The frequency of the freeze-thaw cycle affects root growth in alpine meadows by changing soil moisture and nutrients
Morphological distribution of roots in alpine meadows
As shown in Fig. 2, in XM, the roots of alpine meadows are mainly distributed at the level of 0–20 cm. The root distribution of alpine meadows is wider than that of CM, which is mainly distributed from 0 to 40 cm. The roots of alpine meadows in XM have greater surface area, greater projected area, and greater volume than those in CM. In XM, the distribution law of alpine meadow roots is a horizontal divergence, while the distribution of alpine meadow roots in CM shows a vertical extension, but the roots of alpine meadows all decrease gradually from top to bottom in XM and CM. At the same soil depth, the total length, total area, total projected area and total root volume of alpine meadows in XM are larger than those in CM.
According to the diameter of the roots, the roots of the alpine meadows of the Nagqu River basin can be divided into three types: 0–0.5, 0.5–1 and 1–1.5 mm (Fig. 3 and 4). In CM and XM, the root length of alpine meadows decreases with increasing soil depth. Moreover, 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 largest proportion. smaller proportion. With increasing soil depth, the proportion of roots with a length of 0.5–1 and 1–1.5 mm 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 area and projected area of the roots of alpine grasslands in the Nagqu River basin undergo similar changes with depth. In CM, with increasing soil depth, the proportion of roots with an area of 0 to 0.5 mm gradually increases, and the proportion of roots with an area of 0.5 to 1 and 1 to 1, 5 mm gradually decreases. In the 10–15 cm soil, roots with an area of 0.5–1 mm account for the largest proportion of CM; meanwhile, in XM, roots with an area of 0–0.5 mm account for the largest proportion in the soil of 10–20 cm.
In CM, roots with a root volume of 0.5–1 mm account for the greatest proportion in soil 10–20 cm. However, roots with a volume of 0.5–1 mm account for the greatest proportion in soil 10–15 cm, while roots with a volume of 0–0.5 mm account for the greatest proportion in soil. the ground from 15 to 20 cm.
In short, with the increase in soil depth, the length, area, projected area and root volume of alpine grasslands gradually decrease in the Nagqu River basin, the root proportion from 0 to 0.5 mm increases while the proportion of roots from 0.5 to 1 mm roots decreases. The distribution of alpine meadow roots in CM shows vertical extension, while in XM it shows horizontal divergence. In addition, compared to CM, the total length, total area and total volume of roots from 0 to 0.5 mm in XM increase by 20.95 cm, 1.90 cm2and 0.014 cm3, and the corresponding specific gravity increases by 9.09%, 13.50% and 12.14%. The total length, total area and total volume of roots of 0.5–1 and 1–1.5 mm 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 soil moisture of 0–20, 20–40 and 40–60 cm 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 of the minimum soil moisture in the LFM.
In LFM, as soil depth increases, soil moisture gradually increases. The 10-20cm soil had the lowest moisture content in the LFM of about 9.11%, the 20-40cm soil moisture increased by 27.4%, and the soil moisture by 40 at 60 cm increased by 45.9%. In HFM, the moisture content was 6.31% in the soil from 0 to 20 cm. Humidity was highest in the soil from 40 to 60 cm. Compared to 0–20 cm, the humidity has increased by 10.6%. Moisture was 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 lower moisture content, while the surface (0-20 cm) and deep soil layers (40-60 cm) had a higher moisture content.
Unlike the soil moisture distribution, the soil nutrient distribution in HFM and LFM was the same: soil nutrients gradually decreased from the surface downwards (Fig. 6). In the LFM, the soil from 0 to 20 cm depth had the highest nutrient content and available phosphorus (AP), hydrolyzable nitrogen (HN), available K (AK) and carbon contents of the soil. microbial biomass (MBC) were 2.7, 109.83, 140.11 and 149.38 mg/kg, respectively. With the increase in soil depth, compared to the soil of 0-20 cm, the contents of AP, HN, AK and MBC in the soil of 20-40 cm depth were reduced by 33.33%, 33 .44%, 5.45% and 55.64%, while the content of AP, HN, AK and MBC in the soil of 40-60cm depth was reduced by 31.48%, 31.83%, 11.13% and 66.28%, respectively. In HFM, the nutrient content in the soil layer from 0 to 20 cm was also the highest, and AP, HN, AK and MBC have contents of 3.7, 86.17, 107.42 and 120, 11 mg/kg, respectively. Compared with the soil from 0 to 20 cm, the contents of AP, HN, AK and MBC in the soil from 20 to 40 cm deep decreased by 43.24%, 29.11%, 27.07% and 60. 26%, respectively, while the content of AP, HN, AK and MBC in the soil of 40-60cm depth decreased by 64.86%, 82.79%, 53.04% and 83.88%, respectively.
Meanwhile, at the same soil depth, in LFM, the content of HN, AK and MBC is higher than in HFM. Compared to LFM, the contents of HN, AK and MBC in the soil layer from 0 to 20 cm in HFM are reduced by 21.54%, 23.33% and 19.59%; the HN, AK and MBC in the 20–40 cm range are reduced by 16.43%, 40.86% and 27.98%; and the HN, AK and MBC in 40–60 cm decreased by 80.19%, 59.49%, 61.56%, respectively. However, the AP in the depths 0–20 and 20–40 cm in the HFM is larger 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 of the soil26.
As shown in Figure 7, NFTC, FTCD, FTCF and daily mean temperature difference (DATD) all have significant negative correlation with surface soil moisture in the Nagqu River basin. This shows that soil moisture is not only affected by repeated HTFs. Moreover, the effect of FTC on soil moisture is not transient; the longer the FTC, the greater the impact on soil moisture.
Meanwhile, HN, AK and MBC contents in topsoil have no obvious correlation with NFTC and FTCD but have significant negative correlation with FTCF. This shows that, compared to NFTC and FTCD, FTCF is more suitable for measuring the influence of FTC characteristics on soil properties. With the increase of FTCF, the damage to soil structure increases and the contents of HN, AK and MBC in the topsoil decrease significantly, but PA shows different changes under the influence of microorganisms28.