of organic matter; and (iii) assess predictors for gross N mineralization and immobilization rates in soils. The amount of soil organic C and N were major determinants of C and gross N mineralization, but not of net N mineralization. Carbon mineralization was related to gross N mineralization, but the ratio between the two was not constant. Gross N immobilization was related to the amount of free-light fraction material in the soil with 90% variation explained. For most common organic amendments applied in autumn, our results support the use of total soil organic N and C mineralization as predictors of gross N mineralization from stabilized soil organic matter. In addition, we propose that the amount of free-light fraction material present in the soil in spring is adequate as a predictor of the immobilization potential of the soil, without a need to consider the C-to-N ratio of this material.     

Estimates of Gross Transformation Rates of Dairy Manure N Using 15N Pool Dilution

Abstract

Most measurements of dairy manure nitrogen (N) availability depend on net changes in soil inorganic N concentration over time, which overlooks the cycling of manure N in the soil. Gross transformations of manure N, including mineralization (m), immobilization (i), and nitrification (n), can be quantified using ¹⁵N pool dilution methods. This research measures gross m, n, and i resulting from application of four freeze‐dried dairy manures that had distinctly different patterns of N availability. A sandy loam soil (coarse‐loamy, mixed, frigid Typic Haplorthod) was amended with four different freeze‐dried dairy manures and incubated at 25°C with optimal soil water content. The dilution of ¹⁵ammonium (NH4⁺) during a 48‐h interval (7–9 d and 56–58 d after manure application) was used to estimate m, whereas the dilution of ¹⁵nitrate (NO₃ ^?) was used to estimate n. Gross immobilization was calculated as gross minus net mineralization. Gross mineralization in the unamended soil was similar at 7‐ to 9‐d and 56‐ to 58‐d intervals and was significantly increased by the application of manures. For both amended and unamended soil, m was much greater (i.e., three‐ to nine‐fold) than estimated net mineralization, illustrating the degree to which manure N can be cycled in soil. At the early interval, both m and i were directly related to the manure C input, demonstrating the linkage between substrate C availability and N utilization by soil microbes. This research clearly shows that the application of dairy manures stimulates gross N transformation rates in the soil, improving our understanding of the impact of manure application on soil N cycling.     

Long-term effects of mineral and organic fertilization on soil organic matter fractions and sorghum yield under Sudano-Sahelian conditions

Abstract. Knowledge of changes in soil organic matter (SOM) fractions resulting from agricultural practice is important for decision‐making at farm level because of the contrasting effects of different SOM fractions on soils. A long‐term trial sited under Sudano‐Sahelian conditions was used to assess the effect of organic and inorganic fertilization on SOM fractions and sorghum performance. Sorghum straw and kraal manure were applied annually at 10 t ha^?1, with and without urea at 60 kg N ha^?1. The other treatments included fallowing, a control (no fertilization), and inorganic fertilization only (urea, 60 kg N ha^?1). Fallowing gave significantly larger soil organic carbon and nitrogen (N) levels than any other treatment. Total soil SOM and N concentrations increased in the following order: urea only < straw < control < straw+urea < manure with or without urea < fallow. Farming had an adverse effect on SOM and N status; however, this mostly affected the fraction of SOM >0.053 mm (particulate organic matter, POM). The POM concentrations in the control, straw and urea‐only treatments were about one‐half of the POM concentrations in the fallow treatment. POM concentrations increased in the following order: urea only < control < straw with or without urea < manure with or without urea < fallow. The fraction of SOM <0.053 mm (fine organic matter, FOM) was greater than POM in all plots except in fallow and manure+urea plots. Total N concentration followed the same trend as SOM, but cultivation led to a decline in both POM‐N and FOM‐N. Crop yield was greatest in the manure plots and lowest in the straw, control and urea‐only plots. Results indicate that under Sudano‐Sahelian conditions, SOM, POM and FOM fractions and crop performance were better maintained using organic materials with a low C/N ratio (manure) than with organic material with a high C/N ratio (straw). Urea improved the effect of straw on crop yield and SOM concentration.     

