New Methodologies for the Surface Application of Limestone and Gypsum in Different Crop Systems

: To address the problems of soil acidity (pH values below 4.4) in surface and subsurface soil layers and improve soil chemical fertility, this study evaluated three methodologies for surface application of limestone (LS) (ensuring that calcium (Ca) 2+ occupied 70%, 60% or 50% of cation exchange capacity (CEC) at a depth of 0.0–0.2 m) and gypsum (GP, phosphogypsum) (ensuring that Ca 2+ occupied 60%, 50% or 40% of effective cation exchange capacity (ECEC) at a depth of 0.2–0.4 m). LS and GP were applied in a conventional pasture system (CPS), no-till system (NTS), and agropastoral system (APS) in an Arenic Hapludult in Brazil. Surface application of LS and GP using these three methodologies corrected surface and subsurface acidity and improved soil chemical fertility. Speciﬁcally, Ca 2+ content increased in the CPS, NTS, and APS at a depth of 0.0–0.2 m and in the CPS and APS at a depth of 0.2–0.4 m; sulfur (S)-SO 42 − content and Ca 2+ /ECEC increased in the CPS, NTS, and APS at a depth of 0.2–0.4 m; base saturation (BS) increased and aluminum (Al) 3+ content decreased in the NTS and APS at depths of 0.0–0.2 m and 0.2–0.4 m; and pH, magnesium (Mg) 2+ content, CEC, Ca 2+ /CEC, and Mg 2+ /CEC increased and total acidity decreased in the NTS and APS at a depth of 0.0–0.2


Introduction
Conservation management systems aim to prevent soil erosion or degradation and are widely used in agriculture. No-till systems (NTSs) eliminate the practice of turning the soil and allow plant material to accumulate on the surface for the sowing or planting of the next crop [1]. Agropastoral systems (APSs) incorporate pasture, which takes advantage of the soil correction and residual fertilizer applied to the previous crop. In turn, the successive crop benefits from the physical conditioning of the soil and straw production by the pasture [2]. In these conservation management systems, soil acidity is typically corrected by applying limestone (LS) on the soil surface without incorporation. The low mobility of the dissolution products of surface-applied LS limits its efficiency in reducing acidity in subsurface layers of the soil, resulting in variable loads that depend on the leaching of organic and/or inorganic salts through the soil profile [3]. Surface liming (LS application) creates a soil acidity correction front in the soil that is proportional to dose and time [4,5]. Several studies have shown that the positive effects of superficial liming are most pronounced in the surface layers of the soil [5][6][7][8][9][10][11].
Subsoil acidity limits agricultural productivity by restricting root growth and the absorption of water and nutrients by crops [12][13][14]. Improving soil conditions below the surface layers can increase and/or stabilize crop yield, especially in regions with dry winters that are subject to dry spells. One such region is the Cerrado biome in Brazil, which is also characterized by Ca deficiency in the soil subsurface, frequently in association with Al toxicity [15]. An interesting option for improving the subsurface soil layers in NTSs is the application of gypsum (GP), which has been used in acidic soils as a complement to LS [16,17]. The high mobility of GP has been attributed to its greater solubility and the presence of a stable sulfate anion (SO 4 2− ) [18]. In Europe, the anticipated expansion of soil salinization due to climate change is expected to increase the demand for GP as a soil amendment [19,20].
Surface application of GP in NTSs reduces exchangeable Al 3+ levels, increases Ca 2+ and S-SO 4 2− levels and, in some cases, indirectly raises pH values in subsurface soil layers [6,7,[21][22][23][24][25]. Positive effects of GP application after 24 months have been reported [6], and these effects persisted at 36 months after application [7]. In another study, increased pH was observed at depths of 0.2-0.4 m at 8 months and 0.4-0.6 m at 20 and 32 months after surface application of GP [26]. GP has also been found to increase pH and Ca 2+ and S-SO 4 2− levels and reduce exchangeable Al 3+ levels in the soil up to 18 months after application [20]. The authors attributed this effect to a ligand exchange reaction on the surface of soil particles that involved partial neutralization of acidity [27]. Other studies have observed increases in Ca 2+ levels in the soil profile and Mg 2+ lixiviation [24,26,28,29] and decreases in exchangeable Al 3+ [22,23,30] due to surface application of GP.
There is a need to test new LS and GP dosing methodologies, as surface application of LS does not effectively correct subsurface acidity and the optimal dosage of GP remains unclear. To contribute to solving the problems of acidity in the surface and subsurface layers and improving soil chemical fertility in conventional pasture systems (CPSs), NTSs, and APSs, in this study we tested the following hypothesis: surface application of LS (ensuring that Ca 2+ occupies 70%, 60%, or 50% of CEC at a depth of 0.0-0.2 m) and surface application of GP (ensuring that Ca 2+ occupies 60%, 50%, or 40% of ECEC at a depth of 0.2-0.4 m) can correct surface and subsurface acidity and improve soil chemical fertility.

