Effects Of Different Cultivations And Fertilizations On Soil Carbon And Nitrogen

Soil organic carbon is an important corbon source for plant growth, and play an important role in sustain propoer soil structure. However, it is also a main source of greenhouse gas. Due to its great amount, soil organic coarbon is a key dominate factor in global carbon cycling. Although, Nitrogen is an important element for crop growth. and yield and quality formation, over fertilization can lead to the desease resistance capicity of crops, surface and subsurface water polution, increase of gas N₂O. Soil organic carrbon and it changes in different cropping system and at different N and P fertilization level were investigated by an longterm field experiment located in mid-south Loess Planteau. The main obtained results are as follows.1. Compared to fallow, monocropping or rotation of different crops all increased the soil carbon contents, with total C increased by 27%-140%, organic C increased by 43%-250%, inorganic C increased by 0.2%-70%, and light fraction organic C increased by 34%-993%. Over different cropping systems, the alfalfa monocropping had the highest contents of total C, organic C and light fraction organic C in 0-15 cm soil layers, then was the rotation of the Cereals-Millet and the Pea-Cereals, and the monocropping of maize and wheat had the lowest contents of these C forms. However, alfalfa monocropping only increased the inorganic C contents in the 0-5 cm soil layer. With the increase of soil layer depth, the contents of total C, organic C and light fraction organic C were all observed to decrease, and the greatest decrease occurred in the alfalfa monocropping. This indicated that long term monocropping of alfalfa was an effective measure to increase soil C in dryland soil, especially for the organic C and the light fraction organic C. However, this increase effects were found only in the layers above 15 cm soil depth. Rotations showed greater potential in increasing soil C than monocropping of different crops, while no significant difference was observed between the two different rotations.2. The continuous long-term fallowing system contained the lowest organic N, light fraction organic N and mineral N. Long-period alfalfa cropping with no-till system could promote water storage in topsoil layers, and accumulation of organic and light fraction organic N in 0-20 cm soil layers, but increased exhaustion of water and mineral N in deep soil layers. Continuous monocropping of wheat or maize could all significantly increase soil organic and light fraction organic N contents, and the organic N were increased more by monocropping of wheat. Organic N contents in soil layers of two rotation systems showed no obvious different to monocropping of wheat. Light fraction organic N contents also were not obvious different between two rotations in 0-20 cm soil layers, but they were all significantly higher than wheat or maize monocropping. The amount of soil mineral N was found to depend on the status of crop growing or N fertilizer application at sampling time. Although long term monocropping of legumes without tillage can enhance organic N accumulation by increase the light fraction organic N in soil, the exhaustion of soil water in deep layers is also increased. Rotations of legumes with cereal crops or the shallow and deep root crop rotations are proved to be feasible measures to optimize soil water utilization, increase organic N accumulation in soil, and synergize soil N supply capacity.3. Application of P fertilizer had no significant effects on total soil corbon, but obviously increased organic carbon and total N contents, especially the light fraction organic carbon, organic N in 0-20 cm soil layers. compare of the results over different P rates showed that soil light freaction organic C and N have reached the highest value at P rate of 20 kg P2O5 ha.yr^-1on the basis of 90 kg N ha.yr^-1, and over application of P fertiler did not show any increase in soil light organic N. With the increase of P rates, the increase of soil light fraction orgainc C is more obvious than light graction organic N.4. With the increase of N rates, total N and organic C contents were obvious increased in soil. However, when the N rate was higher than 135 kg .ha^-1, soil total N did not continously incresased with the increase of N rates, the total C even showed a trendancy to decrease. Soil light fraction C and N were found increased with the N rate increase. Soil C/N ratios were observed ranging from 16.6 to 20, and it decreased with the N rate.