1 Introduction
Soil fertility is a major concern owing to the increasing population and rapid economic growth worldwide. Human activities, such as modern agriculture, land-use changes, and pollution increasingly affect natural ecosystems, soil fertility, and nutrient balances (Dumanski and Pieri, 2000; Chapin, 2003). Unsustainable land use is a major factor affecting soil degradation (Liu et al., 2022). Topsoil loss is primarily caused by erosion, which also affects soil structure, biological activity, and nutrient availability (Gomiero, 2016). It is widely known that in conventional farming systems, farmers use animal manures, plant residues, household trash, bush fallow, and other inorganic nutrient sources to maintain soil fertility and soil organic contents (Kugbe, 2019). The spatial distribution of soil fertility varies from the field to larger regional scales and is influenced by both land-use types and soil management practices (Liu et al., 2022; Li et al., 2022). It is crucial for improving sustainable land-use strategies to reveal the spatial variability of soil fertility and its influencing factors. At the field scale, the main variables affecting soil fertility quality are variations in fertilization, cropping systems, and farming techniques.
Vietnam is located in South-east Asia and stretches on the Indochina Peninsula (Hai and Egashira, 2008; Tung et al., 2018). The total inland surface is approximately 33 115 000 ha, but only 28.4% is used for agriculture (Ha, 2010). Strong deforestation and conversion into agricultural land are mainly characterized by the inappropriate use of land in the mountainous region of Vietnam (Cochard et al., 2017). Rice, maize, cassava, acacia, and banana are the main distributed pattern crops in the mountainous region of the Thua Thien Hue Province, Vietnam (Herzberg et al., 2019; Tung et al., 2022). The main factors affecting soil fertility are these farming techniques, cropping systems, and cultivation patterns (Wezel et al., 2002). In addition, inappropriate agricultural practices and continuous cropping without adequate nutrients are occurring in many regions in Vietnam (Ha, 2010; Anh et al., 2014). For example, acacia plantations can improve soil organic carbon (SOC) and total nitrogen (TN) compared to other type of land-use, like grassland, secondary forest, and abandoned land (Sang et al., 2013; Anh et al., 2014; Dong et al., 2014). Changes in soil pH and acidification are caused by deforestation and cultivation (Sisay Golla, 2019). The distribution of P and some exchangeable cations in the soil are affected by the differences in land-use types and management practices (Sang et al., 2013). Soil management requires particularly integrated practices that can increase soil fertility and nutrients. Agriculture in A Luoi District, Thua Thien Hue Province, has ongoing difficulties, like a lack of land for growing crops, soil erosion, intensive precipitation, inorganic fertilizer application, and declining soil fertility, which has resulted in low yield production. However, only a few studies have examined how land use affects soil characteristics. Therefore, this study aimed to determine how various land-use types affected a few specific soil physicochemical characteristics for better soil management decisions and the improvement of yield production.
2 Materials and methodology
2.1 Study site
The study region was in A Luoi District of Thua Thien Hue Province in central Vietnam (Tab.1 and Tab.2). It is located in a mountainous area approximately 80 km from Hue city, and ranges between 16°00′57" N to 16°27′30" N and 107°0′3" E to 107°30′30" E, near Laoseʼs boundary. The district lies in a mountainous region with an average elevation of approximately 600–800 m above sea level. The topography slope of the study site ranged from 20° to 25°, which is not convenient for cropping or intensive livestock production. The climate in this region is characterized by a tropical monsoons with a rainy season in cold winters and relatively dry summers. The mean annual precipitation is higher than 3500 mm, concentrated between September and December (Fig.1), and associating with high-frequency tropical typhoons (Hung et al., 2017; Tung et al., 2018). The mean annual temperature is 24.9°C, with the lowest and highest mean monthly temperatures in January (17.4°C) and June (25.3°C), respectively (Fig.1). The natural vegetation of tropical semi-deciduous monsoon forests and secondary forests is still found in areas where human impact on vegetation is low, and replaced by agricultural land elsewhere. Soils in the study region are mostly siliceous and sandy and are classified into groups (e.g., Acrisols, Cambisols, Alisols, and Leptosols) of red or yellow soils (Sang et al., 2013; Dong et al., 2014). The soil has a high proportion of the coarse content (> 2 mm) derived from siliceous parent materials, such as sandstone, mudstone, granite, and metamorphic rocks (VMI, 2000).
