Biological activity of alpine mountain-meadow soils in the Teberda Reserve

The intensity of the actual and potential CO₂ emissions, nitrogen fixation, denitrification, and methane production were determined by gas chromatography in alpine mountain-meadow soils under different types of phytocenoses at the Teberda State Reserve. The main factors that control the intensity of these processes in the mountain-meadow soils are moisture and accumulation of biophilous elements related to the position of the soils on the biogeochemical catena. The CO₂ emission and intensity of the nitrogen fixation in the soils under Geranium-Hedysarum meadows that occupy transit-accumulative positions were 2–3 times higher that these parameters in the soils under the alpine heathlands and Festuca meadows. The soils under the carpet-like alpine meadows, accumulating moisture and mineral nitrogen, were characterized by the highest intensity of denitrification and methane production.     

Influence of drying of the samples on the transformation of nitrogen and carbon compounds in mountain-meadow alpine soils

The drying of samples of mountain-meadow soils characterized by their permanently high moisture under natural conditions fundamentally changes the concentrations of the labile nitrogen and carbon compounds, as well as the patterns of their microbial transformation. When the soil samples are dried, a four- to fivefold increase in the content of the extractable organic nitrogen compounds, carbon compounds, and inorganic nitrogen compounds is observed, while the content of nitrogen and carbon of the microbial biomass decreases by two-three times. The rewetting of the dried soil launches the process of the replenishment of the nitrogen and carbon reserves in the microbial biomass. However, even after two weeks of incubation, their values were 1.5–2 times lower than the initial values typical of the natural soil. The restoration of the microbial community in the samples of the previously dried soils occurs in the absence of a deficiency of labile organic compounds and is accompanied by their active mineralization and the low uptake of ammonium nitrogen by the microorganisms.     

Seasonal protein dynamics in Alaskan arctic tundra soils

In the arctic tundra of Alaska, plant growth is limited by N supply, especially in tussock tundra. Because proteins are the predominant form of soil organic N, proteolysis is considered to be the rate-limiting step in both the release of amino acids and in N mineralization. To help understand the controls on N availability in tundra soils, and to determine whether proteolysis is controlled by enzyme activity or by substrate availability, we measured soil protein concentrations, and proteolysis rates with and without added protein, every 1-2 weeks through the summer of 2000 and twice in the summer of 2001. Protease activity with added protein (‘potential’) was higher than without added protein (‘actual’). However, differences between the two tended to be driven by relatively brief peaks in potential protease activity. In fact, actual and potential rates were usually similar, suggesting that much of the time proteolysis was not limited by substrate availability, but rather by enzyme activity. Our data suggest that protease activity was actually only substrate limited at the times when it was highest. Potential rates peaked at the same times that soluble proteins were also high. These increases in protease activity and soluble protein concentrations occurred when soil amino acid and NH₄⁺ concentrations were at their lowest, drawn down by the seasonal peaks in root growth. The fact that the peaks in protease activity coincided with the peak in soil amino acid and NH₄⁺ demand strongly suggests that proteolysis was stimulated by high soil amino acid demand, and resulted in increases in soluble protein concentrations caused by the solubilization of larger proteins.     

Dynamics of mineral nitrogen in soils supporting potato crops

Following application of fertiliser-N to the seedbed of potato crops, concentrations of extracted mineral-N were up to 3 times greater than would be anticipated by calculation. The rates at which both NO ₃ ^– -N and NH ₄ ⁺ -N apparently appeared and disappeared in the soil solution were, at various times, also much greater than could be attributable to any transformations resulting from microbial activity. This suggests that the involvement of other factors in this phenomenon must be considered. The effect of certain physical parameters such as water movement, resulting from capillary action and evaporation from the soil surface, may be implicated. We suggest that soil microbes are not directly involved in the early fate of fertiliser-N, primarily due to C-limitation in arable soils. N dynamics in fertilised potato systems require further studies targeting the relationships between nutrient concentrations in soil solution and mass flow of soil water.     

