We studied the effects of liming to increase soil pH from acidic to near-neutral on the degree of phosphorus saturation (DPS), the P saturation index (PSI), Mehlich-3 P (PM3), and water-extractable P (Pw). Soils collected from a long-term highbush blueberry experiment were incubated at 22.5 °C for 93 days after CaCO3 amendment to increase pH values from 4.1, 4.8, and 5.5 to 6 and from 5.8 to 6.5. Liming decreased PM3 by 8%, 6%, 10%, and 11% with increasing initial soil pH. The PM3 concentrations of all the studied soils belonged to the very high class with critical DPS > 25%, which are associated with increased environmental risk of P loss with runoff. For soils with initial pH values of 5.8, Pw was 3.65 mg kg–1, in line with critical DPS > 25%, but decreased to 2.74 mg kg–1 with CaCO3 addition. In contrast, soils with initial pH < 5.5 had lower Pw concentrations and CaCO3 did not significantly decrease Pw at the end of the incubation averaging 1.02, 1.11, and 1.43 mg kg–1 for initial pH 4.1, 4.8, and 5.5, respectively. The low Pw concentrations of soils with initial pH < 5.5 were in line with low PSI (5.2%–6.1%), but did not reflect DPS values > 25%. It is possible that high exchangeable aluminum (Al) (AlM3 > 2500 mg kg–1) enhanced the fixation of phosphate ions from the soil solution, thus reducing Pw. Our results suggest that using PM3 as a sole indicator of environmental risk likely underestimates potential P losses compared with Pw.
Les auteurs ont étudié les effets du chaulage utilisé pour neutraliser le pH d’un sol acide i) sur le taux de saturation du P (TSP), l’indice de saturation du P (ISP), la teneur en P selon la technique Mehlich-3 (PM3) et la teneur du P extractible à l’eau P (Pw). Des échantillons de sol venant d’une expérience de longue haleine sur le bleuet en corymbe ont été incubés pendant 93 jours à 22,5 °C après avoir été amendés avec du CaCO3 de façon à augmenter le pH de 4,1, 4,8 ou 5,5 à 6 et de 5,8 à 6,5. Le chaulage diminue la concentration de PM3 de 8, 6, 10 et 11 %, selon le pH du sol de plus en plus élevé. Dans tous les sols examinés, la concentration de PM3 se retrouve dans la classe la plus élevée, avec un TSP supérieur à 25 %, associé à un risque plus grand de lixiviation par le ruissellement. Pour les sols dont le pH initial était de 5,8, la concentration de Pw s’établissait à 3,65 mg par kg, ce qui est cohérent avec un TSP critique de plus de 25%, mais l’addition de CaCO3 ramène la concentration à 2,74 mg par kg. En revanche, les sols au pH inférieur à 5,5 au départ se caractérisaient par une concentration inférieure de Pw qui n’avait pas été affectée de manière significative par le CaCO3 à la fin de la période d’incubation (concentration moyenne de 1,02, 1,11 et 1,43 mg par kg pour un pH initial de 4,1, 4,8 ou 5,5, respectivement). La faible concentration de Pw observée dans les sols dont le pH initial était inférieur à 5,5 est cohérente avec le faible ISP (de 5,2 à 6,1 %), mais pas avec un TSP inférieur à 25 %. Il se peut qu’une concentration élevée d’aluminium facilement échangeable (AlM3 > 2 500 mg par kg) ait accru la fixation des ions phosphate dans la solution de sol, ce qui a réduit la concentration de Pw. Ces résultats laissent croire que, contrairement à ce qui se produit quand on utilise la concentration de Pw, on sous-estime sans doute les pertes potentielles de P quand on ne se sert que de la concentration de PM3 comme indicateur des risques environnementaux. [Traduit par la Rédaction]
Excessive phosphorus (P) from manure and inorganic fertilizer applications to agricultural soils is the major cause of surface water eutrophication (Sharpley et al. 2015). In many parts of the world, agri-environmental indicators are used to monitor the risk of P loss in cropped soils under intensive nutrient management (Sharpley et al. 2015). The degree of P saturation (DPS) developed in the Netherlands to monitor the risk of P loss for acidic soils is defined as the ratio of the concentration of phosphate extracted by oxalate (POx) to P sorption capacity (PSC) (Breeuwsma and Silva 1992)). The PSC is derived from the concentrations of aluminum (Al) and iron (Fe) extracted by oxalate (AlOx and FeOx; Benjannet et al. 2018). Notably, a significant increase of P solubility at a critical saturation threshold DPS of 25% was established for acidic soils of the Netherlands (Breeuwsma and Silva 1992). The P saturation index (PSI) developed in North America is based on the ratio of P concentrations to Al and/or Fe in Mehlich-3 extracts. The PSI has been widely adopted in North America because Mehlich-3 extraction is a routine analysis in many laboratories (Khiari et al. 2000).
In several Canadian provinces where acidic soils prevail, including Quebec and the Maritimes, the PSI has been used to assess the risk of P loss for almost two decades (Khiari et al. 2000; Benjannet et al. 2018). In British Columbia (BC), the PSI was shown to be a proxy of DPS to assess the risk of P loss in agricultural soils (Messiga et al. 2020, 2021). Both DPS and PSI are related to water-extractable P (Pw) concentration and are ratios of concentrations of the same elements with different extractions (Khiari et al. 2000; Self-Davis et al. 2000). The Pw measures phosphate readily extracted in water, which, in turn, is a potential indicator of P that could be lost from soils in runoff (Khiari et al. 2000; Sims et al. 2002). The current strategies aim at relating the capacity of P loss from soils through desorption from the soil matrix using DPS and PSI as indicators of P potential to be released into the soil solution (Sims et al. 2002). The assessment of risk of P loss based on PSI and Pw with soil P tests such as Mehlich-3 P (PM3) is well understood, but further work is needed to evaluate how these indicators change based on varying soil properties and agricultural practices. Soil pH is a key property that partly controls the relationships between PSI and Pw (Benjannet et al. 2018). Moreover, the PSC used to derive PSI is significantly influenced by soil pH (Penn and Camberato 2019). Previous studies reported better correlations between PSI and Pw for soils characterized by a very narrow range of soil acidity compared with soils with wide pH range (3.4 –7.9; Sims et al. 2002).
