Responses of feldspathic sandstone and sand - reconstituted soil C and N to freeze – thaw cycles

: The Mu Us Desert in China is signi ﬁ cantly a ﬀ ected by seasonal freeze – thaw processes. In order to evaluate the adaptation potential of reconstituted soil with di ﬀ erent proportions of feldspathic sandstone and sand to extreme environment, the laboratory simulation freeze - thaw experiments was conducted to study the characteristics of soil C and N under freeze – thaw condi - tions. The results showed that the content of soil organic matter reached the peak after two cycles of freezing and thawing in T1, T2 and T3, compared to before freeze – thaw cycle, the soil organic matter content increased by 70, 55 and 59%. After ten cycles of freezing and thawing, the content of soil organic matter increased signi ﬁ cantly in T2 and T3. After one cycle of freezing and thawing, soil nitrogen content reached the peak. After ten cycles of freeze – thaw cycle, compared to before freeze – thaw cycle, the contents of ammonium nitrogen increased by 10, 49 and 11%, and the contents of nitrate nitrogen increased by 14, 39 and 34% in T1, T2 and T3. In conclusion, short - term freeze – thaw cycles in the Mu Us Desert signi ﬁ cantly increased the accumulation of soil carbon and nitrogen reconstructed by di ﬀ erent ratios of feldspathic sandstone and sand, and T2 and T3 treatments had better retention performance on soil organic matter and nitrogen, which has a good adaptability to the extreme environment.


Introduction
Soil freezing and thawing is an important factor affecting the release and availability of soil nutrients [1,2].However, soil organic matter and nitrogen are important components of soils, and their content and dynamic balance not only directly affect soil quality and land productivity [3,4], but also have important implications for the carbon and nitrogen cycles in ecosystems [5,6].At present, soil organic matter and nitrogen have become one of the core research types on global change problems [7].Soil freezing and thawing is mainly by destroying soil aggregates and organic matter structure, killing certain microorganisms in the soil, increasing the death and decomposition of fine plant roots, and accelerating the fragmentation of plant residues to release nutrients needed for plant growth [8,9].At the same time, the thawing process following soil freezing often leads to enhanced microbial respiration [10] and increased concentrations of C, N and P nutrients in the soil [11], potentially affecting ecosystem nutrient cycling processes and ecosystem productivity [12].After freezing-thawing cycle, soil coarse mineral particles are fragmented mechanically and fine particles are aggregated, and soil particle size tends to be homogeneous [13].Therefore, the seasonal freezing-thawing alternating action can pulverize large blocks of feldspathic sandstone, which is more conducive to the full mixing of feldspathic sandstone and sand to form reconstituted soil in the Mu Us Desert [14].
The Mu Us Desert annually lasts a freeze period of 4-5 months, and the temperature goes below 0°C in November and gradually returns to above 0°C in March of the following year, with the soil in a frozen and thawed state in the late autumn and early spring seasons.Soils are affected by the temperate cool climate and the seasonal freeze layer, and the resulting freeze-thaw alternation has a significant impact on regional soils [15].In the Mu Us Desert, feldspathic sandstone and sand are two relatively independent natural substances, and they are the main factors causing soil erosion and land desertification.Han et al. [16,17] found that the rich clay minerals in feldspathic sandstone can improve the erosion resistance and water and fertility retention of sandy soils, and after reconstituting feldspathic sandstone and sandy soils into "soil" according to scientific ratios, they found that the optimal mixing ratio of feldspathic sandstone and sand was 1:1-1:5, and its water and fertility retention performance was the strongest [18,19].It has been used in large-scale demonstrations.The effects of freeze-thaw on soil structure and mechanical characteristics have been studied extensively at home and abroad [8,11,13,20,21]; however, studies on the effects of soil freeze-thaw processes on organic carbon and nitrogen mineralization in feldspathic sandstone and sand-reconstructed soils in the Mu Us Desert, China, have not been reported.In this study, we investigated the effects of soil freeze-thaw processes on the carbon and nitrogen contents of different proportions of feldspathic sandstone and sand-reconstituted soils in the Mu Us Desert after land remediation, so as to determine the adaptation potential of different proportions of reconstituted soils to extreme environments and to help understand the soil nitrogen cycling processes in the Mu Us Desert.This study has important practical significance for guiding vegetation management and soil fertility improvement in early spring in seasonal frozen soil region.