Effect of mineral nitrogen fertilizer forms on N2O emissions from arable soils in winter wheat production

Nitrogen fertilizers are supposed to be a major source of nitrous oxide (N₂O) emissions from arable soils. The objective of this study was to compare the effect of N forms on N₂O emissions from arable fields cropped with winter wheat (Triticum aestivum L.). In three field trials in North‐West Germany (two trials in 2011/2012, one trial in 2012/2013), direct N₂O emissions during a one‐year measurement period, starting after application of either urea, ammonium sulfate (AS) or calcium ammonium nitrate (CAN), were compared at an application rate of 220 kg N ha^?1. During the growth season (March to August) of winter wheat, N₂O emission rates were significantly higher in all three field experiments and in all treatments receiving N fertilizer than from the non‐fertilized treatments (control). At two of the three sites, cumulative N₂O emissions from N fertilizer decreased in the order of urea > AS > CAN, with emissions ranging from 522–617 g N ha^?1 (0.24–0.28% of applied fertilizer) for urea, 368–554 g N ha^?1 (0.17–0.25%) for AS, and 242–264 g N ha^?1 (0.11–0.12%) for CAN during March to August. These results suggest that mineral nitrogen forms can differ in N₂O emissions during the growth period of winter wheat. Strong variations in the seasonal dynamics of N₂O emissions between sites were observed which could partly be related to weather events (e.g., precipitation). Between harvest and the following spring (post‐harvest period) no significant differences in N₂O emissions between fertilized and non‐fertilized treatments were detected on two of three fields. Only on one site post‐harvest emissions from the AS treatment were significantly higher than all other fertilizer forms as well as compared to the control treatment. The cumulative one‐year emissions varied depending on fertilizer form across the three field sites from 0.05% to 0.51% with one exception at one field site (AS: 0.94%). The calculated overall fertilizer induced emission averaged for the three fields was 0.38% which was only about 1/3 of the IPCC default value of 1.0%.     

Seasonal dynamics of mineral nutrients by walnut tree fruits

Walnut (Juglans regia L.) tree fruit showed after the endocarp lignification a fast growing stage during which fresh and dry weights increased abruptly. From the beginning of fruit ripening and during the fast sperm growing stage, fresh weight started to decrease while dry weight continued to increase with a reduced growth rate. Dry weights increased in sperm and decreased in exocarp‐mesocarp tissues during the fast sperm growing stage. The material exit from pericarp tissues was completed in the ripe fruit. By contrast, fresh weight continued to decrease in the tissue. Patterns of nutrient accumulation per fruit increased continuously during the fruit growth period. The observed reductions of nutrient accumulations for total nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) in the fruit individuals during the very late fruit stage after fruit ripening, and in conjunction with the pericarp tissues senescence, are supposed to represent mineral nutrient returns from the ripe fruit. Patterns of total N, P, Mg, Fe, and Zn accumulations increased in the exocarp‐mesocarp tissue during the slow sperm growing stage and decreased during the fast sperm growing stage. Potassium accumulation in the tissue increased continuously up to the fruit ripening time. Calcium, Mn, and Cu increased continuously. Patterns of all nutrients in endocarp tissue increased during the slow sperm growing stage and decreased at the fast sperm growing stage. In the sperm tissues, total N, P, Mg, and Ca accumulations increased during the sperm development and slightly decreased in a late stage. The increasing trend of Ca accumulation was temporarily interrupted during the fast sperm growing stage. Iron, Mn, Cu, and Zn accumulations showed no reductions at all. Potassium accumulation was drastically restricted in the tissue with the approach of fruit ripening. Sperm tissues are extraordinary rich in mineral nutrients. Sperm total N, P, Mg, Mn, Zn, Cu, and Fe accumulations represented the 98.1%, 88.2%, 59.2%, 81.5%, 72.3%, 65.6%, and 52.5% of the total nutrients accumulation in the fruit, respectively. Sperm K and Ca accumulations represented only the 13% and 11.6%, respectively. Exocarp‐mesocarp K, Ca, and Mg accumulations represented the 76%, 72% and 37.1% of the total nutrients accumulation in the fruit individual, respectively. Total N and P accumulation in the tissue were detected in very low levels 1.3% and 7%, respectively. Iron, Cu, Zn, and Mn accumulations were detected in the same tissue in ratio values of 27.5%, 22%, 5.4%, and 11%, respectively. Macro‐ and micro‐nutrient accumulations of the endocarp tissues were detected in the lower levels as compared to the other fruit tissues. The estimated values of mineral nutrient returns from the mature fruit individuals were 2.8% for total N, 13% for P, 16.5% for K, 23% for Ca, 12% for Mg, 28.5% for Fe, and 21% for Zn. Manganese and Cu showed no returns at all. The estimated nutrient returns from the sperm tissues were 60% for total N, 67% for P, 22% for K, and 50% for Mg of the total returned nutrient from the fruit individual. The estimated nutrient returns from exocarp‐mesocarp were 100% for Zn, Fe, and Ca, 50% for Mg, 78% for K, 33% for P, and 40% for total N. Calcium, Fe, Mn, Cu, and Zn in the sperm and Mn and Cu accumulations in pericarp tissues showed no returns at all. A restricted nutrient diffusion from exocarp‐mesocarp and sperm tissues to the endocarp tissues is supposed to be possible. These results suggested a pericarp tissue behaviour similar to the old senescing leaves.     

