Potassium Leaching as Affected by Soil Texture and Residual Fertilization in Tropical Soils

Potassium (K) leaching is affected by soil texture and available K, among other factors. In this experiment, effects of soil texture and K availability on K distribution were studied in the presence of roots, with no excess water. Soils from two 6-year field experiments on a sandy clay loam and a clay soil fertilized yearly with 0, 60, 120, and 180 kg ha^?1 of K₂O were accommodated in pots that received 90 kg ha^?1 of K₂O. Soybean was grown up to its full bloom (R2). Under field conditions, K leaching below the arable layer increased with K rates, but the effect was less noticeable in the clay soil. Potassium leaching in a sandy clay loam soil was related to soil K contents from prior fertilizations. With no excess water, in the presence of soybean roots, K distribution in the profile was significant in the lighter textured soil but was not apparent on the heavier textured soil.     

Fate of N-labelled nitrate in a wet summer under different residue management regimes in young hoop pine plantations

A field study was conducted to investigate the fate of ¹⁵N-labelled nitrate applied at 20 kg N ha⁻¹ in a wet summer to microplots installed in areas under different residue management regimes in second-rotation hoop pine (Araucaria cunninghamii) plantations aged 1–3 years in south-east Queensland, Australia. PVC microplots of 235 mm diameter and 300 mm long were driven into 250 mm soil. There were three replications of each of eight treatments. These were areas just under and between 1-year-old windrows (ca. 2–3 m in width) of harvesting residues spaced 15 m apart, and with and without incorporated foliage residues (20 t DM ha⁻¹); the areas just under and between 2- or 3-year-old windrows spaced 10 m apart. Only 7–29% of the added ¹⁵N was recovered from the top 750 mm of the soil profile with the leaching loss estimated to be 70–86% over the 34-day period. The ¹⁵N loss via denitrification was 3.7–6.3% by directly measuring the ¹⁵N gases emitted. The microplots with the incorporated residues at the 1-year-old site had the highest ¹⁵N loss (6.3%) as compared with the other treatments. The ¹⁵N mass balance method together with the use of bromide (Br) tracer applied at 100 kg Br ha⁻¹ failed to obtain a reliable estimate of the denitrification loss. The microplots at the 1-year-old site had higher ¹⁵N immobilisation rate (7.5–24.7%) compared with those at 2- and 3-year-old sites (2.1–3.6%). Incorporating the residues resulted in an increase in ¹⁵N immobilisation rate (24.5–24.7%) compared with the control without the incorporated residues (8.4–14.3%). These findings suggest that climatic conditions played important roles in controlling the ¹⁵N transformations in the wet summer season and that the residue management regimes could also significantly influence the ¹⁵N transformations. Most of the ¹⁵N loss occurred through leaching, but a considerable amount of the ¹⁵N was lost through denitrification. Bromide proved to be an unsuitable tracer for monitoring the ¹⁵N leaching and movement under the wet summer conditions.     

Fungal/bacterial ratios in grasslands with contrasting nitrogen management

It is frequently hypothesised that high soil fungal/bacterial ratios are indicative for more sustainable agricultural systems. Increased F/B ratios have been reported in extensively managed grasslands. To determine the shifts in fungal/bacterial biomass ratio as influenced by grassland management and to find relations with nitrogen leaching potential, we sampled a two-year-old field experiment at an organic experimental farm in the eastern part of The Netherlands. The effect of crop (grass and grass-clover), N application rate (0, 40, 80, ) and manure type (no manure, farm yard manure and slurry) on the F/B ratio within three growing seasons was tested, as well as relations with soil and crop characteristics, nitrate leaching and partial N balance. Biomass of fungi and bacteria was calculated after direct counts using epifluorescence microscopy. Fungal and bacterial biomass and the F/B ratio were higher in grass than in grass-clover. The F/B ratio decreased with increasing N application rate and multiple regression analysis revealed a negative relationship with pH. Bacterial activity (measured as incorporation of [³H]thymidine and [¹⁴C]leucine into bacterial DNA and proteins) showed the exact opposite: an increase with N application rate and pH. Leaching increased with N application rate and was higher in grass-clover than in grass. Partial N balance was more positive at a higher N application rate and showed an inverse relationship with fungal biomass and F/B ratio. We conclude that the fungal/bacterial biomass ratio quickly responded to changes in management. Grasslands with higher N input showed lower F/B ratios. Grass-clover had a smaller fungal biomass and higher N leaching than grass. In general, a higher fungal biomass indicated a lower nitrogen leaching and a more negative partial N balance (or smaller N surplus), but more observations are needed to confirm the relationship between F/B ratio and sustainability.     

