Decomposition in forest and fallow subarctic soils

Summary Large-scale argicultural development in high latitude regions could lead to large losses of soil C due to accelerated decomposition. Changes in decomposition rates of forest floor material upon land clearing in interior Alaska were simulated by measuring, over a 2-year period, changes in mass, cellulose, lignin, and N of forest floor materials and in mass of filter papers and wood in a forest floor and a fallowed field. All materials decomposed slowly at the surface, with about 90% of the original weight remaining after 2 years. Decomposition rates were higher for materials buried in the field than the forest. Cellulose loss in forest floor materials closely followed mass loss, whereas lignin loss was not significant. However, weight loss of wood was rapid when buried in the field, with about 20% of the initial mass remaining after 2 years. Relationships between mass loss of buried forest floor materials and soil degree days were significant (r=70%–80%). Temperature was a major, but not the only factor, controlling decomposition rates. Forest floor materials showed significant N losses, indicating net N mineralization and that N deficiency was not a factor affecting decomposition. C loss to the atmosphere due to decomposition of forest floor materials after forest clearing will be minimal and similar to that in the undisturbed forest if left on the soil surface, but will be substantial if incorportated into the soil. Incorporation is necessary for cropping; thus some accelerated decomposition is unavoidable in clearing subarctic forests for cultivation.     

Decomposition and nitrogen mineralization from legume and non-legume crop residues in a subarctic agricultural soil

An understanding of the C and N dynamics of crop residues is important for efficient nutrient management. The present experiment was conducted to determine the rate of mass and N loss from alfalfa, faba bean, barley, and rape crop residues in a subarctic agricultural soil. Mass, C, and N losses were measured from residues contained in mesh bags and placed on the soil surface or buried 15 cm below the surface. The mass loss from October, 1988, to May, 1989, was the same for surface and buried alfalfa, barley, and rape residues, averaging 40, 20, and 61%, respectively, while surface and buried faba bean residue sustained 30 and 40% mass loss, respectively. The mass loss of the buried residues continued over the summer but not of those placed on the soil surface, resulting in an average 23% greater mass loss of the buried materials after 1 year. The N loss from October to May was similar from the surface and from the buried placements for the alfalfa, faba bean, and rape residues, averaging 11.3, 10.3 and 38.4 g N kg^-1 residue, respectively, while the surface and buried barley lost 2.9 and 4.2 g N kg^-1, respectively. The C:N ratio of all of the residues increased during the winter. These data indicate that the rate of decomposition and N mineralization from crop residues in subarctic environments can equal that measured in temperate climates. Furthermore, the concurrent loss of mass and N combined with an increase in the C:N ratio of the residues suggests that physical rather than biological processes were functioning during the winter. Most of the mass and N loss from these residues occurred during the winter, out of phase with crop demand, thereby creating the potential for N loss from the system and inefficient use of crop residue N.     

Long-term management impacts on soil carbon and nitrogen dynamics of grazed bermudagrass pastures

Managed pastures have potential for C and N sequestration in addition to providing forage for livestock. Our objectives were to investigate changes in soil organic C (SOC) and soil organic N (SON) concentrations and mineralizable C and N in cattle (Bos indicus) grazed bermudagrass [Cynodon dactylon (L.) Pers.] pastures up to 32 y after establishment. Management included low- and high-grazing intensity, fertilization, and winter overseeding with annual ryegrass (Lolium multiflorum Lam.) and clover (Trifolium sp.). Soil (0-15 cm) was sampled 7, 15, 26, and 32 y after establishment of Coastal and common bermudagrass pastures. No significant differences in SOC or SON concentrations were observed between Coastal and common bermudagrass pastures. Grazing strategies played important roles in C and N sequestration, as high-grazing intensity resulted in a lower increase in SOC and SON concentrations over time compared to low-grazing intensity. Increases in SOC were observed up to 26 y, while increases in SON were observed up to 32 y after establishment of bermudagrass pastures. Soil organic C increased 67 and 39% from 7 to 26 y at low-grazing intensity for bermudagrass+ryegrass and bermudagrass+clover pastures, respectively. SOC and SON concentrations did not increase beyond 15 y after bermudagrass establishment at high-grazing intensity. An exception was the Coastal bermudagrass+ryegrass pastures, which exhibited higher SON at 32 y than at 7 y at both grazing intensities. By 32 y, SON increased 83 and 45% in Coastal bermudagrass+ryegrass pastures at low- and high-grazing intensity, respectively, compared to 7 y. The introduction of clover to pastures decreased SOC and SON relative to ryegrass at high- but not at low-grazing intensity. Potentially mineralizable C increased from 7 to 15 y, while mineralizable N increased from 7 to 32 y. Potentially mineralizable N was also greater for bermudagrass+clover than bermudagrass+ryegrass pastures. Long-term increases in SOC and SON concentrations suggest that managed and grazed pastures have strong potential for C and N sequestration.     

