A Preliminary Study on the Determination of the Fertilization Tolerance of an Entisol in the Yuanmou Dry-Hot River Valley Based on Soil Qualities in Plot Scale

Abstract

Using field slope farmland plots, this study planted the typical crop of maize (Zea mays L.) and investigated the effects of varied chemical fertilizer (organic compound fertilizer of potassium nitrate, containing 17% each of nitrogen, phosphorus, and potassium) application levels (0.5 times the common fertilizer amount (CK, 0.75 t·hm⁻² to 2.5 CK) on the soil fertility in the Yuanmou dry-hot River Valley. The results showed that the soil chemical properties, microbial properties, and enzyme activities increased with the increase of fertilizer application levels from CK to 2.0 CK. However, a declining trend showed both under 0.5 CK level and the 2.5 CK level, and higher in fertilizer application level 3 (1.5 CK) and level 4 (2.0 CK) compared to level 1 (1 CK). Soil chemical properties, microbial properties, and enzyme activities in five-degree slope cropland topsoil were higher than these in 10-degree slope cropland topsoil. Five parameters (available N, nitrifying bacteria, inorganic phosphorus bacteria, organic matter, and invertase) in five-degree slope cropland and three parameters (organic matter, ammonifying bacteria, and total P) in 10-degree slope cropland, which had the greatest weight in the principal components analysis, were selected to calculate the soil quality index (SQI). The SQI calculated by integrating all critical parameters indicated that the highest SQI values were found in fertilizer levels 1.5 CK (0.71) and 2.0 CK (0.69), followed by CK (0.64), and the lowest were found in 0.5 CK (0.62) and 2.5 CK (0.61) in five-degree slope cropland soil. The highest SQI values were found in fertilizer levels 1.5 CK (0.26) and 2.0 CK (0.29), followed by CK (0.23), and the lowest were found in 0.5 CK (0.14) and 2.5 CK (0.20) in 10-degree slope cropland soil. The final SQI values implied that the fertilization treatment 2 (CK), fertilization treatment 3 (1.5 CK) and fertilization treatment 4 (2.0 CK) could improve the soil fertility, whereas the fertilization treatment 1 (0.5 CK) and fertilization treatment 5 (2.5 CK) could decrease the soil fertility. In view of the impact of slope, the soil qualities of five-degree slope cropland of five fertilization treatments were higher than in 10-degree slope cropland. The SQI values in five-degree slope cropland soil were found higher than the SQI values in 10-degree slope cropland soil by 68.65%, 64.20%, 62.22%, 57.46%, and 67.01%, respectively. For this study, the range of fertilization tolerance was 0.75–1.50 t·hm⁻² (organic compound fertilizer of potassium nitrate) in 10-degree slope plot scale and 0.75–1.13 t∙hm⁻² in five-degree slope cropland soil.

1. Introduction

The deterioration of soil quality caused by imbalance in the use of chemical fertilizers is a serious problem all over the world [1]. Common fertilization not only can improve crop yields and quality but also can help to improve the soil fertility and the nutrient absorption efficiency, keeping the soil and water free from pollution [2]. Trends and effects in soil fertility of different soil types have been reported in many long-term and short-time fertilizer experiments [3,4,5]. Yuanmou (E 101°35′~102°06′, N 25°23′~26°06′) is located in a dry-hot river valley of the Jinsha River, which is the upper reach of the Yangtse River. Purple soil prevails in the upper reaches of the Yangtze River [6], which is one of the most important soils for agricultural production in Yuanmou. The purple soil (Entisols in the U.S. Soil Taxonomy and Regosols in the FAO soil classification) has great significance of crop planting due to many nutritive elements contained in the soil and high natural fertility. The trends and changes of soil chemical properties, microorganisms, and enzymes in purple soil by fertilizing have been studied by previous scholars [7,8]. Understanding the influence of the different amount of fertilizer application on soil is the key to maintaining both high crop yield and soil and water environmental security. In order to inform fertilization practice objectively, there is an urgent need to comprehensively assess the effects of different amount of fertilizer on soil quality in Yuanmou dry-hot river valley.

The physical, chemical, and biological characteristics of soil have been widely used to evaluate soil quality [9,10,11]. At present, the key indicators of the soil physicochemical parameters, such as soil organic matter, available N, and water holding capacity, which are generally used to evaluate soil quality, change very slowly [12]. Unlike soil physical and chemical properties, soil biological characteristics have the characteristics of their rapid response, high sensitivity, and capacity to provide information that integrates many environmental factors. Therefore, the soil biological characteristics are increasingly used as indicators of soil quality [13]. By combining previous research, applied microbial biomass and soil enzyme activities can evaluate the effects of organic and inorganic fertilization on the microbiological status of different soil types and climates [14]. Soil quality must be assessed by a variety of properties. Since the soil microbial biomass and soil enzyme activities integrate physical and chemical soil properties, the soil biological properties have been considered to be important indicators of soil quality. Modern studies have argued that the correct assessment of soil quality must include both physical, chemical, and biological indicators of soil [10,15].

Herein, fertilization tolerance (FT) is the amount of fertilizer applied to the soil that maintains both high crop yield and soil and water environmental security. That is to say, it is the maximum amount of fertilization that ensures the soil not only has high productivity but also high fertility and secure runoff that would occur latter. Based on this definition, our objective in this study is to determine the fertilization tolerance for purple soil by investigation of respective effects of fertilization on soil quality. Soil quality assessment based on the method of indices has been successfully applied in regional scale and on-farm level [16,17]. It is important to build a simple, sensitive, and workable indicator method for soil-quality evaluation. The method of soil-quality index adopted by Karlen et al. [18], Andrew and Carroll [19], and Bastida et al. [20] is to score and weigh different soil indicators to form an index or multiple linear regression and calculate by linear combination of other indicators [21,22,23]. Zhang et al. introduced this approach into their evaluation of the impact of revegetation types on soil quality on the Loess Plateau, China [24]. Masto et al. established a soil-quality index (SQI) with the aim to evaluate and quantify the long term effect of different fertilizer and farm yard manure treatments in a rotation system with Z.mays, Pennisetum americanun, Triticum aestivum, and Vigna unguiculata in New Delhi [18]. Peng et al. [25] introduced the approach used by Zhang et al. into their evaluation of the effects of vegetation types on soil quality in the Yuanmou dry-hot river valley, China [23]. However, there is little knowledge available concerning the soil quality assessment affected by fertilization by a comprehensive index in the Yuanmou dry-hot river valley.