Site, Soil, Climate, and Treatments
The experiment was carried out at the Advanced Research Center and Development for Rubber and Agroforestry Systems of the Agronomic Institute (IAC) of the São Paulo Agency for Agribusiness Technology (APTA), which is located in the Cerrado biome in the municipality of Votuporanga, São Paulo State, Brazil (20 • 20 S, 49 • 58 W and 510 m altitude). The soil in the experimental area is classified as an Arenic Hapludult (USDA soil classification system) [31], hereafter referred to as Ultisol, with a sandy texture. Each plot was 5 m long and 5 m wide, with a total area of 25 m 2 . The climate in the region is tropical with dry winters (Aw-type according to Köppen's classification). The average annual maximum, minimum, and mean temperatures are 31.2 • C, 17.4 • C, and 24 • C, respectively, and the annual average rainfall is 1328.6 mm. Monthly data on potential evapotranspiration (PET), rainfall (R), and average temperature (T) in Votuporanga from 1 November 2017 to 29 February 2020 are shown in Figure 1. Data on PET for January and February 2020 were not available from the data source used.
The experimental design was randomized complete blocks with four replications and four treatments involving LS and GP application. The details of the LS and GP applications in each treatment are provided in Table 1. The experimental design was randomized complete blocks with four replications and four treatments involving LS and GP application. The details of the LS and GP applications in each treatment are provided in Table 1.

Crop Systems
The CPS was implemented in an area of recovered pasture cultivated with millet (Pennisetum glaucum) (fall-winter season 2008/09), soybean (Glycine max L.) (spring-summer season 2009/10), sunn hemp (Crotalaria juncea) (fall-winter season 2009/10), and maize intercropped with palisade grass (Urochloa brizantha syn. Brachiaria brizantha cultivar Marandu) (spring-summer season 2010/11). Beginning in September 2011, newly weaned beef cattle were introduced into the area and remained until slaughter, and the pasture was not restructured thereafter. A continuous grazing system was used with a stocking rate of 1-2 animal units ha −1 (one animal unit is equal to a live weight of 450 kg) according to the forage offered. The animals were allowed to move freely about the pasture. The cattle were grazed to a certain weight and removed, and a new set of cattle were introduced and grazed to a certain weight. LS (29% CaO and 20% MgO) was surface applied at a dose of 1000 kg ha −1 on 29 January 2016, and GP (17% CaO and 14% S-SO4) was surface applied at a dose of 300 kg ha −1 on 3 February 2016.
The NTS was implemented in the 2009/10 season in an area that was previously used for grain production in a conventional soil tillage system. The crops used in the NTS were soybean, maize, sunn hemp, grain sorghum (Sorghum bicolor), and a sorghum-sudangrass hybrid (S. bicolor × S. sudanense) intercropped with palisade grass and Congo grass (U. ruziziensis syn. B. ruziziensis). In the 2016/17 season, soybean and sunn hemp were grown in the spring-summer and fall-winter seasons, respectively.

