Patterns Of Nutrient Release From Straws Returned To The Field And The Allelopathic Effect Of Rice Straw

Employing nylon net bags, carborundum tubers and plant straw/shoot sections, the decomposition rates and nutrient release patterns of straw returned to the field with different returning methods were studied. Soil fertility and crop yields in the field with straw treatments were examined. The allelopathic effect of rice straw on wheat germination was studied with indoors germination experiments in order to seek the solution to this allelopathic problem and offer practical techniques and gudence to farmers in rice straw mulching areas. The main results were achieved as below:1. Wheat straw and rape shoot were decomposed much faster at the very beginning and slower at the later stages of the experiment. The wheat straw was easier to decompose than the rape shoot, giving a decomposing rate of 66.18% after 100 d for wheat straw against 55.62% for the rape straw. As for the nutrient release rates, it appeared that K was the fastest and followed by P and N from two types of straws. Within the first 10 d, amount of K released accounted for 98.92% for the wheat straw and 98.74 % for rape shoot, very close to its total content. The study further revealed that amount of K released accounted for 94.76% and 95.93 within 6 d. During the decayed process of wheat straw, the damage of straw tissues occurred predominately at thin parietal cells of fundamental tissue and its vascular bundle, epiderms and mechanical tissue had no evident damage.The fundamental tissue of wheat straw and its vascular bundle remained almost intact within first 50 d and decay was observed thereafter. During the decay process of rape shoot, the decomposition of organization structure started after 10 d. The vascular cambium, phloem fiber, parenchyma and epidermis above the secondary xylem, however, were started to decay and fall off within the first 10 d.2. Employing nylon net bags and carborundum tubers, the decomposition rates and nutrient release patterns of rice straw in the field with different returning methods were studied. Results showed that rice straw decomposed much faster at the very beginning than at the later stages of the experiment. The straw decayed faster when it was buried into the soil, yielding a decomposition rate of 76.55% after 210 d against 53.50% for the surface mulching straw. Among the three nutrients, release of K from the straw was the fastest, followed by P and N, regardless of returning methods used. Within the first 10 d, amount of K released accounted for 50.32% for the surface mulching straw and 90.13% for the buried straw. When the straw was mixed into the soil, the major form of available N was nitrate (＞80%), with reminders of NH₄⁺ and soluble organic nitrogen. Meanwhile, the phenomenon of soil microbes grabbing available N was observed during the first 40 d of straw decomposition and it disappeared thereafter, resulting in net N release from the soil with filtration. After N fertilizer was amended with buried straw, it offset the need of microbes for available N to decompose the buried straw. During the decayed process of rice straw, the damage of tissue structure started from thin parietal cell of fundamental tissue and its vascular bundle, while epiderms and mechanical tissue were not evidently damaged.In the early days of straw returning, the damage of tissue structure for the buried straw was more serious than the mulched. As decay proceeded, the changes in epiderms and mechanical tissue and its vascular bundle were not different btween the two different return methods, after the fundamental tissue of rice straw and its vascular bundle were decomposed. 3. Straw returned to the field enabled to reduce soil bulk density, increased soil porosity, improved soil structure, increased soil available nutrient and crop uptake and finally resulted in yield increases. Soil bulk density was reduced by 0.03g/cm³ and soil porosity increased by 0.99% compared to the initial data of the experiment. The soil available phosphorus and potassium were increased, especially for potassium. Rice yield was increased by 5.61% with mulching wheat straw, and by 1.31% with mulching rape shoot. Wheat yield was increased by 6.16% and rape yield by 7.08% with mulching rice straw.4. Allelopathic effect of water soaked rice straw extract on wheat germination was tested. Results showed that the extract significantly depressed germination and elongation of seedling and roots of wheat. It appeared that it showed a stimulating effect of the extract at lower concentrations (0.01 g/ml) and an inhibitory effect at higher concentrations (=0.02g/ml). The inhibition effect was more severe to seedling growth of wheat than its germination. With introduction of plant regulator gibberellin or humic acid to the extract, the inhibitory effect of the extract on wheat germination was relieved. As the concentration of the extract was greater than 0.04g/ml, a serious inhibitory effect was observed and this could not be reversed by addition of gibberellin or humic acid any more.