Tab.1 Site studying and sampling points in A Luoi District, Thua Thien Hue Province. |
| Sampling sites | Longitude | Latitude |
|---|---|---|
| 1 | 107°22′48.7" | 16°04′36.7" |
| 2 | 107°12′26" | 16°13′18.5" |
| 3 | 107°11′12.3" | 16°19′39.7" |
| 4 | 107°13′24.3" | 16°15′37.8" |
| 5 | 107°22′31" | 16°05′23,9" |
| 6 | 107°13′23" | 16°13′12.3" |
| 7 | 107°23′11.1" | 16°19′36.2" |
| 8 | 107°12′23.2" | 16°15′30.9" |
| 9 | 107°14′29.1" | 16°17′45.7" |
| 10 | 107°12′25.8" | 16°15′5.5" |
| 11 | 107°23′26.1" | 16°18′41.3" |
| 12 | 107°23′6.8" | 16°18′52.7" |
| 13 | 107°21′47.1" | 16°06′44.1" |
| 14 | 107°20′28.4" | 16°07′43.7" |
| 15 | 107°19′19.8" | 16°18′54.7" |
| 16 | 107°23′5.1" | 16°05′17.6" |
| 17 | 107°23′43.5" | 16°06′4.5" |
| 18 | 107°20′44.5" | 16°07′39.6" |
| 19 | 107°19′25.2" | 16°07′27.1" |
| 20 | 107°19′0.2" | 16°12′30.8" |
| 21 | 107°20′55.7" | 16°11′47.8" |
| 22 | 107°12′30.3" | 16°19′35.7" |
| 23 | 107°10′19.6" | 16°14′33.3" |
| 24 | 107°11′40.3" | 16°18′34.7" |
| 25 | 107°20′50.6" | 16°08′58.5" |
| 26 | 107°13′6.9" | 16°17′28.4" |
| 27 | 107°12′12.5" | 16°18′58.5" |
| 28 | 107°16′24.7" | 16°12′18.8" |
| 29 | 107°18′39.3" | 16°12′15.6" |
| 30 | 107°17′44.4" | 16°14′58.9" |
| 31 | 107°14′10.6" | 16°14′39.3" |
| 32 | 107°14′51.4" | 16°14′18.6" |
| 33 | 107°12′6.7" | 16°16′10.9" |
| 34 | 107°11′20.4" | 16°13′43.7" |
| 35 | 107°04′12.3" | 16°23′10.8" |
| 36 | 107°24′42.7" | 16°18′21.6" |
| 37 | 107°19′56.4" | 16°12′48.6" |
| 38 | 107°21′10.1" | 16°18′33" |
| 39 | 107°12′57.2" | 16°19′34.2" |
| 40 | 107°11′12.7" | 16°13′14.4" |
| 41 | 107°13′39.5" | 16°12′19.8" |
| 42 | 107°19′21.6" | 16°03′30.1" |
| 43 | 107°13′2.7" | 16°15′17.5" |
| 44 | 107°11′5.1" | 16°21′36.5" |
| 45 | 107°13′59.5" | 16°18′13.2" |
| 46 | 107°12′27" | 16°15′13.1" |
| 47 | 107°11′56.4" | 16°17′4.5" |
| 48 | 107°15′33" | 16°14′10.8" |
| 49 | 107°13′7.5" | 16°15′1.5" |
| 50 | 107°12′15.6" | 16°14′9.4" |
| 51 | 107°10′39.3" | 16°14′44.5" |
| 52 | 107°04′52.3" | 16°24′7.1" |
| 53 | 107°13′3.5" | 16°16′46.7" |
| 54 | 107°14′40.9" | 16°16′3.8" |
| 55 | 107°16′37.9" | 16°14′45" |
| 56 | 107°13′29" | 16°17′8.2" |
| 57 | 107°07′9.2" | 16°22′31.5" |
| 58 | 107°03′28" | 16°23′30.9" |
| 59 | 107°03′38.8" | 16°22′5.2" |
| 60 | 107°06′22.4" | 16°22′11" |
| 61 | 107°05′59.6" | 16°24′37.1" |
| 62 | 107°19′6.3" | 16°18′36.2" |
| 63 | 107°14′23.5" | 16°15′50.2" |
| 64 | 107°14′42.7" | 16°15′2.2" |
| 65 | 107°11′6.2" | 16°19′36.5" |
| 66 | 107°13′10.7" | 16°17′31.6" |
| 67 | 107°16′1.3" | 16°15′41.5" |
| 68 | 107°11′54.7" | 16°20′41.7" |
| 69 | 107°12′12.8" | 16°17′2.1" |
| 70 | 107°10′17.5" | 16°21′36.6" |
| 71 | 107°16′32.5" | 16°15′6.8" |
| 72 | 107°10′47.7" | 16°20′3.4" |
| 73 | 107°11′29" | 16°13′22" |
| 74 | 107°14′29" | 16°15′16.6" |
| 75 | 107°14′2.8" | 16°16′43.2" |
Tab.2 Characteristics of each land use type in A Luoi, Thua Thien Hue Province. |
| Land use | Samples No. | Land use defined | Historical land use management | Time (years) | Elevation (m) | Scale |
|---|---|---|---|---|---|---|
| Acacia | 21 | 5-year-growing cycle cropland | Minimum tillage | 10 | 589−644 | 9 |
| Rice | 10 | 1-year-growing cycle cropland | Intensive tillage fertilizer, irrigation | 20 | 591−752 | 7 |
| Cassava | 17 | 1-year-growing cropland | Intensive tillage | 5 | 588−610 | 1 |
| Maize | 14 | 1-year-growing cropland | Intensive tillage | 2 | 598−672 | 3 |
| Banana | 17 | 3-year-growing cropland | Intensive tillage, garden | 3 | 578−664 | 5 |
