摘要
本研究以1992/1993年开始布置的在东北(黑龙江、吉林、辽宁)、华北(河北、山西)和西北(宁夏、青海、新疆)地区的连续施用钾肥和秸秆还田的八个定位试验为基础,研究连续施用钾肥和/或小麦秸秆还田对土壤钾素状况、土壤对钾素固定能力以及作物产量和品质的影响。所选择的定位试验点包括中国北方有代表性种植制度(小麦单作、玉米单作、小麦—玉米轮作)和主要的土壤类型。试验处理包括连续不施用钾肥(NP)、施用钾肥(NPK或NPK1和NPK2)和在NP或NPK基础上增加小麦秸秆还田(St)。从不同处理试验区采取分别采取0-20 cm和20-40cm土层土壤样品,应用不同浸提剂提取土壤不同形态钾素含量,研究不同形态钾素在土壤的分布特征及其植物有效性;通过不同浓度外源钾素的加入研究土壤对外源钾素的固定能力;应用ASI联合浸提剂测定土壤样品中各种养分含量,并对作物品质指标进行测定。主要结果如下:
1.与只施氮磷肥(NP处理)比较,长期施用化学钾肥(NPK处理)或小麦秸秆还田(NP+St处理)显著提高耕层土壤水溶性钾、非特殊吸附钾、非交换性钾、矿物态钾乃至全钾的含量,对特殊吸附钾含量影响不显著(可能由于所研究土壤中对钾素有特殊吸附能力的2:1黏土矿物的楔形位比较稳定,连续13年施用钾肥和秸秆还田影响不大),钾肥和小麦秸秆还田对各种形态钾的影响不同。土壤不同形态钾含量和比例在不同的土壤层次中表现迥异,钾素投入对0-20 cm层土壤钾素状况影响较大,化学钾肥的施用效果更为明显。外源钾的投入使土壤全钾中有效钾所占比例增大,并可增加速效钾中水溶性钾和非特殊吸附钾的比例,相应的降低特殊吸附钾的比例。作物吸钾量与耕层土壤速效钾和缓效钾含量存在显著的正相关关系。相同施钾措施对各形态钾的贡献程度也因所在地区及种植制度的不同而异,与气候条件、矿物类型、作物消耗钾素量密切相关。
2.钾肥施用和秸秆还田可提高作物的吸钾量,除NPK2和NPK+St处理外,各点其余处理的钾素投入量均低于作物收获部分的移走量,造成土壤钾素不同程度的亏缺,以NP处理的亏缺量最大。华北和西北地区的小麦/玉米一年两季作物轮作的钾素移走量较高,年均移走量达到200-400kg K_2O/hm~2,钾素平衡系数低,土壤钾素亏缺相对严重。小麦秸秆还田对于缓解土壤钾素的消耗作用明显,是一项切实可行的补充土壤钾素的措施。与试验开始时土壤速效钾和缓效钾含量比较,各处理均有明显的差异,不补充钾素的处理(NP)或钾素补充不足的处理(NP+St)的土壤速效钾变化幅度在-51.9 mg/kg~40.6 mg/kg,缓效钾含量变化幅度在-158.6 mg/kg~71.0 mg/kg;而补充钾素量较大的处理(NPK2和NPK+St)土壤速效钾变化幅度在14.1 mg/kg~131.4 mg/kg,缓效钾含量变化幅度在-8.6 mg/kg~150.7 mg/kg,但是其增加或减少的幅度各试验点之间差别较大,受钾素投入量、作物吸钾量、土壤钾素耗竭到基本稳定的平衡点时的“最低水平值”、矿物态钾释放速率等因素的影响。
3.土壤平均固钾能力表现出明显的地带性规律,自西向东土壤对钾的固定能力逐渐增强,在钾浓度为1000 mg/L的环境溶液中,取自西北、华北和东北地区试验点NP处理土壤的平均固钾量分别为348.5 mg/kg、442.9 mg/kg、468.5 mg/kg。长期连续施用钾肥或秸秆还田可显著降低土壤对外源钾素的固定量,不同土壤在连续耗竭或补充钾素后土壤固钾能力的差异可作为判断土壤钾素状况的一种依据。土壤固钾量随环境溶液中钾素浓度的增加而增加,但固定率下降。20-40 cm土层土壤的固钾量高于0-20 cm土层。土壤固钾量与原始土的速效钾、缓效钾、有机质含量均呈
负相关,但其相关程度受试验点和土壤层次影响。
4.长期连续施钾或秸秆还田对土壤中其它养分含量也有程度不同的影响。长期施用钾肥或秸秆还田可提高土壤活性有机质含量与有机质总量,秸秆还田效果更为显著;两种措施均可降低土壤铵态氮、有效钙、有效镁含量,可能与补充的钾离子和这些阳离子的竞争吸附有关。施钾或秸秆还田的土壤全磷含量有下降的趋势,但是土壤有效磷含量增加。土壤速效钾含量与活性有机质和有效磷含量呈显著正相关,而与铵态氮、有效钙、有效镁和有效硼含量呈负相关。
5.钾肥和秸秆还田增产效果因定位试验的地点和作物不同而异,受土壤供钾能力影响较大。总体看,施用钾肥、秸秆还田和秸秆还田配合施钾肥均有明显的增产效应。在华北(小麦施钾平均增产5%-8%,玉米平均增产12%-19%)和东北地区(玉米施钾平均增产10%-20%)的增产效果显著高于西北地区(小麦施钾平均增产4%-6%,玉米平均增产3%-6.5%),其中在青海点施钾效应不显著。不同施钾措施在玉米上的增产效果优于小麦,轮作制度下钾素投入显效时间较早,化学钾肥处理显效早于秸秆还田处理。施钾或秸秆还田显著提高作物体内钾素含量和总移走量,植物体内的钾素80%以上存在于作物秸秆中。施用钾肥和小麦秸秆还田还可提高作物籽粒氮磷和中微量元素的含量,但降低茎叶等营养器官的大部分中微量元素含量,表明钾素施用有增加养分向籽粒转移的功能。
6.长期施用化学钾肥和小麦秸秆还田对小麦和玉米蛋白质、脂肪、可溶性糖、氨基酸、小麦湿面筋含量等方面有不同程度的提高作用,但过量施钾(NPK2处理)其效果明显下降。同一品质指标在不同定位点对钾肥和秸秆施用的反应亦不一致。对作物籽粒多数品质性状的改善,化学钾肥的效果一般优于秸秆还田处理,但秸秆还田对改善某些品质指标的作用也不容忽视。施用钾肥和秸秆还田改善作物品质的作用可能与补充的钾素量有直接关系,同时也可能与不同钾素投入措施对养分平衡和土壤性状的改善有关,机制有待深入研究。