A threshold PSI value of 15% above which the risk of P loss is high corresponding to 25% DPS was obtained across a range of acidic to neutral coarse-textured soils in Quebec (pH 4.6–6.7; Khiari et al. 2000). In Prince Edward Island, PSI values were 19% for soils with pH < 5.5 and 14% for soils with pH > 5.5 (Benjannet et al. 2018). Similarly, in a recent study in BC, PSI values were 18% for soils with pH < 4.7 and 10% for soils with pH > 5.5 (Messiga et al. 2021). At very low pH, P is strongly retained by Fe and Al oxyhydroxides, explaining the higher PSI thresholds, while association with Ca2+ in limed and calcareous soils is less documented (Ige et al. 2005). It is, however, expected that an increase in soil pH would introduce hydroxyl groups (OH–) into the soil solution, which would displace P ions from oxides of Al and Fe (Hinsinger 2001). The end result of these reactions would be an increase in P release in soil solution (Penn and Camberato 2019), which could decrease PSI values.
In many agricultural regions, the excessive application of nitrogen (N) fertilizers has led to acidification of arable soils particularly in the plough layer (Messiga et al. 2013; Wang et al. 2020; Chen et al. 2021). Decreased soil pH has ripple effects on several other soil properties and processes. In acidic soils, soluble Al is increased leading to Al toxicities that inhibit plant root development (Illes et al. 2006; Barrow 2017). Studies have shown that decreased soil pH also affects ammonia (NH3)-oxidizing bacteria and therefore the conversion of NH3 to nitrates (Liu et al. 2014; Wang et al. 2020). In addition, decreased soil pH affects N2O emissions (Wang et al. 2020) and represses soil phosphatase activity (Chen et al. 2021). Because acidic soil pH reduces the productivity and sustainability of many major cropping systems, liming is used to raise the pH to adequate levels. Logically, soil pH increase induced by lime has double effect of increasing the concentration of hydroxyl groups (OH–) and desorption of Al and Fe from the exchange complex, which precipitates into Al and Fe oxyhydroxides that could also increase P sorption (Hinsinger 2001). However, the increase of P availability for root uptake as a result of increased soil pH through lime addition to near-neutral is widely reported (Haynes 1982; Lambers et al. 2008; Barrow 2017; Penn and Camberato 2019). Such increases are also attributed to decreased Al toxicity (Illes et al. 2006). While the benefits of liming are widely recognized, little is known on how increased soil pH upon liming would affect agri-environmental indicators such as PSI and Pw, P sorption characteristics, and therefore the environmental risks of P losses with runoff.
Highbush blueberries, a common cropping system in the BC Fraser Valley, are typically grown in acidic soils (pH from 4.2 to 5.5; Ponnachit and Darnell 2004). However, recent studies have shown that long-term applications of ammonia-based N fertilizers in highbush blueberries further decreased soil pH from the 4.2–5.5 range down to pH 3.8 (Messiga et al. 2018, 2021). At these low pH levels, it is expected that liming is necessary to raise soil pH to optimum levels for highbush blueberry growth and production (Bertrand et al. 1991). Given that blueberry systems in this region have high soil test P levels (van Bochove et al. 2012; Messiga et al. 2021), liming to raise soil pH before a new production cycle may shift soil pH to a range in which P is more readily desorbed, increasing P release from soil to water. A study in New Zealand showed that about 40% of the decrease in soil P upon liming resulted from net mineralization of organic P and adsorption of inorganic P extractable in 0.5 mol L–1 NaHCO3 or NaOH (Perrott and Mansell 1989). As such, there is a need to understand how agri-environmental indicators would be affected by liming in this crop production system.
Highbush blueberry systems offer a unique opportunity to study and improve our understanding of changes in soil pH upon liming on agri-environmental indicators. We hypothesize that applying calcium carbonate (CaCO3) to decrease the acidity of low-pH soils (liming) would (i) enhance the release of phosphate ions from Al and/or Fe oxyhydroxides into the soil solution, and (ii) lower the critical PSI indicating an increased risk of P loss. The objectives of this study were to determine the effects of increasing soil pH from acidic to near-neutral using lime on (i) the DPS, the PSI, and concentrations of PM3 and Pw; (ii) soil P pools determined by sequential fractionation; and (iii) phosphate sorption characteristics.
Materials and methods
Long-term experimental plots
A long-term highbush blueberry experiment, established in 2008 on flat topography at 7.6 m above sea level at the Agriculture and Agri-Food Canada, Agassiz Research and Development Centre (Agassiz RDC) (49°14′N, 121°45′W), was selected for this study. The local climate is characterized as moderate oceanic with relatively cool and dry summers, and warm and rainy winters (British Columbia Agriculture and Food Climate Action Initiative 2015). Annual rainfall ranges from 1483 to 1689 mm with the peak of about 280 – 350 mm received in November. Average daily temperatures range between 3.4 °C in January and 18.8 °C in August (30 years average, 1981 – 2010, Agassiz Climate Daily Almanac-Agassiz CDA station). The soil is a loam textured soil of the Monroe series, classified as Dystric Brunisols (Canadian System of Soil Classification 1998) or Typic Dystroxerepts in the U.S. Soil Taxonomy (Soil Survey Staff 2010). The soil is moderately to well drained, shallow with a weakly developed profile. The 13-year experiment consists of annual ammonium sulfate (21-0-0) application at three rates (100%, 150%, and 200% of the annual recommendation for blueberry based on the BC blueberry production guide) applied either as broadcast or fertigation, for a total of seven treatments including a control (0%), arranged in a randomized complete block design with four replicates (Messiga et al. 2018). General physicochemical properties, shown in Table 1, include: pH, 4.1–5.8; electrical conductivity, 118–311 µs cm–1; total organic carbon, 1.97%–3.74%; total N, 0.15–0.22; PM3, 163–200 mg kg–1; Mehlich-3 extractable Al (AlM3), 2126–3218 mg kg–1; and Fe (FeM3), 166–210 mg kg–1.
