The Formation of Threshold for Wheat Production of Black Soil.

There are thresholds for the effect of SOM on crop production. To determine thresholds quantitatively and the mechanism of these thresholds formation an experiment have been conducted in plots using five soils with different SOM content (23, 59, 72, 82 and 106 g kg^-1) selected from black soils range in Northeast China. The aim of the research are getting rid of the effect of the climate on the crop production from the SOM and to 1) determine quantitatively the thresholds; 2)investigates the mechanism on its formation in physics, chemistry and biology through determining the key soil physical, chemical and biological properties. The results showed that the threshold is 82 g kg^-1 and will change with different N application level. The soil balk density decreased with the SOM from 1.25 to 0.87. The soil water content increased with the SOM at different growing stages. The soil temperature is also decreased with SOM. The pH value at lower SOM level increased with SOM and decreased with SOM at high SOM level. The weight of soil from soil particle size 53μ to 250μ and from 250μ to 2mm increased with SOM; and decrease with SOM under 53μ. Similar to wheat yield, soil biomass C and N increase with SOM and decreased from 82 g kg ^-1. Ammonia N decreased with SOM at different growing stages and nitrate N increased with SOM. The result indicated that soil is in a process of degeneration after reclamation, the change of soil properties are complex.