Comparison of in situ methods to measure N mineralization rates in forest soils

Nitrogen mineralization is a critical ecosystem process that is difficult to measure. Among several in situ methods used to estimate N mineralization rates in soils, the buried bag and covered-cylinder methods are two of the most common. Few studies have compared N mineralization rates from these two in situ methods, and it is still unclear if they provide analogous results. We compared both techniques, and two different core diameters, to determine if the different methods produce comparable results. Contrary to our expectations, larger cores were not more representative than smaller due to the importance of site-specific soil characteristics, especially rockiness. Dissimilarities in means, and weak and inconsistent correlations between methods, suggested that the different methods may not be equivalent. Our results suggest that the method optimization depends on specific site conditions, at least in forest soils, and that comparison among studies using different in situ methods should be made with caution until more standardization is achieved.     

Isoproturon mineralization in an agricultural soil

The impact of soil moisture content and temperature on isoproturon (3-(4-isopropylphenyl)-1,1-dimethyl-urea [IPU]) mineralization activity was assessed on an agricultural soil regularly exposed to this herbicide. Mineralization of ¹⁴C-IPU was monitored on soil microcosms incubated at different temperatures (10°C, 20°C, 28°C) and soil moisture contents (9%, 12%, 15, 18%, 21%, 24%). An increase in temperature and/or soil moisture significantly enhanced the maximum rate and percentage of IPU mineralization while it decreased the lag time before mineralization. The maximum rate and percentage of IPU mineralization respectively ranged from 0.18% day⁻¹ and 9% for the lowest temperature and soil moisture content pair (10°C–9%) to 1.51% day⁻¹ and 27.1% for the highest pair (28°C–24%). Statistics revealed a cross interaction of temperature and soil moisture content on the maximum rate of IPU mineralization. The optimum conditions for IPU mineralization, estimated from the double Gaussian model, were 25.8°C and 24% soil moisture content. The influence of fluctuations in soil moisture content on IPU-mineralization was investigated by subjecting the soil microcosms to drought stress. When IPU was added at the end of the drought stress, it had no statistical effect on IPU mineralization. However, when it was added before the drought stress, two mineralization phases were observed: (1) one corresponding to the drought stress for which mineralization was low and (2) another one observed after restoration of soil moisture content characterized by higher mineralization rate. It can be concluded that climatic fluctuations affect the activity of IPU mineralizing microbial community, and may lead to an increase in IPU persistence.     

Microbial biomass and microbial C:N ratio in bulk soil and buried bags for evaluating in situ net N mineralization in agricultural soils

The in situ net nitrogen mineralization (N_net) was estimated in five agricultural soils under different durations of organic farming by incubating soil samples in buried bags. Simultaneously, soil microbial C and N was determined in buried bags and in bulk soil under winter wheat and after harvest. The aim was to check for variations in soil microbial biomass contents and microbial C:N ratios during the incubation period, and their importance for N_net rates. Microbial C and N contents were highest in soils that had been organically farmed for 41 years, whereas N_net rates were highest in a short‐term organically managed soil that had been under grassland use until 36 years ago. The mean coefficient of variation in the bulk soil for microbial C estimates ranged from 5 to 12 %. Microbial N contents were similar inside buried bags and in the bulk soil at the end of the incubation periods. Under winter wheat during the incubation period until harvest, microbial C contents and microbial C:N ratios (in 10—27 cm depth only) decreased more strongly inside buried bags than in the bulk soil. Following harvest of winter wheat and ploughing, microbial biomass increased while in situ N_net decreased, presumably due to N immobilization. The N_net rates were not correlated with microbial N contents or changes in microbial N contents inside buried bags. At the end of the vegetation period of winter wheat, N_net rates were negatively correlated with microbial C:N ratios. Because these ratios concurrently decreased more inside buried bags than in the bulk soil, the N_net estimates of the buried bag method may differ from the N_net rates in the bulk soil at that time.     

