Spatial distribution characteristics of soil nutrients in walnut orchards based on geostatistical model

Abstract：【Objective】Despite the importance of soil nutrient management in walnut orchards, there is little research focusing on the spatial characterization of soil nutrients and the impact of topography on nutrient distribution and overall fertility. Consequently, the purpose of this study was to elucidate the current state of soil nutrient content and fertility, as well as to investigate the spatial characteristics of soil nutrients in walnut orchards. The findings are intended to inform the division of management units within these orchards, making it easier to establish focused and accurate fertilization strategies. Finally, the study aims to enhance soil nutrient management and contribute to the sustainable and efficient production of walnut orchards.【Methods】In this study, a 5 km grid was used to designate 110 plots of walnut orchards in Baokang County. Soil properties, including pH value, organic matter, hydrolyzable nitrogen, available potassium, and available phosphorus, were tested. Soil fertility was thoroughly evaluated using both statistical analysis and the Nemerow index method. Geostatistical analyses were performed using GS+9.0 software to fit the variogram and select the optimal model. Moran’s index was employedto assess the spatial autocorrelation of soil nutrients in the study area, with local spatial autocorrelation clusters identified using local Moran’s Ⅰ. Kriging method was applied for spatial optimal unbiased interpolation, and spatial distribution maps were created using ArcGIS software.【Results】In Baokang County’s walnut orchards, the average soil pH was 6.68, indicating predominantly neutral to slightly acidic conditions with weak variability. Mean values for hydrolyzable nitrogen (HN), available potassium (AK), organic matter (OM), and available phosphorus (AP) were 109.41 mg·kg^-1, 125.43 mg·kg^-1 , 27.63 g·kg^-1 , and 23.74 mg·kg^-1 , respectively, each exhibiting moderate variability. According to the Nemerow index, the soil fertility index ranged from 0.90 to 2.26, with an average of 1.57. Approximately 78.3% of the land was categorized as moderately fertile, and 20.7% as fertile. In analyzing the facultative properties of soil nutrients, it was found that HN, AK, OM, and AP, as well as the comprehensive soil fertility, predominantly exhibited clustered distributions. Concerning the directional trends: HN concentrations were observed to be higher in the eastern and southern regions, with lower concentrations in the western and northern areas. AK displayed a localized polar distribution. AP concentrations were predominantly higher in the central and eastern regions compared to other areas. Finally, OM exhibited a trend of being higher in the eastern and western regions while lower in the northern and southern parts. These spatial and directional trends provide insights into the heterogeneous distribution of soil nutrients, which is critical for targeted soil management and fertilization strategies in walnut orchards. Notably, HN scores were significantly lower than other soil indices, indicating it as a primary limiting factor in soil fertility. Correlation analysis of soil fertility index and average acreage yield of walnut dry fruits showed that soil fertility was significantly positively correlated with dry fruit yield. The ranges of HN, AK, OM, and AP contents were 8.175, 5.421, 7.440, and 6.634 km, respectively, which were larger than the sampling distances, indicating that they could satisfy the needs of spatial analysis. The coefficients of determination were 0.894, 0.724, 0.742, and 0.844, respectively, which were larger than 0.5, indicating that the fitting effect was good under the corresponding theoretical model, and the spatial variability of soil nutrients was more accurately reflected by the spatial variability. Spatial autocorrelation analysis of the soil revealed that HN and AK block base ratios were 14.4% and 15.6%, respectively, with block base ratios below 25% , suggesting influence predominantly from natural structures. Conversely, OM and AK block base ratios were 35.2% and 39.2%, respectively, indicating influence from both natural structures and stochastic factors. All Moran’sⅠ values of soil nutrients in the study area were positive, denoting a significant spatial aggregation. Z- values of Moran’s index for HN and AK were 4.30 and 3.05, respectively, with p＜0.05, which indicated that there were significant spatial aggregation effects for both of them; while z-values of AP and OM were 0.63 and 0.98, respectively, and the aggregation effect of the two was not significant (p＞0.05). High-value nutrient aggregation areas were primarily in higher elevations like Longping Township, with lower values in lower elevations such as Maqiao and Dianya Townships. Elevation significantly influenced soil fertility, showing a strong positive correlation with HN and AP content, a significant correlation with OM content, and no significant correlation with AK content.【Conclusion】The soil pH in the area, predominantly neutral to slightly acidic, is conducive to walnut growth. Currently, the overall fertility of the walnut orchards ranges from medium to high, indicating a beneficial environment for walnut cultivation. Despite this, there is room for improvement in soil fertility to further enhance walnut yields. It is well-established that better soil fertility correlates with higher yields of walnut dry fruit. The management of walnut orchards at the county scale tends to be basic, with soil nutrient content primarily influenced by natural structures and secondarily by anthropogenic activities. Elevation is found to be an important natural factor that significantly corre-lates with soil fertility, HN, AK and OM, making it a critical parameter for delineating soil nutrient management units. To optimize the comprehensive fertility of orchard soil and, consequently, the yield of walnut orchards, it is necessary to adjust soil management practices. Specifically, increasing the applica tion of nitrogen fertilizer is specifically recommended to address the identified deficiency in HN and boost overall soil fertility.