General properties of soils with initial pH values of 4.1, 4.8, 5.5, and 5.8 collected at 0–30 cm soil depth in a long-term blueberry experiment receiving annual applications of nitrogen fertilizers (ammonium sulfate) at Agassiz RDC in British Columbia, Canada.
Soils were selected according to their variable pH to fit into four descriptive acidity classes (Soil Survey Division Staff 1993): (i) extremely acidic (4.1), (ii) very strongly acidic (4.8), (iii) strongly acidic (5.5), and (iv) moderately acidic (5.8). We used soil samples collected from a long-term highbush blueberry experiment with a history of ammonium-based fertilizer applications that produced a range of soil pH from extremely low to near-neutral (pH = 3.8–6.5). Three treatments of the 13-year blueberry experiment and an adjacent field with identical agri-practices were therefore selected to fit these classes: (i) broadcast N (200% for pH 4.1); (ii) fertigation N (100% for pH 4.8); (iii) control (0% N for pH 5.8); and (iv) an adjacent field under rye grass (pH 5.5). For soil sampling, after discarding the layer of sawdust mulch at the soil surface approximately 3.5 kg of soil was collected down to 30 cm depth using a shovel, and then was placed in a labeled clean 10 L bucket and mixed thoroughly. The fresh soil was sieved (<2 mm) and kept at 4 °C in the laboratory. Two 80 g soil samples taken from each bucket were air dried prior to physico-chemical analyses, including pH, texture, organic matter content, cation exchange capacity (CEC), total N, total S, total C, organic P, total P, PM3, AlM3, FeM3, and other Mehlich-3 extractable cations including Ca (CaM3), and Mg (MgM3). The remaining soil was kept frozen at –20 °C until the beginning of the incubation experiment.
Eight combination treatments (four initial soils (pH 4.1, 4.8, 5.5, and 5.8) × two lime applications (with and without lime)) were used for this study. The lime treatment was selected to reach a target pH of 6.0 for soils initially at pH 4.1, 4.8, and 5.5, and a target pH of 6.5 for the soil initially at pH 5.8. The lime requirement (LR) was calculated using a modified procedure of Laboski and Peters (2012), which used the CaCO3 equivalent ratio to moles of the H+ concentrations to be neutralized from initial pH to target pH. The LR to raise the initial pH to 6.0 was 9.0 g for pH 4.1, 4.0 g for pH 4.8, 0.7 g for pH 5.5, and 0.1 g for pH 5.8. The eight combination treatments, each replicated four times (32 experimental units in total), were (i) pH 4.1 without lime (pH4.1L0), (ii) pH 4.1 with lime (pH4.1L1), (iii) pH 4.8 without lime (pH4.8L0), (iv) pH 4.8 with lime (pH4.8L1), (v) pH 5.5 without lime (pH5.5L0), (vi) pH 5.5 with lime (pH5.5L1), (vii) pH 5.8 without lime (pH5.8L0), and (viii) pH 5.8 with lime (pH 5.8L1). For treatments without lime, 1200 g of soil was mixed in a clean bucket and 250 g was weighed in four Mason jars per treatment. For the treatments with lime, 1200 g of soil was placed in a clean bucket, mixed with CaCO3 (fine powder, 99.95%–100.5% dry basis, pH 8, from Sigma–Aldrich), and 250 g was weighed in four Mason jars per treatment. The 32 Mason jars were closed with parafilm into which 10 holes were punctured with a needle to allow for gas exchange. The Mason jars were arranged in a completely randomized design in a growth chamber (Conviron Adaptis A1000, Winnipeg, Canada) at 22.5 °C for 93 days. The weight of each Mason jar at the start of the incubation was recorded to monitor the soil moisture content. Moisture content was maintained at 60% water-filled pore space to maximize microbial activity (Franzluebber et al. 1999). Each Mason jar was weighed every 3 days to check moisture content, and distilled water was added by a bottle-top dispenser if the moisture content decreased by more than 10%. After 30 and 44 days of incubation, 10 g subsamples were taken from each jar to monitor changes in soil pH. After 30 days of incubation, it was observed that the soils initially at pH 5.8 did not reach the target pH 6.0. An extra 20 mg of CaCO3 was mixed with the soil in the jars for that treatment, which increased the soil pH to ∼6.5.
After 93 days of incubation, the soil in each Mason jar was removed and air dried for chemical analysis. Air-dried soils were analyzed for pH in distilled water (1:2 soil:solution) on a MultiLab IDS (Model 4010-3; Hendershot et al. 1993). The Pw was determined by shaking 2.0 g of air-dried soil mixed in 50 mL centrifuge tubes with 20 mL distilled water for 1 h, centrifuging the mixture at 3500 r min–1 for 10 min, and filtering through Whatman Grade 42 filter paper (Self-Davis et al. 2000). Phosphorus in the extract was determined by the molybdenum blue colorimetric method (Murphy and Riley 1962). The PM3, AlM3, and FeM3 were determined by shaking 2.5 g of air-dried soil with a 25 mL of Mehlich-3 solution (pH 2.3) for 5 min (Mehlich 1984). The POx, AlOx, and FeOx were determined according to Ross and Wang (1993). The concentrations of PM3, AlM3, FeM3, POx, AlOx, and FeOx were assessed with an inductively coupled plasma optical emission spectrometer (ICP-OES). Total organic P was determined by the ignition method (Saunders and Williams 1955). Total P was determined by digestion using a fluxer (M4 Fluxer, Claisse) as described by Kowalenko and Babuin (2014). Phosphorus concentrations in these digests were analyzed by an ICP-OES (ICAP 7000 series, Thermo Scientific).