Crop Systems
The CPS was implemented in an area of recovered pasture cultivated with millet (Pennisetum glaucum) (fall-winter season 2008/09), soybean (Glycine max L.) (spring-summer season 2009/10), sunn hemp (Crotalaria juncea) (fall-winter season 2009/10), and maize intercropped with palisade grass (Urochloa brizantha syn. Brachiaria brizantha cultivar Marandu) (spring-summer season 2010/11). Beginning in September 2011, newly weaned beef cattle were introduced into the area and remained until slaughter, and the pasture was not restructured thereafter. A continuous grazing system was used with a stocking rate of 1-2 animal units ha −1 (one animal unit is equal to a live weight of 450 kg) according to the forage offered. The animals were allowed to move freely about the pasture. The cattle were grazed to a certain weight and removed, and a new set of cattle were introduced and grazed to a certain weight. LS (29% CaO and 20% MgO) was surface applied at a dose of 1000 kg ha −1 on 29 January 2016, and GP (17% CaO and 14% S-SO 4 ) was surface applied at a dose of 300 kg ha −1 on 3 February 2016.
The NTS was implemented in the 2009/10 season in an area that was previously used for grain production in a conventional soil tillage system. The crops used in the NTS were soybean, maize, sunn hemp, grain sorghum (Sorghum bicolor), and a sorghumsudangrass hybrid (S. bicolor × S. sudanense) intercropped with palisade grass and Congo grass (U. ruziziensis syn. B. ruziziensis). In the 2016/17 season, soybean and sunn hemp were grown in the spring-summer and fall-winter seasons, respectively.
The APS was implemented in the 2010/11 season in an area that had been previously used for grain production in a conventional soil tillage system. The crop rotation used in this system was soybean (spring-summer season), sunn hemp (after soybean; fall-winter season), and maize intercropped with palisade grass (after sunn hemp; spring-summer season). In this system, newly weaned beef cattle were introduced sixty days after the harvest of maize intercropped with palisade grass. The cattle remained in the area for 14 months. The area was then closed for thirty days, and the grass was desiccated with glyphosate (a broad-spectrum systemic herbicide). Next, soybean was sown in an NTS on the palisade grass straw. The grazing system was continuous, and the stocking rate was 2-4 animal units ha −1 according to the forage offered. The animals were allowed to move freely about the pasture, and different generations of animals were pastured on the same area.
In the NTS and APS, the maize hybrid Dow AgroSciences 2B587 Power Core™ was sown mechanically on a straw of sunn hemp under no-till, at a row spacing of 0.8 m and density of 6.0 seeds m −1 (0.0167 m plant spacing). Basic fertilization was performed, sowing at a dose of 315 kg ha −1 of 08-28-16 fertilizer (8% N, 28% P 2 O 5 , and 16% K 2 O), with 1.7% Ca and 3.6% S-SO 4 . The first topdressing fertilization of maize was carried out using 20-00-20 fertilizer (20% N and 20% K 2 O) at a dose of 270 kg ha −1 . The second topdressing fertilization of maize was carried out using ammonium sulfate (20% N and 22% S-SO 4 ) at a dose of 250 kg ha −1 .
Palisade grass was sown mechanically in the NTS and APS using 10 kg ha −1 of forage seeds with a cultural value of 50% mixed with simple superphosphate fertilizer (18% P 2 O 5 , 16% Ca, and 8% S-SO 4 ; common practice) at a dose of 60 kg ha −1 . Two rows were sown between rows of the maize crop. The cultural value of seed indicates seed quality and is calculated according to the following formula: cultural value = (%germination × %purity)/100.

Sampling and analysis
Soil samples were collected at depths of 0.0-0.2 and 0.2-0.4 m before the application of the treatments to determine the initial soil characteristics and fertility [35]. The samples were collected at 10 random points in each crop system at depths of 0.0-0.2 and 0.2-0.4 m; thus, 10 subsamples were collected at each depth of the crop system. The 10 subsamples were homogenized and pooled to form a composite sample for each depth of each crop system. Soil was sampled with a metal probe and air-dried before analysis.
pH was evaluated in 0.01 M CaCl 2 2H 2 O solution, and total acidity (H + + Al 3+ ) was determined by titration using 1 N calcium acetate (C 4 H 6 CaO 4 ) solution, pH 7.0. S-SO 4 2− content was determined by the calcium phosphate (Ca 3 (PO 4 ) 2 ) method, and the levels of P, K + , Ca 2+ , and Mg 2+ in the soil were determined by extraction with ion-exchange resin [35]. These results were used to calculate the base saturation (BS) based on the relationship between the content of exchangeable bases in the soil (Ca 2+ , Mg 2+ , and K + ) and the cation exchange capacity (CEC) in cmol c dm −3 , as well as the percentage ratios of K + /CEC, Ca 2+ /CEC, and Mg 2+ /CEC. On 16 April 2018, new soil samples were collected for chemical analysis, determination of fertility [35], and calculation of the doses of LS and GP to be reapplied according to the treatments. The samples were collected at five random points in each plot at depths of 0.0-0.2 and 0.2-0.4 m; thus, five subsamples were collected from each depth of each plot. The five subsamples were homogenized and pooled to form a composite sample of each depth of each plot. Soil was sampled with a metal probe and air-dried before analysis.
Sunn hemp was sown on soybean straw in the NTS at a row spacing of 0.5 m and density of 25 seeds m −1 (0.04 m plant spacing) under no-till.

