Effects of long-term super absorbent polymer and organic manure on soil structure and organic carbon distribution in different soil layers

Highlights

• SAP and OM improved soil structure and increased organic carbon content.

• SAP incresed the TOC and LOC of 0.25-2.0 mm aggregates at the 10-40 cm depth.

• Soil structural stability of SAP was significantly higher than that of OM at 0-10 cm depth.

Abstract

Super absorbent polymer (SAP) and organic manure (OM) may improve soil structure and change soil organic carbon (SOC) composition and agroecosystem functioning. However, the understanding of the effects of SAP and OM on SOC composition, specifically in deeper soil layers, is still not clear. The objectives of this study were to examine the effects of long-term application of SAP and OM (8 years) on changes in SOC and soil structure down to a soil depth of 100 cm. Therefore, in order to investigate the stability of soil structure and the distribution of organic carbon at different soil depths (0-10 cm, 10-20 cm,…, 90-100 cm) under long-term application of SAP, OM, and a control treatment, soil structure and soil organic carbon content were analyzed in mixed soil samples and undisturbed soil samples. The results indicated that with depth, the proportion of large aggregates (0.5-2.0 mm) decreased gradually, while the proportion of small aggregates (<0.25 mm) increased gradually. Compared with the control, SAP treatment was conducive to the increment of>0.5 mm soil aggregates in the 0-30 cm and 40-60 cm soil layers (P < 0.05) and 0.25-0.5 mm in the 0-50 cm soil layer (P < 0.05), and while the OM treatment had the same effect in the 0-30 cm soil layer (P < 0.05). The total organic carbon content (TOC) and labile organic carbon content (LOC) of bulk soil increased (0-20 cm) initially with depth, and then decreased (20-70 cm) approximately. In the 0-50 cm soil layer, the TOC and LOC under SAP treatment were higher than those under OM treatment. The SAP treatment was more beneficial to the increase of the TOC and LOC of 0.5-2.0 mm and 0.25-0.5 mm aggregates in the 10-40 cm soil layer compared with the control and OM treatment (P < 0.05). The SAP treatment was also more beneficial in increasing the contribution rate of organic carbon (CROC) in> 0.5 mm aggregates in 0-40 cm soil depth, while the OM treatment was more beneficial in increasing the CROC of > 0.5 mm aggregates in 0-30 cm soil depth. The SAP treatment improved the stability of the soil structure in 0-30 cm and 40-60 cm depths, and the OM treatment had the same effect in the 0-30 cm depth. Compared with the control and OM treatments, SAP treatment has shown to be the most beneficial in improving soil structure and increasing organic carbon content.

Introduction

Soil aggregation and organic carbon content can be used in an integrated index to evaluate soil quality and supply capacity of soil nutrients (Six et al., 2000a), which maintains the ecological functions of soil (Wang et al., 2010). Soil aggregates are the basic units of soil structure, and store various nutrients in the soil, which promotes the growth of soil microorganisms (Six et al., 2004). A favorable soil structure and high aggregate stability are important in improving soil fertility and moisture, increasing food production, enhancing porosity and decreasing erodibility. Disturbances of soil structure through compaction or tillage can result in the rapid recycling of nutrients, crusting, and reduced water and air availability to roots, while improper and excessive use of fertilizers in dry farming areas increase the risk of soil structure degradation and low resource use efficiency (Bronick and Lal, 2005; Zhao et al., 2014). Therefore, it is important to improve soil aggregation through suitable fertilization practices and soil amendment methods, in order to enhance soil quality and increase agronomic performance (Guo et al., 2018).

A mutually reinforcing relationship exists between soil organic carbon content and soil aggregate quality (Christensen, 1996). In particular, soil organic carbon has an important effect on the number and size distribution of soil aggregates (Eynard et al., 2005). An increase in soil organic carbon content is beneficial for the formation and stability of soil structure (Wang et al., 2012a, 2012b; Liu & Yu, 2011). The physical reinforcement of soil organic carbon by stable aggregates can slow down or prevent mineralization and decomposition of soil organic carbon (Lal & Kimble, 1997; Six et al., 2000b). If organic carbon content decreases, the stability of the aggregates will decrease (Li et al., 2002), resulting in a negative feedback loop. For example, frequent ploughing destroys soil structure and decreases soil organic carbon content (López et al., 2011). On the other hand, soil improvement measures such as the application of organic manure (OM) and super absorbent polymer (SAP) can effectively improve the soil structure and increase the proportion of water-stable aggregates (Yang et al., 2012; Yang et al., 2018), which results in a positive feedback loop.

