Degradation of trichloroacetic acid in Saskatchewan soils

The persistence of ¹⁴C-trichloroacetic acid in three Saskatchewan soils was investigated at different moisture regimes under laboratory conditions at 25 ± 1°C. Following soil extraction with 0.1 m calcium chloride soln, radiochemical analytical procedures were used to monitor the breakdown. At rates equivalent to 4.4 kg/ha, degradation was most rapid in a high organic soil, with complete breakdown occurring within 3 weeks at moisture levels in excess of the wilting point. On a sandy loam and on a heavy clay breakdown was somewhat slower.     

Comparative Analysis of MLR,ANN, and ANFIS Models for Prediction of Field Capacity and Permanent Wilting Point for Bafra Plain Soils

ABSTRACT

Soil hydraulic parameters like moisture content at field capacity and permanent wilting point constitute significant input parameters of various biophysical models and agricultural practices (irrigation timing and amount of irrigation to be applied). In this study, the performance of three different methods (Multiple linear regression – MLR, Artificial Neural Network – ANN and Adaptive Neuro-Fuzzy Inference System – ANFIS) with different input parameters in prediction of field capacity and permanent wilting point from easily obtained soil characteristics were compared. Correlation analysis indicated that clay content, sand content, cation exchange capacity, CaCO₃, and organic matter had significant correlations with FC and PWP (p < .01). Validation results revealed that the ANN model with the greatest R² and the lowest MAE and RMSE value exhibited better performance for prediction of FC and PWP than the MLR and ANFIS models. ANN model had R² = 0.83, MAE = 2.36% and RMSE = 3.30% for FC and R² = 0.81, MAE = 2.15%, RMSE = 2.89% for PWP in training dataset; R² = 0.80, MAE = 2.27%, RMSE = 3.12% for FC and R² = 0.83, MAE = 1.84%, RMSE = 2.40% for PWP in testing dataset. Also, Bayesian Regularization (BR) algorithm exhibited better performance for both FC and PWP than the other training algorithms.     

Soil hydraulic and physico-chemical properties of Ultisols and Inceptisols in south-eastern Nigeria

Understanding soil water dynamics and storage is important to avoid crop failure on highly weathered, porous and leached soils. The aim of the study was to relate soil moisture characteristics to particle-size distributions and chemical properties. On average, Atterberg limits were below 25% in the A-horizon and not more than26.56% in the B-horizon, whereas soil bulk density was between 1.27 and 1.66Mgm^?3. The saturated hydraulic conductivity (K_sat) was generally between 0.20 and 5.43 cm h^?1 in the top soil and <1.31 cm h^?1 in the subsoil. The higher K_sat values for the A-horizons were attributed to the influence soil microorganisms operating more in that horizon. The amount of water retained at field capacity or at permanent wilting point was greater in the B-horizons than in the A-horizons, suggesting that clay accumulation in the B-horizon and evapotranspiration effects in the A-horizon may have influenced water retention in the soils. Soil moisture parameters were positively related to clay content, silt content, exchangeable Mg²⁺, Fe₂O₃ and Al₂O₃, and negatively related to sand content, SiO₂, sodium absorption ratio, exchangeable sodium percentage and bulk density. The low clay content may explain why drainage was so rapid in the soils.     

Nitrification in soils incubated with pig slurry or ammonium sulphate

The effects of temperature, moisture content and the addition of pig slurry on nitrification in two soils were studed. There was no accumulation of NO₂^?-N under the incubation conditions investigated and the accumulation of NO₃^?-N was linear for additions of 50–250 μg NH₄⁺-N g^? soil, either as ammonium sulphate or as pig slurry. Nitrate formation was treated as a single step, zero order process to enable a rate constant to be calculated. Nitrification rate increased with increasing moisture content up to the highest level tested, soil water potential ?8.0 kPa, corresponding to approximately 60% of water holding capacity in both soils. Measurable nitrification was found in both soils at the lowest moisture content (soil water potential ?1.5 MPa) and temperature (5° C) tested. The nitrification rate constant in soils treated with 50 μg NH₄⁺-N g^? soil was not significantly affected (P = 0.05) by the form of ammonium added. Addition of 250 μg NH₄⁺-N as ammonium sulphate caused a marked inhibition of nitrification at all moisture contents and temperatures. Addition of 250 μg NH₄⁺-N as pig slurry caused a marked increase in nitrification rate, the increase being greater at the higher temperatures and moisture contents.     