Decadal Nitrogen Fertilization Decreases Mineral‐Associated and Subsoil Carbon: A 32‐Year Study

Crop residues and manure are important sources of carbon (C) for soil organic matter (SOM) formation. Crop residue return increases by nitrogen (N) fertilization because of higher plant productivity, but this often results only in minor increases of SOM. In our study, we show how N fertilization and organic C additions affected SOM and its fractions within a 32‐year‐long field‐experiment at Puch, Germany. Five organic additions, no‐addition (control), manure, slurry, straw and straw + slurry, were combined with three mineral N fertilization rates (no, medium and high fertilization), which resulted in 1·17–4·86 Mg C‐input ha^‐1 y^‐1. Topsoil (0–25 cm) SOM content increased with N fertilization, mainly because of the C in free light fraction (f‐LF). In contrast, subsoil (25–60 cm) SOM decreased with N fertilization, probably because of roots' relocation in Ap horizon with N fertilization at the surface. Despite high inputs, straw contributed little to f‐LF but prevented C losses from the mineral‐associated SOM fraction (ρ > 1·6 g cm^‐3) with N fertilization, which was observed without straw addition. Above (straw) and belowground (roots) residues had opposite effects on SOM fractions. Root C retained longer in the light‐fractions and was responsible for SOM increase with N fertilization. Straw decomposed rapidly (from f‐LF) and fueled the mineral‐associated SOM fraction. We conclude that SOM content and composition depended not only on residue quantity, which can be managed by the additions and N fertilization, but also on the quality of organics. This should be considered for maintaining the SOM level, C sequestration, and soil fertility. Copyright © 2016 John Wiley & Sons, Ltd.     

Temporal variations of crop residue effects on soil N transformation depend on soil properties as well as residue qualities

Purple soils (Eutric Regosols) are widely distributed in humid subtropical Southwest China. They are characterized by high nitrification activities, with risks of severe NO₃^? leaching. Incorporation of crop residues is considered an effective method to reduce NO₃^? loss. In the present study, we compared the effects of alfalfa, rice straw, and sugarcane bagasse on gross N transformation turnover in a purple soil (purple soil, pH 7.62) compared with those in an acid soil (acid soil, pH 5.26), at 12 h, 3 months, and 6 months after residue incorporation. The gross N transformation rates were determined by ¹⁵N tracing. All tested crop residues stimulated the gross N mineralization rates, but reduced the net mineralization rates in both soils at 12 h after residue incorporation; however, the extent of the effect varied with the crop residue qualities, with rice straw having the strongest effects. Crop residues reduced net nitrification rates by depressing gross autotrophic nitrification rates and stimulating NO₃^? immobilization rates in the purple soil, particularly after rice straw incorporation (net nitrification rate decreased from 16.72 mg N kg^?1 d^?1 in the control to ??29.42 mg N kg^?1 d^?1 at 12 h of residue incorporation); however, crop residues did not affect the gross autotrophic nitrification rates in the acid soil. Crop residue effects subsided almost completely within 6 months, with sugarcane bagasse showing the longest lasting effects. The results indicated that crop residues affected the N transformation rates in a temporal manner, dependent on soil properties and residue qualities.     

Temperature and stubble management influence microbial CO2-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 CO₂-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 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.     

Effects of long‐term repeated mineral and organic fertilizer applications on soil nitrogen transformations

Repeated applications of mineral and/or organic fertilizer will probably affect gross nitrogen (N) dynamics in soils in the long term but only a limited number of observations are available. Here we present results of a ¹⁵N tracing study with soil from the various fertilizer treatments of the Huang‐Huai‐Hai Plain experiment that has been in operation for more than 17 years. Mineral fertilizer in various combinations of N, phosphorus (P) and potassium (K), organic manure (OM) or a mixture of mineral fertilizer and manure had been repeatedly applied for 17 years. The gross N transformation rates were quantified with a ¹⁵N tracing model, which uses a parameter optimization routine based on Bayesian principles. Mineralization of soil organic matter was at least 2.7 times greater in all fertilizer treatments compared with the untreated control (0.67 µg N g^?1 day^?1). While application of mineral N enhanced mineralization from recalcitrant organic N, the application of organic fertilizers stimulated the mineralization of labile organic N. Gross nitrate (NO₃^?) production solely resulted from ammonium (NH₄⁺) oxidation. Compared with the gross NO₃^? production in the control treatment (2.22 µg N g^?1 day^?1), long‐term N applications stimulated gross nitrification by more than 5.3 times. The largest gaseous N emissions were associated with the organic manure treatments. The ratio of gross NO₃^? production to total mineral N consumption, a ratio proposed previously to determine potential NO₃^? loss, was a good indicator except for the treatment without N application. This ratio increased from 0.8 in the control to 2.7 in the mixture of mineral fertilizer and manure treatment. The largest gaseous N emissions (N₂O + NO) (P < 0.05) were generally found at greater ratios. Results clearly showed that various fertilizers have a differential effect on N dynamics and potential gaseous N losses in the long term.     

