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Article

Exploring Phosphorus Fraction Dynamics in Loess Soils: Impact of Long-Term Nitrogen and Phosphorus Fertilization on Cropland and Fallow Land

by
Mohsin Mahmood
1,2,
Sajid Mehmood
1,
Waqas Ahmed
1,
Ahmed Salah Elrys
3,4,5,
Yi Tian
2,
Xiaoli Hui
2,
Anam Ayyoub
6,
Ahmed S. M. Elnahal
7,
Weidong Li
1,*,
Zhaohui Wang
2,* and
Jinshan Liu
2
1
Center for Eco-Environment Restoration Engineering of Hainan Province, College of Ecology and Environment, Hainan University, Haikou 570228, China
2
Key Laboratory of Plant Nutrition and Agri-Environment in Northwest China, Ministry of Agriculture/College of Natural Resources and Environment, Northwest A&F University, Yangling, Xianyang 712100, China
3
Soil Science Department, Faculty of Agriculture, Zagazig University, Zagazig 44511, Egypt
4
College of Tropical Crops, Hainan University, Haikou 570228, China
5
Liebig Centre for Agroecology and Climate Impact Research, Justus Liebig University, 35390 Giessen, Germany
6
College of Life Sciences, Northwest A&F University, Yangling, Xianyang 712100, China
7
Department of Plant Pathology, Faculty of Agriculture, Zagazig University, Zagazig 44511, Egypt
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(16), 12342; https://doi.org/10.3390/su151612342
Submission received: 17 July 2023 / Revised: 6 August 2023 / Accepted: 7 August 2023 / Published: 14 August 2023
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Versions Notes

Abstract

:
Long-term cropping systems require balanced phosphorus (P) management for better yield and environmental sustainability. However, the soil P transformations under fallow rotations with and without long-term nitrogen (N) and P fertilization largely remained unknown. This study evaluated the status of P forms in loess soils in response to varied combined rates of N and P fertilizers, tillage management practices, fallow land systems (natural fallow (NF), and bare fallow (BF)). Four NP treatments (N0P0, control; N0P100, 100 kg P ha−1; N160P0, 160 kg N ha−1; and N160P100), and two treatments with no fertilizer application and crops (NF and BF) were conducted. The treatments N0P100 and N160P100 significantly increased soil total P, inorganic P (Pi), organic P (Po), and Olsen P concentrations compared to the control, NF, and BF treatments. Labile P fractions (NaHCO3-Po and NaHCO3-Pi) were 7.30% and 11.8–12.4% higher in fertilized treatments than in control, NF, and BF treatments. The moderately labile NaOH-Pi was stable in all treatments, but NaOH-Po significantly decreased in the NF (2.60%) and BF (1.40%) treatments compared to the control and fertilized treatments; however, HClD-Pi was 59.1–66.0% higher in NF and BF compared to the control and fertilized treatments. Non-labile P (HClC-Pi and HClc-Po) fractions showed no significant difference between the fertilized and unfertilized treatments. Residual P levels were substantially greater in the P fertilized (N0P100) treatment than in the fallow treatments. The conceptual framework and redundancy (RDA) analysis revealed that the labile (NaHCO3-Pi and NaHCO3-Po) and moderately labile P fractions (NaOH-Po, NaOH-Pi, and HClD-Pi) were substantially associated with Olsen P contents, grain yield, and P uptake. Higher moderate fraction concentrations in fallows and their positive correlation with yield, P uptake, and Olsen P predict the importance of reserved P in these soils upon long-term fertilization, suggesting the utilization of P legacy and optimizing fertilizer applications.

