彭健健 徐坚 王晓晓 傅伟军 张申 彭欣怡 徐懿 姜霓雯 方嘉 吴家森
摘 要:【目的】阐明杨梅主产区土壤肥力的空间异质性,探寻影响土壤肥力的主要因素。【方法】依据杨梅(Myrica rubra)产区分布情况和适宜种植区域分布状况,筛选出浙江省杨梅主产区台州市仙居县和临海市的部分区域作为研究区,利用1 km×1 km网格法布点,采集了100个杨梅表层土壤样品(0~20 cm),通过地统计学方法探究土壤肥力的空间异质性。【结果】研究区杨梅土壤平均pH值4.48,有机碳含量(w,后同)18.21 g·kg-1,碱解氮含量115.01 mg·kg-1,有效磷含量15.57 mg·kg-1,速效钾含量239.23 mg·kg-1。杨梅主产区土壤有机碳含量、碱解氮含量、有效磷含量和速效钾含量低值区主要分布在临海东部,高值区则主要分布在仙居西部;pH值高值区主要分布在仙居东部,低值区则主要分布在仙居和临海的西部。土壤综合肥力评价指数的结果表明,仙居西部和临海西部土壤较肥沃,仙居东部和临海东部土壤较贫瘠。Pearson相关性分析结果表明,树龄与土壤有机碳含量呈显著正相关(p<0.05),海拔与土壤碱解氮含量呈极显著正相关(p<0.01),而坡向与速效钾含量呈显著负相关(p<0.05)。结构方程模型结果表明,树龄对有机碳含量有显著正影响(p<0.05),海拔对碱解氮含量有显著正影响(p<0.05),对速效钾含量有显著负影响(p<0.05),坡向对速效钾含量有显著负影响(p<0.05)。【结论】杨梅主产区土壤酸化和肥力失衡问题严重,应该增加仙居东部和临海东部的肥料投入和加强日常管理,适当控制仙居西部和临海西部的肥料施用量,同时还应增加研究区磷肥的施用。为缓解仙居西部和临海西部土壤酸化问题,可利用生石灰、有机肥和生理碱性肥料进行改良。
关键词:杨梅;土壤肥力;结构方程模型;空间分布;影响因素;土壤肥力评价
中图分类号:S667.6 文献标志码:A 文章编号:1009-9980(2023)07-1421-13
Spatial variation of soil fertility and its influencing factors in Myrica rubra region: A case study in Xianju county and Linhai city
PENG Jianjian1, XU Jian2, WANG Xiaoxiao3, FU Weijun1, ZHANG Shen1, PENG Xinyi1, XU Yi1, JIANG Niwen1, FANG Jia1, WU Jiasen1*
(1School of Environmental and Resource Sciences, Zhejiang A & F University, Hangzhou 311300, Zhejiang, China; 2Bureau of Agriculture and Rural Affairs of Xianju, Xianju 317300, Zhejiang, China; 3Taizhou Wanfeng Forestry Co., Ltd., Xianju 317300, Zhejiang, China)
Abstract: 【Objective】 Soil fertility, the key factor of fruit production, is usually influenced by diverse factors in major producing areas. The study aimed to clarify the vital factors closely associated with the spatial heterogeneity of soil fertility by different comprehensive evaluation systems. The specific goals were to alleviate the common problem of soil acidification to provide a theoretical basis for realizing the precise management of soil nutrients in the main districts of Myrica rubra production. 【Methods】 According to the distribution of M. rubra main production area and suitable planting area, representative producing areas of Xianju county and Linhai city, Taizhou city, Zhejiang province were selected as study sites. 100 top soil (0-20 cm) samples were collected based on an intensive grid of 1 km×1 km. The spatial heterogeneity of soil fertility was studied by geostatistical methods. The soil fertility status of each region was evaluated by the integrated soil fertility evaluation index method, and the impact factors of soil fertility were explored by Pearson correlation analysis and Structural Equation Model (SEM). 【Results】 The results of soil physicochemical experiments showed that the average pH, organic carbon (OC), alkali-hydrolyzable nitrogen (AN), available phosphorus (AP) and available potassium (AK) of M. rubra soils in the study area were 4.48, 18.21 g·kg-1, 115.01 mg·kg-1, 15.57 mg·kg-1 and 239.23 mg·kg-1, respectively. Compared with the average value of soil nutrients in the main producing areas of M. rubra in China, the soil pH and AP were lower than the average value (4.95 and 24.65 mg·kg-1) respectively, and the OC and AK were higher than the average value (19.47 g·kg-1 and 62 mg·kg-1). The soil acidification was serious in our study sites, and the content of AP was generally lower than the average levels. The coefficients of variation of the five fertility indicators were 12.02%, 25.98%, 35.51%, 