This paper compared the soil quality with five fertilization treatments in the plot scale in the Yuanmou dry-hot river valley. The objectives of this study are (a) to assess changes in soil properties, including physical, chemical, microbial, and enzyme parameters after the maize harvested compared to the basic properties of soil before fertilization under different degree slope croplands; (b) to determine a minimum set of soil properties for an SQI which could assess soil quality after applying different amount of fertilization; and (c) to determine which fertilization treatment has the most suitable soil fertility for planting maize in the purple soil regions of degree slope croplands.

2. Materials and Methods

2.1. Study Area

The selected study site is located in the Yuanmou dry-hot river valley (E 101°35′–102°06′, N 25°23′–26°06′) located in Yunnan province, China. This study location is situated in the lower reaches of the Jinsha River, an upper tributary of the Yangtze (Figure 1). This area is characterized by a semi-arid climate, with a mean annual temperature of 21.5 °C and an average annual rainfall of 600 to 800 mm, mostly falling in the period from May to September. The main crops are irrigated rice (Oryza sativa L.) and dry-land maize (Zea mays L.), and the main soils are classified as purple soils (Entisols according to U.S. Soil Taxonomy and Regosols in the FAO soil classification system) and dry-red soils (Chromic cambisol in FAO taxonomy, Ustochrept in US taxonomy), both of which are vulnerable to water erosion. Our testing blocks are located in the town of Zuolin in Yuanmou County, at the Research Station for Gully Erosion and Collapse.

2.2. Field Experiment and Measurements

The experimental plots were designed on the land with 5° and 10° slope which were employed, respectively, and the area of each plot was 25 m² (10 m length × 2.5 m width). Plot boundary walls of height 60 cm (i.e., the soil thickness) surrounding purple soil were constructed using bricks, with a runoff collection system including surface runoff and leaching constructed at the end of every plot in 2010 [26] (Figure 2).

The chemical, microbial, and enzyme properties of the experimental purple topsoil before test fertilization treatment were as presented in

Table 1. Basic properties of the purple soil before fertilization.

Soil Properties	Value	Soil Properties	Value
Total N (g∙kg−1)	0.47 ± 0.02	Ammonifying bacteria (105 cfu ∙ g−1)	10.13 ± 3.18
Total P (g∙kg−1)	0.30 ± 0.01	Nitrifying bacteria (104 cfu ∙ g−1)	6.80 ± 1.35
Available N (mg∙ kg−1)	31.50 ± 2.12	Organic phosphorus bacteria (103 cfu ∙ g−1)	33.95 ± 10.01
Available P (mg∙ kg−1)	1.66 ± 0.02	Inorganic phosphorus bacteria (103 cfu ∙ g−1)	22.47 ± 8.33
Organic matter (g∙ kg−1)	9.45 ± 0.99	Denitrifying bacteria (104 cfu ∙ g−1)	30.70 ± 5.23
Bulk density (g∙cm−3)	1.46 ± 0.22	Invertase (mg Glucose ∙ g−1)	2.03 ± 0.10
pH value	8.12 ± 0.03	Alkaline phosphatase (mg phenol ∙ g−1)	0.23 ± 0.01
Urease (mg NH4+-N ∙ g−1)	1.85 ± 0.26

.

The plots were planted with a monoculture of maize with a density of six plants m⁻², seeding in late April and harvesting in late September 2011, and the aggrotechnical treatments were the same for all plots. Five fertilization treatments were applied with three replications: the common fertilization amount of 750 kg∙ha⁻¹ (CK) as the basic amount of fertilizer, as well as 0.5 CK, 1.5 CK, 2 CK, and 2.5 CK. The applied fertilizer was an organic potassium nitrate compound fertilizer, containing 17.0% each of N, P₂O₅, and K₂O, which is a common used fertilizer in the region. Before sowing in April, all base fertilizers were evenly broadcast onto the soil surface by hand and ploughed into the soil immediately, and no fertilizer was applied into the soil during the whole growth period.

Soil within the plots was sampled after the harvest 2011. Soil samples were collected from the top 15 cm soil depth with a 5 cm diameter stainless-steel corer. Five soil samples were collected along an “S” line at each fertilization level of each plot and mixed to form a composite sample after removing roots, stones, and debris. Each mixed sample was divided into two parts. One part was immediately sieved through a 2 mm mesh and then stored at 4 °C until the soil microbial characteristics were analyzed; the other part was air-dried to analyze soil chemical characteristics.

2.3. Laboratory Measurements

The basic chemical and representative physical properties of the composite soil samples were determined using standard methods. The soil’s pH value was determined with a pH meter at a soil/water ratio of 1:2.5 (soil:water, weight/volume, air-dried soil) suspensions [26]. The soil’s organic matter was measured by titration after digestion with K₂Cr₂O₇-H₂SO₄ solution, and total nitrogen (TN) by the Kjeldahl method (digested by H₂SO₄ and CuSO₄–Na₂SO₄ as the catalytic agent) [26]. The soil’s total phosphorus (TP) was measured by the colorimetric method using ammonium molybdate in hydrochloric acid [26]. The soil’s available nitrogen (AN) was determined using a microdiffusion technique after alkaline hydrolysis [26]. The soil’s available phosphorus (AP) was determined by the Olsen method [26].

The soil’s enzyme activities were measured according to the methods described by Lin [26]. The soil’s enzyme activities were measured according to the methods described by Lin [26]. Urease activity was determined using 5 g of fresh soil (passed through a 1 mm sieve), 5 mL of citrate solution at pH 6.7, and 5 mL of 10% urea solution. Results are expressed as mg NH₄⁺–N∙g⁻¹. Alkaline phosphatase activity was determined using 2 mL of toluene, 10 g of fresh soil, 10 mL of disodium phenyl phosphate solution, and 10 mL of 0.05 buffer at pH 9.6. Results are expressed as mg phenol∙g⁻¹∙h⁻¹. Invertase activity was measured by the Hoffmann–Seegerer method [26]. In a 100 mL flask, 10 g soil was combined with 10 mL 20% sucrose solution in pH 5.5 buffer and shaken vigorously at 37 °C for 24 h. The content of sucrose was measured using a starch indicator following the addition of 0.2 mol∙L⁻¹ Na₂SO₄.

The numbers of cultivable nitrifying bacteria, ammonifying bacteria, nitrite oxidizing bacteria (NOB), denitrifying bacteria, and organic phosphorus bacteria in fresh soil were determined as described by Lin [26]. The numbers of nitrifying bacteria were cultured in a culture medium for nitrite-oxidizing bacteria. The culture medium was filtered by filter paper, and then 5 mL culture medium was taken into the test tube and sterilized for 30 min at 121 °C. Five diluted concentrations of the soil suspensions (10⁻⁶–10⁻²) were selected to inoculate every four test tubes with 1 mL. The soil suspensions were cultured for 14 d at 28 °C. Results are expressed as 10⁴ cfu∙g⁻¹.

