| On dryland of north-western China, where a little rainfall was received with high variability over seasons, apart from utilization by crops, a great deal of fertilizer N applied is found remained in the soil. The residual N has proved to be a threat to environment because it can be lost to atmosphere by the gas of NH3,N2O and so on, or be leached to deep soil and then enter into groundwater. However, as an essential nutrient to crops, residual N was benefit for maintaining soil productivity by compensating soil N pool. Therefore, it is of great importance to study how to prevent residual N loss from soil and how to make it be fully and efficiently used by crops.Straw mulching and plastic-film mulching over the soil surface were two agricultural measures to improve crop yield in arid and semi-arid region. Studying the effects of straw mulching and plastic-film mulching cultivation on soil fertility concerned the sustainability of dry land agriculture. Soil organic matter (SOM) contents were the important index for soil fertility. Light fraction organic matter (LFOM) is one of the labile forms of organic matter in soil, which consists of partially decomposed plant tissue, the hyphea and spores of fungi, seed, the debris of animals and microorganisms, and mineral granules. This pool turns over rapidly due to high C and N content, and serves as a readily decomposable substrate for soil microorganisms and a short-term reservoir of plant nutrients. Also, it is an important indicator of early changes in soil organic matter resulted from soil management. The N in soil LFOM is called light fraction organic N (LFN), which is closely relative to soil N mineralization and is a natural indicator for soil N fertility.Two long-term field experiments were conducted in southern region of Loess Plateau, in which different N rates and planting manners were setup and winter wheat and summer maize were planted. One experiment was to study the action of soil residual N and its utilization by following crops. The other was to study the change of soil organic C and N, light fraction organic C and N, and mineral N in soil on the conditions of straw mulching and plastic-film mulching on soil surface, by compared to conventional cultivation (no mulch). Transform of fertilizer N and straw N to soil organic N and LFN was also investigated by 15N-labeled method. The objective is to provide theory support for optimizing crop cultivation and nutrient management, improving soil fertility, and elevating crop production with high yield and high efficiency.1. Residual mineral N in dryland soil was found increased with the increase of N rates, in which the majority was nitrate N. In the first rotation of winter wheat and summer maize, the amount of residual mineral N in 0-300cm soil profiles could be as high as 449 kg hm-2, of which nitrate N accounted for 93.2%. During the following years of cropping, a part of nitrate N was gradually leached to layers below 300cm by rainfall. The amount of residual ammonium N was greatly affected by rainfall and soil moisture. In the warm summer with plenty of rainfall, the amount of ammonium N was low because the soil moisture was benefit for the nitrification of ammonium N. while the amount is high in dry seasons.2. Soil residual N could improve crop growth and N, P, K nutrients uptake, in which the effects on biomass and N uptake for the following winter wheat and summer maize were more significance. The effects of residual mineral N due to N fertilization could at least last to the fourth rotation of winter wheat and summer maize. Whereas, the residual effect of N fertilization was lied on crop species, climate, rainfall, temperature and so on, and showed variance among seasons and years.3. Measurement of the 15N- labeled fertilizer showed that the total fertilizer N use efficiency was only about 50% over four consecutive year rotation of winter wheat and summer maize. In the first year of rotation, fertilizer N use efficiency by winter wheat and summer maize was 32.3% and 9.3%, respectively, and only about 10% in last 3 years. Fertilizer N recovered by soil was 15.0-19.0 % and the lost accounted for 29.7-38.1% for 4 years. And that, a majority of residual fertilizer N were lost in the first summer maize season.4. During winter wheat growing season, straw mulch on soil surface is benefit for the accumulation of organic N, while plastic-film mulching makes soil organic C and N tend to decompose. In straw mulching cultivation, soil organic N in 0-40 cm soil layer was increased at the wheat dormant stage, and then gradually decreased at turning green stage. In which the amount of organic N was higher than that in conventional cultivation, and the difference was increased from 36 kg hm-2 at seeding to 153 kg hm-2 at harvest. In plastic-film