施氮量和土壤含水量对黑麦草还田红壤氮素矿化的影响

目标 氮素矿化是决定土壤供氮能力的重要生态过程,养分添加和水分在调节土壤的氮转化方面起着重要的作用。探讨施氮和土壤水分对黑麦草还田过程中土壤氮素矿化的影响有利于进一步优化红壤旱地作物生产的水肥管理。 【方法】 通过室内培养试验,研究了施氮量 (0、60、120 mg/kg) 和土壤含水量 (15%、30%、45%) 对红壤旱地黑麦草还田过程中土壤净硝化量、氨化量和氮矿化量的影响。 【结果】 土壤含水量15%时,施氮有利于提高黑麦草还田初期土壤净硝化量,施氮量120 mg/kg抑制了黑麦草还田后期土壤硝化作用。在30%土壤含水量时,施氮量120 mg/kg明显抑制了黑麦草还田后期土壤硝化作用。土壤含水量45%抑制了黑麦草还田初期不同施氮水平下土壤净硝化量,但增加了黑麦草还田91 d时土壤净硝化量,且施氮量60 mg/kg下的净硝化量显著高于120 mg/kg水平下的。土壤净氨化量在整个黑麦草还田过程中均为正值,且呈现多次升高-降低的往复动态变化。土壤净氨化量在三种土壤含水量下均表现为施氮条件下的显著高于不施氮处理。土壤含水量的增加有利于提高施氮量120 mg/kg下黑麦草还田初期土壤的氨化作用,但降低了黑麦草还田后期土壤净氨化量。相比不施氮,三个含水量条件下的施氮处理在黑麦草还田过程中的大部分阶段都显著增加了土壤净氮矿化量,土壤含水量30%条件下土壤净氮矿化量的变化最大。相比土壤含水量15%,30%含水量促进了黑麦草还田中期 (13～57 d) 土壤净氮矿化量的增加,45%含水量抑制了黑麦草还田后期 (73～91 d) 土壤净氮矿化量。 【结论】 红壤区旱地黑麦草还田时应合理施入化学氮肥 (60 mg/kg),在黑麦草还田初期保持较高的土壤含水量 (45%) 能够抑制土壤的氮矿化作用,还田中后期适当降低土壤含水量 (30%)有利于增加土壤氮素的矿化。      

Net nitrogen mineralization and nitrification rates in forest soil in northeastern Taiwan

We used a laboratory incubation approach to measure rates of net N mineralization and nitrification in forest soils from Fu-shan Experimental Forest WS1 in northern Taiwan. Net mineralization rates in the O horizon ranged from 4.0 to 13.8 mg N kg⁻¹ day⁻¹, and net nitrification rates ranged from 2.2 to 11.6 mg N kg⁻¹ day⁻¹. For mineral (10–20 cm depth) soil, net mineralization ranged from 0.06 to 2.8 mg N kg⁻¹ day⁻¹ and net nitrification rates ranged from 0.02 to 2.8 mg N kg⁻¹ day⁻¹. We did not find any consistent differences in N mineralization or nitrification rates in soils from the upper and lower part of the watershed. We compared the rates of these processes in three soil horizons (to a soil depth of 30 cm) on a single sampling date and found a large decrease in both net N mineralization and nitrification with depth. We estimated that the soil total N pool was 6,909 kg N ha⁻¹. The present study demonstrates the importance of the stock of mineral soil N in WS1, mostly organic N, which can be transformed to inorganic N and potentially exported to surface and ground water from this watershed. Additional studies quantifying the rates of soil N cycling, particularly multi-site comparisons within Taiwan and the East Asia–Pacific region, will greatly improve our understanding of regional patterns in nitrogen cycling.     

