不同作用因子下有机无机配施添加DMPP对氮素转化的影响

研究不同作用因子下有机无机配施模式添加DMPP(3,4-二甲基毗唑磷酸盐)对土壤氮素形态转化的影响,为田间氮素管理和高效利用提供科学依据.采用好气恒温土壤培养试验,研究施肥用量、水分条件、环境温度、土壤类型等不同作用因子,对有机无机氮肥配施模式添加DMPP土壤氮素形态转化的影响.结果表明,60 d时,高用量有机无机配施处理比常规有机无机处理铵态氮含量提高89倍,硝态氮减少57.8％;与湿润培养相比,淹水条件下有机无机配施模式添加DMPP,60 d内土壤铵态氮含量持续增加,硝化进程受抑制更显著;60 d时15℃处理铵态氮含量较25℃处理高56倍,硝态氮含量低18倍;60 d时红壤中铵态氮含量分别是小粉土和青紫泥的30倍与31倍,而硝态氮含量仅为二者的55.7％与33.6％.25℃时青紫泥、小粉士和红壤中有机无机配施模式添加DMPP最佳抑制效果在30～ 40 d,有效作用时间可达60 d.DMPP能够明显增加有机无机配施模式土壤中铵态氮含量,有效延长铵态氮在土壤中停留的时间,使硝态氮含量长期维持在较低水平.高施肥水平有机无机配施模式下DMPP的抑制效果更突出.低温环境有利于DMPP对硝化进程的抑制.不同作用因子下,有机无机配施模式添加DMPP对氮素转化影响的深层作用机理值得进一步研究.     

影响砂姜黑土麦田土壤氮素转化的生物学因素 及其对供氮量的响应

砂姜黑土是我国典型的中低产田土壤类型,研究其在土壤微生物驱动下的氮素转化过程及其机制,可为定向调控土壤氮素转化过程,提高氮素利用效率并减少其负面效应提供科学依据。试验设置0 kg·hm~(-2)、120 kg·hm~(-2)、225 kg·hm~(-2)和330 kg·hm~(-2) 4个供氮量,分别于冬小麦越冬期、拔节期、抽穗期、开花期、灌浆期和成熟期测定小麦根际土壤氮转化相关微生物作用(氨化作用、硝化作用和反硝化作用)强度和土壤氮素转化相关酶(脲酶、蛋白酶)活性,土壤净氮素矿化速率、土壤硝态氮和铵态氮含量的变化,研究影响砂姜黑土麦田土壤氮素转化的生物学因素及其对不同供氮量的响应。结果表明,土壤氮素转化微生物及酶活跃时期为拔节到灌浆期,灌浆期之后土壤氨化作用强度、硝化作用强度、脲酶及蛋白酶活性降低;土壤净氮素矿化速率与土壤氮素转化微生物作用强度及酶活性的活跃期较为一致,在开花前后达到最高。除脲酶活性随供氮量增加持续上升外,土壤氮素转化微生物作用强度及蛋白酶活性均随供氮量的增加,在225 kg·hm~(-2)处理下达到最高,进一步增加供氮量至330 kg·hm~(-2),微生物作用强度及酶活性均表现出不同程度的下降。可见,砂姜黑土土壤氮素转化的活跃期与小麦需氮高峰期基本一致,有利于冬小麦的生长。但由于砂姜黑土中土壤硝化作用强度较低,土壤硝化能力有限,从而降低了氮素可利用性,且增加了土壤氨挥发损失的潜在风险。在一定范围内增加供氮量,有利于土壤氮素的转化,但供氮过多(330 kg·hm~(-2))则不利于砂姜黑土供氮能力的提高。     

Soil nitrogen mineralization as affected by water and temperature interactions

Summary The hypothesis that water and temperature interact to influence the rate of soil N mineralization was studied in laboratory incubation experiments with two contrasting soils. Small sample rings (10 mm tall, 50 mm diameter) were packed to uniform bulk density with 1–2 mm aggregates of Plano silt loam and Wacousta silty clay loam. Samples were brought to five different water potentials (–0.1, –0.33, –0.5, –1.0, –3.0 bars) using pressure-plate techniques, and the undisturbed sample rings were then incubated at 10–35°C for 3, 10 or 14 days. The concentration of soil exchangeable NH₄ ⁺-N and NO₃ ^–-N was measured at the end of each incubation period on replicate samples. The Q₁₀ of N mineralization was approximately 2 for all tested water potentials. Soil N mineralization was linearly related to water content or log water potential, but no water-temperature interaction was evident. The Q₁₀ was constant with water content, and the scaled water content-N mineralization relationship was constant with temperature. We recommend the use of scaling approaches for assessing interactive effects between water and other environmental factors on N turnover in soils.     