Populations of denitrifying bacteria were estimated by the method of Lin [26]. The denitrifying bacterium was cultured in a culture medium for denitrifying bacteria. Nine mL culture medium was taken into the same test tube and put into Du fermentation tube inverted. Nine mL culture medium was sterilized for 30 min at 121 °C. Five diluted concentrations of the soil suspensions (10⁻⁷–10⁻³) were selected to inoculate every four test tubes with 1 mL. The soil suspensions were cultured for 14 d at 28 °C. Results are expressed as 10⁴ cfu∙g⁻¹.

The ammonification bacteria were cultured in a peptone culture medium. 5 mL culture medium was taken into the same test tube and sterilized for 30 min at 121 °C. Four diluted concentrations of the soil suspensions (10⁻⁹–10⁻⁶) were selected to inoculate every four test tubes with 1 mL. The soil suspensions were cultured for 3 or 5 d at 28 °C. Results are expressed as 10⁶ cfu∙g⁻¹.

The organic phosphorus bacteria were cultured in Monkina organic phosphorus culture medium for organic phosphorus bacteria. A 5 mL culture medium was taken into the same test tube and sterilized for 30 min at 121 °C. Four diluted concentrations of the soil suspensions (10⁻⁷–10⁻⁴4) wereselected to inoculate every four test tubes with 1 mL. The soil suspensions were cultured for 5 d at 28 °C. Two mL of 40 g∙L⁻¹ ammonium molybdate reagent was added into the soil suspensions and then mixed at 100 °C for 2 min. After two minutes, the soil suspensions were taken out. Results are expressed as 10³ cfu∙g⁻¹.

The inorganic phosphorus bacteria were cultured in tricalcium phosphate inorganic phosphorus culture medium for inorganic phosphorus bacteria. Three diluted concentrations of the soil suspensions (10⁻⁶–10⁻⁴) were selected to inoculate every test tube with 1 mL. The soil suspensions were cultured for 7 d at 28 °C. Calculate the total number of colonies and bacterial colonies with transparent circle. Results are expressed as 10³ cfu∙g⁻¹.

2.4. Soil Quality Index (SQI)

SQI was calculated using the method reported by Andrews et al. [27] and Bastida et al. [20]. This method involves the following steps: (i) selection of appropriate parameters; (ii) transformation and weighting of parameters; and (iii) combining the scores into an index. Principal component analysis (PCA) was employed in the choice of appropriate parameters and their weighting. Within each PC, the highest weighted variable is selected for indexing, along with variables having an absolute value within 10% of the highest weighted variable. If more than one variable was retained under a single PC, multivariate correlation was employed to determine if the variables could be considered redundant and, therefore, eliminated from the SQI. If the highly loaded factors were not correlated, then all the highly loaded factors were used to calculate the SQI value. On the contrary, if the highly loaded factors were well correlated, then the variable with the highest factor loading (absolute value) was used to calculate the SQI value. We then summed up the weighted SQI variables scores for each observation using the following equation:

$$\mathrm{SQI}={\displaystyle \sum _{i=1}^n{W}_i{Y}_i}$$

(1)

Where W_i is the PCA weighing factor and Y_i is the indicator score. The equation was finally normalized to get a maximum SQI of one. The following Equations (2) and (3) are used to calculate the value of W_i and Y_i.

$$W_i=C_i/({\displaystyle \sum _{i=1}^n{C}_i})$$

(2)

Where C_i is the cumulative percentage of weight coefficients corresponding to the evaluation index for the evaluation of the main branch of the corresponding number. The C_i is obtained from the results of the PCA that analyzed all the values of the soil parameters (soil chemical properties, microbial quantity, and enzyme activity).

$$Y=\frac{a}{1+{(x/x_0)}^b}$$

(3)

Where x is the value of the soil parameter (soil chemical properties, microbial quantity, and enzyme activity), x₀ is the mean value of the soil parameter, a is the maximum value (in our case, a = 1), and b is the slope values of the equation. In the PCA, the factor loadings of the parameters were ranked as “+” (more is better) or “−” (less is better) depending on whether a higher value was considered “good” or “bad” in terms of soil function. We obtain curves that fit a sigmoid tending to 1 for all the proposed parameters, −2.5 and 2.5 were used as the b values for “+” and “−”, respectively.

2.5. Statistical Analysis

All statistical analyses were carried out using the analysis of variance (ANOVA) which was conducted to investigate comparisons of soil chemical properties, soil microbial quantity, and soil enzyme activities between the various fertilization treatments on 5° and 10° slope farmland, respectively. Least significant differences (LSD) at p = 0.05 were tested to identify any significant differences between treatments.

3. Results

3.1. Soil Chemical Properties

Table 2. Soil chemical properties of different fertilization treatments.

Slope	Treatments	Total N (g∙kg−1)	Total P (g∙kg−1)	Available N (mg∙kg−1)	Available P (mg∙kg−1)	pH	Organic Matter (g∙kg−1)
5°	0.5 CK(T1)	0.64 ± 0.02 cd	0.34 ± 0.02 c	104.74 ± 4.41 c	17.21 ± 2.65 d	7.75 ± 0.01 a	10.78 ± 0.12 c
	1.0 CK(T2)	0.73 ± 0.06 c	0.46 ± 0.02 bc	135.55 ± 5.26 b	26.54 ± 5.57 c	7.33 ± 0.03 ab	12.49 ± 0.43 bc
	1.5 CK(T3)	0.87 ± 0.01 a	0.64 ± 0.01 a	164.07 ± 4.25 a	33.22 ± 4.09 a	7.19 ± 0.03 ab	12.78 ± 0.25 b
	2.0 CK(T4)	0.81 ± 0.02 ab	0.51 ± 0.02 b	143.67 ± 10.50 b	29.56 ± 6.89 b	7.06 ± 0.05 ab	14.06 ± 1.17 ab
	2.5 CK(T5)	0.76 ± 0.02 bc	0.45 ± 0.01 bc	110.25 ± 4.03 c	19.03 ± 3.94 d	7.01 ± 0.05 ab	15.11 ± 0.17 a
10°	0.5 CK(T1)	0.41 ± 0.01 c	0.45 ± 0.01 c	97.33 ± 2.38 b	26.03 ± 0.29 c	7.95 ± 0.05 a	9.75 ± 0.60 c
	1.0 CK(T2)	0.49 ± 0.04 b	0.73 ± 0.02 b	128.84 ± 5.26 b	47.80 ± 0.58 b	7.43 ± 0.03 ab	10.46 ± 0.34 c
	1.5 CK(T3)	0.55 ± 0.05 a	0.74 ± 0.01 b	133.67 ± 16.34 ab	49.21 ± 0.70 b	7.33 ± 0.03 ab	11.99 ± 1.53 c
	2.0 CK(T4)	0.55 ± 0.02 a	0.82 ± 0.03 a	149.24 ± 3.34 a	57.39 ± 0.38 a	7.25 ± 0.20 ab	13.39 ± 0.40 b
	2.5 CK(T5)	0.44 ± 0.01 c	0.72 ± 0.03 b	107.67 ± 2.75 b	37.70 ± 0.39 c	7.14 ± 0.01 ab	14.75 ± 0.23 a