A rapid procedure for estimating nitrogen mineralization in manured soil

 A routine soil testing procedure for soil N mineralization is needed that is rapid and precise. Not accounting for N mineralization can result in the over-application of N, especially in soils with a history of manure application. Our objectives were to compare results from a recently proposed rapid laboratory procedure with: (1) long-term N mineralization under standard laboratory conditions, and (2) actual forage N uptake from soil receiving dairy cattle (Bos taurus) manure in a 2-year field study. The rapid procedure is based on the quantity of CO₂-C evolved during 24 h under optimum laboratory conditions following the rewetting of dried soil. Dairy cattle manure was surface applied beginning in 1992 at annual rates of 0, 112, 224, or 448 kg N ha^–1 to field plots on a Windthorst fine sandy loam soil (fine, mixed, thermic Udic Paleustalf) near Stephenville, Texas (32°N, 98°W). Results of the one-day CO₂ procedure were highly correlated with soil N mineralized from samples collected in March of 1995 (P=0.004) and 1996 (P<0.001) and with forage N uptake (P<0.001) both years of the study. Residual inorganic N in the same soil samples was poorly correlated with soil N mineralization and forage N uptake. Received: 23 February 2000     

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.     

Nitrogen transformation from three soil organic amendments in a sandy soil

Abstract

A sandy soil was amended with various rates (20 – 320 g air-dry weight basis of the amendments per kg of air-dry soil) of chicken manure (CM), sewage sludge (SS), and incinerated sewage sludge (ISS) and incubated for 100 days in a greenhouse at 15% (wt/wt) soil water content. At the beginning of incubation, NH₄-N concentrations varied from 50 – 280 mg kg^?1 in the CM amended soil with negligible amounts of NO₃-N. Subsequently, the concentration of NH₄-N decreased while that of NO₃-N increased rapidly. In soil amended with SS at 20 – 80 g kg^?1 rates, the NO₃-N concentration increased sharply during the first 20 days, followed by a slow rate of increase over the rest of the incubation period. However, at a 160 g kg^?1 SS rate, there were three distinct phases of NO₃-N release which lasted for160 days. In the ISS amended soil, the nitrification process was completed during the initial 30 days, and the concentrations of NH₄-N and NO₃-N were lower than those for the other treatments. The mineralized N across different rates accounted for 20 – 36%, 16 – 40%, and 26 – 50% of the total N applied as CM, SS, and ISS, respectively.     

Changes in hot water soil extracts brought about by nitrogen immobilization and mineralization processes during incubation of amended soils

Summary During incubation of an acid cambisol and an alkaline fluvisol, amended with glucose and nitrate, hot water soil extracts were analysed for N content, ultraviolet absorption, and fluorescence. Humic substances in the hot water extracts and in a neutral sodium pyrophosphate extract were fractionated on polyvinylpyrrolidone and measured spectroscopically. Changes in the hot water and pyrophosphate extract compositions were related to changes in microbial biomass, as estimated by substrate-induced respiration, and the hexosamine content of soil hydrolysates. During the incubation, the microbial population in each type of soil developed quite differently, according to the soil pH. Microbial growth and death in the alkaline soil sample induced a maximum of hot-water-extractable ultraviolet-absorptive non-fluorescent substances. The fluorescence of the hot water soil extract increased steadily with incubation time even after the microbial activity was reduced. A similar increase in fluorescence, in accord with the ultraviolet absorption, was found during incubation of the acid soil samples. After 95 days of incubation, the hot-water-extractable fluorescent and ultraviolet-absorptive substances were reduced. N immobilization induced an increase, and N mineralization a decrease, in dissolved organic N. The relative increase in humic substances in the hot water soil extract was much higher than in the pyrophosphate extract. Therefore, humic material, produced by microbial growth and death, is obviously extractable with hot water.     