Calculations of degree of P saturation and P saturation index
The DPS was calculated by using the molar ratio of POx, AlOx, and FeOx (Breeuwsma and Silva 1992):Breeuwsma and Silva 1992). We used an average αm value of 0.5 to calculate DPS (Benjannet et al. 2018).
The P saturation index was calculated using two variations, PSI1 and PSI2, using PM3, AlM3, and FeM3, as follows (Khiari et al. 2000):
Phosphate sorption experiments
Phosphate sorption experiments were also conducted on air-dried soils (Messiga et al. 2021). In brief, solutions with a range of P concentrations were formulated in a matrix of distilled water and 100 µL of toluene (to inhibit microbial activity) for batch equilibration with the soil. Initial equilibrating solution P concentrations were 0, 10, 20, 50, 100, and 150 mg P L–1 as KH2PO4. Sorption batches consisted of 2.0 g air-dried soil mixed in 50 mL centrifuge tubes with 30 mL of the equilibrating solution. The tubes were gently shaken end-over-end for 24 h at room temperature, centrifuged at 3500 r min–1 for 10 min, and then filtered through Whatman Grade 42 filter paper. An aliquot of the supernatant solution was used to determine P in solution by ICP-OES.
Soil P fractionation
Finally, soil P fractionation was performed on the remaining air-dried soil by sequentially extracting P according to a modified method of Chang and Jackson (1958) as described by Zhang and Kovar (2009). Briefly, 0.5 g of finely ground soil (0.2 mm) was sequentially extracted by 1 mol L–1 NH4Cl (NH4Cl-P), 0.5 mol L–1 NH4F (NH4F-P), 0.1 mol L–1 NaOH (NaOH-P), a mixture of 0.3 mol L–1 sodium citrate dihydrate, Na3C6H5O7.2H20 + 1 mol L–1 NaHCO3 for reductant-soluble P (CBD-P), and 0.25 mol L–1 H2SO4 (H2SO4-P) at room temperature. For the purpose of precision, the inorganic P fractions are described by their extraction method (Barrow et al. 2020; Guppy 2021). Phosphorus concentrations in the extracts were analyzed by the molybdenum blue colorimetric method (Murphy and Riley 1962).
Sorption data fitting
Sorption data for each soil pH and lime treatments were fitted using Langmuir and Freundlich isotherms. Sorption isotherms were parameterized using SAS PROC NLIN version 9 (SAS Institute Inc. 2010). The Langmuir isotherms resulted in the best goodness of fit for all soil pH and lime treatments. The Langmuir isotherm is described as follows:Messiga et al. 2020). The goodness of fit of the sorption isotherms to experimental data was assessed by the R2 and root mean square deviation (RMSD). The RMSD was calculated with the following equation:
To statistically compare the Smax (not ka because of autocorrelations) parameter between not limed and limed treatments, we calculated the following F ratio:
Data of soil P analysis, DPS, PSI, and P fractionation were analyzed statistically using one-way Analysis of variance (ANOVA) with random effects as replicates and the eight treatments as fixed effects using SAS PROC MIXED (SAS Institute 2010). Data were first checked for assumptions of ANOVA using Shapiro–Wilk test and Q–Q plots for normality and for equal variance using residual plots and Bartlett's tests. Differences between least square means for treatment pairs (without lime vs. with lime at each initial pH level) were tested at significance level of P = 0.05. Nonlinear regression techniques were applied to pH, PM3, Pw, DPS, PSI1, and PSI2 using NLIN procedure of SAS. The fit of the regression curves was evaluated using R2 to determine how well the curve explains measured data variation. Principal component analysis (PCA) was carried out using SAS PROC PRINCOMP (SAS Institute 2010) to establish the relationships between agri-environmental indicators and chemical properties of soil and P fractions.
Liming effects on soil pH, concentrations of Mehlich-3 P, and water-extractable P, and agri-environmental indicators
As projected by the LR calculations, soil pH was increased from 4.1 to 6.0, 4.8 to 6.0, 5.5 to 6.0, and 5.8 to 6.5, respectively, after 93 days of incubation with additions of CaCO3 (Fig. 1a). The initial soil pH value did not change after 93 days of incubation. (Fig. 1a). The concentrations of PM3 ranged from 146 to 200 mg kg–1 and were not significantly changed by liming (Fig. 1b). The concentrations of Pw ranged from 1.0 to 3.65 mg kg–1 and were significantly affected by additions of lime only for soils with initial pH of 5.8 (Fig. 1c). The DPS, PSI1, and PSI2 were not affected by additions of lime for any of these soils, regardless of initial soil pH (Figs. 1d–1f). On average, the DPS values were 27%, 26%, 25%, and 27% for soils at initial soil pH of 4.1, 4.8, 5.5, and 5.8, respectively (Fig. 1d). On average, the PSI1 values were 6.0%, 5.2%, 5.6%, and 6.1%, respectively (Fig. 1e). On average, the PSI2 values were 5.8%, 5.0%, 5.4%, and 5.8%, respectively (Fig. 1f).
Liming effects on the relationships between concentrations of water-extractable P and agri-environmental indicators
There were two patterns of the relationships between concentrations of Pw and agri-environmental indicators at the end of incubation depending on the initial soil pH. At initial soil pH ≤ 5.5, the concentrations of Pw were related to agri-environmental indicators, PM3, DPS, PSI1, PSI2, PSC, and CaM3, by quadratic regressions (Figs. 2a–2f). The slopes of the quadratic regressions were positive for PM3 (Fig. 2a), DPS (Fig. 2b), and PSC (Fig. 2e), but negative for PSI1 (Fig. 2c), PSI2 (Fig. 2d), and CaM3 (Fig. 2f). In addition, the R2 of the regressions were 0.25 for PM3, 0.49 for DPS, 0.22 for PSI1, 0.23 for PSI2, 0.23 for PSC, and 0.58 for CaM3. In contrast, at initial pH = 5.8, there was no trend in the distribution pattern of the concentrations of Pw with PM3, DPS, PSI1, PSI2, and PSC except the decreasing trends following additions of lime and the increased soil pH (Figs. 2a–2e). However, at initial pH = 5.8, the concentrations of Pw decreased with increasing CaM3 following additions of lime (Fig. 2f).