Comparison of different methodologies for field measurement of net nitrogen mineralization in pasture soils under different soil conditions

 Net mineralization was measured in free-draining and poorly drained pasture soils using three different field incubation methodologies. Two involved the use of enclosed incubation vessels (jar or box) containing C₂H₂ as a nitrification inhibitor. The third method confined soil cores in situ in an open tube in the ground, with an anion-exchange resin at the base to retain leached NO₃ ^– (resin-core technique, RCT). Measurements were made on three occasions on three free-draining pastures of different ages and contrasting organic matter contents. In general, rates of net mineralization increased with pasture age and organic matter content (range: 0.5–1.5 kg N ha^–1 day^–1) and similar rates were obtained between the three techniques for a particular pasture. Coefficients of variation (CVs) were generally high (range: 10.4–98.5%), but the enclosed incubation methods were rather less variable than the RCT and were considered overall to be the more reliable. The RCT did not include C₂H₂ and, therefore, newly formed NO₃ ^– may have been lost through denitrification. In a poorly drained pasture soil, there were discrepancies between the two enclosed methods, especially when the soil water content approached field capacity. The interpretation of the incubation measurements in relation to the flux of N through the soil inorganic N pool is discussed and the drawbacks of the various methodologies are evaluated. Received: 18 November 1999     

Laboratory vs. in situ resin‐core methods to estimate net nitrogen mineralization for comparison of rotation and tillage practices

Properly estimating soil nitrogen (N) mineralization as a consequence of different agronomic practices would result in better soil N fertility management. In this study, we tested the differences between laboratory and in situ resin‐core incubation methods for estimating soil net N mineralization for long‐term burley tobacco (Nicotiana tobacum L .) tillage and rotation systems. The laboratory incubation method used crushed, homogenized, litter‐free soil samples, and the in situ resin‐core incubation method used an intact soil core with the inclusion of any plant residue below or above ground. Comparisons showed that no‐tillage had significantly increased soil net N mineralization compared to conventional tillage with the laboratory incubation method, while there was no significant difference between tillage methods with the in situ resin‐core method. This indicates that soil pretreatment in the laboratory incubation method can create an “artificial tillage effect” for soil previously managed with no‐tillage, resulting in overestimated soil net N mineralization. The rotation comparison showed that different crop sequences had no impact on measured net N mineralization with the laboratory incubation method. However, a preceding soybean crop did significantly increase net soil N mineralization compared to preceding corn when measured with the in situ resin‐core method. This suggests that discarding plant residue in the laboratory incubation method can neglect the potential effect of plant residue on soil N mineralization. Therefore, it is important to be aware that soil pretreatment may influence soil N mineralization estimates, potentially resulting in flawed decisions for soil N fertility management.     

Salinity and nitrogen mineralization in soil

A laboratory study determined the effects of salinity on ammonification, nitrification and mineral N accumulation in incubated soils. NH⁺₄-N, NO^?₂-N and NO^?₃-N were measured periodically for 102 days in unamended soil of varying salinity and in soil amended with farm compost, mustard oil cake or urea. Increased salinity progressively retarded ammonification but did not suppress it completely. Nitrification was retarded, suppressed or inhibited completely by salinity, the effect depended on both the amount of salt and the type of amendment added to the soil. The amount of mineral N that accumulated generally decreased with increased salinity.     

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.     

Field measurement of net nitrogen mineralization of manured soil cropped to maize

We evaluated the in situ net nitrogen (N) mineralization in a soil cropped to maize and fertilized for 11 years with cattle slurry or farmyard manure, both common on livestock farms of the Po River valley in Northern Italy. The net N mineralization of the tilled soil layer was measured in six consecutive incubation periods after manure application, for a total of 12 weeks, using the polyethylene buried bags technique. Results showed that net N mineralization was followed by N immobilization and finally, by mineralization whose rate increase until maize flowering. On average, net N mineralization was 70.4 kg N ha⁻¹, with the majority being released during the last measurement period. The time and extent of net N mineralization and plant N uptakes were not affected by fresh manure application. Instead, the effect of past management increased the maximum net N mineralization rate obtained with farmyard manure. The buried bag technique probably underestimates the total amount of mineralized N available for crop growth because it excludes the presence of the plant.     

