Agr50350 1050..1059

This study examines the effects of straw management and winter flooding on soil N dynamics and crop N uptake in California rice (Oryza sativa L.) systems. Experiments were established in two locations in northern California with main plot treatments being winter flooding or no flooding; and four straw management practices (burn, remove, incorporate, and roll) as subplot treatments. Fertilizer was applied to the plots at the recommended levels for each site, but within each plot a zero N microplot was established. Total straw inputs before winter flooding averaged 7000 kg ha for the incorporate and roll treatments, 4200 kg ha for the remove, and 1600 kg ha for the burn treatment (straw N ranged from 11 to 66 kg ha). Before flooding the field for planting, there was 9% less straw in winter flooded plots compared to nonflooded plots. Straw incorporation resulted in more rapid straw decomposition compared to other treatments where the straw was not incorporated. Furthermore, potentially mineralizable N and soil extractable N was higher in the winter flooded treatments and where rice straw was retained. Crop response varied between sites but our results suggest that N fertilizer recommendations could potentially be reduced by 20 kg ha if straw is incorporated and winter flooded. IN California and the southern USA, various environmental and economic factors are resulting in a change in the way rice straw has historically been managed. California legislation to phase down rice straw burning to 25% of total acreage has required major changes in straw management. Furthermore, in California, and more recently the southern USA (Anders et al., 2005), the rice industry and conservation groups have embraced the concept of winter flooding to provide substitute wetlands for waterfowl. The effect of these alternative winter straw management practices under flooded or nonflooded winter conditions on straw decomposition and N cycling are not known. Large amounts of straw in the field before spring field operations is a potential concern as it can interfere with field operations and may immobilize N. Three avenues of straw disposal are possible: burning, removal for offsite use, and in-field decomposition. Removing straw by burning or for off-farm use alleviates these concerns, but burning is only available on a limited area and there is a limited market for rice straw, therefore, in-field decomposition is the only viable alternative for many farmers. Moisture, aeration, and temperature are principle soil factors determining the rate of organic residue decomposition. Below a certain critical moisture level biological processes are arrested, while at high moisture levels anaerobic conditions result. Decomposition under anaerobic conditions is thought to be slower than under aerobic conditions (Tate, 1979). In a lab study, Pal and Broadbent (1975) reported that 14% more straw C was lost from soil at 60% water holding capacity compared to 150% water holding capacity after 4 mo incubation. Similar results were found by Clark and Gilmore (1983). However, there are few field studies comparing aerobic vs. anaerobic decomposition. In one such study, Neue and Scharpenseel (1987) using C labeled rice straw found no difference in straw decomposition in submerged vs. aerobic soils. Low temperatures retard organic residue decomposition (Sain and Broadbent, 1977; Pal and Broadbent, 1975) but temperature has much smaller influence on decomposition rates in saturated than unsaturated soil (Clark and Gilmore, 1983). The potential for N immobilization under flooded soil conditions is thought to be less than in aerobic soils (Arharya, 1935a, 1935b; Broadbent and Nakashima, 1970). However, in pot and field studies, N immobilization has resulted in N deficiencies in rice (Bacon et al., 1989; Becker et al., 1994; Huang and Broadbent, 1989; Rao and Mikkelsen, 1976). Broadbent and Nakashima (1970) found significant N immobilization regardless of the N concentration of the straw residue in a lab study. Based on results from field studies, Williams et al. (1968) reported that a straw N concentration of 0.54% N was the critical level of straw N determining whether or not N immobilization would affect yield response in single growing seasons. However, it is also important to consider when straw is incorporated in relation to when rice is planted. Williams et al. (1968) incorporated straw immediately before sowing. Rao and Mikkelsen (1976) found that incubating straw in soils for 15 to30ddecreased N immobilization. In a field study, Bacon et al. (1989) also found that incorporating straw residue in the fall increased N availability from the straw and soil resulting in increased rice yields compared to when the straw was incorporated shortly before harvest in the spring. Two experimental sites were established in California to examine the long-term effects of straw management and winter flooding on rice production. Experiments were initiated in 1993 (Colusa County) and 1994 (Butte County) and continued through 1999. A number of reports have been published from these experiments which focus on the effects of straw and winter flood management practices on: microbial populations (Bossio and Scow, 1995), methane emissions (Fitzgerald B. Linquist and J. Hill, Dep. of Plant Sciences, Univ. of California, Davis, CA 95616; and S. Brouder, Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-1105. Received 21 Dec. 2005. *Corresponding author ([email protected]). Published in Agron. J. 98:1050–1059 (2006). Nutrient Cycling and Uptake doi:10.2134/agronj2005.0350 a American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: ExN, soil extractable nitrogen; F, flooded; PMN, potentially mineralizable nitrogen; NF, nonflooded. R e p ro d u c e d fr o m A g ro n o m y J o u rn a l. P u b lis h e d b y A m e ri c a n S o c ie ty o f A g ro n o m y . A ll c o p y ri g h ts re s e rv e d . 1050 Published online June 5, 2006