| 06 Aug 2025
Effects of intensified freeze-thaw frequency on dynamics of winter nitrogen resources in temperate grasslands
Abstract. In seasonal snow-covered temperate regions, winter serves as a crucial phase for nitrogen (N) accumulation through persistent mineralization processes. Climate warming has accelerated snowmelt and intensified freeze-thaw cycle frequency (FTC), potentially altering the availability of winter N sources for plants. We simulated intensified FTC regimes (increased 0, 6, and 12 cycles) in situ across two contrasting temperate grasslands, employing dual-labeled isotopes (15NH415NO3) to quantify winter N dynamics. Our results showed that intensified FTC significantly enhanced soil net ammonification rates and inorganic N levels in early spring, while net nitrification rates remained stable. This suggests that frequent FTC may provide a substantial N source for soil microorganisms and plant growth. Notably, soil microbial biomass N increased despite microbial C limitation, indicating efficient microbial N competition that restricted plant access to winter N sources. Intensified low-frequency FTC did not affect plant 15N acquisition, whereas high-frequency FTC significantly reduced plant 15N acquisition. Importantly, the impacts of FTC on plant 15N acquisition varied among functional types. Dominant cold-tolerant species (perennial bunch grasses and semi-shrubs) increased 15N acquisition, likely due to earlier root activity, while subordinate species (perennial rhizome grasses and forbs) exhibited reduced acquisition. In conclusion, while intensified FTC did not lead to the loss of winter N sources, it restructures N availability by favoring microbial retention and creating competitive hierarchies among plants in temperate grasslands. The high-frequency FTC-induced shifts in partitioning of winter N resources could substantially influence grassland productivity and community structure, highlighting the critical need to integrate winter climate change effects into temperate grassland ecosystem models.
This manuscript presents a well-done designed and highly relevant study that investigates the effects of intensified freeze-thaw cycles (FTC) on winter nitrogen dynamics in temperate grasslands. The application of an in situ 15N tracer approach across two contrasting grassland sites represents a significant methodological strength, providing direct insights into the fate of winter N sources. The central finding—that intensified FTC restructures winter N availability by enhancing microbial retention and altering plant competitive hierarchies, rather than causing simple N losses—is novel, compelling, and has important implications for predicting ecosystem responses to winter climate change. The study is timely, addresses a critical knowledge gap, and possesses innovation.
While the scientific foundation of the work is strong and the conclusions are broadly supported by the data, the manuscript in its current form requires significant revision to fully realize its potential. I am enthusiastic about the potential of this manuscript to make a valuable contribution to the field. The necessary revisions are needed and primarily focused on presentation and interpretation. I am confident that after a thorough revision addressing the points above and those detailed in the specific comments, this manuscript will be suitable for publication in BG journal.
Major concerns:
(1) Structural and Narrative Flow: The organization of the Results and Discussion sections could be optimized to create a more logical. Specifically, the order of presenting findings could be rearranged to better guide the reader from the ecosystem-level outcome (N retention) down to the underlying mechanisms (soil processes, microbial uptake, plant competition).
(2) The Results section currently contains lengthy lists of percentage changes.
(3) While generally clear, the manuscript requires a thorough proofread for grammatical consistency, conciseness, and precise scientific terminology.
(4) Discussion: The Discussion would benefit from stronger integration between sections (e.g., explicitly linking microbial decoupling to plant responses) and a more focused interpretation of the statistical analyses (e.g., highlighting the key drivers from the random forest analysis, rather than listing correlations).
Specific comments:
Introduction:
This Introduction presents a solid foundation with a clear research gap and well-defined hypotheses. The following suggestions aim to enhance the logical flow, sharpen the focus on the core scientific problem, and strengthen the overall narrative leading to the study’s objectives.
The transition between some paragraphs could be smoother to create a more compelling "storyline." For instance, the jump from the importance of winter processes (Paragraph 1) to global warming (Paragraph 2) could be more seamlessly connected.
Paragraph 3 effectively lists the competing processes but could be restructured to more sharply emphasize the central paradox or tension.
The important fact that temperate grasslands "cover nearly 40 % of China’s terrestrial ecosystems" is currently placed after the hypotheses, where it serves less of a motivational purpose.
The hypotheses are well-founded but contain extensive lists of referenced mechanisms, which can make them appear somewhat list-like and dilute their core predictive statement.
M&M:
The rationale for the FTC treatment levels (+6 and +12 cycles) is somewhat scattered between the site description and the experimental design sections. The reader must combine information from Table 1 and the text to fully understand the basis for these choices.
The sampling description in section 2.3 contains some ambiguity. It mentions excavating soil blocks "containing dominant plant species" (suggesting targeted sampling) but also collecting soil cores "randomly" (suggesting random sampling). It is unclear if plant and soil samples were co-located within the same 1m x 1m subplot.
Section 2.1 directs the reader to Table 1 but then proceeds to list much of the same data (precipitation, temperature, plant species) in the text. This creates redundancy.
Results:
Subsections like 3.2 and 3.4 contain long lists of percentage changes (e.g., "increased by 25.0% and 24.0%... increased by 44.3% and 58.6%...") without high-level synthesis.
Section 3.5 reads like a list of correlation coefficients, which is difficult to interpret.
Discussion:
The last paragraph of 4.4 contains a slightly repetitive list of reasons for the subordinate species' disadvantage. The powerful concept of "temporal niche partitioning" is present but could be emphasized as the overarching explanation.
Figure 7:
Please remove the plus signs (+) preceding the percentage values on the y-axis labels .
Minor comments:
Line 338: Modify to read “In contrast to the positive effects on microbial recovery, HFTC significantly reduced plant 15N acquisition in both grasslands. LFTC had no significant effect on plant 15N recovery ...”.