Spatial and temporal processes affecting nitrogen availability at the landscape scale

Nitrogen dynamics in soils are affected by spatial and temporal processes. Drainage class is generally regarded to be the most significant source of variability for N in temperate humid climates. A 5-year study was conducted including four rates of N fertilizer and three drainage classes within a 15 ha maize (Zea mays L.) field. Variance component analysis showed that N response was minimally affected by drainage class, but showed strong yearly variations, apparently related to early-season precipitation. Annual field-averaged economic optimum N rates had a range of 65 kg ha⁻¹ with lower rates being associated with years with low early-season precipitation. A calibrated LEACHMN model and site-specific weather data were used to evaluate the effects of early-season weather conditions on N rate and availability. During wet years, soil N availability was reduced by approximately 35–50 kg ha⁻¹ compared to dry years, largely independent of drainage class. For well-drained soils, most losses were attributed to leaching (especially in years with wet early-season), while poorly drained soils mainly experienced denitrification. It is concluded that limited benefits may be gained from spatially variable N applications within fields based on drainage class or soil type, but considerable economic and environmental gains are possible from yearly adjustment of supplemental N rates based on model simulations of N dynamics using information on early-season weather conditions.     

Some tools for parsimonious modelling and interpretation of within-field variation of soil and crop systems

Parsimony is a guiding principle in scientific investigation which goes back to the medieval schools. It is proposed in this paper that, in different guises, the principle of parsimony (using no more complex a model or representation of reality than absolutely necessary) is essential for agricultural research at the scale of the field or larger regions. Two examples of the principle are given, illustrated with examples from research on precision agriculture:

1. The spatio-temporal variability of a sequence of yield maps is considerable, and often defeats simple approaches to interpretation and analysis. Reducing the variability to a small number of basic temporal patterns with spatial expression allows useful information to be extracted. The normalised fuzzy partition entropy is a parsimonious criterion appropriate for identifying the number of distinct patterns in the data. It is shown in a case study how this can aid interpretation of a complex data set.

2. Models of the joint effect of different factors on crop yield can take various forms. A simple assumption is that the different factors are additive, the most complex models describe interactions. Justus von Liebig’s ‘Law of the minimum’ is a model of little complexity and may often be a parsimonious and powerful tool for modelling crop responses. A comparison of these models on some crop response data, using the Akaike information criterion to measure the parsimony of the different models, is given.

     

Factors contributing to temporal stability in spatial patterns of water content in the tillage zone

The rates of many biological processes vary across an agricultural landscape in response to the spatial patterns of water content in the tillage zone. Although, water content varies temporally through the growing season, the combined effects of soil properties, landscape attributes, tillage or position relative to the crop row on the temporal variation in the spatial pattern in soil water content are not well understood. We measured the soil water content (0–0.20 m) regularly through three growing seasons at 32 positions along each of two transects in a side-by-side comparison of corn under conventional tillage (plowing and secondary tillage) and no till in order to identify factors with the strongest influence on the spatial patterns in water content. The tillage comparison traversed a landscape in which the clay content (cl) varied from 5.8 to 37.4% and the organic carbon content (OC) varied from 0.9 to 3.9%. The spatial pattern in water contents during wetting and drying events were temporally stable, as reflected in R²>0.7 of correlation analysis of water contents on successive measurement dates. Multiple regression analyses indicated that the water contents, averaged over all measurement dates, were positively correlated with cl and ln(OC) and were smaller in the row than the inter-row position. The reduction in water content due to conventional tillage was diminished with increasing OC. However, application of multiple regression analyses to each set of water contents measured on a given day for each year indicated that the impact of soil properties, tillage and position relative to the row varied within and among seasons.     