Simulating nitrogen dynamics in soils using a deterministic model

Abstract. LEACHN is a deterministic model for simulating nitrogen dynamics in soil. Transport processes are based upon numerical solutions to the Richards equation for water flow and the convection-dispersion equation for solute transport. Transformations of urea, ammonium, nitrate and three organic pools are included, and the influence of water content and temperature can be reflected. Lack of measured input data sometimes limits the more general use of models such as these. Approaches to estimating data values using soil survey information and a limited number of measured data are discussed. Simple model sensitivity studies and a limited number of field measurements can guide the choice of input data values and lead to simulations that reflect the main features of the field soil nitrogen regime. Such an approach provides initial values for a modelling exercise, and improves intuition regarding the relative importance of processes and interactions in the field nitrogen cycle.     

Seasonal dynamics of mineral nutrients by walnut tree reproductive organs

The dry weight accumulation per male and female flower as well as the concentration per gram of dry weight and the accumulation of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) were determined in walnut tree (Juglans regia L.) catkins and female flowers at the stage of flower bud and during the flower development. Catkin emergence was accompanied by a very fast hydration of the tissues. After the catkin matured, the fresh and dry weights were reduced. The female flower development period was accompanied by the dry and fresh weight increase. Total N, P, K, Fe, Mn, Cu and Zn concentrations in catkin buds were detected at lower levels, Mg in equal levels, and Ca at higher levels as compared to the nutrient concentrations in young growing leaves. The estimated values of the ratio NCmfb/NCygl were: total N = 0.54, P = 0.83, K = 0.56, Ca = 1.5, Mg = 1.0, Fe = 0.46, Mn = 0.71, Cu = 0.85, and Zn = 0.60. Nutrient concentration in female flower buds was detected in almost equal levels with the exception of total N and Fe. The estimated values of the ratio: NCffb/NCygl were: total N = 0.57, P = 1.1, K = 1.17, Ca = 1.06, Mg = 0.9, Fe = 0.47, Mn = 1.0, Cu = 0.92, and Zn = 0.85. Total N, P, Mn, Cu, and Zn accumulations in the catkin were increased during the fast growing phase and decreased after catkin maturing. Potassium, Mg, and Fe accumulation continued to increase in the mature catkin. Calcium accumulation decreased at a very late mature catkin phase. Total N, P, and K accumulation rates during the catkin fast growing phase were higher than the dry weight accumulation rate. Calcium, Mg, Fe, Mn, Cu, and Zn accumulation rates at the same period were lower or equal to dry weight accumulation rates. In mature catkins, the total N, P, Mn, Cu, and Zn depletion rates were higher than the dry weight depletion rate. The continual increase of K, Ca, Mg, and Fe accumulation in mature catkin resulted in the increase of nutrients concentration also. Total N and P showed the highest remobilization values from mature catkin of 51.4% and 45%, respectively. Calcium, K, Mg, Cu, Mn, and Zn remobilization values estimated to be 22.1%, 7.5%, 3.2%, 45.3%, 33.4%, and 31.8%, respectively. Iron showed no remobilization at all. Nutrients remobilization from catkins as compared to the leaves had almost similar values for total N, Zn, and Cu, higher for P, Ca, and Mn, and lower for Mg, Fe, and K. Accumulation of all nutrients in female flowers increased after fertilization. The dry weight accumulation rate was higher than the nutrient accumulation rates.     