Soil nitrogen availability as affected by fallow-maize systems on two soils in Kenya

Simple methods for the measurement of nitrogen (N) availability are needed to assess the effect of low-input, organically based land management systems on the N supply of tropical soils. Our objectives were to determine the effect of contrasting land-use systems (LUS) on soil N availability and to identify measures of N availability that correlated with maize (Zea mays L.) grain yield. The LUS at the two sites in Kenya involved growth of a maize crop following 17 months of either: (1) Sesbania sesban (L.) Merr. tree growth (sesbania fallow), (2) natural regrowth of vegetation without cultivation (natural fallow), (3) three crops of unfertilized maize (maize monoculture), or (4) bare uncultivated soil (bare fallow). Soil was collected before the post-fallow maize crop was sown. The LUS had no effect on total soil N or amount of N in the heavy fraction soil organic matter (SOM) (>150 μm, >1.37 Mg m^–3). Sesbania and natural fallows, as compared to maize monoculture, increased the N in light fraction SOM (>150 μm, <1.13Mgm^–3), N in intermediate fraction SOM (>150 μm, 1.13 to 1.37 Mg m^–3), ammonium-N and aerobic N mineralization at a depth of 0–15 cm. Maize yields were highest following the sesbania fallow. Nitrate-N, inorganic-N (ammonium plus nitrate) and anaerobic N mineralization correlated with maize grain yield at both sites. The relationship between maize yield and pre-season nitrate-N improved when the depth of soil sampling was increased to 1 m at one site (an Alfisol), but did not improve at the site with anion adsorption in the subsoil (an Oxisol). The sesbania fallow was more effective than the natural fallow in increasing available soil N. Maize yield was better related to pre-season nitrate than N in size-density fractions of SOM. Received: 5 May 1997     

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.     

水分含量对秸秆还田土壤碳矿化和微生物特性的影响

An 80-d incubation experiment was conducted to investigate straw decomposition,the priming effect and microbial characteristics in a non-fertilized soil(soil 1) and a long-term organic manure-fertilized soil(soil 2) with and without13 C-labeled maize straw amendment under different moisture levels. The soil 2 showed a markedly higher priming effect,microbial biomass C(Cmic),and β-glucosidase activity,and more abundant populations of bacteria and fungi than the soil 1. Also,soil CO2 emission,Cmic,β-glucosidase activity,and bacterial and fungal population sizes were substantially enhanced by straw amendment. In the presence of straw,the amount of straw mineralization and assimilation by microbes in the soil at 55% of water holding capacity(WHC) were significantly higher by 31% and 17%,respectively,compared to those at 25% of WHC. In contrast,β-glucosidase activity and fungal population size were both enhanced as the moisture content decreased. Cmicdecreased as straw availability decreased,which was mainly attributed to the reduction of straw-derived Cmic. Amended soils,except the amended soil 2 at 25% of WHC,had a more abundant fungal population as straw availability decreased,indicating that fungal decomposability of added straw was independent of straw availability. Non-metric multidimensional scaling analysis based on fungal denatured gradient gel electrophoresis band patterns showed that shifts in the fungal community structure occurred as water and straw availability varied. The results indirectly suggest that soil fungi are able to adjust their degradation activity to water and straw availability by regulating their community structure.     

Carbon and nitrogen mineralization of non-composted and composted municipal solid waste in sandy soils

A sterilized, but undecomposed, organic by-product of municipal waste processing was incubated in sandy soils to compare C and N mineralization with mature municipal waste compost. Waste products were added to two soils at rates of 17.9, 35.8, 71.6, and dry weight and incubated at for 90 d. Every 30 d, nitrate and ammonium concentrations were analyzed and C mineralization was measured as total CO₂-C evolved and added total organic C. Carbon mineralization of the undecomposed waste decreased over time, was directly related to application rate and soil nutrient status, and was significantly higher than C mineralization of the compost, in which C evolution was relatively unaffected across time, soils, and application rates. Carbon mineralization, measured as percentage C added by the wastes, also indicated no differences between composted waste treatments. However, mineralization as a percentage of C added in the undecomposed waste treatments was inversely related to application rate in the more productive soil, and no rate differences were observed in the highly degraded soil. Total inorganic N concentrations were much higher in the compost- and un-amended soils than in undecomposed waste treatments. Significant N immobilization occurred in all undecomposed waste treatments. Because C mineralization of the undecomposed waste was dependant on soil nutrient status and led to significant immobilization of N, this material appears to be best suited for highly degraded soils low in organic matter where restoration of vegetation adapted to nutrient poor soils is desired.     

