NITRATE AND POTASSIUM DYNAMICS IN PROFILES OF SOILS CULTIVATED WITH FERTIGATED SUGARCANE CROPS 1

- The objective of this study was to evaluate the distribution of NO 3- and K + in the profile of soils cultivated with sugarcane crops, after application of different nitrogen (N) and potassium (K 2 O) rates via subsurface drip fertigation, in Teresina, PI, Brazil. The experiment was conducted at the experimental area of Embrapa Meio-Norte, using a randomized block design with four replications. The treatments were arranged in split plots, with N+K 2 O rates (60+120, 180+120, 120+60, 120+180, and 120+120 kg ha -1 of N + K 2 O, respectively) in the plots, and soil layers (0.0-0.2, 0.2-0.4, and 0.4-0.6 m), for quantification of NO 3- and K + concentrations in the soil, in the subplots. The soil of the experimental area was a Typic Hapludult of sandy-loam texture. The distribution of nitrate (NO 3- ) and potassium (K + ) in the soil layers after the N+K 2 O applications were evaluated. The NO 3- and K + concentrations in soils under sugarcane crops in the plant crop cycle is higher at the beginning and at the end of the cycle, respectively. The NO 3- and K + concentrations in the soil solution are dependent on the evaluation time, soil layer, and N and K 2 O rates applied via fertigation. The highest concentrations of NO 3- (264 mg L -1 ) and K + (377 ppm) were found in the 0.0-0.2 m soil layer. No leaching of NO 3- or K + to deepest soil layers (>0.4 m) was found.


INTRODUCTION
The use of traditional soil fertilization for sugarcane crops is common due to its practicality and lower technical requirement, which may lead to disordered applications. This practice is characterized by applications of high fertilizer rates at low frequencies, causing risk of nutrient leaching to areas with no root systems, especially nitrate (NO 3 -) and potassium (K + ) (VITTI et al., 2005). Intensive and often disordered fertilizations with insufficient and unbalanced nutrients generate economic losses and aggravate possible environmental impacts, which is not in agreement with the principles of agronomic science (FREITAS et al., 2007).
Thus, the use of fertigation is recommended; this system allows a better control of the distribution of nutrients throughout the sugarcane cycle. Nutrient uptake by plants, such as sugarcane, is limited by soil, temperature, solar radiation, and precipitation (OLIVEIRA et al., 2011). The Brazilian northeastern coast presents rainfall concentrated within four months, thus, hindering the crops' productive capacity and the plants' efficiency in absorbing available soil water and nutrients (SANTOS et al., 2009). K + is the most exported nutrient by sugarcane, reaching extraction of approximately 55% only by the culms (ROSSETTO et al., 2008). Oliveira et al. (2010) evaluated the exportation of nitrogen (N) in sugarcane crops and found that this nutrient was one of the most exported, reaching extraction of 51% by the culms.
The N supply to sugarcane should be monitored considering its use by the plant, and nutrient loss through several processes, including nitrification. Silva et al. (2006) evaluated the N balance for N-fertilizer recommendation and found large loss of ammonium because of the lack of rainfall after application and the high temperatures during fertilization, which reduced mineral N contents, nitrification, and, thus, the N availability to the sugarcane crop.
Part of the NO 3 leachate to layers below 0.2 m is possibly partially absorbed by sugarcane plants, considering that a significant part of their root system can reach deep soil layers. According to Silva -Olaya, Cerre and Cerre (2017), 61% of the root biomass of sugarcane plants is in the 0.0-0.02 cm soil layer, and Camargo et al. (2017) found that the effective depth of the sugarcane root system is at 0.6 m. This context denotes the need for studies regarding the distribution of NO 3 and K + in different layers of the soil profile after application of N and K fertilizers.
Losses of NO 3 in the soil profile should be monitored, especially in intensive production systems with high use of nitrogen fertilizers, for the planning of measures that increase the use efficiency of the applied N focused on the management of an economically sustainable production system, with quality products and a minimal negative environmental impact (MENDES et al., 2015). NO 3 leaching increases depending on soil physical properties and use of intensive farming practices and high amount of water for irrigation (ANDRADE et al., 2009). K + leaching is common because of rainfall events (ROSOLEM et al., 2003); this loss is due to its high solubility, which may result in unsatisfactory crop yields.