Responses of the bacterial and fungal biomass in a grassland soil to multi-year applications of dairy manure slurry and fertilizer

Plots of a tall fescue (Festuca arundinacea) sward in the south coastal region of BC, Canada, were treated with dairy manure slurry or fertilizer at 50 or 100 kg NH₄-N ha⁻¹ up to four times per year for six consecutive years; control plots received no manure or fertilizer. The length of fungal hyphae and abundance of bacterial cells were determined by direct counting at 19 sample dates during the fourth (1997), fifth (1998) and sixth (1999) application years. Bacterial abundance was significantly greater in manured soil than in fertilized and untreated soils. In contrast, hyphal length was significantly greater in untreated soil than in manured and fertilized soils. In subplots that ceased to receive manure in 1998, bacterial abundance remained greater through 1998 and 1999 than in previously fertilized plots, indicating that the 4 year cumulative effect of manure was detectable for at least two growing seasons after applications cease. The apparently negative effect of manure and fertilizer on fungal hyphae also appeared to persist through 2 years after applications ceased. Bacterial abundance increased after an initial application of manure for 1 year to previously untreated plots, but not to levels comparable to plots treated with manure continuously from 1994 to 1998.Increases in bacterial abundance, during the one to three week intervals immediately following individual applications of manure, were inconsistent and other factors, such as soil moisture, temperature and perhaps crop phenology appear to have had strong effects on the timing of these microbial responses. Annual means for bacterial abundance and total microbial biomass in the continuous manure treatment were similar for all 3 years. This suggested that the manure-induced increase in microbial biomass probably reached a plateau between one and 3 years after applications commenced. The large bacterial populations along with abundant carbon substrates in manured soil, relative to fertilized soil, were probably capable of immobilizing influxes of mineral N, explaining the observations that less leaching occured from manured than from fertilized soils.     

Effect of incorporation depth and soil climate on straw decomposition rate in a loamy Podzoluvisol

Several studies on the decomposition of straw using the mesh bag technique were carried out under field conditions in south Estonia. Straw of spring and winter cereals (Hordeum vulgare L., Secale cereale L., Triticum aestivum L.) was compared over different periods and at two depths (5 and 20 cm). Calculation of straw decomposition was based on the ash-free organic matter weight loss. Depending on experimental and climatic conditions the decomposition rate fluctuated from 42% to 75% during the 1st year. The three experimental factors incorporation period, incorporation depth and straw species all had a significant effect on the decomposition of straw. The decaying processes proceeded more rapidly in the upper layer and during the initial months; later there were no differences in decomposition rate between the two depths. During the period without vegetation, i.e. from freezing (December) to thawing (April) of the soil, the decomposition rate was 6–7% and during autumn it was about 10%. Greater loss of straw occurred early in the growing season and in the upper layer. The release of nitrogen from straw was slow and N quantity in straw increased due to microbiological immobilization during the 1st year.     

Water and temperature dynamics in a clay soil under winter wheat: influence on straw decomposition and N immobilization

Summary Winter wheat grown on a clay soil was subjected to one of four treatments. The control was not irrigated; the drought treatment had screens to divert rainwater; the irrigation and irrigation/fertilization treatments were irrigated using a drip-tube system with liquid fertilizer (200 kg N ha^-1 year^-1) applied daily in the irrigation/fertilization treatment according to predicted plant uptake. All other treatments also received 200 kg N, but as a single application of bag fertilizer. Soil temperature was monitored. Soil moisture was measured using gravimetric samplings and a capacitance method. Litter bags with barley straw were buried at 10 cm depth in the spring and sampled repeatedly during the growing season. Decomposition rates were calculated assuming exponential decay and that water-soluble components were immediately decomposed or leached from the litter bags. Rates were highly dependent on soil moisture, and the constants ranged from 0.11% day^-1 in the drought treatment to 0.55% day^-1 in the irrigation/fertilization treatment. A simulation model with driving variables based on Q ₁₀ temperature dependence and a log/linear relationship between soil water tension and activity was fitted to the data. The control and drought treatments showed high climate-corrected decomposition constants. The high values were attributed to low and erratic mass loss due to drought, and to low precision in the conversions from water content to tension in the dry range. The irrigated treatments showed good fits, and there was little or no difference in decomposition rates between the two irrigated treatments. The N dynamics of the straw differed considerably between treatments, and the ranking of plots in terms of net immobilization in the straw was control>irrigation/fertilization>irrigation>drought.     

