Crop yield and soil available potassium changes as affected by potassium rate in rice–wheat systems

Highlights

• We determined crop yield and soil K changes in a K deplete experiment.

• Wheat yield loss was much higher than rice in unfertilized plot.

• Soil K changes for NaTPB was more close to the theoretical soil available K changes.

Abstract

Large areas of the arable soils of the world are deficient in potassium (K) due to the low application rate of K fertilizer. However, current soil test methods cannot precisely determine soil available K changes. From 2009 to 2014, a field experiment was conducted in a rice–wheat cropping system in the Yangtze Plains using five K rates. The objectives were to determine the responses of wheat and rice yield to different K rates and to compare the soil available K changes extracted by three methods (ammonium acetate (NH₄OAc), boiling nitric acid (HNO₃), and sodium tetraphenylboron (NaTPB)). Long periods without application of K fertilizer markedly decreased wheat yield by 47% and rice yield by 15% compared with local farmers’ K practices (FKP, 90 and 120 kg K₂O ha⁻¹ for wheat and rice, respectively). The FKP achieved optimal yields for wheat and rice; however, only 160% of FKP achieved a positive K balance for the cropping system. Soil-extractable K consistently decreased with increasing cropping rotations where the K rate was below 160% FKP for the three extraction methods. The extractable and changed amounts for the NH₄OAC and HNO₃ methods were significantly lower than that for the NaTPB method. The soil K changes for NaTPB were closer to the theoretical soil available K changes (TAKC) derived from an apparent K balance. The NaTPB method could be useful for accurately determining changes in soil available K in cropping systems.

Introduction

In the past few decades, potassium (K) has become the “forgotten” nutrient with regard to environmental quality, drawing less research attention than nitrogen and phosphorus (Öborn et al., 2005, Römheld and Kirkby, 2010). A negative K balance is extremely challenging for food security at regional and even global scales (Lal, 2009, Cakmak, 2010), and presents a challenge in most developing and many developed countries. Recently, a study found that 75% of rice fields in south China and 66% of the soil in the Australian wheat belt suffered from K deficits (Römheld and Kirkby, 2010, Zhang et al., 2010, Wu et al., 2013). One of the biggest causes of crop yield stagnation and low nutrient efficiency in modern intensive agriculture can be attributed to K depletion in the soil (Regmi et al., 2002, Ladha et al., 2003).

In soil-crop systems, the magnitude of a crop’s growth response to the application of K fertilizer varies with soil available K levels, the K input rate (Wu et al., 2013), the soil K-supplying capacity (Dobermann et al., 1996b, Ladha et al., 2003), the status of straw recycle use (Zhao et al., 2014), and the crop species (Cassman et al., 1989). Bijay-Singh et al. (2004) reported the yield of rice and wheat increased due to application of fertilizer potassium in soils testing less than 100 mg K kg⁻¹ of NH₄OAc extractable K. Potassium input rates are generally much lower than K uptake rates for cereal crops (Regmi et al., 2002). According to a recent study on the Yangtze Plain by Wu et al. (2013), the typical farmers’ K rate of 90–120 kg ha⁻¹ is applied in the rice-wheat system. Importantly, the amount of K accumulated in straw can account for nearly 80% of total K uptake (Panaullah et al., 2006), and the quantity and quality of straw returned to the fields also significantly affects K balance for many cropping systems. Unfortunately, in developing countries, straw is typically removed from the field, as it is used for other purposes such as household cooking or animal feed (Regmi et al., 2002, Miao et al., 2010). Even in the North China Plain, where there is a higher degree of mechanization, the percentage of returned field straw only accounts for 50–60% of crop systems due to the incurred additional cost (Gao et al., 2009). Recently, the emergence of biofuels has made this situation much worse (Propheter and Staggenborg, 2010). These integrated suboptimal behaviors will progressively deplete soil K fertility and cause significant soil available K changes.

Potassium application rate and crop K uptake immediately influence the soil available K content (Regmi et al., 2002, Römheld and Kirkby, 2010). Hence, soil available K change is a closely related parameter that directly reflect K budget in soil-crop system. Long term negative soil K changes are non-sustainable for agricultural system (Wang et al., 2007). Accurately determining changes in soil available K is important when developing optimal K management strategies. In terms of methodology, soil tests for fertilization recommendations typically extract exchangeable K with ammonium acetate (NH₄OAc). Exchangeable K often exhibits a close correlation with crop K uptake, as Johnston and Krauss (1998) showed for winter wheat and field beans in a silty, clay loam soil in the United Kingdom. However, Römheld and Kirkby (2010) reported that the K-NH₄OAc method can often, but not always, offer a precise indicator of the available K status of the soil because certain rates of interlays of labile K are available for crop uptake. Dobermann et al. (1996a) reported that current recommendations for K addition using NH₄OAc method in most intensive irrigated rice domains are insufficient to replace K removal. The most common method for determining nonexchangeable K is based on hot nitric acid (HNO₃) extraction via the replacement of K⁺ by H⁺ (Øgaard and Krogstad, 2005, Li et al., 2015). Additionally, Wang et al. (2007) recently found an improved method for simulating the process of available K depletion in crop roots using sodium tetraphenylboron (NaTPB). This method accurately evaluates available K released in the soil from non-exchangeable K via the formation of K precipitation in a soil solution. However, few studies have been conducted to compare the three extraction methods in terms of a precise prediction of available K changes in rice–wheat cropping systems. Field K depletion experiments can help achieve this goal.

In this study, we used a field experiment with varying levels of K to study the K-deficient status of a wheat–rice cropping system in China. The experiment covered five complete wheat–rice cropping rotations, from the 2009 rice season to the 2014 wheat season. The specific objectives were: (1) to determine the wheat and rice yield and apparent K balance in relation to different K levels; (2) to monitor the dynamics of soil extractable-K content under the three methods; and (3) to determine the relationship between theoretical soil available K changes based on K fertilizer input and K uptake and actual soil extractable-K changes for the three methods.