1. Introduction

Phosphorus (P) is one of the essential macronutrients that limits plant productivity and soil is a vital P source [1]. Excessive nitrogen (N) application, P fertilizers, and low P use efficiency in cropping systems have resulted in the depletion of P stocks in soils [2]. Soil P is differentiated based on its availability to plants, and enumerating the effects of fertilized and unfertilized lands and soil P fractionation may lead to a better comprehension of soil P cycling and bioavailability [3].
Previous studies have revealed that higher application of N and P fertilizers under long-term regimes could supply adequate available P for plant growth for several years but also can increase P accumulation in soils [4]. In addition, low P input could decrease total soil P fixation. However, in long-term cropping systems, the P transportation from soil to crops requires further investigation, particularly during growing periods when N and P fertilizers are not applied and soil remains fallow [5,6]. The application of phosphorus (P) fertilizer is a crucial factor in maintaining soil fertility and improving crop yields. It not only affects the P concentration status in soils but also enhances various soil processes such as biological, physiological, and chemical processes [7]. Previous studies have shown that excessive P application can lead to P fixation caused by various microbial activities, such as decomposition and mineralization, particularly in long-term cropping systems [8,9]. Furthermore, excessive P can also inhibit the mineralization of organic P due to phosphatase synthesis [10,11]. The application of N fertilizers affects soil P chemistry directly or indirectly through nitrification and mineralization of Po and Pi fractions [12]. For instance, studies showed that increasing the amount of soil N improves P use efficiency [13,14,15]; additionally, other research has indicated that N fertilization can induce P limitations by decreasing its availability for plant uptake [5]. Additionally, the indirect impacts of N application on soil P forms and cycling might vary depending on the soil type and by altering the soil characteristics and microbial activity [16,17,18].
Only a few research works have investigated the effect of land use systems under long-term fertilization on total P distribution only [19,20]. The natural fallow (NF) and bare fallow (BF) formation also affects soil P distribution. Long-term inorganic fertilization releases P into the available pools, where it can be taken up by plants, while about 70% adsorbed on the surface of soil particles by occlusion with different ions. Bare lands restrict P mobilization and increase P fixation due to anthropogenic activities and changes in soil texture and structure, as well as the production of free calcium (Ca) and iron (Fe) oxides with a strong P sorption affinity [21]. Long-term N and P fertilization and tillage practices enhanced the availability of soil nutrients in land-use systems [22].
Under long-term fertilized and unfertilized fields, residual-P and different inorganic (Pi) and organic (Po) fractions of P can be critical factors for influencing applied P recovery [23]. In situations where the P availability is limited, higher fertilization rates are necessary for the land to achieve the critical P levels required for optimal plant growth [4]. Fields receiving tillage practices up to once a year have shifted different P forms from fertilized to unfertilized land (fallows) where the N and P application doses should reserve as a legacy and should not be greater than the amount of P taken from the soil by the crop at harvest [24]. Because a large portion of the essential P is supplied by the soils enhanced through previously applied fertilizer in a stable form, application of N and P fertilizers to these soils can be omitted for a while or can be less than P lost with the harvested crop [25].
Due to changes in land use and environmental factors, P is transformed over the years from available to non-available and organic forms [26]. The conversion of cropland into bare lands decreased P availability and increased the proportion of non-labile P forms [27]. Accelerated soil system change due to land-use and tillage practices decreased organic matter by at least half [28], providing an organic material for releasing nutrients, such as the accessible P [29,30].
It is well known that imbalanced long-term N and P application can adversely affect soil quality and productivity [31]. A higher application rate of N causes soil acidification which increases the fixation of P owing to the increased mobilization of Fe and aluminum (Al). However, a detailed study of the soil P transformations upon fallow rotation with and without long-term P and N fertilization is lacking. Therefore, the response of soil P chemistry to N and P fertilization and different fallow land systems needs to be additionally explored using the long-term field trials.
The study aimed to (i) assess the impact of comparing fallows with different P and N inputs on soil P fractions and (ii) to assess P fractions′ relationship with Olsen P in the soil during the long-term experiment (12 years) after the fallows using the Tiessen and Moir P fractionation scheme. We hypothesized that N fertilization would alter the P forms in the soil and produce distinct patterns when compared to P fertilization. We also assumed that variations in P forms and concentrations might be linked to the way soil chemical characteristics respond to repeated applications of N and P. As a result of the substantial study, fallow (NF and BF) rotations will leave a substantial P legacy.

2. Materials and Methods

2.1. Site Description

A field trial was conducted in 2004 using the typical management regimes employed by local farmers at the research farm of Northwest A&F University (34°17′59″ N, 108°4′12″ E), Yangling, Shaanxi Province, China. This area is located on the southern Loess Plateau at an elevation of approximately 520 m. The geomorphology is the third-level terrace of the Wei River (the biggest tributary of the Yellow River), and the field is a flat land with no water erosion. The subhumid and drought-prone climate makes this a typical rain-fed area in China, and winter wheat (Triticum aestivum L.) is one of the dominant cereal crops. The average annual rainfall is 580 mm, approximately 60% of which occurs in July, August and September, and the average annual temperature is 13 °C. The study site is rainfed and the yearly precipitation contributed 58% of the total irrigation. The investigation soil site was classified as Calcareous Udic Haplustalf according to (USDA system). The soil’s basic parameters were as follows: pH was 8.24, organic matter was 13.8 g kg−1, available P was 15.0 mg kg−1, bulk density was 1.24 g cm−3, and total N was 1.07 g kg−1. Basic soil properties were investigated on soil samples from 2004 to 2015 at a depth of 0 to 20 cm.

2.2. Experimental Design

A randomized complete block design long-term field experiment with four replications was initiated. The study had four N and P fertilization treatments (N0P0, control; N0P100, 100 kg P2O5 ha−1; N160P0, 160 kg N ha−1; and N160P100) with winter wheat (Xiaoyan 22 cultivar) and two treatments without fertilizer application and wheat cultivation (NF and BF). The NF is a natural renewal of vegetation, known as the traditional uncultivated fallows, that are still widely utilized by local farmers, while the BF is a bare fallow with no vegetation growth. In the BF treatment, vegetation was manually removed to ensure vegetation-free land. Each plot was 40 m2 (4 × 10 m), having a buffer zone of 1.0 m between plots and of 2.0 m between blocks. The N fertilizer source was urea (46% N), while the triple super phosphate (46% P2O5) was used as a P source. In each treatment plot, manual fertilization was performed on the soil’s upper layer during the sowing process and was followed by ploughing with a rotavator. Seeds were sown in mid-October of 2016 and 2017 and harvested in the first week of June. The additional crop management techniques applied were in line with those employed by local farmers. The pesticides Chlorpyrifos and Phoxim were used as insecticides, and the herbicide Tribenuron (by FMC) was used for weed control. No anthropogenic intrusion, fertilizer application, or tillage practices were done in the NF treatment so that the natural vegetation could grow throughout the study period. However, in the BF treatment, to keep the land vegetation-free, vegetation was removed and remained fertilizer-free.