Effect of biochar,clay substrate and manure application on water availability and tree-seedling performance in a sandy soil

In order to develop a method for extensive pomiculture on marginal soils in semiarid Brazil, a field experiment was conducted to study the impacts of the soil conditioners biochar, clay substrate and goat manure on soil physical parameters of a sandy soil and on seedling performance of Spondias tuberosa Arruda. Manure significantly increased total porosity, soil water content and reduced bulk density of the sandy soil. Water content at field capacity (θ_fc) and at permanent wilting point (θ_pwp) were increased due to manure application. Neither biochar nor clay substrate had a significant impact on the soil physical parameters. Biochar combined with clay substrate led to lower soil water content and significantly reduced the period of retaining atmospheric water. Due to a strong correlation (R² = 0.75) between θ_fc and θ_pwp, the available water capacity within all treatments remained unchanged. Amelioration and initial nutrient supplies had no effect on seedling survival and stem growth of S. tuberosa during the 23-month experiment. This underlines the nondomesticated character of the available plant material of S. tuberosa. The independence of the seedling performance of soil management makes S. tuberosa an interesting species for low-input orchards and for reforestation within the Caatinga.     

Influence of temperature,humidity and inoculation on the degradation of14C-labeled 2-aminobenzimidazole in soil

Degradation of¹⁴C-labeled 2-aminobenzimidazole was measured in sandy loam soils by means of the evolution of¹⁴C into CO₂. The correlation between soil temperature and 2-AB-degradation was investigated utilizing a temperature gradient incubator for temperatures between 1 and 40°C with a water content of 100% of field capacity. Within the temperature interval 1 to 20°C, the evolution of¹⁴C was exponentially related to the inverse of the absolute temperature, in accordance with the Arrhenius' equation. Maximum evolution of¹⁴C was at 22°C while between 25 and 35°C the evolution remained almost constant and at 40°C it was almost nil. The correlation between soil water content and 2-AB-degradation was measured at 25°C in air dry soil and with water contents varying from 5 to 41% (equivalent to 28 to 227% of field capacity (FC)). From 28 to 94% of FC an exponential increase in the evolution of¹⁴C was observed while the evolution was slightly decreasing in soil with water contents above this level. Degradation of 2-AB in soil was greatly enhanced when the soil was inoculated with liquid or with soil aliquots from a soil perfusion apparatus through which 2-AB had been percolating for 6 mo. This indicates the presence of organisms able to decompose 2-AB in the pre-treated soil and in the perfusing water.     

Soil salts reduce hydration of polymeric gels and affect moisture characteristics of soil

Abstract

Hydration of a hydrophilic crosslinked polystyrene (gel) in deionized water at 65 g of gel per g of soil was studied. Four different soil types were selected, three were light‐textured soils (loamy sand and sandy clay loam) and the other a clay. Container capacity, field capacity, permanent wilting point, and available water for these soils were measured for gel contents equal to 0,0.1, 0.2, and 0.3%. Absorption capacity of the gels when incorporated into the soil decreased considerably compared to its absorption in pure water. This was attributed to the increased ionic strength of soil moisture and the formation of additional ionic crosslinks in the gel network due to the presence of multivalent ions in the soils. It was found also that container capacity, field capacity, and available water capacity increased with the gel content in soil, whereas the permanent wilting point was not significantly affected.     

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.     