Comparison of clod and core methods for determination of soil bulk density

Abstract

An interagency forest monitoring program has been initiated on a systematic network of forested plots often accessible only by foot traffic along a compass line. Extensive site classification and physiochemistry data are being collected for vegetation and soil indicators of forest health. In order to select a suitable technique for obtaining an estimate of mineral soil bulk density, conventional clod and core methods were compared across a wide range of forest soils within the Coastal Plain of southeastern Virginia. Replicate soil clods and cores were collected from two to four horizons within each of six pedons representing different soil series. Following analysis, the sample data were used to evaluate main and interaction effects due to differences in the method, series, horizon, and texture effects. Differences between the clod and core data were highly significant on average (P < 0.001) as were all of the main effects. Both methods exhibited high precision (average CV < 5%) within the individual horizons sampled. The two data sets were highly correlated (r = 0.98), and the regression equation used to predict clod bulk density with data from the core method is: Clod BD = (1.011 x Core BD) + 0.068; standard errors of ±0.042 and ±0.048 for the slope and intercept, respectively. From an operational standpoint, the core method appears to have many advantages over the clod method for sampling in remote locations.     

Temperature sensitivity of nitrogen mineralization in agricultural soils

As concerns continue to mount over nitrogen (N) use efficiency and the fate of excess N lost to the environment, the need for accurate assessments of the contribution of non-fertilizer sources is increasing. Nitrogen mineralization from soil organic matter (SOM) can supply large quantities of plant-available N through a growing season and is affected by numerous factors including soil temperature and soil properties. The objective of this study was to determine the temperature sensitivity of N mineralization in soils under annual crops in California and to determine whether growing region and soil properties affect the temperature sensitivity of N mineralization. Nitrogen mineralization was assessed for 55 soils from different regions in central and northern California by incubating undisturbed soil cores at 5, 15, and 25 °C for 10 weeks. Modeling was done by fitting the data to both an S-shaped curve and a Q₁₀ function. Q₁₀ values averaged 2.87 and ranged from 2.67 to 3.15 in the different regions. While the magnitude of N mineralized varied greatly, the temperature response did not differ significantly across regions and was not affected by SOM content and quality, texture, electrical conductivity, or pH. Coupling the temperature response curve with daily soil temperature data measured in the field highlighted the differences in regional and seasonal mineralization rates and underscored the importance of taking soil temperature into account when determining N mineralization rates for N budgets. This is especially important in regions where crops are grown year-round.     

Comparison of biological and chemical methods to predict nitrogen mineralization in animal wastes

Summary Biological and chemical methods to predict the level of plant-available N in animal manure were investigated under laboratory and growth-chamber conditions. Two biological methods (maize cultivation in pots and incubation of soil-waste mixtures) and four chemical methods (N extraction by autoclaving, 0.5 N KMnO₄, pepsin, and 6 N HCl) were compared for their accuracy in determining the availability of N in 10 samples of animal manure applied to soil. The autoclaving, permanganate, and pepsin methods were able to predict N availability in this group of wastes. Total N mineralized in a soil amended with different samples of animal manure ranged from 0 to 311 mg N kg^-1 soil. Expressed as a percentage of organic N added to the soil, mineralized N range from 0 to 39%. The poultry manure samples gave higher mineralization rates than the other types of manure tested. In general, mineralized N became immobilized during the incubation process, except for the second poultry manure sample, which showed an initially rapid then a a slow release of mineral N, and pig manure sample 2, which showed a slow initial rate, followed by a rapid increase, and then a slow rate of N release. The first-order exponential model used was able to describe the pattern of N mineralization in pig manure sample 2, poultry manure sample 2 but not the other samples.     