Therefore, the objective of this study was to evaluate the distribution of NO 3 and K + in the profile of soils cultivated with sugarcane crops, after applications of different nitrogen (N) and potassium (K 2 O) rates via subsurface drip fertigation, in Teresina, PI, Brazil.

MATERIAL AND METHODS
The experiment was conducted at the experimental area of the Brazilian Agricultural Research Corporation (Embrapa Meio-Norte), in Teresina, PI, Brazil (5°5'21"S, 42°48'6"W, and altitude of 72 m). The region presents average air temperature of 27.6 °C and annual average rainfall of 1,349 mm. The climate of the region is C1sA'a', according to the Thornthwaite and Mather climate classification, characterized as dry sub-humid, megathermal, with moderate water surplus in the summer and a concentration of the potential evapotranspiration (32.2%) from September to November (BASTOS; ANDRADE JÚNIOR, 2014).
A randomized block experimental design with four replications was used to evaluate the distribution of NO 3 and K + in the soil profile. The treatments were arranged in split plots, with N+K 2 O rates in the plots, and soil layers in the subplots.
The sugarcane variety RB 92579 (plant crop cycle) was used. This variety is recommended for irrigated crops and covers an expressive area in the Teresina microregion. The planting was done on June 06, 2014, using double-row spacing (0.5 m between plant rows and 2.0 m between double rows), and 15 buds per linear meter. The soil was prepared with one plowing and one harrowing.
A subsurface drip irrigation system was used, with two meters between drip lines, drippers spaced 0.6 m apart, flow rate of 2.3 L h -1 , and pressure of 200 kPa; the drip lines were buried 0.25 m deep in the center of the double rows.
Fertigation with N and K 2 O started on August 04, 2014 (60 days after planting) and lasted six months (August 2014 to January 2015); it was applied once a week, totaling 24 applications throughout the sugarcane crop cycle. The application of the treatments throughout the crop cycle was divided according to the distribution presented in table 2. Based on the soil chemical analysis, 2 Mg ha -1 of lime (total neutralizing power of 90%) was applied at 90 days before planting. The P 2 O 5 was applied at a rate of 100 kg ha -1 -30% at planting (triple superphosphate), and 70% via fertigation (monoammonium phosphate) distributed monthly. The micronutrients applied were boric acid (4.5 kg ha -1 ), zinc oxide (7 kg ha -1 ), copper oxide (6 kg ha -1 ), manganese oxide (11 kg ha -1 ), and sodium molybdate (1 kg ha -1 ); they were applied via fertigation split in six applications during the crop cycle.
The N and K 2 O rates were applied using a hydraulic injector (positive displacement pump).
The fertigation flow to the plots was turned on and off by hydraulic registers installed at the beginning of the row of each experimental plot to control the irrigation depths and N and K 2 O rates.
The irrigation depth was established based on crop evapotranspiration (ETc), using the reference evapotranspiration (ETo) estimated by the Penman-Monteith method, based on daily climatic data from an automatic weather station installed at the site (Embrapa Meio-Norte) and on crop coefficients of sugarcane established for the region (ANDRADE JÚNIOR et al., 2017). The irrigation application frequency was three times a week (Mondays, Wednesdays, and Fridays).
The plots were 60 m² and consisted of three 10-meter double rows. The area of the plot evaluated for NO 3 and K + concentration in the soil solution consisted of two plant rows, from where the soil samples were taken for analysis. Simple soil samples (~0.5 kg) were collected at 6 different times (105,122,194,227,276, and 326 days after planting -DAP) in each replication and soil depth (0.0-0.2, 0.2-0.4, and 0.4-0.6 m), using a Dutch auger at 0.2 m from the dripper line, totaling 360 samples. This sampling method was used to collect soil samples from both the sugarcane root system region and near the region where the fertilizers were applied.
The soil samples were, then, disaggregated and passed through a 2-mm mesh sieve to reduce the size of the soil particles. The disaggregated soil samples were subjected to a saturated paste preparation, following the methodology of Richards (1954). A sub-sample of 0.25 kg was taken from each sample and placed in a 0.5 L beaker. Then, 100 mL of distilled water was slowly added with the aid of a wash bottle, always homogenizing the soil with a stainless-steel spatula until the paste became saturated.