By using the eight fixed sites K fertilization field trials established in 1992/1993 in Northeast(Jilin, Liaoning and Heilongjiang), Northcentral(Shanxi and Hebei) and Northwest(Ningxia,Qinghai and Xinjiang) regions, effect of long-term application of K fertilizer and wheat straw returned to soil on soil K status, on K fixation capacity of soils, and on crop yield and quality were investigated. The fixed site field trials were selected to represent the main cropping systems(single cropping of winter wheat, single cropping of spring maize, winter wheat and summer maize double cropping) and main soil types in the north of China. The treatments in the fixed site field trials include check with N and P fertilizer applied only(NP), KC1 application with different rates(NPK, or NPK1 and NPK2), and with wheat straw returned to soil(NP+St, NPK+St). Soil samples were taken from 0-20 cm and 20-40 cm in soil profile from field plots with different treatments. By using different extraction solutions, the amount of soil K in different forms, their distribution in various forms and their availability to plants were evaluated. Potassium fixation capability of the selected soil samples were determined by soil K adsorption study. The main findings were as following:
1. Compared with NP treatment, long-term application of K fertilizer and wheat straw increased content of water-soluble K, non-specifically adsorbed K, non-exchangeable K, mineral K and total K of the soil. But the content of specifically adsorbed K was not significantly affected, indicating that the amount of specifically adsorbed sites for K, the wedge site of the 2:1 clay minerals, was not affected by the different treatments. The effects of K fertilization and straw returned to soil on different forms of soil K and their distribution varied greatly from location to location because of the difference in soil and the environment conditions. Potassium input either with KCl or straw showed greater influence on different form of K in top layer(0-20cm) than that in 20-40 cm layer soil. The effect of KCl application was more significant compared with straw returned to soil treatment. Potassium input from either K fertilization or straw returned to soil significantly increased the proportion of water soluble K and non-specifically adsorbed K, hence increased the proportion of available K in the total soil K. Significant correlation was found between amount of potassium taken up by crops and the available K and slow released K levels in the top layer soil. The effect also differs from site to site because of differences in cropping systems, in climatic conditions, in soil clay mineralogy.