2.3. Sample Collection and Analysis

2.3.1. Plant and Soil Sampling and Basic Analysis

Soil sampling was carried out for the years 2016 and 2017 by selecting four locations within each plot at a depth of 0–20 cm using a 5 cm diameter auger after the wheat harvest. A composite soil sample was taken from each plot by mixing the individual soil samples collected. According to Henriksen and Selmer-Olsen (1970) [32], available N (Nitrate-N) was extracted using 1 mol L−1 KCl, while available P was extracted using 0.5 mol L−1 NaHCO3 (AA3, SEAL, Germany). Using water: soil ratio of 2.5 and CO2-free deionized water, soil pH was extracted and measured using a pH electrometer (MP511, Shanghai, China) (Bao 2000). Soil organic matter was determined with the potassium dichromate-sulfuric acid (K2Cr2O7-H2SO4) oxidation method [33]. For the determination of grain yield and P uptake, 100 plants from each plot at four random locations (4 × 1 m2) were taken at the time of crop harvest.

2.3.2. Soil P Fractions Analysis

The Hedley-modified Tiessen and Moir sequential fractionation method [34] was utilized to fractionate the soil P. This scheme exploits the strong extractants to differentiate different organic (Po), inorganic (Pi), and occluded forms of P (residual P; Pr). The following extractants were used sequentially to extract one gram of soil sample from a 50 mL centrifuge plastic tube: (1) 30 mL sodium bicarbonate (NaHCO3; 0.5 mol L−1; pH 8.5) to extract relatively labile Pi and Po absorbed onto soil surfaces, (2) 30 mL sodium hydroxide (NaOH; 0.1 mol L−1) to Al phosphates separate crystalline and amorphous Fe, besides P that is tightly linked to Al and Fe compounds, and (3) in highly weathered soils, the moderately insoluble Ca-P minerals apatite, Fe-P, and Al-P were extracted using 30 mL hydrochloric acid (HCl; 1 mol L−1). Samples were shaken for 16 h in a mutual shaker following each additional extractant. The extracts were then centrifuged to remove the supernatant before being run through filter paper (Whatman 42) for P analysis. Soil sample residues were eventually digested by using (30% H2O2) and concentrated (H2SO4) to recover more stable forms of Po and comparatively insoluble Pi forms (Pr). After assimilating aliquots with H2O2 and H2SO4, the total P in HCl, NaOH, and NaHCO3 extracts was measured, while the organic P was estimated as ( O r g a n i c   P = t o t a l   P − i n o r g a n i c   P ) . The blue molybdate-ascorbic acid technique was employed to assess the amounts of Pr and Pi in all extracts following [35].
As a result, there were seven P fractions: labile (NaHCO3-Pi and NaHCO3-Po), relatively labile (NaOH-Po, NaOH-Pi, and HClD-Pi), non-labile HClC-P (HClc-Po and HClc-Pi), and Pr._Total Pi was calculated as the sum of P in NaHCO3-Pi, NaOH-Pi, and HCl-Pi fractions. Total Po was defined as the sum of P in NaHCO3-Po, NaOH-Po, and HClc-Po fractions. The UV spectrophotometer was utilized to assess all P fractions.

2.4. Data Analysis

The least significant differences (LSD) and analysis of variance (ANOVA) test were performed to determine significant differences at p 0.05, using SPSS 21.0 (SPSS, Chicago, IL, USA). Using Canoco 5.0 software, redundancy analysis (RDA) was conducted to investigate the soil properties and P fractions correlations. To explore the relationship between crop yield, P uptake from cropland, and Olsen P from fallow land, conceptual framework analysis was performed using RStudio software.

3. Results

3.1. Crop Yields and Soil Properties

Long-term cropping and fertilization showed no significant effect on soil pH and soil contents of NH4+-N and NO3−-N (Table 1). The N0P100 and N160P100 treatments significantly (p < 0.05) increased Olsen-P content compared to the fallow treatments (NF and BF). Separately fertilized crop land treatments had significant (p < 0.05) effects on yield and P uptake (Table 2). The co-addition of inorganic N and P fertilizers (N160P100) significantly (p < 0.05) increased grain yield and P uptake compared to the solo treatments (N0P100 and N160P0) and the control treatment (Table 2).

3.2. Soil Total P, Pi, Po, and Pr Fractions

Soil total P, Po, and Pr concentrations significantly increased in response to N and P additions (Figure 1). The single addition of N (N160P0) decreased total P by 6.91%, Po by 11.9%, and Pr by 44.6% compared to the combined addition of N and P (N160P100). Compared to fertilized treatments, total inorganic P concentration was higher in BF (464 mg kg−1) and NF (443 mg kg−1) treatments (Figure 1). Compared to the control treatment, soil total P content significantly increased in all treatments: N0P100 (19.7%), N160P100 (12.0%), NF (4.20%), and BF (8.90%). Total Pi content was stable in fertilized treatments, but it increased in NF (22.2%) and BF (27.9%) treatments compared to the control (Figure 1). Compared to the control treatment, soil total Po and Pr contents significantly (p < 0.05) increased by 52.3% and 21.3% in the N0P100 treatment, respectively, but insignificant differences were observed in other treatments (Table 2). The contribution of Pi was 66.5–73.7% of the total P in the fertilized treatments while it was more than 86.0% in the unfertilized (NF, BF) treatments. Soil Po contributed 26.3–32.0% of the total P in the fertilized treatments and 13.0% in the unfertilized fallow treatments (Figure 4). The contribution of Pr of the total P was 11.0% in the fertilized and unfertilized treatments.

