NITRATE LEACHING IN DRYLAND AGROECOSYSTEMS AS INFLUENCED BY SOIL AND CLIMATE GRADIENTS

Farmers in dryland agriculture areas of the Central Great Plains have characteristically practiced alternate crop-fallow to stabilize yields. Large amounts of N released soon after sodbreaking and more recent fertilizer additions may have contributed to N movement below crop root zones. Improved water conservation techniques during fallow periods increases the possibility of NO3 leaching below root zones in modern-day crop-fallow systems. Soil topography and water supply may affect leaching potential. This study was conducted to test hypotheses regarding landscape position effects and potential evapotranspiration on depth of water and NO3 penetration. Three sites in eastern Colorado with equal annual precipitation, but with mean potential evapotranspiration varying from 1000 to 1900 mm yr-1 were evaluated. A soil catena was sampled at each site to test landscape effects. All sites were previously managed under tilled wheat-fallow systems for at least 50 yr. All summit (upland) sites had higher water and NO3 contents below their crop root zones than nearby native prairie sites. Soil water and NO3 content of soil profiles to a depth of 9 m were not related to slope position, but there was an inverse relationship between water and NO3 content of soil profiles and potential evapotranspiration. The total NO3-N in cultivated profiles below the normal root zone of crops varied from 126 kg ha-1 at the northern site to 47 kg ha-1 at the southern site. Although some NO3 leaching had occurred, it did not appear to be a major problem for any particular landscape position or climatic zone on cultivated soils.