Sampling and analysis
On 24 April 2019, new soil samples were collected for chemical analysis, determination of fertility [35], and calculation of the doses of LS and GP to be reapplied according to the treatments following the same methodologies used in the previous year.

2019/20 season
The treatment schedule in the 2019/20 season with details of soil and crop management is shown in Table 4.  Treatments   T1  T2  T3  T4  T1  T2  T3  T4  T1  T2  T3  In the NTS, on 12 November 2019, palisade grass was sown in a broadcast system (manually) at a density of 1000 points of cultural value ha −1 .
In the NTS, the maize hybrid Forseed FS533 PWU was sown mechanically on sunn hemp straw under no-till at a row spacing of 0.5 m and density of 4 seeds m −1 (0.25 m plant spacing). Basic fertilization was performed, sowing at a dose of 250 kg ha −1 of 08-28-16 fertilizer (8% N, 28% P 2 O 5 , and 16% K 2 O), with 1.7% Ca and 3.6% S-SO 4 . The first topdressing fertilization of maize was carried out using 20-00-20 fertilizer (20% N and 20% K 2 O) at a dose of 250 kg ha −1 . The second topdressing fertilization of maize was carried out using ammonium sulfate (20% N and 22% S-SO 4 ) at a dose of 250 kg ha −1 .

Sampling and Analysis
On 16 June 2020, new soil samples were collected for chemical analysis and determination of fertility [35] following the same methodologies used in the previous year.

Statistical Analysis
The data of soil chemical attributes at each soil depth (0.0-0.2 and 0.2-0.4 m) in each season (2017/18, 2018/19, and 2019/20) were submitted to ANOVA (F test), and the averages were compared by the Tukey test (p < 0.05) using the software Assistat Version 7.7, [36], Campina Grande, Brazil. The standard errors of the means of significant changes in soil chemical attributes were also analyzed in graphical format using the software Microsoft Excel (2016).

2017/18 season
In the CPS, there were no differences (p < 0.05) in soil chemical attributes at depths of 0.0-0.2 and 0.2-0.4 m among the treatments, despite the significant values of F for Ca 2+ content and Ca 2+ /CEC at a depth of 0.0-0.2 m.
In the APS, at a depth of 0.0-0.2 m, Ca 2+ /CEC increased by 12.34 percentage points in T1 compared with the control (T4). In addition, S-SO 4 2− content increased by 10.5 mg dm −3 in T1 and by 4.0-4.5 mg dm −3 in T2 and T3 compared with the control; P content increased by 9.5 mg dm −3 in T3 compared with T2; and Ca 2+ content increased by 0.53 cmol c dm −3 in T1 compared with the control. Mg 2+ and K + content and Mg 2+ /CEC did not differ (p < 0.05) among T1, T2, and T3. By contrast, compared with the control, K + /CEC decreased by 0.89-1.05 percentage points in T1, T2, and T3. At a depth of 0.2-0.4 m, S-SO 4 2− content increased by 26.25 mg dm −3 in T1 and by 5.5-7.5 mg dm −3 in T2 and T3 compared with the control; P content increased by 9.75-11.5 mg dm −3 in T1 and T3 compared with T2; Ca 2+ /CEC increased by 8.42 and 7.24 percentage points in T1 and T2, respectively, compared with the control; Mg 2+ /CEC increased by 2.65 percentage points in T2 compared with T3; and Ca 2+ /ECEC increased by 16.88 percentage points in T1 compared with the control. In addition, Mg 2+ content increased by 0.15, 0.13, and 0.11 cmol c dm −3 in T1, T2, and T3, respectively, compared with the control, and Mg 2+ /CEC increased by 19.0-21.0 percentage points in T1 and T2 and 15.5 percentage points in T3. OM content increased by 6.5 g dm −3 in T2 compared with the control. Despite a significant F value, P content did not differ (p < 0.05) among the treatments. At a depth of 0.2-0.4 m, S-SO 4 2− content increased by 3.67 and 9.17 mg dm −3 in T2 and T3, and Ca 2+ /CEC increased by 6.5-11.33 percentage points in T1, T2, and T3 compared with the control. In addition, compared with the control, BS increased by 16.67 percentage points in T2 and 8.0-9.0 percentage points in T1 and T3; Mg 2+ /ECEC increased by 3.5-7.67 percentage points in T1 and T2; Ca 2+ content increased by 0.6 cmol c dm −3 , Mg 2+ content increased by 0.4 cmol c dm −3 , and Al 3+ content decreased by 0.2 cmol c dm −3 in T2.