灌溉方法对保护地土壤有机氮矿化特性的影响

采用Stanford和Smith提出的长期间歇淋洗通气培养法,对连续7a采用渗灌、滴灌和沟灌灌溉,栽培番茄的保护地不同剖面层次土壤的有机氮矿化特点进行了研究。渗灌管为发汗式半软管,埋深为30cm;渗灌、滴灌和沟灌灌水方法及施肥、田间管理同当地农业生产。当20cm深处的土壤水吸力达到40kPa时开始灌水,渗灌和滴灌每次的灌水量是沟灌灌水量的1/2。试验结果表明,土壤矿化氮含量随着土层深度的增加而降低。从累积矿化氮量—时间曲线变化的趋势看,可将保护地土壤0～50cm剖面分为三个层次,其中渗灌与滴灌处理相似,为0～20cm、20～40cm和40～50cm土层,而沟灌处理则为0～30cm、30～40cm和40～50cm土层。保护地不同层次土壤有机氮的矿化可以用Twopool模型表达。比较不同灌溉处理,在0～10cm土层,易矿化有机氮含量(N1)表现为滴灌>沟灌>渗灌,易矿化有机氮矿化速率(k1)常数也以滴灌处理最大,说明滴灌更有利于表层土壤易矿化有机氮的形成。与渗灌和沟灌相比,长期使用滴灌灌溉有利于改善保护地土壤有机氮的品质。     

Nitrogen mineralization from an organically managed soil and nitrogen accumulation in lettuce

The potential of an organically managed Cambic Arenosol to supply nitrogen (N) from either an applied commercial organic fertilizer (granulated hen manure), a compost produced on‐farm, or four different mixtures of both fertilizers was studied in a laboratory incubation and a pot experiment with lettuce. In the incubation experiment, a significant higher apparent N mineralization occurred after hen‐manure application (53.4% of the organic N applied) compared to compost (4.5%) or mixed‐fertilizer application (8.7% to 16.7%). The apparent N mineralization in a mixed treatment consisting of compost and half rate of hen manure (15.4% of the organic N applied) was significantly higher than that estimated based on the N mineralization for compost and hen‐manure treatments (7.6%), proving that a combined application of both fertilizers enhanced organic‐N mineralization when compared to separate fertilizer supply. In the pot experiment, a higher lettuce fresh‐matter yield was obtained with hen manure (1.9 kg m^–2) than with compost (1.7 kg m^–2) or unfertilized control treatment (1.3 kg m^–2). Combined application of compost with only a half rate of hen manure led to yields (2.0 kg m^–2) equal to those obtained with only hen manure. A good correlation was observed between the N‐mineralization incubation data and the N accumulated by lettuce plants in the pot experiment (r = 0.983). Hence, in the organic production of baby‐leaf lettuce, a mixture of compost and hen manure appears to be a good fertilization alternative, since it allows a reduction by half of the typical amount of commercial fertilizer usually applied (granulated hen manure), cutting fertilization costs, and providing an amount of available N that allows maintaining lettuce yields.     

Carbon and nitrogen mineralization in subarctic agricultural and forest soils

Summary C and N mineralization potentials were determined, in a 12-week laboratory incubation study, on soil samples obtained from recently cleared land which had been cropped to barley for 4 years (field soils) and from nearby undisturbed taiga (forest soils). Treatments for the cropped soils were conventional and no-tillage with and without crop residues removed. An average of about 3% of the total C was evolved as CO₂ from the field soils compared with > 10% and 4% for the upper (Oie) and lower (Oa) forest-floor horizons, respectively. Significantly more C was mineralized from the Ap of the no-till treatment with residue left on the surface than from the other field Ap horizons. Both forest-floor horizons showed rather long lag periods for net mineralization compared with the field soils, but at the end of the incubation, more mineral N was recovered from the forest Oie despite a rather wide C:N ratio, than from the field soils. After 12 weeks about 115, 200 and 20 g mineral N/g soil were recovered from the field Ap, the forest Oie and the forest Oa horizons, respectively. Very little C or N was mineralized from the B horizon of the forest or the field soils. Nitrification was rapid and virtually complete for the field soils but was negligible for both forest-floor O horizons.Paper no J-188 of the Journal Series of the Alaska Agricultural and Forestry Experiment Station     