The AHP is carried out in accordance with earlier studies (Herzberg et al., 2019; Canco et al., 2021) |
2.2 General characteristics of the farmers in A Luoi District
The general characteristics of the farmers in A Luoi District are listed in Tab.3. Paco and Cotu are the main ethnicities with averages of 48% and 30%, respectively. The average age of the household head ranged from < 30 to > 60 years and was divided into four groups. The group under 30 years old accounted for 15%, 30–45 years old accounted for 52%, 45–60 years old accounted for 23%, and > 60 years old accounted for 10.31%. Education levels of the farmers were primary school, secondary school, high school, and bachelor's degree with an average of rate 36%, 30%, 28%, and 6%, respectively. The main economic activity in A Luoi District is farming.
Tab.3 Characteristics of agricultural farmers in A Luoi District, Thua Thien Hue Province. |
| Characteristics | Number (household) | Rate (%) | Scale | |
|---|---|---|---|---|
| Ethenic | Kinh | 17 | 9 | 9 |
| Ta Oi | 25 | 13 | 3 | |
| Co Tu | 59 | 30 | 5 | |
| Pa Co | 93 | 48 | 7 | |
| Gender | Male | 89 | 46 | 5 |
| Female | 105 | 54 | 4 | |
| Age of head household | Under 30 years | 29 | 15 | 5 |
| From 30 to 45 years | 100 | 52 | 7 | |
| From 45 to 60 years | 45 | 23 | 9 | |
| Over 60 years | 20 | 10 | 3 | |
| Education level | Primary school | 69 | 36 | 3 |
| Secondary school | 58 | 30 | 5 | |
| High school | 54 | 28 | 7 | |
| Bachelor | 13 | 7 | 9 |
2.3 Soil characterization
Soil parameters were surveyed in the agricultural fields of the district (e.g., acacia, rice, cassava, maize, and banana fields). Soils were sampled from different dominant land-use types. For each land-use type, soil samples were collected at a depth of 0–10 cm. Before the laboratory analysis, the samples were sieved through a 2 mm screen after being air-dried.
Soil bulk density (BD) was determined from cores that had been dried at 105°C to constant weight (Jabro et al., 2020). The soil pH was measured by mixing an aliquot of samples with water or 1 mol L−1 potassium chloride solution, at a soil/liqid ratio of 1:5 (weight soil: liquid volum mixture). A glass electrode was used to determine the value for each sample (HI5521, Hana Instrument Co., Ltd, Vietnam). The particle-size distribution of soils was determined using a combination of sieving and pipette methods (Gee and Or, 2002). Total nitrgen content was analyzed by means of a Kjeldahl method (UDK-129 Distillation Unit, VELP Scientifica Srl, Italy). Total phosphorus content was analyzed using the color comparison method using spectrophotometer (Yoke Sepectrophotometer UV1700, Sanghai Yoke Instrument Co., Ltd, China). The total potassium content in the sample was determined by using Flame Photometer (FP640, Shanghai Drawell Scientific Instrument Co., Ltd, China). Exchangeable Al3 + and H+ using the titration method (Abreu et al., 2003). In order determine CEC value, the residual soil after ammonium acetate extraction was first washed with 96% ethanol, and the remaining NH4+ was finally extracted with NaCl 0.1 mol L−1 solution (Burt, 2004). The NH4+ solution was then determined using Kjeldahl method (UDK-129 Distillation Unit, VELP Scientifica Srl, Italy). The rganic carbon content was analyzed using the Walkley–Black wet oxidation procedure and the soil organic matter content was determined from organic carbon (Nelson and Sommers, 1996; Batjes, 2014).
