ارزيابي تاثير خرده هاي چوب در كنترل رواناب و هدررفت خاك - يك مطالعه ي آزمايشگاهي در مقياس كرت

Evaluating the effects of wood shreds on runoff and soil loss control- An experimental study in plot scale

فرسايش خاك يك مشكل جهاني است كه به طور جدي منابع طبيعي، رفاه بشر و حتي حيات انسان را تهديد مي كند. روش هاي زيادي براي حفاظت خاك در جهان وجود داردكه اين روش ها داراي هزينه زيادي هستند و متناسب با طبيعت نيست. مالچ پاشي يكي از روش هاي مديريتي در حفاظت خاك است و سبب كاهش اثرات قطرات باران، جلوگيري از جريان هاي سطحي و فرسايش خاك مي شود. در اين ميان استفاده از مالچ هاي آلي سبب بهبود بافت و ظرفيت نفوذپذيري خاك مي شوند و از آلودگي محيط زيست جلوگيري مي كنند. اين مطالعه به بررسي اثر مالچ چوبي گونه توسكاي ييلاقي با اندازه طولي 4 سانتي متر و عرض 5 /1 سانتي متر در سه درصد پوشش (0، 30 و 70 درصد) تحت باراني با شدت 50 ميلي متر بر ساعت به مدت 20 دقيقه در پلاتي با ابعاد 2 متر طول، 1 متر عرض و 2 /0 متر عمق در خاك لومي سيلتي پرداخته است. نتايج آزمون LSD و t جفتي در درصد پوشش هاي مختلف نشان داد كه بين پوشش هاي مختلف در اندازه 4 سانتي متر، در سطح اطمينان 99 درصد در ميزان متغيرهاي رواناب، بار رسوب، غلظت رسوب و ضريب هرزآب اختلاف معني دار وجود دارد (01 /0>P) . همچنين ميزان فرسايش از 60 تا 97 درصد بسته به ميزان درصد پوشش مالچ كاهش يافت. نتايج اين پژوهش مي تواند براي مديريت بهتر اراضي جنگلي و همچنين حفاظت خاك مفيد باشد.

Soil erosion is a global problem that seriously threatening natural resources، human welfare and even life human. Globally، there are many methods for soil conservation including biological، mechanical and bio-mechanical methods، which these methods are expensive and they are not environmental friendly. Mulching is one of the management practices to reduce the impact of raindrops on soil erosion and control surface runoff. From these، organic mulches improve soil texture and infiltration capacity and avoid from environmental pollution. Mulching also decreases crusting of the soil due to rainfall impact also reduces erosion by absorbing the kinetic energy of the raindrops. Wood mulches have been developed from wood manufacturing waste (e.g.، wood strands such as Wood Straw، wood shreds or wood chips) made from burned trees or forest thinning operations، and shredded forest floor material from nearby eroded areas. Although these wood-based mulches are unlikely to harbor non-native seeds، their greater density can increase the cost of transportation to the site and aerial application as compared to straw mulch. Laboratory studies established that wood strands have greater resistance to wind displacement as compared to agricultural straw، and both wood strands and wood shreds provide equal or greater protection from erosion as compared to agricultural straw mulch at equal areal coverage rates. Foltz and Wagenbrenner (2010) reported that a 50% cover of wood shred mulch، with small (<25 mm length) pieces removed by sieving، reduced sediment yields nearly as well as 70% cover when it was tested using indoor rainfall and overland flow simulations. In a recent field study، manufactured wood strands and agricultural (wheat) straw were tested on burned hillslope plots at two sites—the Colorado Front Range and south-central Washington (Robichaud et al.، 2013). Although both mulch treatments increased total ground cover to more than 60% immediately after application، the wheat straw mulch cover decreased nearly twice as fast as the wood strand mulch. Wood strand mulch significantly reduced sediment yields at both sites and the wheat straw mulch significantly reduced sediment yields at the Washington site but not at the Colorado site. In addition، wood strands reduced sediment yields for up to 4 years. The purpose of this study is investigation of wood strand as mulching on soil loss، runoff and sediment concentration using laboratory rainfall simulation experiments. This study were investigated the effects of Alnussubcordata wood shreds with having longitudinal length 4 cm and width 1.5 cm in three cover percentage (0، 30 and 70) under the rain intensity of 50 mm/h for 20 min in a plot with 2 m long، 1 m wide and 0.2 m depth in a silt loam soil. There were three replicates for each treatment. All experiments were conducted on silty loam soil. Soil was sieved through a 6-mm mesh screen to remove large diameter particles and organic material. Soil moisture content was measured before and after each rainfall simulating by oven-drying soil samples at 105 Ċ. The moisture content of wood strand measured after each pre- and post-rainfall simulation. Wood strands were oven dried at 105 ºC to determine the amount of water absorbed during rainfall simulations. A simulated storm that produced raindrop impact was applied using rainfall simulator with BEX ¼ S 6/5 nozzle to deliver a raindrop size distribution and energy approximating natural rainfall. A rainfall rate of 50 mm/h for a duration of 20 min was chosen to ensure that the entire plot area was contributing runoff and has a return period of 50 years. Sediment concentration graph were taken each 5 minute to determine runoff rates and sediment concentrations. Sediment concentration was determined by oven-drying of sediment samples for 24 h at 105 ºC (221 F). Presence (or absence) of rills was recorded following each rainfall simulation. Each run was conducted using new soil and mulch. Statistical analyses were performed using SPSS 16. A one-way analysis of variance (ANOVA) was performed to determine differences between various covers in runoff volume، soil loss، sediment concentration and runoff coefficient. LSD and paired t-test results showed that there are significant different in runoff، sediment concentration، sediment yield and runoff coefficient between different cover percentages at the 95% level of confidence (p<0.05). Results showed soil erosion rate decreased from 60 to 97 percent depending on the amount of coverage. The average runoff rate in size of 4 cm showed significance decrease with increasing percentage cover of control plot to 30 and 70%، from 14.09 mm/h to 10.425 mm/h، and then 3.6 mm/h respectively. Average soil loss in size of 4 cm showed significance decrease with increasing cover percentage from 126.544 g/m2 to 29.076 g/m2 and then 3.168 g/m2 respectively. The results of this study can be used for better management of forest land; also can be useful to protect soil in this area.