Soil salinity in Aceh after the December 2004 Indian Ocean tsunami

The 2004 Indian Ocean tsunami inundated about 37,500 ha of coastal farmland in Aceh, and crops planted after the tsunami were severely affected by soil salinity. This paper describes the changes of soil salinity over time on tsunami affected farms and the implications for resuming crop production after natural disasters.Soil salinity and salt leaching processes were assessed across the tsunami affected region by measuring soil apparent electrical conductivity (ECa) using an electromagnetic induction soil conductivity instrument (EM38) combined with limited soil analysis. The ECa was measured 5 times between August 2005 and December 2007 in both the vertical (EMv) and horizontal (EMh) dipole orientations at 23 sites across Aceh. The level of salinity and direction of salt movement were assessed by comparing changes in mean profile ECa and relative changes in EMv and EMh.Eight months after the tsunami the average soil salinity in the 0-1.2 m soil depth varied from ECe 22.6 to 1.6 dS m⁻¹ across sites in the affected region and three years after the tsunami it varied from 13.0 to 1.4 dS m⁻¹. Soil salinity tended to be higher in rice paddy areas that trapped saline tsunami sediments and held seawater for longer periods. Leaching of salts occurred slowly by both vertical displacement and horizontal movement in surface waters. Hence, soil salinity persisted at a level which could reduce crop production for several years after the 2004 tsunami. High soil salinity persisted three years after the tsunami even though there had been more than 3000-7000 mm of accumulated rainfall to leach salts. The slow leaching is likely to have been due to the loss of functional drainage systems and general low relief of the affected areas.Monitoring of soil salinity with EM38 assisted local agricultural extension agencies to identify sites that were too saline for crops and determine when they were suitable for cropping again. The methodology used in this study could be used after similar disasters where coastal agriculture areas become inundated by seawater from storm surges or future tsunamis.     

灌溉方式对保护地土壤磷素淋失风险的影响

自连续12年以相同试验方案、不同灌溉方式进行灌溉试验的保护地采集土壤样品,对不同灌溉处理土壤磷素淋失风险进行评价,并对影响土壤磷素淋失临界值大小的因素进行了探讨。灌溉处理设滴灌、沟灌和渗灌三种灌溉方式,采样深度为0~80 cm。结果表明,0~20 cm沟灌、渗灌和滴灌处理土壤的磷素淋失临界值Olsen-P含量分别为59.44 mg kg-1、65.39 mg kg-1和68.57 mg kg-1;而20~40 cm层次的土壤淋失值分别为60.61 mg kg-1,66.8 mg kg-1和70.58 mg kg-1;40~80cm土层则无临界值存在。影响土壤磷素淋失临界值Olsen-P含量的主要因素有土壤pH和有机质、活性Fe、活性Al、有效磷含量;土壤pH值越大、有机质、活性铁、活性铝和Olsen-P含量越高,磷素淋失临界值越大。对三种灌溉处理表层土壤磷素淋失风险进行综合评价,其风险大小顺序为沟灌、渗灌和滴灌,这提示人们在保护地生产中要充分注意土壤磷素有效性,通过选择合理的灌溉方式、改善施肥技术以加强保护地土壤水肥管理,保证作物生产高效、优质和降低环境风险。     

模拟雨水连续淋洗下土壤化学性状动态变化特征的研究

本文从研究正常降雨对土壤性状的影响出发,进行了模拟雨水对酸性紫色土和石灰性紫色土的连续淋洗试验。试验结果表明在模拟雨水连续淋洗150天的过程中,土壤中钙、镁、钾、钠等盐基离子几乎都显示出一定程度的净淋溶特征,土壤中各盐基离子的动力学变化特征几乎都与抛物线扩散方程吻合。淋洗结束后土壤酸度未发生显著变化,但两种土壤CEC值均有所降低。     