Carbon or nitrogen content at each soil depth (0–10 cm) were calculated using the formula described by Batjes (2014):
Carbon or nitrogen content (Mg ha−1) = C-content layer (g kg−1) × BDlayer (g cm−3) × soil depth (m) × 10−3 Mg kg−1 × 104 m2 ha−1.
2.4 Household survey procedure
We focued on crop yield, land-use types, and land size, which are the main criteria for choosing a suitable crop to develop farmers' incomes in A Luoi District. To obtain data, we randomly selected the household heads of farms in the district for a survey. A total of 194 were willing to participate in the research and were interviewed at their homesteads using questionnaires, if any of the household heads were absent, another member was interviewed. We assumed that their cropping patterns included paddy terraces, dynamic agriculture, the ability of market access, and government services. Land-use types are probably influenced by the presence of labor, indirect costs (fertilizer and seeds), soil properties, and land management practices, among others. Acacia, cassava, banana, and rice maize were chosen as suitable crops to maintain the economy of this region
2.5 Data analysis
Data analysis was performed for each land-use type (acacia, rice, cassava, maize, and banana). The Kruskal–Wallis test was performed to check the normality of the data before statistical analysis. A general linear model was subjected to a one-way ANOVA test of the statistical analysis system to compare significant differences among the soil parameters, crop yields, and land-use systems. Significant differences among the groups were tested using Tukey’s test, if the original data followed a normal distribution, or Bonferroni’s method if the original data had a non-normal distribution. The normality test, one-way ANOVA, and multiple comparison analyses were conducted using IBM SPSS 20 software (IBM Corp., Armonk, NY, USA). The selection of suitable crops was evaluated by the Analytic Hierarchy Process (AHP) based on criteria of weather conditions, characteristics of land use types, cropping yield, agricultural farmer characteristics, and soil properties. The higher weighting of each criterion by AHP is indicated the different level in the physical attributes of crop selections.
3 Results and discussion
3.1 Selected soil characteristics
The soil parameters in different agricultural fields in this study, and corresponding statistical analyses, are summarized in Tab.4. The values obtained for most of the listed properties were significant under different land use types. The surface soils from acacia, rice, maize, cassava, and banana fields were, in general, acidic with the averages pH of H2O and KCl being 4.9 and 4.0 respectively. The lower soil pH agricultural plantations might be associated with the uptake of basic cation by the tree roots that enhances the accumulation of Al3+ and H+ (Molla et al., 2022). In addtion, the lower in value of soi pH in the agricultural soils was due to poor managed cultivation. The pH (H2O) value of the rice soil was significantly lower than that of other plantations. The inappropriate use of inorganic fertilizers in rice fields and ongoing cultivation techniques may be to blame for the pH variations. This result agrees with previous studies that reported pH values in the soils were different under different land use types (Muche et al., 2015; Fekad et al., 2020). The exchangeable Al3+ content was relatively high (4.9 cmol kg−1), the coefficient variation exceeding 59% on average; hence soil acidity was considered a serious constraint for agricultural production. The findings demonstrated that these regions had higher exchangeable acidity contents due to intensive farming, excessive precipitation, and the use of inorganic fertilizers. The average soil texture was clay loam to clay, and the particle size distribution differed the various soils. The clay content was higher in the acacia and cassava plantations (40% and 44%, respectively) than that in the rice, maize, and banana plantations (24%, 23%, and 36%, respectively). The highest sand content (56%) was recorded in the rice plantation. These results were probably due to the deposition of alluvial areas along rice fields near streams and rivers. Previous studies have mentioned that non-stop cropping and intensive land use have some effects on the particle size distributions and these changes are related to cultivation period (Nanganoa et al., 2019; Bufebo and Elias, 2020).
