Nitrogen pools and fluxes in diversified cropping systems.

摘要

Achieving high crop yields requires a large supply of plant available nitrogen (N), yet losses of inorganic N from agriculture are deleterious to environmental quality. A significant portion of agricultural N losses could be prevented if large soil inorganic N pools were not needed to satisfy crop N demand. Alternative N management strategies that consider N fluxes like gross N mineralization in addition to N pools should be investigated, as they could conceivably reduce the size of soil inorganic N pools while still providing sufficient N for crop production. Diversified cropping systems may be able to utilize such alternative N management strategies to reduce N losses and increase crop productivity. Characterization of the effects of cropping systems on crop N uptake, soil inorganic N pools, and N fluxes will enable testing of the importance of N dynamics in diverse compared to simple cropping systems. Understanding the relative rates of crop N uptake and inorganic N production by mineralization of soil organic matter could determine the potential for internal N cycling to fulfill crop N demand. Furthermore, if consistent and easy to measure predictors of N mineralization could be identified, estimations of N mineralization could be widely utilized for both research and agricultural management purposes.;The Marsden Farm cropping systems experiment compares diverse and simple corn-based cropping systems, and is utilized here to investigate the effects of cropping systems on N pools and fluxes. Over a 12-year period, corn grown in diversified cropping systems required 5.7-fold less synthetic N fertilizer than corn grown in a simple cropping system, yet yielded 4% more grain. It is also likely that nitrate leaching was reduced, as spring soil NO3 - concentrations at 1.2 m depth were on average 33% lower in the diversified systems. Further investigations focused on a 2 year period, and revealed that neither soil inorganic N pool size nor potential net N mineralization rate could explain crop N uptake. There was a positive relationship between gross N mineralization and corn N status late in the growing season, but this relationship was consistent across cropping systems and thus did not explain the cropping system effect on yield. Other potential explanations such as corn rooting characteristics, soil moisture status, and corn-microbe interactions should be investigated as causes of the cropping system effect. Gross N mineralization rate was found to be much greater than peak corn N uptake at Marsden, approximately 5-fold higher, which suggests that corn could potentially fulfill much of its N demand by tapping into internal soil N fluxes. However, the crop's ability to access this N supply will depend on how well it can compete with inorganic N consumption processes such as microbial immobilization, denitrification, and leaching; this topic deserves future research attention. Finally, the Marsden experiment and 5 other cropping systems experiments were used to examine predictors of potential gross and net N mineralization. Results suggested that the quantity and quality of soil organic matter could serve as effective predictors of N mineralization rates. Multiple linear regression models were able to predict both gross N mineralization and net N mineralization (R 2=0.8) although the predictors were different for gross and net mineralization, which indicated that different factors influenced these processes. Predictions were valid over the range of sites and management strategies investigated.