Note: values with the same letter are not significantly different at the p < 0.05 level.

shows the effects of the different fertilization treatments on total N (TN), total P (TP), available N (AN), available P (AP), pH, and organic matter (OM) in 5- and 10-degree-slope croplands topsoil. In the five-degree-slope cropland topsoil, the highest TN, TP, AN, and AP were found in the topsoil of 1.5 CK, followed by 2.0 CK, CK and 2.5 CK, whereas the lowest was in the topsoil of 0.5 CK. In the 10-degree-slope cropland topsoil, the highest TN, TP, AN, and AP were found in the topsoil of 2.0 CK, followed by 1.5 CK, CK and 2.5 CK, whereas the lowest was in the topsoil of 0.5 CK. Both in 5- and 10-degree slope croplands topsoil, the highest OM and pH were found in the topsoil of 2.5 CK and 0.5 CK, respectively.

Soil pH was lower at higher fertilization treatment in 5- and 10-degree-slope croplands topsoil. However, differences were not significant at different fertilization treatments (Table 2). Generally, the pH values in five-degree-slope cropland topsoil showed the lower values than in 10-degree-slope cropland topsoil (on the order of 0.20, 0.10, 0.14, 0.19, and 0.13 units in five-degree-slope land topsoil lower than in 10-degree-slope land topsoil from 0.5 CK to 2.5 CK, respectively). Compared to basic pH of the purple topsoil before fertilization (8.12 ± 0.03, Table 1), the pH value reduced within the range of 0.37–1.11 and 0.17–0.98 both in 5- and 10-degree-slope croplands topsoil from 0.5 CK to 2.5 CK, respectively.

The soil organic matter (OM) significantly increased from 0.5 CK to 2.5 CK. The highest OM values were detected in the topsoil of 2.5 CK both in 5- and 10-degree-slope croplands (Table 2). OM contents in five-degree-slope cropland topsoil were higher than that in 10-degree-slope cropland topsoil by 37.42%, 40.29%, 37.50%, 26.06%, and 2.40% from 0.5 CK to 2.5 CK, respectively. Compared to basic OM of the purple topsoil before fertilization (9.45 g∙kg⁻¹, Table 1), the concentrations of OM in five-degree-slope cropland topsoil increased from 1.33 g∙kg⁻¹ to 5.66 g∙kg⁻¹, and the concentrations of OM in 10-degree-slope cropland topsoil increased from 0.30 g∙kg⁻¹ to 5.30 g∙kg⁻¹ from 0.5 CK to 2.5 CK.

Available N (AN) and P (AP) contents in the topsoil reflected the level of available nutrients in the topsoil under different amendments with organic compound fertilizer of potassium nitrate. Compared to basic available N of the purple topsoil before fertilization (31.50 mg∙kg⁻¹, Table 1), the fertilizing significantly increased available N content from 73.24 to 132.57 mg∙kg⁻¹ in five-degree-slope cropland topsoil, and 65.83 to 117.74 mg∙kg⁻¹ in 10-degree-slope cropland topsoil, respectively (Table 2). The contents of available P increased from 15.55 to 31.56 mg∙kg⁻¹ in five-degree-slope cropland topsoil, and 24.37 to 55.73 mg∙kg⁻¹ in 10-degree-slope cropland topsoil after fertilizing, respectively. The AN and AP contents increased under 1.5 CK and 2.0 CK as compared to control CK. For 2.5 CK and 0.5 CK, a significant reduction in AN and AP contents were observed. Available N in five-degree-slope cropland topsoil were higher than that in 10-degree-slope cropland topsoil from 4.95% to 9.04% with 0.5 CK to 2.5 CK, but available P in five-degree-slope cropland topsoil were lower than that in 10-degree-slope cropland topsoil from 21.59% to 61.33%.

In this short-time field test, the usage of fertilizer triggered a significant TN and TP increase. The highest TN levels occurred in fertilization 1.5 CK in five-degree-slope cropland topsoil (0.87 g∙kg⁻¹) and 10-degree-slope cropland topsoil (0.55 g∙kg⁻¹), respectively (Table 2). Soils with a low dose of fertilizer (0.5 CK) and a high dose of fertilizer (2.5 CK) showed total P contents in a range from 0.34 to 0.64 g∙kg⁻¹ and 0.45 to 0.82 g∙kg⁻¹ in 5- and 10-degree-slope croplands soil, respectively (Table 2). The concentration of TN in five-degree-slope cropland topsoil were higher than in 10-degree-slope cropland topsoil within the range of 57.72% to 67.70% from 0.5 CK to 2.5 CK. Total P contents in 10-degree-slope cropland topsoil were higher than those in five-degree-slope cropland topsoil 23.09%, 36.14%, 21.10%, 30.35%, and 37.66% from 0.5 CK to 2.5 CK, (Table 2). The contents of TN increased 0.17−0.40 and −0.03–0.08 g∙kg⁻¹ in 5- and 10-degree-slope croplands topsoil from 0.5 CK to 2.5 CK compared to basic TN of the purple topsoil before fertilization (0.47 g∙kg⁻¹, Table 1), respectively. The contents of TP increased 0.04−0.34 and 0.15–0.52 g∙kg⁻¹ in 5- and 10-degree-slope croplands topsoil from 0.5 CK to 2.5 CK compared to basic TP of the purple topsoil before fertilization (0.30 g∙kg⁻¹, Table 1), respectively.

3.2. Soil Microbial Quantity

Changes in soil microbial quantities (ammonifying bacteria, nitrifying bacteria, denitrifying bacteria, organic phosphorus bacteria and inorganic phosphorus bacteria were shown in Figure 3a–c. In the five-degree-slope cropland topsoil, the highest quantities of ammonifying bacteria, nitrifying bacteria, denitrifying bacteria, organic phosphorus bacteria, and inorganic phosphorus bacteria were found in the topsoil of T3 (1.5 CK), while the highest quantities of all bacteria were found in the topsoil of T4 (2.0 CK) in 10-degree-slope cropland soil (Figure 3a–c).