Rotation and tillage effects on available soil water for winter wheat in a semi-arid environment

The recent adoption of conservation farming systems in the semi-arid Canadian prairies opens up the possibility of replacing the traditional fallow period with non-cereal crops (oilseeds, legumes). However, information on changes to soil water regimes by inclusion of these crops, especially in combination with zero tillage, is sparse. A study was initiated in 1984 on a sandy clay loam soil at Lethbridge, Alberta, to investigate the performance of winter wheat (Triticum aestivum L.) under conventional, minimum and zero tillage in monoculture and in 2-year rotations with fallow, canola (Brassica campestris L.) or lentils (Lens culinaris Medic.)/flax (Linum usitatissimum L.). Conventional tillage in the Lethbridge region is shallow cultivation (10 cm) with a wide-blade (sweep) cultivator. Continuous cropping greatly depleted soil water reserves, resulting in some crop failures. Averaged over 10 years, available water for establishment of winter wheat in fall was least after canola (45 mm), followed by continuous winter wheat (59 mm), lentils/flax (74 mm) and fallow (137 mm). In this semi-arid region, the effect of rotation on soil water was much greater than that of tillage. Zero tillage had relatively little impact on available water to 1.5 m depth. However, once the experiment had been established for 6–7 years, available water in the 0–15 cm depth under winter wheat in spring was greatest under zero tillage. Precipitation storage efficiency during the fallow year was generally unaffected by tillage system.     

Cotton lint yield variability in a heterogeneous soil at a landscape scale

Landscape variability associated with topographic features affects the spatial pattern of soil water and N redistribution, and thus N uptake and crop yield. A landscape-scale study was conducted in a center pivot irrigated field on the southern High Plains of Texas in 1999 to assess soil water, soil NO₃-N, cotton (Gossypium hirsutum L.) lint yield, and N uptake variability in the landscape, and to determine the spatial correlation between these landscape variables using a state-space approach. The treatments were irrigation at 50 and 75% cotton potential evapotranspiration (ET). Neutron access tubes were placed at a 15-m interval along a 710 m (50% ET) and 820 m (75% ET) transect across the field. Soil NO₃-N in early spring was autocorrelated at a distance varying between 60 and 80 m. Measured soil volumetric water content (WC), total N uptake, and lint yield were generally higher on lower landscape positions. Cotton lint yield was significantly correlated to soil WC (r=0.76), soil NO₃-N (r=0.35), and site elevation (r=−0.54). Differences of site elevation between local neighboring points explained the soil water, NO₃-N and lint yield variability at the micro-scale level in the landscape. Soil WC, cotton lint yield, N uptake, and clay content were crosscorrelated with site elevation across a lag distance of ±30–40 m. The state-space analysis showed that cotton lint yield was positively weighted on soil WC availability and negatively weighted on site elevation. Cotton lint yield state-space models give insights on the association of soil physical and chemical properties, lint yield, and landscape processes, and have the potential to improve water and N management at the landscape-scale.     

A comparison of two methods to predict the landscape-scale variation of crop yield

Landscape-scale variation is a source of information that increasingly is being taken into consideration in agricultural and environmental studies. Models that encompass and interpret this variation in fields and across contrasting management practices have the potential to improve the landscape management of agroecosystems. Our objective was to compare the results of two approaches, analysis of covariance (ANCOVA) and state-space modeling, to determine the factors affecting grain yield in three crop rotations [pea (Pisum sativum L.)–wheat (Triticum aestivum L.)–barley (Hordeum vulgare L.), canola (Brassica napus L.)–wheat–barley, and wheat–wheat–barley] at two sites in Saskatchewan, Canada. Crop rotations were established in adjacent 30 m×80 m plots arranged in a randomized complete block with five replicates. Variables that were expected to affect resource availability and pest infestations in wheat (second rotation phase) or barley (third rotation phase) were measured. Each sampling point was classified according to landscape position as either a shoulder or footslope. Landscape position was considered as a cross-classified treatment along with crop rotation, and analyzed using ANCOVA procedures. State-space modeling was conducted on a single transect connecting sampling points across all of the rotations and replicates at each site. ANCOVA frequently indicated that grain yield and other measured variables differed between landscape position across all rotations, or in a specific crop rotation. For example, grain yield, soil water content, soil N availability during the growing season, and the incidence of common root rot were higher in the footslopes than the shoulders in all of the crop rotations at one of the sites. However, the landscape position effect for grain yield was never fully explained by the landscape position effects detected for the other variables (e.g., higher soil water content in the footslopes did not correspond with higher grain yields in footslope positions at both sites). State-space modeling indicated that most of the measured variables contributed to the prediction of landscape-scale variation for grain yield in the pea–wheat rotation; whereas only leaf and root disease incidences explained landscape-scale variation in the wheat–wheat rotation. The selective omission of data indicated that state-space modeling was accounting for the varied importance of the predictors across the landscape; i.e., localized response functions. The major reason that ANCOVA did not explain landscape-scale variation of grain yield across the different crop rotations may be because it was unable to account for highly localized variation. However, there is evidence from other studies that the ANCOVA approach is appropriate when the response functions explaining grain yield do not vary significantly within the study area. This situation is most likely to occur in studies with smaller experimental areas. Future research conducted at scales reflecting ‘real world’ field conditions (i.e., study units representative of producer’s fields) should consider the use of state-space modeling or alternative statistical techniques that are designed to address and predict the complex and dynamic nature of landscape-scale processes.     