Studies have shown that SAP can reduce soil penetration resistance (John et al., 2005), increase soil aggregation, and aid in the conservation of soil organic matter (Goebel et al., 2005; John et al., 2005). SAP are hydrophilic and can absorb and retain 1000 times more water or aqueous solutions than their original size and weight (Sojka and Entry, 2000). Thus, the application of SAP to soil may increase water retention capacity and nutrient use efficiency (Lentz and Sojka, 1994; Lentz et al., 1998), while reducing water loss (Al-Omran and Al-Harbi, 1997). Results have shown that the SAP can improve soil structure (Terry & Nelson, 1986; Sojka et al., 2007), increase the relative proportion of water stable aggregates (Yang et al., 2009), improve the stability of soil structure (Yang, 2011), and enhance the porosity (Liu et al., 2003; Han et al., 2010). However, previous studies on SAP have been focused on its effects on soil hydro-physical and chemical properties (Nadler et al., 1996; Zhang and Miller, 1996), soil water content (Bai et al., 2010), soil erosion control and irrigation management (Sojka et al., 1998), and plant growth and production (Busscher et al., 2009; Islam et al., 2011). Tian et al. (2019) investigated the effect of different polymer materials on soil aggregates and soil organic carbon, and found that the modified polymer improved soil aggregate water-stability and increased soil organic carbon content. However, few studies have investigated the effects of SAP, especially under long-term application, on the different organic carbon fractions such as total organic carbon and labile organic carbon, and on the contribution rate of organic carbon in different diameter aggregates to total organic carbon in deep soil.

Organic manure (OM), such as from pigs, cows, chickens and so forth, has been widely used throughout the world for centuries as a fertilizer for farming, especially in ancient China. The application of manure to soils can provide additional nutrients that facilitate the growth of plants (Edmeades, 2003). Manure can also improve soil structure (i.e., increase soil porosity, and aggregate stability but decrease bulk density), enabling the soil to hold more nutrients and water, and thus achieving higher agricultural productivity (Celik et al., 2010; Rasool et al., 2008; Zhou et al., 2013). However, reports regarding the effects of organic manure application on soil aggregation, particularly in long-term field experiments, are inconsistent. For example, it is widely reported that OM application can increase soil aggregation considerably especially for macroaggregates due to increased soil organic carbon (SOC) accumulation (Rasool et al., 2008; Tripathi et al., 2014; Yan et al., 2013) and OM along with inorganic fertilizers can improve soil structure as well (Zhou et al., 2017). However, some studies have shown that excessive animal manure (such as pig manure and cattle manure) has a negative (Guo et al., 2018; Zhang et al., 2016) or neutral effect (Xie et al., 2015) on the formation of large macroaggregates, and consequently results in a lower mean weight diameter (MWD) of aggregates, thus decreasing soil structural stability. As for chicken manure, Yang et al. (2018) found that the application of straw mulch and organic manure improved soil porosity and the stability of soil structure.

There have been numerous studies on the changes of soil aggregate composition and organic carbon content under OM, but few on the effect of SAP on organic carbon content, especially under long-term application. Furthermore, most of these prior studies focused on soil structure and soil total organic carbon (TOC) in cultivated soil. However, the distribution of TOC, labile organic carbon (LOC), different diameter aggregates and the contribution rate of organic carbon (CROC) in aggregates to total soil organic carbon under the long-term application of SAP and OM in the 0-100 cm soil layer are not clear. Therefore, it is necessary to study in detail the effect of long-term farming and continuous application of SAP and OM on organic carbon composition, distribution of soil aggregates and different organic carbon fractions with the soil profile. The aim of this study was therefore to (1) determine the effect of long-term application of SAP and OM on the distribution of different sizes of soil aggregates, aggregate stability (MWD, Geometric mean diameter (GMD), Fractal dimension (D)), TOC, LOC and the CROC to total organic carbon; and (2) to evaluate the relationships between different soil organic carbon fractions, proportion of different sizes of aggregates, and soil structure stability.