紫色水稻土轻组有机质的季节动态研究

以位于重庆市北碚区西南大学试验农场(30°26′N,106°26′E)的紫色水稻土为研究对象,利用重液(NaI,密度1.8 g·cm-3)对土壤中轻组组分进行提取,对土壤中轻组有机质在整个油菜生长季的季节变化情况进行分析与讨论。结果表明:表层(0~30 cm)土壤轻组物质(LF)的含量为2.95%~5.51%,平均值为4.38%;土壤轻组有机碳含量(LFOC)和轻组氮含量(LFN)的变化范围分别为1.44~3.72 g·kg-1和0.08~0.17 g·kg-1,其平均值分别为2.79 g·kg-1和0.14 g·kg-1。LFOC具有明显的季节变化(P<0.05),其含量在油菜生长中期最高,其次是生长后期,而在生长初期最低;LFN的季节变化趋势与LFOC一致,但季节差异性不显著(P>0.05)。轻组有机碳分配比例(LFOC/SOC)的变化范围为9.21%~24.47%,具有明显的季节变化(P<0.05),其变化趋势与LFOC的季节变化一致;而轻组氮的分配比例(LFN/TN)变化范围为4.55%~12.58%,无明显的季节变化。轻组C/N比值季节变化范围为18.52~25.04,平均值为20.66,全土C/N比值的变化范围为9.04~14.36,平均值为11.66,说明轻组有机质的生物可利用性较土壤总有机质高。相关分析表明,轻组有机碳、氮含量分别与根系生物量、根系碳含量、根系氮含量呈极显著(P<0.01)或显著(P<0.05)正相关;回归分析表明,土壤轻组有机碳、氮含量变化的40%~60%可由根系生物量、根系碳氮含量决定,说明根系是调控紫色水稻土轻组有机碳、氮季节变化的主要生态因子。     

Seasonal dynamics of mineral nutrients and carbohydrates by walnut tree leaves

The dry weight accumulation per leaf as well as the concentration per gram of dry weight and the accumulation of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) were determined in walnut tree leaves (Juglans regia L.) during a complete life cycle. Additionally, the dynamics of plant nutrient concentration in leaf petiole sap and carbohydrate accumulation in leaves were studied in relation to the main life cycle events of the walnut tree. Total N, P, K, Cu, and Zn concentrations decreased, whereas that of Ca, Mg, and Mn increased during the season. Iron concentration fluctuated around a mean value. Total N, P, K, Mg, and Cu concentrations detected in younger mature leaves were at the sufficient level, whereas Ca, Fe, Mn, and Zn concentrations were at higher levels as compared to those previously reported. All the detected nutrient accumulations increased abruptly during leaf ontogeny and leaf maturation until a maximum level was attained in the younger mature leaves. Similarly, sucrose, glucose, and fructose accumulation were observed at the same period. The rates of total N, P, Cu, and Zn accumulation were lower than the rates of the observed dry matter accumulation and nutrient concentration dilution. Potassium and Mn accumulation rates were almost equal, whereas those for Ca and Mg were higher as compared to the dry matter accumulation rate. The fast embryo growing phase resulted in a considerable decrease in dry weight, total N, P, K, Cu, Zn, and carbohydrate accumulation, and to a lesser degree in Ca, Mg, and Mn accumulation. Nutrient accumulation reduction in leaves by the influence of the growing fruits were estimated to be: total N 52%, K 48%, P 29.5%, Mg 16.3%, Ca 15%, Fe 51.2%, Cu 55.2%, Zn 37.3%, and Mn 5.4% of the maximum nutrient value of the younger mature leaves. Old leaves preserved nutrients before leaf fall as follows: total N 25.4%, P 45%, K 31%, Ca 74.8%, Mg 76.5%, Mn 89.2%, Fe and Zn 50%, and Cu 37%. Nutrient remobilization from the senescing old leaves before leaf fall were: total N 22.6%, P 25.5%, K 21%, Ca 10.2%, Mg 7%, Fe 3.2%, Mn 5.4%, Cu 8%, and Zn 13.3% of the maximum value in the younger mature leaves. In early spring, the absorption rates of N, P, and Ca were low while those of Mg, Fe, Mn, Cu, and Zn were high. During the fast growing pollen phase, the N, P, Fe, Mn, Cu, and Zn concentrations were reduced. Calcium concentration is supposed to be more affected by the rate of transpiration rather than during the growing of embryo. Calcium and Mg concentrations in the sap were negatively correlated. The detected K concentration level in the sap was as high as 33 to 50 times that of soluble N, 12 to 21 times to that of P, 5 times to that of Ca, and 10 to 20 times to that of Mg. The first maximum of starch accumulation in mature leaves was observed during the slow growing embryo phase and a second one after fruit ripening. Old senescing leaves showed an extensive carbohydrate depletion before leaf fall.     