The kinetics of extraction of soil-applied metoxuron by methanol and its biological implications

The effect of three levels of moisture (saturation, field capacity, 50% field capacity) and two temperatures (10 and 27 °C) on the extraction of sil applied metoxuron by methanol was determined for an alfisol and a vertisol. Technical grade metoxuron applied at 20 μg g^?1 of soil, was extracted by shaking with methanol for 30 min and assayed spectrophotometrically after correcting for soil blanks. There was an initial steep fall in the extractable portion for about 40 to 60 days followed by a much slower decrease in all the cases upto 150 days. The time of 50% extractability of the added herbicide (t₅₀) was lower at 27 °C than at 10 °C, the difference being more marked for the alfisol. The effect of moisture which varied from 40 to 10% for vertisol and 30 to 7.5% for alfisol was not significant. The specific rate of decrease of extractable metoxuron was similar at both temperaturs for the vertisol, and was significantly lower for the alfisol at 10 °C. The observed pattern of the kinetics of methanol available metoxuron is interpeted in terms of an initial rapid partitioning of the herbicide into labile and nonlabile compartments followed by a probable slow release and degradation path. The specific reaction rates for the different steps have been estimated and correlated with other soil properties and their implication on the bioavailability of the herbicide by considering methanol extractability as a suitable index is discussed.     

Respiratory substrate availability plays a crucial role in the response of soil respiration to environmental factors

We examined the response of the temperature coefficient (Q₁₀) for soil respiration to changes in soil temperature and soil moisture through a laboratory incubation experiment. Two types of soils differing in vegetation and moisture status were collected and incubated under two temperatures (10 and 30 °C) and two soil moisture regimes (35 and 75% of water holding capacity, WHC) for 5 weeks. Before and after the incubation experiment, the temperature coefficient of soil respiration was measured using soda-lime method by changing temperature in a water bath. For both soils, the mean Q₁₀ values of the respiration rate were 2.0 in the 30 °C and 2.3 in the 10 °C soil treatments. Higher temperature with lower soil moisture treatment significantly decreased the Q₁₀ value, whereas lower temperature with higher soil moisture treatment significantly enhanced the Q₁₀ value (ANOVA, p < 0.05). These results indicate that soils became less sensitive to temperature when incubated under higher temperature with higher moisture conditions, and more sensitive in lower temperature with higher moisture conditions.There was a significant correlation (r² = 0.67, p < 0.05) between water-soluble carbon (WSC) and soil respiration rate. However, the correlation between soil respiration rate and microbial biomass carbon (MBC) was weak (r² = 0.27, p > 0.05). Although incubation temperature and moisture accounted for 40 and 29% (as r² × 100%), respectively, of variations in Q₁₀, soil water-soluble carbon content alone could have explained 79% of the variation, indicating that the availability of respiratory substrate, rather than the pool of soil microorganisms, played a crucial role in the response of the temperature coefficient to environmental factors. These results suggest that biotic factors should also be taken into consideration when using the Q₁₀ function to predict the response of soil respiration to global warming.     

几种常用绿地改良材料对土壤水分特征的影响

分析了几种常用绿地土壤改良材料及其不同配比对土壤水分特征曲线和水分常数的影响,结果表明:利用RETC软件对各配比土壤水分特征曲线van Genuchten方程的参数拟合效果较好,R2均大于0.99;随着砂粒含量的增加,土壤田间持水量降低,土壤中水分有效性比例增加,但砂粒粒径对土壤水分常数影响不显著;绿化植物废弃物能提高土壤田间持水量和有效水含量,降低土壤凋萎含水量;绿化植物废弃物还能提高有效水占田间持水量的比例,以20%绿化植物废弃物的用量为最大,为49.59%;聚丙烯酰胺(PAM)虽然能提高土壤田间持水量,但阻碍土壤水分的释放,降低土壤水分的有效性;脱硫石膏可以增加土壤田间持水量和水分的有效性。综合而言,以70%土、10%砂、20%绿化植物废弃物和0.5 kg/m~3脱硫石膏的配比相对最佳。     

Potential of sawdust and corn cobs derived biochar to improve soil aggregate stability,water retention,and crop yield of degraded sandy loam soil