100.35%, and 53.74%, respectively. Except for the strong variation of AP, the others were all moderate, and all fertility indicators were greatly affected by fertilizer application. The nutrient-impoverished study area with low OC, AN, AP and AK were mainly distributed in the east region of Linhai City, and the nutrient-rich area with high OC, AN, AP and AK were distributed in the west region of Xianju County. The district with high pH was located in the East of Xianju county, while the district with low pH was located in the West of Xianju county and Linhai city. The convergent management of M. rubra forest had resulted in significant aggregation in high nutrient zones and acidification problems. The integrated soil fertility evaluation index showed that 91% of the soil samples in M. rubra forest reached medium and high fertility levels, only 9% of the soil samples were at low fertility level. Specifically, the Western part of Xianju county and the Western part of Linhai city were relatively fertile, while the Eastern part of Xianju county and the Eastern part of Linhai city were relatively impoverished. The results of the Pearson correlation analysis showed that the tree age was significantly positively correlated with the soil OC (p<0.05), altitude was significantly and positively correlated with the soil AN (p<0.01), while the correlation between slope and AK was significantly negative (p<0.05). The results of the Structural Equation Model showed that the tree age had a significant positive effect on OC (p<0.05), the altitude had a significant positive effect on AN (p<0.05), a significant negative effect on AK (p<0.05), and slope aspect had a significant negative effect on AK (p<0.05). With the growth of M. rubra trees, the soil pH showed an overall trend of decreasing, indicating that the soil acidification caused by excessive intensive management and fertilization application was becoming increasingly serious. As the aging of M. rubra trees, the standing environment continued to be stable, and the carbon source was continuously replenished by anthropogenic activities. Consequently, the accumulation rate of organic matter was greater than the decomposition rate, and the soil OC showed a rising trend, so that the organic matter kept accumulating. The results of various research methods showed that the anthropogenic fertilization application was the dominant factor affecting the soil nutrients, and elevation, slope direction and tree age were also important factors affecting the soil fertility. 【Conclusion】 The increase of the fertilizer input and reinforce field management in the East M. rubra producing areas of Xianju county and Linhai city is suggested to relieve soil acidification and fertility imbalance. While in the West of Xianju county and Linhai city, it is better to reduce the fertilizer application appropriately. Besides, in our study area, more phosphorus fertilizer is needed because the overall soil phosphorus level is lower than the average value of main M. rubra production areas soils in China. However, in order to alleviate the problem of soil acidification caused by the excessive fertilizer application in the Western region of Xianju county and Linhai city, application of the quicklime, organic fertilizer and physiological alkaline fertilizer is needed for soil improvement.