3.3. Organic P (Po) Fractions

The Po fractions (NaHCO3-Po and NaOH-Po) concentrations were significantly (p < 0.05) higher in the treatments of N0P100 and N160P100 than in the treatments of N160P0 and BF (Figure 2). HCl-Po organic fraction was significantly higher in the fertilized treatments (N0P100 and N160P100) than in the treatments of control, NF, and BF (Figure 2). Compared to the control treatment, all organic fractions increased in N0P100 and N160P100 treatments, but decreased in NF and BF treatments. Among Po fractions, HClc-Po concentration was the highest, accounting for 5.30–13.9% of the total P, followed by NaHCO3-Po and NaOH-Po fractions (Figure 4).

3.4. Inorganic P (Pi) Fractions and Olsen P

The Pi fractions (NaHCO3-Pi, HClD-Pi, and HClC-Pi) concentrations were influenced by the combined application of N and P fertilizers. However, NaOH-Pi fraction showed no significant difference among fertilized, NF, and BF treatments (Table 2). Soil NaHCO3-Pi and HClC-Pi were significantly lower in NF and BF treatments than those in the fertilized treatments. In contrast, HClD-Pi was significantly higher in NF and BF treatments than that in the fertilized treatments (Figure 3). Compared to the control treatment, NaHCO3-Pi and NaOH-Pi increased (p < 0.05) by 150% and 13.1% in the fertilized treatment of N0P100, respectively, but HClD-Pi increased by 59–66% in the treatments of NF and BF and contributed 59% of the total P (Figure 4). NaOH-Pi and HClD-Pi in BF were significantly higher than those in NF. The proportion of Pi fractions was found in the order of HClD-Pi > NaHCO3-Pi > NaOH-Pi > HClc-Pi.
Olsen-P was significantly higher in the fertilized treatments of N0P100 (13.1 mg kg−1) and N160P100 (20.3 mg kg−1) than that in other treatments (Figure 5). Compared to the control treatment (4.1 mg kg−1), Olsen-P increased by 215%, 392%, 55%, and 33% in the treatments of N0P100, N160P100, NF, and BF, respectively.

3.5. Relationships among P Fractions and Soil Properties

We have probed the conceptual framework for the grain yield and P uptake content with P fractions. Positive correlations were found among grain yield, P uptake, and P fractions (HClD-Pi, NaHCO3-Pi, NaHCO3-Po, NaOH-Po, and HClc-Po). However, Pr and HClC-Pi were indirectly associated with P uptake and grain yield (Figure 6A). Similarly, NaHCO3-Pi, HCl-Po, and NaHCO3-Po directly affected the Olsen-P. Olsen-P was indirectly affected by the Pr, which had a strong coefficient with HClD-Pi. Olsen-P also had a direct effect on HClc-Pi. All P fractions, except Pr and HCl-P, showed significant associations and the highest correlation was recorded between NaHCO3-Pi and NaOH-Pi (Figure 6B). Furthermore, except HCl-Pi, available P was substantially linked with P fractions. This could imply that any changes in the individual P fractions might generate a variation in available P.
We also used RDA to test the relationship between P fractions and soil characteristics. The RDA showed significant effects of soil properties on P fractions distribution (Figure 7). Soil properties (organic matter and Olsen-P) showed positive and close relationships with labile (NaHCO3-Pi and NaHCO3-Po) and moderately labile (NaOH-Pi, NaOH-Po, and HClC-Pi) P fractions. However, Pr and HClD-Pi fractions displayed a negative correlation with soil contents of NO3−-N and NH4+-N, and positively correlated to soil available K and pH (Figure 7). The RDA-1 and RDA-2 explained 22.5% and 40.8% out of the total variance, respectively.