The soil solution extracts were, then, removed from the saturated paste using a filter paper placed on a porcelain Büchner funnel coupled to a beaker, which was set to a pressure of -80 kPa generated by a vacuum pump. A 15-mL aliquot of the solution extract was taken from each sub-sample and placed in 50-mL capped glass containers for subsequent determination of NO 3 and K + ion concentrations, which was made according to the Kjeldahl (NO 3 -) and photometry (K + ) methods (SILVA et al., 2010).
The results regarding the dynamics of the NO 3 and K + ions in the soil profile were subjected to analysis of variance, and the means were compared by the Tukey's test (p>0.05). Statistical analyses were done using the SAS 14.1 program (SAS INSTITUTE, 2016).

RESULTS AND DISCUSSION
The cumulative monthly data of ETc, applied irrigation depth (ID), rainfall (R), and sum of ID and R throughout the experimental period are presented in Figure 1. The sugarcane water requirement was fully met by the application of ID+R during the experiment period. The total ID used was 657.0 mm and the total rainfall measured was 849.2 mm, totaling 1,506.2 mm. The water requirement of sugarcane plants is 1,500 to 2,500 mm (MARIN, 2019), which must be evenly distributed during their vegetative development (BRASIL, 2016); and the amount of water needed for the maximum production potential of the crop is 1,200 to 1,300 mm (DOORENBOS; KASSAM, 2000). Therefore, this was not a limiting factor for the expression of the crop productive potential under the conditions of the present experiment.
The analysis of variance for the NO 3 concentrations over the three soil layers and six sampling times throughout the sugarcane cycle, after application of different N rates via subsurface drip fertigation, is shown in Table 3. Table 3. Analysis of variance for nitrate (NO 3 -) concentration (mg L -1 ) in three soil layers collected at six different times (days after planting -DAP) during the sugarcane crop cycle. DAP = days after planting; DF = degrees of freedom; CV = coefficient of variation; ns = not significant (p>0.05), * = significant at 0.05 ≥p>0.01, ** = significant at p≤0.01.
The interaction between treatments and soil layers was significant (p<0.05) for NO 3 concentrations in all evaluated sampling times, indicating that NO 3 concentration varied in the soil layers in each sampling time depending on the N rates applied.
The highest NO 3 concentrations in the 0.0-0.2 m soil layer (Table 4) were found at 227 DAP (culm growth stage), despite the reduction from 25% to 20% in the rates of N applied during this period ( Table 2). The highest NO 3 concentration was found at 326 DAP (maturation stage and harvest), denoting that sugarcane extracted less NO 3 from the soil during this period, since the N demand of sugarcane plant decreases significantly in this stage (PENATTI, 2013). Joris (2015) evaluated NO 3 concentrations in different layers of soils cultivated with sugarcane crops, after application of different N rates, and found increases in NO 3 concentration in the 0.1-0.2 m soil layer as the sampling time increased, which was confirmed by the results obtained in the present study.
Regarding the 0.0-0.2 m soil layer, when the plants were at 105 DAP (establishment stage), higher NO 3 concentrations were found in the treatments T2 (180 kg ha -1 N + 120 kg ha -1 K 2 O) and T4 (120 kg ha -1 N + 180 kg ha -1 K 2 O). Similar result was found at 194 DAP (tillering stage), when the treatments received the highest percentage of N rates. However, these higher NO 3 concentrations in T2 and T4 were not constant throughout the sugarcane crop cycle, due to lower nitrogen uptake by the crop at initial vegetative stages. Joris (2015) evaluated applications of different N rates on sugarcane crops to evaluate NO 3 concentrations in the soil profile and found similar results: the application of the highest N rates (180 kg ha -1 ) did not result in the highest NO 3 concentrations (16 mg kg -1 ), which was reached with application of lower N rates (120 kg ha -1 ). Considering the 0.0-0.2 m soil layer, significantly higher NO 3 concentration was found only for the treatment T4, when the plants were at 194 DAP; whereas at 227 DAP, higher NO 3 concentrations were found for T4 and T5 (120 kg ha -1 of N + 120 kg ha -1 of K 2 O).
