Dynamics of soil potassium fractions under conservation agriculture in maize-based cropping systems

Introduction

Maize is one of the most important cereal crops in the world after rice and wheat and contributes significantly to food and fodder production (Shiferaw et al., 2011). Globally, maize is cultivated over 187 million hectares in 166 countries and accounts for nearly 40% of total grain production (FAOSTAT, 2019) [3] . In India, maize is grown on about 9.6 million hectares and plays an important role in crop diversification under different agro-climatic conditions (Dass et al., 2020; Islam et al., 2018) [1,6] . Major maize-based cropping systems include maize-wheat, maize-mustard, maize-chickpea, and maize-maize. However, maize productivity in India is lower than the global average due to intensive conventional tillage and dependence on rainfed conditions, which deteriorate soil quality and increase production costs (Dass et al., 2020; Nandan et al., 2019; Sharma et al., 2019) [1,12,13] . Conservation agriculture practices such as zero tillage (ZT) and permanent raised bed (PB) have emerged as sustainable alternatives for improving soil health, water use efficiency, and crop productivity (Friedrich et al., 2012; Jat et al., 2014) [4,9] . Potassium (K) is an essential plant nutrient present in soil in four forms: water-soluble, exchangeable, non-exchangeable, and mineral K, which remain in dynamic equilibrium. Water-soluble and exchangeable K are readily available to plants, whereas non-exchangeable K acts as a reserve source under intensive cropping systems (Madar et al., 2020) [11] . Tillage practices influence the distribution and availability of different K fractions due to soil disturbance and crop growth (Deubel et al., 2011) [2] . To evaluate the effect of tillage and cropping systems on soil K dynamics, a long-term experiment initiated in 2008 was studied during 2018-19 in Inceptisols of the Indo-Gangetic Plains of India.

The experiment consisted of three tillage practices: permanent raised bed (PB), zero tillage-flat (ZT), and conventional tillage-flat (CT), along with four maize-based cropping systems, namely maize-wheat-mungbean (MWMb), maize-chickpea-sesbania (MCS), maize-mustardmungbean (MMuMb), and maize-maize-sesbania (MMS). Soil samples collected from 0-5, 5-15, and 15-30 cm depths after harvest of summer crops were analyzed for different K fractions using standard methods described by Jackson (1967, 1973) [7,8] , Hanway and Heidel (1952) [5] , and Wood and De Turk (1941) [14] .

Materials and Methods

Experimental Design and Treatments

The study was laid out in a split-plot design with three replications. Main plot treatments consisted of three tillage and crop establishment practices: permanent raised bed with residue retention (PB), zero till flat with residue retention (ZT), and conventional till flat with residue retention (CT). Four maize-based cropping systems were assigned to subplots: maize-wheat-mungbean (MWMb), maizechickpea-sesbania (MCS), maize-mustard-mungbean (MMuMb), and maize-maize-sesbania (MMS). Each subplot measured 16.5 m × 4.0 m. The long-term experiment was initiated in 2008, and observations for the present study were recorded during 2018-19.

Field Preparation and Crop Management

Initially, the experimental field was laser-leveled and deep tilled up to 30 cm depth to remove compact layers. Permanent beds of 67 cm width with 37 cm top width and 15 cm furrows were prepared and maintained annually using a raised-bed planter. Conventional tillage plots were prepared through disc harrowing, rotation, and tine cultivation before sowing, whereas crops in ZT plots were directly seeded using a zero-till multi-crop planter. Crop residue management was followed under conservation agriculture principles. From 2008 to 2015, nearly 30% crop residue was retained, while from 2015 onwards complete residues of mungbean and sesbania were incorporated or retained in the field. The quality protein maize hybrid HQPM-1 was sown during kharif and winter seasons using a seed rate of 20 kg ha⁻¹ with recommended crop spacing and management practices.

Collected soil samples were air-dried, ground, and processed for the determination of different potassium (K) fractions. Water-soluble K was extracted using a soil-towater ratio of 1:5 following the procedure described by Jackson (1973) [8] . Exchangeable K was determined by shaking the soil samples with neutral normal ammonium acetate solution (1:5 ratio) for 5 minutes as outlined by Hanway and Heidel (1952) [5] . Boiling 1N HNO₃ in a 1:10 soil-to-solution ratio was used to extract HNO₃-soluble K according to the method of Wood and DeTurk (1941) [14] . Potassium concentration in all extracts was measured using a flame photometer. Non-exchangeable K was calculated by subtracting available K from HNO₃-extractable K.