4. Discussion

4.1. Soil P Fractions in N and P Fertilized Croplands and Fallow Lands

Our work clearly demonstrates that, in long-term cropping systems, the availability and amount of organic and P inorganic forms were strongly influenced by the type of land use. Land use types and long-term fertilization management substantially affect P availability and its status in term of accumulation in soils [20,36]. Our findings showed that in croplands, the long-term N and P fertilized treatments (N0P100 and N160P100) cause a significant rise among Pi forms, such as labile Pi (NaHCO3-Pi, an inorganic bioavailable fraction), non-labile Pi (HClC-Pi), relatively labile Pi (HClD-Pi and NaOH-Pi), and Pr (Table 1). This may be related to the extensive use of N and P fertilizer on the surface under long-term fields, resulting an increase in labile, non-labile, and moderate labile Pi forms [37,38]. The relatively labile Pi such as HClD-Pi and NaOH-Pi provided a higher proportion under solo N or P fertilized treatments; this may be due to high accumulation due to fixation with different ions such as Ca and Mg in soils. Different findings demonstrate links between these forms and suggest that long-term fertilizing considerably impacts the soil P fractions. Our results are consistent with different studies conducted in China and Brazil under long-term fertilized and fallow conditions. Zhang et al. (2018) [2] found that intermediate-Pi (NaOH-Pi and HClD-Pi) were higher in the fertilized and fallow lands. Mahmood et al. (2021) [39] also indicated that solo P fertilizers increased the NaHCO3-Pi, NaOH Pi, and HClD-Pi forms. Meanwhile, we also found higher inorganic P forms specifically (HClD-Pi) in fallow land soils (Figure 2). These results suggest that the fallow land part of a long-term field study significantly affects the soil P fractions cycling by cycling by different anthropogenic activities and environmental factors. Notably, there is a higher proportion of inorganic P legacy in fallow land and a lower proportion in crop land, which is clear evidence of P utilization by the winter wheat crop. DeBruler (2019) and Koutika (2013) [40,41] stated that wheat-maize crops have diverse impacts on the soil’s P fractions. Thus, it is important to understand the effects of lands being fallow under long-term cropping system on soil P cycling so that it can be utilized in the future.
As anticipated, according to the findings, fallow treatment (NF and BF) considerably enhanced the total quantity of inorganic phosphorus in the soil (Figure 1). Labile Pi fraction (NaHCO3-Pi) represented 7.30% and 2.60–2.80% of the total P in NF and BF treatments, respectively (Figure 4). In the unfertilized treatments (NF and BF), the moderate labile Pi fraction (HClD-Pi) was found to be the highest contributor (59.9%) to the total Pi and P, whereas NaOH-Pi remained stable in both fertilized and unfertilized treatments. This was probably caused by the P rates, which may have had various impacts on P forms and their accumulation, as well as mobility in the upper (20 cm) layer of soils, while in fallow land (NF and BF) that was likely due to tillage practices, rainfall, and anthropogenic activities [42,43]. Stability and higher accumulation of HClD-Pi in the NF and BF might be due to land use and occlusion among the soil particles. Our study has consistency with the previous studies of [44,45], who have reported that apatite-P (HClD-Pi) fractions were not affected in fallow land. The large percentage change into HClD-Pi content observed in this study showed that these fractions could contribute to the long-term P cycling (Figure 4). HClD-Pi fraction primarily comprises insoluble and fixed P types including Ca-, Fe- and Al-bound P [46,47], representing the moderate and non-labile forms of P fractions in the soil. Bi et al. (2018) [48] revealed that P bonds with Ca reduce the amount of P available for crops. According to our research findings, the concentrations of HClC-Pi showed a different pattern than those of the other P fractions and were highest in the control treatment (N0P0) (Figure 3). Moreover, our study observed that NaHCO3-Pi fractions were substantially higher in the higher P application treatments (N0P100 and N160P100) and lower in the NF and BF treatments. This could be a result of (I) ongoing fertilization, which could affect the P binding in soil by increasing available Pi and (II) N fertilizer addition which also mobilizes the absorbent nutrient and ultimately increasing soil P availability [49]. These results are in agreement with those of Dobermann et al. (2002), [50] who found that N and P fertilization affect the fractions of Po and Pr but mainly increased Pi. Sources of P and N fertilization with higher P concentrations can have a considerable impact on optimizing the Po contents by mineralization process in soils, and the P adsorption in organic materials, which is likely to be transformed into accessible Pi due to the mineralization process, could be the cause of the Po retention [51]. NaHCO3-Po increased significantly in response to the P application rate alone or with N (N160P100), while non-significant change was observed in NF and BF treatments. Among the organic fractions, non-labile HClC-Po was the highest contributor to the total P (Figure 2). The HClC-Po rose under the high P treatment in a manner similar to that of the relatively labile P fraction (Figure 2). Additionally, prior research has demonstrated that the HClC-Po and NaHCO3-Po fractions greatly increased with the application of various inorganic fertilizers, while the NaOH-Po fractions only slightly increased [52]. Our findings are in line with earlier research by [53,54], which demonstrated that ongoing P and N fertilization boosted P fixation and resulted in a rise in non-available Po pools. These results can be explained as there is also input of crop residue incorporation in the fertilized treatment which decreases soil pH and induces the production of Al- and Fe-associated complexes by reducing the soil’s availability of P. Additionally, our findings aligned with earlier studies by [55,56], who reported that when long-term crops were grown without fertilizer, the soil’s Po concentration declined, whereas the addition of inorganic fertilizer greatly increased the soil’s labile Pi and Po contents.
Because of the higher levels of soil organic matter and the assimilation of crop residue, the cropland had higher contents of organic P (Po) than the fallow did [57]. RDA analysis showed a significant positive association between soil organic matter and Po, which endorses this elucidation since Po accumulation is the soil’ results from the substantial P legacy and crop residue [58]. However, larger HClc-Po accumulation may also improve Po preservation in fertilized soils as earlier research revealed that the majority of the Po is linked to Al and Fe oxides [59]. However, we found no discernible difference in the distribution of organic P (Po) on fallow land, suggesting that fallow land induces Po accumulation instead of changes in exchangeable Al and Fe as the primary mechanism leading to increased Po on fertilized cropland. Therefore, stimulating microbial biomass to mineralize soil P related with organic matter and crop residues is a potential approach to balance soil Pi availability and crop demand.
Furthermore, there is a strong correlation between soil Pi concentrations, microbe abundance (such as bacteria and fungi), and the process of Po mineralization. As a result of the higher bioavailability of P in fertilized cropland, crops might undergo lower Po mineralization compared to fallow land. To better understand the effects of land use on soil Po concentrations, it is most insightful to investigate the individual fractions comprising the Po pool separately [60]. The most stable forms of Po are represented by NaOH-Po and HClc-Po, which are firmly coupled with Al and Fe minerals. We revealed that NaOH-Po and HClc-Po were higher and balanced in fertilized land than fallow ones across all tested treatments, suggesting that biogeochemistry controls the distribution of Po fractions. The Po is firmly attached to the outer and inner particle surfaces in these geochemical processes or forms precipitates with metal ions [61]. The Po was extracted using HClc and NaOH is therefore only anticipated to be available over an intermediate or extended period. The NaHCO3-Po is assumed to originate from organic substances that microorganisms may easily mineralize. The distribution of soil NaHCO3-Po, on the other hand, was co-controlled by geochemical, biological, and anthropogenic activities, as evidenced by a significantly higher trend in NaHCO3-Po in the soil and combined N and P fertilized treatments among the treatments, indicating that biological processes in fertilized and fallow lands may have diverse functions.
Residual P is primarily comprised of insoluble forms bonded with Al- and Fe- [62,63], considered recalcitrant forms of P which is strongly fixed and cannot be directly uptaken by the plants. Our findings showed that the Pr increased slightly with solo P addition and increased by 21.0% and significantly higher than the N combined with P fertilizer application compared with the N0P0 and unfertilized fallow (NF and BF) treatments (Figure 4). These findings align with the findings of, which reported that the co-application of NP fertilizers influences P fractions and increases the insoluble Pi and residual Pr. Our investigation’s variation in Pr fraction suggests that the residual fraction may be subject to long-term P cycling (Figure 1). The presence of several processes in the soil system may cause this process. P fertilizers adsorbing on primary minerals can significantly influence Pr [64]. The lower Pr proportion in the fallow land (NF and BF) (Figure 4) may be related to the effects of fertilization, which led to the saturation of P in the form of residuals that may eventually transform into pools of moderately or labile P (NaOH-Pi and HClD-Pi fractions) or labile P (NaHCO3-Pi fractions), respectively. The results from long-term field study also validate our results that no fertilizer addition and fallow rotation decrease the availability of P and Pr by altering the P compositions [65].