Tab.4 Selected soil parameters of agricultural soils in A Luoi District, Thua Thien Hue Province. |
| Items | Bulk density | pHKCl | Clay | Silt | Sand | P2O5 | K2O | OC | N | H+ | Al3+ | CEC | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Acacia | 1.29 | 4.9 | 4.0 | 40 | 33 | 27 | 0.05 | 0.19 | 21.4 | 1.9 | 1.0 | 5.7 | 19 |
| Max | 1.55 | 5.7 | 4.4 | 55 | 46 | 66 | 0.49 | 0.71 | 44.9 | 2.6 | 1.5 | 12.0 | 25 |
| Min | 1.03 | 4.2 | 3.6 | 16 | 18 | 10 | 0.00 | 0.04 | 6.7 | 0.5 | 0.4 | 1.7 | 13 |
| SD | 0.12 | 0.5 | 0.2 | 13 | 8 | 20 | 0.11 | 0.18 | 12.6 | 0.7 | 0.3 | 2.9 | 11 |
| CV (%) | 9 | 10 | 5 | 32 | 24 | 75 | 110 | 95 | 59 | 53 | 30 | 50 | 55 |
| Rice | 1.39 | 4.4 | 3.9 | 24 | 21 | 56 | 0.10 | 0.28 | 14.9 | 1.0 | 1.2 | 3.2 | 21 |
| Max | 1.57 | 5.3 | 4.9 | 44 | 44 | 75 | 0.13 | 0.60 | 16.5 | 1.8 | 2.2 | 4.2 | 34 |
| Min | 1.20 | 3.8 | 3.4 | 17 | 4 | 33 | 0.01 | 0.10 | 3.4 | 0.5 | 0.4 | 1.7 | 13 |
| SD | 0.12 | 0.6 | 0.5 | 8 | 13 | 15 | 0.03 | 0.14 | 4.3 | 0.4 | 0.7 | 0.8 | 7 |
| CV (%) | 9 | 13 | 12 | 34 | 61 | 26 | 60 | 50 | 43 | 36 | 57 | 25 | 33 |
| Cassava | 1.43 | 5.2 | 4.0 | 44 | 37 | 19 | 0.05 | 0.16 | 21.0 | 1.5 | 1.8 | 8.0 | 18 |
| Max | 1.57 | 5.6 | 4.4 | 51 | 53 | 29 | 0.28 | 0.49 | 45.4 | 2.3 | 2.8 | 12.1 | 33 |
| Min | 1.20 | 4.4 | 3.6 | 18 | 23 | 13 | 0.01 | 0.05 | 4.2 | 0.4 | 0.8 | 2.3 | 11 |
| SD | 0.14 | 0.3 | 0.2 | 9 | 7 | 5 | 0.08 | 0.12 | 11.3 | 0.4 | 0.6 | 3.2 | 5 |
| CV (%) | 10 | 6 | 5 | 19 | 19 | 26 | 80 | 63 | 54 | 24 | 33 | 40 | 10 |
| Maize | 1.40 | 4.7 | 4.2 | 23 | 26 | 52 | 0.03 | 0.22 | 13.4 | 1.0 | 1.1 | 3.1 | 18 |
| Max | 1.57 | 5.7 | 5.3 | 40 | 50 | 79 | 0.08 | 0.33 | 16.8 | 2.2 | 2.6 | 5.0 | 31 |
| Min | 1.20 | 3.8 | 3.4 | 18 | 2 | 24 | 0.00 | 0.10 | 9.6 | 0.8 | 0.1 | 2.5 | 3 |
| SD | 0.10 | 0.5 | 0.6 | 6 | 13 | 14 | 0.03 | 0.08 | 2.4 | 2.4 | 0.7 | 1.1 | 10 |
| CV (%) | 7 | 13 | 14 | 24 | 49 | 27 | 100 | 36 | 18 | 195 | 62 | 35 | 57 |
| Banana | 1.410 | 4.9 | 4.0 | 36 | 33 | 31 | 0.05 | 0.19 | 20.2 | 1.4 | 1.0 | 3.4 | 16 |
| Max | 1.70 | 5.3 | 4.8 | 5 | 50 | 70 | 0.33 | 0.33 | 37.0 | 2.5 | 2.1 | 6.0 | 28 |
| Min | 1.20 | 4.6 | 3.6 | 12 | 18 | 12 | 0.00 | 0.06 | 11.1 | 0.5 | 0.4 | 1.2 | 11 |
| SD | 0.15 | 0.2 | 0.4 | 15 | 9 | 20 | 0.07 | 0.09 | 7.5 | 0.5 | 0.4 | 1.3 | 6 |
| CV (%) | 11 | 5 | 9 | 40 | 26 | 63 | 140 | 47 | 37 | 39 | 36 | 38 | 36 |
| Average | 1.38 | 4.9 | 4.0 | 35 | 31 | 34 | 0.05 | 0.20 | 18.2 | 1.4 | 1.2 | 4.9 | 18 |
| Max | 1.70 | 5.7 | 5.3 | 55 | 53 | 79 | 0.49 | 0.71 | 45.4 | 2.6 | 2.8 | 12.1 | 31 |
| Min | 1.03 | 3.8 | 3.4 | 12 | 2 | 10 | 0.00 | 0.04 | 3.4 | 0.4 | 0.1 | 1.2 | 9 |
| SD | 0.14 | 0.5 | 0.4 | 14 | 11 | 21 | 0.08 | 0.13 | 10.0 | 0.6 | 0.6 | 2.9 | 10 |
| CV (%) | 10 | 10 | 9 | 39 | 35 | 61 | 160 | 65 | 55 | 43 | 50 | 59 | 54 |
Unit: BD: g cm−3; soil texture, P2O5 and K2O: %; OC and N: g kg−1: CEC, Al3+ and H+: cmol kg−1. |
