Sign up for email alerts of new Fluid Journal issues!
Fluid Journal : Fluid Journal 1996-1998
Dr.PaulFixen Beware of N L Beware of N L Beware of N L Beware of N L Beware of N Lockup in R ockup in R ockup in R ockup in R ockup in Reactivated eactivated eactivated eactivated eactivated CRP F CRP F CRP F CRP F CRP Fields ields ields ields ields Carbon to nitrogen ratios higher than 30:1 could temporarily immobilize soil or fertilizer N in high-residue fields. Winter 1996 Figure 1. Changes in soil nitrate levels during decomposition of low N residues, Sabey, University of Illinois. 4to8weeks Amount New Nitrate Level CO2 Nitrate Time CO2 evolution Z Z Z Z Z Z Z Z Z While the Conservation Reserve Program (CRP) is uncertain, most agree that at some point a significant acreage currently in CRP will be brought back into production. We already know peak contract expirations for 36 million acres of CRP will occur in the fall of 1996. While nutrient management will not be the major challenge faced by most farmers in bringing CRP land back into production, the potential for nutritional problems does exist and could reduce profitability and/or contribute to water quality problems. Our purpose here is to address some of these concerns, plus look at some ongoing projects involving managing post-CRP fields. Depressed N availability C:N ratio. Our major nutrient concern centers on the effects of organic plant residues on N manage- ment. The quantity of residue accumu- lated in CRP fields can be very large. For example, Nebraska researchers have estimated above-ground levels of 4 to 5 tons/A for a bromegrass CRP field in northeast Nebraska. Grass residues grown in low N environments usually have wide carbon to nitrogen ratios (C:N) compared to soil microorganisms or stable organic matter. If the C:N ratio is greater than 30:1, soil or fertilizer N can be temporarily immobilized during residue decomposition. The traditional illustration of this concept was developed by Sabey (Figure 1). Initially, the increase of energy supply caused by the residue addition stimulates microbial activity as indicated by the increased CO2 evolu- tion. The growing population of het- erotrophic microorganisms removes nitrate from the soil and the nitrate level is depressed until the energy supply is exhausted. At this point, the microbes die and their decomposing bodies gradually return soil nitrate to a higher level than it was initially. Depression period. The overall contribution of the residue to plant- available N is positive, but a period of depressed N availability occurs along the way. The critical question related to N management following CRP is how deep is the depression period and how long does it last? Even though Figure 1 shows four to eight weeks, the actual duration is probably much more vari- able. Depth and duration will depend primarily on the quantity of residue, the actual C:N ratio of the above, and below-ground residue--as well as its particle size, degree of soil incorporation or tillage, and soil moisture and tempera- ture conditions following killing of the sod. Fortunately, some studies are far enough along to offer examples of the timing of the depression period for specific conditions. Much more data will be available during the next couple of years. Effects of tillage Primary tillage had a marked effect on fertilizer response by corn following six years of an alfalfa/smooth brome sod in east central South Dakota. If the sod was plowed, starter plus sidedressed UAN produced no yield response (Table 1). Modest response was measured with a chisel system. A 32-bu/A response occurred under no-till. By the spring of the second year, soil nitrate levels had increased to approxi- mately 200 lbs/A in the top two feet in the moldboard plow and chisel systems where fertilizer had been applied. Second year responses were similar even though no N was sidedressed in the plowed or chiseled systems and only 20 lbs/A was applied in the no-till. A second no-till treatment was initiated in 1991 (Table 1). Successful no-till production was dependent on fertiliza- tion at this location where the initial sod was composed of more alfalfa than grass. Plowed plots showed no fertilizer response. Rotation effects In southwestern Minnesota, some- what greater N was applied following CRP than when following continuous corn because of the lower initial nitrate levels of the CRP plots (Table 2). The CRP plots were nearly all grass the preceding three years. Corn received 15 lbs/A of starter N at planting. Remaining N was broadcast as urea in early June and incorporated by cultiva- tion. Nitrate in the tile drainage was
Fluid Journal 1993-1995
Fluid Journal 1999-2001