4.2. Effect of Soil Properties on P Fractions and Crop Yield

In the study, long-term fertilization with nitrogen (N) and phosphorus (P) did not have a significant impact on soil pH, nitrate (NO3−-N), ammonium (NH4+-N), and available potassium (K) levels (Table 1). However, both grain yield and P uptake in croplands showed a remarkable increase in response to P and N fertilization (Table 2). Interestingly, the lack of response in soil available N and K concentrations in croplands to P and N fertilization aligns with previous findings from studies on calcareous soils in China, indicating that varying rates of N and P applications did not influence the availability of N and K in the soil. [66]. Furthermore, this might be related to localized fluctuations in precipitation and temperature and slight variations in the soil’s available K and N that are indistinguishable from the soil’s heterogeneous fertility [67]. However, available K and NO3−-N were significantly increased in fallows (NF and BF) treatments but were negatively correlated with Po and Pi fractions (Figure 7); this could be due to the soil compaction and natural vegetation [68].
In the long-term cropping system, the N0P100 treatment’s long-term N application considerably enhanced the Olsen-P concentration in the soil (Figure 5), which also raised P uptake and grain yield (Table 2). Furthermore, grain yield was unaffected by the addition of more P fertilizer, suggesting that plants can utilize the legacy P in the soil [69]. Although there were no significant variations in pH across treatments in these plots, it is also possible that soil acidification produced by N application led to a drop in P solubility, which in turn led to a fall in P availability and soil Olsen-P content. It is well known that soil pH significantly impacts soil P solubility [70,71]. In agricultural soils, a pH decrease caused by N fertilization can mobilize the exchangeable Al and Fe while reducing the exchangeable Mg and Ca [66,72], which have a direct impact on P solubility. Here, a correlation between soil pH and moderate P fractions contents was found (Figure 7).
Due to the long-term nature of this study, we initially hypothesized that the fallow phase in one year would not have any impact on the soil’s P chemistry. However, the croplands under long-term cropping revealed substantial impacts on the soil’s Olsen-P, Po concentrations, total P, crop yield, and P uptake (Table 2, Figure 5). However, the conceptual framework shows a linkage among different P fractions (organic and inorganic), Olsen P and grain yield (Figure 6A,B), which endorse the role of all P forms in plant growth and uptake. Our results show that the BF and NF revealed the highest levels of moderately labile P, whereas the fallows highly contributed to the total Pi and P concentrations. This was thought to be because crop residues were broken down during the fallow phase and Po was mineralized, increasing the availability of labile P to the crop that followed the fallows as a legacy. Our results exhibited an increase in total P and total Po during fallow; however, on the other hand, this suggests a higher breakdown of crop residues but not a greater mineralization of Po. Other research has shown that fallow land adjacent to farmland can impact soil structure and soil microorganisms, leading to increased nutrient legacy sources [73]. This could also enhance soil P linked with soil’s microbial biomass, such as labile P and its mobility. This was thought to be caused by the fallow phase’s mineralization of Po and the breakdown of crop residues, which increased the availability of labile P to the crop after fallows as a preserved legacy. However, the current study’s observation that total P and total Po increased after fallow suggests that crop residues decomposed more quickly but that Po mineralization did not. According to other studies, the fallow land around the cropland could impact the soil’s microbial biomass and soil structure and cause a higher source of nutrients legacy [74], which might also boost soil P related to soil microorganisms, such as labile P and its mobility.
Overall, management and soil P cycling is crucial for soil health and better crop yield. As a result, analyses of the P forms’ distribution under the long-term cropping system and fallow phases are crucial for assessing soil P cycling and developing mediations to lower the P loss risk. Our results indicated that P pools of various fractions and their distributions varied between the fertilized cropland and fallow land, suggesting that soil P fractions could be used to detect changes in the characteristics of soil P behavior. Moreover, the major physicochemical traits in fallow and plant indices in cropland are sensitive to variations in P buffer capacity and soil P pools, according to substantial connections between soil features (such as organic matter, AN, Fe, Al, and pH), grain yield, P uptake, and P fractions. Furthermore, an accumulation of soil P in fallow land and its strong correlation with labile P suggest higher importance of P legacy. Consequently, it is possible to evaluate the behavior of soil P in various land-use and cropping systems using soil P fractions, particularly labile-P and Po, as well as important soil physicochemical parameters. We discovered that all soils in the study area from fertilized cropland and fallow land had higher Pi and lower Po levels, confirming that the risk of P loss in long-term fertilization is still substantial independent of the types of land use under various cropping systems.
Therefore, it is possible to evaluate the behavior of soil P in various land-use and cropping systems using soil P fractions, particularly labile-P, and Po, as well as important soil physicochemical parameters. We found that all soils in the study area from fertilized cropland and fallow land had higher Pi and lower Po levels, confirming that the risk of P loss in long-term fertilization is still substantial independent of the types of land use under various cropping systems.
Finally, it is widely believed that the key to improving the balance between crop yield, stable economy, and protection of soil health is to achieve the best synchronicity between soil NP application and crop P demand by accounting for the complex chemistry between plant uptake, accumulation in soils, and management practices. Farmers usually select fertilization rates and crop management based on their previous experiences. However, the optimum combination of NP rates is strongly recommended, which varies among sites and growing seasons under the long-term winter wheat cropping system [75,76]. In the present study, a positive approach was taken for cropland treatments by suggesting a balanced application of N and P rates under long-term wheat cultivation. Our study observed the highest yield of 5845 kg ha−1 in the N160P100 treatment, possibly due to the highest P uptake in the current treatment (Table 2). These N and P application practices have been successful in optimizing the application rate and obtaining the targeted yield [77].