Frequent addition of wheat straw residues to soil enhances carbon mineralization rate

In many ecosystems, residues are added frequently to soil, in the form of root turnover and litter fall. However, in most studies on residue decomposition, residues are added once and there are few studies that have investigated the effect of frequent residue addition on C mineralization and N dynamics. To close this knowledge gap, we mixed mature wheat residue (C/N 122) into soil at a total rate of 2% w/w once at the start (R1×), every 16 days (R4×), every 8 days (R8×) or every 4 days (R16×). Un-amended soil served as control. All treatments were mixed every 4 days. Soil respiration was measured continuously over the 80-day incubation. Inorganic N, K₂SO₄-extractable C and N, chloroform-labile C and N (as an estimate of microbial biomass C and N), soil pH and microbial community composition were assessed every 16 days. Increasing frequency of residue addition increased C mineralization per g residue. Compared to R1×, cumulative respiration per g residue at the end of the incubation (day 80) was increased by 57, 82 and 92% in R4×, R8× and R16×, respectively. The largest differences in soil respiration per g residue occurred in the first 30 days. Despite large increases in cumulative respiration, frequent residue addition did not affect inorganic N or K₂SO₄-extractable N concentrations, chloroform-labile C and N or soil pH. Compared to the control, all residue treatments resulted in increases in chloroform-labile C and N and soil pH but decreased inorganic and K₂SO₄-extractable N. Microbial community composition was affected by residue addition, however there were no consistent differences among residue treatments. It is concluded that experiments with single residue additions may underestimate residue decomposition rates in the field. The increased C mineralization caused by frequent residue additions does not appear to be due to an increased microbial biomass or changes in microbial community composition, but rather to increased C mineralization per unit biomass.     

Effect of Baythroid on nitrogen transformations in soil

Laboratory incubation experiments were conducted in soil to study the influence of the insecticide Baythroid on immobilization-remineralization of added inorganic N, mineralization of organic N, and nitrification of added NH _inf4 ^su+ -N. Baythroid was applied at 0, 0.4, 0.8, 1.6, 3.2, and 6.4 g g^-1 soil (active ingredient basis). The treated soils were incubated at 30°C for different time intervals depending upon the experiment. The immobilization and mineralization of N were significantly increased in the presence of Baythroid, the effect being greater with higher doses of the insecticide. Conversely, nitrification was retarded at lower doses of Baythroid and significantly inhibited at higher doses. The results of these studies suggest that excessive amonts of insecticide residues affect different microbial populations differently, leading to changes in nutrient cycling.     

Effect of chemical composition on the release of nitrogen from agricultural plant materials decomposing in soil under field conditions

Summary In two field experiments, plant materials labelled with ¹⁵N were buried separately within mesh bags in soil, which was subsequently sown with barley. In the first experiment, different parts of white clover (Trifolium repens), red clover (T. pratense), subterranean clover (T. subterraneum), field bean (Vicia faba), and timothy (Phleum pratense) were used, and in the second, parts of subterranean clover of different maturity. The plant materials were analysed for their initial concentrations of total N, ¹⁵N, C, ethanol-soluble compounds, starch, hemicellulose, cellulose, lignin, and ash. After the barley had been harvested, the bags were collected and analysed for their total N and ¹⁵N. In the first experiment the release of N was highest from white clover stems + petioles (86%) and lowest from field bean roots (20%). In stepwise regression analysis, the release of N was explained best by the initial concentrations of lignin, cellulose, hemicellulose, and N (listed according to decreasing partial correlations). Although the C/N ratio of the plant materials varied widely (11–46), statistically the release of N was not significantly correlated with this variable. The results of the second experiment using subterranean clover of different maturity confirmed those of the first experiment.     