Key words: Myrica rubra; Soil fertility; Structural equation model; Spatial distribution; Influencing factors; Evaluation for soil fertility
土壤肥力的高低与农林业发展密切相关,在提升农作物的产量和品质中扮演着重要角色[1-2]。土壤的形成受到各种人为和自然因素的影响[3-4],最终发育形成不同类型。由于影响因子的差异,土壤性质在不同地理位置上存在一定的差异[5-6],即空间异质性。前人对土壤性质的影响因素进行了大量研究。姜霓雯等[7]在研究清凉峰自然保护区土壤肥力的空间变异时,揭示了影响土壤肥力指标关键的影响因子是海拔。Wang等[8]在研究影响青藏高原土壤肥力的生物因子(畜牧)和非生物因子(环境因子)时发现,影响土壤肥力关键的因素是过度放牧。同时,探寻土壤肥力的主要影响因子,对掌握土壤肥力变化根源和调控土壤肥力水平都有着重要的科学意义。
杨梅(Myrica rubra)是原产于我国亚热带的珍贵水果,主要集中在浙江、江苏、福建和湖南等地栽培,口感独特、营养价值高,有延缓衰老、抗癌和抗氧化等功效。杨梅中含有丰富的膳食纤维,后者有助于调节肠道运动,同时还有降低血脂和胆固醇的作用[9]。浙江省杨梅的产量、种植面积和产值均位列全国首位,栽培面积在2019年达到8.88万hm2,产量多达61.84万t,产值高达46.38亿元[10]。“杨梅经济”的繁荣发展也带来了一定的环境问题,如化肥的大量施用造成了土壤酸化和养分失衡等问题[11-14],严重影响了土壤质量、杨梅产量及品质[15]。目前,前人对于影响杨梅土壤肥力因素的研究集中在生草栽培[16]、施肥处理[17]、微生物[18]和林龄[19]等方面,缺乏对杨梅土壤肥力空间分布及受地形因子影响程度的研究。因此,掌握林地土壤肥力的分布状况及其影响因素,对促进经济林生长与经济效益增长有着不可或缺的帮助。
为探明杨梅土壤肥力的影响因素,解决杨梅主产区土壤养分不均和酸化问题,笔者在本研究中以浙江省杨梅主产区土壤为研究对象,探究杨梅主产区林地土壤养分的空间异质性和分布规律,并根据相关性分析和结构方程模型结果,探寻土壤肥力的不同影响因子。本研究为杨梅经济林土壤质量管理、精准施肥提供理论及技术基础,推进杨梅果实增产与品质升级,实现杨梅经济林科学高效的管理和经营。
1 材料和方法
1.1 研究区与采样地概况
杨梅是浙江省台州市一大农业经济亮点产业,台州市的杨梅种植面积在2019年高达2.95万hm2,产量多达27.6万t,产值超过24.5亿元[20]。台州市杨梅产区是中国最为著名的杨梅产区之一,占据浙江省1/3以上的种植面积和果实产量。仙居县和临海市杨梅种植面积大、产量多,因此被称为“中国杨梅之乡”,笔者选取浙江省杨梅主产区仙居县和临海市为研究区。
仙居县位于浙江省东部,28°30′~29° N,120°~121° E,海拔1~1370 m,亚热带季风气候,平均降水量2000 mm,全年平均气温18.3 ℃,无霜期在240 d左右,年日照时数1 786.2 h,土壤为红壤土类[19]。2019年,仙居县的种植面积为9200 hm2,投产面积为8330 hm2,总产量达9.5万t,产值达7.2亿元,是浙江省台州市第一杨梅主产区[21]。
临海市位于浙江省东南沿海,28°40′~29°04′ N,120°49′~121°41′ E,海拔10~1200 m,亚热带季风气候,平均降雨量1 710.4 mm,全年平均气温17.1 ℃,无霜期241 d,平均蒸发量1 231.4 mm。2019年,临海市的种植面积8.87×103 hm2,年产量8.8万t,产值6.4亿元,临海市是浙江省最大的杨梅生产基地之一。
1.2 样品采集和分析