5. Conclusions

We found that excessive long-term fertilization practices left a great P legacy in the soils. Fallows (NF and BF) no-tillage practices under long-term NP fertilization strongly influence the P status in the soils. Soil concentrations of Pi were higher than Po under long-term fertilized crop and fallow (NF and BF) land-use systems. A higher concentration of HClD-Pi in the NF and BF indicates that it can be utilized when no fertilizer is applied. In long-term fields, lack of continuous cropping and residue input was responsible for the P fixation and low available P content. The direct and indirect relationship between Olsen-P and all organic and inorganic P fractions indicates the significant contribution of these P fractions in P availability in the soil. The observed differences in the distribution and amount of P highlight the importance of maintaining P reserves in soils by balancing NP fertilizer applications. Our study strengthened the significance and utilization of reserved P legacy in soils under long-term fertilization.

Author Contributions

Conceptualization, M.M.; methodology, M.M. and Z.W.; validation, M.M. and Z.W.; formal analysis, M.M.; investigation, Y.T. and W.L.; resources, Z.W., X.H. and A.S.M.E.; data curation, M.M., W.A., S.M., A.S.E. and A.A.; writing—original draft preparation, J.L. and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by (The National Natural Science Foundation of China NSFC-31860728), Launch Fund of Hainan University High level Talent (RZ2100003226), Hainan Province Science and Technology Special Fund (ZDYF2021SHFZ071), Innovation Platform for Academicians of Hainan Province (YSPTZX202124), Key R & D projects in Hainan Province (ZDYF2021XDNY185). The study was also financially supported by the National Natural Science Foundation of China (32072669), China Agricultural Research System (CARS-3), and the National Key Research and Development Program of China (2018YFD0200400).