The land-use types had a significantly greater impact on bulk density value (P ≤ 0.05), as shown in Tab.4. Acacia plantation had low mean value of bulk density (1.29 g cm−3), and high mean value was observed in the rice, maize, cassava, and banana plantations (1.4 g cm−3). This was probably due to the intensive tillage cultivation that would have possibly caused better bulk density values in the rice, maize, cassava, and banana plantations than in the acacia plantation. The lower BD was probably related to high inputs of OM and plant residues from acacia litter biomass and low impacts of rain droplets on the surface soil layer. Whereas, higher BD in the soils could be related to tramping effect of animals during plowing and after harvest. Furthermore, the increasing exposure of the soil to direct temperature and raindrop effects can be attributed to the increasing BD. Previous studies have provided evidence that the soil nutrients and related soil processes, such as leaching, oxidation, erosive oxidation, and mineralization, are impacted by land use and soil management practices (Lal, 2018; Ghosh, 2019). In addition, the vegetative biomass is a factor that impacts the soil organic carbon content in acacia-cultivated land as reported by Hung et al. (2017). After cultivating untilled soils, soil quality typically deteriorates due to significant alteration caused by land use change (Wang et al., 2001). The cause of the relatively high bulk density of soil in the rice, maize, and banana plantations could be due to the low clay content (Tab.4). This was comparable to earlier studies relating influences land luse types and land management on soil properties, organic carbon, and bulk density (Haghighi et al., 2010). Other previous studies also found that the highest and lowest BD were in the cultivated and forest lands, respectively (Bewket and Stroosnijder, 2003).
Soil organic matter (OM) content was significantly affected by the land-use types (Tab.4). OM variability was significantly higher in acacia and cassava (21 g kg−1), which was due to the high clay content in the acacia- and cassava-cultivated soils (Fig.2). In addition, the high productivity and leguminous properties of acacia plantations may be linked to the recovery of soil nutrients and the speeding up of nutrient cycling in soils (Dong et al., 2014; Hung et al., 2017). Another reason was that the higher OM could be due to the addition of plant litter to the soil and reducing the rate of risk of erosion. However, the lower OM from rice and maize soils could be the removal of crop residues for energy and animal feed and soil disturbances during plowing and harvesting that expose the soils. The results were similar to previous studies that have reported a lesser amount of organic carbon in cultivated soils than in grass soil (Clemens et al., 2010; Anh et al., 2014). TN was also significantly affected by land-use type and land managment. The mean TN ranged from low (1.0 g kg−1) in rice soils to high (1.9 g kg−1) in acacia soils (Tab.4). The high TN in acacia soil may be associated with the capacity of the species to restoration atmospheric N through symbiosis with root-nodulating bacteria (Giller, 1993; Hung et al., 2017). The result of this study was in agreement with previous studies reported that TN was affected by different land-use systems (Trakooyingcharoen et al., 2012; Sang et al., 2013).