Results and Discussion

Tillage and crop establishment methods significantly affected different potassium (K) fractions at 0-5, 5-15, and 15-30 cm soil depths after eleven years of continuous experimentation. Water-soluble K (WS-K), the most readily available form of potassium for plants, was significantly higher under conservation agriculture practices compared to conventional tillage. Permanent raised bed (PB) tillage recorded the highest WS-K values of 36.1, 29.5, and 23.5 mg kg⁻¹ at 0-5, 5-15, and 15-30 cm depths, respectively, whereas the lowest values of 29.3, 24.2, and 18.0 mg kg⁻¹ were observed under conventional tillage-flat (CT-flat). Increased WS-K under PB and ZT-flat may be attributed to residue retention, reduced soil disturbance, improved soil aggregation, and enhanced mineral weathering under conservation agriculture practices. Similar findings were also reported under long-term conservation tillage systems due to better nutrient recycling and improved soil physical properties. Cropping systems also exerted a significant influence on WS-K content. Among the cropping systems, maizechickpea-sesbania (MCS) and maize-wheat-mungbean (MWMb) recorded significantly higher WS-K content across all soil depths, while maize-mustard-mungbean (MMuMb) showed the lowest values (28.7, 23.6, and 9.93 mg kg⁻¹ at 0-5, 5-15, and 15-30 cm depths, respectively). Inclusion of legumes such as chickpea, mungbean, and sesbania in the cropping sequence improved biomass addition, organic matter accumulation, and biological activity, which enhanced nutrient release and K availability in soil. Exchangeable K, which serves as the immediately available K pool for crop uptake, was also significantly influenced by tillage and cropping systems. Higher exchangeable K was observed under PB and ZT-flat systems due to increased cation exchange capacity and improved retention of nutrients under conservation agriculture practices (Deubel et al., 2011; Sharma et al., 2009) [2,13] . Continuous residue retention and minimum soil disturbance under PB and ZTflat enhanced soil organic carbon and reduced nutrient losses, thereby maintaining a greater amount of exchangeable K compared to CT-flat. These findings are in close agreement with the observations of Lakaria et al. (2012). Long-term conservation agriculture practices also significantly increased non-exchangeable K content in soil. Under PB tillage, non-exchangeable K increased from 1259 to 1514 mg kg⁻¹ at 0-5 cm depth, from 1177 to 1436 mg kg⁻¹ at 5-15 cm depth, and from 1128 to 1298 mg kg⁻¹ at 15-30 cm depth compared to CT-flat. The PB and ZT-flat treatments remained statistically at par and superior to CTflat at all soil depths. Among the cropping systems, MCS recorded the highest non-exchangeable K followed by MWMb and MMS. This could be due to better root growth and higher biomass contribution from legumes, which accelerated residue decomposition and released organic acids and carbon dioxide that facilitated the weathering of soil minerals and release of K into soil solution (Deubel et al., 2011) [2] . Chickpea roots are also known to absorb and recycle greater amounts of K, contributing to improved K dynamics in soil (Katyal et al., 1987) [10] . The distribution of K fractions clearly indicated nutrient stratification under conservation agriculture practices, with greater accumulation of available K forms in surface soil layers due to residue retention and minimal soil inversion. In contrast, conventional tillage resulted in comparatively lower K fractions because of intensive soil disturbance and faster depletion of nutrients. Total K content, however, did not vary significantly among tillage practices and cropping systems, suggesting that long-term cultivation practices had limited influence on total soil K reserves, which are largely governed by soil mineralogy. Moreover, the interaction effect between tillage practices and cropping systems on K fractions was non-significant, indicating that the independent effects of tillage and cropping systems were more dominant than their combined effects on soil K dynamics. The study indicated that permanent raised bed (PB) and zero tillage-flat (ZT-flat) practices, along with maize-chickpeasesbania (MCS) and maize-wheat-mungbean (MWMb) cropping systems, were the most effective in enhancing different forms of soil potassium in the Indo-Gangetic Plains. These conservation agriculture practices improved soil nutrient availability, created a favorable crop growth environment, and contributed to sustaining maize productivity over the long term.

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