Phosphorus mineralization in subarctic agricultural and forest soils

Summary Potential P and C mineralization rates were determined in a 12-week laboratory incubation study on subarctic forest and agricultural soil samples with and without N fertilizer added. There was no significant difference in net inorganic P produced between N fertilized and unfertilized soils. The forest soil surface horizons had the highest net inorganic P mineralized, 32 mg P kg^-1 soil for the Oie and 17 mg P kg^-1 soil for the Oa. In the cropped soils net inorganic P immobilization started after 4 weeks and lasted through 12 weeks of incubation. Cumulative CO₂–C evolution rates differed significantly among soils, and between fertilizer treatments, with the N-fertilized soils evolving lower rates of CO₂–C than the unfertilized soils. Soils from the surface horizons in the forest evolved the highest rates of CO₂–C (127.6 and 89.4 mg g^-1 soil for the Oie and Oa horizons, respectively) followed by the cleared uncropped soil (42.8 mg g^-1 soil C), and the cropped soils (25.4 and 29.0 mg g^-1 soil C). In vitro soil respiration rates, or potential soil organic matter decomposition rates, decreased with increasing time after clearing and in accord with the degree of disturbance. Only soils with high potential C mineralization rates and high organic P to total P ratios, mineralized P by the end of the study. Mineralizable P appeared to be associated with readily mineralizable organic C.     

Role of organic fractions on C decomposition and N mineralization of animal wastes in soil

The relative contributions of water-soluble, water-non-soluble, Van Soest-soluble, and neutral detergent fiber (NDF) fractions of pig slurry (PS), cattle slurry (CS), cattle farmyard manure (FYM), and composted cattle farmyard manure (CFYM) to the overall C and N mineralization of the raw wastes were studied by incubating treated soil for 107 days at 15°C under non-limiting N conditions. The C or N mineralization of soluble fractions was calculated from the difference between C or N mineralization of the raw and non-soluble fractions. The organic N content of raw wastes ranged from 15 to 32 mg N g⁻¹ dry matter and organic C to organic N ratio from 13 to 29. The water-soluble fraction (SOL_W) was close to 100 mg C g⁻¹ raw waste C for CS, FYM, and CFYM but reached 200 mg C g⁻¹ for PS. The Van Soest-soluble fraction (SOL_VS) was the main fraction for PS, CS, and CFYM (>500 mg C g⁻¹ raw waste C) but only 303 mg C g⁻¹ raw waste C for FYM. Both soluble and non-soluble fractions contributed to C decomposition of slurries, with half to more than half of the decomposed C derived from the degradation of soluble compounds. Most of the C decomposed from FYM was derived from the large NDF fraction, but the contribution from the water-soluble C to the decomposition was also significant. Carbon mineralization of CFYM was due to the degradation of the NDF fraction, whereas soluble C did not contribute. Amounts of N mineralized or immobilized by raw wastes and non-soluble fractions at the end of incubation were significantly correlated (P < 0.01) with their organic C to organic N ratio. The contribution of the Van Soest-soluble fraction to N mineralization varied greatly between the four wastes. Finally, large differences in the C degradability and N availability of the water and Van Soest-soluble fractions were demonstrated.     

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

This 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.     

Effect of increasing rates of 15N‐labelled fertilizer on recovery of fertilizer N in plant and soil N pools in a pot experiment with winter wheat

