Soil Mineral Nitrogen And Its Effects On Crop Growth And Nutrient Use In Winter Wheat And Summer Maize Rotation On Dryland

Application of nitrogen (N) is an important measure for increasing crop yields. Rational application of N fertilizer is of great significance in elevating crop yields and maintaining soil productivity. However, long-term and unreasonable input of N fertilizer may lead to a great deal of mineral-N (Nmin) residue in cropland soils. Nitrate-N is the main N form of fertilizer N residual in arid soil, it can move with water in soil profile and the runoff water over ground, and threaten the quality of ground and underground water. It can also be transformed to N2O gas and then pollute atmosphere. In addition, excessive N in the soil will decrease the recovery of N fertilizer by crops, even restrain crop growth and reduce crop yields. In this research, field experiments were carried out with test crops of winter wheat and summer maize, to study the balance and dynamics of mineral-N in dryland soils and its availability for crop at different N rates. We also discussed the accumulation and translocation of dry biomass, N, P and K in the vegetative parts of summer maize during late growth stages from grain-filling to harvest. The main purpose of this study was to optimize the management of N fertilizer in winter wheat and summer maize rotation, and reduce the residue of fertilizer-N and nitrate leaching in soils on dryland. The main conclusions are as follows: 1. The accumulation of mineral-N in preplant soil significantly affected crop yields and the efficiency of N fertilizer applied. Higher accumulation of mineral-N could lead to a decrease in fertilizer N recovery by winter wheat and summer maize, which was only 39% and 5～22%, respectively. Most of the fertilizer N remained in soil profile in the form of Nmin, or loss by other ways. The balance of soil N budget in the soil of winter wheat and summer maize rotation showed that fertilizer N use efficiency was decreased, and soil residual Nmin was increased with the increase of N rates. The apparent loss of fertilizer N tended to decrease at high N rates. 2. Cropping and fallowing significantly affected the accumulation and distribution of soil moisture and nitrate-N, but no effects was found on soil ammonium-N. When planting summer maize, soil moisture and nitrate-N mainly moved toward upper soil layers due to the crop transpiration and nutrient uptake, and nitrate-N residual in soil was largely decreased. In the contrary, storage of moisture in the subsoil was significantly increased by fallowing, and then nitrate was leached and residual in the subsoil layers with the downward moving soil water. 3. Fertilizer N rates remarkably influenced nitrate-N in 0-1 m soil layer, while nitrate-N in 1-2m soil layer and ammonium-N in 0-2m soil layers were not affected by N rates. In early growth stages of summer maize, nitrate-N was mainly distributed in 0-60cm soil layer, and then was gradually leached down, but not to the layers below 1-1.2 m at harvest. It was only during elongation stage that the N rates affected ammonium-N in soil profile, which was in a very small amount, evenly distributed in the 0-2 m soil profile, and not affected by application of N fertilizer at other stages. 4. The amount of N fertilizer significantly influenced the accumulation of dry matter and N and P uptake and their translocation in plant of summer maize during the late growth stages. The synthesis of dry matter and nutrient uptake was depressed when no N fertilization, and dry matter and nutrients in grain at harvest were found mainly transported from that accumulated in the vegetative parts of the plant before grain-filling. Appropriate application of N fertilizer could enhance synthesis of dry matter and uptake of N and P from soil during the late stages, and elevate the grain yield and N and P accumulation in grain. Over-application of N could not only decrease the grain yield, but also lead to a significant decrease in N and P uptake due to the relatively high accumulation of these nutrients in the stages before grain-filling. Thereafter, the capacity of nutrient uptake from soil by plant was decreased, and N and P in grain at harvest were also mainly from vegetative parts. Since the accumulation of K in grain at harvest was much less than that in vegetative parts, its translocation from vegetative parts to grain during the late growth stages of summer maize was not affected by N rates.