Seasonal dynamics of carbon and nitrogen pools and fluxes under continuous arable and ley-arable rotations in a temperate environment

Improved understanding of the seasonal dynamics of C and N cycling in soils, and the main controls on these fluctuations, is needed to improve management strategies and to better match soil N supply to crop N demand. Although the C and N cycles in soil are usually considered to be closely linked, few data exist where both C and N pools and gross N fluxes have been measured seasonally. Here we present measurements of inorganic N, extracted soluble organic N, microbial biomass C and N, gross N fluxes and CO₂ production from soil collected under wheat in a ley‐arable and continuous arable rotation within a long‐term experiment. The amounts of inorganic N and extracted soluble organic N were similar (range 5–35 kg N ha⁻¹; 0–23 cm) but had different seasonal patterns: whilst inorganic N declined during wheat growth, extracted soluble organic N peaked after cultivation and also during maximal stem elongation. The microbial biomass was significantly larger in the ley‐arable (964 kg C ha⁻¹; 0–23 cm) than the continuous arable rotation (518 kg C ha⁻¹; 0–23 cm) but with no clear seasonal pattern. In contrast, CO₂ produced from soil and gross N mineralization showed strong seasonality linked to soil temperature and moisture content. Normalization of soil CO₂ production and gross N mineralization with respect to these environmental regulators enabled us to study the underlying influence of the incorporation of fresh plant material into soil on these processes. The average normalized gross rates of N mineralized during the growing season were 1.74 and 2.55 kg N ha⁻¹ nday⁻¹ in continuous arable and ley‐arable rotations respectively. Production rates (gross N mineralization, gross nitrification) were similar in both land uses and matched rates of NH₄⁺ and NO₃⁻ consumption, resulting in periods of net N mineralization and immobilization. There was no simple relationship between soil CO₂ production and gross N mineralization, which we attributed to changes in the C : N ratio of the mineralizing pool(s).     

Influence of Long‐Term Sodic‐Water Irrigation,Gypsum, and Organic Amendments on Soil Properties and Nitrogen Mineralization Kinetics under Rice–Wheat System

Abstract

Influence of long‐term sodic‐water (SW) irrigation with or without gypsum and organic amendments [green manure (GM), farmyard manure (FYM), and rice straw (RS)] on soil properties and nitrogen (N) mineralization kinetics was studied after 12 years of rice–wheat cropping in a sandy loam soil in northwest India. Long‐term SW irrigation increased soil pH, exchangeable sodium percentage (ESP), and sodium adsorption ratio (SAR) and decreased organic carbon (OC) and total N content. On the other hand, application of gypsum and organic amendments resulted in significant improvement in all these soil properties. Mineralization of soil N ranged from 54 to 111 mg N kg^?1 soil in different treatments. Irrigation with SW depressed N mineralization. In SW‐irrigated plots, two flushes of N mineralization were observed; the first during 0 to 7 d and the second after 28 d. Amending SW irrigated plots with GM and FYM enhanced mineralization of soil N. Gypsum application along with SW irrigation reduced cumulative N mineralization at 56 days in RS‐amended plots but increased it under GM‐treated, FYM‐treated, or unamended plots. Nitrogen mineralization potential (N_o) ranged from 62 to 543 mg N kg^?1 soil. In the first‐order zero‐order model (FOZO), the easily decomposable fraction ranged from 5.4 to 42 mg N kg^?1 soil. Compared to the first‐order single compartment model, the FOZO model could better explain the variations in N mineralization in different treatments. Variations in N_o were influenced more by changes in pH, SAR, and ESP induced by long‐term SW irrigations and amendments rather than by soil OC.     