Regarding the 0.2-0.4 m soil layer, the NO 3 concentrations found from 105 DAP to 194 DAP were lower than those found in the 0.0-0.2 m. This result was due to the root system development and performance in this soil layer at 122 DAP (sugarcane establishment stage) and to the higher initial nutrient consumption of the plants during this period. The NO 3 concentration in this layer increased from 227 DAP (culm growth stage), presenting the highest values at the last evaluation (326 DAP). Coelho et al. (2014) evaluated NO 3 concentrations in the soil profile of fertigated soils and found similar results; they reported increases in mean NO 3 concentrations in the soil solution at the 0.0-0.30 m layer from the beginning of the crop cycle, which lasted for 11 months. The NO 3 concentrations found in treatments T1 (60 kg ha -1 N + 120 kg ha -1 K 2 O) and T2 were significantly different at 194 DAP, presenting the lowest and highest NO 3 concentrations, respectively. These treatments received the lowest and highest N rates, respectively, in this period, which explains the differences in NO 3 concentrations. The highest NO 3 concentrations in the 0.2-0.4 m soil layer were found at 326 DAP; despite the reduction in N rates applied in this period, the values were higher because of the greater rainfall volumes occurring during this period and decreases in N consumption by the sugarcane plants (FRANCO et al., 2008), resulting in more available NO 3 in the soil solution. Regarding the 0.4-0.6 m soil layer, the NO 3 concentrations were higher at 326 DAP, which was a similar result to those found for the 0.0-0.2 e 0.2-0.4 m. Studies on NO 3 quantification at the end of the crop cycle of sugarcane crops under conventional management conditions (without constant soil coverage) showed that the contribution of nitrogen fertilizers to the total N absorbed by sugarcane is low (10% to 16%) at this final stage (GAVA et al., 2001;TRIVELIN;VICTORIA;RODRIGUES, 1995;FRANCO et al., 2008). A higher N consumption at initial crop stages and a lower NO 3 concentration in the soil were confirmed at 122 DAP, when the treatment T2 (highest N rate) resulted in the lowest NO 3 concentrations. However, at 194 DAP, T2 presented higher NO 3 concentrations in the soil than T3, T4, and T5. The plants were over half of their cycle at 194 DAP, with decreasing demand for N; thus, the results are in agreement with those of Franco et al. (2011), who evaluated the 15 N use efficiency by sugarcane ratoon crop and found that after fertilization, at 30-60 DAP (initial stage of the crop cycle), the sugarcane plants absorbed practically all N accumulated during the whole crop cycle.
Higher NO 3 concentrations were expected in the 0.4-0.6 m soil layer because of the leaching process. However, no significant difference in NO 3 concentrations was found when compared to the other evaluated layers. This denotes the efficiency of the fertigation and subsurface irrigation system. The results confirm those found by Gil et al. (2008), who reported that subsurface drip irrigation presents high uniformity of water application, reducing the percolation of NO 3 -. Despite the proportionally lower nutrient rates applied, the highest NO 3 concentrations in the three evaluated soil layers were found in the last evaluation times. Franco et al. (2011) evaluated N fertilization in sugarcane and found higher N extraction at the beginning of crop cycle, usually up to 90 days, due to a subsequent decrease in the crop N demand. with sugarcane crops and found differences in NO 3 concentrations in the soil solution at depths of 0.3, 0.6, and 0.9 m according to the treatments and sampling times, and found increases in NO 3 contents in all evaluated soil layers as the N rates applied was increased, which is in agreement with the results found in the present study. These results denote the proportionality between N application and NO 3 concentrations; as in the evaluation at 197 DAP, when the treatment T2 received the highest N rate and presented the highest NO 3 concentrations in the 0.2-0.4 m (63.9 mg L -1 ) and 0.4-0.6 m (47.68 mg L -1 ) soil layers (Table 4).
According to Oliveira et al. (2001), differences in NO 3 concentrations over time and soil depths can be attributed to rainwater percolation. The sum of the rainfall depths recorded and the irrigation depths applied was higher (1,500.02 mm) in June (Figure 1).
The analysis of variance for K + concentrations in the soil layers at the six sampling times throughout the sugarcane crop cycle is shown in Table 5. This analysis showed significant difference between treatments and soil layers in all evaluation times, showing that the soil depth is also a significant factor for K + distribution. n.s = not significant (p>0.05), * = significant at 0.05 ≥p>0.01, ** = significant at p≤0.01; DF = degrees of freedom; CV = coefficient of variation.
