Responses of aggregates and associated soil available phosphorus, and soil organic matter in different slope aspects, to seasonal freeze–thaw cycles in Northeast China

Highlights

• Soil aggregate stability in shady slope was more sensitive to freeze-thaw cycles.

• Well relationships exist between the aggregate fractions and available phosphorus.

• Soil organic matter of partial aggregate sizes were sensitive to freeze-thaw cycles.

Abstract

Freeze-thaw is an important factor that affects soil aggregate size and integrity in mid-high latitudes or mountainous regions, which in turn impacts soil erosion. However, there are limited studies on the responses of aggregates and associated soil available phosphorus (AP), as well as soil organic matter (SOM) in different slope aspects to seasonal freeze–thaw cycles (FTCs). In this regard, soil samples at different depths (0–5 [D₅], 5–10 [D₁₀], 10–15 [D₁₅], and 15–20 cm [D₂₀]) were collected from sunny (SW) and shady (NE) slopes in Meihekou, Northeastern China. Temporal changes in aggregate size distribution, AP (extracted with 0.5 mol L⁻¹ NaHCO₃), and SOM of SW and NE were analyzed during one FTCs season (from October 2017 to March 2018). Seasonal FTCs significantly (p < 0.05) influenced aggregate size distribution and associated AP content. The results indicated that although there were similar patterns of change in soil aggregate size distribution over time between SW and NE, NE was more sensitive to seasonal FTCs than SW. The greatest reduction and increment rates were observed in aggregates sized > 5 mm and < 0.25 mm, respectively, at D₅ in both SW (reduction rate = 44.9%, increment rate = 63.1%) and NE (reduction rate = 73.3%, increment rate = 76.3%). The mean weight diameters were significantly (p < 0.05) decreased by 0–22.6% in SW and 13.4–43.7% in NE at different soil depths after the seasonal FTCs. Good linear function relationships were observed between the proportion of aggregate fractions and associated AP content for aggregate sizes < 0.25 mm (r² = 0.38**), 1–2 mm (r² = 0.32**), and 2–5 mm (r² = 0.42**) in SW and for > 0.25 mm (r² = 0.68**), 0.25–1 mm (r² = 0.38**), 1–2 mm (r² = 0.52**), and 2–5 mm (r² = 0.64**) in NE during the seasonal FTCs. The associated AP content showed positive correlations with the proportion of aggregates sized <0.25 mm and 0.25–1 mm and negative correlations with the proportion of aggregates sized 1–2 mm, 2–5 mm, and >5 mm in both SW and NE. The SOM was less sensitive to seasonal FTCs. After the seasonal freeze–thaw period, only the SOM content of partial aggregate sizes decreased significantly (p < 0.05) in NE, whereas there were no such significant changes in SW. Our results demonstrated that slope aspect is a key driver of the different responses of aggregates and associated AP and SOM content during the seasonal FTCs and that slope information could be useful for exploring soil erosion and nutrient loss in freeze–thaw agricultural ecosystems.

Introduction

As one of the important units of soil structure, soil aggregate is the primary factor influencing pore characteristics (Mangalassery et al., 2013) and mechanical resistance (Ye et al., 2017), which consequently affects air-change characteristics (Wei et al., 2016), sediment transport capacity (Liu et al., 2020), and retention of water (Wei et al., 2019) and nutrients (Jiang et al., 2010). Therefore, the stability and integrity of soil aggregates are important factors for soil productivity and consequent crop production (Edwards, 2013). Many investigations have reported that high aggregate stability is beneficial for reducing soil erosion and improving soil fertility (Lu et al., 2016). As a result, changes (i.e., decrease or increase) in aggregate size have gradually become an area of research interest. The soil aggregate characteristic is vulnerable, and threatened by land-use changes (Saha et al., 2011) and climatic stresses, such as the alteration of freeze–thaw in cold regions.

Cold regions in mid-high latitudes and mountainous areas suffer severe seasonal freeze–thaw cycles (FTCs); this strongly affects the soil structure, including the stability of soil aggregates. Most studies have reported that FTCs break down large aggregates and reduce the strength and size of aggregates, which results in lower aggregate stability (Cécillon et al., 2010, Dagesse, 2013, Ma et al., 2019). However, because of the different experimental conditions used in these studies, the responses of soil aggregate changes to FTCs varied. Some researchers have found that FTCs increased large-sized aggregates and decreased small-sized aggregates, which improved soil aggregate stability (Lehrsch, 1998, Wang et al., 2020, ZHANG et al., 2016). The soil type (Kværnø and Øygarden, 2006), initial soil water content (Wang et al., 2012), vegetation type (Xiao et al., 2019), soil degradation conditions (Ma et al., 2019), and freeze–thaw conditions (Oztas and Fayetorbay, 2003) have all been considered factors that impact the relationship between FTCs and soil aggregates, which might have further led to inconsistent conclusions. Slope aspect is also an important factor affecting the physico-chemical and biological properties of soil (Begum et al., 2010, Yuan et al., 2019) by influencing solar radiation, temperature, moisture content, and evaporation and consequently affecting the intensity of soil erosion (Li et al., 2019).

Natural and anthropogenic activities, as mentioned previously herein, can influence soil aggregate development, which is a relevant indicator of nutrient cycling in soil (Jiang et al., 2011, Guidi et al., 2017, Bedel et al., 2018). For example, negative correlations have been observed between soil organic matter (SOM) content and macroaggregates (Yin et al., 2015), whereas positive correlations have been found between organic carbon and aggregate size (Jiang et al., 2011, Qu et al., 2018). The differences between the cited results indicate that the relationship between nutrient content and aggregate size is complex and could depend on additional factors, such as soil type, moisture content, freeze–thaw time, and topography (Ozgul et al., 2012, Zhou et al., 2020). Further, Han et al. (2019) reported that the highest nitrogen and phosphorus contents were found in microaggregates (250–53 μm).

Under the climatic conditions of Northeast China, seasonal freeze–thaw affects soil physical properties and increases soil erodibility, increasing subsequent snowmelt erosion during the spring thawing period. In the Mollisol region of Northeast China, snowfall caused 13.3–24.9% of snowmelt runoff and 5.8–27.7% of snowmelt sediment yields over 1 year (Jiao et al., 2009). This not only results in soil degradation but also further threatens sustainable development. Therefore, research on the mechanism of soil erosion and nutrient loss during the seasonal freeze–thaw period in this region is of immense importance. To date, several researchers have generated important findings on the different characteristics of soil aggregates (Jin et al., 2019, Ma et al., 2019, Wang et al., 2020) and associated nutrients (Yin et al., 2015, Jiang et al., 2018, Lu et al., 2019) in Northeast China. However, there are few studies of the influence of FTCs on soil aggregates and associated nutrients during the seasonal freeze–thaw period, especially in different slope aspects. In the present study, field experiments were conducted in Northeast China to explore the temporal changes (from October 2017 to March 2018) in aggregate size distribution and available phosphorus (AP) and SOM in soils collected from cultivated land with different slope aspects during the seasonal freeze–thaw period. The aim of this study was to understand the variation in aggregate size distribution and further identify its associations with AP and SOM and finally, to provide a basis for the mechanism behind soil erosion and nutrient loss in this region.