Effect of organic manures,crop residues and green manure (Sesbania aculeata) on nitrogen and phosphorus transformations in a sandy loam at field capacity and under waterlogged conditions

Summary The influence of the water regime on mineralization and immobilization of N and P was investigated in a calcareous sandy loam incubated with cattle, poultry and green manure (Sesbania aculeata), and wheat and rice straw in a pot experiment. At field capacity, N released from poultry and green manure during the first 4 weeks of incubation was 45% and 59%, respectively. During the next 12 weeks, only around 40% more organic N was mineralized from both sources. In contrast, addition of cattle manure resulted in a period of net N immobilization lasting up to 4 weeks. By the end of 16 weeks of incubation only about 19010 of the added N was mineralized. High rates of N immobilization were observed during the first 4 weeks of incubation of rice or wheat straw with C/N ratios of 78 and 85, respectively. The N mineralization kinetics of poultry and green manure and of untreated soil showed an initial fast reaction followed by a slow release of inorganic N and could be described by two simultaneous first-order reactions. Under waterlogged conditions mineralized N was lost simultaneously in significant amounts possibly through nitrification — denitrification reactions. At field capacity, the largest amount of Olsen P was accumulated in the soil amended with poultry manure, followed by cattle manure. Results from other treatments did not differ much from those of the untreated soil. About 15% of P from poultry manure was mineralized during the 1st week of incubation. In contrast to the field-capacity moisture regime, marked increases in Olsen P in the soils amended with green manure and crop residues were observed under water-logged conditions.     

Effects of soil moisture on gross N transformations and N2O emission in acid subtropical forest soils

Soil moisture changes, arising from seasonal variation or from global climate changes, could influence soil nitrogen (N) transformation rates and N availability in unfertilized subtropical forests. A ¹⁵?N dilution study was carried out to investigate the effects of soil moisture change (30–90 % water-holding capacity (WHC)) on potential gross N transformation rates and N₂O and NO emissions in two contrasting (broad-leaved vs. coniferous) subtropical forest soils. Gross N mineralization rates were more sensitive to soil moisture change than gross NH₄ ⁺ immobilization rates for both forest soils. Gross nitrification rates gradually increased with increasing soil moisture in both forest soils. Thus, enhanced N availability at higher soil moisture values was attributed to increasing gross N mineralization and nitrification rates over the immobilization rate. The natural N enrichment in humid subtropical forest soils may partially be due to fast N mineralization and nitrification under relatively higher soil moisture. In broad-leaved forest soil, the high N₂O and NO emissions occurred at 30 % WHC, while the reverse was true in coniferous forest soil. Therefore, we propose that there are different mechanisms regulating N₂O and NO emissions between broad-leaved and coniferous forest soils. In coniferous forest soil, nitrification may be the primary process responsible for N₂O and NO emissions, while in broad-leaved forest soil, N₂O and NO emissions may originate from the denitrification process.     

Depressed gross mineralization and immobilization of nitrogen and changes in microbial population in soil after heat treatment

Partial sterilization causes a change in N mineralization in soil. An increase in the net rate of N mineralization was reported in soil with chloropicrin applied to it (Rovira 1976), and has been well known in soil fumigated with chloroform to measure the microbial biomass N (Jenkinson and Ladd 1981). The gross rate of N mineralization increased in soil inoculated with fresh soil following fumigation with chloroform (Shen et al. 1984). The increased rate of N mineralization has been attributed to the rapid decomposition of organisms killed by partial sterilization (Jenkinson 1966). On the other hand, Nira et al. (1996) reported that the application of a fumigant in a field depressed the gross rates of N mineralization and immobilization in spite of the increase in the net rate of N mineralization. These results suggested that the increase in the net rate of N mineralization by partial sterilization is presumably due to the change in the ratio of N mineralization to immobilization. However, the residues of a fumigant may depress gross N transformation in the field, because the residues may continue to influence microbial activity long after the original treatment (Jenkinson 1966). Some effects of partial sterilization without residues on gross N mineralization remain to be determined.     