The interaction between treatments and soil layers was significant (p<0.05) at 105, 122, 194, and 227 DAP, indicating variations in K + concentrations throughout the soil profile in each sampling time according to the K 2 O rates.
The treatment T4 had the highest K 2 O rate and presented the highest K + concentrations in the 0.0-0.2 m layer in four of the six evaluation times ( Table 6). The significantly different K + concentrations denote their proportional response to K 2 O application, mainly considering the K + increases in the soil solution between 105 and 122 DAP (sprouting and establishment stages). Flores et al. (2012) evaluated the K + distribution in soils under sugarcane crops and found similar results, with K 2 O applications affecting the K + concentration in the soil solution of the surface layer the K + concentration in the soil solution increased in the 0.0-0.2 m layer as the K 2 O rates were increased.
Increases in the K + concentrations in the soil surface layer as the K 2 O rates are increased were expected; the loss of K + by leaching is small in clayey soils with high cation exchange capacity (CEC) (MIELNICZUK, 1982).
T2 presented a high K + concentration (377 ppm) in the 0.0-0.2 m soil layer at 122 DAP. This result can be explained by the higher K 2 O rate applied at 105 and 122 DAP (sprouting and establishment stages). Moreover, this soil layer presented almost 2-fold the K + concentration (0.23 cmolc dm -3 ) when compared to the 0.4-0.6 m layer (0.12 cmolc dm -3 ), and more than 3-fold that of the 0.2-0.4 m layer (0.7 cmolc dm -3 ) ( Table 1). The experimental error may have affected this result, considering the number of samples collected (60 per evaluation time).
The highest K + concentrations were found at the first evaluation times (105,122,and 194 DAP) and the lowest at the last ones (227,276,326 DAP) in all evaluated soil layers. Thus, the K + consumption in the early stages of the sugarcane crop cycle is lower than in the final stages. Vitti et al. (2005) recommended K 2 O fertilization at higher rates before the sugarcane crop canopy closure because of its high consumption at final stages, as observed in the last evaluations for all soil layers, with decreases in K + concentrations due to the higher consumption of K + by the sugarcane plants. Table 6. Potassium concentration (ppm) in the 0.0-0.2, 0.2-0.4, and 0.4-0.6 m soil layers after application of different K 2 O rates, evaluated at six sampling times (days after planting -DAP).
Means followed by different letters in the columns are different by the Tukey's test at 5% probability.
The treatment T4, which received the highest K 2 O rate, presented the highest K + concentrations in the 0.2-0.4 m soil layer in three of the six evaluation times, showing a proportionality between K + concentrations and K 2 O rates applied. Similar results were found by Rossetto et al. (2004) in an experiment also in a Typic Hapludult (Argissolo Vermelho), showing that the application of K 2 Obased fertilizers increased the K + concentration in the soil solution.
Higher K + concentrations were expected in the 0.4-0.6 m layer (Table 6) when compared to the other evaluated layers, which would characterize the leaching process, mainly when considering the rainfall events occurred. The highest K + concentrations in the 0.4-0.6 m soil layer was found at 122 DAP (establishment stage), which corresponds to less than half of the sugarcane cycle; similar results were found for the other soil layers evaluated in the present study.

CONCLUSIONS
The NO 3 concentration in soils under sugarcane crops in the plant crop cycle is higher at the beginning of the crop cycle, whereas the K + concentration is higher at the end of the crop cycle. The NO 3 and K + concentrations in the soil solution are dependent on the evaluation time, soil layer, and N and K 2 O rates applied via fertigation.
The highest NO 3 concentration (264 mg L -1 ) was found in the 0.0-0.2 soil layer at 236 days after planting (DAP), corresponding to the harvest time. The highest K + concentration (377 ppm) was found in the 0.0-0.2 soil layer at 122 DAP, corresponding to the crop establishment stage. No NO 3 and K + leaching to deep soil layers (>0.4 m) was found; similar concentrations were found for these ions in the three evaluated soil layers, denoting the good efficiency of fertilizer application via fertigation for sugarcane crops.