After fertilization, the populations of ammonifying bacteria, nitrifying bacteria, denitrifying bacteria, organic phosphorus bacteria, and inorganic phosphorus bacteria in different slope croplands topsoil increased significantly. The populations of ammonifying bacteria, nitrifying bacteria, denitrifying bacteria, organic phosphorus bacteria, and inorganic phosphorus bacteria in five-degree-slope croplands topsoil were 3.08, 3.56 and 3.09, and 2.00 and 2.15 times more than basic populations of microorganism from 0.5 CK to 2.5 CK, respectively. Similarly, the populations of ammonifying bacteria, nitrifying bacteria, denitrifying bacteria, organic phosphorus bacteria, and inorganic phosphorus bacteria in 10-degree-slope cropland topsoil were 2.00, 2.58, 1.53, 2.38, and 2.50 times more than basic populations of microorganism from 0.5 CK to 2.5 CK, respectively.

The populations of ammonifying bacteria in 5- and 10-degree-slope croplands topsoil were within 13.23–79.50 10⁶cfu∙g⁻¹ and 13.60–53.30 10⁶cfu∙g⁻¹, respectively. Under the same fertilization treatment, the populations of ammonifying bacteria in five-degree-slope cropland topsoil were higher than in 10-degree-slope cropland topsoil. Similarly, the populations of nitrifying bacteria (11.50–42.03 10⁴cfu∙g⁻¹) and denitrifying bacteria (11.20–23.13 10⁴cfu∙g⁻¹) in five-degree-slope cropland topsoil with all fertilization treatments were larger than nitrifying bacteria (58.80–119.43 10⁴cfu∙g⁻¹) and denitrifying bacteria (28.53–118.20 10⁴cfu∙g⁻¹) in 10-degree-slope cropland topsoil. In contrast, the populations of organic phosphorus bacteria (54.57–80.94 10³cfu∙g⁻¹) and inorganic phosphorus bacteria (44.85–57.06 10³cfu∙g⁻¹) in five-degree-slope cropland topsoil were less than organic phosphorus bacteria (79.24–98.66 10³cfu∙g⁻¹) and inorganic phosphorus bacteria (54.49–68.60 10³cfu∙g⁻¹) in 10-degree-slope cropland topsoil.

3.3. Soil Enzyme Activities

From Figure 4, the activity of alkaline phosphatase, invertase, and urease in the topsoil among different fertilization treatments in different degree slope croplands topsoil were found. In five-degree-slope cropland topsoil, urease, alkaline phosphatase and invertase activities were highest under the 1.5 CK with values of 4.35 mg NH₄⁺-N∙g⁻¹, 0.37 mg Phenol∙g⁻¹ and 4.40 mg Glucose∙g⁻¹ respectively, and fertilization treatment 2.0 CK came second with 3.49 mg NH₄⁺-N∙g⁻¹, 0.33 mg Phenol∙g⁻¹ and 3.49 mg Glucose∙g⁻¹, respectively. The lowest values of 2.68 mg NH₄⁺-N∙g⁻¹, 0.30 mg Phenol∙g⁻¹ and 2.95 mg Glucose∙g⁻¹ in 0.5 CK, respectively. In 10-degree-slope cropland topsoil, the highest activities of urease (4.40 mg NH₄⁺-N∙g⁻¹), alkaline phosphatase (0.41 mg Phenol∙g⁻¹), and invertase (9.22 mg Glucose∙g⁻¹) were also found in the 2.0 CK, and the lowest activities of urease (2.95 mg NH₄⁺-N∙g⁻¹), alkaline phosphatase (0.30 mg Phenol∙g⁻¹), and invertase (5.35 mg Glucose∙g⁻¹) were also found in the 0.5 CK. In general, compared to CK, the highest of all soil enzyme activities were in 2.0 CK and 1.5 CK, whereas the lowest were in 0.5 CK and 2.5 CK for 5° and 10° slopes, respectively.

The activities of urease in 10-degree slope cropland topsoil were higher than in five-degree-slope cropland topsoil 9.12%, 13.24%, 19.88%, 2.13%, and 3.51% from 0.5 CK to 2.5 CK, respectively. Alkaline phosphatase activities in 10-degree-slope cropland topsoil showed 44.96%, 56.89%, 55.97%, 52.33%, and 59.35% higher than in five-degree-slope cropland topsoil, respectively. The invertase activities in 10-degree-slope cropland topsoil were evenly higher than in five-degree-slope cropland topsoil 6.86%.

3.4. Soil Quality Index (SQI)

The PCA of all the soil parameters was used to calculate the SQI.

Table 3. Principal components analysis of soil quality indicators having significant differences in 5- and 10-degree-slope lands under different fertilization treatments.

	5°					10°
Component	PC1	PC2	PC3	PC4	PC5	Component	PC1	PC2	PC3
Eigenvalue	4.061	2.777	1.962	1.559	1.157	Eigenvalue	4.872	3.525	2.269
Variance (%)	29.006	19.835	14.016	11.134	8.267	Variance (%)	34.802	25.177	16.210
Cumulative (%)	29.006	48.841	62.857	73.991	82.285	Cumulative (%)	34.802	59.979	76.188
Eigenvectors a
TN	0.846	0.256	0.084	0.106	0.197	TN	0.880	0.322	−0.242
TP	−0.243	0.674	0.513	−0.027	−0.011	TP	−0.061	0.391	0.811
AN	0.952	0.078	−0.108	−0.004	0.133	AN	0.803	0.378	−0.226
AP	−0.056	0.614	0.423	0.141	0.468	AP	0.135	0.362	0.801
pH	0.075	−0.671	0.392	−0.451	0.084	pH	0.506	−0.631	−0.253
OM	−0.389	−0.096	0.090	0.802	0.317	OM	0.911	0.283	−0.099
AB	−0.456	0.299	0.404	−0.531	−0.274	AB	0.051	−0.969	0.074
NB	−0.271	−0.784	0.306	0.198	0.082	NB	0.217	0.196	−0.466
DB	0.745	−0.035	−0.259	−0.116	0.154	DB	−0.171	0.917	0.098
OPB	−0.395	0.696	−0.284	0.120	−0.278	OPB	−0.026	0.629	−0.411
IPB	−0.147	−0.027	−0.791	0.105	−0.247	IOPB	0.134	0.438	−0.339
Urease	0.846	−0.099	0.243	0.052	−0.288	Urease	0.883	−0.289	0.135
Invertase	0.034	0.349	−0.384	−0.490	0.523	Invertase	0.850	−0.037	0.437
Alkaline P	0.699	0.305	0.321	0.278	−0.417	Alkaline P	0.863	−0.164	0.247

Note: Bold-faced and underlined parameters are considered highly weighted and were included in the soil quality index (SQI) calculation. SQI (5°) = 0.29Y_{Available N} + 0.20Y_{Nitrifying bacteria} + 0.14Y_{Inorganic phosphorus bacteria} + 0.11Y_{Organic matter} + 0.08Y_Invertase. Normalized SQI (5°) = SQI (5°)/0.82 = 0.35Y_{Available N} + 0.24 Y_{Nitrifying bacteria} + 0.17 Y_{Inorganic phosphorus bacteria} + 0.13Y_{Organic matter} + 0.10 Y_Invertase. SQI (10°) = 0.35Y_{Organic matter} + 0.25Y_{Ammonifying bacteria} + 0.16Y_{Total P}. Normalized SQI (10°) = SQI (10°)/0.76 = 0.46Y_{Organic matter} + 0.17Y_{Ammonifying bacteria} + 0.21Y_{Total P.}

showed the first five PCs in five-degree-slope cropland, and the first three PCs in 10-degree-slope cropland were examined with eigenvalues ≥1.