Simulation of the dynamics of different forms of radiocesium in soils of terrestrial ecosystems

A refined imitational model of the seasonal dynamics of radiocesium in soils of forest ecosystems is suggested. This model has been used to predict the dynamics of biologically available and unavailable forms of radiocesium in an artificially contaminated sandy soddy-podzolic soil of an oak ecosystem for a period of 50 years.     

pH regulation of carbon and nitrogen dynamics in two agricultural soils

Soil pH is often hypothesized to be a major factor regulating organic matter turnover and inorganic nitrogen production in agricultural soils. The aim of this study was to critically test the relationship between soil pH and rates of C and N cycling, and dissolved organic nitrogen (DON), in two long-term field experiments in which pH had been manipulated (Rothamsted silty clay loam, pH 3.5-6.8; Woburn sandy loam, pH 3.4-6.3). While alteration of pH for 37 years significantly affected crop production, it had no significant effect on total soil C and N or indigenous mineral N levels. This implies that at steady state, increased organic matter inputs to the soil are balanced by increased outputs of CO₂. This is supported by the positive correlation between both plant productivity and intrinsic microbial respiration with soil pH. In addition, soil microbial biomass C and N, and nitrification were also significantly positively correlated with soil pH. Measurements of respiration following addition of urea and amino acids showed a significant decline in CO₂ evolution with increasing soil acidity, whilst glucose mineralization showed no response to pH. In conclusion, it appears that changes in soil pH significantly affect soil microbial activity and the rate of soil C and N cycling. The evidence suggests that this response is partially indirect, being primarily linked to pH induced changes in net primary production and the availability of substrates. In addition, enhanced soil acidity may also act directly on the functioning of the microbial community itself.     

生物质炭对不同pH值土壤矿质氮含量的影响

为了揭示生物质炭作为土壤调理剂添加后对土壤矿质氮形态、含量等土壤性质的影响,该研究利用芒草分别在350和700℃裂解制得生物质炭,发现2个温度尤其是700℃制得的生物质炭,对NH4+有很强的吸附能力,但对NO3-的吸附能力很弱。将生物质炭分别加入到酸性(pH值为3.8)和碱性(pH值为7.6)土壤中,25℃下室内培养180d。结果表明,生物质炭提高了土壤全氮含量,酸性和碱性土壤分别平均提高了22%和17%;但使土壤铵态氮含量大幅降低至接近仪器检测限水平;生物质炭对土壤硝态氮含量的影响因生物质炭和土壤类型而异。生物质炭对土壤矿质氮形态和含量的影响,显然与生物质炭对铵的吸附作用、提高土壤pH值、增强氨挥发损失,以及形成微生物量氮等密切相关。该研究可为开展生物质炭基氮素新型肥料及制剂等方面的科学研究提供参考。     

