Macropores in a compacted soil impact maize growth at the seedling stage: Effects of pore diameter and density
Introduction
Soil compaction is a global problem, leading to an increase in soil bulk density and a decrease in soil porosity (Batey and McKenzie, 2006). Reportedly, 32% of European subsoils are compacted (Horn and Fleige, 2009). Compacted soils usually have poor air and water permeability with high mechanical impedance (Valentine et al., 2012). Root growth was limited when the soil penetration resistance reached 2 MPa (Bengough et al., 2011). Xiong et al. (2020) found that the root volume of maize cultivar (Zhengdan958) significantly decreased under compacted soils. Tracy et al. (2012) reported that soil compaction resulted in a decrease in total root length and root surface area in tomato. In addition, a study by Obour and Ugarte (2021) revealed that soil compaction could reduce wheat, corn and soybean yields by up to 6%, 34% and 34%, respectively. As a result, alleviating the adverse effects of soil compaction on crop growth is important for ensuring food production.
It is reported that soil compaction can be effectively alleviated by cover crops with deep roots (Chen and Weil, 2010, Rosolem and Pivetta, 2017). The method of using root system of cover crops to improve soil pore structure has been termed bio-drilling or bio-tillage (Cresswell and Kirkegaard, 1995, Zhang and Peng, 2021). The tap-rooted crops such as forage radish can create root channels in the compacted soil (Chen and Weil, 2010), whereas the fine roots may perform well in increasing the porosity of soil (Hudek et al., 2021). Many studies have reported that creating macropores in compacted soils could be helpful in alleviating the negative effects of soil compaction on crop growth (Pfeifer et al., 2014, Colombi et al., 2017, Atkinson et al., 2020). Macropores can be regarded as potential pathways for roots to bypass soil with high strength and consequently, facilitate the absorption of water and nutrients in the subsoil by roots (Jakobsen and Dexter, 1988, McKenzie et al., 2009, Athmann et al., 2019). Some studies reported that crop roots tended to grow in the macropores in compacted soils, and root length and volume were increased due to the presence of macropores (Atkinson et al., 2020, Haling et al., 2011, Pfeifer et al., 2014).
The impacts of bio-tillage on succeeding crop growth usually vary with cover crop species (Han et al., 2016). Different cover crops have different root architectures, resulting in diverse macropore characteristics. For example, crop roots with larger diameter will lead to an increase of large-sized pores (McCallum et al., 2004, Kautz et al., 2014, Han et al., 2017), while the roots with smaller diameter will increase the small-sized pores (Huang and Gao, 2000). The effects of various sizes of pores on crop growth may be different. The promotion of crop growth by artificial macropores with diameters of 0.8-, 1- or 1.25-mm has been reported (Atkinson et al., 2020, Pfeifer et al., 2014, Colombi et al., 2017). However, Stirzaker et al. (1996) pointed out that macropores with 3.2-mm diameter might limit root growth in wet soils. Moreover, Bauke et al. (2017) observed that the 6-mm diameter macropores in the subsoil did not increase the wheat growth. The inconsistent effects of macropores on root growth seem to be related to the macropore diameter, which is one of the most important pore characteristics that directly affects plant growth. For example, Passioura (2002) reported that the leaf area of young barley decreased with increasing pore diameter in a hard soil. Jakobsen and Dexter (1988) also found that the grain yield decreased as the pore diameter increased from 0.4 to 12.8 mm using a computer model. In addition, it has been reported that macropore density also has a significant effect on crop performance (Perkons et al., 2014). Koch et al. (2021) revealed that wheat shoot biomass significantly enhanced with increasing pore density under a subsoil moisture of 30% water holding capacity, but not for root biomass. Using a computer model, Jakobsen and Dexter (1988) found that the wheat yield declined with increasing biopore density. In general, determining macropore characteristics that promote crop growth has important guiding significance for the improvement of compacted soils.
To date, it has always been a challenge to observe and quantify the spatial relationship between roots and macropores well. With the development of X-ray computed tomography (CT), some researchers have begun to directly analyze the interaction between roots and pores (Pfeifer et al., 2014, Colombi et al., 2017, Atkinson et al., 2020). Using X-ray CT, Zhou et al. (2021) found that the percentage of roots growing in macropores increased with soil depth due to a decrease in macroporosity in the subsoil. Dexter (1986) reported that the probability of roots encountering 1 mm pores increased linearly with pore density. These studies have indicated that root-macropore interactions are greatly dependent on soil pore characteristics. Colombi et al. (2017) reported that the wheat roots tended to cross the macropores compared with the soybean and maize colonizing the macropores in a soil column experiment. Dexter (1991) observed that roots preferred to grow towards air-filled macropores in an anaerobic soil. Atkinson et al. (2020) found that roots usually colonized the macropores and changed growth direction in compacted soils, but roots only crossed the macropores in loose soils. These studies mainly revealed the interactions between roots and pores were affected by crop species, oxygen concentration and soil bulk density, ignoring the effects of macropore characteristics on root-pore interactions.
Thus, the objectives of this study were to investigate the effects of macropore diameter, density, pore wall surface area and macroporosity on maize growth in a compacted soil and to reveal the interaction of roots and macropores as affected by macropore characteristics. Our hypotheses were (1) that macropore diameter and density impacted maize growth and (2) that the number of macropores colonized or crossed by roots increased with increasing macropore density. We believe this study can improve our understanding of the functions of macropore characteristics on root growth in compacted soils, and will be beneficial for cover crop management in terms of bio-tillage.
Section snippets
Experimental design
The soil was sampled at a depth of 0–20 cm soil layer under a wheat-maize cropping system at Longkang farm, Anhui Province, China (33° 32′ N, 115° 59′ E). This soil is classified as a clayey Vertisol (IUSS WG WRB, 2015), in which soil bulk density at the 0–20 cm plow layer ranged from 1.3 to 1.6 g cm−3 (Xiong et al., 2021). The soil properties were shown in Table 1. The soil was air-dried and sieved to < 2 mm for the column experiment. Sieved soils were packed into polyvinyl chloride cylinders.
Soil physical conditions
The presence of 0.5-, 1- and 2-mm macropores did not significantly reduce the soil penetration resistance compared with that in the Control treatment (Fig. S2; P > 0.05). Under the same macropore wall surface area (i.e., 2513 mm2) or the same macroporosity (i.e., 0.342%) conditions, the soil penetration resistance did not vary with the macropore diameter (Fig. S2; P > 0.05). The macropore density had no effect on the soil penetration resistance (Fig. S2; P > 0.05). Thus, the presence of
Effect of macropores on soil physical conditions
In our study, the presence of macropores did not significantly affect the penetration resistance of compacted soil (Fig. S2). This result was consistent with a soil column experiment of Colombi et al. (2017), who also found that the soil mechanical impedance was not affected by macropores. The soil penetration resistance was not affected by vertical macropores in these two studies, which may be related to the measurement of penetration resistance parallel to artificial macropores. However,
Conclusions
This study demonstrates the effect of macropore characteristics in a compacted soil on maize growth using X-ray computed tomography. The 0.5- and 1-mm macropores performed much better in improving maize growth and nitrogen uptake in the compacted soil compared with the 2-mm macropores. At a given macropore diameter (i.e., 0.5-mm), a macropore density (i.e., 7000 m−2) significantly enhanced maize growth relative to other two macropore density conditions (i.e., 1750 and 28,000 m−2). Thus, a
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.
Acknowledgements
This work was granted by National Natural Science Foundation of China (41725004; 41771264; 41930753), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA28010401), and Youth Innovation Promotion Association of CAS (2021311).
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