2020年8月,采用1 km×1 km网格法布设采样点[22-23],选取气候最适宜杨梅生长的区域为研究区,结合杨梅林地的分布情况,在台州仙居、临海分别选取了64个和36个坐标小区块,根据初步定位好的100个GPS位点,在每个杨梅经济林研究区样地附近采用5点取样法分别取每个样点位于0~20 cm土层土壤1 kg,同时调查杨梅地径、树龄、施肥及其他管理措施,将土样混合均匀后采用四分法取1 kg土样装于塑封袋中,密封保存带回实验室。土壤样品于通风干燥处风干7 d,研磨充分后分别过2 mm、0.149 mm筛,于阴凉干燥处,置于密封塑料袋中,编号封口保存。土壤pH值以及有机碳、碱解氮、有效磷和速效钾含量根据《土壤农化分析》内的方法测定[24]。采样点杨梅在10、15、20和25 a以上4個树龄段所占比例分别为11%、41%、36%和12%,坡度范围为2°~30°,各个坡向均有分布。
1.3 地统计学
地统计学以变异函数为主要研究工具,研究土壤在空间分布上的随机性、连续性和空间相关性等自然现象[25]。普通克里格法是地统计学中最为常用的无偏最优估计的插值法[26],本研究中,通过采用普通克里格方法来绘制土壤理化性状的空间分布图[27-28],直观地反映出土壤理化性状的空间异质性。
[γh=12N(h)i=1N(h)Zxi+h-Z(xi)2][29]。 (1)
式中,[γh]表示间隔为[h]时的半方差;[h]表示样本间距;[Z(xi)]和[Zxi+h]分别表示在[xi]和[xi+h]时变量[Z(x)]的实测值;[N(h)]表示空间间隔为[h]时的点对数。
当半方差函数[γh]随着样本间距[h]的增加而增加,趋于一个常数后恒定不变[30],该常数即基台值(C0+C);在[h]=0时半方差值即为块金值(C0),表示由于采样误差和尺度过小的采样等随机因素引起的空间异质性[31]。最常见的半方差函数理论模型有线性模型、指数模型、球状模型和高斯模型[32]。
1.4 土壤肥力评价模型的选取和计算
结合前人研究成果,本研究选取了土壤肥力质量指标中最具代表性的5个重要指标,分别是pH值及有机碳、碱解氮、有效磷和速效钾含量[33]。
由于土壤肥力评价指标实测值量纲的不同,对数据进行归一化处理,利用隶属度函数将各项指标转换为0.1~1之间的无量纲值[34],实现土壤各肥力指标的量纲归一化。利用因子分析法确定肥力指标的权重,评价指标公因子方差所占比例即为权重值。
抛物线型:
[Wi0.9x-x3x4-x3+0.1 x3
S型:
[Wi 1.0 x≥x20.9x-x1x2-x1+0.1 x1≤x 式中,[Wi]为各个肥力指标的隶属度;[x]为各个肥力指标的实测值;[x1]、[x2]、[x3]和[x4]为各个肥力指标的转折点(表1)。 1.5 土壤综合肥力评价 对各项指标进行评价后,需将单因素评价结果转换为由各指标所构成的土壤综合肥力评价结果。研究对各土壤肥力指标的隶属度值进行加权求和,最终计算土壤综合肥力评价指数[35],公式如下: [IFI=i=1nWiNi]。 (4) 式中, IFI为土壤综合肥力评价指数;[Wi]为第[i]项指标的隶属度值;[Ni]为第[i]项指标的权重值。 1.6 结构方程模型 结构方程模型(structural equation model,SEM)是一种基于变量的协方差矩阵分析变量间关系的方法,整合了因子分析、路径分析和回归分析等多种方法,通过假设影响路径,对复杂的数据进行处理,有效地揭示了因子之间的因果关系[36-37]。进行结构方程模型拟合分析之前,需对土壤肥力及其影响因子数据的可信度进行验证[38]。相关性分析得到的相关系数不能表明因果关系,相较于相关性分析,结构方程模型的优势在于处理多变量的同时,还能处理变量之间的间接因果关系。通过卡方([χ2])检验对构建的模型进行拟合优度评价,当卡方检驗p>0.05,各参数中,CFI(comparative fit index,比较拟合指数)和GFI(goodness of fit index,拟合优度指数)>0.9,RMR(root mean square residual,残差均方根)<0.05,RMSEA(root mean square error of approximation,近似误差均方根)<0.08时,SEM拟合效果较为理想[39]。 