Nitrogen Mineralization Under 3 And 8-year Old Stands Of Lucerne In The Qingyang Loess Plateau, Gansu

Inclusion of lucerne into farming systems has been proposed as a strategy to increase sustainability of agriculture in the Qingyang Loess Plateau, China. To understand the nitrogen mineralization of lucerne pasture: (1) soils from 0- to 10-cm,10- to 30-cm ,30-to 60-cm and 60-to 90-cm were taken under 3-and 8-year old stands of lucerne at regreening and 1^st,2^nd and 3^rdd flowering cuts in 2005. A incubation experiment was conducted with a combination of three temperatures (5℃,15℃,25℃) and three soil moistures (30% ,50% and 70% of the field capacity). (2):An in-situ field incubation was used during June to October in same year. The NO3-N and NH4-N were measured before and after incubation and net N mineralization rate calculated. The results were as follows:1. In laboratory incubation experiment ,there were positive linear regressions between net N mineralization, net nitrification rate and temperature and moisture conditions. The N mineralization had a little response below 15℃, however, there was a rapid increase of net mineralization rate at 25℃ during August to October, which may be a function of the activity of the microbal biomass at 15℃. Net mineralization and nitrification rate appeared the paek value at 25℃. The highest net mineralization rate was found at the combination of 25℃/30 FC for two age stands of Lucerne at the 2nd and 3rd cut. There were different respond patterns to temperature and moisture combination for NO₃-N and net nitrification rate from two age stands of lucerne among the season., The highest NO3-N and net nitrification rate were detected under 25℃/60FC for 8-year stand of Lucerne, 15℃/40FC for 3-year stand of Lucerne at regreen stage. There was no strong interactions between temperature and moisture at the 1^st and 3^rd cut for 3-year stand lucerne.2. Nitrate-N significantly decreased with increasing soil depth (p<0.01). The potential of N mineralization below 60- cm depth under 8-year stand of lucerne was 30.9%-57.01 % higher than that under 3-year stand of lucerne .3. The NO₃-N , net nitrification rate and net mineralization rate under 8-year stand of lucerne were higher than those under 3-year old lucerne within the season (except for the regreen stage), indicating that 8-year stand of lucerne had a higher N supplying capacity than 3-year stand of lucerne under ideal conditions.4. Microbal biomass carbon from 0- to 10-cm in soil was significantly affected by soil moisture content and less effected by temperature across the range of temperature and moisture investigated. The highest microbal carbon waspresented under 70FC (except incubated at 5°C). Eight year old lucerne had accumulated more soil microbal biomass carbon than under three year old lucerne.5. There was a significant seasonal dynamic of N mineralization rate and Nitrification rate. The peak value appered at 2nd cut stage. Nitrogen immobilization was found at the 3 rd cut stage, which related to the microbal biomass activity.6. Nitrate-N under 3-year and 8-year stands of lucerne in-situ accounted for 86% and 48% after 7-day lab incubation during August to October, suggesting that the potential of nitrification of 8- year stand of Lucerne was limited under field condition because of soil water depletion.7. From this data it could be suggested that the length of a lucerne phase in a farming system could be less than four years, which will both benefit soil water conservation and better N supply to next crops.