Effect of tillage and contour diking on sorghum establishment and yield on sandy clay soil in Sudan

Sandy clay soils locally known as gardud are less arable despite their better production potential than other infertile and exhausted sand soil in western Sudan due to surface physical constraints such as low infiltration and workability. Field experiments were, therefore, conducted on these soils to determine effect of two relatively new tillage systems to the area and two widely practised systems on soil bulk density, porosity, water storage and sorghum (Sorghum bicolor L.) plant growth and yield. Contour diking at specified spacings was superimposed on the four tillage treatments for impounding surface runoff. These systems were (1) chisel ploughing, (2) broadbed and furrow, (3) ridge-furrow, and (4) traditional no-till (control). Distances of the contour diking were at four levels: zero, 5, 10 and 20 m. Soil bulk density, porosity and water content were significantly affected by the different tillage systems and contour diking. Combined analysis of three successful seasons out of five seasons showed that plant establishment and growth were highly significant. Yield component data (percent of plants reaching physiological maturity, grain and straw) indicated significant differences among treatments tested for the three seasons of the experiment. The 3-year mean grain yield for chisel plowing pooled over the four contour diking distances produced 1448 kg/ha which was greater by 72%, 107% and 384% than broadbed and furrow, ridge-furrow and no-till, respectively. Similarly, the 10-m contour-diking-distance mean grain production (917 kg/ha) over the tillage systems was 16%, 5% and 30% for more than zero, 5-m and 20-m distances, respectively. These results indicate that chisel and 10-m contour diking are appropriate and suitable for gently sloping compacted sandy clay soils of western Sudan, and superior to widely practised ridge-furrow and no-till surface configuration.     

加拿大西部起伏地貌的地形指数与产量变异性

Understanding the relationships between topographic indices and crop yield variability is important for soil management and crop production in rolling landscape. Two agricultural fields at Alvena and Hepburn, Saskatchewan, Canada were selected to examine how topographic indices were related to wheat yield under two topographic and weather conditions in the Canadian prairies. The landscapes of the two sites are classified as hummocky and the dominant soil type is an Aridic Ustoll. The relationships among yield, topography, soil, and weather were analyzed using wheat (Triticum aestivumL.) grain yield from Alvena in 2001 (dry year) and 2004 (wet year) and from Hepburn in 1998 (dry year). Topographic/soil indices included relative elevation, wetness index, upslope length, curvature, soil organic matter, and soil moisture storage before seeding. The results indicated that, in the dry years, the correlation coefficients between upslope length and grain yield were 0.79 for the typical rolling landscape (Alvena) in 2001 and 0.73 for shallow gentle rolling landscape (Hepburn) in 1998. In the wet year (2004), the relationships between yield and topographic/soil attributes were not as strong as in dry years. Therefore, upslope length was the best yield indicator for the two landscapes in dry years, whereas no topographic indices were highly correlated to crop yield in wet years. Those topographic indices seemed useful in identifying the yield variability and delineating the proper management zone.     

Management practice to optimize wheat yield and water use in changing climate

ABSTRACT

Field experiments for six seasons (2008–2013) for present time slice (PTS; 2008–2013) and simulation studies for mid-century (MC; 2021–2050) were carried out to assess different planting dates, varieties and irrigation schedules in addressing the impact of climate change on grain yield and water use efficiency (WUE) in bread wheat (Triticum aestivum L.). During field experimentation, WUE (averaged over other treatment) was unaffected by planting date; however, it was 6% higher in late variety (V₁) than early variety (V₂). Simulation study suggested that in MC, increase in maximum and minimum temperatures compared to PTS would reduce wheat yield by 17–27%. In MC, WUE would be reduced by 14.8% due to shortening of crop duration (1–11 days). The reduction in WUE could be ascribed to relatively more reduction in yield (22%) than evapotranspiration (ET) (4%). The WUE in MC3 (2041–2050) was relatively more than MC1 (2021–2030) and MC2 (2031–2040) due to more yield and less ET. Delaying planting date of wheat crop by 15–30 days in this region emerged as the best adaptation measure to tackle climate change impact for sustaining yield and having higher WUE in MC.     