2. Potassium fertilizer application and straw returned to soil increased plant K uptake. The amount of plant K uptake in NP, NP+St, NPK1 treatments in all field trials were greater than the corresponding K input, resulting in a negative balance of soil K budget, which was more severe with NP treatment. Positive balance was only found with NPK2 and NPK-St treatments. The highest removal of soil K was found from the field trial of winter wheat-summer maize double cropping system in Hebei province of Northcentral region, with annual removal of 200-400 kg K_2O/ha. Straw provided part of the K uptake by crops, reduced the depletion of soil K. Compared with available K and slowly released K levels in the original soils collected at the beginning of the fixed site experiments, the available K and slowly released K contents in the treatments without K input(NP treatment) or with low level of input(NP+St) were reduced, while that in high K input treatment(NPK2 or NPK+St) increased. Change degree of soil available K was-51.9 mg/kg~40.6 mg/kg and slowly available K was-158.6 mg/kg~71.0 mg/kg in treatment of NP or NP+St, however, change degree of soil available K was 14.1 mg/kg~131.4 mg/kg and slowly available K was-8.6 mg/kg~150.7 mg/kg in treatment of NPK2 and NPK+St. The influence of K input on soil K status varied remarkably due to the differences in K input, K uptake by crops, and the dynamic process of K transformation in the soils.
3. Significant different K fixation capacity among the selected soil samples were found, with clear trend of K fixation capacity increasing when moving the location from the west toward the east. When the K concentration in the surrounding solution was 1000 mg/L, the average K fixation by soils from NP treatment were 348.5 mg/kg、442.9 mg/kg、468.5 mg/kg for soils from Northwestern, Northcentral and Northeastern regions, respectively. Continuous application of K fertilizer or straw returned to soil significantly reduced K fixation capacity of the soils. Soil K fixation capacity increased with the increase of K concentration in the surrounding solution, but the percentage of the total K in the solution fixed by soils was reduced with increase of K concentration. The K fixation capacity in subsoil layer(20-40 cm) was greater than that in the top layer soil(0-20 cm). Significantly negative correlation was found between soil K fixation capacity and soil available K, slowly released K, and soil organic matter contents, which was influenced by the different soil conditions at various locations.
4. Long-term application of K fertilizer and straw returned to soil also affected content of other nutrients in the soil. Soil active organic matter and total organic matter contents were increased with both K fertilization and straw returned to soil treatment, with straw returned to soil showed more significant influence. Continuous application of K fertilizer and straw returned to soil reduced soil NH_4~+-N, available Ca and Mg contents, which might be attributed to the possible competition of added K~+ with NH_4~+, Ca~(2+) and Mg~(2+) for the same adsorption site in the soil colloid system. The total P content in treatment with K fertilizer application and with straw returned to soil decreased, but the available P increased. The possible reason was that the balanced fertilization in NPK treatment and the addition of organic matter in the straw returned to soil treatment increased total P removal from the soil with increased crop yields, at the same time, mobilized P in the soil-crop system. Significant positive correlation was found between soil available K and soil active organic matter, while negative correlation found with NH_4~+-N, available Ca, available Mg and available B contents.
5. The effect of K fertilizer application and straw returned to soil on crop yield varied with location and cropping system, and influenced by potassium supplying power of soil. In general, application of K fertilizer and straw returned to soil increased crop yield. The average yield increase by K fertilizer application ranged from 5% to 8% for winter wheat and 12% to 19% for summer maize in field trials from Northcentral region, and that for spring maize located in Northeastern region ranged from 10% to 20%. The effect on crop yield in Northwestern region was relatively small, with average yield increase from 4% to 6% for wheat and from 3.0% to 6.5% for maize. No significant yield increase from K application was found in the field trial located in Qinghai province. It appeared that maize was more tentative to K application than wheat, and double cropping system in Northceantral region accelerated soil K depletion and resulted in significant crop yield response to K application soon after the establishment of the field trials. Application of K fertilizer and straw returned to soil treatments significantly increased K content in the plants and the total K removal by crops. More than 80% of K in plants was in the straw or stalk. Application of K fertilizer and straw returned to soil also increased content of N, P and the secondary nutrients and micro-nutrients in kernels, but reduced content of most micro-nutrients in leaves and stalks, indicating that K application might have improved the translocation of nutrients from vegetative parts to kernels.
6. Long-term application of K fertilizer and straw returned to soil increased content of crude protein, lipid, soluble sugar and amino acid of wheat and maize, and wet gluten of wheat. But the effect was reduced when K fertilizer was over used(in NPK2 treatment). The effect varied from location to location, and the K fertilizer treatments generally showed greater effect than straw returned to soil treatment. The positive effect of K fertilization and straw returned to soil on crop product quality might attribute partly to the direct influence of improved K nutrition in the plants, and might also affected by improved balance nutrition and soil condition. The associated mechanism needs to be further investigated.
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