Our results showed soil characteristics similar to those of previous studies. The soils were thought to have low in OM and TN contents as well as low soil pH. The findings demonstrated that acacia soils had relatively higher levels of organic carbon and nitrogen than soils from other types of land use. The total nitrogen content found in this study was comparable to that found in earlier studies conducted in Vietnam (Tab.4). Our results showed that the acacia soils had a higher level of TN than that of other soils. Acacia plants have been shown to be able to fix atmospheric nitrogen through symbiosis with root-nodulating bacteria, which has been linked to the high concentration of TN in acacia soils (Giller, 1993; Brockwell et al., 2005). In addition, TN can also be better in acacia soils because of the excessive nitrogen content in the litter and root biomass (Brockwell et al., 2005; Selassie et al., 2015). The results also showed that the organic carbon content in acacia soil was relatively higher than that in rice and maize soils but did not significantly differ from those in cassava and banana soils (Tab.4). The greater carbon content in acacia plantations has been connected to the retention of all residues at harvest (Hardiyanto and Nambiar, 2014; Bich et al., 2018), and the extra C leached as dissolved organic C to mineral soils through litter decomposition (Novara et al., 2015). However, the value of C in acacia soil was not significantly different from that of cassava and banana soils, mainly because of similar clay content (Tab.4).
TP was noticeably higher in the rice soil compared to the other soils (Tab.4). The excessive TP in the soil of rice fields (0.10%) was because of the non-stop application of mineral P fertilizer for some years as indicated with the aid of using exclusive farmers in the area. However, acacia and maize soils had no application of P fertilizer, as indicated by the low total P content. Previous studies have reported that the phosphorus content of soils was affected by land-use type (Liu et al., 2006; Deng et al., 2018; Liu et al., 2018). Exchangeable K+ content was significantly higher in the rice soil than in the other soils. The high exchangeable K+ content in rice soil was possibly because of non-stop cultivation and inorganic farming practices. The cultivation using inorganic fertilizer with forming K+ could be strongly affected the distribution of K in the soil system. The CEC values of the soil samples did not differ under different land use types. CEC values were in the range of 16 cmol kg−1 in banana soil to 21 cmol kg−1 in rice soil. Previous works have reported the depletion of exchangeable bases due to non-stop intensive cultivation and the application of acid-forming inorganic fertilizers, which decreased the CEC in cultivated land (Muche et al., 2015; Molla et al., 2022).
Tab.5 shows the carbon content of the first 10 cm under for various land-use types. In the agricultural plantation, intensive cultivation techniques and absence of an organic layer led to a generally low C and N concentration. In terms of C storage under different land-use types, C content was higher in the acacia, banana, and cassava plantations than in the rice and maize plantations. These results were similar to the most previous ideas on how soil C content was affected by soil cultivation (Jacinthe and Lal, 2005; Wu et al., 2022). It could be examined that the rice field, which more intensively affected the flooding of the A-Sap river, may have caused a richer sand content texture (Tab.4). Due to the adsorption of organic compounds at clay surfaces, these conditions were harmful for the rich inorganic matter protection (Kalita et al., 2016; Nath et al., 2018). The majority of the carbon was recovered in the heavy mineral fraction, indicating a significant interaction with the mineral phase and a possibly higher stabilization of the carbon. The lower C content in the rice and maize fields, whose texture was sandier than that of the other fields (Tab.4), provides additional support for this. Another cause was erosion that occurred at maize fields in this region under heavy rain conditions (Wezel et al., 2002). The C:N ratio in acacia soil was significantly lower than that of other soils. This was possibly due to the loss of nitrogen from the agricultural systems. Our result was similar to previous studies that reported a decreased level of soil N in the rice, maize, and cassava fields after shifting cultivation from fallow land (Vien, 2003; De et al., 2008).