Discussion
Effects of the treatments on S-SO 4 2− content The increases in S-SO 4 2− content illustrate the importance of combining GP amendment with LS application to improve S-SO 4 2− content at depths of a 0.0-0.4 m layer in CPSs, NTSs, and APSs. GP was previously reported to improve the availability of exchangeable SO 4 2− in the entire soil profile [24,37,38]. In addition, several studies have found that the tandem application of LS and GP provides nutrients and is an alternative strategy for improving the root environment in the early years of cultivation, when the effects of LS have not yet reached the subsurface layers due to its poor solubility and mobility [26,39].

Effects of the treatments on pH
Combining LS and GP provided better conditions for LS to act on the soil solution and increase pH [40]. Liming is the most efficient methodology for increasing pH [29], particularly in the surface layer (depth of 0.0-0.1 m) [41,42]. GP indirectly corrects soil pH in deeper layers in the soil because it contains SO 4 2− , which displaces OH − from soil colloids into solution [26].
Effects of the treatments on Ca 2+ content, Ca 2+ /CEC, and Ca 2+ /ECEC Increasing Ca 2+ content at soil depths of 0.0-0.2 m and 0.2-0.4 m is important due to the role of Ca 2+ in root growth [13], including cell division [43], and because plants absorb Ca 2+ almost exclusively via the roots [44]. Ca 2+ absorbed by superficial roots cannot meet the needs of deep roots located in environments with poor levels of this nutrient [45]. The continuity of channels in NTSs [46] promotes the descent of Ca 2+ [47], and no soil turning was performed in the crop systems in the present study. LS application has previously been reported to increase Ca 2+ content [45,48], as has GP application [24,29,47,49,50]. Moreover, previous studies have shown that combined application of LS and GP can increase Ca 2+ content [40,51,52]. The combined application of LS and GP enhances the descent of Ca 2+ and Mg 2+ , which are added by liming, in the soil profile [53,54] via the formation of ionic pairs with SO 4 2− [55] dissociated from GP. This ion descends easily in the soil profile, carrying K + , Mg 2+ , and, mainly, Ca 2+ along with it [47,56,57]. Increasing and redistributing Ca 2+ , Mg 2+ , and K + at greater depths in NTSs is important for alleviating the chemical impairment of root development and for promoting water deficit resistance in maize and soybean crops [42]. In addition, the presence of grazing cattle in the CPS and APS increased the effects of surface application of LS at greater depths by promoting Ca 2+ and Mg 2+ leaching in the soil profile through the formation of low-molecular-weight organic acids. These organic acids are released during the decomposition of animal waste, mainly feces, or are exuded by pasture plants during grazing [58]. Grazing of palisade grass leads to a higher regrowth rate, as this grass is characterized by a voluminous fasciculated root system. The greater regrowth of tillers after grazing increases the volume of pores in the soil, allowing the movement of cations within the soil profile.
Effects of the treatments on Mg 2+ content and Mg 2+ /CEC Mg 2+ and Ca 2+ compete for negative charges in soil, and Ca 2+ is preferred in exchange sites [59]. As LS contains a high percentage of Ca (31% CaO), it provides a high Ca 2+ concentration in the soil, which promotes the displacement of Mg 2+ from exchange sites. Mg 2+ is less strongly retained due to its greater hydrated radius and lower electronegativity. This displacement allows greater movement of Mg 2+ within the profile and formation of the MgSO 4 0 ion pair [24,60]. Movement of Mg 2+ within the soil profile is also promoted by water-soluble compounds originating from the residues of previous crops [61]. Compared with the initial content of Mg 2+ in the soil, LS and GP increased Mg 2+ content at a depth of 0.0-0.2 m by 1.0 and 0.53 cmol c dm −3 in the NTS and APS, respectively. These results are consistent with previous studies that have found increases in Mg 2+ content with the application of GP [42]. However, another study found that GP improved soil fertility in the soil profile but that Mg 2+ migrated down over a period of three years, regardless of application [38]. GP application reduces Mg 2+ in the surface layers, leading to accumulation in the subsurface layers [26]. Thus, the methodology used in this study to apply LP and GP promotes a more uniform distribution of Mg 2+ along the soil profile.