Abstract

Poor soil structure is the main cause of soil degradation; however, biochar the solid carbon-rich production of pyrolysis biomass could improve the soil structure. Biochar from the feed stock sawdust (SD) and corn cobs (CC) was pyrolyzed at 450?°C. Wheat was grown as a test crop and treatments were control, NPK, SDB1% (sawdust biochar), CCB1% (corn cobs biochar), SDB0.5%+CCB0.5%, SDB1%?+?½ NPK, CCB1%?+?½ NPK. The higher growth, higher grain and dry matter yield were displayed by biochar?+?NPK. The lowest pH, the higher organic matter, available P and available K were observed in SDB0.5%+CCB0.5%. However, the highest total N (1.43?g kg^?1) was by NPK treatment. The biochar increased plant available water contents, water contents at field capacity and permanent wilting point, soil porosity and decreased bulk density. The highest stable aggregates were in SDB0.5%+CCB0.5%. Biochar application was found as a useful practice for soil sustainability     

Nitrous oxide production in aerobic soils under varying pH,temperature and water content

Laboratory studies were conducted to evaluate the effect of soil pH, temperature and water content on the rate of nitrification and on the amount of N₂O evolved from samples of Plano silt loam soil. The rate of nitrification of added NH₄⁺-N increased with increasing soil pH (4.7, 5.1 and 6.7), temperature (10, 20 and 30°C) and water content (0.1, 0.2 and 0.3 m³ m^?3). At soil water contents of 0.1 and 0.2 m³ m^?3, corresponding to 18 and 36% water-filled pore space, respectively, N₂O evolution was proportional to NO₃^? production. Approximately 0.1–0.2% of the nitrified N was evolved as N₂O-N. At 0.3 m³ m^?3 water content (54% water-filled pore space) and 20 and 30°C, the ratio of N₂O-N evolved to N nitrified was significantly higher (range of 0.3–1.1%).An additional experiment was conducted using diurnally fluctuating temperatures (10–30°C). The pattern of N₂O evolution was markedly different when the system was sampled at 10 and 30°C than at 20°C. The apparent N₂O emission rates were approximately equal for 12-h periods during which the temperature increased from 10 to 30°C or decreased from 30 to 10°C. In contrast, the apparent N₂O emission rates were significantly lower for the 12-h period when the incubation flasks were sampled at 20°C following the daily minimum temperature compared to the 12-h period when the samplings were at 20°C following the daily maximum temperature. This provides additional evidence that temperature fluctuation in the surface soil is a factor in-observed diurnal variations in N₂O emissions under field conditions.Our findings indicate that an interaction of three factors (soil pH, temperature and water content) affects the amount of N₂O evolved during nitrification in soils. In relatively dry soils, estimated N₂O production of ca. 0.1–0.3% of the N nitrified may be sufficiently accurate. Much higher N₂O output can be expected following rainfall or irrigation. Diurnal variability in N₂O fluxes from soils due to fluctuating temperature is an additional uncertainty in quantifying N₂O production in field soils.     

Effect of Varying Soil Temperatures on the Degradation of Methabenzthiazuron,Isocarbamid and Metamitron

The ¹⁴C-labelled herbicidal active ingredients methabenzthiazuron, isocarbamid and metamitron were subjected to decomposition for 10 to 12 weeks in a degraded loess soil at 65% of maximum water holding capacity. To simulate the field situation, the standardised soil temperature of 0, 10 and 20°C were increased several times by 5 or 10°C, either daily or weekly. The rates of ¹⁴CO₂ evolution clearly showed the delay in degradation as a consequence of lowering the temperature or of the retarded microbial activity. For all 3 herbicidal compounds, a strongly reduced degradation was readily observed at 10°C, and at 0°C degradation stopped almost entirely. Daily temperature increases had only a weak stimulating influence. At the standardised temperature of 20°C, however, daily temperature increases, or temperature increases lasting for a longer period, by 5 or 10°C effected a marked increase in the rates of ¹⁴CO₂-evolution. On the other hand, the higher temperatures led to lower extractability of residual herbicide in the soil. In the case of isocarbamid and metamitron, about 90% of the extracted radioactivity still represented the unchanged active ingredient, whilst, in the case of methabenz-thiazuron, this fraction was between 97 and 100%.     