A further examination showed that the relationships among agri-environmental indicators were described by linear regressions (Figs. 3a–3d). The linear regression between PSI1 and PSI2 was characterized by a positive slope = 0.96 and R2 = 0.99 and was not affected by liming (Fig. 3a). Similarly, the relationships between DPS and PSI1, PSI2, and PM3 were not affected by liming and were described by linear regressions with positive slopes = 0.39 and R2 = 0.52 for DPS vs. PSI1 (Fig. 3b), positive slopes = 0.38 and R2 = 0.53 for DPS vs. PSI2 (Fig. 3c), and positive slopes = 20.5 and R2 = 0.45 for DPS vs. PM3 (Fig. 3d).
Liming effects on concentrations of aluminum, iron, and calcium
There was a decreasing trend of the concentrations of AlM3 and FeM3 in soils with increasing soil pH. The addition of CaCO3 further decreased the concentrations of AlM3 and FeM3 by 24%, 11%, 2.3%, and 17% for initial soil pH = 4.1, 4.8, 5.5, and 5.8, respectively. The relationships between the concentrations of AlM3 and FeM3 vs. soil pH were not affected by the addition of CaCO3 and were described by linear regressions with negative slope = –329.61 and R2 = 0.62 for AlM3 (Fig. 4a), and negative slope = –8.06 and R2 = 0.33 for FeM3 (Fig. 4b).
In contrast, there was an increasing trend of the concentrations of CaM3 in soils with increasing soil pH. The addition of CaCO3 further increased the concentrations of CaM3 by 5.5 times, 2.4 times, 1.4 times, and 1.8 times for soils with initial pH 4.1, 4.8, 5.5, and 5.8, respectively. The relationship between the concentrations of CaM3 and soil pH was not affected by the addition of CaCO3 and was described by a power regression with a positive slope = 0.48 and R2 = 0.44 (Fig. 4c).
There was no specific trend in the concentrations of AlOx and FeOx with initial soil pH as observed with AlM3 and FeM3. On average, the concentrations of AlOx were 321, 332, 237, and 320 mmol kg–1 for initial soil pH values of 4.1, 4.8, 5.5, and 5.8, respectively, indicating no effect of soil pH or CaCO3 additions (Fig. 4d). The concentrations of FeOx were 87, 81, 59, and 83 mmol kg–1 for initial soil pH values of 4.1, 4.8, 5.5, and 5.8, respectively, indicating no effect of soil pH or CaCO3 additions (Fig. 4e).
Liming effects on phosphorus sorption characteristics
The experimental data of the sorption study for all soils at the end of the 93 days of incubation were best fitted by the Langmuir equation, with R2 values 0.97 and 0.99 (Fig. 5). The Smax and ka values were affected by liming only for soils with initial pH values of 4.1 and 5.8 (Figs. 5a and 5d). For soils with initial pH 4.1, the Smax value was 1382 mg P kg–1 without lime but significantly decreased to 1243 mg P kg–1 after liming (Table 2; Fig. 5d). For soils with initial pH 5.8, the Smax value was 1093 mg P kg–1 without lime but significantly increased to 1114 mg P kg–1 after liming (Table 2; Fig. 5d).
Comparison of the parameters of the Langmuir equation for the four soils at their initial pH values following additions of lime after 93 days of incubation.
For the other soils, the Smax value was 1244 mg P kg–1 without lime and 1267 mg P kg–1 with lime with an average Smax value of 1254 mg P kg–1 for initial soil pH 4.8; 745 mg P kg–1 without lime and 795 mg P kg–1 with lime with an average Smax value of 777 mg P kg–1 for initial soil pH 5.5; and 1093 mg P kg–1 without lime and 1114 mg P kg–1 with lime for initial soil pH 5.8 (Figs. 5b and 5c).
Liming effects on phosphorus fractions, organic P, and total P
At the end of the 93 days of incubation, the concentrations of NH4Cl-P significantly decreased with addition of CaCO3 for soils with initial pH 4.1 only, by 50.4% (Fig. 6a). The decreasing trends of NH4Cl-P in soils with the other initial pH values were not significant (Fig. 6a). The concentrations of NH4F-P significantly decreased with addition of CaCO3 for soils with initial soil pH 4.1 only, from 59 to 29 mg kg–1 (Fig. 6b). The concentrations of NaOH-P significantly decreased with addition of CaCO3, from 324 to 247 mg kg–1, for initial soil pH = 4.8 (Fig. 6c). The decreasing NaOH-P trends observed with the other initial pH with addition of CaCO3 were not significant. The concentrations of the reductant-soluble P fraction (CBD-P) significantly increased with addition of CaCO3, from 26 to 218 mg kg–1, at initial soil pH = 4.1, and significantly decreased with addition of CaCO3, from 364 to 21 mg kg–1 and from 103 to 23 mg kg–1, at initial soil pH 5.5 and 5.8, respectively, with no significant change for soils initially at pH 4.8 (Fig. 6d). The concentrations of the H2SO4-P fraction were not significantly affected by addition of CaCO3 (Fig. 6e). The corresponding organic P fractions at different sequential extracts were not influenced by liming.
At the end of the 93 days of incubation, soil organic P and total P were not affected by additions of lime. On average over the two lime additions, organic P concentrations were 339–444, 427 mg kg–1 (Fig. 7a), while total P concentrations were 1181–1492 mg kg–1 (Fig. 7b).