In five-degree-slope cropland, the highly weighted variables in PC-1 were available N (AN), total N (TN), and urease. Available N (AN) had the highest factor loading and was highly correlated with total N (TN) and urease (r > 0.70), and thus, only the available N (AN) was retained for the SQI. Under PC-2, nitrifying bacteria (NB), organic phosphorus bacteria (OPB), total P (TP), pH, and available P (AP) were highly weighted. The highest factor loading was Nitrifying bacteria (NB) but not significantly correlated with organic phosphorus bacteria (OPB), total P (TP), pH, and available P (AP) (r < 0.2), and the variabilities of organic phosphorus bacteria (OPB), total P (TP), pH, and available P (AP) exceeded the 5% of the variabilities; thus, these variabilities were eliminated from the SQI. In the same way, the inorganic phosphorus bacteria (IPB) in PC-3, organic matter (OM) in PC-4 and invertase in PC-5 were chosen for calculating the SQI. The final PCA chosen for SQI were available N (AN), nitrifying bacteria (NB), inorganic phosphorus bacteria (IPB), organic matter (OM), and invertase.

In 10-degree-slope cropland, the highly weighted variables in PC-1 were organic matter (OM), total N (TN), Alkaline P, and urease. Organic matter (OM) had the highest factor loading and was highly correlated with total N (TN), Alkaline P, and urease (r > 0.70), and thus, only the organic matter (OM) was retained for the SQI. Under PC-2, only ammonifying bacteria (AB) was highly weighted, and the ammonifying bacteria (AB) were retained for the SQI. In the same way, the total P (TP) in PC-3 was chosen for calculating the SQI. The final PCA chosen for SQI were organic matter (OM), ammonifying bacteria (AB), and total P (TP).

The final PCA chosen for SQI in 10-degree-slope cropland were organic matter (OM), ammonifying bacteria (AB), and Total P (TP) (Table 3, Figure 5 and Figure 6).

The final polynomial for the SQI in 5- and 10-degree-slope croplands were calculated using Equation (1), as follows:

SQI (5°) = 0.35Y_{Available N} + 0.24 Y_{Nitrifying bacteria} + 0.17 Y_{Inorganic phosphorus bacteria} + 0.13Y_{Organic matter} + 0.10 Y_Invertase

SQI (10°) = 0.46Y_{Organic matter} + 0.17Y_{Ammonifying bacteria} + 0.21Y_{Total P}

where Y were determined based on Equation (3). The average values, curve types, and transforming equations of parameters are presented in

Table 4. Average values and normalization equations of the scoring curves.

5°
Parameters	Available N (AN)	Nitrifying Bacteria (NB)	Inorganic Phosphorus Bacteria (IPB)	Organic Matter (OM)	Invertase (INTS)
Average (x0)	110.54	76.67	71.73	8.29	40.24
Curve Type *	+	-	-	+	+
Normalization Equation	Y = 1/(1 + (x/110.54)−2.5)	Y = 1/(1 + (x/76.67)2.5)	Y = 1/(1 + (x/71.73)2.5)	Y = 1/(1 + (x/8.29)−2.5)	Y = 1/(1 + (x/40.24)−2.5)
10°
Parameters	Organic matter (OM)		Ammonifying bacteria (AB)		Total P (TP)
Average (x0)	12.97		108.42		0.49
Curve Type *	+		-		+
Normalization Equation	Y = 1/(1 + (x/12.97)−2.5)		Y = 1/(1 + (x/108.42)−2.5)		Y = 1/(1 + (x/0.49)−2.5)

Note: * (+) means “more is better”, (-) means “less is better”.

.

The five components and three components soil functions in 5- and 10-degree-slope croplands contributing to the overall SQI data were shown in Figure 7 and Figure 8a,b separately.

In both slope croplands, two fertilizer levels, 1.5 CK and 2.0 CK, showed the highest SQI values, followed by CK (0.64), and the lowest were found in 0.5 CK (0.62) and 2.5 CK (0.61) (Figure 7). The values of SQI in five-degree-slope land among different fertilization treatments were higher than the values in 10-degree-slope land.

The radar diagrams in Figure 8a,b is the plot of the soil quality index (normalized 0–1), ratings of the soil functions, and nonlinear scores of the major soil quality indicators. Lines crossing the axes are the different fertilization treatments. Compared with the index (axis), the line at the edge of the web represented better soil quality than the line toward the origin. There were significant differences of influence ratio among the different fertilization treatments and in different slope croplands. In five-degree-slope cropland, application of 2.5 CK produced a higher rating for the OM than 2.0 CK; however, this was not reflected in the soil’s nutrients of AN, which signaled that the potential AN was likely to rise at 2.0 CK. Similarly, in 10-degree-slope cropland, soil AB was higher at 2.5 CK fertilization level than at 1.5 CK level; however, soil organic matter content was higher at 2.0 CK level. This also conveys the message that soil organic matter TP and AB could not be used as the measurement standard of soil quality both in 5- and 10-degree-slope croplands alone.

4. Discussion

4.1. Soil Chemical Properties

Different amount of fertilizer had significant effect on soil properties, mainly due to its contribution of organic matter and nutrients of N, P, and K to the soil [28,29,30]. In the present study, under the different fertilization treatments, all of the soil chemical properties increased significantly in 5- and 10-degree-slope croplands topsoil, while pH declined with increasing fertilization compared to the basic properties of the topsoil. Many studies reported the topsoil pH decreased under long term application of fertilization [31,32,33,34]. However, topsoil pH was not significantly changed in the initial three years but did significantly decrease by more than 1.0 ∆pH in the 19 years under an oat/maize/beans/wheat random rotation in the infertile volcanic soil in Mexico [32]. In this study, topsoil pH did not rapidly decrease at Zuolin in the initial first year (Table 2). The reduction in pH varied with fertilization treatments and ranged from 0.05 to 0.62 units in five-degree-slope land topsoil and 0.10–0.52 units in 10-degree-slope land topsoil. The nitrification of NH₄⁺ in soil and uptake of N as NH₄⁺ by the crops could cause the decrease of soil pH after fertilization [35].