Overview of the study area
The study area is located at the southern edge of the Mu Us Desert, in the middle reaches of the Wuding River (109°28′58″-109°30′10″E, 38°27′53″-38°28′23″N), at an altitude of 1,206-1,215 m.The climate is mainly temperate arid and semi-arid types, with annual rainfall ranging from 200 to 600, sunshine for 2,500-3,000 h throughout the year, with a sunshine percentage as high as 70-80%.
The annual average temperature in the sandy area is 6.0-8.5°C, the average temperature in January is −9.5 to 12°C, the average temperature in July is 22-24°C, and the annual average frost-free period is 120-300 days.Feldspathic sandstone and sand in the Mu Us Desert are distributed interspersedly, and the soil type is mainly aeolian soils, with a total nitrogen content of 0.075%, a total phosphorus content of 0.63 g kg −1 , a total potassium content of 26.51 g kg −1 and an organic matter content of 0.03%.

Research methodology
The experimental soil samples were collected from the field scientific observation test plots of feldspathic sandstone amended wind-sand soil in Xiao Jihan Township, Yuyang District, Yulin City, Shaanxi Province, with each plot being 12 m long × 5 m wide.The original sandy soil was covered with a layer of 30-cm-thick reconstituted soil in each test plot according to the needs of the study, with volumetric ratios of 1:1 (T1), 1:2 (T2) and 1:5 (T3) of feldspathic sandstone to sandy soil, respectively.Below 30 cm was the original local aeolian soils 0:1 (T0).Soils were collected from the 0 to 30 cm surface layer and immediately placed in a 0-4°C freezer and taken back to the laboratory for freeze-thaw incubation tests.Local irrigation water was also collected, and the test samples were moisture corrected.The main physicochemical properties of the reconstituted soil and the original aeolian soil for the different treatments are detailed in Table 1.
The collected soil samples treated with T0, T1, T2 and T3 were removed from grassroots and other debris, airdried, and then passed through a 2 mm sieve for freezethaw culture experiments.The five samples of 500 g each were prepared for each treatment and placed in 20 round aluminium boxes.In order to keep the freeze-thaw conditions in the chamber closer to the natural state, i.e. temperature fluctuations start as close as possible to the top layer of soil, the aluminium boxes were covered with asbestos mesh to achieve better insulation.The soil was completely frozen at −15 to −5°C for a 24-h period and then placed at 15°C for 24 h, and this constituted one freeze-thaw cycle.Five samples from each treatment, replicated three times, were used to determine the organic matter, nitrate and ammonium nitrogen content of the soil of before freeze-thaw, one cycle, two cycles, five cycles and ten cycles, respectively.The T1 freeze-thaw-treated soil samples were recorded as M0, M1, M2, M5 and M10; the T2 soil samples were recorded as A0, A1, A2, A5 and A10; the T3 soil samples were recorded as S0, S1, S2, S5 and S10; and the T0 soil samples were recorded as H0, H1, H2, H5 and H10.M0, A0, S0 and H0 were control samples and were not subjected to freeze-thaw treatment.In order to simulate the actual soil freeze-thaw conditions, the moisture content of the soil samples needed to be moisture corrected by adjusting the soil moisture content to 60% of the maximum field holding capacity, and then, during the experiment, the lost water was continuously replenished by weighing method to maintain the corresponding moisture condition of the test soil samples.Soil samples were extracted with 2 mol L −1 KCl solution (5:1 water to soil ratio) before the test and the nitrate, and ammonium nitrogen content was determined using a fully automated intermittent chemical analyser (Cleverchem 200, Germany), while the soil moisture was measured by gravimetric method.Soil organic matter content was determined using the potassium dichromate volumetric methodexternal heating method [22].

Data processing
SPSS 13.0 statistical analysis software was used for T-test to analyse the differences of soil organic matter, ammonia nitrogen and nitrate nitrogen in T1, T2 and T3 treatments after different freeze-thaw cycles.There were significant differences (P < 0.05) and no significant differences (P > 0.05).The experimental data were collated and plotted using Excel 2010 and SigmaPlot 12, respectively.