mulching cultivation, soil organic N in 0-40 cm soil layer was rapidly reduced from turning green stage to harvest. The amount of organic N at dormant and harvest stages was 185 and 106 kg hm-2 less than that in conventional cultivation. For soil organic C in 0-40cm soil layer, it was increased from seeding to turning green stage and then reduced in straw mulching cultivation. In plastic-film mulching cultivation, it was increased from seeding to anthesis and then decreased. Compared with conventional cultivation, the amount of soil organic C in straw mulching cultivation was increased by 5525 kg hm-2 in winter, while decreased by 4507 and 2760 kg hm-2 at anthesis and harvest stages, and that in plastic-film mulching cultivation was decreased by 5203 and 2704 kg hm-2 at anthesis and harvest stages.5. In straw mulching cultivation, a plenty of LFOM in soil enhances the regulation of soil N-supply capacity. The amount of LFC and LFN in 0-40cm soil layer were both decreased from wheat seeding to the green turning stage, and then increased by the harvest. Compared to conventional cultivation, LFN and LFC in straw-mulched soil were higher at any time, and the average differences were 359 and 20 kg hm-2, respectively. Whereas, those in plastic-film mulched soil were lower, with the average difference being 219 and 8 kg hm-2, respectively. The results indicated that LFN was mineralized in cold winter and provide N for crops, and with the increase of temperature, the fresh organic materials in soil began to be decayed and formed new LFOM, which could immobilize soil N temporally in the same time.6. Straw mulching and plastic-film mulching could effectively maintain soil water, and soil moisture in 0-40cm soil layer were more 6 and 9mm than that in conventional cultivation at wheat dormant stage. However, soil nitrate N was decreased by 90.6 and 132.6 kg hm-2, and ammonium N by 16.3 and 13.8 kg hm-2 at wheat dormant stage, compared to conventional cultivation. This showed that straw mulching and plastic-film mulching improved soil moisture condition in winter, and promoted wheat to take up and use soil N. soil mineral N was closely correlated to LFN. Especially in no N fertilization, there was significant positive correlation among soil mineral N, LFN and LFC. This indicated that LFOM has more important function when soil N is scarce.7. In straw mulching or plastic-film mulching cultivation, N fertilization makes soil organic C tended to decrease and soil organic N was unchanged. As a result, the ratio of organic C to N was decreased. On the condition of high N fertilization, the amount of soil LFN in straw mulching cultivation was increased and LFC was unchanged, while the amount of LFC in plastic-film mulching cultivation was decreased and LFN was unchanged. Consequently, the ratio of LFN to LFC was both decreased. The result showed that when application high amount of N fertilizer to soil, mulching straw on soil surface promoted the accumulation of soil LFN, while mulching plastic-film on soil surface accelerated the decomposition of LFOM.8. After summer fallow, the amount of LFOM, LFN, and LFC in 0-5cm soil layer were enhanced, and those in 5-40cm layers were reduced for all treatments. This indicated that the fresh organic materials, such as stubble and the casts of leaf, stem, and roots, remained in soil surface was decomposed during summer fallowing due to high rainfall and temperature. Thus, LFOM at different forming phase occurred. Yet, the old LFOM was decomposed and released its N. However, the mineral N from LFN was leached to subsoil by frequent rainfall in summer.9. Aside from crop use, fertilizer N and straw N can be effectively transformed to soil organic N and LFN. At winter wheat harvest, The amount of fertilizer N immobilized by SOM in 0-40cm layer was most in straw mulching cultivation, the next in conventional cultivation, and the least in plastic-film mulching cultivation, and the same to soil fertilizer N in LFOM in each layers of 0-40cm. Undergoing a summer fallowing, fertilizer N in SOM and LFOM were both reduced, and that 100% reduced fertilizer N in SOM contributed to LFN mineralization on the conditions of straw mulching and plastic-film mulching cultivations, and only about 46.9% on the conventional cultivation. For straw mulching cultivation, 10.2% of N from straw covered on field surface was transported to wheat shoot, 64.2% was immobilized by SOM in 0-40cm layer, and 23.7% entered into LFOM. During summer fallowing stage, the amount of straw N in SOM and LFOM in soil surface layer was increased, and that in lower soil layers were reduced. This indicated that the undecomposed straw on soil surface at winter wheat harvest was formed new LFOM during summer fallowing, and the old LFOM was decomposed.
|
PDF Full Text Request