生物质炭对旱地红壤理化性状和水力学特性的影响

[目的]研究生物质炭对旱地红壤基本理化性质及水分特征曲线的影响,为红壤地区土壤改良提供依据。[方法]分层测定不同生物质炭施用量水平下的土样容重、孔隙度和有机碳含量,采用原状土压力膜法分层测定土壤的水分特征曲线。[结果]施用生物质炭能显著降低土壤的容重,提高土壤的孔隙度及有机碳含量,且随着施用量的增加,土壤容重逐渐降低,孔隙度及有机碳含量逐渐提高;随着生物质炭施用量的增加,土壤饱和含水量、田间持水量和有效水含量逐渐增加,凋萎系数逐渐减小,施用生物质炭30t/hm2的土壤处理饱和含水量、田间持水量和有效水含量最高;生物质炭施用量与土壤饱和含水量、田间持水量和有效水含量呈极显著正相关关系,与凋萎系数呈极显著负相关关系。[结论]施用生物质炭能显著提高红壤田间持水量和有效水含量。     

甘肃省不同气候类型区土壤水分特性

为揭示甘肃省不同气候区不同质地土壤的容重、田间持水量和凋萎湿度的差异,对观测资料的适用性和推广价值进行评价。通过对77个站点10—100 cm土壤水分资料的分析,结果表明:甘肃省全省的土壤容重范围为0.89～1.79 g/cm~3,平均值为1.36 g/cm~3,表层土壤容重与深层土壤容重差异显著(P0.05),半湿润区、半干旱区浅层土壤容重更易受到外界环境及人为活动的干扰。甘肃省大部田间持水量由西北向东南呈增加趋势,田间持水量的最大值为36%～40%,分布在高寒湿润区10—50 cm土层,全省10 cm与20 cm土层田间持水量差异较小,相关系数为0.96,与其他层次差异较大,50 cm土层很可能是甘肃地区土壤田间持水量的分界层。各土层凋萎湿度最大值均出现在冷温带干旱区、高寒半干旱半湿润区中部,不同层次间田间持水量与凋萎湿度呈极显著相关。甘肃省全省大部分地区主要以壤土为主,除此之外干旱区主要以砂壤土为主,半干旱区主要以砂壤土与黏壤土为主,半湿润区主要以粉壤土与黏壤土为主。探讨不同气候区不同层次间土壤容重、田间持水量和凋萎湿度的差异,以期为保障地上生产力、提高水分利用效率提供数据支撑。     

Immobilization and mineralization of nitrogen in soils incubated with pig slurry or ammonium sulphate

Nitrogen mineralization and immobilization were investigated in two soils incubated with ammonium sulphate or pig slurry over a range of temperatures and moisture contents. A reduction in the mineralization of soil organic N was observed in soils incubated with 100 μg NH₄⁺-Ng^?1 soil as ammonium sulphate at 30°C but not at lower temperatures. Addition of 100 μg NH₄⁺-N g^?1 soil as pig slurry resulted in a period of nett immobilization lasting up to 30 days at 5°C. Although the length of the immobilization phase was shorter at higher temperatures the total N immobilized was similar. The subsequent rate of mineralization in slurry-treated soils was not significantly greater (P = 0.05) than in untreated soils. There was no evidence of any subsequent increased mineralization arising from the immobilized N or slurry organic N for up to 175 days. The rate of immobilization was found to increase with increasing moisture content, though the period of nett immobilization was shorter, so that the amount of N immobilized was similar over a range of moisture contents from 10 to 40%. Approximately 40% of the NH₄⁺-N in the slurry was immobilized under the incubation conditions used.     