Significance of microbial biomass for carbon and nitrogen mineralization in soil

C and N mineralization was quantified in an incubation experiment with two samples containing different amounts of microbial biomass. The samples from two layers (0–20, 20–30 cm) of an arable luvisol from loess were fertilized with nitrate, mixed with ¹⁴C-labelled straw and incubated for 52 days at different O₂ levels. Decreasing O₂ concentrations (21, 2, 1 and 0% O₂) in soil conducted a decrease in C and N mineralization. More C and N were mineralized in samples with a higher initial microbial biomass. The differences in microbial biomass were still present at the end of the experiment, but more proliferation was detected in samples with the lower initial microbial biomass, leading to equal ratios between microbial biomass-C and soil organic C in both soils.     

杜马斯燃烧法和凯氏定氮法在土壤全氮检测中的比较研究

选用 6组不同全氮含量的土壤样品(梯度为 0%～0.06%、0.06%～0.1%、0.1%～0.2%、0.2%～0.3%、0.3%～ 0.4%和 0.4%～ 0.5%)和 4份土壤有效态成分分析标准物质为试验材料,分别采用杜马斯燃烧法和凯氏定氮法测定其全氮含量,并对两种方法测定结果的精密度、准确度和相关性及检测成本进行了比较分析。对土壤有效态成分分析标准物质全氮含量的检测,两种方法测定结果都接近于真实值,比较两种方法的相对标准偏差(RSD)和相对误差(RE)值,可确定杜马斯燃烧法精密度更高,且准确度更好;对于含氮量范围为 0%～ 0.06%的样品,两种方法的测定值之比(D/K)为 0.692～ 0.965,杜马斯燃烧法测定结果的变异系数为 0.00%～ 9.96%,测定结果不稳定,两种方法的测定值间存在显著的线性相关,但 R2仅为 0.9049;而含氮量大于 0.06%的样品,两种方法的测定值之比(D/K)为 0.940～ 1.104,变异系数均小于 5%,测定结果间存在显著的线性相关(R2=0.9979)。在检测成本方面,完成相同数量样品检测,杜马斯燃烧法所需时间和人力都约是凯氏定氮法的1/2。因而,杜马斯燃烧法是一种环保、高效的土壤全氮检测方法,对于含氮量范围为 0%～ 0.06%的土壤样品,凯氏定氮法的测定结果更接近于真实值,而对于含氮量大于 0.06%的土壤样品,杜马斯燃烧法更加适用。     

Studies on mineralization and immobilization of nitrogen in soil

Mineralization and immobilization processes were studied in a soil treated with ammonium sulphate, calcium ammonium nitrate, urea, urea ammonium phosphate or suphala along with unhumified dung. The conversion of fertilizer nitrogen into non-KCI extractable fractions occurred within two weeks and was relatively rapid in the samples treated with urea and urea ammonium phosphate. The mineral nitrogen content during incubation studies was highest in calcium ammonium nitrate treated soils. Amino acid nitrogen appeared to be the main fraction involved in the immobilization and mineralization of nitrogen in soil. Dung behaved as an efficient nitrification inhibitor and slowed the release of nitrogen from fertilizers.     

Calculation of gross nitrogen immobilization and mineralization in soil

In a laboratory experiment, soil was treated with ¹⁵NH₄- and ¹⁵NO₃-N compounds at various times during the incubation. Several approaches in the calculation of gross immobilization and gross mineralization between two sampling dates were compared and a new method was developed. It is based on a dynamic simulation model designed to interpret isotopic data and an optimization procedure used to determine the best fit between model output and experimental data. This flexible method was used to examine the validity of the assumptions usually made when calculating the gross transformations.
The results indicate that seriously erroneous estimates of the gross transformations can follow if it is assumed that remineralization of immobilized N does not occur. Less serious errors can result from the assumption that both opposing processes occur at a constant rate during the interval between sampling dates.
The combined use of the model and the optimization procedure has several advantages over traditional methods and some of the gross transformation estimates reported would not have been obtainable using older methods.