1.7 数据的软件处理与分析 采用SPSS 24.0和Excel 2016对试验数据进行整理、计算和表格的绘制;利用ArcGIS 10.7软件采用克里金插值法绘制土壤肥力指标空间分布图;利用GS+7.0进行半方差模型拟合和地统计分析;利用Pearson法研究土壤各项养分指标之间的相关性,使用R 3.3.3进行相关性图形绘制;使用Minitab 19进行Box-Cox转换。 笔者在进行数据分析之前,使用3倍标准差法(阈值法)剔除异常值,在剔除异常值时,根据实际情况判断异常值,分析异常值的原因。如果数据能够说明问题,则可选择性保留;如果是由系统误差或人为误差所导致,则可选择剔除。 在本研究中要求数据符合正态分布,避免降低研究对实际情况的估算精度。由表2可知,pH值和有机碳的原始数据服从正态分布(p>0.05),采用峰度、偏度和Kolmogorov-Smirnov(K-S)联合法对其余3种数据进行多种检验和处理[40],对碱解氮含量数据进行Box-Cox转换,对速效钾和有效磷含量进行对数转换,转换后,各土壤肥力指标数据均满足正态分布(p>0.05)。 2 结果与分析 2.1 杨梅主产区土壤肥力指标的描述性统计分析 描述性统计分析结果如表3所示。杨梅林地土壤pH值为3.71~5.55,有机碳含量10.47~32.41 g·kg-1,碱解氮含量55.30~255.50 mg·kg-1,有效磷含量0.19~64.99 mg·kg-1,速效钾含量66.00~632.00 mg·kg-1。由于适宜杨梅生长的土壤有机碳含量为11.6~29.0 g·kg-1[41],因此采样点91%的土壤有机碳含量适宜杨梅的生长。当变异系数小于10%时,属于弱变异,自然因素占主导作用;当变异系数在10%~100%,属于中等变异,自然和人为因素共同影响;当变异系数大于100%时,属于强变异,人为因素占主导作用。土壤pH值及有机碳、碱解氮、速效钾含量的变异系数均大于10%且小于100%,在空间上表现为中等变异性;有效磷含量的变异系数大于100%,在空间上表现为强变异性。 2.2 杨梅主产区土壤肥力指标的空间分布特征 研究区杨梅土壤养分的空间分布结果显示,仙居东部地区杨梅土壤pH值较高,仙居和临海西部地区土壤pH值较低,研究区大部分区域土壤pH值均小于4.5,属于强酸性土壤;整体上看,土壤酸性自仙居东部高值区域向四周逐渐增强,其中仙居西部和临海西部的土壤酸化问题最严重。杨梅土壤有机碳含量则表现为仙居整体高于临海,高值区域集中分布在仙居,表现显著的集聚效应。杨梅土壤碱解氮含量高值区域集中分布在仙居西部,由西部向东部逐渐降低,整体上呈现西高东低的分布特征,在仙居西部表现明显的集聚效应。杨梅土壤有效磷含量在仙居和临海的西部地区较高,仙居整体上较低,呈现点状分布的特征。杨梅土壤速效钾含量在仙居的含量整体大于临海,呈现斑块破碎状分布的特征。从养分的整体分布状况来看,仙居和临海的杨梅土壤高养分区域表现出明显的聚集现象。 2.3 杨梅主产区土壤综合肥力评价 土壤综合肥力评价法能够帮助当地的农林管理者、科研人员和决策人员更直观和深入地了解当地的土壤肥力状况[33]。研究选取土壤pH值、有机碳含量、碱解氮含量、有效磷含量和速效钾含量作为计算土壤综合肥力的5个指标。表4为各肥力评价指标被分配的公因子权重和方差的值,不同指标的权重值差异较大,有机碳含量、碱解氮含量和有效磷含量被分配的权重较高(0.254、0.247和0.220),pH值和速效钾含量被分配到的权重为0.105和0.174。 将计算的土壤肥力指数结果利用ArcGIS 7.0通过克里金插值法绘制分布图。当土壤肥力指数(integrated soil fertility evaluation index,IFI)>0.47时,说明土壤肥力属于中、高质量水平[35],在杨梅主产区仙居和临海中,有91%的土壤肥力属于中、高质量水平,仅9%的土壤肥力水平处于低质量水平。中、高肥力水平的土壤主要分布在仙居西部和临海西部,低肥力水平的土壤较少,主要分布在仙居和临海东部地区。主产区杨梅土壤肥力水平整体较高,大部分地区土壤已达到肥沃水平,仅少数地区土壤较贫瘠。 