Influence of the sanitary sewage application method to closed-end furrows on the macronutrient extraction capacity and productivity of elephant grass

Raw sanitary sewage (RS) used in fertigation can be an important source of plant nutrients and irrigation water, reducing mineral fertilizer needs and treatment costs, especially where sewage is continuously available. Common application techniques include spraying and drip, but furrow irrigation can be advantageous for raw wastewater due to lower emitter-clogging risk and improved sanitary safety relative to spraying. However, furrow fertigation often leads to non-uniform nutrient distribution along furrows, with accumulation near the inlet when application position is fixed, complicating system management. A critical aspect for system sizing and safety is the crop’s capacity to extract nutrients from the aerial part, which indicates the extent to which applied nutrients (and sodium) are removed from the soil. For elephant grass (Pennisetum purpureum), a key Brazilian forage, macronutrient extraction depends on fertilization and management, and numerous studies have evaluated fertilizer doses, types, cutting ages, seasons, and cultivars. Nonetheless, responses of elephant grass to direct application of sanitary sewage have not been addressed. This study evaluates whether alternating the RS application position in closed-end, level furrows improves dry matter yield and macronutrient extraction (N, P, K) relative to non-alternating application and to conventional mineral fertilization over three continuous years.

Prior work on surface fertigation highlights challenges in achieving uniform solute distribution along furrows, with tendencies for nutrient and pollutant accumulation near the inlet under conventional fixed-position fertigation. Modeling and field studies have examined one- and two-dimensional water flow and solute transport in furrow and basin systems, showing nonuniform and unsteady transport influenced by flow depth, soil adsorption, and application method; nitrate mobility is particularly problematic. Comparisons of treated sewage effluent fertigation versus mineral fertilization in other forages (e.g., bermudagrass) suggest wastewater can partially satisfy N requirements when applied to meet water demand, though treatment typically lowers nutrient concentrations vs raw sewage. Studies on elephant grass under various fertilizer regimes report macronutrient accumulation patterns and yields influenced by N and K supply, cutting interval, and season, but not specifically under raw sewage fertigation. Literature also indicates that increasing furrow sinuosity and appropriate hydraulic management can reduce deep percolation losses and improve solute distribution.

Location: The experiment was conducted at COPASA ETE-Onça (sewage treatment station) near Ribeirão do Onça, Santa Luzia, Minas Gerais, Brazil (19°49′20.6" S, 43°53′46.6" W; elevation 852 m). The region has a humid tropical climate with summer rains. The soil, formed from compacted material, resembled an Inceptisol (USDA Soil Taxonomy) but could not be properly classified. Climate data (mean temperature, relative humidity, precipitation) during the study were obtained from INMET. Crop and design: Elephant grass (Pennisetum purpureum) was planted from stalks in June 2016. The experiment followed a completely randomized design with 4 treatments and 7 sampling regions along the furrow length (every 5 m from 5 to 35 m), with 7 replicates per treatment. Each plot (72 m²) comprised three 40 m long level, closed-end furrows spaced 0.6 m apart and four planted rows; border furrows/rows were excluded from analyses. Treatments: (i) TWN: conventional mineral fertilization (CMF) plus irrigation with treated water (TW), without alternating application position; (ii) TWA: CMF + TW with alternation of application position between furrow ends; (iii) TFN: fertigation with raw sanitary sewage (RS) complemented with TW, without alternation; (iv) TFA: fertigation with RS complemented with TW, with alternation. Alternation meant switching the application point between the two furrow ends on different events, enabled by level furrows. Fertilization and irrigation: CMF plots received 70 kg ha⁻¹ year⁻¹ P (at the beginning of each year) and 300 kg ha⁻¹ year⁻¹ of N and K split after each cutting, using single superphosphate, urea, and KCl, per state recommendations. RS fertigation was applied weekly at a dose set by Na loading of 300 kg ha⁻¹ year⁻¹. Complementary irrigation (TW) for RS plots and irrigation for CMF plots were applied weekly based on crop evapotranspiration. Operational flow rates respected non-erosive limits to ensure rapid infiltration and minimal surface exposure of RS. Sampling and measurements: The study spanned ~3 years (June 2016–July 2019) starting after a uniform cut. About five cuts per year were made when plots reached ≥1 m height; plants were cut to ≥15 cm stubble. At each cut, dry matter (DM) yield was measured from 1 m² quadrats placed every 5 m along rows. Samples of the aerial part (stems and leaves) were dried at 65 °C, weighed for DM, ground, and analyzed for total nitrogen (TN), phosphorus (P), and potassium (K) contents. TN was by Kjeldahl after sulfuric digestion; P and K were measured after nitric-perchloric digestion (P by colorimetry/UV-Vis; K by flame emission spectrophotometry), according to EMBRAPA protocols. Wastewater characterization: RS was sampled weekly prior to application to determine Na, total Kjeldahl nitrogen (TKN), total phosphorus (TP), and total potassium (TK). TKN by Kjeldahl after sulfuric digestion; TP by UV-Vis after sulfuric digestion; K by ion chromatography after sulfuric digestion; Na by ion chromatography after 0.22 µm filtration, following APHA methods. Statistical analysis: ANOVA followed by Tukey test (5% significance) compared treatment means for DM yield and macronutrient extraction/content across time and along furrow length. Linear regression (F-test at 5% significance) assessed trends along furrow length for RS treatments (TFN, TFA). Analyses and plots used STATISTICA v7.