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.     

Microbial biomass and C mineralization in agricultural soils as affected by atrazine addition

This study examines the effects of atrazine on both microbial biomass C and C mineralization dynamics in two contrasting agricultural soils (organic C, texture, and atrazine application history) located at Galicia (NW Spain). Atrazine was added to soils, a Humic Cambisol (H) and a Gleyic Cambisol (G), at a recommended agronomic dose and C mineralization (CO₂ evolved), and microbial biomass measurements were made in non-treated and atrazine-treated samples at different time intervals during a 12-week aerobic incubation. The cumulative curves of CO₂–C evolved over time fit the simple first-order kinetic model [Ct = Co (1 − e ^−kt )], whose kinetic parameters were quantified. Differences in these parameters were observed between the two soils studied; the G soil, with a higher content in organic matter and microbial biomass C and lower atrazine application history, exhibited higher values of the total C mineralization and the potentially mineralizable labile C pool than those for the H soil. The addition of atrazine modified the kinetic parameters and increased notably the C mineralized; by the end of the incubation the cumulative CO₂–C values were 33–41% higher than those in the corresponding non-added soils. In contrast, a variable effect or even no effect was observed on the soil microbial biomass following atrazine addition. The data clearly showed that atrazine application at normal agricultural rates may have important implications in the C cycling of these two contrasting acid soils.     

Mineralization of nitrogen in Danish soils,as affected by short-, medium- and long-term annual inputs of animal slurries

At four sites across Denmark with varying medium- and long-term histories of annual slurry applications, N turnover rates and crop N use efficiencies were measured in 2000. No significant effect of medium-term (in this study, 3 years) annual slurry applications on gross N turnover was observed. However, a significant effect of long-term (in this study, >25 years) annual slurry applications was observed. At one site in Denmark with short-term (4 days before measurement) slurry application, N turnover was measured in 2001. Gross N turnover was 4–5 times higher in the slurry-amended soil compared to the unamended soil. In both years, net N turnover was unaffected by the slurry application. Generally, the crops had higher use efficiency of slurry NH₄ ⁺-N than of mineral fertilizer-N, indicating that the crops were able to extract slurry organic-N, independently of the net mineralization. The measured net N mineralization rate was generally higher than the difference between gross rates. The application of ¹⁵NH₄ ⁺ to soil (a prerequisite for the determination of N mineralization and N immobilization turnover), probably stimulated the gross N immobilization rate in soil with little native NH₄ ⁺, since NH₄ ⁺ is the substrate for immobilization. The results suggest that gross immobilization estimates should be interpreted with caution.     

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     

添加葡萄糖对不同肥力黑土氮素转化的影响

氮是作物生长必需的大量营养元素,增施化学氮肥,是农业生产采取的主要增产措施之一。我国的氮肥消费量已占世界总消费量的约30%,但我国农业中氮素的生产效率趋于下降,而带来的农业环境污染则趋于加重。提高氮素利用率,降低其对环境的负面影响,在保障粮食安全的同时兼顾生     