Previous reports suggest that the fertilized soil is gradually enriched with OM in its surface horizon [36,37]. In our study, the organic matter in 5- and 10-degree-slope croplands topsoil were increased compared to the basic soil OM of the purple topsoil before fertilization. The 2.5 CK (1.88 t∙ha⁻¹) brought 15.11 g∙kg⁻¹ OM compared to the content of OM (12.49 t∙ha⁻¹) in the soil of CK (0.75 t∙ha⁻¹). López-Bellido et al. showed that the 100 and 150 kg N∙ha⁻¹ rates gave rise to higher OM contents than did 50 kg N∙ha⁻¹ [38]. Different nitrogen application rates stimulated labile organic forms of soil microorganisms, which could further promote SOM mineralization [39,40,41,42]. The soil shoulder with slope of 20–30% is seriously damaged by heavy rainfall (2000–3000 mm/annul) under the effect of runoff, and the intensities of such disturbances further aggravate at back slope of 30–50%, leading to a shallow soil depth. This phenomenon might negatively impact the moisture retention capacity of the soil over the years [43]. In our study, the content of organic matter in five-degree-slope cropland soil was more than in 10-degree-slope cropland soil. Li et al. also found that the degree of soil moisture in higher degree slope land had exceeded the degree of soil moisture in lower degree slope land because of the smaller the slope the smaller evaporation. Therefore, the content of organic matter decreased with the increasing of slope land degree [43].

Levels of soil TN, AN, TP, and AP were greatly affected by the various fertilization treatments [36]. Among the fertilization treatments, the concentrations for TN and AN were 0.5 CK < 2.5 CK < CK < 2.0 CK < 1.5 CK. The TP and AP stocks also followed similar trends with the highest values for 1.5 CK and the lowest for 0.5 CK. The high level of the AN content in the soil under 1.5 CK (Table 2) could be caused by a higher TN content resulting from the conservation of active or passive fractions of the organic matter [44]. Some authors described a similar coefficient between AN and PN (R² = 0.67) while others, using a wider range of conditions, obtained a higher one [45,46].

4.2. Soil Microbial Biomass

Microbial biomass had been successfully applied to evaluate the effects of organic and inorganic fertilization on the microbiological status of different soil types and soil quality [14,47]. In the short-term experiments, fertilization had significant effects on the population, composition, and function of soil microorganisms [48,49]. In the present study, actual quantity of ammonifying bacteria increased with the increasing of fertilization. Soil organic compound fertilizer significantly stimulated the ammonifying bacteria growth, and the ammonification was produced and strengthened [50,51,52]. The quantity of nitrifying bacteria and denitrifying bacteria enhanced with the increase of fertilizer as shown by many previous studies [53,54]. The large amount of applied fertilizer does not only change the pH value of the soil but also takes a lot of effective moisture away from the soil. The lower soil moisture and high content of NO₃-N were not beneficial to survival of nitrifying bacteria and denitrifying bacteria and inhibited the growth of nitrifying bacteria and denitrifying bacteria [55]. The quantities of organic phosphorus bacteria under different fertilizer treatments were significantly different. The population sizes and specific functions of organic phosphorus bacteria and inorganic phosphorus bacteria may be different under different fertilization managements [56]. Luo et al. observed that the numbers of organic phosphorus bacteria and inorganic phosphorus bacteria could be increased by applying organic fertilizer and chemical fertilizers combined with appropriate proportion [56]. The soil’s organic phosphorus bacteria and inorganic phosphorus bacteria quantities increased significantly in the 1.5 CK and 2.0 CK but decreased significantly when fertilizer was applied excessively and there was an insufficient amount of the 2.5 CK and 0.5 CK. The organic phosphorus bacteria and inorganic phosphorus bacteria quantities increased with higher amount and quality of total phosphorus and available P input, which are known effects of manure application [56]. In conclusion, balanced chemical fertilizers (N, P, and K) could enhance nutrient availability to crops but they also affect microbial communities in the soil [57,58,59].

The different spatial distribution of soil fertility and the content of nitrogen and phosphorus in runoff are uneven due to the different slopes of cropland soil in hilly slope [60]. Our results indicated that the actual quantities of ammonifying bacteria, nitrifying bacteria, and denitrifying bacteria in five-degree-slope cropland topsoil were higher than in 10-degree-slope cropland topsoil. The increase in degree of disturbance (soil erosion due to runoff water) in hill topography significant negatively influenced soil moisture; thus, the soil moisture plays a major role in sustaining the cropland ecosystems through its effect on soil biochemical attributes [60]. The low soil moisture restrained the soil microbial biomass. With the exception of the actual quantities of ammonifying bacteria, nitrifying bacteria and denitrifying bacteria, the organic phosphorus bacteria and inorganic phosphorus bacteria quantities in 10-degree-slope cropland topsoil were higher than those in five-degree-slope cropland topsoil. The relatively higher soil TP and available P may have resulted in greater organic phosphorus bacteria and inorganic phosphorus bacteria quantities [56].

4.3. Soil Enzyme Activities

Soil enzymes could comprehensively reflect soil microbial status and soil physical and chemical conditions; therefore, it was considered as an effective sensor to study the impact of environmental change on soil quality [61,62]. In this study, 1.5 CK and 2.0 CK increased urease and acid phosphatase activities, the same as several studies [63,64,65]. Many previous results showed that soil urease activity was extremely positively correlated with soil organic matter content, TN, AN, and AP [66,67]. In the present study, the activities of urease were higher within the range of fertilizer amount 0.75–1.50 t∙hm⁻² (CK-2.0 CK) than those in the amounts of fertilizer 0.38 t∙hm⁻² (0.5 CK) and 1.88 t∙hm⁻² (2.5 CK). Urease activity decreased at higher rates of N fertilization probably due to the uptake of NH₄⁺ by soil microorganisms that made the synthesis of urease unnecessary [68,69]. With the exception of urease, many previous studies found alkaline phosphatase enzyme was significantly related to AN, TP, and AP content [70,71]. Soil pH affects alkaline phosphatase activity by affecting the concentration of inhibitors or activators in soil solution and the effective concentration of the substrate [72]. Niemi et al. found that some enzymes were found to be clearly favored by acidic conditions, while some were found to be favored by alkaline conditions in all of the soil samples examined [73]. The alkaline phosphatase activity increased as the pH increased, and the soil urease activity showed extremely positive correlation with soil pH value [74]. Invertase activity displayed an increase in response to amounts of fertilizer in 1.5 CK and 2.0 CK addition in the topsoil, which is in agreement with the results of Wang et al. [75]. The invertase activity was significantly related to total N and organic matter [71,75,76]. Li et al. found the activity of invertase is significantly related to the activation of organic matter to microbe, and higher organic matter content directly increases the organic C of soil, which has a significant effect on invertase [77].