Effects of freezing-thawing cycles on reconstructed soil organic matter content
The freeze-thaw cycle had a significant effect (P < 0.005) on the organic matter content of the feldspathic sandstone and sand reconstituted soils (Figure 1).The organic matter content of the three reconstituted soils before freezethaw was T2 > T3 > T1, and as the freeze-thaw cycle increased, the organic matter content of the soils in the three treatments showed an overall trend of increasing and then decreasing.The organic matter content of the soils increased significantly at the beginning of the freeze-thaw cycle, and the organic matter content of the soils of the three treatments reached a peak at the second cycle of the freeze-thaw cycle, in the order of T1 > T2 > T3.Before freeze-thaw cycle, the organic matter content of the soils of the T1, T2 and T3 reconstructions increased by 70, 55 and 59%, respectively.After two cycles, the soil organic matter content began to decrease; after ten cycles, the organic matter content of T1, T2 and T3 reconstituted soils was 0.89, 0.74 and 0.77%, respectively, with an overall levelling off, but the organic matter content still increased overall compared to before freeze-thaw cycle, with no significant difference between T2 and T3 (P > 0.005).

Effects of freezing-thawing cycles on nitrate nitrogen content in reconstructed soils
As can be seen from Figure 2, with the increase in the freeze-thaw cycle, the nitrate-nitrogen content of the soils of the three treatments showed a trend of first increasing, then decreasing and then steadily increasing.Before freeze-thaw and at the beginning of the freezethaw cycle, the nitrate-nitrogen content of the soils in all three treatments was T2 > T1 > T3.The nitrate-nitrogen content of the reconstituted soil increased significantly with increasing freeze-thaw cycles at the beginning of the freeze-thaw cycle, with the most pronounced increase reaching a peak at one cycle of freeze-thaw.
Compared to the pre-freeze-thaw treatment, soil nitrate-N content increased by 1.30, 1.52 and 1.49 times in the T1, T2 and T3 treatments, respectively.After two cycles of freeze-thaw, the nitrate-nitrogen content of the soils in the three treatments decreased significantly and reached a minimum value.After five cycles of freeze-thaw, the overall trend of the samples with all treatments increased.Compared to the period before freeze-thaw, after ten cycles of freeze-thaw, the nitrate-nitrogen content of the soils in the T1, T2 and T3 treatments increased by 14, 39 and 34%, respectively, with the content showing: T2 > T3 > T1, with the rate of increase in the nitratenitrogen content of the soils in the T2 and T3 treatments being more significant.

Effects of freezing-thawing cycles on the ammonium nitrogen content in reconstructed soils
With the increase of freeze-thaw cycle, the distribution of ammonium nitrogen content of reconstituted soils in the three treatments changed, and the ammonium nitrogen content of reconstituted soils in different proportions showed a trend of first increasing, then decreasing and then increasing steadily.During freeze-thaw cycles 0-1, the soil ammonium nitrogen content of the three treatments was T3 > T2 > T1.During freeze-thaw cycle 1, the soil ammonium nitrogen content of the T1, T2 and T3 treatments increased significantly and reached the peak, which was 1.41, 1.64 and 1.44 times higher than that before the freeze-thaw treatment, respectively.Then, the ammonium nitrogen content of the soils in the three treatments then began to decline, reaching a minimum value after two cycles of freeze-thaw, after which the content showed a steady increase.Compared with freeze-thaw cycle 0, after ten cycles of freeze-thaw, soil ammonium nitrogen content increased by 10, 49 and 11% in the T1, T2 and T3 treatments, respectively, with T2 > T3 > T1, with T2 and T3 treatments having significantly higher soil ammonium nitrogen content than T1 (Figure 3).