Modelling carbon and nitrogen dynamics in a bare soil with and without straw incorporation

Incorporating straw into the ploughed layer of soil affects the dynamics of carbon and nitrogen. A precise quantification of its short‐term effect in agricultural fields is difficult because biological and physical processes interact and take place simultaneously. As an alternative to experimentation, investigations have turned to simulations using mechanistic models, and we have taken this approach. The goal of our study was to test a mechanistic and one‐dimensional model of transport and biotransformation (PASTIS) against a data set obtained in a field experiment in northern France. We tested carbon and nitrogen dynamics by measuring C mineralization rates, the rates of gross immobilization and mineralization of N (using ¹⁵N tracing), and inorganic pools of N in the soil profile during 1 year in a bare soil with or without addition of wheat straw. Most of the model parameters were determined in independent experiments. We estimated the biological parameters from incubation experiments in the laboratory. The simulated results were in good agreement with experimental data, particularly for gross N rates. Hypotheses concerning the pathway of microbial assimilation and the dependence of decomposition on the size of the biomass were tested. The simulated net N immobilization due to addition of straw (8000 kg dry matter ha^?1) reached a maximum of 64 kg N ha^?1 after 2 months, whereas the observed value was 66 kg N ha^?1. The model indicated that after 13 months the incorporation of straw had reduced the net amount of nitrogen mineralized by 13% and the amount of leached nitrate by 27%. The sensitivity analysis to the depth of straw incorporation indicated that the deeper was the incorporation the less was the leaching and the mineralization of nitrogen.     

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.     

Grain yield and crop N offtake in response to residual fertilizer N in long‐term field experiments

Organic inputs [e.g. animal manure (AM) and plant residues] contribute directly to the soil organic N pool, whereas mineral N fertilizer contributes indirectly by increasing the return of the crop residues and by microbial immobilization. To evaluate the residual effect of N treatments established in four long‐term (>35 yr) field experiments, we measured the response of barley (grain yield and N offtake at crop maturity) to six rates (0, 30, 60, 90, 120 and 150 kg N/ha) of mineral fertilizer N (N_new) applied in subplots replacing the customary long‐term plot treatments of fertilizer inputs (N_prev). Rates of N_prev above 50–100 kg N/ha had no consistent effect on the soil N content, but this was up to 20% greater than that in unfertilized treatments. Long‐term unfertilized plots should not be used as control to test the residual value of N in modern agriculture with large production potentials. Although the effect of mineral N_prev on grain yield and N offtake could be substituted by N_new within a range of previous inputs, the value of N_prev was not eliminated irrespective of N_new rate. Provided a sufficient supply of plant nutrients other than N, the use‐efficiency of N_new did not change significantly with previous mineral N fertilizer rate. The residual effect of mineral N fertilizer was negligible compared with the residual effect of N from AM and catch crop residues.     

Mechanisms of soil N dynamics following long-term application of organic fertilizers to subtropical rain-fed purple soil in China