Effects of the treatments on Al 3+ content
Once in the soil solution, the ion Ca 2+ can react in the soil exchange complex, shifting Al 3+ , K + , and Mg 2+ to the soil solution. In turn, these ions can react with SO 4 2− to form AlSO 4 + (which is less toxic to plants) and the neutral pairs K 2 SO 4 0 , MgSO 4 0 , and CaSO 4 0 , which have great mobility in the soil profile [62]. The dissociation of GP releases Ca 2+ , which binds to organic carbon, making SO 4 2− available in the soil solution to replace OH − . These chemical changes in the soil solution also interfere with the increase in pH and reduction of acidity by Al 3+ [22].

Effects of the treatments on K + content
The decreases in K + content and K + /CEC at a depth of 0.0-0.2 m in the NTS and APS due to LS and GP application are related to K + lixiviation within the soil profile, which is promoted by GP [51]. By contrast, GP application did not promote K + lixiviation in the CPS, corroborating previous findings [45,47,52,63]. The absence of an effect of GP application on K + lixiviation can be explained by the low formation of the K 2 SO 4 0 ionic pair (0.2% of total solubility) [64].

Effects of the treatments on total acidity
The reduction in total acidity in the NTS and APS may have been due to the movement of fine particles of LS through the continuous pores within the soil profile (which are the result of root system decomposition) [10] and the formation of ionic pairs between sulfate or nitrate and Ca 2+ and Mg 2+ from LS [65]. Decreasing total acidity is important to avoid restrictions on the expansion of the root system, which would impair access to water and nutrients found in the deepest layers of the soil [52]. The reduction in total acidity may also be related to the formation of complexes of Ca 2+ and Mg 2+ with soluble organic compounds released by the decomposition of plant biomass deposited on the soil surface (carboxylic and phenolic radicals) [66,67]. In the present study, all cultivations were carried out under no-till on the residues of the previous crops, as the NTS was established in the 2009/10 season and the APS was established in the 2011/12 season.

Effects of the treatments on BS
The increases in BS and CEC in the NTS and APS were promoted by the supply of Ca 2+ and Mg 2+ by LS, which contained 31% CaO and 21% MgO, and GP, which contained 17% CaO, and by the movement of Ca 2+ and Mg 2+ in the soil profile. Previous studies have reported that combining GP with LS enhances the vertical movement of Ca 2+ in the soil profile [40,54,68,69]. GP application has also been shown to increase BS [45,49,50]. In addition, changes in BS are always greatest after the grazing season (winter) [70]. However, this improvement in BS is temporary because the decomposition of plant residues increases soil acidity, which was minimized in the present study by the use of three applications of LS [71].

Conclusions
In the present study, three methodologies for the surface application of LS and GP in CPSs, NTSs, and APSs were evaluated. Surface application of LS (ensuring that Ca 2+ occupies 70%, 60%, or 50% of CEC at a depth of 0.0-0.2 m) and GP (ensuring that Ca 2+ occupies 60%, 50%, or 40% of ECEC at a depth of 0.