Indicators of nitrate leaching loss under different land use of clayey and sandy soils in southeastern Oklahoma

Evidence of increasing nitrate (NO3−) leaching losses from soils under various land-use systems has elevated the interest and need to find better land-management practices. An essential step is to find and understand the common parameters that closely relate to leaching losses across a wide range of land-use systems. The overall objective of this study was to relate NO3− leaching losses to various properties across soil types and land-use systems. We chose two locations in southeastern Oklahoma with different soil types. The land-use systems were old-field (abandoned grassland), forest and cultivated arable crop. Parameters determined were soil biomass C, biomass N, total C, total N, extractable mineral N, pH, moisture, CO2 emission and lysimeter-drained water NO3− concentration. Firstly, stepwise multiple linear-regression analyses were performed between biannual NO3− leaching losses and monthly measured independent variables. Secondly, annual and biannual timescale-independent variables and respective NO3− leaching losses were used to perform a regression model. Monthly measured parameters explained over 75% of the biannual NO3− leaching losses, but often with different explanatory variables. Land use was the dominant parameter on monthly models, while soil total C, total C/N ratio, extractable NH4+ and NO3− concentrations and water flux were the most frequently included parameters. When annual- and biannual-mean parameter values were used in regression, respective NO3− leaching losses were explained by somewhat similar parameters to monthly models. Together, these parameters explained only 63, 53 and 44% of the total variation in NO3− leaching loss for the first, second and biannual period, respectively. These models explained a consistently lower amount of variation than the monthly models. Additionally, more than 70% of the total variation could be explained by NO3− concentration and the tested soil-type related parameters. Results from this study revealed that annual and biannual trends of NO3− leaching losses across land-use systems can be explained by a number of parameters collectively rather than a single parameter. Thus, there is no single parameter that can be used to fully predict NO3− leaching losses across different land-use systems on a monthly, annual and biannual basis. Additionally, there is no specific month or season for sampling to better predict NO3− leaching losses across different soil and land-use systems.