2.4 杨梅主产区土壤肥力指标相关性分析 Pearson相关性分析可以有效地反映土壤肥力指标与影响因子之间的关系。如图1所示,海拔与碱解氮含量呈极显著正相关(p<0.01),坡向与速效钾含量呈显著负相关(p<0.05),树龄、地径与土壤有机碳含量呈显著正相关(p<0.05)。随着树龄的增长,杨梅立地环境趋于稳定,随着杨梅周围枯枝落叶等有机物料的分解,表层土壤腐殖质不断积累,有机碳含量随着树龄的增长而增加[42]。而坡度对土壤肥力指标的影响不显著,但坡度对杨梅生长有着重要影响,因为杨梅根系不耐积水,适宜在排水效果良好的山坡上种植。 2.5 杨梅主产区土壤肥力指标和影响因素的结构方程模型路径分析 模型的适配度检验结果显示:[χ2]=24.020,df=26,n=100,p=0.575,CFI=1.000,GFI=0.951,RMR=0.000,RMSEA=0.000,表明本研究中模型和数据的适配度较高、拟合情况理想,可以满足研究和分析的要求。杨梅土壤肥力指标与影响因素的结构方程模型路径图如图2所示。影响因素中海拔、坡向和树龄均对各项肥力指标有着显著的影响,其中树龄对有机碳含量有显著正影响(p<0.05),路径系数为0.248;坡向对速效钾含量有显著负影响(p<0.05),路径系数为-0.197;海拔对碱解氮含量有显著正影响(p<0.05),路径系数为0.204;对速效钾含量有显著负影响(p<0.05),路径系数为-0.214。从路径图可以看出,各影响因素对有效磷含量的直接影响并不显著,但能通过对有机碳含量和碱解氮含量的直接影响进而间接影响有效磷含量,土壤pH值与各项影响因素之间的路径系数均较小,pH值的变化主要是严重的人为施肥管理干扰引起的。 2.6 树龄对杨梅主产区土壤肥力指标的影响 树龄和地形因子(海拔、坡度、坡向)是影响土壤肥力的因素,在对不同梯度树龄的杨梅土壤肥力指标进行单因素方差分析时,有较显著的差异和变化特征,各个梯度的海拔、坡度和坡向之间均不存在显著差异(p>0.05),因此对树龄进行具体分析。 不同树龄对土壤肥力指标的影响如图3所示,树龄分为10、15、20、25 a以上4个阶段。土壤pH值随着树龄的增长呈现降低的趋势,15、20、25 a以上的杨梅土壤pH值显著低于10年生杨梅土壤。随着树龄的增长,由于化肥长期过度使用,根系吸收后的酸根离子与土壤中的氢离子结合生成酸,导致土壤板结[43],土壤呈现酸化的趋势,土壤酸化不利于土地的可持续利用,严重影响土壤肥力质量。土壤有机碳随着树龄的增长逐渐增加,25 a以上杨梅土壤有机碳与15、20年生杨梅土壤差异显著。随着经营年限增加,立地环境稳定,人为补充碳源,土壤中有机物质积累速率大于分解速率,土壤的有机碳含量逐年增加。 3 讨 论 3.1 楊梅主产区土壤肥力特征 本研究中的杨梅各土壤养分平均值主产区(仙居和临海)与全国杨梅主产区相比,土壤pH值低于全国杨梅主产区土壤平均值(4.95),有机碳含量高于全国杨梅主产区平均值(11.29 g·kg-1),有效磷含量低于全国杨梅主产区平均值(24.65 mg·kg-1),速效钾含量高于全国杨梅主产区平均值(62 mg·kg-1)[41]。通过与土壤养分平均值比较,发现本研究中研究区杨梅土壤酸化问题严重。土壤酸化会降低土壤养分活性,抑制作物对养分的吸收,严重影响杨梅果实的产量和品质[44],还会促进土壤中有害元素的释放和有害微生物的滋生,加大杨梅病害发生的概率,造成根系发育不良[45]。土壤有效磷含量整体较低,叶柳欣等[19]在研究中发现磷素是杨梅生长的限制性元素,实地调查显示,果农在管理过程中,偏重于施用氮肥和钾肥,缺少对磷肥的施用。研究结果说明人为活动成为影响土壤肥力和分布的一大重要因素。这与孙玉冰等[46]和牛文鹏等[47]的研究结果一致。张彬等[48]在研究苹果种植区时发现,土壤速效钾含量的变异程度最大,其原因是果园人为施肥,而本研究中的杨梅林人为施肥活动较频繁,受人为施肥影响,有效磷变异程度较大,对杨梅林地土壤中有效磷含量影响较大。然而磷素对杨梅的生长同样很重要,合成蛋白质需要大量磷素[49],磷素缺乏会限制杨梅的生长[50]。