Effects of long-term super absorbent polymer and organic manure on soil structure and organic carbon distribution in different soil layers
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.
Section snippets
Study site
The study area is located in Yuzhou county, Henan, China (113°03’- 113°39’ E, 33°59’- 34°24’ N, and 116 m above the sea level). The 30-year average precipitation in this area is 674.9 mm, and more than 60% of the rainfall occurs during the summer. According to the USDA texture classification system, the soil texture is sandy loam (59.1% sand, 22.5% silt, and 18.4% clay). Before the experiments, the average soil organic matter content in the cultivation layer was 12.3 g kg-1, total nitrogen was
Effect of different treatments on the proportion of different size class aggregates assessed by mass
As shown in Fig. 2, we can see that the proportion of 0.5-2.0 mm aggregates was the largest (P < 0.05), accounting for 20% to 40% in the 0-30 cm soil layer. The proportion of >2.0 mm aggregates was lowest (P < 0.05) compared with other size soil aggregates across various soil depths. With an increase in soil depth, the proportion of 0.5-2.0 mm aggregates decreased gradually, while the proportion of 0.053-0.25 mm and 0-0.053 mm aggregates increased gradually.
In the 0-30 cm soil layer, the
Effects of long-term application of SAP and OM on aggregate structure of soil
The improvement of soil structure after the application of SAP might be due to the cementing effect, granular shrinkage after water absorption and release cycles, and the promotion of crop root growth (Yang, 2011; Li et al., 2015). However, previous studies showed the effect of short-term application of SAP on soil properties mainly happening in top soil or location of application of SAP in the soil. The depth of influence of these processes increased with the application of SAP over time.
Conclusion
After long-term SAP and OM application, soil aggregates and organic carbon content increased across the soil profile. We found: (1) Compared with the OM treatment, the SAP treatment had a better effect on the increment of the proportion of soil aggregates, TOC and LOC in bulk soil or aggregates and the CROC to total organic carbon of bulk soil: SAP treatment was conducive to the increment of >0.5 mm soil aggregates in the 0-30 cm and 40-60 cm soil layers and 0.25-0.5 mm in the 10-50 cm soil
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
This work was financially supported by the National Key R & D Program of China (2017YFD0301102), the National Natural Science Foundation of China (U1404404), Key R & D, promotion projects in Henan Province(182102110060) and the Re-USe of Treated effluent for agriculture (RUST)project of the Netherlands Organisation for Scientific Research (NWO).
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