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to extend their sincere appreciation for the financial support from the National Natural Science Foundation of China (32072669), China Agricultural Research System (CARS-3), and the National Key Research and Development Program of China (2018YFD0200400).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Changes in total inorganic P (total Pi), total organic P (total Po), total P, and residual P (Pr) to different N and P applications and natural vs. bare fallow soil conditions. Error bars show the standard error. Different letters above the columns indicate significant differences at p < 0.05.
Figure 1. Changes in total inorganic P (total Pi), total organic P (total Po), total P, and residual P (Pr) to different N and P applications and natural vs. bare fallow soil conditions. Error bars show the standard error. Different letters above the columns indicate significant differences at p < 0.05.
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Figure 2. Changes in organic P (Po) fractions in response to different N and P fertilization rates and bare vs. natural fallow soil conditions. Different letters above the columns indicate significant differences at p < 0.05.
Figure 2. Changes in organic P (Po) fractions in response to different N and P fertilization rates and bare vs. natural fallow soil conditions. Different letters above the columns indicate significant differences at p < 0.05.
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Figure 3. Changes in inorganic P (Pi) fractions in response to different N and P fertilization rates and bare vs. natural fallow soil conditions. Different letters above the columns indicate significant differences at p < 0.05.
Figure 3. Changes in inorganic P (Pi) fractions in response to different N and P fertilization rates and bare vs. natural fallow soil conditions. Different letters above the columns indicate significant differences at p < 0.05.
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Figure 4. Relative proportion of organic (Po) and inorganic (Pi) P fractions in total P under different N and P fertilization rates and bare vs. natural fallow soil conditions.
Figure 4. Relative proportion of organic (Po) and inorganic (Pi) P fractions in total P under different N and P fertilization rates and bare vs. natural fallow soil conditions.
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Figure 5. Changes in Olsen-P in response to different N and P fertilization rates and bare vs. natural fallow soil conditions. Different letters above the columns indicate significant differences at p < 0.05.
Figure 5. Changes in Olsen-P in response to different N and P fertilization rates and bare vs. natural fallow soil conditions. Different letters above the columns indicate significant differences at p < 0.05.
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Figure 6. (A) A proposed conceptual framework showing the linkage among available P (Olsen-P), and different organic (Po) and inorganic (Pi) P fractions. (B) A proposed conceptual framework showing the linkage among grain yield, P uptake, and different organic (Po) and inorganic (Pi) P fractions. The linkages are based on the relationships between different variables. The color of each line refers to the degree of relationship between the variables.
Figure 6. (A) A proposed conceptual framework showing the linkage among available P (Olsen-P), and different organic (Po) and inorganic (Pi) P fractions. (B) A proposed conceptual framework showing the linkage among grain yield, P uptake, and different organic (Po) and inorganic (Pi) P fractions. The linkages are based on the relationships between different variables. The color of each line refers to the degree of relationship between the variables.
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Figure 7. Relationships between soil properties and P fractions by redundancy analysis. Soil pH, available N, Olsen P (OP), NH4+-N, and available K are explanatory variables that showed significant effect on P fractions (response variables).
Figure 7. Relationships between soil properties and P fractions by redundancy analysis. Soil pH, available N, Olsen P (OP), NH4+-N, and available K are explanatory variables that showed significant effect on P fractions (response variables).
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Table 1. Effect of long-term N and P fertilization on soil pH, available N, P, K, NH4+-N, and soil organic matter concentrations (average 2016–2017). Data represent the mean and different letters indicate significant differences (p < 0.05).
Table 1. Effect of long-term N and P fertilization on soil pH, available N, P, K, NH4+-N, and soil organic matter concentrations (average 2016–2017). Data represent the mean and different letters indicate significant differences (p < 0.05).
TreatmentpHNO3–-NNH4+-NOlsen PAvailable KOrganic Matter
(H2O)(mg kg−1)(mg kg−1)(mg kg−1)(mg kg−1)(g kg−1)
N0P08.27 ab2.95 b0.32 a4.13 c155.04 b14.71 bc
N0P1008.26 ab1.78 b0.11 a13.03 b154.65 b14.99 ab
N160P08.15 b6.88 a0.62 a3.70 c146.84 b13.47 cd
N160P1008.19 ab7.20 a0.61 a20.33 a147.28 b15.21 ab
NF8.29 a7.06 a0.32 a6.41 c171.25 a16.21 a
BF8.31 a8.79 a0.46 a5.52 c167.67 a12.97 d
Table 2. Different P fractions (mg kg−1), P uptake and grain yield (kg ha−1) in each fertilized cropland and fallow treatments for the average of the years 2016 and 2017. Data represent the mean (n = 3) and different letters indicate a significant differences (p < 0.05).
Table 2. Different P fractions (mg kg−1), P uptake and grain yield (kg ha−1) in each fertilized cropland and fallow treatments for the average of the years 2016 and 2017. Data represent the mean (n = 3) and different letters indicate a significant differences (p < 0.05).
TreatmentNaHCO3-PiNaHCO3-PoNaOH-PiNaOH-PoHClD-PiHClc-PiHClc-PoResidual-PTotal PGrain YieldP Uptake
N0P017.1 b33.2 bc22.9 ab39.9 b190.4 d74.0 a56.2 b58.0 b491.8 d2671 b9 c
N0P10042.8 a69.7 a26.0 a46.5 a189.0 d63.6 ab80.8 a70.4 a588.8 a2938 b13 b
N160P013.1 c27.3 c24.5 a27.2 c165.5 e53.5 bc58.8 b54.4 bc424.3 e2964 b6 d
N160P10040.5 a68.3 a19.4 ab35.6 b216.3 c49.8 c72.4 a48.7 c550.9 b5845 a17 a
NF13.5 bc27.5 c16.7 b13.3 d303.1 b51.6 bc28.6 c58.0 b512.3 c....
BF15.1 bc36.0 b19.3 ab7.7 d316.2 a52.5 bc28.1 c60.4 b535.3 b....
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Mahmood, M.; Mehmood, S.; Ahmed, W.; Salah Elrys, A.; Tian, Y.; Hui, X.; Ayyoub, A.; Elnahal, A.S.M.; Li, W.; Wang, Z.; et al. Exploring Phosphorus Fraction Dynamics in Loess Soils: Impact of Long-Term Nitrogen and Phosphorus Fertilization on Cropland and Fallow Land. Sustainability 2023, 15, 12342. https://doi.org/10.3390/su151612342

AMA Style

Mahmood M, Mehmood S, Ahmed W, Salah Elrys A, Tian Y, Hui X, Ayyoub A, Elnahal ASM, Li W, Wang Z, et al. Exploring Phosphorus Fraction Dynamics in Loess Soils: Impact of Long-Term Nitrogen and Phosphorus Fertilization on Cropland and Fallow Land. Sustainability. 2023; 15(16):12342. https://doi.org/10.3390/su151612342

Chicago/Turabian Style

Mahmood, Mohsin, Sajid Mehmood, Waqas Ahmed, Ahmed Salah Elrys, Yi Tian, Xiaoli Hui, Anam Ayyoub, Ahmed S. M. Elnahal, Weidong Li, Zhaohui Wang, and et al. 2023. "Exploring Phosphorus Fraction Dynamics in Loess Soils: Impact of Long-Term Nitrogen and Phosphorus Fertilization on Cropland and Fallow Land" Sustainability 15, no. 16: 12342. https://doi.org/10.3390/su151612342

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