The effect of increasing rates of ¹⁵N‐labelled Ca(NO₃)₂ (N₀ = no N application, N₃₀₀ = 300 mg N/pot; N₆₀₀ = 600 mg N/pot; N₉₀₀ = 900 mg N/pot) on recovery of fertilizer N in winter wheat plants and soil (total soil N, soil microbial biomass N [N_mic], extractable organic N [N_org]) and on N mineralization (NM_soil) was investigated at milk‐ripe growth stage in a pot experiment. The N rates were equally split at tillering, stem elongation and ear emergence. Fertilizer N recovered in crops increased with increasing N rates (N₃₀₀: 223.5 mg N/pot [74.5% of applied fertilizer N], N₆₀₀: 445.6 mg N/pot [74.3%], N₉₀₀: 722.1 mg N/pot [80.2%]). NM_soil slightly increased from N₀ (43.8 mg N/pot) to N₉₀₀ (75.6 mg N/pot) indicating that N application enhanced availability of soil‐derived N for the plants. However, in fertilized treatments NM_soil is balanced by immobilization and losses (non‐recovered fertilizer N). Therefore the effective soil N mineralization is indicated by apparent net N mineralization (ANNM = NM_soil — fertilizer N immobilization — lost fertilizer N). Fertilizer N immobilization in soil increased from N₃₀₀ (38.7 mg N/pot) to N₆₀₀ (60.7 mg N/pot) and N₉₀₀ (65.5 mg N/pot). Lost fertilizer N increased from N₃₀₀ (14.8 mg N/pot) to N₆₀₀ (56.7 mg N/pot) and N₉₀₀ (62.1 mg N/pot). As a consequence negative ANNM values were calculated at N₆₀₀ and N₉₀₀. Due to the small differences between N₆₀₀ and N₉₀₀ fertilizer N immobilization and lost fertilizer N did not increase linearly with increasing N rates, i.e. both processes were limited by factors other than N rate. Only 5.6—7.4% of the immobilized fertilizer N was recovered in N_org and 5.4—9.3% in N_mic soil pools. It is assumed that most of the immobilized fertilizer N was in non‐extractable organic N forms. N_mic and N_org were weak indicators for the extent of fertilizer N immobilization.     

Gross nitrogen transformation rate in soil is affected by coastal wetland cultivation

Abstract

A ¹⁵N tracing experiment was carried out to investigate gross N dynamics in coastal wetland soils. The results showed that the cultivation of coastal wetland has resulted in a significant increase in the gross mineralization rate, gross nitrification rate and gross microbial immobilization of ammonium rate (p<0.01). However, the effect of cultivation on the gross microbial immobilization of nitrate rate was not significant. The gross nitrification rate was much higher than nitrate immobilization (p<0.01), indicating that cultivation of coastal wetland could increase the risk of losses of N from the soil through leaching.     

Decomposition of paddy straw in soil and the effect of straw incorporation in the field on the yield of wheat

In a laboratory experiment, incorporation of paddy straw in soil immobilized native as well as added fertilizer N and about half of the immobilized N was mineralized after 90 days of straw incorporation. Straw and N application alone or in combination increased biomass carbon, phosphatase and respiratory activities of the soil. Microbial biomass carbon and phosphatase activity were maximum at 30 days of straw decomposition. In field trials, incorporation of paddy straw 3 weeks before sowing of wheat significantly increased the wheat yield at Sonepat district in a clay loam soil while no such beneficial effect was observed in a sandy loam soil at Hisar.     

Microbial growth efficiencies across a soil moisture gradient assessed using C-acetic acid vapor and N-ammonia gas

One integrative measurement of microbial activity in soils is the efficiency by which microbes convert assimilated carbon (C) into biomass C. This efficiency, called the microbial growth efficiency (Y), is a key physiological characteristic that regulates soil carbon sequestration, nutrient immobilization, and greenhouse gas emissions. Changes in rainfall patterns and soil water content as the result of global climate change have the potential to influence microbial activity and lead to changes in Y and thus, nutrient cycling at the ecosystem level. Unfortunately, little information is available on how environmental variables such as soil moisture influence Y. We have developed a new method for injecting ¹³C-labeled carbon as acetic acid vapor into soil that will allow measurement of microbial growth efficiency (as Y_C) without increasing soil moisture content. We compare Y determined with this new approach with an alternate method where injected ¹⁵N-labeled ammonia gas is used to quantify microbial N immobilization, and microbial growth efficiency is calculated based on microbial C:N and respiration rate (as Y_N). We also include injections of a solution containing labeled ammonium and acetate in our experiment to compare the results of our vapor methods with more commonly employed liquid-based methods. The ¹³C-acetic acid vapor, which was supplied to soils with soil moisture content ranging from 0.05 to 0.21 g H₂O g⁻¹ soil, was readily assimilated and respired by microbes. Between 0.10 and 0.21 g H₂O g⁻¹ soil (−0.60 to −0.04 MPa), values of Y_C averaged 0.46, and were significantly lower than values of Y_N, with average values of 0.58. Over this range, soil moisture content had no significant effect on either Y_C or Y_N. However, at the lowest soil moisture content (0.05 g H₂O g⁻¹ soil; <−6.0 MPa), Y_C and Y_N diverged substantially, suggesting that in very dry soils, constraints on microbial growth cause differential uptake of C and N resources.