Effects of straw incorporation and potassium fertilizer on crop yields, soil organic carbon, and active carbon in the rice–wheat system

Highlights

• Soil organic C and active C fractions were compared in organic and inorganic K source.

• Straw returning increased SOC than inorganic K via increasing active C contents.

• Straw incorporation increased yield more in wheat than rice over without K fertilizer.

Abstract

Active soil organic carbon (SOC) fractions play a key role in agricultural soil fertility. However, the effects of potassium application and straw incorporation on SOC and active SOC fractions as well as the relationships among these factors in a rice–wheat system are less well-studied. Hence, the objective of this study was to analyse the effects of potassium fertilization and straw incorporation on SOC sequestration, active carbon fractions, and crop yields in a long-term (6 years) field experiment. Four treatments were examined: no addition of potassium fertilizer and straw (CK), straw incorporation only (SI), potassium-fertilizer application only (K), and straw incorporation plus a recommended amount of potassium fertilizer (SI + K). SOC, dissolved organic carbon (DOC), light fraction organic carbon (LFOC), microbial biomass carbon (MBC), easily oxidizable carbon (EOC), crop yields, and the carbon pool management index (CMI) were determined. After 6 years, SOC content and labile C fractions in the SI treatment increased significantly, by 7.95–25.0 % and 23.6–185 %, respectively, compared to the CK treatment. Significant and positive correlations were observed between SOC, DOC, LFOC, MBC, EOC, and the CMI (r = 0.449–0.899, P < 0.01). Among the five C fractions, LFOC, DOC, and EOC were the most sensitive indicators of changes in SOC induced by incorporating straw with or without potassium fertilizer. The highest SOC contents and labile C fractions were observed in the SI + K treatment. Rice yields increased from 8.55 and 7.59 t ha⁻¹ in the CK plots to 9.56 and 8.54 t ha⁻¹ with SI + K at Guangde (GD) and Jiangyan (JY) sites, respectively. Wheat yields increased from 0.53 and 5.33 t ha⁻¹ in the CK plots to 5.86 and 6.43 t ha⁻¹ with SI + K at GD and JY sites, respectively. Overall, based on crop yields and C storage, straw incorporation in combination with a moderate amount of potassium fertilizer appears to be the best practice for improving soil fertility and productivity in the rice-wheat cropping system.

Introduction

Soil comprises the largest pool of terrestrial organic carbon (C), and it plays a paramount role in the global C cycle (Jobbágy and Jackson, 2000). Soil organic C (SOC) content and quality are key indicators of the physical, chemical, and biological characteristics of soil as well as its processes (Liu et al., 2010; Duval et al., 2013; Lu, 2015). Understanding the SOC dynamics is important for the long-term sustainability of agro-ecosystems and the environment, because SOC provides the nutrients that crops need, facilitates water retention, transforms soil physical properties, and improves the productivity and activity of soil organisms. However, short- and medium-term changes in SOC in response to altered management practices are difficult to measure due to high background levels and natural soil variability (Haynes, 2005; Xu et al., 2011; Reddy et al., 2017). Instead, the active C in soil are early indicators of the impact of newly introduced management techniques on soil quality (Yan et al., 2007; Zhu et al., 2015).

According to the turnover rates of SOC fractions, SOC can be divided into labile organic C and stable organic C (Majumder et al., 2008). The labile fraction of SOC serves as an energy source for soil microorganisms and a nutrient bank for plants. Different components of the labile soil organic matter pool have been studied as early indicators of changes in soil quality due to changes in management practices (Blair et al., 1995; Liu et al., 2003; Wander, 2004; Xu et al., 2011; Zhu et al., 2015; Hu et al., 2017), including particulate organic C (POC), dissolved organic C (DOC), microbial biomass C (MBC), easily oxidizable carbon (EOC), and light fraction organic C (LFOC). These labile organic C fractions are transferred between fresh plant residues and stable organic matter within short periods of time (Benbi et al., 2015). The effects of rice-cultivation time and land use on labile organic matter in paddy soil have also been reported (Zhang and He, 2004). Additionally, chemical fertilizers (Meng et al., 2017), straw, and manure have been widely applied to soil to increase rice yields and improve C sequestration (Yan et al., 2007; Gong et al., 2009a, 2009b; Xu et al., 2011).

SOC content in agricultural systems can be improved by the adoption of suitable management practices (e.g., fertilizer application and straw return). In the past decades, many studies have focused mainly on the soil C turnover in response to nitrogen (N) and phosphorus (P) fertilizer application than potassium (K) fertilizer (Yan et al., 2007; Liu et al., 2010; Meng et al., 2017), because N, P fertilizer may affect more crop grain and straw yield than K fertilizer (Liu et al., 2003). Only a few studies paid attention to the soil active C fractions and SOC in response to long-term inorganic- and organic-K fertilizations management. Crop straw is an important organic fertilizer resource, as it contains abundant C and K (Zhao et al., 2014). The quantity and quality of returned straw are important considerations in the establishment of a K-management strategy to ensure crop productivity (Li et al., 2020). A 10-year of straw returning significantly increased the SOC content, but short-term (1–2 year) of straw incorporation to the fields have diverse results (Xu et al., 2011). In the soil–plant system, straw incorporation can potentially increase SOC storage (Liu et al., 2014a, 2014b; Chen et al., 2018; Li et al., 2018) and available K content (Lu et al., 2017; Li et al., 2020) as well as mitigate the impacts of climate change (Lal, 2004; Jiang et al., 2017; Ghosh et al., 2018). Lu et al. (2017) reported that continuous straw removal resulted in K deficiency, especially when it was carried out in combination with improper K fertilizer application for 5 years, resulting in considerably reduced wheat and rice grain yields. Furthermore, K management strategy and straw production (thereby C budget) could differ with different soil K level especially for the non-exchangeable K soil types. It is important and necessary to understand the response of the crop production and soil C pool with non-exchangeable K soil types. Here, we hypothesized that straw incorporation combined with the addition of chemical K fertilizer would improve crop productivity and SOC content by increasing the labile organic C fractions in a rice–wheat (RW) cropping system compared with the sole addition of K fertilizer or straw in a given soil nonexchangeable type.

The RW rotation system is the main crop rotation system in the rice-growing region along the middle-to-lower reaches of the Yangtze River in China. This cropping system plays a crucial role in ensuring China’s food security and comprises one of the largest agricultural production systems in the world (Timsina and Connor, 2001). However, long-term intensive cultivation has resulted in a substantial decrease in SOC in croplands and a rapid decline in soil fertility (Yu et al., 2006; Zhao et al., 2020), largely from the continuous removal of straw for other purposes such as cooking and feeding animals as well as a lack of labour for processing the straw. The soil K content is affected by the type and quality of clay as well as soil texture (Rosolem et al., 2010). Large crop yield gaps existed due to different organic- and inorganic-K management strategies and soil non-exchangeable K levels (Li et al., 2020). Sustainable soil management is needed to feed the increasing world population without jeopardizing the quality of natural resources. Therefore, we established a 6-year (2012–2018) field experiment to investigate a consecutive RW rotation system in two fields located along the middle-to-lower reaches of the Yangtze River. The objectives of our study were to assess the effects of adding K fertilizer and/or straw on crop yields, SOC, and labile organic C fractions as well as the relationships between SOC, labile SOC fractions, and the C-pool management index (CMI) using a long-term RW cropping field experiment with chemical fertilization and straw incorporation as treatments.