Freeze–thaw cycle frequency affects root growth of alpine meadow through changing soil moisture and nutrients

Morphological distribution of alpine meadow roots

As shown in Fig. 2, in XM, the alpine meadow roots are mainly distributed at the level of 0–20 cm. The distribution of alpine meadow roots is wider than that of CM, which is mainly distributed from 0 to 40 cm. The alpine meadow roots in XM have a larger surface area, a larger projected area, and a larger volume than that in CM. In XM, the distribution law of alpine meadow roots is horizontal divergence, while the distribution of alpine meadow roots in CM shows vertical extension, but the alpine meadow roots all decrease gradually from the top to the bottom in the XM and CM. At the same soil depth, the total length, total surface area, total projected area, and total volume of alpine meadow roots in XM are larger than those in CM.

According to the diameter of the roots, the alpine meadow roots in the Nagqu River Basin can be divided into three types: 0–0.5, 0.5–1, and 1–1.5 mm (Figs. 3 and 4). In CM and XM the length of the alpine meadow roots decreases with the increase in the soil depth. Additionally, at a soil depth of 0–60 cm, roots with a length of 0–0.5 mm account for the largest proportion, while roots with a length of 1–1.5 mm account for the smallest proportion. With the increase in soil depth, the proportion of the roots with lengths of 0.5–1 and 1–1.5 mm roots gradually decreases or even disappears, while the proportion with a length of 0–0.5 mm gradually increases.

As shown in Figs. 3 and 4, the surface area and the projected area of the alpine meadow roots in the Nagqu River Basin experience similar changes with depth. In CM, with the increase in soil depth, the proportion of roots with a surface area of 0–0.5 mm gradually increases and the proportion of roots with a surface area of 0.5–1 and 1–1.5 mm gradually decreases. In the 10–15 cm soil, roots with a surface area of 0.5–1 mm account for the largest proportion in CM; meanwhile, in XM the roots with a surface area of 0–0.5 mm account for the largest proportion in the 10–20 cm soil.

In CM, the roots with a volume of 0.5–1 mm roots account for the largest proportion in the 10–20 cm soil. However, the roots with a volume of 0.5–1 mm account for the largest proportion in the 10–15 cm soil, while the roots with a volume of 0–0.5 mm account for the largest proportion in the 15–20 cm soil.

In short, with the increase in soil depth, the length, surface area, projected area, and volume of the alpine meadow roots gradually decrease in the Nagqu River Basin, the proportion of 0–0.5 mm roots increases while the proportion of 0.5–1 mm roots decreases. The distribution of alpine meadow roots in CM shows a vertical extension, while in XM it shows horizontal divergence. In addition, compared with CM, the total length, total surface area, and total volume of 0–0.5 mm roots in XM increase by 20.95 cm, 1.90 cm², and 0.014 cm³, and the corresponding specific gravity increases by 9.09%, 13.50%, and 12.14%. The total length, total surface area, and total volume of 0.5–1 and 1–1.5 mm roots in XM show smaller changes, and the corresponding specific gravity decreases.

Distribution of soil moisture and nutrients

As shown in Fig. 5, at the same soil depth, compared with LFM, the moisture in the 0–20, 20–40, and 40–60 cm soil in HFM was reduced by 30.74%, 52.89%, and 47.52%, and even the maximum soil moisture in the HFM was lower than that in the minimum soil moisture in LFM.

In LFM, as the soil depth increases, the soil moisture gradually increases. The 10–20 cm soil had the lowest moisture content in LFM of around 9.11%, the 20–40 cm soil moisture increased by 27.4%, and the 40–60 cm soil moisture increased by 45.9%. In HFM, the moisture content was 6.31% in the 0–20 cm soil. The moisture was the highest in the 40–60 cm soil. Compared with the 0–20 cm, the moisture was increased by 10.6%. The moisture was the lowest in the 20–40 cm soil, being reduced by 13.3% compared to the 0–20 cm soil. Therefore, HFM and LFM have different soil moisture distributions. In the 0–60 cm soil layer of HFM, the middle soil (20–40 cm) had a lower moisture content, while the surface (0–20 cm) and deep soil layers (40–60 cm) had higher moisture contents.

In contrast to the distribution of soil moisture, the distribution of soil nutrients in HFM and LFM was the same: the soil nutrients gradually decreased from the surface to the bottom (Fig. 6). In LFM, the 0–20 cm-depth soil had the highest nutrient content, and the available phosphorous (AP), hydrolysable nitrogen (HN), available K (AK), and microbial biomass carbon (MBC) contents were 2.7, 109.83, 140.11, and 149.38 mg/kg, respectively. With the increase in soil depth, compared with the 0–20 cm soil, the contents of AP, HN, AK, and MBC in the 20–40 cm-depth soil were reduced by 33.33%, 33.44%, 5.45%, and 55.64%, while the content of AP, HN, AK, and MBC in the 40–60 cm-depth soil were reduced by 31.48%, 31.83%, 11.13%, and 66.28%, respectively. In HFM, the nutrient content in the 0–20 cm soil layer was also the largest, and the AP, HN, AK, and MBC has contents of 3.7, 86.17, 107.42, and 120.11 mg/kg, respectively. Compared with the 0–20 cm soil, the contents of AP, HN, AK, and MBC in the 20–40 cm-depth soil decreased by 43.24%, 29.11%, 27.07%, and 60.26%, respectively, while the contents of AP, HN, AK, and MBC in the 40–60 cm-depth soil decreased by 64.86%, 82.79%, 53.04%, and 83.88%, respectively.

Meanwhile, in the same depth of soil, in LFM the content of HN, AK, and MBC is greater than that in HFM. Compared with the LFM, the contents of HN, AK, and MBC in the 0–20 cm soil layer in HFM are reduced by 21.54%, 23.33%, and 19.59%; the HN, AK, and MBC in the 20–40 cm are decreased by 16.43%, 40.86%, and 27.98%; and the HN, AK, and MBC in the 40–60 cm decreased by 80.19%, 59.49%, 61.56%, respectively. However, the AP in the 0–20 and 20–40 cm depths in the HFM is greater than in the LFM. It may be that the higher FTCF causes more damage to Bradyrhizobium, Mesorhizobium, and Pseudomonas in the soil of the Nagqu River Basin, the competitiveness of Bacillus decreases and the abundance increases, while the phosphate-dissolving ability of Bacillus may lead to an increase in the phosphorus content in the soil²⁶.

Correlation analysis

As shown in Fig. 7, the NFTC, FTCD, FTCF, and daily average temperature difference (DATD) all have a significant negative correlation with the top soil moisture in the Nagqu River Basin. This shows that the top soil moisture is not just affected by repeated FTC. Additionally, the effect of FTC on the top soil moisture is not transient; the longer FTC exists, the greater the impact on the top soil moisture will be.

Meanwhile, the contents of HN, AK, and MBC in the top soil have no obvious correlation with NFTC and FTCD but have a significant negative correlation with FTCF. This shows that, compared with NFTC and FTCD, FTCF is more suitable for measuring the influence of FTC characteristics on soil properties. With the increase in FTCF, the damage to the soil structure increases and the contents of HN, AK, and MBC in the top soil significantly decrease, but the AP shows different changes under the influence of microorganisms²⁸.

Source: Ecology - nature.com