温度对厌氧条件下不同pH水稻土氮素矿化的影响

氮矿化反应是土壤生态系统氮素循环的重要环节之一,决定了土壤氮素的可利用性。温度和pH是影响氮素矿化的重要环境因子。为研究厌氧条件下温度对不同pH水稻土氮素矿化的影响,本文以两种不同pH的水稻土为试验对象,在厌氧条件下,设置15℃、25℃、37℃和50℃4个温度,结合一级反应动力学方程式和有效积温式研究温度对土壤氮素矿化势、矿化速率、矿化程度和矿化势/全氮等矿化参数的影响。结果表明,两种土壤氮素矿化势均随着温度的升高而增大。在15~37℃范围内,两种土壤的矿化速率以及矿化程度均随着温度升高而增大,且同种温度下两土壤差异不显著。但在37~50℃范围内,随着温度的升高,两种pH土壤矿化速率以及矿化程度有增大也有减小,差异达1%显著水平。说明在高温范围内,不同pH土壤氮素矿化对温度的响应有很大差异。4个温度下,矿化势/全氮的值均随温度升高而增大,说明有机氮的品质随温度升高而提高。通过温度与矿化参数的相关性分析发现,在15~37℃范围内,各矿化参数与温度均呈正相关,且相关性极显著(P0.01);但37~50℃时,各矿化参数与温度相关性均较小,有的为负相关。本试验测定各培养周期的pH,发现在培养过程中,两种土壤pH波动不大,对土壤氮矿化的变化无影响。结果表明,厌氧条件下,尽管中、低温时不同pH水稻土氮素矿化对温度有相似的响应,但高温时不同pH土壤的氮素矿化显著不同。     

Relationship between mineralization and immobilization of nitrogen influenced by various conditions of submerged soils

The relationship between mineralization of soil nitrogen and immobilization of added nitrogen in submerged soils were studied under various soil conditions in a laboratory experiment. Selected factors which constituted soil conditions were content of soil organic nitrogen, quantity of nitrogen addition, temperature, soil drying, puddling, and period of incubation. Each factor contained some treatments in it.

The ratio of mineralization to immobilization, M/I, was relatively constant under various soil conditions. The values of M/I were around 2 except the soil drying treatments, indicating that the amount equivalent to about half of mineralized nitrogen is immobilized simultaneously under nitrogen added condition. Even so, if considered in detail, treatments that stimulate the microbial activities were observed to have increasing effect on M/I.

The ratio of additional mineralization caused by nitrogen addition to immobilization, ΔM/I, is considered to be an index to know the influence of nitrogen addition on the quantitative change of soil nitrogen level. This value was around 1 in the soil without any application of organic matters, and around 0.6 in the soil receiving straw compost for 10 years. Factors that have close relations with the value of ΔM/I were soil organic nitrogen level, temperature, soil drying, and period of incubation.     

Carbon and nitrogen mineralization of sewage sludge compost in soils with a different initial pH

Soil properties may affect the decomposition of added organic materials and inorganic nitrogen (N) production in agricultural soils. Three soils, Potu (Pu), Sankengtzu (Sk) and Erhlin (Eh) soils, mixed with sewage sludge compost (SSC) at application rates of 0 (control), 25, 75 and 150 Mg ha⁻¹ were selected from Taiwan for incubation for 112 days. The aim of the present study was to examine the effects of SSC application rates on the carbon decomposition rate, N transformation and pH changes in three soils with different initial soil pH values (4.8–7.7). The results indicated that the highest peaks of the CO₂ evolution rate occurred after 3 days of incubation, for all treatments. The Pu soil (pH 4.8) had a relatively low rate of CO₂ evolution, total amounts of CO₂ evolution and percentage of added organic C loss, all of which resulted from inhibition of microbial activity under low pH. For the Pu and Sk soils, the concentration of NH₄⁺-N reached its peak after 7–14 days of incubation, which indicated that ammonification might have occurred in the two soils with low initial pH values. NO₃⁻-N rapidly accumulated in the first 7 days of incubation in the Eh soil (pH 7.7). The direction and extent of the soil pH changes were influenced by the N in the SSC and the initial soil pH. Ammonification of organic N in the SSC caused the soil pH to increase, whereas nitrification of mineralized N caused the soil pH to decline. Consequently, the initial soil pH greatly affected the rate of carbon decomposition, ammonification and nitrification of SSC.     

土壤氮素矿化对烤烟产量和尼古丁含量的影响

Nitrogen （N） supply is the most important factor affecting yield and quality of flue-cured tobacco （FCT）. A field experiment and an in situ incubation method were used to study the effects of soil N mineralization in the later stages of growth on yield and nicotine content of FCT in Fenggang and Jinsha, Guizhou Province. The yield and market value of FCT at Fenggang were much lower than those at Jinsha. However, the nicotine content of middle and upper leaves was much higher at Fenggang than at Jinsha when the same rate of fertilizer N was applied, which might be due to a higher N supply capacity at the Fenggang site. At later stages of growth （7-16 weeks after transplanting）, the soil net N mineralization at Fenggang （56 kg N ha^-1） was almost double that at Jinsha （30 kg N ha^-1）. While soil NH4-N and NO3-N were almost exhausted by the plants or leached 5 weeks after transplanting, the N taken up at the later growth stages at Fenggang were mainly derived from soil N mineralization, which contributed to a high nicotine content in the upper leaves. The order of soil N contribution to N buildup in different leaves was： upper leaves 〉 middle leaves 〉 lower leaves. Thus, soil N mineralization at late growth stages was an important factor affecting N accumulation and therefore the nicotine content in the upper leaves.     