In this study, a ¹⁵N tracing incubation experiment and an in situ monitoring study were combined to investigate the effects of different N fertilizer regimes on the mechanisms of soil N dynamics from a long-term repeated N application experiment. The field study was initiated in 2003 under a wheat-maize rotation system in the subtropical rain-fed purple soil region of China. The experiment included six fertilization treatments applied on an equivalent N basis (280 kg N ha⁻¹), except for the residue only treatment which received 112 kg N ha⁻¹: (1) UC, unfertilized control; (2) NPK, mineral fertilizer NPK; (3) OM, pig manure; (4) OM-NPK, pig manure (40% of applied N) with mineral NPK (60% of applied N); (5) RSD, crop straw; (6) RSD-NPK, crop straw (40% of applied N) with mineral NPK (60% of applied N). The results showed that long-term repeated applications of mineral or organic N fertilizer significantly stimulated soil gross N mineralization rates, which was associated with enhanced soil C and N contents following the application of N fertilizer. The crop N offtake and yield were positively correlated with gross mineralization. Gross autotrophic nitrification rates were enhanced by approximately 2.5-fold in the NPK, OM, OM-NPK, and RSD-NPK treatments, and to a lesser extent by RSD application, compared to the UC. A significant positive relationship between gross nitrification rates and cumulative N loss via interflow and runoff indicated that the mechanisms responsible for increasing N loss following long-term applications of N fertilizer were governed by the nitrification dynamics. Organic fertilizers stimulated gross ammonium (NH₄⁺) immobilization rates and caused a strong competition with nitrifiers for NH₄⁺, thus preventing a build-up of nitrate (NO₃⁻). Overall, in this study, we found that partial or complete substitution of NPK fertilizers with organic fertilizers can reduce N losses and maintain high crop production, except for the treatment involving application of RSD alone. Therefore, based on the N transformation dynamics observed in this study, organic fertilizers in combination with mineral fertilizer applications (i.e. OM, OM-NPK, and RSD-NPK treatments) are recommended for crop production in the subtropical rain-fed purple soils in China.     

Mineralization of Three Organic Manures Used as Nitrogen Source in a Soil Incubated under Laboratory Conditions

Abstract

The rate and timing of manure application when used as nitrogen (N) fertilizer depend on N‐releasing capacity (mineralization) of manures. A soil incubation study was undertaken to establish relative potential rates of mineralization of three organic manures to estimate the value of manure as N fertilizer. Surface soil samples of 0–15 cm were collected and amended with cattle manure (CM), sheep manure (SM), and poultry manure (PM) at a rate equivalent to 200 mg N kg^?1 soil. Soil without any amendment was used as a check (control). Nitrogen‐release potential of organic manures was determined by measuring changes in total mineral N [ammonium‐N+nitrate‐N (NH₄ ⁺–N+NO₃ ^?–N)], NH₄ ⁺–N, and accumulation of NO₃ ^?–N periodically over 120 days. Results indicated that the control soil (without any amendment) released a maximum of 33 mg N kg^?1soil at day 90, a fourfold increase (significant) over initial concentration, indicating that soil had substantial potential for mineralization. Soil with CM, SM, and PM released a maximum of 50, 40, and 52 mg N kg^?1 soil, respectively. Addition of organic manures (i.e., CM, SM, and PM) increased net N released by 42, 25, and 43% over the control (average). No significant differences were observed among manures. Net mineralization of organic N was observed for all manures, and the net rates varied between 0.01 and 0.74 mg N kg^?1 soil day^?1. Net N released, as percent of organic N added, was 9, 10, and 8% for CM, SM, and PM. Four phases of mineralization were observed; initial rapid release phase in 10–20 days followed by slow phase in 30–40 days, a maximum mineralization in 55–90 days, and finally a declined phase in 120 days. Accumulation of NO₃ ^?–N was 13.2, 10.6, and 14.6 mg kg^?1 soil relative to 7.4 mg NO₃ ^?–N kg^?1 in the control soil, indicating that manures accumulated NO₃ ^?–N almost double than the control. The proportion of total mineral N to NO₃ ^?–N revealed that a total of 44–61% of mineral N is converted into NO₃ ^?–N, indicating that nitrifiers were unable to completely oxidize the available NH₄ ⁺. The net rates of mineralization were highest during the initial 10–20 days, showing that application of manures 1–2 months before sowing generally practiced in the field may cause a substantial loss of mineralized N. The rates of mineralization and nitrification in the present study indicated that release of inorganic N from the organic pool of manures was very low; therefore, manures have a low N fertilizer effect in our conditions.     