Effect of rewetting air-dried soils on pH and accumulation of mineral nitrogen

The effect of rewetting a number of air-dried soils on pH and on accumulation of mineral-N was examined in a laboratory incubation study. When rewetted-soils were incubated at 25°C three patterns of change in soil _pH and in accumulation of mineral-N were observed. Ammonification and nitrification proceeded together in soils with pH values greater than 6.0; soil pH decreased whilst concentrations of nitrate rose and those of ammonium remained low. By contrast, in soils with pH values less than 5.0, although ammonification proceeded there was no appreciable nitrification; soil pH increased whilst concentrations of ammonium rose and those of nitrate remained very low. In a third group of soils with pH values between 5.0 and 5.5, there was a delay in nitrification, but ammonification was not retarded; soil _pH initially rose as concentrations of ammonium increased, but when nitrification subsequently commenced the _pH decreased, concentrations of nitrate rose and those of ammonium declined. When microbial activity in rewetted soils was inhibited by incubation at 3°C, or in a chloroform atmosphere at 25°C, there was little change in concentrations of ammonium and nitrate, and soil pH remained relatively constant.
Such changes in soil _pH, induced by ammonification and nitrification, are likely to have important consequences to soil chemical studies where _pH-dependent reactions are being studied using rewetted soils. Changes in pH can be minimized by using field moist rather than air-dried soils.     

不同形态氮肥在坡耕地雨季土壤氮素流失动态特征

为有效控制紫色丘陵区坡耕地氮素流失,采用二因素四水平随机区组试验,利用模拟径流小区观测方法,研究氮肥形态对坡耕地雨季土壤氮素流失动态的影响,结果表明,整个雨季氮素流失量占玉米季氮肥施用量的3.29%,其中玉米生长季的氮素流失量占氮肥施用量的1.63%,玉米收获后径流氮素流失量占氮肥施用量的1.65%。氮素流失动态差异受氮肥形态和施肥距径流产生时间间隔的共同作用。在攻苞肥施用之前的5月22日,各氮肥形态处理相比差异不显著;在接近作物收获的7月30日和8月4日,铵态氮处理和酰胺态氮处理两次总的氮素损失量(3.65和4.35 kg/hm2)较其它处理高。玉米收获后的9月16日,硝态氮肥处理和缓控释肥处理的氮损失分别是2.95和3.17 kg/hm2,较铵态氮肥处理的氮损失低119.95%和103.96%,较酰胺态氮肥处理的氮素损失低85.04%和71.60%。可见,施用硝态氮肥和缓控释肥能够有效降低攻苞肥施用后的径流氮流失量,而铵态氮加地膜以及酰胺态氮肥处理增加攻苞肥施用后壤中流和总径流氮流失浓度和流失量。从有效控制农业面源污染的角度考虑,建议玉米生产上,在肥料种类选择时以缓控释肥和硝态氮肥为主。     

The effects of earthworm functional group diversity on nitrogen dynamics in soils

A novel experimental design, based on the simplex model, was used to study the effects of earthworm functional group diversity on nitrogen dynamics in soils. This mesocosm experiment was carried out at two densities of earthworm and at two levels of food supply. Leachate was collected and analysed over a 20-week period. Soil nitrogen content was measured when the soil columns were destructively sampled on week 30.Results showed the presence of variation among the functional groups in their effects on N dynamics and that both population densities and levels of resource availability were significant. Ammonium concentrations in leachate were generally higher with anéciques and endogées; the opposite was true for nitrate-N, which under certain circumstances was higher with epigées. A significant synergy between the endogées and anéciques was shown in terms of nitrate in soil. Earthworm biomass was important in some instances. For example, larger amounts of soil nitrate were present at higher densities of endogées. A varying effect of food supply was seen for the three functional groups. For example, there were reduced concentrations of nitrate-N in the leachate from the anecic monocultures at low levels of food supply, while increased amounts were leached from the epigées at low food supply. Greater concentrations of ammonium-N were leached from anéciques monocultures at low levels of food supply. Increased food supply resulted in increased amounts of soil nitrate-N in monocultures of both epigées and endogées. It was apparent that nitrogen transformations and amounts available in soil water are dependant on the composition of the earthworm community.     

