واکنش شاخص‌های رنگدانه‌ای پوسته زیستی، ماده آلی و نیتروژن خاک به تغییر کاربری اراضی و جهت جغرافیایی.

Introduction and Goal
Sustainable ecosystems play a fundamental role in providing ecosystem services and maintaining the balance of biogeochemical cycles. However, the widespread conversion of the rangelands with natural vegetation to rainfed agricultural lands, particularly in arid and semi-arid regions, has disrupted the equilibrium of these systems and accelerated soil degradation processes. Such transformations have not only affected soil physical structure and fertility, but also impaired vital functions of the surface layer, including biological soil crusts (biocrusts). Despite the importance of these crusts in particle stabilization and carbon sequestration, the simultaneous response of their chemical and biological indices to Land-use changes (from rangeland to active rainfed and then abandoned rainfed) has received limited attention. In this context, monitoring photosynthetic pigments and their ratios as stress-sensitive indicators can provide valuable insights into the physiological status of microbial communities. Therefore, the present study was designed and conducted to investigate changes in soil organic matter, total nitrogen, and pigment indices (chlorophylls, carotenoids, and related ratios) in biocrusts across the three aforementioned Land-uses, as well as to evaluate the effect of geographical aspect on these variables in a semi-arid watershed.
Materials and Methods
This study was conducted in the Khanqah Sorkh watershed located in Urmia County, West Azerbaijan Province, which has a cold semi-arid climate. To assess the effects of Land-use change, three Land-use types including natural rangeland (reference), active rainfed land converted from natural rangeland, and abandoned rainfed land (with 2–3 years since cessation of the rainfed agriculture) were studied in two geographical aspects (northern and southern). Sampling was based on a factorial design using a systematic random-compound method; accordingly, for each of the six Land-use, aspect combinations, six independent replicates from the 0–2 cm soil layer (active biocrust zone) were collected, yielding a total of 36 composite samples. Samples were immediately transferred to the laboratory in insulated, dark containers and stored at 4°C to prevent degradation of light-sensitive pigments. In the analytical phase, soil organic matter and total nitrogen were determined using the Walkley-Black and Kjeldahl methods, respectively. Additionally, concentrations of chlorophyll-a, chlorophyll-b, and carotenoids were measured following extraction with 90% acetone and centrifugation, using a spectrophotometer at specified wavelengths. Subsequently, three key ratio indices, chlorophyll-a to chlorophyll-b, chlorophyll-a to carotenoid, and carotenoid to total chlorophyll, were calculated to evaluate the physiological status and microbial community composition of biocrusts. Data were analyzed using two-way ANOVA, Tukey's post-hoc test, and Pearson correlation coefficient in SPSS software after verifying statistical assumptions.
Results and Discussion
Two-way ANOVA revealed that Land-use had a significant effect (p<0.01) on variables including soil organic matter, total nitrogen, chlorophylls, carotenoids, and pigment ratios. Geographical aspect also showed significant effects on most variables; however, the Land-use × aspect interaction was significant only for the carotenoid to total chlorophyll ratio. Mean comparisons indicated that soil organic matter and total nitrogen contents were higher in rangeland than in active and abandoned rainfed lands, resulting in 23–47% reduction in organic matter and 21–27% reduction in nitrogen relative to rangeland. Similarly, chlorophyll contents were higher in rangeland than in rainfed lands, with 60–80% reductions recorded for these indices. Carotenoids also exhibited a similar declining pattern, though the magnitude of reduction (42–49%) was comparatively milder. Regarding pigment ratios, the chlorophyll-a to carotenoid ratio was higher and the carotenoid to total chlorophyll ratio was lower in rangeland compared to rainfed lands. Pearson correlation analysis confirmed significant positive correlations between chlorophylls and both soil organic matter (r= 0.82–0.83) and total nitrogen (r= 0.79–0.81). In contrast, the carotenoid to total chlorophyll ratio showed significant negative correlations with these chemical variables. These patterns indicate that the decline in photosynthetic biomass and the shift in metabolic strategy toward defense mechanisms occur concurrently with reductions in soil carbon and nitrogen stocks in converted lands.
Conclusion and Suggestions
Land-use change in arid regions has impaired biocrusts functions and challenged the stability of carbon and nitrogen cycles. This study evaluated the response of chemical and pigment indices in biocrusts to Land-use changes and geographical aspect. Findings demonstrated that land conversion caused 40–80% reductions in soil organic matter, total nitrogen, and photosynthetic biomass, with no significant recovery observed even after 2–3 years of abandonment. In contrast, pigment ratios effectively reflected microbial strategy shifts toward stress-defense mechanisms. The positive effect of northern aspect in preserving moisture and photosynthetic potential confirmed the moderating role of microclimate in mitigating degradation impacts. Given the strong correlations between pigments and carbon–nitrogen stocks, monitoring these indices offers a rapid, non-destructive tool for assessing soil health. The inefficiency of passive land abandonment in restoration underscores the necessity of active bioremediation approaches. It is recommended that land management prioritize strategies to enhance photosynthetic microorganisms, modify microclimate on sun-exposed slopes, and integrate pigment-based monitoring. Future research should focus on long-term evaluation of restoration interventions and microbial community dynamics.