施肥对红壤稻田硫素演变及供硫能力的影响

研究了进行 15年的不同施肥结构对红壤稻田土壤硫素肥力演变和对土壤供硫能力的影响。结果表明 ,不同施肥结构明显影响土壤全硫、有机硫、无机硫和供硫潜力 (无机硫加部分有机硫 ,40℃KCl浸提 )的演变时间进程以及土壤有机硫的潜在矿化量、矿化常数和矿化半减期 ,稻草还田和有机肥施用有利于土壤硫的积累 ,提高土壤供硫能力 ;连续盆栽幼苗试验结果表明 ,作物吸硫量以稻草还田区 有机肥加化肥区 全化肥区 无肥区 ,与土壤全硫、供硫潜力及土壤硫潜在矿化量顺序一致     

Soil microbial biomass and nitrogen dynamics in a turfgrass chronosequence: A short-term response to turfgrass clipping addition

A mechanistic understanding of soil microbial biomass and N dynamics following turfgrass clipping addition is central to understanding turfgrass ecology. New leaves represent a strong sink for soil and fertilizer N, and when mowed, a significant addition to soil organic N. Understanding the mineralization dynamics of clipping N should help in developing strategies to minimize N losses via leaching and denitrification. We characterized soil microbial biomass and N mineralization and immobilization turnover in response to clipping addition in a turfgrass chronosequence (i.e. 3, 8, 25, and 97 yr old) and the adjacent native pines. Our objectives were (1) to evaluate the impacts of indigenous soil and microbial attributes associated with turf age and land use on the early phase decomposition of turfgrass clippings and (2) to estimate mineralization dynamics of turfgrass clippings and subsequent effects on N mineralization of indigenous soils. We conducted a 28-d laboratory incubation to determine short-term dynamics of soil microbial biomass, C decomposition, N mineralization and nitrification after soil incorporation of turfgrass clippings. Gross rates of N mineralization and immobilization were estimated with ¹⁵N using a numerical model, FLAUZ. Turfgrass clippings decomposed rapidly; decomposition and mineralization equivalent to 20-30% of clipping C and N, respectively, occurred during the incubation. Turfgrass age had little effect on decomposition and net N mineralization. However, the response of potential nitrification to clipping addition was age dependent. In young turfgrass systems having low rates, potential nitrification increased significantly with clipping addition. In contrast, old turfgrass systems having high initial rates of potential nitrification were unaffected by clipping addition. Isotope ¹⁵N modeling showed that gross N mineralization following clipping addition was not affected by turf age but differed between turfgrass and the adjacent native pines. The flush of mineralized N following clipping addition was derived predominantly from the clippings rather than soil organic N. Our data indicate that the response of soil microbial biomass and N mineralization and immobilization to clipping addition was essentially independent of indigenous soil and microbial attributes. Further, increases in microbial biomass and activity following clipping addition did not stimulate the mineralization of indigenous soil organic N.     

Nitrogen Mineralization of Two Manures as Influenced by Contrasting Application Methods under Laboratory Conditions

The decomposition and the associated nitrogen (N) dynamics of organic N sources are affected by their contact with soil. While several authors have examined the effect of surface application or incorporation of crop residues on their decomposition rate, less information is available about the relationship between the placement of animal manure and their N mineralization rate. This study investigated the influence of chicken manure and cattle manure placement on soil N mineralization. The manures were incorporated or surface applied at 175 mg N kg^?1, and N release was periodically determined over 56 days by measuring inorganic N [nitrate (NO₃ ^?) N and ammonium (NH₄ ⁺) N] in a 2 M potassium chloride (KCl) extract at a ratio of 1:10 (w/v). Results indicated that the control soil released a maximum of 64 mg N kg^?1 soil at day 21, a sixfold increase over the initial concentration, which indicates its substantial mineralization potential. Manure treatments showed an initial increase in net NO₃ ^?-N content at the start of the experiments (until day 7) before an extended period of immobilization, which ended at day 21 of the incubation. A small but positive net N mineralization of all manures was observed from 28 days of incubation. At each sampling time, the mean mineral N released from the control was significantly less (P < 0.01) than surface-applied chicken manure, incorporated chicken manure, and surface-applied cattle manure. Treatments exceptions were at days 21 and 28 where N immobilization was at its peak. In contrast, incorporation of cattle manure showed a different N-release pattern, whereby the mineral N amount was only significantly greater than the control soil at days 42 and 56 with 84 and 108 mg N kg^?1 soil respectively. Incorporation of chicken manure and cattle manure did not favor nitrification as much as surface application and cattle manure caused a much greater immobilization when incorporated than when surface applied.