Impact of soil compaction upon soil water balance and maize yield estimated by the SIMWASER model

The capability of the soil water balance model SIMWASER to predict the impact of soil compaction upon the yield of maize (Zea mays L.) is tested, using the results of a field experiment on the influence of soil compaction by wheel pressure upon soil structure, water regime and plant growth. The experimental site was located on an Eutric Cambisol with loamy silt soil texture at an elevation of 260 m in the northern, semi-humid sub-alpine zone of Austria. Within the experimental field a 7 m wide strip was compacted by a tractor driven trailer just before planting maize in May 1988. Compression effects due to trailer traffic resulted in distinct differences of physical and mechanical soil parameters in comparison with the uncompressed experimental plots down to a depth of about 30 cm: bulk density and penetration resistance at field capacity were increased from 1.45 to 1.85 g/cm³, and from 0.8 to 1.5 MPa, respectively, while air-filled pore space as well as infiltration rate were appreciable lowered from about 0.08–0.02 cm³/cm³ and from 50 to 0.5 cm per day, respectively. The overall effect was a clear depression of the dry matter grain yield from 7184 kg/ha of the non-compacted plot to 5272 kg/ha in the compacted field strip. The deterministic and functional model SIMWASER simulates the water balance and the crop yield for any number of crop rotations and years, provided that daily weather records (air temperature, humidity of air, global radiation, wind and precipitation) are available. Crop growth and soil water regime are coupled together by the physiological processes of transpiration and assimilation, which take place at the same time through the stomata of the plant leaves and are both reacting in the same direction to changes in the soil water availability within the rooting zone. The water availability during rainless seasons depends on the hydraulic properties of the soil profile within the rooting depth and on rooting density. Rooting depth and density are affected by both the type of the crop and the penetration resistance of the soil, which depends on the soil moisture status and may be strongly increased by soil compaction. The model SIMWASER was able to simulate these effects as shown by the calculated grain yields, which amounted in the non-compacted plot to 7512 and to 5558 kg dry matter/ha in the compacted plot.     

水肥耦合对温室盆栽黄瓜产量与水分利用效率的影响

采用一种负水头供水控水盆栽装置进行水分精确控制,通过设定装置的不同供水吸力控制不同的土壤含水量,研究不同水肥供应对温室黄瓜生长发育的影响及水肥间的耦合效应。试验分别设3种供水吸力:3 kPa(W1)、5kPa(W2)和7 kPa(W3);3种施肥水平:N 600、P 300、K 300 kg/hm2(F1),N 900、P 450、K 450 kg/hm2(F2)和N 1200、P600、K 600 kg/hm2(F3)。试验结果表明,W1、W2、W3处理控制的土壤含水量分别为26.25%、20.81%、15.35%。在F1、F2和F3的处理下,黄瓜的生长速率和产量随着土壤水分的增加而增加,表现为W1W2W3;在W1与W2处理下,施肥水平越高,黄瓜生长速率、叶片光合速率、干物质积累量以及产量与水分利用效率越高,表现为F3F2F1;且增加施肥对提高水分利用效率并不以增加植株耗水量为代价。而在W3处理下,植株干物质积累量、叶片光合速率和产量的高低顺序为F2F1F3,F3过高的施肥量抑制了植株的生长。试验结果还表明,水肥互作效应对黄瓜产量与水分利用效率有显著的影响。     

Exploring optimal soil mulching to enhance maize yield and water use efficiency in dryland areas in China

Water and nutrient availability significantly limits global crop production, especially for dryland agriculture in arid and semi-arid regions. To explore the optimal soil mulching options for the Loess Plateau in China, a 3-year field study was conducted to investigate the effects of various soil mulching practices on soil temperature and the water use and grain yield of spring maize. The treatments included traditional flat farming (CK), narrow plastic film mulch (NM), wide plastic film mulch (WM) and narrow plastic film mulch?+?maize straw mulch between rows (MS). The results showed that MS treatment increased consistently soil temperature during the initial stages of maize growth, and more importantly, it reduced diurnal temperature variation. MS also increased in soil water storage by 10.1%, leading to the highest water use efficiency (WUE?=?30.9?kg?ha^?1?mm^?1) over CK on 3 year average. MS significantly increased maize yield and net income of farmers by up to 20%, compared to CK. In conclusion, optimisation of soil mulching strategies significantly enhanced crop yield and water productivity in dryland agriculture in China. Our study provides important guidance for exploring better soil management practice for dryland agriculture in the other regions of the world.     