Relationships between Pw, Mehlich-3 P, agri-environmental indicators, inorganic P fractions, and chemical soil properties
The PCA showed that Pw was highly correlated to PSI1, PSI2, and CaM3, whereas PM3, AlM3, FeM3 inorganic P fractions and chemical soil properties were highly correlated following the addition of CaCO3 (Fig. 8). The eight combinations of initial soil pH × addition of CaCO3 were separated into four groups for their effects on the studied properties, including (i) pH4.1L0 and pH4.8L0, (ii) pH4.8L1, (iii) pH5.5L0 and pH5.5L1, and (iv) pH4.1L1, pH5.8L0, and pH5.8L1. Groups I and II positively affected NH4Cl-P, AlM3, FeM3, and ka, but negatively affected Pw, PSI1, PSI2, and CaM3; Group III positively affected H2SO4-P and NaHCO3-P, but negatively affected DPS, Al-P, Fe-P, PSC, Smax, and PM3; Group IV positively affected Pw, PSI1 and PSI2, CaM3, and pH. Pearson's correlation among Pw, PM3, agri-environmental indicators (PSI1, PSI2, and DPS), inorganic P fractions (NH4Cl-P, NH4F-P, NaOH-P, NaHCO3-P, and H2SO4-P), and chemical soil properties (AlM3, FeM3, CaM3, PSC, ka, and Smax) were in line with the results of PCA (Table 3). The concentrations of Pw were positively correlated with CaM3, PSI1, and PSI2. Increasing soil pH was associated with decrease in AlM3, FeM3, and NH4Cl-P more strongly at initial soil pH values of 4.1 and 4.8 (Table 3). The concentrations of H2SO4-P were negatively correlated with ka and Smax. Remarkably, the Pw were positively associated with PSI1 and PSI2 than DPS (Table 3).
Pearson's correlation matrix.
Low soil pH and relationship between concentrations of PM3, Pw, and agri-environmental indicators
The concentrations of PM3 across all soils (Fig. 1b) fall into the “very high” P class (above 150 mg kg–1) and represented agricultural soils typically found in the BC Fraser Valley (Kowalenko et al. 2007; Reid and Schneider 2019). According to the literature and local recommendations, these high PM3 concentrations are associated with increased risk of environmental impact due to P losses with runoff (Kowalenko et al. 2007; Reid and Schneider 2019). A value of DPS > 25% (Fig. 1d) is considered the critical limit above which P runoff is enhanced in acidic soils (van der Zee et al. 1987), and all the studied soils had DPS values above this critical limit. However, only soils with initial pH 5.8 had concentrations of Pw of 3.8 mg kg–1 (Fig. 1c), which is close to critical concentration values set at 4.0 mg kg–1 across different agro-ecosystems in BC (van Bochove et al. 2012) and 3.7 mg kg–1 in silage corn and blueberry plantings (Messiga et al. 2021) in the Fraser Valley. The soils with initial pH 5.8 have not received any N fertilizers for the past 10 years (Messiga et al. 2018). The other soils with initial pH values of 4.1, 4.8, and 5.5 had concentrations of Pw below the critical level (Fig. 1c), which contrasted with their PM3 and DPS values, indicating significant pH effects on the solubility of P and assessment of the risk of P losses with runoff. The soils with initial soil pH values of 4.1, 4.8, and 5.5 received ammonium sulfate at different rates, which resulted in the decreased soil pH and reduced changes in Pw concentration patterns compared with soils with initial pH 5.8 (Messiga et al. 2018). The significant decline with liming of Pw at pH 5.8 could be explained by the dissociation of P ions leading to speciation shifts. At pH 6.5, phosphate will occur as HPO42– more than as H2PO4–, which will increase phosphate sorption (Chang and Overby 1986; Han 2020). The mismatch between concentrations of PM3, DPS, and concentrations of Pw as influenced by soil pH indicates the need to improve our understanding of the functioning of these indicators to assess the risk of P losses with runoff across the range of cropping systems and management practices found in the Fraser Valley.
The low PSI1 and PSI2 (Figs. 1e and 1f), which also contrasted with high concentrations of PM3 (150–200 mg kg–1) and DPS > 25% (Figs. 1b and 1d), were in line with low Pw concentrations associated with soils at initial pH ≤ 5.5 (Fig. 1c). The discrepancy between the indicators of risk provided by DPS and PSI1 and PSI2 as they relate to Pw and PM3 of soils with initial pH ≤ 5.5 could be explained by a combination of factors inherent to the extraction solutions and the chemistry of P, Al, Fe, and Ca, and to some extent, phytates driven by soil pH. We found that the concentrations of AlM3 in soils with initial pH values of 4.1 and 4.8 were higher than soils with initial pH values of 5.5 and 5.8 (Table 2; Figs. 4a and 4b). On average, the concentration of AlM3 was 3056 mg kg–1 in soils with initial pH values of 4.1 and 4.8 compared with 2594 mg kg–1 in soils with initial pH values of 5.5 and 5.8 (Table 2; Figs. 4a and 4b). The concentrations of AlM3 with initial soil pH ≤ 5.5 followed the same trend as PM3 (Fig. 1b). At these low initial pH levels, Al3+ cations present in the primary minerals are easily expelled in the soil solution where they form secondary minerals of Al oxides and oxyhydroxides. These secondary Al oxides and oxyhydroxides further react and precipitate with phosphate ions (Penn and Camberato 2019). In addition, the low initial pH prevailing in the soil solution increases the amount of pH-dependent charges at the edges of Al oxides and oxyhydroxides and other broken bonds of silicate minerals onto which the fixation of phosphate ions is enhanced (Penn and Camberato 2019). It is therefore possible that at low initial pH, the combination of high concentrations of Al oxyhydroxides provides large surface area for adsorption sites and large pH-dependent charges enhance the retention of phosphate ions in the soil solution, thus reducing the concentration of Pw. Furthermore, the opposite trends between the concentrations of Pw and PM3 and AlM3 indicate that phosphate ions retained at the surfaces of Al oxides and oxyhydroxides are not soluble in water and therefore are not accounted for by the colorimetric blue methods used in their assessment (Murphy and Riley 1962; Self-Davis et al. 2000; Messiga et al. 2021).
