Mid-Continent Fall Temperatures at the 10-cm Soil Depth

Recommendations for applying nitrogen (N) fertilizer in autumn involve delaying applications until daily soil temperature at 10-cm depth is #10 C. Daily soil temperature data during autumn were examined from 26 sites along a transect from 36 to 49 N latitude in mid-continent USA. After soils first cooled to #10 C (First10), temperatures usually rebounded for varying amounts of time before the final date, at which they decreased to #10 C (Last10) for the remainder of the winter. Because N may be lost during warm episodes between First10 and Last10, understanding the extent, duration, and timing of these warm periods is important. Soils at sites between 41 and 44 N latitude in mid-continent USA accumulated more degreedays (base 10 C) between First10 and Last10 than soils north or south of this region. Thus, if N fertilizer is applied too early, soils in Iowa, South Dakota, and southern Minnesota are more susceptible to N losses compared with soils in Missouri or North Dakota and northern Minnesota. Progressively more conservative guidelines for applying N fertilizer in autumnmay be to wait until the dates of (i) average Last10, (ii) Last10 plus one SD, or (iii) Last10 plus two SD units. All of these alternative dates can be estimated reliably through simple polynomial equations for latitudes between 35 and 50 . MANY cropped fields in the corn and wheat belts of the USA receive applications of manure or ammonia-based fertilizers in the autumn. Fall applications are performed primarily to take advantage of drier soils and lower fertilizer prices and to reduce spring work loads (Sawyer 2001). A common recommendation is that these amendments should not be applied until soil temperature at 10-cm depth decreases below 10jC (Creswell et al., 2002, Hanna and Sawyer, 2001, Rehm 2002, NDSU Extension Service, 2006). This restriction on the timing of nitrogen (N)-laden materials to soil is intended to reduce the chances of conversion by soil microorganisms of ammonia-N to nitrate N and N2, which can be lost through leaching and denitrification, respectively. Most soil microbes are considerably more active above than below 10jC (Alvarez et al., 1995). Although nitrification is a two-step process involving chemoautotrophic and heterotrophic bacteria, the recommendations are based primarily on temperatures that have been observed to inhibitNitrosomonas spp., the primary converters of ammonium into nitrite. For instance, Grunditz and Dalhammar (2001) showed substantially reduced activities of Nitrosomonas spp. at temperatures below 10jC, although slight activity continued to 5jC. Several states provide valuable guides to farmers and fertilizer applicators regarding the dates that soil temperatures at 10 cm (or 15 cm) recede below 10jC. Maps of individual states often are delineated by application zones, typically along north-to-south gradients (e.g., Rehm, 2002; Anonymous, 2005). These zones specify ranges of time in the autumn during which soil temperatures commonly decrease to #10jC and when N-fertilizers may be applied with less risk of N losses. Despite such guidelines, N losses remain important environmental and agronomic problems for fall-applied N fertilizers (Randall and Vetsch, 2005a, 2005b; Vetsch and Randall, 2004). Soil temperature is a highly dynamic microclimatic variable. Soil temperature varies dramatically diurnally and with depth. Temperatures at shallow depths vary much more than those at deeper depths, but even at 10 cm, average soil temperature ranges greatly from one day to the next. Such variability among sequential dates is due primarily to daily differences in solar radiation and air temperature (Spokas and Forcella, 2006) and to precipitation, humidity, and wind speed (Flerchinger and Saxton, 1989). Because several variables contribute to determining the temperature of a soil, the probability is low that a soil will remain #10jC on first lowering to this level in the autumn. Indeed, even casual reviewing of average daily soil temperatures (10-cm depth) from Midwestern sites indicates that 10jC often is exceeded after the first occurrence of soil temperature #10jC. Contrasting examples of average daily soil temperatures in autumn are shown in Fig. 1 for Grand Forks, ND, during 2 yr. The ideal situation for fall N-fertilizer applications occurred in 1999. In this year, soil temperature dropped below 10jC on 29 September and stayed below this temperature for the remainder of the autumn and winter. In contrast, in 2003, soil temperature first dropped below 10jC on 25 September but rebounded above 10jC during three episodes (26–27 September, 3– 13 October, and 20–21 October). The frequency and extent of these high-temperature episodes are a concern for fall N applications and N losses. The objective of this study was to examine the times and variability of soil temperature dynamics along a transect of sites from northern Minnesota and adjacent North Dakota to southern Missouri (Fig. 2). Of specific interest were the dates when soil temperatures first drop below 10jC in the autumn and stay below 10jC for the remainder of the winter. Other variables of interest included the number of days that soil temperatures exceeded 10jC between the former dates and the heat units associated with those days. The results are expected to be of interest to all those who consider fall application of N-rich materials to soils in mid-continent USA. USDA-ARS, North Central Soil Conservation Research Lab., 803 Iowa Ave., Morris, MN 56267. Received 18 Sept. 2006. *Corresponding author ([email protected]). Published in Agron. J. 99:862–866 (2007). Fertilizer Management doi:10.2134/agronj2006.0264 a American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: First10, the first day in autumn that the average soil temperature dropped to or below 10jC; Last10, the last day in autumn that daily temperature was above 10jC. 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 . 862 Published online May 11, 2007