Carbon mineralization in soils of different textures as affected by water-soluble organic carbon extracted from composted dairy manure

The water-soluble organic C in composted manure contains a portion of labile C which can stimulate soil microbial activity. The objective of this experiment was to evaluate the effects of water-soluble organic C extracted from composted dairy manure on C mineralization in soil with different textures. Three soils with textures varying from 3 to 54% clay were amended with 0 to 80 mg water-soluble organic C kg^–1 soil extracted from a composted dairy manure and incubated for 16 weeks at 23°C. The total amount of C mineralized was greater than the amount of C added in the three soils. Differences in mineralizable C with and without added water-soluble organic C were approximately 13–16 times, 4.8–8 times, and 7.5–8 times greater than the amount of C added to clay, loam, and sand soils, respectively. The results of this experiment suggest that immediately following composted manure applications, C mineralization rates increase, and that most of the C mineralized comes mainly from the indigenous soil organic C pool.CLBRR contribution No. 94-71     

Mineralization of carbon and nitrogen from cowpea leaves decomposing in soils with different levels of microbial biomass

Soils with greater levels of microbial biomass may be able to release nutrients more rapidly from applied plant material. We tested the hypothesis that the indigenous soil microbial biomass affects the rate of decomposition of added green manure. Cowpea (Vigna unguiculata L.) Walp.] leaves were added to four soils with widely differing microbial biomass C levels. C and N mineralization of the added plant material was followed during incubation at 30°C for 60 days. Low levels of soil microbial biomass resulted in an initially slower rate of decomposition of soil-incorporated green manure. The microbial biomass appeared to adjust rapidly to the new substrate, so that at 60 days of incubation the cumulative C loss and net N mineralization from decomposing cowpea leaves were not significantly affected by the level of the indigenous soil microbial biomass.     

The influence of nitrogen on atrazine and 2,4-dichlorophenoxyacetic acid mineralization in grassland soils

The influence of fertilizer N on the mineralization of atrazine [2-chloro-4(ethylamino)-6(isopropylamino)-s-triazine] and 2,4-D (2,4-dichlorophenoxyacetic acid) in soils was assessed in microcosms using radiometric techniques. N equivalent to 0, 250, and 500 kg N as NH₄NO₃ ha^-1 was added to three grassland soils. Compared to the control, the 250- and 500-kg treatments suppressed mineralization of atrazine by 75 and 54%, respectively, and inhibited mineralization of 2,4-D by 89 and 30%, respectively. Active fungal biomass responded to the N treatments in an opposite manner to herbicide mineralization. Compared to the control, the 250- and 500-kg treatments increased the active fungal biomass by more than 300 and 30%, respectively. These results agree with other observations that N can suppress the decomposition of resistant compounds but stimulate the primary growth of fungi. The degree of suppression was not related to the amount of N added nor to the inherent soil N levels before treatment. The interaction between the N additions and the active fungal biomass in affecting herbicide mineralization suggests that N may alter microbial processes and their use of C sources and thus influence rates of herbicide degradation in the field.     

Relationships between soil thermal units, nitrogen mineralization and dry matter production in pastures

Abstract. Nitrogen (N) is of enviromental concern if it leaches or is released as nitrous oxide (N₂O,). In order to utilize N efficiently in grazed pasture systems, the fluxes of N from various sources need to be quantified. One flux is N mineralization from organic sources. Previous work has examined incubation and chemical extraction of soils as methods to determine N mineralization potential. This paper re-examines new and previously published data on net mineralization, with the aim of examining the relationships between soil thermal units, net N mineralization (measured using acetylene incubations) and dry matter production in pastures. Net N mineralization is expressed as N turnover (net N mineralization as a % of total soil N). Relationships are developed between soil thermal units, dry matter production, and N turnover. These relationships have potential in advising farmers on potential N mineralization from soil organic matter. A second use of such relationships is the modelling of N transformations in pasture systems. Further work should explore the effect of soil moisture on such relationships and examine the relationship between soil thermal units and uptake of N by pasture.