اثرات عملیات کشاورزی حفاظتی و تغییر فصل نمونه‌برداری بر بعضی از ویژگی‌های منفذی خاک

کشاورزی حفاظتی در نظر بسیاری از محققان، یکی از روش های پایدارسازی کشاورزی است. در این نوع کشاورزی، حداقل به‌هم‌خوردگی خاک و نگهداری حداکثر پوشش گیاهی در سطح آن از اجزای مهم است. این مطالعه بلند مدت در یک طرح آزمایشی بلوک‏های کامل تصادفی با چهار تکرار، برای بررسی اثرات عملیات فوق بر ظرفیت نگهداری رطوبت (vwc) و جرم مخصوص ظاهری خاک (bd) در دو منطقه و در دو فصل پاییز و بهار در کشور دانمارک انجام شد. تناوب r2 تناوبی از گیاهان زمستانه با حفظ بقایای گیاهی، تناوب r3 مخلوطی از گیاهان زمستانه و بهاره با حذف بقایا، و تناوب r4 همان مخلوط گیاهان مشابه r3 با حفظ بقایای گیاهی است. هر تناوب شامل تیمارهای شخم گاوآهن سنتی تا عمق بیست سانتی متر (mp)، دیسک تا عمق 8-10 سانتی متر (h) و کشت مستقیم (d) است. در میانه پاییز 2013، نمونه‏برداری خاک از عمق های 8-4 و 16-12 و در فصل بهار 2014 از عمق 8-4 سانتی‌متری انجام شد. سپس منحنی رطوبتی خاک و bd تعیین شد. تیمار شخم d، مقدار bd را در هر دو عمق به طور معنی‏داری افزایش داد (1.31 و 1.38 gr.cm-3). در عمق 8-4 سانتی متر، تیمارهای شخم حداقل (d و h) در مقایسه با mp رطوبت بیشتری در خود نگهداری کردند (38.6 تا 23 درصد در مقابل 35.8 تا 22 درصد). رطوبت خاک در محدوده رطوبت قابل استفاده برای گیاه، در تیمار کشت مستقیم به طور معنی‌داری بالاتر از تیمار کشت سنتی بود. نمونه‌برداری در دو فصل پاییز و بهار اثرات متفاوت داشت. اثر مثبت نگهداری بقایای گیاهی بر ظرفیت نگهداری آب در خاک در محدوده رطوبتی حدود اشباع در منطقه فولوم، می‌تواند به کاهش تولید روان آب و کمک به کنترل فرسایش آبی منجر شود.

the effect of conservation agriculture practices and two sampling seasons on some soil pore characteristics

1- introductionconservation agriculture is considered by many researchers as a sustainable agriculture strategy. in this type of agriculture, continuous minimum mechanical soil disturbance (no tillage or reduced tillage), permanent organic soil cover (residues or cover crops) and diversification of crop species grown in sequences and/or associations are important components. this 11-year long-term (longitudinal) study was conducted to investigate and quantify individual and combined effects of conservation agriculture measurements (namely, plant rotation, crop residue retention, and conservation tillage) on soil volumetric water content (vwc) and soil bulk density (bd) of two sandy loam soils in a temperate region, denmark. the possible effects of different sampling seasons (autumn and spring) on the study results are also investigated. it was hypothesized that there would be a positive effect of residue retention and diverse rotation on especially the no-tillage treatment.2- methodologyin a randomized complete block experimental design with four replications, an 11-year experiment was conducted in two research areas of denmark. three crop rotations/residue management treatments were compared. tillage was included as a split plot factor. the rotation r2 is a rotation of winter crops (mainly cereals) with the retention of plant residues, the rotation r3 is a mixture of winter and spring plants (mainly cereals) with the removal of residues, and the rotation r4 is the same mixture of plants similar to r3 with the retention of residues. each rotation includes traditional plowing treatments to a depth of 20 cm (mp), harrowing to a depth of 8-10 cm (h) and direct drilling (d). in mid-autumn of 2013, and early spring of 2014, soil samples were taken by cylinder from the depths of 4-8 and 12-16 cm (in 2014 only 4-8 cm depth were sampled). the amount of vwc, in matric potentials from 0 to 100 kilopascals, as well as the apparent soil bulk density were measured. also, the possible effects of different sampling seasons (autumn and spring) on the study results are investigated.3- results tillage system and residue management, significantly affected soil properties studied. direct drilling significantly increased bulk density at both depths (1.31 and 1.38 gr.cm-3). the amount of vwc of the soil was also significantly different in tillage and plant residue treatments. at the depth of 4-8 cm, minimum tillage treatments (d and h) retained more moisture compared to mp (38.6-23% vs 35.8-22%). this trend was different in the depth of 12-16 cm and mp treatment showed the highest vwc in all suctions. the treatment of plant residue retention (r4) also caused the lowest amount of bd, especially at the depth of 12-16 cm. this treatment also significantly increased the vwc of the soil in macroporosity at both depths. the interaction effect of tillage and residue management showed a trend of lower bd where d and mp were combined with residue retention (r4) compared to their combination with residue removal (r3). the positive effect of retaining plant residues on increasing the water holding capacity in soil in a range of moistures that are available for plants was shown only in the spring sampling (2014) in both investigated areas. in the autumn sampling (2013), this effect was observed only at foulum area and in low suctions (macroporosity).4- discussion & conclusions the above observations seem reasonable considering the depth of soil that is disturbed in different tillage systems of this study. in the h treatment, which creates the lowest bulk density at foulum area, at a depth of 4-8 cm and has a lower bulk density, plowing (without turning) is done with a disk to a depth of 8-10 cm. it is obvious that the depth of 4-8 cm is located in this depth range and the reduction of bulk density is expected. in treatment d, where minimum soil disturbance occurs at this depth, higher bulk density is expected. in the lower soil depth, i.e. 12-16 cm, mp has resulted in the lowest soil bulk density compared to the two minimum tillage systems, d and h. the cause may be attributed to the plowing and soil inversion at a depth of 20 cm in this treatment. these observations are consistent with the results of several other studies (ball et al., 1994; bescansa et al., 2006; hill et al., 1985; schjønning & rasmussen, 2000). in this regard, tollner et al. (1984) studied the effect of no tillage and plowing with a mouldboard on the soil bulk density and reported that at a depth of 15 to 25 cm, bulk density in the no tillage system was higher than that of the mouldboard plowing, while at a depth of 30 to 40 cm, bulk density of the soil was higher in the mouldboard plowing system than no tillage system.the positive effect of plant residues on reducing the soil bulk density after 11 years of implementing the treatments corresponds with the results presented by blanco-canqui and lal (2007) and lal (2000). they reported a significant increase in soil porosity after 10 years of application of 8 and 16 tons/ha per year of wheat and rice residues to topsoil. however, at zero kpa suction (saturation) and 1 kpa suction, the r4 treatment shows an increasing trend in the amount of water retention in the soil at both investigated depths. this means that plant residue management has increased the amount of moisture that can be stored in macropores. the results of this study are consistent with the results obtained previously from the same long-term test, especially at lower suctions (abdollahi et al., 2014). maintaining soil moisture in low suctions (macroporosity) in spring sampling (2014) at both sites and in autumn 2013 at foulum is not very effective in terms of meeting plant needs. however, it is a desirable feature from the point of view of reducing the amount of runoff production and water erosion.