Fertilizer Nitrogen Recovery in Irrigated Upland Cotton

Field studies were carried out for the purpose of evaluating fertilizer nitrogen (N) recovery of upland cotton by use of the difference technique. The treatments under study included :i)check (no fertilizer N applied), ii)standard approach (preplant and sidedress), iii)feedback approach (based upon soil and plant factors), and iv)2X feedback approach. The studies were carried out at two locations Maricopa (MAC) and Marana (MAR). MAC is a low elevation location with a coarser textured soil compared to the MAR location. 'DPL -20' was the variety used in both locations, except for the early years at MAC where 'DPL -90' was used. The sources of fertilizer N were urea and ammonium sulphate, which were sidedress and split applied. In general, for the MAC location the final N fertilizer rates (NFR) applied were higher than for MAR due to higher yield potentials. The total N uptake increased as the NFR increased. The N use efficiency (NUE) values were reduced as NFR increased. The N fertilizer uptake (NFU) showed a decreasing pattern in the first years, and then an increasing trend; which was coupled with rather high amounts of N taken up in the check plot (soil N mineralized). When the N uptake in the check plot was high, NFU values were low, and vice versa. At MAC the N uptake in the check plot, apparently due to mineralized soil N revealed a slight increasing trend during the first years and then, after the fourth year, a rapid reduction of the mineralized soil N (check plot). A similar pattern was observed for MAR, although the total amount ofNtaken up was smaller compared to the MAC location. Introduction In desert agricultural areas water and N are the two major agronomic inputs. It is important for farmers to use N fertilizer efficiently to maintain an optimum return in yield for the amount of N fertilizer provided. Also, from an environmental standpoint, it is important to manage N fertilizer so as to minimize nitrate (NO3) leaching losses. Fertilizer N recovery by a crop can be estimated by two different techniques: 1)the difference (or indirect) technique, and 2)the 15N (or direct) technique. With the difference technique an apparent recovery fraction (ARF) is calculated from the amounts of N taken up by the crops in unfertilized (NP,,) and fertilized plots (NP), and the amount of N fertilizer applied (NF) (Alcoz, et. al., 1993; Harmsen and Moraghan, 1988; Rao et. al., 1991; Rao et. al., 1992; Roberts andHanzen, 1990) according to equation 1: ARF = ( N1slPo) / NF (1) The difference method generally provides higher fertilizer N recovery estimates than the 15N technique (Carefoot et. al., 1993; Harmsen and Moraghan, 1988; Hauck and Bremner, 1976; Rao et. al., 1991). The basic assumption associated with the difference method is that immobilization-mineralization and other N transformations which are operativeduring the course of the experiment are the same for both fertilized and unfertilized plots. This assumption is often questioned because fertilized plots have shown to have greater N mineralization and root development than unfertilized plots ( Harmsen and Moraghan, 1988; Westerman and Kurtz, 1974). Relatively low fertilizer N recoveries have been reported for cotton (Gossypium hirsutum L.), which has been increased by using nitrification inhibitors (Freney et. al., 1993). Field experiments were established from 1989 through 1995 at the University of Arizona Maricopa Agricultural Center (MAC) and Marana Agricultural Center (MAR) with the following objectives: 1)estimate the NUE of several rates of N fertilizer on upland cotton using the difference technique; 2)compare the NUE of upland cotton grown in two locations