Effect of diversified cropping systems on crop yield, legacy, and budget of potassium in a subtropical Oxisol

Highlights

• Cropping system diversification increases crops grain yield.

• Equating maize-soybean proportion (50/50%) increment the K budget.

• Diversifying crops is the key for using K more efficiently in subtropics.

Abstract

Diversified cropping systems can benefit plant growth and yield, but their impacts on K budget and legacy in no-till Oxisols are still poorly understood. This study, based on a long-term (19-years) field experiment, aimed to evaluate the effect of cropping system diversification as well as the proportion of maize in summer season on crop yields, and on soil K forms, use efficiency, budget, and legacy K in a subtropical Rhodic Hapludox under no-till in Southern Brazil. Five treatments were evaluated: two monocropping of maize and soybean in sucession to barley, and three crop rotations with 21%, 26% and 47% of maize in rotation with soybean in summer, combined with up to four winter crops (barley, wheat, canola, and oat). Soil (0–0.20 m) was sampled at the 19th year for determination of soil K forms, and all records of K fertilization and grain yields, as well as the initial soil analysis, were retrieved, and soil K accumulation, K budget, K usage efficiency, and K legacy were calculated. Across the 19-years and in comparison to monocropping, the diversification of cropping systems increased barley (33%) and maize (52%) yields, while a 10% increment was observed for soybean yield by increasing the proportion of maize in summer season from 0% to 47%. Increasing K input via fertilization as a consequence of the higher proportion of maize in summer was the main factor related to observed increment on potentially available K content in soil and on the potential use of legacy K from 6 to 10 years. Maximum K budget (−160 kg ha⁻¹) and use efficiency (114 kg grains kg⁻¹ K applied) were obtained by equating maize-soybean proportion (50/50%) in summer and diversifying winter crops. According to our findings, intercalating maize and soybean in summer and diversifying winter crops are the key for using K more efficiently in subtropical agriculture.

Introduction

Potassium (K) is the most abundant cationic nutrient in plant tissues (up to 10% of the plant dry weight), sometimes absorbed in greater quantities even than nitrogen (N) – the most required nutrient by plants (Epstein and Bloom, 2004). The current estimate of global K reserves is 208 billion tons (U.S. Geological Survey, 2020), sufficient for a period of 5800 years if the current global consumption (36 million tons) is maintained. However, Brazil possess only 1% of this reserve, with an annual production of only 167 thousand tons of K fertilizers (U.S. Geological Survey, 2020), much less than its consumption of 5.4 million tons per year (Heffer et al., 2017). Therefore, Brazilian agriculture is very vulnerable and highly dependent on imports of potash fertilizers. Moreover, one of the most current challenges of agriculture is to intensify food production to meet the growing world population demand, which requires using natural resources and agricultural inputs efficiently and sustainably (Fischer and Connor, 2018). Therefore, as potash mineral fertilizers are a non-renewable resource, the management of this nutrient is something that requires a better attention in order to improve the efficiency of applied K.

Understanding the K dynamics in soil is important for planning its proper management. Some young to moderately developed soils originated from granites and volcanic rocks have great amount of primary and secondary minerals rich in K, such as feldspars, micas, and illite, which might slowly buffer the soil solution (Öborn et al., 2005). In those soils, much of the K taken up by plants comes from these minerals, which makes crops less dependent on K fertilization (Paola et al., 2016). With the advancement of weathering, K reserves decrease to such an extent that in heavily weathered tropical soils, such as Oxisols, the crops are almost entirely dependent on external K sources (Moterle et al., 2019). However, subtropical Oxisols can be considered as intermediate in the scale of soil weathering and might have non-negligible reserves of K-bearing primary minerals and 2:1 clay minerals, which directly affects the dynamics and availability of K to crops, thus, impacting on K fertilization (Bortoluzzi et al., 2013, Caner et al., 2014, Bertolazi et al., 2017).

Potassium in soil is found in three main forms: dissolved in the soil solution as K⁺; (b) adsorbed to the functional groups of clay minerals, oxides, and organic matter, forming surface complexes with different degrees of energy; and (c) as part of the structure of primary minerals or in 2:1 clay interlayers, which both are highly resistant to weathering (structural K). The bioavailability of these forms decreases in the same order (Zörb et al., 2014). All of them are in equilibrium in the soil, and as soluble K is taken up by plants, it is buffered successively by the other forms (Öborn et al., 2005). Likewise, when there is excess K in the solution, K tends to be stored in 2:1 clay mineral interlayers and other potentially available K forms but with high energy bonding complexes (Bortoluzzi et al., 2005). This process is reversible and these K forms might functioning as a K reserve to be used in long-term (Schneider et al., 2013), what is known legacy K.

Some literature reports indicate that dicotyledonous usually allocate a higher proportion of their aboveground total K in the grains (generally between 0.3 and 0.6) (Fageria et al., 2001, Reza. Farrokh et al., 2012, Tamagno et al., 2017) than monocotyledonous (<0.3) (Fageria et al., 2010, Setiyono et al., 2010, Kuzmanova et al., 2014, Zhan et al., 2016). As a consequence, monocots commonly require higher amounts of K in their aboveground biomass, but removes less by grains (by unit of grain harvest) in comparison of dicots, consequently returning greater amounts of K to soil after harvest. In this way, the type of crops does influence both fertilization and K removal and, thus, the composition of cropping systems might influence the accumulation and legacy of K in the soil, assuming that K in excess will be stored in soil. Therefore, if K storage occurs even in soils with low cation exchange capacity, such as Oxisols (Firmano et al., 2019a, Firmano et al., 2019b), it can be expected that the K usage efficiency in agricultural systems will be increased if cropping systems intercalate crops with higher and lower abilities to use K, for instance soybean [Glycine max (L.) Merr.] and maize (Zea mays L.), respectively.

According to Rengel and Damon (2008), the optimal nutrient management strategy for resilient and sustainable agroecosystems should gather soil fertilization and efficient genotypes. Therefore, we hypothesized that the efficiency of K use from fertilizers and/or from the soil legacy K to produce grains depends on the number and diversity of species in cropping system, and that higher K inputs via fertilization will promote K accumulation in available and non-exchangeable forms in a subtropical no-till Rhodic Hapludox. In this context, this study aimed to analyze the impact of cropping systems with different levels of diversification and proportion of maize in summer on grain yields of crops (maize, soybean, barley, wheat, canola, and oat) and on forms, use efficiency, budget, and legacy K in soil.