Correction of cone index for soil water content differences in a coastal plain soil

Soil penetration resistance (cone index) varies with water content. The field variation of water content could mask treatment differences. The correction of cone index data to a single water content would help prevent this. We used equations from TableCurve^™ software and from the literature to correct cone indices for differences in soil water contents. Data were taken from two field experiments where cotton (Gossypium hirsutum L.) was grown using conventional and conservation tillage without irrigation, and beans (Phaseolus vulgaris L.) were grown using conventional tillage with microirrigation. Boundary conditions based on hard, dry and soft, wet soils were imposed on the equations. Equations fit the data with coefficients of determination ranging from 0.55 to 0.92 and error mean squares from 1.37 to 6.35. After correction, cone index dependence on water content was reduced. A single-equation correction did not always fit the data across all treatments. Separate corrections, based on treatment, might be required. When corrections required multiple equations, differences may be real or may be a manifestation of the correction differences. In this case, the correction may not be feasible (unless some future work can coordinate different equations and assure a uniform correction).     

Long-term effects of a single compaction by heavy field traffic on yield and nitrogen uptake of annual crops

The long-term effects of soil compaction by heavy traffic on crop growth were examined in field experiments on a heavy clay (Vertic Cambisol) and an organic soil (Mollic Gleysol). There were three treatments: one pass and four repeated passes with a tandem axle load of 16 Mg, with wheel tracks completely covering the plot area, and a control without experimental traffic. Both loadings compacted the soils to a depth of 0.4–0.5 m. For 9 years after the loading, spring cereals (oats, wheat and barley) were the main crops grown. Yield, moisture content at harvest, thousand-kernel and bulk weight and nitrogen uptake of crops were determined each year. Although lodging of crops in the control and sometimes also in the treatment with one pass complicated the interpretation of results, especially for the organic soil, compaction clearly did affect crop production. For several years after the loading, it decreased yields and nitrogen uptake of crops and lowered seed moisture contents at harvest. Effects of the compaction were especially marked on the clay soil in the first 3 years and the rainy sixth year. Taken as a mean of the first 8 years, compaction of the clay soil with four passes reduced the yields by 4% and nitrogen uptake of annual crops by 9%. Compaction of the organic soil with four passes decreased the yield by 1% and nitrogen yield by 4%, as a mean of the first 5 and the last 3 years. The bulk weight or the thousand-kernel weight of yields was not notably affected by the compaction.     

温室滴灌黄瓜产量和水分利用效率对水分胁迫的响应

为确定滴灌条件下温室黄瓜的适宜灌水方案,该文基于20 cm标准蒸发皿的累计水面蒸发量设计不同灌水处理,研究了滴灌条件下不同灌水处理(充分灌水T1,轻度水分亏缺T2,中度水分亏缺T3)对不同种植季节温室黄瓜生理特性、耗水量(Evapotranspiration,ET_c)、产量及水分利用效率(Water Use Efficiency,WUE)的影响,且于2017年8—12月(秋冬季)和2018年3—7月(春夏季)分别对不同灌水处理下土壤水分状况、作物生理指标、耗水量、产量和WUE等指标进行了系统的田间试验观测及分析。研究结果表明,随着灌水量的减小,温室黄瓜产量和WUE均呈降低趋势,不同程度的水分亏缺对黄瓜不同生育期ET_c有一定的抑制作用,在黄瓜生长任一阶段发生水分亏缺均会降低黄瓜植株的茎流速率、光合速率及气孔导度,进而可能影响黄瓜干物质的运转与积累,其中在作物生长中期,温室黄瓜茎流速率及产量对水分亏缺响应最为显著。黄瓜平均单果质量、果茎、果长和单株坐果数均随灌水量的降低而减小,黄瓜果实畸形比例随不同生长阶段水分亏缺的增大而增大。不同种植季节温室黄...     