The excessive fertilization applied in the soil leads to excessive nitrogen in soil, and the excessive nitrogen led to the effective components of ammonia nitrogen in fertilization and enzymatic product reaching a balance and restraining the urease activity [78]. The excessive phosphate fertilizer inhibited not only the hydrolysis of phosphorus-containing compounds but also the soil microbes and the synthesis of phosphatase under the plant root [79,80]. As we all know, the insufficient fertilization could lead to the deficiency of nitrogen and phosphorus in soil, and the lack of nitrogen and phosphorus would inhibit enzyme activity in soil.

There are intercommunications and inter-relationships among different soil enzymes [81]. Bi et al. (2006) found the urease, acid phosphatase, invertase, and catalase have significant positive straight-line correlation, and the loss of water and soil causes the loss of all enzyme in soil [80]. Li et al. (2006) showed the declined trend of enzyme in losing soil with the higher-degree slope land [81]. The quantities of enzymes left in 10-degree-slope cropland topsoil were higher than the quantities of enzymes left in five-degree-slope cropland topsoil. Many previous studies found the activity of enzyme have significant relationship with the nutritional components in soil [81].

4.4. Soil Quality Index (SQI)

The aim of Masto et al. constituted that the soil quality index (SQI) was to evaluate and quantify the long-term effect of different fertilizer and farm-yard manure treatments in a rotation system in New Delhi [3]. The research of Masto et al. showed that the bulk density, water holding capacity, pH value, electrical conductivity, bioavailable nutrients, organic matter, microbial biomass, and crop yield were used as the indicators to evaluate the soil quality, and it was proven that the indicators of soil quality in inorganic fertilization and organic improvement treatment were higher than those in unfertilized or unmodified soil effectively [3]. In the present study, five soil parameters were ultimately chosen as the indicators of soil quality for the SQI calculation: available N, nitrifying bacteria, inorganic phosphorus bacteria, organic matter, and invertase in five-degree-slope land, and three soil parameters as the indicators of soil quality for the SQI calculation: organic matter, ammonifying bacteria, and total P in 10-degree-slope land. Almost all of these indicators were also suggested by many researchers [3,82]. In the present study, the order of soil qualities both in 5- and 10-degree-slope croplands topsoil decreased in the following order: 2.0 CK (1.50 t∙hm⁻²) > 1.5 CK (1.13 t∙hm⁻²) > CK (0.75 t∙hm⁻²) > 2.5 CK (1.88 t∙hm⁻²) > 0.5 CK (0.38 t∙hm⁻²). The organic matter, available nutrients, microbial quantities, and soil enzymes of fertilization treatment 1.5 CK and treatment 2.0 CK were much higher than those of treatments 0.5 CK and treatment 2.5 CK, which showed that soil quality index increased with fertilization level within a certain range (0.5 CK to 2.0 CK), and declined with deficient and over-fertilization (0.5 CK and 2.5 CK) in the present study. Indeed, high soil quality occurred in the 1.5 CK and 2.0 CK. The results were similar to presented earlier by Masto et al. [3].

The SQI of five-degree-slope cropland topsoil was higher than in 10-degree-slope cropland topsoil (Figure 5). The results were similar to presented earlier by Rangeh et al., who found that higher slope position with associated increase in degree of hill topography significantly negatively influenced soil quality [43]. The fertilizer loss rate increased with the increase of slope. More than 90% of soil nitrogen exists as organic matter, and soil nitrogen is closely relative to soil carbon [83]. Therefore, higher soil nitrogen might directly or indirectly increase the organic matter content, and the organic matter content resulted in significantly higher enzyme activities. Soil P is absorbed by crops and accelerates the growth of crops, meaning soil enzymes might be involved in the process of soil P being absorbed by crops. As the substrate of biochemical reactions, soil P must be associated with soil enzyme [73]. The amount of fertilizer left does not only change the pH value of the soil but also takes a lot of effective moisture away from the soil. The lower moisture of soil was not beneficial to survival of nitrifying bacteria and inhibited growth. The cumulative effects of above factors might be responsible for the observation that the soil quality of five-degree-slope land was higher than that in 10-degree-slope land.

5. Conclusions

Different fertilization treatments significantly affect the chemical and microbial properties of the purple topsoil in Yuanmou dry-hot valley. Both in 5- and 10-degree-slope croplands soil, the results showed that chemical properties, microbial quantity, and enzymes of soil were increased with the increase of fertilization level but decreased at a high level of fertilization with 2.5 times the traditional amount. Five parameters (available N, nitrifying bacteria, inorganic phosphorus bacteria, organic matter, and invertase) in five-degree-slope cropland and three parameters (organic matter, ammonifying bacteria, and total P) in 10-degree-slope cropland, The greatest weight samples in PCA were selected to calculate the soil quality index (SQI).

The SQI calculated by integrating all critical parameters indicated that the highest SQI values were found in fertilizer levels 1.5 CK (0.71) and 2.0 CK (0.69), followed by CK (0.64), and the lowest were found in 0.5 CK (0.62) and 2.5 CK (0.61) in five-degree-slope cropland soil. The highest SQI values were found in fertilizer levels 1.5 CK (0.26) and 2.0 CK (0.29), followed by CK (0.23), and the lowest were found in 0.5 CK (0.14) and 2.5 CK (0.20) in 10-degree-slope cropland soil. The order of the soil quality in five-degree-slope cropland soil was 1.5 CK > 2.0 CK > CK > 2.5 CK > 0.5 CK; nevertheless, 2.0 CK > 1.5 CK > CK > 2.5 CK > 0.5 CK was the order of the soil quality in 10-degree-slope cropland. In light of the impact of slope, the soil qualities of five-degree-slope cropland of five fertilization treatments were higher than in 10-degree-slope cropland. The SQI values in five-degree-slope cropland soil were found higher than the SQI values in 10-degree-slope cropland soil by 68.65%, 64.20%, 62.22%, 57.46%, and 67.01%, respectively.

According to effect of different fertilization rates on soil chemical and microbial properties by the different degree slope, the present study suggests that the range amounts of fertilizer with 0.75–1.50 t∙hm⁻² in 10-degree-slope cropland and 0.75–1.13 t∙hm⁻² in five-degree-slope cropland soil (organic compound fertilizer of potassium nitrate) are the determined fertilization tolerance for slope farmland.