This project is evaluating grower field management practices on nutrient cycling in an effort to improve fertility management guidelines. This work is currently examining how different early season water-management practices affect nitrogen use efficiency. A controlled experiment near Gridley has documented nitrogen losses and reduced fertilizer nitrogen use efficiency due to early drainage for herbicide application. Twenty-two fields around the Sacramento Valley were monitored to better understand the impact of early season drains on nitrogen dynamics throughout rice-growing areas. Research was also conducted at the “systems” research site at the Rice Experiment Station. More detail on this work follows.

One of the greatest challenges facing California rice growers continues to be effective weed management. Herbicide-resistant weeds, combined with increased restrictions on herbicide applications, are limiting the effectiveness of traditional weed control strategies. Consequently, practices are changing and alternatives requiring an early-season flooding, draining, and reflooding are in increasing use.

This change in early-season water management also has direct implications for nitrogen fertility management. Current guidelines were developed for fields that are continuously flooded throughout the growing season. The impact of evolving, alternative strategies is not well understood but is of critical importance to growers for efficient nitrogen management. Growers have little information on how early season water management affects nitrogen fertility in their soils and, thus, how to improve nitrogen management.

Research in 2007 was conducted both on-farm and at the alternative stand establishment study site at the Rice Experiment Station. The scope of 2007 studies expanded to include both more detailed studies in grower fields, as well as intensive sampling on a large number of fields from throughout the Sacramento Valley.

In the on-farm studies near Gridley, two side-by-side grower fields – one with straw incorporated, the other straw burned – were examined. Treatments included surface, subsurface and top-dressed nitrogen applications. Due to the potential for nitrogen loss when fields are drained, it is likely that more nitrogen will be required as a top-dressed application after reflooding.

Preliminary results showed that early season growth – as measured by biomass – was reduced in the drained treatment. In both incorporated and burned fields, nitrogen use efficiency was higher in the undrained than the drained treatment. Furthermore, nitrogen use efficiency was consistently higher in subsurface applications than either surface applications (40% vs. 28%) or top-dressed nitrogen (40% vs. 11%). In the burned field, nitrogen uptake and yields were higher in the undrained treatment, but in the incorporated field they were not statistically different.

To relate findings to a broader area – and to quantify nitrification and nitrate loss rates – researchers monitored 22 conventional fields with different soil types throughout the valley that received an early season drain. Seven organic fields that were subjected to extended draining and drying periods for weed control were also monitored. Soil samples were taken on average every three to four days during draining and flooding events to document changes in nitrogen status.

The amount of nitrate accumulated in the soil during a drain varied widely across the valley. Where nitrate did accumulate, results consistently showed about one third the amount of nitrate in the surface two inches with the remaining two thirds in the two- to six-inch soil depth. This shows that nitrogen placed in both the surface and the subsurface is susceptible to nitrification and eventual loss. Because most growers drain their fields just after seeding, the rice stand is typically not large enough to take up a significant amount of nitrate. Most of the nitrate that disappears following reflooding is likely lost to the atmosphere through the process of denitrification. Most, if not all, accumulated nitrate is lost within a week of reflooding.

At the RES alternative stand establishment study site, several experiments conducted in 2006 were repeated in 2007. The first made use of varying early-season water management built into the treatments to examine a wide range of weed control strategies.

The second experiment examined nitrogen rates, the fourth year it was repeated. A range of rates up to 200 pounds/acre is being studied to generate yield response curves for each system. With the exception of the wet, stale-seedbed system, yield results were similar, a finding consistent with previous years. A nitrogen deficiency appears to limit yields in the wet, stale-seedbed system, and at lower nitrogen rates in some of the other alternative systems.

Researchers recommend that growers using alternative stand establishment systems optimize the drain period to as short a period as possible to limit nitrogen loss, but long enough to ensure effective weed seed germination for the stale seedbed practice. Extended drain periods can also result in drought stress to rice plants. In 2008, research will focus on the timing and placement of nitrogen fertilizer on grower fields where early drains are used.