聚丙烯酰胺施用对碱土和非碱土水力传导度的影响

聚丙烯酰胺（PAM）可增加土壤结构的稳定性，但它对土壤水力传导能力的影响尚不清楚。通过室内土柱淋洗试验，研究了PAM施用量和施用方式对不同碱度土壤饱和水力传导度的影响。PAM施用量为0、1／5000、1／2000和1／1000（PAM与于土重之比），施用方式为混合施用和表面覆盖施用，土壤为碱土和非碱土两种。试验结果显示，在未施用PAM的条件下，非碱土水力传导度大于碱土。PAM混合施用显著地减小了土壤的稳定水力传导度，但它对碱土的影响程度大于对非碱土的影响。PAM覆盖施用降低了非碱土的稳定水力传导度；但碱土的稳定水力传导度随PAM施用量的增加先减小后增大。为了维持良好的土壤水力传导性能，应尽可能地减小PAM的施用量，或在非碱土地上采用混合施用而在碱土地上采用地表覆盖的施用方式。     

脱硫石膏淋洗次数对铵态氮迁移特征的影响

基于室内土柱穿透试验,设置4组试验处理:3组施加脱硫石膏,不同次数淋洗处理;1组无脱硫石膏无淋洗对照处理。通过分析NH+4、Ca2+、Na+3种离子相互关系及土壤水力传导度的变化情况,研究脱硫石膏及不同淋洗次数对碱土养分离子铵态氮迁移转化的影响。结果表明:碱土混合脱硫石膏后未经淋洗的情况下会抑制碱土对NH+4的吸附效果,但随着穿透前被淋洗次数的增加,NH+4在盐碱土中穿透曲线的斜率减小,吸附作用增强。从节水控盐、促进土壤肥力吸收、抑制土壤次生盐碱化的因素来看,根据试验结果推测铵肥不适合与脱硫石膏同时施加到碱土土壤中。建议在施肥前,拌施脱硫石膏的碱土宜采用秋浇与春灌相结合的方式,每次淋洗662.7 m3/hm2水量,这种淋洗方式在保证改良效果,促进肥料吸收的基础上,还可以适当减少秋浇灌水量,合理分配灌区有限的淡水资源。     

Conservation tillage and macropore factors that affect water movement and the fate of chemicals

A thorough understanding of how conservation tillage influences water quality is predicated on knowledge of how tillage affects water movement. This paper summarizes the effects of conservation tillage on water movement and quality mainly based on long-term experiments on Luvisols at the North Appalachian Experimental Watershed near Coshocton, OH, USA. Conservation tillage can have a much larger effect on how water moves through the soil than it does on the total amount percolating to groundwater. Soil macroporosity and the proportion of rainfall moving through preferential flow paths often increase with the adoption of conservation tillage and can contribute to a reduction in surface runoff. In some medium- and fine-textured soils most of the water that moves to the subsoil during the growing season (May–October) is probably transmitted by macropores. If a heavy, intense storm occurs shortly after surface application of an agricultural chemical to soils with well-developed macroporosity, the water transmitted to the subsoil by the macropores may contain significant amounts of applied chemical, up to a few per cent, regardless of the affinity of the chemical for the soil. This amount can be reduced by an order of magnitude or more with the passage of time or if light rainstorms precede the first major leaching event. Because of movement into the soil matrix and sorption, solutes normally strongly adsorbed by the soil should only be subject to leaching in macropores in the first few storms after application. Even under extreme conditions, it is unlikely that the amount of additional adsorbed solute transported to groundwater will exceed a few per cent of the application when conservation tillage is used instead of conventional tillage. In the case of non-adsorbed solutes, such as nitrate, movement into the soil matrix will not preclude further leaching. Therefore, when recharge occurs during the dormant season thorough flushing of the soil, whether macropores are present or not, can move the remaining solutes to groundwater. Thus, the net effect of tillage treatment on leaching of non-adsorbed solutes should be minimal.     

稻田氮素淋洗损失研究

采用田间小区试验,连续2年研究了施氮量、分子膜对稻田氮素淋洗损失和氮素平衡的影响,结果表明:稻田以铵态氮淋洗损失最少,90cm处浓度低于0.2mg/L;水稻移栽和烤田后硝态氮浓度达到峰值,以90cm处浓度为0.2～0.9mg/L;整个稻季氮素淋洗量为3.2～5.6kg/hm2,占施氮量的1.4%～2.5%。