不同形态氮素添加下三江源区高寒草甸草场氮素分配与利用

【目的】研究三江源区高寒草甸牧草对不同形态氮素的吸收利用和残留氮素在土壤中的去向及分配,以期为制定三江源区高寒草甸草场养分科学添加方案提供理论依据。【方法】于2020年6月至2021年9月,在青海省称多县高寒草甸试验站开展¹⁵N田间微区示踪试验,试验设置3个不同氮素形态处理,分别为(¹⁵NH₄)₂SO₄、Ca(¹⁵NO₃)₂、CO(¹⁵NH₂)₂,各处理的氮素施用量均为N 300 kg/hm²。分析了施肥当年和次年不同形态氮素在高寒草甸牧草地上部、地下部中的含量,及在0—15、15—30 cm土层土壤中的去向及分配。【结果】1)在施肥当年,与Ca(¹⁵NO₃)₂、(¹⁵NH₄)₂SO_4<...     

Seasonal soil nitrogen dynamics in rice‐wheat cropping systems of Nepal

The rice‐wheat annual double cropping system occupies some 0.5 million ha in the Himalayan foothills of Nepal. Alternating soil drying and wetting cycles characterize the 6–10 weeks long dry‐to‐wet season transition period (DWT) after wheat harvesting and before wetland rice transplanting. Mineral fertilizer use in the predominant smallholder agriculture is low and crops rely largely on native soil N for their nutrition. Changes in soil aeration status during DWT are likely to stimulate soil N losses. The effect of management options that avoid the nitrate build‐up in soils during DWT by N immobilization in plant or microbial biomass was studied under controlled conditions in a greenhouse (2001/2002) and validated under field conditions in Nepal in 2002. In potted soil in the greenhouse, the gradual increase in soil moisture resulted in a nitrate N peak of 20 mg (kg soil)^–1 that rapidly declined as soil moisture levels exceeded 40 % water‐filled pore space (equiv. 75 % field capacity). Similarly, the maximum soil nitrate build‐up of 40 kg N ha^–1 under field conditions was followed by its near complete disappearance with soil moisture levels exceeding 46 % water‐filled pore space at the onset of the monsoon rains. Incorporation of wheat straw and/or N uptake by green manure crops reduced nitrate accumulation in the soil to < 5 mg N kg^–1 in pots and < 30 kg N ha^–1 in the field (temporary N immobilization), thus reducing the risk for N losses to occur. This “saved” N benefited the subsequent crop of lowland rice with increases in N accumulation from 130 mg pot^–1 (bare soil) to 185 mg pot^–1 (green manure plus wheat straw) and corresponding grain yield increases from 1.7 Mg ha^–1 to 3.6 Mg ha^–1 in the field. While benefits from improved soil N management on lowland rice are obvious, possible carry‐over effects on wheat and the feasibility of proposed options at the farm level require further studies.     

Seasonal nitrogen dynamics in lowland rice cropping systems in inland valleys of northern Ghana

Farmers in the inland valleys of northern Ghana are challenged with nitrogen (N) deficiency as a major production constraint of rainfed lowland rice (Oryza sativa L.). With extremely low use of external inputs, there is a need to efficiently use the systems' internal resources such as native soil N. Largest soil nitrate‐N losses are expected to occur during the transition between the dry and wet season (DWT) when the soil aeration status changes from aerobic to anaerobic conditions. Technical options avoiding the build‐up of nitrate are expected to reduce N losses and may thus enhance the yield of rice. A field study in the moist savanna zone of Ghana assessed the in situ mineralization of native soil N, the contribution of nitrate to the valley bottom by sub‐surface flow from adjacent slopes, and the effects of crop and land management options during DWT on seasonal soil N_min dynamics and the yield of lowland rice. Large amounts of nitrate accumulated during DWT with a peak of 58 kg ha⁻¹ in lowland soils, of which 32 kg ha⁻¹ were contributed from the adjacent upland slope. Most of this nitrate disappeared at the onset of the wet season, possibly by leaching and denitrification upon soil flooding. While the incorporation of rice straw (temporary immobilization of soil N in the microbial biomass) had little effect on soil N conservation, growing a crop during DWT conserved 22–27 kg of soil N ha⁻¹ in the biomass and Crotalaria juncea supplied an additional 43 kg N ha⁻¹ from biological N₂ fixation. Farmers' practice of bare fallow during DWT resulted in the lowest rice grain yield that increased from 1.3 (2.2) to 3.9 t ha⁻¹ in case of the transition‐season legume. Growing a pre‐rice legume during DWT appears a promising option to manage N and increase lowland rice yields in the inland valleys of northern Ghana.