Our results also indicate that Mehlich-3 solution extracts more Al and P at low pH compared with high pH (Figs. 4a and 4b). This is explained by high Al on the cation exchange complex at low pH and the increased Ca onto negatively charged reacting surfaces at high pH that facilitate phosphate adsorption (Barrow 2017). At low pH, the acidity of Mehlich-3 solution promotes the dissolution of Al oxides and oxyhydroxides such as gibbsite, thus increasing the concentration of AlM3 (Penn et al. 2018). The affinity of F– with Al further increases the concentration of AlM3 by complexing additional Al oxides and oxyhydroxides resulting into the formation of AlF3 that is detected upon analysis of the Mehlich-3 extract by ICP (Mehlich 1984). These increased concentrations of AlM3 associated with low soil pH and Mehlich-3 solution decrease the values of PSI1 and PSI2 (Figs. 1e and 1f) relative to PM3 and DPS values (Figs. 1b and 1d;Figs. 3b–3d).
The concentrations of AlOx did not vary with initial soil pH (Fig. 4d) compared with AlM3, indicating a more stable reaction of the ammonium oxalate extracting solution on Al and Fe over the range of acidic pH (Van der Zee et al. 1987). The oxalate extraction dissolves amorphous Al and Fe at the surfaces of which P is retained and not the crystalline Al and Fe (McKeague and Day 1966). These amorphous Al and Fe are made up mainly of Al and Fe hydrous oxides originating from pedogenesis and weathering processes (McKeague and Day 1966). Our results therefore confirm that at low soil pH as those observed in our study, Mehlich-3 extraction tends to overestimate the concentrations of AlM3, which may result into significant decrease in PSI values.
The literature also shows that Al and Fe bound phytates could be dissolved by Mehlich-3 extracting solution particularly at low-pH conditions (Penn et al. 2018). However, we did not find significant changes in total organic P or total P among soils with varying initial soil pH to support the role of phytates in the studied soils (Figs. 7a and 7b). In addition, we did not observe any changes in P sorption characteristics of the soils, which may be ascribed to compounds other than Ca added with lime (Figs. 5a and 5d). At the pH conditions prevailing in our soils, the acidity of Mehlich-3 solution had little effect on Fe because Fe oxides and oxyhydroxide minerals such as goethite and hematite remain partly insoluble. This is also well demonstrated by the 1:1 regression between PSI1 and PSI2 (Fig. 3a) and similar relations between DPS vs. PSI1 and DPS vs. PSI2 (Figs. 3b and 3c). Similar observations were made on a range of soils in the Fraser Valley with Fe having minor effect on PSI (Messiga et al. 2021). The minor role of Fe could also be associated with the pedogenesis of the soil as Dystric Brunisols (Canadian System of Soil Classification 1998) or Typic Dystroxerepts (Soil Survey Staff 2010), which are moderately developed with the absence of appreciable quantities of Fe sesquioxides within the solum (Smith et al. 2011). The high concentrations of AlM3 and PM3 at low initial soil pH are therefore driven by the solubility of Al oxides and oxyhydroxides and further enhanced by the acidity of Mehlich-3 extracting solution (Tran et al. 1990).
Increasing soil pH and changes in agri-environmental indicators
The concentrations of AlM3 significantly reduced with addition of CaCO3 (Fig. 4a). Liming can increase OH– ions that increase precipitation of Al(OH)3 (Haynes 1982; Penn and Camberto 2019). The Ca derived from CaCO3 added to the soil decreased AlM3 concentrations, which is consistent with the neutralizing power of CO32– towards the acidic component (HNO3 and CH3COOH) of Mehlich-3 extraction (Zhang and Kovar 2009). In addition, the added Ca further reacted with fluoride (F–) used in the Mehlich-3 solution, which likely fixed some phosphates from the solution leading to decreased PM3 concentrations in limed soils (Penn et al. 2018). It is interesting to observe that the addition of CaCO3 with its neutralizing power on the acidic component in Mehlich-3 extraction, which decreased AlM3, did not affect the concentrations of AlOx (Figs. 4a, 4b, 4d, and 4e). This result further supports the more specific reaction of the ammonium oxalate extracting solution on Al and Fe over the range of acidic pH (Van der Zee et al. 1987). One objective of this study was to understand how in soils with initial low pH, the release of phosphate ions previously retained onto Al oxides and oxyhydroxides surfaces into the soil solution would change upon increase in soil pH induced by addition of lime. We had expected that decreased AlM3 upon liming would result in high PM3 and Pw concentrations and thus increased PSI values and risk of P losses by runoff. Our results showed that PSI values did not significantly change with increased soil pH from low to near-neutral following applications of CaCO3. Therefore, the decreased concentrations of AlM3 were not translated into increased PSI values and then potential risk of P losses (Figs. 1e and 1f). The lack of PSI change observed in our study could be primarily due to re-association of P with Ca added with CaCO3 (Fig. 4c). The increase in the concentration of CaM3 with addition of CaCO3 was described by a power regression indicating a more than proportional increase of CaM3 with increasing soil pH (Fig. 4c). The P sorption data showed that the effect of Ca was influenced by the initial soil pH and thus the LR or amount of CaCO3 needed to reach the target pH. We found that the addition of CaCO3 affected the sorption characteristics of soils at initial pH values of 4.1 and 5.8 (Figs. 5a and 5d) but not at initial pH values of 4.8 and 5.5 (Figs. 5b and 5c). A comparison of the parameters of the Langmuir equations describing the sorption data of the soils at the respective initial pH upon addition of CaCO3 showed that Smax and ka values were only influenced at initial pH values of 4.1 and 5.8 (Figs. 5a and 5d).