相较于茶园土壤和水稻土壤,林地土壤的磷吸附能力较差[51],因此应加强对磷肥的搭配施用。对于过多施用化肥导致杨梅土壤酸化的问题[52-53],可以利用生石灰、有机肥和生理碱性肥料(如草木灰)调控土壤pH值,缓解土壤酸化[54-55]。 3.2 杨梅主产区土壤肥力空间分布特征 由于研究区横跨范围较广,种植的作物种类多样,土地管理和利用方式差异大,因此相邻区域土壤肥力差异大[56],空间分布不连续。研究区内部的地势由西南向东北降低,四周则由大山包裹形成盆地,而有机碳含量和碱解氮含量高值区大都分布在高海拔地区。相关研究表明,地形因子导致温差、日照和水分的差异是土壤肥力空间异质性的主导因素[57]。 由实地调查可知,杨梅果农在肥力管理中,为了节省成本和省时省力,过度偏施氮肥和钾肥,而磷肥的施用量较少,虽然杨梅对氮素和钾素的需求较大,但磷素也是促进植物蛋白质合成的重要元素之一。有机肥在日常施用过程中由于费时、费力和肥效慢等缺点,难以统一大量施用,化肥的滥用进而导致土壤酸化问题的形成,酸化导致肥料利用率下降,增加肥料采购成本,严重时还会导致杨梅的质量和产量下降。土壤综合肥力指数不仅取决于肥料施用量和肥料吸收效率,还受到多种影响因素的作用。杨梅果农应该根据当地土壤的实际情况,及时调整肥料施用结构,重新制定专门的改良方案,在管理中节省生产成本,提升杨梅果实品质,促进杨梅产业的健康发展。 3.3 杨梅主产区土壤肥力影响因素 本研究中,pH值与各项土壤肥力指标之间均呈负相关,但仅与有效磷含量呈显著负相关(p<0.05),说明土壤酸化在一定程度上影响着土壤有效养分的含量,影响植物吸收土壤中的有效养分。过量施用化肥虽然能增加土壤中的养分,但带来的土壤酸化问题也会导致肥料利用率的下降,施用生物质炭可有效缓解土壤酸化问题,还可以增加土壤有效养分[58]。除有机碳含量和速效钾含量不存在显著正相关外(p>0.05),有机碳含量、碱解氮含量、有效磷含量和速效钾含量两两间均存在显著正相关(p<0.05),其中有机碳含量和碱解氮含量之间的相关系数(0.509)最高。有机碳含量和土壤供氮有着密切的关系,充分说明土壤有机碳是碱解氮的源泉[59]。随着树龄的增长,碱解氮含量呈现先减少后增加(高-低-高)的趋势,与刘顺等[60]在研究土壤碱解氮含量随着树龄变化时有相似的结果。这是由于10~15 a阶段,杨梅进入盛产期,氮素的需求量大,树龄在15 a后随着肥料的大量施用,氮素逐渐积累。有效磷含量呈现出先增加后降低(低-高-低)的趋势,与刘顺等[61]对陈山红心杉土壤的研究结果相同,可能是由于杨梅的需磷高峰期在15 a之后。速效钾含量随着树龄的增加变化波动较大,不同树龄杨梅的土壤无显著差异和明显规律。钾元素由于其移动性强的特性,在不同年份和气候易受径流、淋溶等影响而流失[33]。通过对比结果可以发现,在经济林土壤管理中,人为施肥是影响土壤养分有效态的主导因素[31] 。 杨梅生长地的海拔、坡向和坡度不同,会导致杨梅生长环境的光照、温度和湿度的差异,影响土壤中有效养分的释放[62],最终影响到杨梅的生长发育、果实的产量、品质和成熟时间。根据杨梅喜阴耐湿的特点,在杨梅园的选址时,应根据杨梅品种的成熟时间,选取不同海拔。同时在空气湿度大、排水效果良好的阴(半阴)坡上种植杨梅,果实的品质会有显著的提升。当具备一定经济条件时,杨梅果农可以搭建设施大棚。设施栽培可以针对性地解决外界对杨梅生长的干扰问题,控制果实成熟的时间,提升果实的品质,还能起到防虫防风,提前、推迟和延长收获时间等作用[63]。 4 结 论 杨梅主产区土壤肥力主要受到人为施肥影响,土壤酸化和肥力失衡问题严重,需要在仙居和临海东部增加肥料施用量,仙居和临海西部则需要适当降低化肥的施用比例,主产区还需增加磷肥的施用,为缓解土壤酸化问题,可利用生石灰、有机肥和生理碱性肥料(如草木灰)改良。 参考文献 References: [1] 刘占锋,傅伯杰,刘国华,朱永官. 土壤质量与土壤质量指标及其评价[J]. 生态学报,2006,26(3):901-913. 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