Tab.5 Carbon stocks under different land use types in A Luoi District, Thua Thien Hue Province. |
| No | Land use type | C:N | Stocks of carbon (Mg ha−1) |
|---|---|---|---|
| 1 | Acacia (21) | 11.76 | 31 ± 17A |
| 2 | Rice (10) | 14.30 | 14 ± 7B |
| 3 | Cassava (17) | 14.46 | 28 ± 17A |
| 4 | Maize (14) | 14.71 | 19 ± 3AB |
| 5 | Banana (17) | 14.44 | 29 ± 12A |
3.2 Cropping systems, yield, and suitable crops in A Luoi District
Agricultural land in mountainous regions may be used for cash crops such as acacia, cassava, banana, rice, and maize. The different land-use types, yield, and its contributed cash values in A Luoi District are listed in Tab.6 and Tab.7. The coefficient of variation was mostly below 50%, which indicates small variability among the samples for each land-use type in A Luoi District.
Tab.6 The yield of crops in A Luoi District, Thua Thien Hue Province. |
| Items | Acacia (n=151) | Rice (n = 87) | Maize (n = 62) | Cassava (n = 124) | Banana (n = 40) |
|---|---|---|---|---|---|
| Maximum (Mg ha−1) | 143 | 5 | 5 | 16 | 58 |
| Minimum (Mg ha−1) | 42 | 2 | 3 | 6 | 15 |
| Average (Mg ha−1) | 92 | 4 | 5 | 12 | 39 |
| Standard deviation (Mg ha−1) | 43 | 0.6 | 0.7 | 2.4 | 12 |
| CV (%) | 47 | 16 | 14 | 21 | 29 |
Tab.7 Cropping system distribution in A Luoi District, Thua Thien Hue Province |
| No | Crops | n | Percentage (%) | AHP weighting | Rank | |
|---|---|---|---|---|---|---|
| Hectare* | Household interview | |||||
| 1 | Acacia | 151 | 94a | 33a | 0.32 | 1 |
| 2 | Cassava | 124 | 1.9b | 27a | 0.07 | 4 |
| 3 | Rice | 87 | 1.6b | 19ab | 0.31 | 2 |
| 4 | Maize | 62 | 0.7b | 13b | 0.05 | 5 |
| 5 | Banana | 40 | 1.9b | 8b | 0.19 | 3 |
* Value from household interviewed. |
The choice of suitable crops and cropping systems are often very delicate, as shown in Tab.6 and Tab.7. These selections were based on expectation of farmers, local government and expertise supporting. Acacia was the most interesting crop, with a size area and number of households of 94% and 33%, respectively. Acacia yield was calculated to be approximately 93 Mg ha−1 after five years of cultivation. Acacia was frequently cited as a plant species that can grow easily in this area as a cash crop. Cassava crop accounted for approximately 27% of farmers who participated in this survey, and its yield value varied from 6 to 16 Mg ha−1. However, rice, maize, and banana still dominated this area. Yield of rice crop was from 2 to 5 Mg ha−1, with an average of 4 Mg ha−1. Average yield of maize was approximately 5 Mg ha−1 (minimum of 3 Mg ha−1 and maximum of 5 Mg ha−1). A total size area of banana was smaller compared to other land use types because farmers have cultivated over the last three years and, it was accounted for approximately 9%. A minimum of banana yield was 15 Mg ha−1 while maximum yield was 58 Mg ha−1. In comparison, the crop yield in A Luoi was relatively low compared to other regions because of the natural and social conditions (Dong et al., 2014; Hedlund et al., 2004; Müller, 2003; Hai and Hung, 2018; Bich et al., 2018). There were many things to take into account, including the number of years of farming experience, education level, number of visits by extension agents, farm size, dependence ratio, region, production knowledge, and household characteristics (age, religion, size, and gender of household head), and level of specialization (whether a farmer farmins full- or part-time).
4 Conclusion
Our findings imply that the rice and maize had sandy soils, whereas the acacia and cassava had clay to loam soils. In comparison to the other soils, the acacia soil’s mean bulk density value was significantly higher. TN were higher in the acacia soils than those in the rice, maize, and banana soils. Carbon content was significantly higher in the acacia, cassava, and banana soils than that in the rice and maize soils. The mean of exchangeable K+ of the soil was significantly higher in rice soils and was affected by land-use type. The exchangeable acidity content was high in the soil probably because of intensive precipitation. However, the CEC value was not affected by land-use types or land management. The selected properties of this study are similar to those of previous studies. The most suitable crops in A Luoi District could be acacia, cassava, banana, and rice plantations, along with their various cropping system and relative yield. Overall, the development of agriculture with careful management of soil properties is necessary in the mountainous regions of Vietnam.