Growth,yield and water use efficiency of chickpea (Cicer arietinum): response to biochar and phosphorus fertilizer application

Field experiments were conducted during summer (2013/2014) and winter (2014) in two different soil types to evaluate the effect of biochar and P fertilizer application on growth, yield, and water use efficiency of chickpea. Soil types include Rhodic Ferralsols (clay) in Thohoyandou and Leptic Cambisols (loamy sand) in Nelspruit, South Africa. Treatments consisted of a factorial combination of four biochar levels (0, 5, 10 and 20 t ha^?1) and two phosphorus fertilizer levels (0 and 90 kg ha^?1) arranged in a randomized complete block design and replicated three times. Biochar application at 5 t ha^?1 significantly increased biomass, grain yield and water use efficiency of biomass production (WUE_b) in the clay soil compared to 10 and 20 t ha^?1. However, the increase was attributed to the addition of P fertilizer. Biochar application had no effect on yield components in the loamy sand soil, but P fertilizer addition increased number of seeds/pod in the loamy sand soil and number of pods/plant in the clay soil. Biochar and P fertilizer application on growth and yield of chickpea varied in soil types and seasons, as the effect was more prominent in the clay soil than the loamy sand soil during the summer sowing.     

黑土农田水肥条件对作物产量及水分利用效率的影响

长期定位试验研究黑土农田不同水肥作物产量和水分利用效率的变化结果表明,大豆对水分条件变化反映较敏感,而玉米对养分条件变化反映较敏感,适宜水分条件下3种作物随施肥水平的提高,其产量与水分利用效率相应增加;有机 无机肥配施处理3种作物随水分含量的增加,其产量与水分利用效率相应提高     

RZWQM2模型模拟地膜覆盖时间对南疆棉田水分利用效率及产量的影响

为准确预知地膜覆盖与作物的匹配情况,该研究针对南疆棉田,在大田试验基础上应用根区水质模型(Root Zone Water Quality Model-version 2,RZWQM2),对2016-2017年籽棉产量及水分利用效率(Water Use Efficiency,WUE)等数据进行模型参数率定与验证,利用验证...     

中国西北旱地小麦和玉米提高产量与水分利用率的氮肥管理效果

A field experiment with four treatments and four replicates in a randomized complete block design was conducted at the Changwu Experimental Station in Changwu County, Shaanxi Province, of Northwest China from 1998 to 2002. The local cropping sequence of wheat, wheat-beans, maize, and wheat over the 4-year period was adopted. A micro-plot study using ^15N-lahelled fertilizer was carried out to determine the fate of applied N fertilizer in the first year. When N fertilizer was applied wheat （years 1, 2 and 4） and maize （year 3） grain yield increased significantly （P 〈 0.05） （〉 30%）, with no significant yield differences in normal rainfall years （Years 1, 2 and 3） for N application at the commonly application rate and at 2/3 of this rate. Grain yield of wheat varied greatly between years, mainly due to variation in annual rainfall. Results of ^15N studies on wheat showed that plants recovered 36.6%-38.4% of the N applied, the N remained in soll （0-40 cm） ranged from 29.2% to 33.6%, and unaccounted-for N was 29.5%-34.2%. The following crop （wheat） recovered 2.1%- 2.8% of the residual N from N applied to the previous wheat crop with recovery generally decreasing in the subsequent three crops （beans, maize and wheat）.     

旱地地膜和秸秆双元覆盖栽培下小麦产量与水分效应

通过覆盖措施提高水分利用率对旱地农业生产具有重要意义。该文采用田间对比试验,研究了旱地冬小麦几种覆盖栽培下产量、水分利用率、土壤水分剖面和硝态氮的分布的差异。结果表明,地膜和秸秆双元覆盖模式下小麦籽粒产量比对照增产12.11%～17.65%,水分利用效率(WUE)比常规栽培提高7.2%～30.8%,土壤0～20 cm土层的含水量提高到12%～16%,硝态氮含量提高到4.70～10.17 mg/kg。地膜和秸秆双元覆盖模式能够显著的提高作物产量和水分利用率,并显著增加耕层土壤中水分含量和硝态氮含量,减轻了土壤剖面硝态氮的淋溶累积。