Nitrogenase activity associated with pasture grasses in Northern New South Wales

Nitrogenase activity associated with roots of grasses was initially examined at 67 sites in New South Wales using an enriched malate medium. Twenty six of the 39 grass species examined produced at least 10 nmol C₂H₄cm^?1 root h^?1—a level accepted as positive presumptive evidence of N₂-fixation: 40 of the 288 samples exceeded 100nmol C₂H₄h^?1. The seasonal N₂ase activity of up to 4 grass species collected in soil cores at 6 sites was measured over 16 months.Activity at field moisture levels, but incubated at 30°C was greatest for cores collected in summer months. Activity was increased in 33.5% of samples by raising soil moisture to field capacity. No single species of grass consistently supported higher activity than any other.Nitrogenase activity was compared in cores of Kikuyu grass (Pennisetum clandestinum) watered to in excess of field capacity and allowed to drain for between 7 and 28 days before re-watering. Activity declined rapidly in the first 7 days and although recovery was also rapid, integration of N₂ase activity over time showed a loss of 20 and 61% for 7 and 28 days drainage respectively. N₂ase activity was greatest at 30°C.Maximum N₂ase activity in field samples was only 246 nmol C₂H₄ core^?1 h^?1 indicating that fixation of N₂ would not be of agronomic significance.     

The effects of organic residue quality on growth and reproduction of <Emphasis Type="Italic">Aporrectodea trapezoides</Emphasis> under different moisture conditions in a salt-affected agricultural soil

We studied the effects of organic residues with different C/N ratios and soil moisture contents on the growth and reproduction of the earthworm Aporrectodea trapezoides to investigate potential measures to increase its population in a salt-affected agricultural soil. The experiment consisted of eight treatments in a fully factorial design: low or high C/N ratio organic residue, soil moisture at 75 or 95% field capacity (FC), and salinity (as electrical conductivity (EC)) of 3.07 or 4.77 dS m^?1. It was carried out under controlled laboratory conditions for 4 months. In the low C/N ratio organic residue application, there was a significantly greater mean total dry weight and number of clitellate individuals of A. trapezoides, regardless of the soil moisture and salinity content, which may be due to the greater soil microbial biomass and dissolved organic N (DON) derived from the low C/N ratio organic residue. Generally, more cocoons were found in the application of low C/N ratio clover residue at months 2 and 4. At an EC of 3.07 dS m^?1 and moisture content of 75% field capacity (FC), significantly more hatchlings were found when low C/N ratio clover residue was applied compared to the high C/N ratio wheat residue. High soil moisture content (95% FC) resulted in a significantly greater mean total dry weight of A. trapezoides at months 2 and 4 and significantly more clitellate individuals and cocoons at month 4 compared to the low soil moisture content (75% FC), but only when the low C/N ratio residue was applied. In contrast, high soil moisture content (95% FC) resulted in significantly less hatchling numbers at an EC of 3.07 dS m^?1, only when the low C/N ratio residue was applied. These results suggest that the organic residue type and soil moisture content can regulate the growth and reproduction of the earthworm A. trapezoides, which should help to improve the recovery of their populations in salt-affected agricultural soil.     

Optimum temperatures for soil respiration along a semi-arid elevation gradient in southern California

Soil respiration (R) has not been adequately studied at temperatures above 35 °C, which are common temperatures for soils in the southwestern United States and may be important for C dynamics in semi-arid regions. While frequently excluded from ecosystem models or set to 35 °C, the optimum temperature for soil R is poorly understood. Optimum temperatures are likely controlled by substrate availability, soil moisture content, and previous climate. To quantify the optimal temperature for soil R and hypothesized relationships, we collected soils from beneath and between plant canopies at three sites along a semi-arid elevation gradient. Processed soil samples were incubated at three soil moisture contents and soil R was measured at 6 temperatures, successively (25–55 °C). From these data, an activation energy for reaction kinetics and deactivation energy for enzyme functionality model was used to generate soil R curves from which two parameters were derived: R_max, the maximum rate of soil R and T_opt, the optimum temperature for soil R. R_max was significantly greater for soils at the highest elevation and at medium soil moisture content. T_opt was greater than 35 °C at all locations. In addition, T_opt was significantly greater for soils with greater amounts of SOM but not significantly different along the elevation gradient or at different moisture contents. These results support inclusion of much higher optimum temperatures than currently used in many ecosystem and land-surface models and provide support for explaining variation in T_opt as regulated by substrate quantity within a site and general insensitivity across climate differences.