Indeed, the Smax and ka values decreased for initial pH 4.1 and increased for initial pH 5.8 upon addition of CaCO3. The low Smax and Ka values obtained with addition of CaCO3 at initial pH 4.1 shows that the reduction of Al sorption sites was not compensated by the activity of added Ca. Finally, we would also argue that the decreased concentrations of Pw after changing the initial soil pH from 4.1 to 6 are due to the amount of Ca added to the soil with CaCO3 to reach the target pH (Fig. 4c). Indeed, less Ca was added to the soil with initial soil pH = 4.8 and 5.5 to reach the target soil pH = 6. Our results are in line with previous works on Mehlich-3 extractions and the declining AlM3 concentrations reported with increased soil pH (Penn et al. 2018; Messiga et al. 2021). Another important aspect of our study is the high concentration of Pw at initial soil pH 5.8 despite similar total organic and total P across the range of soils (Figs. 1a, 7a, and 7b). This is an indication of the inhibition activity of Al oxides and oxyhydroxides at high pH (Penn et al. 2018).
Increased soil pH and phosphorus fractionation
The use of NH4Cl solution (Chang and Jackson 1958) could explain the contrast between NH4Cl-P and concentrations of Pw with increasing initial soil pH (Figs. 1c and 6a). Gianello and Amorim (2015) showed that NH4Cl solution performs well at extracting Al and other cations in acidic soils indicating the contribution of some amount of P associated with Al (Figs. 6a and 6b). The decrease in the concentrations of NH4Cl-P and NH4F-P with additions of CaCO3, particularly for soils with initial pH 4.1 (Fig. 5a), is consistent with the trend of Pw concentrations (Fig. 1c). The strong adsorption of F– on soil colloids at pH > 5.5 explains the decrease in NH4F-P with increasing pH, which would otherwise displace adsorbed P ions into the solution (Chang and Overby 1986). It is worth noting that the decrease in NH4Cl-P and NaOH-P following addition of CaCO3 for soils with initial pH 4.1 was translated into increased concentrations of NaHCO3-P (Fig. 6d). We also found similar trends among the other P fractions and CBD-P in soils with initial pH values of 5.5 and 5.8 (Fig. 6e), although the change was not significant. These results are consistent with the role of Ca in sorption characteristics of the soil upon addition of CaCO3, particularly Smax and ka values for soils at initial pH values of 4.1 and 5.8 (Figs. 5a and 5d). It is therefore possible that the CBD-P and H2SO4-P represent the sink of P accumulation from other fractions upon liming. The behavior of NaOH-P showed less significant variations with addition of CaCO3, which is consistent with the low solubility of Fe at the initial pH under consideration. This result is consistent with the small variation of FeM3 and FeOx with soil pH (Figs. 4b and 4e).
Relationship between P and agri-environmental indicators, chemical soil properties, and P fractions
From the PCA results, soil pH was inversely related to AlM3, FeM3, and NH4Cl-P (Fig. 8). The decrease in AlM3 and FeM3 with increasing soil pH has several consequences, including an increase in the concentration of Pw and PSI1 and PSI2. However, the close relationship between Pw and CaM3 indicates that phosphate ions released by Al and Fe oxides and oxyhydroxides with increased soil pH were later re-associated with Ca. In the conditions of our study, it is difficult to state whether CBD-P and H2SO4-P represent transient fractions of higher solubility that would buffer the solution P following uptake by plants or transport with runoff or leaching. To the best of our knowledge, the solubility of H2SO4-P at target pH selected for this study is high compared with that of NH4F-P and NaOH-P (Barrow 2017). It is well known that the availability of P to plants in acidic soils increases with liming of soil (Penn and Camberato 2019). The present study shows that co-precipitation of phosphate anions with Ca2+ can temporarily buffer this pH-induced increase in P availability. It would be interesting to understand how other liming materials such as dolomitic lime or MgCO3 with less affinity with phosphate would affect the concentration of Pw and PSI values upon changes in soil pH.
At the end of the incubation experiment, soil pH was successfully raised to the target values with addition of CaCO3. We observed decreasing trends of PM3 and Pw concentrations with additions of CaCO3, but the extent was only significant for Pw in soils with initial pH = 5.8 probably because extra lime was added and final soil pH reached 6.5. These results highlight the effects of pH on PSI indicators and are supported by high concentrations of AlM3 in the range of 3000 mg kg–1 at soil pH = 4.1 and 4.8. The concentrations of AlM3 and FeM3 at all initial soil pH decreased when soil pH was increased to the target values, but the effect on Pw was not significant. Our relationship analysis highlighted a close link between Pw and CaM3, as well as opposite relationships of H2SO4-P and NaHCO3-P vs. NH4F-P and NaOH-P fractions. These relationships also indicate that the decreasing trend of Pw concentrations could be due to the association of phosphate ions with Ca2+ added by the liming, acting as a sink for P released by NH4F-P and NaOH-P fractions. The double role of liming through the increasing pH and Ca2+ concentration in soils should certainly deserve joint investigation when studying liming effects on P-related agri-environmental indicators.
AJM thanks Agriculture and Agri-Food Canada for funding this work through an A-Base program (Project ID: J-002266—Solutions for carryover of legacy P in the Fraser Valley and Hullcar Valley). We thank Dr. Barbara Cade-Menun (Swift Current RDC) for her inputs and comments that helped improve the quality of the manuscript. We also thank Jessica Stoeckli and Dean Babuin from the Agassiz RDC research support unit for their assistance with analyses using ICP.
S.N.: Conceptualization, Data curation, Formal analysis, Methodology, Writing – original draft, Writing - review & editing. A.J.M.: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Writing – review & editing. J.-T.C.: Writing – review & editing. S.M.S.: Supervision, Writing – review & editing.