تغییرات حاصلخیزی خاک در منطقه‌ی میان‌کوه شهرکرد در پی تخریب جنگل‌های بلوط

تغییر کاربری اراضی، راهکاری آسان است که کشاورزان به صورت گسترده از آن در راستای یافتن منابع جدید برای تولید بیشتر محصولات کشاورزی استفاده می کنند. جنگل‌تراشی از بارزترین شکل‌های تغییر کاربری اراضی به ویژه در بخش‌های جنوب‌غربی ایران است. در پژوهش حاضر تلاش شد با بررسی تغییرات واکنش خاک، درصد مواد آلی، نیتروژن کل، فسفر قابل دسترس و پتاسیم تبادلی خاک تحت پوشش سه کاربری جنگل نیمه‌متراکم بلوط، کشت دیم گندم و اراضی دیم رها شده (بایر) در منطقه ی میان‌کوه در جنوب‌غربی شهرکرد، اثر تغییر کاربری اراضی بر حاصلخیزی خاک بررسی شود. برای این منظور، 30 نمونه خاک سطحی (لایه ی صفر تا 25 سانتی‌متر) برداشت و مقادیر ویژگی‌های مورد اشاره در آنها اندازه‌گیری شد. نتایج تجزیه ی واریانس، گویای اثر معنی‌دار نوع کاربری اراضی بر تمامی ویژگی‌های مورد بررسی به جز واکنش خاک بود. تبدیل اراضی جنگلی به دیم‌زار سپس رهاسازی آنها، به کاهش مقادیر میانگین مواد آلی خاک از 2.33 به حدود 1.0 درصد و کاهش میانگین نیتروژن کل خاک از 0.11 به 0.05 درصد منجر شد؛ در حالی ‌که مقادیر فسفر با کاهش جزئی همراه بود و مقادیر پتاسیم تبادلی خاک در اثر تغییر کاربری اراضی از 615.0 میلی‌گرم بر کیلوگرم در اراضی جنگلی به 633.4 میلی‌گرم بر کیلوگرم در دیم‌زار افزایش یافت. معنی‌دار شدن نتایج آزمون چندمتغیره ی هاتلینگ در تایید و تکمیل نتایج مقایسه های میانگین، بر تاثیر نامطلوب تغییر کاربری بر کیفیت حاصلخیزی اراضی منطقه ی مورد مطالعه دلالت داشت. به نظر می‌رسد در مقایسه با تغییر کاربری اراضی از جنگل نیمه‌متراکم بلوط به کشت دیم گندم، رهاسازی اراضی دیم خسارت‌های بیشتری را متوجه کیفیت خاک خواهد کرد. همچنین نتایج نشان داد که به دلیل پیچیدگی محیط خاک، مقایسه ی خاک‌های تحت کاربری‌های متفاوت بر حسب هر خصوصیت خاک به‌طور جداگانه، الزاماً به نتایج قابل اعتمادی منتهی نخواهد شد، بلکه روش‌های آماری چندمتغیره‌ای مانند آزمون هاتلینگ، با ماهیت پیچیده و مرکب خاک سازگاری بیشتری دارند.

Variations of Soil Fertility Following the Oak Deforestation in Miankooh Region, Shahrekord

Extended abstract1 IntroductionLanduse change has been extensively considered by farmers as an easily available approach with the aim of finding new resources for producing more agricultural products. Deforestation, the most common type of landuse change, has led to the extensive parts of productive lands in different parts of the world being highly degraded. Rainfed farming can be listed as one of the most important substitutes for the forests which have been destroyed. In this scenario, as time goes on, the productivity of the land might be decreased due to the problems arising out of inappropriate land management strategies or inexorable climate changes, leading to the lands being abandoned. Given the prevalence of this issue in the Southwest part of Iran, the present study aimed to monitor the changes of selected soil fertility properties following the oak deforestation in the Miankooh region, Southwest Shahrekord City. 2 MethodologyThe study area is located in 31 ° 44 prime; N and 50 ° 34 prime; E, 150 kilometers far from Shahrekord City, Southwest Iran. The area has a semiarid climate with an annual average temperature and rainfall of 16.3 °C and 439.5 mm, respectively. This area is naturally covered with semidense oak forests, considerable parts of which have been partly destroyed and then assigned to rainfed wheat by local farmers. A substantial loss in crop yield in recent decades, mainly due to the decreased precipitation, has persuaded the young farmers to abandon the less productive lands. Aimed at assessing the impact of landuse change on soil fertility, a total of 30 surface soil samples (0 -25 cm) were collected from the lands under wheat cultivation, semidense oak forest, and abandoned lands. The soil samples were airdried and sieved through a 2mm sieve and then analyzed for soil pH, organic matter content (OM), total nitrogen (N), available phosphorus (P), and exchangeable potassium (K) using standard methods. The data was statistically analyzed applying the Duncan test at the probability level of 5%, oneway ANOVA, and the multivariate approach of the Hotelling test.3 Results The studied soils contained a fairly high content of clay and can be classified as alkaline soils, possibly due to their relatively high content of calcium carbonates. A high soil buffering capacity caused by the accumulation of calcium carbonates in surface layers would have contributed to enabling the studied soil to undergo different uses without any significant changes in soil pH. Following the landuse change from oak forest to rainfed wheat and then to abandoned (uncultivated) lands, soil organic matter and total nitrogen decreased from 2.33% and 0.11% to 1.0% and 0.05%, respectively. This observation may be arisen out of the fact that the soils under wheat cultivation receive a relatively deep plowing almost every year, which can substantially reduce the percentage of soil total N or OM by mixing the rich surface layer of the soil with the less enriched subsurface layer. As far as the abandoned soil is considered, receiving the negligible inputs of organic matter caused a further decrease in soil total N and OM. The investigated soils benefited from acceptable amounts of available P so that the insignificant fall in their P contents due to landuse change may not affect their fertility potential. When it comes to K, unlike soil OM and N, it significantly rose from 615.0 mg/kg in forest lands to 633.4 in the soils under wheat cultivation. It appears that the addition of manure to surface soil by local farmers has provided a good source of this important nutrient. Applying the multivariate approach of the Hotelling test aimed at simultaneous comparing of different landuse types according to all of the studied soil properties, it was observed that the three types of land use investigated could be considered as different units.4 Discussion ConclusionsEven though deforestation generally leads to soil degradation, some soil fertility properties might remain unchanged or even be improved based on the management strategies applied. In fact, applying purposeful and appropriate techniques in the field may be helpful to alleviate the unavoidable consequences of landuse change. Compared to substituting the semidense oak forests with rainfed farming, abandoning the croplands may result in more problematic soils. The pretty fast degradation of the crucial soil fertility properties caused by landuse change gives a serious warning about the possibility of further destruction of soils in the area studied, which may make them entirely unsuitable for any kind of agricultural use. Considering the complexity of soil environment, comparing the soils under different land use in terms of each individual soil property may not necessarily lead to reliable results. Accordingly, multivariate approaches, like the Hotelling test, maybe more consistent with the complex nature of the soil.