Reconstructing soil organic content changes
Freeze-thaw alternation can affect soil organic matter content, but the results vary depending on the soil type and the research method [23,24].In this research, it was found that with the increase in the freeze-thaw cycle, the soil organic matter content of T1, T2 and T3 treatments showed a trend of increasing and then decreasing, and the rate of change of organic matter content in different treatments was different.This is similar to the findings of Hao et al. [25], who found that soil organic matter content increased after 1-3 cycles of freeze-thawing and began to decrease after 6 cycles.Therefore, the short-term effect of freeze-thaw cycling on soil organic matter content is obvious, with significant effects of freeze-thaw frequency.This is due to the fact that changes in soil organic matter under the freeze-thaw cycle mainly originate from changes in soil microorganisms [25,26].The increase of organic matter content in the early stage of freeze-thaw cycle is due to the death of some microorganisms in the soil due to the severe freezing temperature.These killed microorganisms release some small molecular organic matter in the decomposition process, which increases the content of organic matter in the soil.On the other hand, the stability of soil aggregates is an important factor to determine the content of soil organic matter.Freezing-thawing breaks the stability of soil aggregates and causes the organic matter wrapped and adsorbed by the soil to disaggregate ahead of time, and soil organic matter content increased [26].After several freeze-thaw cycles, the content of organic matter in the reconstructed soil gradually decreased.On the one hand, the absolute death amount of microorganisms gradually decreased because they had adapted to the temperature change of the outside environment; accordingly, the amount of organic matter released by microorganisms is also decreasing.Second, with the development of freeze-thaw cycle experiment, the microorganisms living in the soil gradually decompose and utilize the original organic matter, resulting in the reduction of the content of organic matter in the soil [27].

Reconstructing soil nitrogen changes
There is no universal conclusion as to whether nitrate and ammonium nitrogen content in soils increases or decreases under the freeze-thaw cycle, depending on the parent soil forming material, the study area and the mode of analysis.In this study, a significant decrease in nitrate and ammonium nitrogen was found in the soils of the T1, T2 and T3 treatments after two cycles of freezethaw.There are mainly several reasons: (1) The nitrate and ammonium nitrogen in the soil was used by the small amount of plant roots remaining during the indoor simulated freeze-thaw test.(2) Nitrogen is sequestered by surviving or nascent microorganisms in the soil, especially under the effect of mild freeze-thaw alternations, to which the microorganisms are highly resistant.
(3) Loss of inorganic nitrogen from reconstituted soil infiltrates during indoor simulations.(4) Loss of gaseous nitrogen in reconstituted soils may also lead to a decrease in the ammonium nitrogen content of the soil.
Starting from cycle 5 of the freeze-thaw cycle, the nitrate and ammonium nitrogen contents in the soils of the T1, T2 and T3 treatments all began to show a steady increase, indicating that the effect of multiple freeze-thaw cycles can increase the nitrate and ammonium nitrogen contents in the soil.This is due to the fact that some microorganisms are adapted to survive at low temperatures, and when the frozen soil thaws at elevated temperatures, the residual microorganisms use the sufficient substrate provided by the dead microorganisms, which stimulates microbial activity and facilitates the process of reconstituting the mineralization of soil organic nitrogen, thus promoting the increase of soil nitrate and ammonium nitrogen contents during the freeze-thaw cycle [28,29].Freppaz et al. [30] and Chen et al. [31] showed that the freeze-thaw cycling process may lead to the release of + NH 4 -N from previously unavailable organic and inorganic colloids in the soil.Wang et al. [32] found that the disruption of soil aggregates during the freeze-thaw cycle resulted in an increase in extractable organic matter and mineral N in the soil.

Conclusions
The diurnal freeze-thaw cycle in the late autumn and early spring seasons of the Mu Us Desert enhances soil microbial activity, and the mineralization of carbon and nitrogen in the soil is still ongoing.It was found that the short-term freeze-thaw cycle could promote the increase of soil organic matter, nitrate nitrogen and ammonium nitrogen, and the adaptation potential of T2 and T3 treatments to extreme environment was higher than that of T1 treatments.After freezing and thawing cycles, the soil organic matter content, nitrate nitrogen content and ammonium nitrogen content increased significantly in T2 and T3 treatments, and the soil nutrient retention performance is better.The freeze-thaw cycle increases the mass fraction of organic matter and inorganic nitrogen in the soil, which is conducive to providing a large amount of nutrients for the growth of plants in early spring, and plays an important role in improving the fertility of the soil.This study has important scientific value for guiding the improvement of soil fertility and sustainable agricultural development in frozen soil area.

Figure 1 :
Figure 1: Changes of organic matter content in reconstructed soils with different freezing-thawing cycles.Letters above the bars indicate the significance of the differences (at 0.05 level) between treatments; the small bar shows standard deviation.Bars in different treatments but with the same letters are not significantly different at P > 0.05.

Figure 2 :Figure 3 :
Figure 2: Changes of nitrate nitrogen content in reconstructed soil with different freezing-thawing cycles.

Table 1 :
Main physical and chemical properties of soils reconstructed by feldspathic sandstone and sand