- RS nutrient inputs at Na-based dosing: Mean RS concentrations and annual fertigation inputs (mean ± SD; n=130): Na 71.4 mg L⁻¹ → 300 kg ha⁻¹ yr⁻¹ (by design); N 148.9 ± 33 mg L⁻¹ → 647 kg ha⁻¹ yr⁻¹; P 17.8 ± 6 mg L⁻¹ → 86 kg ha⁻¹ yr⁻¹; K 38.5 ± 11 mg L⁻¹ → 176 kg ha⁻¹ yr⁻¹. Compared to CMF, N input via RS was ~2× higher, P was similar, and K roughly half the CMF recommendation. - Dry matter yield (DM, Mg ha⁻¹ yr⁻¹; Fig. 2): TFA achieved the highest mean DM yield at 29.9, a 30% increase over TFN (20.2). TWN, TWA, and TFN did not differ significantly (range 17.5–22.2). - Macronutrient extraction totals (kg ha⁻¹ yr⁻¹; Fig. 2): TFA: N 688, P 102, K 508 (significantly greater than other treatments). TWN/TWA maxima: N up to 389, P up to 66.8, K up to 311. TFN averaged N 404, P 70.7, K 283. Under TFA, extracted N, P, K met or exceeded amounts applied via RS, indicating near 100% recovery and reduced leaching risk; under TFN, applied N (647) exceeded extraction (404), implying accumulation or leaching risk near inlet. - Tissue nutrient contents (% of DM; Table 3): TFA had higher N (2.3%) and K (1.7%) than TWN/TWA (N 1.7–1.8%; K 1.3–1.4%); P content did not differ significantly among treatments (0.30–0.35%). - Spatial distribution along furrows (Table 4): Under TFN (fixed inlet), DM yield and N, P, K extraction decreased with distance from the inlet; higher values near the beginning (e.g., N: 551–673 kg ha⁻¹ yr⁻¹ at 5–10 m vs 193 kg ha⁻¹ yr⁻¹ at 35 m). Under TFA, high and relatively uniform values were maintained along the length (e.g., N: 564–811; K: 493–551). - Regression analysis (Table 5): For TFN, significant negative linear trends with distance for DM (y = -0.37x + 27.59; R²=0.8895; p=0.0014), N (y = -14.65x + 711.01; R²=0.8778; p=0.0019), P (y = -2.04x + 113.13; R²=0.6746; p=0.0235), K (y = -5.29x + 388.98; R²=0.8348; p=0.0040). For TFA, regressions were not significant and had low R² (<0.30), indicating more uniform distribution. - Cutting frequency: Management enabled ~5 harvests per year, with shorter intervals in warmer, wetter months. - Contextual findings: Yields around 30 Mg ha⁻¹ yr⁻¹ under high N (approaching 700 kg ha⁻¹ yr⁻¹) align with literature reports for elephant grass; K may be yield-limiting under RS-only fertigation (176 vs recommended 300 kg ha⁻¹ yr⁻¹).

Alternating the RS application position between furrow ends (TFA) substantially improved nutrient distribution along closed, level furrows, leading to higher and more uniform dry matter productivity and macronutrient uptake relative to non-alternating application (TFN) and to CMF-irrigated controls. The improved performance under TFA stems from minimizing solute concentration gradients along the furrow and reducing localized nutrient accumulation at the inlet, thereby enhancing root-zone nutrient availability across the plot. The elevated N input via RS (≈647 kg ha⁻¹ yr⁻¹) compared to CMF (300 kg ha⁻¹ yr⁻¹) likely contributed to the higher yields and tissue N under TFA. Under TFN, the strong negative spatial gradients and lower total extraction versus input for N indicate risk of nutrient leaching or accumulation near the inlet due to poor lateral distribution and possible deeper percolation. Although P inputs were modest (≈86 kg ha⁻¹ yr⁻¹), TFA enabled higher P extraction than TFN, reinforcing the role of application alternation in enhancing accessibility. K inputs via RS (≈176 kg ha⁻¹ yr⁻¹) were below crop recommendations (≈300 kg ha⁻¹ yr⁻¹), suggesting K may have limited potential yield; supplementation could further raise productivity. The lack of significant regressions for TFA with low R² corroborates more homogeneous nutrient distribution and plant performance along furrows. Overall, the findings address the management question by demonstrating that alternating application position is an effective, simple operational strategy to increase productivity and nutrient recovery in furrow fertigation with raw sewage, reducing environmental risks associated with nutrient leaching.

Raw sanitary sewage provides considerable nutrient inputs suitable for agricultural use when application rates and system management are properly controlled. Over three years, alternating the application position of RS at the ends of closed, level furrows (TFA) significantly increased elephant grass dry matter yield and macronutrient extraction compared to non-alternating RS application (TFN) and CMF-irrigated controls. Alternation raised mean DM yield by about 30% (from 20.2 to 29.9 Mg ha⁻¹ yr⁻¹) and achieved mean extractions of 688 kg N, 102 kg P, and 508 kg K ha⁻¹ yr⁻¹, matching or exceeding RS-applied macronutrient inputs and thereby lowering leaching risk. Alternating application position is thus a recommended operational improvement for furrow fertigation with raw sewage. Future work could assess K supplementation strategies, explore effects of different flow rates and soil textures, and validate findings across other soils, climates, and crop systems.

- Potassium input via RS (≈176 kg ha⁻¹ yr⁻¹) was below crop recommendations (~300 kg ha⁻¹ yr⁻¹), potentially limiting yield; the study did not include supplemental K in RS treatments. - Results are from a single site with a specific soil (compacted material, similar to Inceptisol) and humid tropical climate; generalizability to other soils/climates may be limited. - Productivity and nutrient extraction are influenced by furrow flow rates and soil texture; these operational and site factors can alter nutrient distribution and were not systematically varied. - Fixed Na-based dosing determined RS application volume; RS composition variability over time could affect nutrient inputs. - Closed, level furrow configuration and plot scale may not capture performance under different field layouts or larger scales.