Objectives:

1.To determine the nitrogen balance within large agricultural fields. This involves an experimental and a modeling objective of the nitrogen cycle as it relates to water and nitrogen management in relation to cotton lint yield.

2.To determine the spatial and temporal variability of N within a large field.

A. Reporting Period:

1 September 1997 - 31 August 1999

B. Summary of Progress (Address all applicable objectives; data must be included)

Objective 1: Quantify the spatial and temporal variability of factors that can be addressed by precision agriculture practices.

Lint Yield as a Function of Variability of Soil Factors in a Large Field. Field heterogeneity may affect yield potential, irrigation responses, and N transport. A water and N balance study was conducted in a large cotton field beginning in the spring 1998 to determine lint yield and N uptake potential related to local water and N input. Two irrigation levels, 50% ET and 75% ET, were applied from north to south across the field. In 1998, lint yields increased significantly with increasing irrigation (P > 0.001). There was neither an effect of N inputs nor interaction between water and N inputs (P > 0.68). However, the covariance of the model was significant on lint yield and N uptake (P > 0.0001). Therefore, a state-space approach was used to identify the spatial soil variability along transects. Lint yield, soil water, P2O5 content, and elevation were correlated in space. The state-space equations were determined through multivariate autoregressive processes to quantify the spatially correlated parameters that create spatial difference in a heterogeneous soil. Results showed that the natural landscape variability could affect lint yield and N uptake potential related to water and N use. Furthermore, the variability of our field measurements can be described with state-space models.

Soil Water Distribution. Water distribution in the soil profile varied greatly with irrigation level, soil depth, and distance across the field as shown in Fig. 1. Compared to the 50% ET water level, soil water in the 75% ET transect generally contained more water and water contents were more variable. Soil water content at squaring (August, 1998) within the 0.0 - 1.5 m layer in the 75% ET transect was >15% than in the 50% ET transect. Water distribution was heterogeneous in all soil layers with the same irrigation level along each transect. In general, soil in the middle of the field contained more water compared to soil in the south and north sides, and lower water contents were measured at the north end. Variability in soil water distribution was linked to field topography.

Cotton Irrigation Response. In 1998, total average N uptake by cotton was 157 and 189 kg ha-1, respectively, for the two irrigation levels, showing a dependence on water input. Cotton lint yield in both transects varied greatly with irrigation level and within a short distance across the field (Fig. 2), demonstrating a similar variation trend as for soil water shown in Fig. 1. Mean cotton lint yield in 1998 was 962 kg ha-1 in the 75% ET transect and 704 kg ha-1 in the 50% ET (Fig. 2), an increase of 37%. There is a drop of 6.7 m in the 50% ET area and of 7.2 m in the 75% ET area, from the north plateau to the middle of the field. Cotton lint yields were higher in the low elevation area (Fig. 2), where the soil was generally wetter throughout the growing season (Fig. 1). Cotton lint yields were lower on the slope and the plateau (Fig. 2), where soil is more exposed to heat, solar irradiance, and perhaps erosion. The latter could cause the removal of organic matter, silt, and clay from the plow layer resulting in loss of water holding capacity and nutrients.

Cotton lint yield standard error (SE) and range were most variable compared to other response parameters. Tests of the mixed effects of irrigation and N fertilizer on soil and cotton responses showed that soil water content, lint yield, and N uptake increased significantly with increasing irrigation (Table 1), especially lint yield (P > 0.0012).

However, there was neither an effect of N input nor interaction between water and N inputs (Table 1), and the covariance of the model residual (Table 2) was significant on soil water content, lint yield and N uptake (P > 0.0001). This covariance could consist of influences from other independent variables such as topography, texture, or soil nutrients rather than N.